Search

Background: allogeneic HSCT is the only potentially curative treatment for many non-malignant diseases (NMD), either inherited or acquired. However, many patients lack an HLA-matched donor (familiar (MFD) or unrelated (MUD)) and the outcome of children transplanted from an HLA-haploidentical relative (haplo) was historically inferior to that of transplants from a MFD or a MUD. We previously published promising results in a cohort of 23 children with NMD given this type of allograft (Bertaina et al., Blood 2014), demonstrating a low transplant-related mortality (TRM) and high cure rates. Here, we report the outcome of a large cohort of children affected by NMD who received a TBdepl-haploHSCT at our Center (NCT01810120). Patients and methods: Between February 2011 and June 2020, 80 consecutive patients affected by NMD received TBdepl-haploHSCT from an HLA-partially matched relative at Ospedale Pediatrico Bambino Gesù in Rome, Italy. Patients had many different disorders (see Table for details on patient- and transplant-related characteristics). Median time from diagnosis to transplant for the whole cohort was 12 months (range 1-177), while it was 2.5 months (range 1.3-11.2) for SCID patients. All patients, including children with SCID, received a conditioning regimen, which varied according to the original disease. Pre-transplant anti-thymocyte globulins (from day -4 to day -2) were given to modulate bi-directional donor/recipient alloreactivity, while rituximab (on day -1) was administered to prevent PTLD. Moreover, no post-transplant pharmacological GvHD prophylaxis was given. Results: fifty-eight patients (72.5%) achieved primary donor cell engraftment, while 3 patients experienced secondary graft failure (GF); the cumulative incidence of either primary or secondary GF was 27.8% (95% CI 17.2-37.0). Median time to neutrophil and platelet recovery was 13.5 (range 9-33) and 10 days (range 7-51), respectively. As expected, GF occurred more frequently in children with disorders known to be associated with an increased GF risk (i.e., HLH, thalassemia, SAA or osteopetrosis) (see also Figure 1A). Three children (4%) experiencing GF died because of infectious complications before retransplant. Sixteen of the 22 patients with either primary or secondary GF were successfully retransplanted (2 with a mismatched unrelated cord blood unit, the other having received a second TBdepl-haploHSCT from either the same donor or the other parent). Since 3 other patients died [all because of infectious complications, 2 due to disseminated adenovirus infection and 1 to CMV pneumonia)], TRM is 7.8% (95% CI 1.6-13.7). Eighteen patients experienced acute GVHD of any grade, the cumulative incidence of this complication being 22% (95% CI 13.5-31.8); 10/18 patients developed grade II acute GVHD (no patient developed grade III or IV aGVHD), this resulting into a cumulative incidence of 12.9% (95% CI 6.6-21.4). Only one patient at risk developed mild chronic GVHD. Twenty-two and 7 patients developed clinically-relevant (i.e., with a viral load > 1000 copies/ml and/or requiring specific antiviral-treatment) CMV and adenovirus infection, respectively, at a median time of 4 (range 0-16) and 1 (range 1-4) weeks from HSCT. Time averaged area under the curve (i.e., viral burden under the curve/weeks at risk for infection) for CMV and ADV are reported in Figure 1B. With a median follow-up of 36 months (range 2 - 110), the 5-year probability of overall survival and event-free survival for the entire cohort of patients is 92.1% (95% CI 83.3-96.4) (Figure 1C) and 68.1% (95% CI 56.4-77.2), respectively. Considering the 16/22 given a successful 2nd allograft, the 5-year disease-free survival is 88.4% (95% CI 78.9-93.8). Details on reconstitution of CD3+, CD4+ and CD8+ lymphocytes are reported in Figure 1D. Conclusions: TBdepl-haploHSCT is an effective option for children with different NMD. GF (either primary or secondary) is a challenging problem in a sub-group of patients at risk (i.e., those with HLH, thalassemia, SAA or osteopetrosis): thus, new strategies to overcome this problem are desirable. However, a second transplant is able to rescue most of these patients. Prompt availability of this type of transplant, limiting infectious risk, low incidence of both acute and chronic GvHD preserving a good quality of life in patients makes this strategy an attractive choice in patients with NMD. Figure 1 Disclosures Merli: Bellicum Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; SOBI: Consultancy, Membership on an entity's Board of Directors or advisory committees; Jazz: Honoraria; Sanofi-Genzyme: Honoraria; Atara Therapeutics: Honoraria. Algeri:BlueBird Bio: Membership on an entity's Board of Directors or advisory committees; Atara Therapeutics: Membership on an entity's Board of Directors or advisory committees. Locatelli:Jazz Pharmaceeutical: Speakers Bureau; Medac: Speakers Bureau; Miltenyi: Speakers Bureau; Bellicum Pharmaceutical: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

CD8+ T-cell activation has been demonstrated to distinguish patients with primary and infection-associated hemophagocytic lymphohistiocytosis (HLH) from patients with early sepsis. We evaluated the activation profile of CD8+ T cells in patients with various forms of secondary HLH (sHLH), including macrophage activation syndrome (MAS). Peripheral blood mononuclear cells from children with inactive systemic juvenile idiopathic arthritis (sJIA, n = 17), active sJIA (n = 27), MAS in sJIA (n = 14), infection-associated HLH (n = 7), and with other forms of sHLH (n = 9) were analyzed by flow cytometry. Compared with patients with active sJIA, in patients with MAS and sHLH of different origins, beside a significant increase in the frequency of CD38high/HLA-DR+CD8+ T cells, we found a significant increase in the frequency of CD8+ T cells expressing the CD4 antigen (CD4dimCD8+ T cells). These cells expressed high levels of the activation markers CD38 and HLA-DR, suggesting they were a subset of CD38high/HLA-DR+CD8+ T cells, as well as of the activation/exhaustion markers CD25, PD1, CD95, and interferon-γ. The frequency of CD4dimCD8+ T cells strongly correlated with most of the laboratory parameters of MAS severity and with circulating levels of CXCL9 and interleukin-18. These findings were confirmed in a prospective replication cohort in which no expansion of any particular T-cell receptor Vβ family in CD3+ T cells of patients with sHLH was found. Finally, frequency of CD4dimCD8+, but not of CD38high/HLA-DR+CD8+ T cells, significantly correlated with a clinical severity score, further supporting the involvement of these cells in MAS/sHLH pathogenesis.

Background: A major cause of mortality in patients receiving hematopoietic stem cell transplantation (HCT) is acute graft-versus-host disease (GVHD), a multiorgan disorder that includes the skin, liver and gastrointestinal tract. We have previously identified elafin, a protease inhibitor overexpressed in inflamed epidermis, as a diagnostic biomarker of GVHD in the skin, the most commonly involved GVHD organ. However, our initial study was limited to a subset of patients with isolated skin GVHD. The main driver of nonrelapse mortality (NRM) in HCT patients is GI GVHD. Two biomarkers, Regenerating islet-derived 3a (REG3α) and Suppressor of tumorigenesis 2 (ST2), have since been validated as biomarkers of GI GVHD that predict long-term outcomes in patients treated for GVHD. We undertook this study to determine the utility of elafin as a prognostic biomarker of acute GVHD in the general population of previously unstudied acute GVHD patients, and to compare it to ST2 and REG3α. Study Design: 526 patients who received systemic corticosteroid treatment for skin GVHD were analyzed from the Mount Sinai Acute GVHD International Consortium (MAGIC), which includes patients from 25 HCT centers. We used ELISA to measure serum concentrations of elafin, ST2 and REG3α. Patients were divided randomly into equal training and validation sets; and we developed a competing risk regression model for 6-month NRM using elafin concentration in the training set. We developed additional models for 6-month NRM using concentrations of ST2 and REG3α, or the combination of all three biomarkers as predictors. We then constructed ROC curves to evaluate the predictive accuracy of each model and to analyze the ability of each model to stratify patients into high- and low-risk groups. We analyzed the cumulative incidence of 6-month NRM and overall survival in each model and compared the accuracy of each model in the validation set. Results: The area under the receiver operating curve (AUROC) for elafin alone was 0.55 whereas it was 0.75 and statistically superior (P = 0.02) for the combination of ST2 and REG3α. The combination of 3 biomarkers produced an AUROC of 0.76 that was not significantly better than the two biomarker model (P = 0.10). Elafin concentrations, either alone or in combination with ST2 and REG3α, did not produce higher hazard ratios of NRM (data not shown). Patients in the low-risk elafin group paradoxically demonstrated a higher incidence of 6-month NRM, although this difference was not statistically significant (17% vs. 11%, P=0.19), and both overall survival at 6 months (68% vs. 68%, P>0.99) and four-week response (78% vs. 78%, P=0.98) were similar in the low- and high-risk elafin groups (Figure 1). As demonstrated in previous data sets, the combination of ST2 and REG3α divided patients into two groups with a nearly five-fold difference in NRM (6.7% vs. 31%, P Conclusion: We demonstrated that serum elafin concentrations measured at the initiation of systemic treatment for acute GVHD in a multicenter population of patients treated systemically for acute GVHD do not predict 6-month NRM, overall survival, or treatment response. As seen in previous studies, serum concentrations of the GI GVHD biomarkers ST2 and REG3α were significant predictors of NRM and the addition of elafin levels did not improve their accuracy. These results underscore the importance of GI disease in driving NRM in patients who develop acute GVHD. Figure 1. Cumulative incidence of nonrelapse mortality and overall survival in high and low risk groups Six-month cumulative incidences of nonrelapse mortality (NRM) in high (solid line) and low (dotted line) risk groups defined by optimized biomarker thresholds (upper panels) and six-month overall survival estimated using the Kaplan-Meier method (lower panels). (A) Cumulative incidence of NRM (14%) and overall survival (75%) in the total validation set (N=263). (B) Cumulative incidence of NRM in the low (N=150) and high (N=113) elafin group (17% vs. 11%, P=0.19). Overall survival in the low and high elafin group (68% vs. 68%, P > 0.99). (C) Cumulative incidence of NRM in the low (N=175) and high (N=88) ST2 + REG3a group (6.7 vs. 31%, P < 0.001). Overall survival in the low and high ST2 + REG3a group (77% vs. 51%, P < 0.001). (D) Cumulative incidence of NRM in the low (N=180) and high (N=83) elafin + ST2 + REG3a group (7.0 vs. 30%, P < 0.001). Overall survival in the low and high elafin + ST2 + REG3a group (79% vs. 64%, P < 0.001). Figure 1 Figure 1. Disclosures Ozbek: Viracor: Patents & Royalties: GVHD biomarker patent with royalties from Viracor. DeFilipp: Omeros, Corp.: Consultancy; Incyte Corp.: Research Funding; Regimmune Corp.: Research Funding; Syndax Pharmaceuticals, Inc: Consultancy. Grupp: Novartis, Kite, Vertex, and Servier: Research Funding; Jazz Pharmaceuticals: Consultancy, Other: Steering committee, Research Funding; Novartis, Roche, GSK, Humanigen, CBMG, Eureka, and Janssen/JnJ: Consultancy; Novartis, Adaptimmune, TCR2, Cellectis, Juno, Vertex, Allogene and Cabaletta: Other: Study steering committees or scientific advisory boards. Hexner: Blueprint medicines: Membership on an entity's Board of Directors or advisory committees, Research Funding; Tmunity Therapeutics: Research Funding; PharmaEssentia: Membership on an entity's Board of Directors or advisory committees. Kitko: Co-investigator on two NIH grants as part of the cGVHD consortium: Research Funding; Vanderbilt University Medical Center: Current Employment; PER: Other: PER - CME educational talks about GVHD; Horizon: Membership on an entity's Board of Directors or advisory committees. Qayed: Novartis: Honoraria; Mesoblast: Honoraria; Medexus: Honoraria. Reshef: ilead, BMS, Precision, Immatics, Atara, Takeda, Shire, Pharmacyclics, Incyte: Research Funding; Bayer: Consultancy; Gilead and Novartis: Honoraria; BMS, Regeneron, TScan, Synthekine, Atara, Jasper, Bayer: Consultancy. Levine: Incyte: Consultancy, Research Funding; Viracor: Patents & Royalties: GVHD biomarker patent with royalties from Viracor; Mesoblast: Consultancy, Research Funding; Equillium Bio: Membership on an entity's Board of Directors or advisory committees; X4 Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Talaris Therapeutics: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Omeros: Membership on an entity's Board of Directors or advisory committees; Symbio: Membership on an entity's Board of Directors or advisory committees; Biogen: Research Funding; Kamada: Research Funding. Ferrara: Eurofins Viracor: Consultancy, Other: Royalties. Chen: Incyte: Consultancy; Gamida: Consultancy.

Background: WAS is a rare X-linked recessive disorder, characterized by thrombocytopenia with low platelet volume, recurrent infections, eczema, autoimmunity, vasculitis and increased incidence of malignancies. Patients with classical WAS have chronic morbidities, severely impaired quality of life and a decreased life expectancy. Allogeneic HSCT is the only well-established curative treatment, gene therapy being still an experimental approach. Excellent outcome have been reported in patients transplanted at early age from either matched related and unrelated donors, with 5-year overall survival (OS) exceeding 90%. However, experience with HLA-haploidentical HSCT is limited and has been historically associated with inferior results. Haploidentical HSCT after selective depletion of α/β+ T-cells and CD19+ B-cells (TBdepl-haploHSCT) was shown to be safe and effective in children with multiple types of non-malignant disorders (Merli et al, Blood Adv 2021). To further optimize this approach and accelerate the recovery of adaptive immunity, we conducted a phase I/II trial evaluating the safety and efficacy of post-transplant infusion of a titrated number of donor T-cells transduced with the inducible caspase-9 (iC9) suicide gene (BPX-501, or rivogenlecleucel, cells) in children with either malignant or non-malignant disorders (ClinicalTrials.gov identifier: NCT02065869). We report the outcome of a cohort of 12 children affected by WAS who received a TBdepl-haploHSCT at our Center. Patients and methods: Between 2014 and 2021, 12 patients affected by WAS received a TBdepl-haploHSCT from an HLA-partially matched relative at Ospedale Pediatrico Bambino Gesù in Rome, Italy. Patients and transplant characteristics are depicted in Table 1. All patients received a myeloablative conditioning regimen, combining pharmacokinetic-adjusted busulfan with Thiotepa and Fludarabine. Pre-transplant anti-thymocyte globulins (from day -4 to day -2) were given to modulate bi-directional donor/recipient alloreactivity, while rituximab was administered on day -1 to prevent PTLD. No post-transplant pharmacological GvHD prophylaxis was employed. Eight subjects, enrolled in NCT02065869 trial, additionally received post-transplant infusion of BPX-501 cells (dose: 1x10 6 cells/kg) at a median time of 16 days after HSCT (range 12-20). Results: 11 patients achieved primary donor cell engraftment, while one patient experienced secondary graft failure (GF), likely triggered by CMV reactivation. Median time to neutrophil and platelet recovery was 15 (range 8-33) and 10 days (range 9-16), respectively. The patient with secondary GF was successfully re-transplanted with a second TBdepl-haploHSCT from the same donor. Grade I/II skin acute GvHD (aGvHD) occurred in 3 patients, the cumulative incidence of aGvHD being 25.9% (95% CI 0-47.7). None of these 3 patients required activation of iC9 with rimiducid and no cases of chronic GVHD (cGvHD) were observed. No patient died. With a median follow-up of 58 months (range 1 - 78), the 5-year probability of OS and event-free survival is 100% and 90.7% (95% CI 50.8-98.7), respectively. Considering the successful second allograft, the 5-year disease-free survival is 100%. Platelet recovery is reported in Figure 1A. Two months after the allograft, all evaluable subjects had a platelet count above 50.000/microL (median 202.000, range 53.000-353.000). All subjects but one have full donor chimerism at last follow-up, the remaining patient having stable mixed chimerism (40% of donor cells) without any WAS manifestation. All subjects with a follow-up of at least 6 months are independent of immunoglobulin replacement. Details on reconstitution of lymphocytes subsets are reported in Figure 1B,C,D. Conclusions: TBdepl-haploHSCT after Bu-based conditioning regimen is an highly-effective curative option for children with WAS, being characterized by high-engraftment rate with fast recovery of both neutrophils and platelets, low incidence of aGvHD and no occurrence of cGVHD. Given the prognostic impact of age in determining HSCT outcome in WAS and prompt availability of haploidentical family donors, our data suggest that this approach should be offered without delay to those patients with WAS who lack a matched donor. Infusion of BPX-501 cells contribute to accelerate the recovery of adaptive T-cell immunity, further increasing the safety of the procedure. Figure 1 Figure 1. Disclosures Merli: SOBI: Consultancy; JAZZ: Consultancy. Locatelli: Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio, Inc.: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyl: Honoraria.

Background: Pediatric myelodysplastic syndromes (MDSs) are a heterogeneous group of clonal disorders, accounting for less than 5% of childhood hematologic malignancies. Usual indications to HSCT are: MDSs with excess of blasts, MDSs secondary to previously administered chemoradiotherapy and RCC associated with monosomy 7, complex karyotype, severe neutropenia, or erythrocyte/platelet transfusion dependence [Locatelli & Strahm, Blood 2018]. We previously demonstrated that TBdepl-haploHSCT is a suitable option for children with acute leukemia, with outcomes comparable to those reported in studies using either an HLA-identical sibling or an unrelated volunteer as donor. Here we present the results of this approach in children with MDSs. Patients and methods: Between February 2013 and February 2021, 23 children with MDSs other than juvenile myelomonocytic leukemia received TBdepl-haploHSCT from an HLA-partially matched relative at Ospedale Pediatrico Bambino Gesù, Rome, Italy or at IRCCS Fondazione Policlinico San Matteo, Pavia, Italy as part of a prospective study (#NCT01810120). All patients were prepared to the allograft using a fully-myeloablative conditioning regimen including a combination of cytotoxic drugs and/or total body irradiation (TBI). Anti-T-lymphocyte globulin (ATLG) was used before transplantation (12 mg/kg total dose, from days -5 to day -3) to modulate bi-directional donor/recipient alloreactivity. Rituximab (200 mg/sqm) was administered on day -1 to prevent post-transplantation EBV-induced lymphoproliferative disorders (PTLD). No patient received any post-transplant pharmacological GvHD prophylaxis. Results: Characteristics of patients enrolled in the study are shown in Table 1 (which reports also donor and graft characteristics). Median follow-up of surviving patients is 4.2 years (range: 0.5 - 8.5 years). Seventeen children were affected by refractory cytopenia of childhood (RCC) (2 cases occurring in the context of inherited bone marrow failure syndromes: one had GATA2 deficiency and the other SAMD9L mutation), while 1 and 5 were affected by MDS with excess of blasts 1 (EB1) and EB2 (one had GATA2 deficiency), respectively. Median time to neutrophil and platelet recovery was 14 (range 10-19) and 11 (range 9-14) days, respectively, with four patients (3 with RCC and 1 with EB2) experiencing primary graft failure, the cumulative incidence of this complication being 17.3% (95% CI 0.3-31.5). All these 4 patients were rescued with a second TBdepl-haploHSCT from the same or the other parent. Cumulative incidence of grade II-III acute GvHD was 11.4% (95% CI 0-25.2). One patient developed skin and gut GvHD after the second TBdepl-haploHSCT, while for all other patients skin was the sole organ involved; no case of grade IV GvHD was observed. One patient developed moderate chronic GvHD [cumulative incidence 5.2% (95% CI 0-14.8)], which completely resolved with low-dose steroids and ruxolitinib. Notably, no patient died for transplant-related complications. Six patients experienced CMV, 2 HHV-6 and 1 adenoviral infection/reactivation; one patient developed lung aspergillosis, which resolved with specific treatment. One patient affected by EB2, not in remission at time of transplant, relapsed 27 months after HSCT, the 5-year cumulative incidence of relapse being 7.1% (95% CI, 0-19.7); she eventually died after failing a second HSCT. The 5-year probability of overall and event-free survival were 92.3% (95% CI 56.6 -98.9) and 76.3% (95% CI 51.3-89.6) (Figure 1A and B), respectively. Five-year disease-free-survival was 90% (95% CI 47.3-98.5). Because of the low number of events, no prognostic factor related to OS and EFS was found. In particular, the MDS variant did not influence the patient's outcome. The median CD3+ cell count on day +30, +90, +180 and +360 were 113, 171, 558 and 1307/mcl, respectively. Conclusions: These data indicate that TBdepl-haploHSCT is a safe and effective transplant option also in children with MDS. Indeed, the low risk of both non-relapse mortality and acute/chronic GvHD makes this approach particularly attractive in the pediatric setting. Moreover, this haplo strategy compares favorably with T-cell replete approaches [Suo et al., 2020]. Figure 1 Figure 1. Disclosures Merli: JAZZ: Consultancy; SOBI: Consultancy. Locatelli: Miltenyl: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio, Inc.: Consultancy; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Acute myeloid leukemia (AML) is a heterogeneous disease where selected subgroups of patients, linked by the presence of biological and clinical high-risk features, are candidates to receive allogenic hematopoietic stem cell transplantation HSCT) as post-remission consolidation treatment. The achievement of morphological complete remission (CR) before HSCT is an important pre-requisite to optimize the chance of successful post-transplant outcome. Minimal residual disease (MRD) assessment by quantitative polymerase chain reaction (q-PCR) has been shown to increase the ability to monitor therapy response in AML, improving prognostic accuracy and allowing to refine transplant strategies. Although MRD assessment was shown to have potential benefit when measured after induction and consolidation therapy courses, its role before HSCT remains to be fully elucidated. In order to contribute to better clarify this issue, we conducted a q-PCR I-BFM-AML collaborative study to measure MRD in bone marrow samples collected within 5 weeks prior to HSCT of 108 pediatric AML patients harboring one of the main recurrent AML aberrancies t(8;21)(q22;q22); RUNX1-RUNX1T1, inv(16)(p13.1q22)/t(16;16)(p13.1;q22); CBFB-MYH11, t(9;11)(p22;q23); KMT2A-MLLT3 or FLT3-ITD. Sixty patients underwent HSCT in first complete remission (CR1) with an overall survival (OS) of 84% versus 54% for the 48 transplanted in CR2 achieved after an initial relapse. Sixty patients showed q-MRD negativity (defined as a value lower than 2.1x10-4 calculated by ROC curve analysis with respect to diagnosis or relapse), whereas in 48 patients we detected q-MRD levels >2.1x10-4. Five-year OS after HSCT was 83% for patients with q-MRD negativity, while that of patients with q-MRD above the cutoff was 57% (p=0.012). As regards, cumulative incidence of relapse (CIR), q-MRD above the cutoff was associated with a high risk of recurrence (26% versus 10% for patients with q-MRD 2-log versus 73% for q-MRD2.1x10-4 and 2-log, HR). This combined stratification by q-MRD resulted into a better subdivision of the OS probability, which was 83%, 69% and 39% for LR, IR and HR respectively (p=0.004). Finally, a multivariate Cox regression model revealed that, together with CR status at time of the allograft (CR2, hazard ratio 4.4, p=0.001), q-MRD was an independent factor (hazard ratio 0.5, p=0.001) predicting HSCT outcome. In conclusion, this study supports the role of q-MRD pre-HSCT as a useful prognostic tool in childhood AML, able to provide information to tailor transplant strategies involving conditioning regimen intensity and graft-versus-host disease prophylaxis. Disclosures Reinhardt: AbbVie: Consultancy; Novartis: Consultancy, Other: Institutional Research Funding; Jazz: Consultancy, Other: Institutional Research Funding; Celgene: Consultancy, Other: Institutional Research Funding; bluebird bio: Consultancy; Roche: Consultancy, Other: Institutional Research Funding; Biotest: Other: Institutional Research Funding; Novo Nordisk: Other: Institutional Research Funding; Behring: Other: Institutional Research Funding. Merli:Bellicum Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; SOBI: Consultancy, Membership on an entity's Board of Directors or advisory committees; Jazz: Honoraria; Sanofi-Genzyme: Honoraria; Atara Therapeutics: Honoraria.

Introduction Juvenile myelomonocytic leukemia (JMML) is a rare pediatric hematological disease, characterized by aberrant proliferation of myeloid precursors and hypersensitivity to GM-CSF stimulation. Mutations in PTPN11, N/K-RAS, CBL, or NF1 genes are found in ~90% of the cases. Allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative therapy for those patients harboring somatic PTPN11 mutations, while in patients with somatic NRAS mutations, transplantation need a careful evaluation, as these patients may have spontaneous resolution of the disease. Here, we show data from a deep-sequencing analysis enlightening typical molecular features of CD33+/34+ and CD14+ primary cells from NRAS and PTPN11 mutated patients compared with healthy donors (HDs). Patients and Methods Four Cord Blood samples from healthy donors (HDs) and 16 Bone Marrow (BM) samples from PTPN11 (N=8) and NRAS (n=8) mutated JMML patients were collected after approvals from International Review Boards of each Institution. CD33+/34+ and CD14+ cells were magnetically sorted following MACS protocols (Miltenyi). TruSeq and NextSeq kits (Illumina) were used for libraries preparation in order to perform the global transcriptomic and smallRNAs sequencing analyses on Illumina platform, according to the company protocols. StringTie and DESeq2 were used for differential expression analysis and REDItools python scripts for RNA editing analysis. Results In our first analysis, we screened ~60000 gene transcripts, finding different expression signatures between HDs, PTPN11 and NRAS samples. We built Expression Heat-Maps reporting the top 100 most statistically significant (Fig. 1, 2; adjusted p < 0.001) deregulated sequences in all the possible pairwise comparisons. Among the top 100 transcripts, we identified differential expression of 5 different genes: ZNF185, MRLPL30 resulted upregulated in PTPN11 vs NRAS CD14+ cells while CD36, RAG2 and CAMK2A resulted downregulated. Interestingly, comparison of PTPN11 and NRAS cohorts in CD33+/34+ subset, these 5 transcripts showed the opposite expression trend. Moreover, among the upregulated genes, we identified a subset of PRAME paralog transcripts in both NRAS and PTPN11 patients when compared to HDs. Seven transcripts (PRAMEF4, 5, 6, 9, 11, 15, 26) were shared between CD33+/34+ and CD14+ populations, while PRAMEF7, 8, 20, 21 resulted upregulated in CD33+/34+ cells and PRAME, PRAMEF22, 23, 25 were upregulated in CD14+ cells, as compared to HDs. In addition, we found aberrant upregulation of Hemoglobin γ chains 1 and 2 (HBG1, 2, i.e. those needed for fetal hemoglobin) in all JMML vs HDs in CD14+ cells, while in CD33+/34+ a downregulation of Hemoglobin α and β chains (HBA1,2 and HBB) in PTPN11 samples vs NRAS was detected. We also report an enhanced LIN28B expression in JMML samples vs HDs but only in CD33+/34+ cells. A global ADAR-dependent RNA editing analysis revealed differences in CD33+/34+ cells showing an enhanced Adenine to Inosine conversion in NRAS patients vs PTPN11 and HDs. Finally, microRNAs sequencing reveals a significant upregulation of miR-144-3p in all JMML samples and cell lineages compared to HDs, while the miR-146a-5p was upregulated only in CD14+ cells. Also the miR-22 showed decreased expression in PTPN11 vs HDs in CD33+/34+ subset, while the miR-29a-3p was downregulated in NRAS vs HDs in CD14+ cells. Discussion We report different expression and editing signatures among samples cohorts and cellular lineages, showing novel insights in JMML molecular biology. Upregulation of several PRAME paralog genes depicts an interesting array of possible targets for immunotherapies (e.g., using genetically modified T cells with a PRAME-specific T-cell receptor), although their expression have to be further validated. Differences in the expression of microRNAs subsets, LIN28B and subunits of the HbA and HbF already described in JMML, but here related to specific mutations and cell lineages, could help to better understand the pathogenic processes and peculiar clinical behaviors. Finally, the novel evidence of differential RNA editing in JMML opens new perspectives for further studies regarding differential protein isoforms expression and regulation as well as microRNA targeting. Disclosures Niemeyer: Celgene: Consultancy. Locatelli:Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy. Merli:Novartis: Honoraria; Sobi: Consultancy; Amgen: Honoraria; Bellicum: Consultancy.

Background: For children with AL candidate to receive an allograft and lacking a suitable HLA-matched donor, HLA-haplo-HSCT after abTCD may represent a valid alternative. Due to delayed recovery of adaptive T-cell immunity with abTCD-haplo-HSCT alone, we conducted a phase I/II trial testing the safety and efficacy of post-transplant infusion of a titrated number of donor-derived T cells transduced with the iC9 gene (rivogenlecleucel; ClinicalTrials.gov id: NCT02065869) in children with malignant and non-malignant diseases. Here, we report on the cohort of 70 patients with AL treated in Italy with abTCD-haplo-HSCT+rivogenlecleucel, comparing the results with those of 88 patients given abTCD-haplo-HSCT alone and previously published by our group (Blood 2018; 132:2594-2607). Patients and methods: Patients (age < 18 years) were transplanted between 2010 and 2018. Patient and disease characteristics are shown in Table 1. Median age at HSCT was 6 years (range 0.3-18), and median follow-up was 71 and 31 months for abTCD-haplo-HSCT and abTCD-haplo-HSCT+rivogenlecleucel, respectively. Compared to the control group, abTCD-haplo-HSCT+rivogenlecleucel recipients were transplanted more recently and from a younger donor, and received a higher number of CD34+ cells (Table 1). Diagnosis did not differ between the 2 groups, acute lymphoblastic leukemia (ALL) being the most frequent diagnosis. All patients were transplanted in morphological complete remission (CR1 and CR2) and received myeloablative preparation. Graft composition is reported in Table 1; notably all patients received >10x106 CD34+cells/Kg and Results: Graft failure occurred in 2% of patients in each group. Median time to neutrophil and platelet engraftment was 14 (6-23) and 11 (5-56) days, with no differences between groups (p=0.28). Rivogenlecleucel were infused at a median time of 21 days (range 11-59). Treatment was well tolerated; no infusion-related side effects were recorded. Cumulative incidence (CI) of 100-day grade II-IV acute GvHD was 18.9% vs 15.9% (p=0.77) and CI of 1-year chronic GvHD was 6.9% vs 5.7% (p=0.56) in abTCD-haplo-HSCT and abTCD-haplo-HSCT+rivogenlecleucel, respectively. The 4-year non-relapse-mortality (NRM) was significantly lower in abTCD-haplo-HSCT+rivogenlecleucel (1.4% vs 8%, p=0.05) (Figure 1). There was no statistically significant difference in the 4-year CI of relapse (RI) (17% vs. 25%, p=0.30), respectively. Disease recurrence was the most common causes of death in both groups, viral and fungal infections being the most frequent non-relapse fatalities. The 4-year overall survival (OS) and leukemia-free survival (LFS) was 70% vs 87%, p=0.01 (Figure 2) and 67% vs 82%, p=0.05, for abTCD-haplo-HSCT and abTCD-haplo-HSCT+rivogenlecleucel, respectively. There was no difference in 4-year CI of CMV reactivation between the 2 groups (p=0.68), median time to CMV reactivation being 29 and 30 days (p=0.29), respectively. Once infused, rivogenlecleucel expanded (mainly in the CD8+ subset), reaching a peak at 9 months after infusion. At 6-months, median CD3+, CD3/CD4, CD3/CD8, CD3-/CD56 and CD20/CD19 count/microL were 820, 265, 225, 141, 171, for abTCD-haplo-HSCT and 898, 294, 288, 214, and 161 for abTCD-haplo-HSCT+rivogenlecleucel, (p=ns, p=ns, p=0.02, p=0.03, p=ns), respectively. The advantage in the recovery of CD3/CD8 and CD3-/CD56 after abTCD-haplo-HSCT+rivogenlecleucel persisted at 1 year (p=0.01, p=0.03, respectively). In multivariable analysis, abTCD-haplo-HSCT+rivogenlecleucel was associated with better OS (HR 0.27, p=0.003) and LFS (HR 0.40, p=0.001); there was also a trend for lower relapse risk (HR 0.50, p=0.098). Age below the median value at HSCT (HR 2.62, p=0.01), CR1 at HSCT (HR 0.35, p=0.03) and use of irradiation in the conditioning regimen (HR 0.32, p=0.02) were other factors correlating with OS and LFS. Conclusions: These data confirm that the infusion of donor-derived rivogenlecleucel is safe and well tolerated. Rivogenlecleucel cells infusion contributed to enhance recovery of cytotoxic T and NK cells, improving patients NRM and OS/LFS. Rivogenlecleucel (with the possibility of inducing apoptosis of donor T cells) may facilitate the cellular therapy approaches aimed at optimizing immune recovery after HSCT. Disclosures Merli: Amgen: Honoraria; Bellicum: Consultancy; Novartis: Honoraria; Sobi: Consultancy. Algeri:Bluebird bio: Consultancy, Honoraria; Miltenyi: Honoraria; Atara Biotherapeutics: Consultancy, Honoraria. Woodard:Bellicum Pharmaceuticals, Inc: Employment, Other: Stock, Stock options. Locatelli:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BluebirdBio: Consultancy.

Prognosis of a significant proportion of patients with chemotherapy-refractory or multiply-relapsed CD30+ Non-Hodgkin's Lymphoma (NHL) or Hodgkin lymphoma (HL) still remain poor. Targeting CD30 with monoclonal antibodies in HL and anaplastic large cell lymphoma was shown to induce remarkable clinical activity; however, occurrence of adverse events (mainly neuropathy) may result into treatment discontinuation in many patients. Immunotherapeutic approaches targeting CD30 by chimeric antigen receptor (CAR) has been demonstrated to be of value in two independent clinical trials, although clinical benefit was sub-optimal. We designed a new CAR construct characterized by an anti-CD30 single-chain variable-fragment cassette (AC10), linked to CD3ζ by the signaling domains of two costimulatory molecules, namely either CD28.4-1BB or CD28.OX40. The inducible Caspase-9 (iCasp9) safety switch was included in both constructs with the goal of promptly controlling undue toxicity. As a selectable marker, we added in frame the CD34 antigen. The in vitro anti-tumor efficacy was evaluated by using either the NHL cell line: Karpas299, or the HL cell lines: L428, in both short-term cytotoxic assay (51Cr release assays) and long-term co-cultures for 6 days. Supernatant from co-culture experiments was analyzed by Elisa. We assessed the antitumor effect of CAR.CD30 T cells in a in vivo NSG mouse model engrafted i.v. with lymphoma FF-luciferase cell lines Karpas299 or L428, and monitored tumor growth by IVIS Imaging system. For tumor re-challenging, mice of the NHL model surviving until day +140, were i.v. infused with 0.2x106 Karpas299 cells, and subsequently followed for additional 110 days. Persistence of CAR.CD30 T cells was evaluated, together with a deep characterization of memory profile of T cells. Independently from the costimulatory domains CD28.OX40 or CD28.4-1BB, the generated retroviral vectors showed similar transduction efficiency of T cells (86.5±5.1% and 79.3±5.3%, respectively). Nevertheless, CD28.OX40 costimulatory domains was associated with more stable expression of the CAR over time, during extensive in vitro culture (84.72±5.30% vs 63.98±11.51% CD28.4-1BB CAR T cells at 30 days after transduction; p=0.002). For both CAR constructs, we did not observe any significant difference in the suicide gene iCasp9 activity, both in vitro and in vivo. In short-term cytotoxic assay, both CAR.CD30 T cells significantly and specifically lysed CD30+ NHL and HL tumor cell lines. In long-term co-culture, CD28.OX40 showed a superior anti-lymphoma in vitro activity as compared to CD28.41BB T cells, when challenged at very high tumor/effector ratio (8:1) (for Karpas 299; p=0.03). Moreover, the antigen stimulation was associated to higher levels of Th1 cytokine production, with CD28.OX40 T cells secreting a significantly higher amount of IFNγ, IL2 and TNFα as compared to CD28.41BB T cells (p= 0.040; p=0.008; p=0.02; respectively). Bioluminescence in HL (L428) tumor-bearing mice, treated with NT T cells, rapidly increased up to 5 log in less than 50 days and mice either died or were sacrificed due to morbidity. The best outcome was observed in mice treated with CD28.OX40, as three out of five mice were still alive at the experimental end-point of day+165, as compared with mice treated with CD28.4-1BB (60% vs 0%, p=0.0021). In NHL (Karpas 299) mouse models, CD28.OX40 had an extensive anti-tumor control superior to that of CD28.41BB T cells, leading to a significant reduction of tumor bioluminescence at day 45 (3.32x10 vs 2.29x10, p=0.04). The median survival of mice treated with NT and CD28.4-1BB CAR T cells was 45.5 and 58 days respectively, but undetermined for mice treated with CD28.OX40 CAR T cells (p=0.0002). After 140 days, cured mice were re-challenged with Karpas 299; mice were followed for other 100 days. Bioluminescence analysis showed rapid progression of the tumor in the control mice cohort, as well as in CD28.4-1BB treated mice. In contrast, in CD28.OX40 treated mice, at day+240 days, 4 out of 6 mice were tumor-free, resulting into a statistically significant survival benefit (p=0.0014). Only in mice treated with 28.OX40 T cells, we observed a long-lasting persistence of circulating CAR-T cells up to day +221. In summary, we have developed a novel CAR.CD30 construct displaying features that make it a particularly suitable candidate for a clinical trial in patients suffering from CD30+ tumors. Disclosures Merli: Novartis: Honoraria; Sobi: Consultancy; Amgen: Honoraria; Bellicum: Consultancy. Locatelli:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BluebirdBio: Consultancy; Miltenyi: Honoraria; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

β-thalassemia is one of the most common monogenic blood disorders worldwide, and is highly prevalent in Mediterranean countries. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has been the only curative treatment for transfusion-dependent β-thalassemia (TDT; the most severe disease form) for many years, but it is limited by donor availability and has a significant risk of morbidity and mortality. We conducted a chart review of patients with β-thalassemia who underwent allo-HSCT (N=80) at the Bambino Gesù Children's Hospital, Rome, Italy, between March 2011 and August 2018. Median (range) age at allo-HSCT was 5.5 (0.3-20.0) years [ In total, 18 (22.5%), 28 (35.0%), and 34 (42.5%) patients received allo-HSCT from human leukocyte antigen (HLA)-identical sibling donors, HLA-haploidentical donors, and unrelated donors (fully matched donor: n=28, donor with a single HLA disparity: n=6), respectively. Of these donors, 42 (52.5%) were carriers for thalassemia-associated mutations. In total, 53 (66.3%) donors and 35 (43.8%) recipients were cytomegalovirus-positive. Bone marrow was the stem cell source in 51 cases (63.8%), while 28 patients received an alphabeta T-cell depleted peripheral blood haploidentical HSCT (35.0%); the remaining child (1.3%) received both bone marrow and cord blood from the same related donor. All patients continued to receive transfusions immediately after allo-HSCT; however, only 7 (8.8%) received a transfusion in the 3 to 12-month post-transplantation period (2 due to underlying disease; 5 due to other reasons including GI bleeding). Median (range) time to reach transfusion-free status was 3.8 (1.1-47.8) weeks. Median (interquartile range) hemoglobin levels at 6 and 12 months after allo-HSCT were 10.9 (10.2-11.9) and 11.9 (10.6-13.0) g/dL, respectively. The cumulative incidences of primary and secondary graft failure were 10.0% and 12.5% at 24 months (HLA-identical donor: 0% and 11.1%, haploidentical donor: 17.9% and 3.6%, unrelated donor: 8.8% and 20.6%). Eleven out of 14 patients experiencing graft failure were successfully rescued with a second allograft, while 2 patients were not retransplanted due to parental decision and 1 patient died after the engraftment of the second allograft. Eight patients developed grade II-IV acute graft-versus-host disease (GVHD) and one patient developed moderate chronic GVHD. Cumulative incidence rates of grades II-IV and III-IV acute GVHD were 12.7% and 8.0% at 24 months (HLA-identical donor: 0% and 0%, haploidentical donor: 7.3% and 0%, unrelated donor: 23.8% and 18.8%). Three patients (3.8%) died of transplant-related causes (1 case each of hemophagocytic lymphohistiocytosis, sepsis, and multi-organ failure [the patient receiving the second allograft]) with a median (range) time from transplantation to death of 8.7 (3.7-11.0) months. Of these patients, all had been transplanted from an unrelated donor and 2 had reached sustained full-donor chimerism. The probability of overall and event-free (event defined as either death or primary/secondary graft failure) survival was 96.2% and 81.2% at 24 months (HLA-identical sibling donor: 100% and 88.9%, haploidentical donor: 100% and 78.6%, unrelated donor: 91.2% and 79.4%). The probability of thalassemia-free survival (event defined as either death or primary/secondary graft failure not rescued by a second allograft) was 93.7% at 24 months (HLA-identical sibling donor: 100%, haploidentical donor: 92.9%, unrelated donor: 91.2%). In this large single-center cohort of children with predominantly TDT, allo-HSCT led to beneficial outcomes for most patients, resulting in the discontinuation of transfusions with 93.7% of patients being thalassemia-free. Nevertheless, HSCT is still associated with GVHD, graft failure, and mortality, and only 22.5% of patients had an HLA-identical sibling donor, illustrating a key limitation of allo-HSCT. Emerging research is addressing such barriers to treatment. Disclosures Merli: Novartis: Honoraria; Sobi: Consultancy; Amgen: Honoraria; Bellicum: Consultancy. Algeri:Miltenyi: Honoraria; Atara Biotherapeutics: Consultancy, Honoraria; Bluebird bio: Consultancy, Honoraria. Gruppioni:Bluebird bio: Employment, Equity Ownership. Kommera:Bluebird bio: Employment, Equity Ownership. Maa:Bluebird bio: Employment, Equity Ownership. Locatelli:Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria.

Systemic glucocorticoids are the principal treatment for acute graft-versus-host disease (GVHD), which remains the major cause of non-relapse mortality (NRM) after allogeneic hematopoietic cell transplantation (HCT). However, there are no validated biomarkers that measure a patient's response to glucocorticoid therapy, and thus response is evaluated by the change in clinical symptom severity. A major weakness in the predictive power of clinical responses is that changes to all organs are weighted equally even though the major driver of NRM is irreversible damage to the crypts of the GI tract. Recent studies from the Mount Sinai Acute GVHD International Consortium (MAGIC) have validated an algorithm probability (MAP) that combines serum concentrations of two biomarkers of GVHD (REG3α and ST2) to generate an estimated probability of 6 month NRM for individual patients. The MAP has been considered a "liquid biopsy" that estimates the damage caused by GVHD to crypts throughout the lower GI tract (Hartwell et al., JCI Insight, 2017; Major-Monfried et al., Blood, 2018). We hypothesized that the change in MAP between start of treatment and 28 days later could serve as a response biomarker for GVHD and might compare favorably to the change in clinical symptoms that measures response to GVHD treatment, which is widely used as a surrogate for long term survival and is the primary endpoint in most GVHD treatment trials (Martin et al., BBMT, 2009; MacMillan et al., Blood, 2010). We prospectively collected serum samples and clinical staging from 368 sequential HCT patients who received systemic treatment for acute GVHD in one of 20 MAGIC centers between January 2016 and February 2018. We measured the serum concentrations of REG3α and ST2 before and after systemic therapy for acute GVHD and computed MAPs, the changes in MAPs, and clinical responses for each patient. MAPs of patients who experienced 6 month NRM showed significantly greater increases than MAPs of patients who survived (p=0.0004). In patients whose MAPs at the start of treatment were low (Ann Arbor 1, MAP < 0.141) or intermediate (Ann Arbor 2, 0.141 ≤ MAP ≤ 0.290), 6 month NRM clustered among those who had the greatest increases in MAP after 28 days (Fig 1A,B). In patients with high MAPs at the start of treatment (Ann Arbor 3, MAP > 0.290), those who survived tended to have the largest decreases in MAP (Fig 1C). These changes in MAP suggested crossing a single threshold could predict risk of mortality. We found that patients whose MAPs rose above a threshold MAP of 0.290 (5% of Ann Arbor 1, 27% of Ann Arbor 2) had significantly worse survival compared to those who remained below it, whereas the large number patients with initially high MAPs that remained above the threshold (66% of Ann Arbor 3) had a large increases in mortality (Fig 2). When measured at day 28, the MAP was significantly more accurate in predicting NRM than the gold standard of the clinical response, with areas under the receiver operating characteristic curve (AUC) of 0.86 and 0.70, respectively (p We conclude that the MAP is, to our knowledge, the first validated laboratory test to serve as response biomarker for the treatment for acute GVHD and a more accurate predictor of survival than clinical response after four weeks of treatment. The MAP may serve as a novel endpoint and an important complement to changes in clinical symptom severity in future trials of GVHD treatment. Disclosures Srinagesh: National Institutes of Health: Research Funding. Ozbek:Viracor: Patents & Royalties: Biomarker Patent. Ayuk:Novartis: Honoraria, Other: Advisory Board, Research Funding. Aziz:Doris Duke Charitable Foundation: Research Funding. Defilipp:Incyte: Research Funding. Grupp:Novartis: Consultancy, Research Funding; Roche: Consultancy; GSK: Consultancy; CBMG: Consultancy; Novartis: Research Funding; Kite: Research Funding; Servier: Research Funding; Jazz: Other: study steering committees or scientific advisory boards; Adaptimmune: Other: study steering committees or scientific advisory boards; Cure Genetics: Consultancy; Humanigen: Consultancy. Hexner:novartis: Research Funding. Kitko:Mallinckrodt: Honoraria; Novartis: Consultancy, Honoraria. Mielke:EBMT/EHA: Other: Travel support; ISCT: Other: Travel support; Miltenyi: Consultancy, Honoraria, Other: Travel and speakers fee (via institution), Speakers Bureau; Jazz Pharma: Honoraria, Other: Travel support, Speakers Bureau; IACH: Other: Travel support; Kiadis Pharma: Consultancy, Honoraria, Other: Travel support (via institution), Speakers Bureau; DGHO: Other: Travel support; Bellicum: Consultancy, Honoraria, Other: Travel (via institution); GILEAD: Consultancy, Honoraria, Other: travel (via institution), Speakers Bureau; Celgene: Honoraria, Other: Travel support (via institution), Speakers Bureau. Merli:Sobi: Consultancy; Amgen: Honoraria; Novartis: Honoraria; Bellicum: Consultancy. Pulsipher:Amgen: Other: Lecture; Miltenyi: Research Funding; Bellicum: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz: Other: Education for employees; CSL Behring: Membership on an entity's Board of Directors or advisory committees; Adaptive: Membership on an entity's Board of Directors or advisory committees, Research Funding; Medac: Honoraria. Qayed:Bristol-Myers Squibb: Honoraria. Reshef:Pfizer: Consultancy; Magenta: Consultancy; Kite: Consultancy, Research Funding; Atara: Consultancy, Research Funding; BMS: Consultancy; Pharmacyclics: Consultancy, Research Funding; Incyte: Consultancy, Research Funding; Celgene: Research Funding; Shire: Research Funding. Levine:Incyte: Consultancy, Research Funding; Biogen: Other: non-financial support; Viracor: Patents & Royalties: biomarker patent; Ironwood: Honoraria; bluebird bio: Consultancy; National Cancer Institute: Research Funding; Novartis: Honoraria; Kamada: Research Funding. Ferrara:National Institutes of Health: Research Funding; ViraCor: Consultancy; Incyte: Consultancy; Kamada: Consultancy; Mallinckrodt: Consultancy; Enlivex: Consultancy; Xenikos: Consultancy; CSL Behring: Consultancy.

Survival rates of children with relapsed/refractory (r/r) BCP-ALL remain unsatisfactory and little progress has been made in the past 2 decades. Similarly, relapse of childhood B-NHL is usually associated with an aggressive disease and poor outcomes. Targeted immunotherapy with T-cells genetically modified to express a CD19-directed CAR showed an unprecedented antitumor efficacy, leading to the recent FDA and EMA approval of two CD19-CAR products for treatment of BCP-ALL and B-NHL. Relevant toxicities have, however, been reported, mainly related to the development of severe Cytokine Release Syndrome (CRS) and/or of neurotoxicity. At Ospedale Pediatrico Bambino Gesù (OPBG) in Rome, we developed a clinical-grade, 2nd generation, CD19-specific CAR construct, including 4.1bb as costimulatory domain and the inducible caspase-9 safety switch (iC9-CD19-CAR), vehiculated by a retroviral vector, to conduct an academic, phase I/II clinical trial in patients (age 1-25 yrs) affected by BCP-ALL or B-NHL. We now report on the results of the phase I and of the first 8 patients treated in the phase II portion of the study, in terms of feasibility, toxicity, maximum tolerated/recommended dose (MTD/RD) and data on response rate and biological correlates. The phase I, dose-escalation portion of the study included 3 dose levels (DL), namely: DL1, 0.5×106; DL2, 1.5×106; DL3, 3.0×106 CAR+ T cells per kg of recipient body weight. In the phase II portion, patients were treated at the RD identified in the phase I, namely 3.0×106 CAR+cells/kg. All patients received a lymphodepleting regimen consisting of fludarabine and cyclophosphamide for 3 days and iC9-CD19-CAR T cells were subsequently administered as single infusion. Patients were monitored for toxicity, expansion and persistence of iC9-CD19-CAR T cells. Seventeen children were enrolled into the trial and received iC9-CD19-CAR T cells between January 2018 and June 2019. Data were analyzed as of July 20, 2019. The characteristics of the patients are detailed in table 1. The designed dose concentration was successfully produced for all the enrolled patients and we did not observe any production failure. The median transduction rate in the drug product was 54% (range 21-73), while the median vector copy number was 3.8 (range 2.8-6.2). During the phase I portion of the study, no dose limiting toxicities (DLTs) have been recorded, defining the MTD as 3.0×106 CAR+ T cells per kg of recipient body weight. The treatment was overall tolerated and all the toxicities were reversible, the most severe being grade 3-4 neutropenia, thrombocytopenia and/or anemia, occurring in 16/17 (94.1%) patients; in 13/16 patients (81.2%) the hematological toxicity developed before the infusion and persisted after the administration of CAR T cells. Cytokine release syndrome (CRS) occurred in 10/17 patients (58.8%) and was overall moderate, reaching grade 3 (Lee criteria) in one patient only. Notably, none of the patients developed neurotoxicity and no activation of the safety switch was required. All patients were assessed for response at 4 weeks from iC9-CD19-CAR T cell infusion and 13/15 (86.7%) patients with ALL achieved complete remission (CR) with negativity of minimal residual disease (MRD), including 2/3 patients receiving the DL1, 9 patients who had failed a previous allogeneic haematopoietic stem-cell transplantation (HSCT) and 6 patients that had previously received blinatumomab, as CD19-directed immunotherapy. The iC9-CD19-CAR T cells expanded in vivo and were detectable by both flow-cytometry and molecular biology in the blood (Fig.1), bone marrow and cerebrospinal fluid of the responders. One CD19-negative relapse 3 months after infusion was recorded, while 3 additional patients relapsed with CD19+ leukemia blasts. Four patients received HSCT while in CR with MRD negativity because of regrowth of normal CD19+ B cells. The 18-month probability of overall survival for the BCP-ALL cohort is 72.2% (Fig.2). One of the 2 B-NHL patients showed a partial response. Our data indicate that iC9-CD19-CAR T cell in an academic setting is feasible, safe and extremely effective in treating highly resistant/relapsed BCP-ALL. In our trial, no major or life-threatening toxicities were observed and, despite the moderate CRS recorded, high rates of CR were achieved, suggesting that the combination of a retroviral platform and 4.1bb as costimulation is able to mediate a potent antitumor effect Disclosures Merli: Amgen: Honoraria; Novartis: Honoraria; Sobi: Consultancy; Bellicum: Consultancy. Algeri:Bluebird bio: Consultancy, Honoraria; Atara Biotherapeutics: Consultancy, Honoraria; Miltenyi: Honoraria. Locatelli:Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy; Miltenyi: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees.

Background Allogeneic Hematopoietic Stem Cell Transplantation (HSCT) constitutes a curative treatment for children with malignant and non-malignant disorders. HLA-partially matched haploidentical (haplo) donors represent a viable alternative option for those children who lack an HLA-compatible donor. T-cell depletion approaches with positive (CD34 selection) or negative selection (alpha/beta T-cell and CD19+ B-cell-depletion) may allow engraftment of donor cells with a low risk of GvHD; however, success is limited by delayed immune recovery, increasing the risk of fatal infections. Infusion of unmanipulated donor T cells (DLI) to accelerate immune recovery is associated with high risk of fatal GVHD. In contrast, adoptive transfer of donor T cells genetically manipulated to include a safety switch can be a suitable strategy to render DLI safer and more widely applicable. BPX-501 is an allogeneic product consisting of T cells modified to express the inducible caspase-9 (iC9) safety switch. The polyclonal natural of these modified T cells can provide viral immunity following stem cell transplant, with the unique ability to promptly and durably resolve GvHD symptoms following the administration of rimiducid, an inert, lipid-permeable compound that rapidly induces dimerization and activation of iC9, inducing apoptosis of the gene modified T cells. Aims To evaluate the safety and efficacy of rimiducid in the treatment of GvHD following administration of BPX-501 T cells in pediatric patients with malignant or non-malignant disorders given an αβ T-cell receptor and B-cell depleted haplo-HSCT. A key objective of this study is to assess the activity of rimiducid infusion following onset of GvHD which is refractory to standard of care therapies. Methods Two multicenter (US [NCT03301168] and EU [NCT02065869]), prospective trials utilized αβ-T-cell and B-cell-depleted haplo-HSCT followed by infusion of a titrated number of donor lymphocytes genetically modified with iC9 (BPX-501 T cells) in patients with malignant or non-malignant disorders. BPX-501 T-cells were planned to be infused on day14+/-4 after the allograft. No post-transplant pharmacological GvHD prophylaxis was employed. Patients who develop GvHD resistant to conventional steroid therapy could receive ≥1 dose of dimerizing rimiducid to activate the iC9 safety gene. The efficacy evaluable population is defined as any patient who received ≥ 1 dose of rimiducid for the treatment of GvHD and had a follow up response assessment. Results At the time of clinical cut-off (June 30th, 2018) 249 patients with a malignant (41.4%) or non-malignant disorder (58.6%) were enrolled. The conditioning regimens varied according to the original disease and were Treosulfan-based (14.9%), Busulfan-based (28.9%), TBI-based (34.9%) or other (19.3%). The donor was a parent in 229 children (92 %), a sibling in 17 (6.8 %), and a half-sibling in the remaining 3 (1.2 %). The median time to BPX-501 infusion was 18 days (10 - 66 days). Fifty-two patients developed Grade I-IV aGvHD (cumulative incidence [CI] 21.9 % [95% confidence interval (CoI): 16.7 - 27.2]). Twenty-six patients developed Grade II-IV aGvHD (CI 10.9 %). Five patients developed Grade III-IV aGvHD (CI 2.1 %). Eight patients developed cGvHD (CI of 4.6% [95% CoI: 1.3 - 7.8]). Twenty-one patients met the rimiducid efficacy evaluable population definition. An overall clinical response rate of 86% was observed. A CR or PR to rimiducid was observed in 12 and 6 patients, respectively. Median time to initial response was 2 days (1-61 days). Median number of doses received was 1 (1 - 2). At a median follow-up of 7.8 months (2.3 - 30.8 months), 77% of the initial responders were still in either complete (n=8) or partial response (n=6). Conclusion These data suggest that administration of rimiducid for treatment of steroid-refractory GvHD represents a novel and highly effective treatment approach in pediatric patients with non-malignant or malignant disorders who received a αβ-T-cell and B-cell depleted haplo-HSCT followed by infusion of BPX-501 cells. The administration of rimiducid in children given BPX-501 T cells allows for effective control of GvHD occurring after the adoptive transfer of genetically modified T cells. Disclosures Slatter: Medac: Other: Travel assistance. Merli:Neovii Biotech: Honoraria; AMGEN: Honoraria. Aldinger:Bellicum Pharmaceuticals, Inc.: Employment. Locatelli:Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy.

Introduction. Severe acute graft-versus-host disease (GVHD) remains the most relevant complication after allogeneic HSCT. Although its incidence in the pediatric population is lower than in adults, children with severe acute GVHD and who do not respond to first-line treatment with systemic steroids still have a poor prognosis. The exact incidence of steroid-refractory acute GVHD in children is still not precisely defined, as well as the risk of non-relapse mortality (NRM) due to steroid-refractory acute GVHD. Aim of our study was to analyze the frequency of acute GVHD unresponsive to first-line steroid treatment in children and adolescents given allogenic HSCT, to describe the second line treatment employed, and the outcome of patient with this complication. Patients and methods. This retrospective study included patients younger than 18 years at the time of transplantation and given a first allogeneic HSCT between 2010 and 2015 in one of the HSCT Centers of the Italian Association for Pediatric Hematology / Oncology (AIEOP). Overall, 1608 patients (59% M and 41% F) were analyzed. Median age at HSCT was 8 years (range 0.2 - 18) 1084 (67%) were affected by malignant diseases and 524 (33%) by non-malignant disorders. The donor was an HLA-matched family donor (MFD) in 28% of cases, an unrelated donor (UD) in 52% and an HLA-haploidentical family donor in 20%. In MFD transplants Cyclosporine (CSA) was used as GVHD prophylaxis in 30% of cases and the combination of CSA + short-term methotrexate (MTX) in 48%. 75% of UD transplant recipients received CSA + MTX + anti-thymocyte globulin (ATG) as GVHD prophylaxis, and 25% other drug combinations. Ex vivo T-cell depletion of the graft was employed in most patients given a HLA-haploidentical HSCT (79% of transplants), and high-dose post-transplant cyclophosphamide in 11% of cases. Results. The cumulative incidence (CI) of grade II-IV acute GVHD was 31%, while that of grade III-IV acute GVHD was 10%. The overall incidence of chronic GVHD was 13% and that of extensive chronic GVHD was 6%. The CI of NRM was 14% for grade 0 acute GVHD patients, 9% for grade I, 11% for grade II, 26% for grade III and 68% for grade IV (P < 0.001). Of the 491 patients with grade II-IV acute GVHD, 250 (51%) required a second-line treatment after first-line steroid therapy (30% of grade II, of 75% grade III and 83% of grade IV patients). Acute GVHD requiring second-line treatment was more frequent in UD transplant recipients (21% of patients) than in matched sibling or haploidentical donor recipients (7% and 13% respectively, P < 0.01), while age at HSCT and diagnosis (malignant vs. non-malignant disease) were not associated with this complication. Second-line treatment was extracorporeal photochemotherapy in 60% of patients, mofetil mycophenolate (MMF) in 46%, mesenchymal stromal cells (MSC) in 12%, monoclonal antibodies (MoAbs) in 5% and other treatments in 28%; 32% of patients received more than one second line treatment. Overall NRM was 13% for patients with grade 0-I acute GVHD, 15% for grade II-IV responding to steroids, and 25% for grade II-IV patients requiring second-line therapy (P < 0.001). The addition of a second-line treatment partially decreased NRM only in patients with grade IV acute GVHD, but the difference was not statistically significant (66% vs. 78%; P = 0.313). In multivariable analysis, grade III (HR = 1.84; P = 0.044) and grade IV acute GVHD (HR = 7.07; P < 0.001), and the use of an UD (HR = 1.63; P = 0.007) were associated with an increased NRM, while the use of a second-line treatment did not decrease this risk (HR = 0.80; P = 0.368). Conclusions. Despite being less frequent than in adults, severe steroid-refractory acute GVHD is still associated with a very high NRM also in pediatric patients. Second or third-line treatments adopted so far have not been effective in improving control of the complication and in decreasing NRM. The prospective evaluation of acute GVHD biomarkers (such as ST2 and REG3α) could help in identifying patients at higher risk of NRM. Prospective studies are warranted to define new treatment modalities that could decrease the mortality rate associated with the most severe form of disease. Disclosures Zecca: Chimerix: Honoraria. Locatelli:bluebird bio: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria.

Background Allogeneic hematopoietic stem cell transplantation (HSCT) is a well-established treatment for children with Acute Leukemia (AL). For patients lacking a compatible matched related or unrelated donor, HLA-haploidentical HSCT (haplo-HSCT) from a relative represents a viable alternative. Promising results were reported with a novel method of selective depletion of αβ T and B cells (Locatelli, Blood 2017). This approach is associated with limitations such as suboptimal adaptive immune reconstitution, increased risk of infection and disease relapse. BPX-501 is an allogeneic product consisting of T cells modified to express the inducible caspase-9 (iC9) safety switch and truncated CD19 to allow monitoring and expansion of BPX-501 following transplant. The polyclonal nature of the BPX-501 provides broad virus and tumor-specific immunity, while the safety switch provides the unique ability to promptly and durably resolve graft-versus-host disease (GvHD) symptoms following the administration of rimiducid which induces dimerization and activation of iC9, inducing apoptosis of BPX-501. Aims To evaluate the safety and efficacy of BPX-501 administered after a αβ T and B-cell depleted haplo-HSCT in pediatric patients with AL in morphological complete remission (CR). The objective was to determine whether BPX-501 infusion can increase relapse-free survival (RFS) and overall survival (OS) through an enhanced graft-versus-leukemic (GvL) effect, while maintaining a low risk of GvHD. Methods This multicenter US (NCT03301168) and EU (NCT02065869), prospective trial utilizes αβ-T and B-cell-depleted haplo-HSCT followed by infusion of donor lymphocytes genetically modified with iC9 safety switch (BPX-501) in patients with malignant or non-malignant disorders. A subset of patients had acute high-risk leukemias (AML and ALL). BPX-501 was planned to be infused on day14±4 after the allograft. No post-transplant pharmacological GvHD prophylaxis was employed. Patients who develop GvHD resistant to conventional steroid therapy could receive ≥1 dose of rimiducid to activate iC9. The efficacy-evaluable population (EEP) was defined as any patient with AL who received HSCT, BPX-501 infusion, and at least one follow-up assessment. Results At clinical cut-off (June 30, 2018), 100 patients (EU: 75, US: 25) with AL met the EEP definition. Median follow-up was 14.7 mos (1 - 40.6 mos). Key baseline characteristics are shown in Table 1. The median time for neutrophil and platelet engraftment was 16 (15 - 17) and 12 (11 - 12) days, respectively. Four patients (4.1% [95% CI: 0 - 8%]) experienced primary graft failure. Of 96 evaluable patients, 21 patients developed Grade I-IV aGvHD (21.7% [95% CI: 13.5 - 29.8%]). Five patients developed Grade III-IV aGvHD (3.1% [95% CI: 0 - 6.5%]). Of 82 evaluable patients, 12 patients developed cGvHD (18.1% [95% CI: 8.2 - 22%]), with only three cases being moderate-severe. Rimiducid was administered to 10 patients with steroid-resistant acute GvHD. Best overall clinical response of CR or PR post-rimiducid administration was seen in 8 patients (80%). Among responding patients, 7 patients (87.5%) had a CR. Six patients died after transplantation (6.6% [95% CI: 1.4 - 11.7%]). Relapse Free Survival (RFS) was 82.2% (95% CI: 74.5 - 89.7%). Overall Survival (OS) was 90.1% (95% CI: 83.9 - 96.3%). Efficacy outcomes (TRM, RFS and OS) in AL subsets (AML and ALL) are shown in Table 2. CD3+ and CD3+CD4+ T cells above 500 cells/ml were achieved by 180 and 270 days, respectively. IgA and IgM levels achieved normal values by 180 days. The percentage of circulating and median absolute BPX-501 cells at Day 100 were 9.96% ± 11.6% (0 - 54.9%) and 85.58 ± 165.57 cells/ul (0 - 1001 cells/ml), respectively. Conclusion The adoptive transfer of BPX-501 following αβ-T and B-cell depleted haplo-HSCT represents a novel and highly effective transplantation strategy for pediatric patients with AL. Compared to data from children receiving only αβ T and B-cell depleted haplo-HSCT or matched unrelated donor HSCT (https://bloodcell.transplant.hrsa.gov/research/transplant_data/us_tx_data/survival_data/survival.aspx), this novel approach resulted in a comparable risk of transplant-related mortality and a lower risk of recurrence. Rimiducid was also an effective treatment for patients who developed steroid-resistant GvHD. Disclosures Locatelli: bluebird bio: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Qasim:Orchard: Equity Ownership; Autolus: Equity Ownership; Servier: Research Funding; Bellicum: Research Funding. Nemecek:Novartis Pharmaceuticals Corporation: Other: advisory boards.

Background αβ T- and B-cell depleted HLA-haploidentical HSCT (αβ haplo-HSCT) is an alternative promptly available and virtually applicable to any children with malignant and non-malignant disorders lacking an HLA-matched donor or in need of an urgent allograft (Locatelli et al. 2017). Through this approach, recipients can benefit immediately after transplantation from the anti-leukemia effect mediated by donor natural killer (NK) and γδ T cells, which confers protection also against infections. Nonetheless, even with this type of HSCT, the recovery of adaptive immunity is suboptimal. We have developed a further refinement of the approach based on the post-transplant infusion of a titrated number of donor T cells transduced with the inducible suicide gene iC9 (BPX-501 cells). This strategy has the potential to accelerate the recovery of adaptive immunity and to restore an efficient T-cell mediated graft-versus-leukemia (GvL) effect without the risk of inducing uncontrollable graft-versus-host disease (GvHD). Since reconstitution of a naïve T- and B-cell repertoire plays a key role in the long-term ability to respond to a broad range of pathogens, as well as to tumor antigens, we quantified T-cell receptor and kappa chain-deleting recombination excision circles (TREC and KREC), which represent reliable surrogate of T and B-cell neogenesis respectively. Study design and patients We analyzed samples of 48 children (M/F: 26/22) with acute leukemia (31 ALL, 17 AML) given an ab T-cell and B-cell depleted haplo-HSCT after a myeloablative regimen followed by the infusion of a titrated number of donor BPX-501 cells (1 million/Kg) after a median of 27 days (range: 11-87). Median age at transplantation was 8.3 years (range, 0.9-18). Thirty-two and 16 patients did or did not receive a preparative regimen containing total body irradiation (TBI). Anti-T lymphocyte globulin (ATLG Grafalon®, Neovii Biotech, 12 mg/Kg) was administered from day -4 to -2 for preventing graft rejection and GvHD. Moreover, to reduce the risk of EBV-related PTLD, on day -1, patients received rituximab (200 mg/m2). We analyzed T-cell reconstitution by measuring the TREC, small DNA excision circles of the TCR d locus deleted during recombination of the a-locus and present in the majority of functional ab-T-cells (sjTREC), and during the TCR b-chain recombination (bTREC). We also analyzed B-cell reconstitution by measuring coding-joint (Cj), signal-joint KREC (sjKREC) which reflect newly produced naïve B cells and the mean number of B-cell division (n) using the formula n=LOG(Cj/sjKREC)/LOG2. We performed real-time quantitative PCR, as recently described (Arruda et al. 2018), on genomic DNA extracted from PBMC collected at 6 different time points (before and 1, 3, 6, 12 and 18 months after the allograft). Results Recovery of thymic function started at 3 months after transplant: sjTREC and bTREC amounts reached and overcame pre-transplant values at 6 months, continuing to increase until 18 months after HSCT. Number of B-cell divisions obtained by the measurement of cj and sjKREC found in peripheral blood matched with the newly-generated B-cells bone marrow output and showed the same kinetics of sj and bTREC, rising from the 3rd month after HSCT. All these data correlate with immunophenotyping findings, which show a progressive increase over time of both total CD3+ and ab-T-cells. Furthermore, we observed a negative impact on both T- and B-cell neogenesis played by aGvHD (occurring in 14 patients), which became significant from 3 months after HSCT for sjTREC (p=0.013) and at 6 months for sjKREC (p= 0.03). We did not observe any impact of the use of TBI during the preparative regimen on T- and B-cells generation in this cohort. Conclusions BPX-501 cell infusion aims to contribute to the acceleration of immune reconstitution after a/b haplo-HSCT. In this study, we show that naïve T- and B- cells are quickly and efficiently generated in the early post-transplant period. This finding represents an interesting result considering the fully myeloablative conditioning regimens with the large use of TBI and the Ta/b and B-cells depletion (also in vivo by the administration of Rituximab) received by the patients. We observed an expected detrimental effect of aGvHD on immune reconstitution, but we emphasize the low frequency of this complication in this transplant setting despite the absence of any post-transplant pharmacological prophylaxis. Disclosures Foster: Bellicum: Employment, Equity Ownership. Locatelli:bluebird bio: Consultancy; Miltenyi: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Relapse of malignancy and lethal graft versus host disease (GVHD) are the principal causes of failure of allogeneic hematopoietic cell transplant (HCT). Recently we have shown that at seven days after HCT an algorithm using two serum biomarkers (ST2 and REG3α) can predict severe GVHD (Hartwell et al. JCI Insight 2017). We determined whether serial testing (in the first month following HCT) of patients with low probability biomarkers would improve the predictive accuracy of the algorithm and identify patients with different risks of relapse and lethal GVHD. Patients who received an HCT at 18 centers in the Mount Sinai Acute GVHD International Consortium (MAGIC) for hematologic malignancy and who supplied three blood samples were divided into a training set and validation set with equal numbers of lethal GVHD events, which was defined as death from GVHD or infection during treatment for GVHD. Patients in the training set (n=702) underwent HCT from January 1, 2006 until June 30, 2015, whereas patients in the validation set (n=906) underwent HCT from July 1, 2015 to May 1, 2017. Serum samples were analyzed using the previously published algorithm of two biomarkers up to three times (day 7, day 14, day 28 or GVHD onset, if onset occurred within the first 28 days). The algorithm generates a predicted probability of lethal GVHD between 0 and 1 for each patient. Patients were categorized as either low probability (LP) or high probability (HP) for lethal GVHD. HP thresholds of 0.20 and 0.16 were used to classify patients with and without GVHD symptoms, respectively (once categorized as HP, patients remained in that category and were not retested). All results were similar between training and validation sets, and we present here the validation set results. Serial testing identified 28% of patients as HP with a three-fold greater cumulative incidence of lethal GVHD at one year (13% vs 4%, p Early development of GVHD (by day 28) is a risk factor for lethal GVHD. Therefore, we next plotted RFS (dashed line), relapse (solid line), and lethal GVHD (dotted line) rates in patients who developed GVHD by day 28. 25% of patients with GVHD were categorized as HP and had a cumulative incidence of lethal GVHD more than four times higher (28%) than that of relapse (6%); however the risks were reversed for the 75% of patients who were LP, where relapse (15%) occurred twice as often as lethal GVHD (7%) (Figure 3B). In patients who did not develop GVHD in the first month, this reversal of risks was even more dramatic. Approximately half (53%) of the entire validation cohort did not develop GVHD by day 28 and was LP at all three evaluations. These patients had an exceptionally low risk of lethal GVHD and thus they relapsed (25%) much more often than they died from GVHD (3%). When malignancies were classified according to risk for relapse by the disease risk index (DRI) (Figure 3C), the probability of relapse was three fold higher than lethal GVHD in malignancies with a low DRI (12%), six fold higher for intermediate DRI (20%), and eleven fold higher for high/very high DRI (33%). We conclude that a serial monitoring strategy using GVHD biomarkers for one month after HCT is able to identify two groups of patients with very different risks of lethal GVHD and relapse. For these patients, the intensity of immunosuppression after day 28 could be tailored according to the probabilities of developing lethal GVHD and relapse in the context of clinical trials. Disclosures Aziz: Doris Duke Charitable Foundation: Research Funding. Ayuk:Therakos (Mallinckrodt): Honoraria; Novartis: Honoraria; Celgene: Consultancy; Gilead: Consultancy. Chen:REGiMMUNE: Consultancy; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees; Magenta Therapeutics: Consultancy; Takeda Pharmaceuticals: Consultancy. Merli:Neovii Biotech: Honoraria; AMGEN: Honoraria. Roesler:Sanofi: Other: Travel, Accommodations, Expenses; Amgen: Equity Ownership; Jazz Pharmaceuticals: Other: Travel, Accommodations, Expenses; Immunomedics: Equity Ownership; Biogen: Equity Ownership; Merck: Consultancy; Pfizer: Consultancy. Kitko:Novartis: Consultancy, Honoraria; Mallinckrodt: Honoraria, Other: Travel, Accommodations, Expenses. Qayed:Novartis: Consultancy. Wölfl:Bristol-myers Squibb: Equity Ownership; Novartis: Equity Ownership; Taheda: Equity Ownership; Juno: Equity Ownership; Neovii: Other: Travel, Accommodations, Expenses. Mielke:Celgene: Speakers Bureau; DGHO: Speakers Bureau; EHA: Speakers Bureau; Kiadis Pharma: Speakers Bureau; Miltenyi: Speakers Bureau. Wudhikarn:Takeda Oncology: Other: Travel, Accommodations, Expenses. Nakamura:Celgene: Honoraria; Molmed: Honoraria; Merck: Consultancy; Pharmacyclics: Consultancy; Atara: Consultancy; Jazz Pharmaceuticals: Consultancy. Pulsipher:CSL Behring: Consultancy; Novartis: Consultancy, Honoraria, Speakers Bureau; Adaptive Biotech: Consultancy, Research Funding; Amgen: Honoraria. Reshef:Pfizer: Consultancy; Atara Biotherapeutics: Consultancy; Kite Pharma: Consultancy; Takeda Pharmaceuticals: Consultancy; Bristol-Myers Squibb: Consultancy; Incyte: Consultancy. Levine:Therakos: Consultancy; Novartis: Consultancy; Bluebird: Consultancy; Incyte: Consultancy; Kamada: Research Funding; Viracor: Patents & Royalties. Ferrara:Incyte: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses; Xenikos: Consultancy, Other: Travel, Accommodations, Expenses; Kamada: Consultancy, Research Funding; Viracor: Consultancy, Patents & Royalties.

Background: Allogeneic hematopoietic stem cell transplantation (HSCT) from an HLA-haploidentical relative (haplo-HSCT) is a suitable option for children/young adults with acute leukemia (AL) either relapsed or at high-risk of treatment failure and in urgent need of an allograft. A novel method of graft manipulation based on the selective, negative depletion of αβ T and B cells has been recently developed. We published the results of a prospective trial (ClinicalTrial.gov identifier: NCT01810120) enrolling 80 children with AL transplanted until September/2014 using this approach (Locatelli, Blood 2017). In the present analysis, we update those results, evaluating also additional patients given haplo-HSCT after that date. Patients and methods: Analyzed are 111 children with AL enrolled in the trial; median age is 10 years (range 0.9-22.2). Eighty-two (74%) and 29 (26%) patients had acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML), respectively; they were transplanted between 09/2011 and 05/2018. All children were transplanted in complete morphological remission and received a fully myeloablative preparative regimen. Details on patients' characteristics, as well as on the number of HSC and lymphocyte subsets infused, are shown in Table 1. The donor was mainly chosen according to immunological criteria, giving priority to NK-cell alloreactivity, evaluated according to the killer immunoglobulin-like receptor (KIR)/KIR-Ligand mismatch in graft-versus-host direction model, KIR B haplotype, higher B-content score and size of NK alloreactive subset. Anti-T lymphocyte globulin (ATLG Grafalon®, Neovii Biotech) was administered at a dose of 12 mg/Kg from day -5 to -3 for preventing graft rejection and graft-versus-host disease (GvHD). Moreover, to reduce the risk of EBV-related post-transplant lymphoproliferative disorder (PTLD), on day -1, patients received rituximab (200 mg/m2) for in vivo depletion of both donor and recipient B cells. No patient was given any post-transplantation pharmacological GvHD prophylaxis. Results: Median follow-up of surviving patients is 47 months (range: 2 months - 7.7 years). All patients but two successfully engrafted and the median time to neutrophil and platelet recovery was 13 (range 9-22) and 11 (range 8-20) days, respectively. Acute GvHD occurred in 28 patients; it was of grade I and grade II severity in 9 and 19 patients, respectively. Skin was the sole organ involved in all patients but one, who had gut involvement. The cumulative incidence of grade I-II acute GvHD was 25% (95% confidence interval, CI, 17-33). Four out of the 91 patients at risk developed chronic GvHD, in all cases of limited severity, the cumulative incidence of this complication being 5% (95% CI, 1-9). Six patients died for transplant-related complications, this resulting into a 5-year cumulative incidence of transplant-related mortality (TRM) of 6% (95% CI, 2-11). Twenty-three patients relapsed at a median time of 186 days (range 60-1012) after transplantation, the 5-year cumulative incidence of relapse being 24% (95% CI, 16-33). The 5-year probability of overall and leukemia free survival (LFS) were both above 70%, as shown in Figure 1A and 1B, respectively. The 5-year probability of LFS in children with ALL and AML was 69% (95% CI, 57-79) and 73% (95% CI, 52-86), respectively (Figure 1C). Use of total body irradiation (TBI) during the preparative regimen was associated with better patient's outcome (Figure 1D), since it protected against the risk of leukemia recurrence [18% (95% CI, 10-28) vs. 45% (95% CI, 22-66) in patients who did or did not receive TBI, respectively, p Conclusions: This study, reporting long-term outcome of a large population of children/young adults with AL, confirms that αβ T- and B-cell depleted haplo-HSCT is an effective option for patients in need of an urgent allograft and lacking an HLA-identical donor. While TRM is impressively low, the main cause of treatment failure is leukemia recurrence, whose incidence can be lowered by the use of TBI during the conditioning regimen. The remarkably low risk of chronic GvHD renders the approach attractive also in terms of patient's quality of life. Figure 1 Figure 1. Disclosures Locatelli: Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria.

In view of the exciting results reported in patients with CD19+ malignancies given CAR T cells, it is expected that a continuously growing number of patients will be offered this treatment and, thus, will be exposed to gene-modified products. Since the techniques of gene manipulation are relatively new, some of the risks associated to CAR T therapy may be unpredictable. Recently, two patients who relapsed with CD19-, CAR-expressing leukemia were reported, this observation being interpretable in light of an inadvertent leukemic cell transduction with the second generation CAR.CD19 lentivirus during CAR T cell manufacturing (Lacey, ASH, 2016 128:281). Immunoglobulin heavy chain sequencing analysis of 17 additional infusion products also identified the leukemic clonotypes in six additional products (86%). In vitro and in vivo experiments proved that these CAR+ leukemic clones were not killed by CAR.CD19 T cells (Ruella, ASH, 2017 130:4463). Since lentiviruses proved to be superior for transduction of quiescent hematopietic stem cells due to their ability to infect non-dividing cells, we hypothesized that CAR-T cell manufacturing based on the genetic modification of T cells by gammaretroviral vector could theoretically represent a safe approach. Peripheral blood or bone marrow (BM)-derived mononuclear cells of patients with >40% of blasts at diagnosis (CD45dim+/CD34+/CD19+/CD22+/CD10+), were transduced with a retrovirus encoding for a second generation CAR.CD19.41bb.z in frame with a suicide gene (i.e., inducible caspase 9, iC9) employed in the academic Clinical Trial (NCT03373071) run at the Bambino Gesù Children's Hospital, Rome, Italy. Patient-derived CAR-T cells showed a phenotype not significantly different from that found on CAR-T cells generated by healthy-donors (data not shown). In particular, we demonstrated that both flow-cytofluorimetry and RealTime-quantitative PCR (with a sensitivity up to 10-5) failed to identify leukemic cells in the final CAR-T cell products generated from Bcp-ALL patients. To generate an in vitro model of CAR+ leukemic cells, we genetically modified CD19+ RAJI and DAUDI cell lines with the bicistronic retroviral vector carrying both second generation CAR.CD19 and the suicide gene iC9 (iC9.CAR-RAJI and iC9.CAR-DAUDI). We demonstrated the possibility of promptlyeliminating CAR+ leukemic cells, through exposure to 20nM of AP1903 of iC9.CAR-DAUDI and iC9.CAR-RAJI cells. Indeed, very early activation (6 hours) of the suicide gene iC9 resulted into a significant reduction in the percentage of CAR+ RAJI leukemic cells (Fig.A). The presence of iC9.CAR.CD19 molecule on leukemic cells precluded the detection of the CD19 antigen, whereas cells retain the expression of all other specific B-lineage markers. CD19 antigen started to be detectable 72 hours after AP1903 exposurewhen CAR negative leukemic cells become preponderant. To demonstrate that CD19 antigen was not down-regulated, but only masked by CAR molecule in iC9.CAR-RAJI and iC9.CAR-DAUDI cell lines, we measured CD19 mRNA, showing no significant modification with respect to wild-type (WT) RAJI and DAUDI cell lines. Moreover, iC9.CAR-RAJI and iC9.CAR-DAUDI cell lines were effectively eliminated by CAR.CD19 T cells (12.5±13.7% and 3.4±4.3% residualleukaemia, respectively) at the same extent of WT cell line (0% and 0.08±0.1%, residual leukaemia, respectively; p>0.05 Fig.B). To assess if patient-derived iC9.CAR.CD19 T cells were able to generate leukemia in vivo mouse model, NSG female mice were infused i.v. with 10x106 CAR-T cells and control NT-T cells. Mice were monitored for a total period of 250 days, by recurrent bleed. Simultaneously, another cohort of mice was infused with patient-derived BM cells (5x106) and monitored for the same time. Mice infused with Bcp-ALL BM cells developed leukemia-phenotype,with 82% of cells expressing hCD45dim and hCD19. By contrast, mice receiving patient-derived CAR-T cells showed a lowpercentage of CD45+ cells (0.1±0.01%), all CD3+. Despite the long period of observation, we did not detect any expansion of hCD19+ cells in this animal cohort. Taken together these data suggest that the use of a retroviral platform, associated with the presence of iC9 suicide gene, contributes to the genesis of a highly functional and safe CAR-T product, even when the production starts from a biological material characterized by high contamination of leukemic blasts. Disclosures Locatelli: Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; bluebird bio: Consultancy; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees.

Background: Allogeneic HSCT from either an HLA-identical sibling or an unrelated donor is a potentially curative treatment for patients with hemoglobinopathies and erythroid disorders (ED), such as Thalassemia Major (TM), Sickle Cell Disease (SCD) and Diamond-Blackfan Anemia (DBA). The limited historical experience with HLA-haploidentical HSCT in this setting has reported a disease-free survival probability lower than that reported using HLA-matched donors. In the last few years, we have developed a novel method of graft manipulation, based on the selective depletion of α/β+ T-cells and CD19+ B-cells (ClinicalTrial.gov identifier: NCT01810120), which was shown to be safe and effective in children with multiple types of non-malignant disorders (Bertaina el al, Blood 2014). To further optimize this approach through the acceleration of the recovery of adaptive immunity, we designed an ongoing phase I/II trial aimed to test the safety and efficacy of post-transplant infusion of donor T-cells transduced with the iC9 suicide gene (BPX-501 cells) in children with either malignant or non-malignant disorders (ClinicalTrials.gov identifier: NCT02065869). As the transduced gene contains sequences for the CD19 marker, BPX-501 cells (CD3+/CD19+) can be easily tracked in peripheral blood. We report on 10 children with hemoglobinopathies and ED who were enrolled in the phase II portion of the study. Patients and methods: Five patients were males and 5 were females, and median age at diagnosis and at HSCT was 5.34 and 9.52 years (range 2.33-11.71), respectively. Seven patients had TM (all bo/bo), 2 DBA and 1 SCD. All 10 patients were transfusion-dependent and receiving iron-chelation therapy before haplo-HSCT. Among the thalassemia patients, 4 patients belonged to class I and 3 to class II of the Pesaro classification. All patients were transplanted from a parent. Median number of CD34+ and αβ+ T-cells infused was 22.5 x 106/kg and 0.3 x 105/kg, respectively. In all patients, conditioning regimen included busulfan (16 mg/Kg), thiotepa (10 mg/Kg) and fludarabine (160 mg/m2). Rabbit ATG (12 mg/Kg over 3 days, from day -4 to day -2) was administered to prevent graft-versus-host disease (GvHD) and graft failure and Rituximab (200 mg/ m2 on day -1) was administered to prevent EBV-related lymphoproliferative disorders. No post-transplantation GvHD prophylaxis was given. Median follow-up is 301 days (range 41-420 days). Basic phenotyping of circulating lymphocytes was assessed by flow cytometry on fresh heparinized peripheral blood samples at 10, 20, 30, 60, 90, 120 and 150 days post haplo-HSCT. Results: After haplo-HSCT, the median time to reach neutrophil and platelet recovery was 14 days (range 11-28) and 10 days (range 8-12), respectively. After engraftment of the allograft, BPX-501 cells were infused (dose: 1x106 cells/kg) at a median time of 13.5 days after HSCT (range 10-26). Nine of the 10 patients maintained sustained donor engraftment, reaching full chimerism. The patient who experienced secondary graft failure was successfully re-transplanted from the same parent and he is full donor chimeric with transfusion-independence. Grade I/II skin acute GvHD occurred in 2 patients (at 31 and 59 days after HSCT, respectively). There was no occurrence of chronic GVHD. Remarkably, no patient has died and none of the patients have been re-hospitalized after initial discharge. The last erythrocyte transfusion was administered on day +7 post-transplant (range 4-33 days). At last follow-up, the median hemoglobin value of these patients was 11.35 gr/dL (range 10.2-13.4). BPX-501 cells expanded after infusion and still persist in all patients at last follow-up. All children are alive and transfusion-independent. Details on T cell, NK cell and B cell recovery are shown in Figure 1 (Panel A-D). Conclusions: Children with hemoglobinopathies and DBA can benefit from curative haplo-HSCT after depletion of α/β T-cells followed by infusion of BPX-501 cells, which, expanding and persisting over time, contribute to speed immune recovery of adaptive T-cell immunity, thus rendering the procedure safer. Figure 1 Figure 1. Disclosures French: Bellicum Pharmaceuticals: Employment, Membership on an entity's Board of Directors or advisory committees. Moseley:Bellicum Pharmaceuticals: Employment, Membership on an entity's Board of Directors or advisory committees.

Background: Haplo-HSCT after depletion of α/β T and B cells is a suitable and effective option for those children with acute leukemia (AL) who need an allograft and lacking an immediately available HLA-identical donor. With this approach, recipients can benefit immediately after transplantation from the anti-leukemia effect mediated by donor natural killer (NK) and γd T cells, which can also protect against infections. A further improvement of the results achievable with this platform could achieved with a faster adaptive T-cell immunity recovery, which play a key role to augment the graft-versus-leukemia effect and the capacity to fight infections. In light of these considerations, we designed a phase I/II trial aimed at testing the safety and efficacy of post-transplant infusion of donor-derived T cells transduced with the new iC9 suicide gene (BPX-501) in children with either malignant or non-malignant disorders (NCT02065869). Remarkably, after the activation and transduction with the retroviral iC9 construct, BPX501 cells switch the phenotype towards a preferential CD45RO pattern. Patients and methods: The phase I portion of the trial consisted of a classical 3+3 design with 3 cohorts, receiving escalating doses of BPX-501 cells of 2.5x105, 5x105, and 1x106 cells/kg, respectively. Patients included in the phase II portion were planned to receive the recommended dose identified during the phase I part of the study.Enrollment of patients started in December 2014; so far, 25 patients with AL in morphological complete remission (CR) have been enrolled. Twenty patients had acute lymphoblastic leukemia (ALL) and 5 acute myeloid leukemia (AML). Details on patient, donor and transplant characteristics are reported in table 1. All patients transplanted in CR1 had either poor cytogenetic/molecular characteristics or high levels of minimal residual disease at the end of induction therapy, both factors predicting a high relapse rate. All patients were given a fully myeloablative conditioning regimen (table 1). Before haplo-HSCT, children received rabbit anti-thymocyte globulin (ATG NEOVII, 12 mg/Kg over 3 days, from day -4 to day -2) to prevent both graft-versus-host disease (GvHD) and graft failure, and Rituximab (200 mg/ m2 on day -1) to prevent EBV-related lymphoproliferative disorders. No post-transplantation GvHD prophylaxis was administered. Results: All patients engrafted and no secondary graft failure was recorded. Median time to neutrophil and platelet recovery was 18 days (range 10-22) and 11 days (range 9-13), respectively. Once documented the engraftment of donor cells, BPX-501 T lymphocytes were infused at a median time of 17 days (range 13-52) after the allograft. Two patients were enrolled in the phase I portion of the study; one each received 2.5x105 and 1x106 cells/kg. The remaining 23 children were treated in the phase II, where the recommended dose was 1x106 cells/kg. However, since we did not observe any acute GvHD requiring the infusion of the dimerizing agent (Rimiducid/AP1903) activating iC9 gene in the first 15 children receiving 1x106 cells/kg, we decided to emend the protocol to further increase the BPX501 cell dose infused to 2 and 4x106 cells/kg. Thus, the last 6 patients were enrolled in these 2 last dose levels (3 patients each). Six and 3 patients developed grade II-IV acute and chronic GvHD, respectively. In one child, given 4x106 cells/kg, we infused rimiducid for steroid-resistant grade II skin acute GvHD, with complete resolution of the disease in 24 hours. The cumulative incidence of grade II-III acute and chronic GvHD are shown in figure 1A and B, respectively. Median follow-up of these 25 children is 8 months (range 1-19 months). One of them died due to chronic GvHD-associated bronchiolitis obliterans and one child with ALL transplanted in CR2 relapsed; the cumulative incidence of non-relapse mortality and leukemia recurrence are shown in figure 1C. The probability of disease-free survival at 15 months is 87% (figure 1D). Once infused, BPX501 cells expanded and persisted over time in both peripheral blood and bone marrow. Conclusion: Overall, these data indicate that the infusion of BPX-501 cells in children with AL given selectively manipulated haplo-HSCT results in low non-relapse mortality and chronic GvHD. Although the median observation time is still limited, the cumulative incidence of disease recurrence is promising. Table 1 Table 1. Figure 1 Figure 1. Disclosures Stanson: Bellicum pharmaceuticals: Employment. Moseley:Bellicum Pharmaceuticals: Employment, Membership on an entity's Board of Directors or advisory committees.

Introduction. Allogeneic HSCT currently represents the only consolidated curative approach for Fanconi anemia (FA) patients, with best results observed in the HLA-identical sibling setting. For patients lacking an HLA-matched related or unrelated donor, haploidentical HSCT virtually assures the opportunity for nearly all patients to benefit from HSCT, offering the advantage of immediate accessibility to the transplant procedure. In order to overcome the limitation of delayed immune recovery, historically associated with this type of allograft, in the last few years we developed a novel method of ex vivo graft manipulation, consisting of the negative depletion of T-cell receptor (TCR) αβ+ T-lymphocytes and CD19 B-cells from peripheral blood stem cells (PBSC) grafts (ClinicalTrial.gov identifier: NCT01810120) (Bertaina el al., Blood 2014). Here we report our analysis in a subgroup of FA patients given TCRαβ/CD19-depleted haploidentical HSCT at our Institution. Patients and methods. Ten consecutive FA patients (6 girls and 4 boys) underwent a TCRαβ/CD19-depleted HSCT from an HLA-haploidentical relative between September 2011 and July 2015. Median age at diagnosis was 6.6 (range 2.7-22.0) years and median age at time of transplantation was 8.1 (range 4.4-22.2) years. The conditioning regimen included Cyclophosphamide 300 mg/m2/day and Fludarabine 30 mg/m2/day for 4 consecutive days (days -6 to -3), with 200 cGy single-dose TBI. Pretransplantation Fresenius® ATG was administered at a dose of 4 mg/kg/day for 3 consecutive days (days -5 to -3) in order to prevent both graft failure and graft-versus-host disease (GVHD). All patients received Rituximab 200 mg/m2 to reduce the risk of Epstein-Barr virus-related post-transplant lymphoproliferative disorders. Selective removal of TCRαβ+ and B-cells was performed on G-CSF-mobilized donor PBSC through labeling with biotinylated anti-TCRαβ antibodies and anti-CD19 antibodies, followed by incubation with anti-biotin antibodies conjugated to paramagnetic beads (CliniMACS; Miltenyi Biotec, Bergisch Gladbach, Germany). No immunosuppressive therapy was administered as post-transplantation prophylaxis against GVHD. Results. The TCRαβ/CD19-depletedgrafts contained a median of 20.40 x106/kg (range 15.80-33.40) CD34+ cells, 5.60 x106/kg (range 1.78-69.60) CD3+ lymphocytes, 0.021 x106/kg (range 0.002-0.043) TCRαβ+ lymphocytes, 5.60 x106/kg (range 1.78-69.60) TCRγδ+ lymphocytes, 0.036 x106/kg (range 0.013-0.079) CD20+ lymphocytes, and 45.30 x106/kg (range 16.2-177.0) NK cells. Engraftment with sustained full donor chimerism was achieved in 9 out of 10 patients, the cumulative incidence of graft rejection being 10% (95% CI, 0-26.8). The patient who rejected his first allograft achieved a complete engraftment after a second HSCT from one-antigen mismatched unrelated donor. No secondary graft failures were observed. The median time for neutrophil and platelet engraftment was 12 days (range, 9-15) and 9 days (range, 8-12), respectively. No patient experienced acute or chronic GVHD in the follow-up period. No transplant-related deaths occurred in our cohort. With a median follow-up of 28 months (range 13.2-39.1), the Kaplan-Meier estimates of OS and DFS were both 100%, while the EFS probability was 90% (95% CI, 47.3-98.5). Discussion. These data suggest that haploidentical HSCT after removal of TCRαβ+ and CD19+ lymphocytes is able to guarantee engraftment with excellent OS and DFS in patients affected by FA. Moreover, given the very low incidence of both acute and chronic GVHD, which has been shown to contribute to the increased risk of developing late post-transplantation malignancies in FA patients, this approach can be considered a very attractive option for FA patients in need of an allograft and lacking an HLA-identical family donor. Disclosures No relevant conflicts of interest to declare.

Introduction. Although rabbit anti-thymocyte globulin (ATG) is largely used for preventing immune-mediated complications in patients given allogeneic hematopoietic stem cell transplantation (HSCT) from a matched unrelated donor (MUD), the optimal dosage of this drug in children is still undefined. We conducted an academic open-label, multicenter, randomized trial comparing two different dosages of rATG in children with hematological malignancies transplanted with either bone marrow (BM) or peripheral blood stem cell (PBSC) from a MUD selected using high-resolution typing for HLA class I/II (i.e. A, B, C, DRB1) loci. The study aimed at testing whether a higher dose of rATG (30 mg/Kg over 3 days, from day -4 to day -2) was superior to a lower dose (15 mg/Kg over 3 days, from day -4 to day -2) in terms of grade II-IV acute graft-versus-host disease (GvHD) prevention. Secondary end-points included cumulative incidence (CI) of chronic GvHD, non-relapse mortality (NRM), disease recurrence and event-free survival (EFS). The drug was kindly provided by Fresenius/Neovii. Patients and methods. Inclusion criteria were: diagnosis of acute or chronic leukemia in remission/chronic phase, non-Hodgkin lymphoma or myelodysplasia; age at HSCT 0-19 years; availability of an unrelated donor selected using high-resolution molecular typing of HLA-A, B, C and DRB1 loci, completely matched or with a single allelic disparity at one of the HLA loci or with 2 allelic disparities, or with one antigenic disparity; use of G-CSF-mobilized PBSC or BM-derived hematopoietic stem cells. From January 2008 to September 2012, 180 patients were enrolled in the study. Eight patients, 5 randomized to the rATG 30 mg/kg group and 3 to the 15 mg/kg group, did not proceed to transplant due to further relapse before HSCT. The remaining 172 patients, 84 belonging to the 30 mg/kg group and 88 belonging to the 15 mg/kg group, were transplanted and included in this analysis; 94 children had acute lymphoblastic and 42 acute myeloid leukemia. The 2 randomization groups were comparable for all demographic and transplant-related variables evaluated (see Table 1). Patients were stratified according to the stem cell source (BM vs. PBSC), degree of HLA compatibility with the donor (unrelated donor perfectly matched or with 1 allelic disparity vs. unrelated donor with 2 allelic disparities or with 1 antigenic disparity) and disease risk group (standard- vs. high-risk, SR and HR). All patients received a fully myeloablative preparative regimen. All patients received cyclosporine-A and short-term methotrexate as post-transplant GvHD prophylaxis. The trial was registered at ClinicalTrials.gov (NCT00934557). Data were analyzed as of January 31st, 2016. Results. With a median follow-up of 4.5 years (range 3.3-7.6 years), the 100-day CI of grade II-IV acute GvHD in the high and low rATG group was 29% and 36%, respectively (P=NS, Figure 1A); the CI of extensive chronic GvHD in the two groups was 6% and 10%, respectively (P=NS, Figure 1B). The CI of NRM was 19% and 9% in the high and low rATG group, respectively (P=0.09). Children receiving the higher rATG dosage had a greater incidence of viral reactivations as compared to those who received the lower dosage. The difference was statistically significant for EBV reactivation (37% vs. 23%; P = 0.038) and for Adenovirus reactivation (12% vs. 1%; P = 0.004). The overall CI of disease recurrence was 17% and did not differ between high and low dose rATG. The 5-year EFS for the whole cohort of patients was 69%; it was 61% and 77% for children given high and low dose rATG, respectively (P=0.028, Figure 1C). EFS was 78% and 55% in the SR and HR groups, respectively (P=0.001, Figure 1D). EFS of the 136 children with acute leukemia given either high or low dose rATG was 60% and 77%, respectively (P=0.049). In multivariate analysis on EFS, the following variables were associated with an unfavorable outcome: rATG dose of 30 mg/kg (relative risk 1.90; P=0.026), HLA mismatch > 1 allele (relative risk 2.08; P=0.01) and HR disease (relative risk 2.46; P=0.0015). Conclusions. Our data indicate that, in children with hematological malignancies transplanted from a MUD selected through high-resolution HLA typing, the use of low dose rATG results into decreased incidence of NRM and better EFS. Low dose rATG is able to spare life-threatening viral infections, without significantly increasing the incidence of acute and chronic GvHD. Table 1 Table 1. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Background: We previously reported that haplo-HSCT after depletion of α/β T cells / CD19 B cells is a suitable option for patients with PIDs, with a high success rate (Bertaina et al., Blood 2014). However, in many patients, we observed a delay in the recovery of adaptive immunity, sometimes resulting in life-threatening or even fatal events, providing the rationale for exploring an innovative approach based on the post-transplantation infusion of donor T cells, genetically modified with the novel suicide gene iC9 (BPX-501 cells), with the aim of accelerating reconstitution of adaptive immunity. As the transduced gene contains sequences for the CD19 marker, BPX-501 cells (CD3+/CD19+) can be easily tracked in peripheral blood. Patients and study design: In this multicenter, prospective phase I-II trial (enrolling both malignant and non-malignant diseases; sponsored by Bellicum Pharmaceuticals®, ClinicalTrials.gov identifier: NCT02065869) 20 children with PIDs have been enrolled. Nine children had Severe Combined Immune Deficiency (SCID), 5 Wiskott-Aldrich syndrome (WAS), 2 Chronic Granulomatosis Disease (CGD) and one each of 4 other PIDs detailed in Table 1, which also reports relevant patient-, donor- and transplant- characteristics. All patients were transplanted after depletion of donor α/β T cells and CD19 B cells, employed to prevent graft-versus-host disease (GvHD) and post-transplant lymphoproliferative disorders (PTLD). Details on cell subsets infused with the graft are reported in Table 1. No patient was given any post-transplantation GvHD prophylaxis. The phase I portion of the trial consisted of a classical 3+3 design with 3 cohorts, with escalating doses of BPX-501 cells of 2.5x105, 5x105, and 1x106 cells/kg, respectively. Patients in the phase II portion received the highest dose identified during the phase I portion (1x106 cells/kg). Results: All patients engrafted and no secondary graft failure was recorded. The median time to neutrophil and platelet recovery was 16 (range 11-35) and 10 days (range 7-14), respectively. BPX-501 cells were infused at a median time of 15 days (range 13-56) after the allograft. Four patients were enrolled in the phase I portion of the study; one received 2.5x105 and 3 received 1x106 cells/kg. The remaining 16 children were treated in the phase II part, with a dose was 1x106 cells/kg. Five children experienced Grade I (3 patients) or Grade II (2 children) acute GvHD, which resolved with either topical or systemic steroids in 3 patients. The other 2 cases resolved after the infusion of Rimiducid (AP1903) which activated the iC9 suicide gene. The cumulative incidence of acute GvHD is shown in Figure 1A. Two of the 17 patients at risk (i.e. those with a follow-up longer than 100 days after transplantation) developed mild (skin only) chronic GvHD. The median time to discharge was 36 days (range 26-180). Eight patients (40%) experienced one episode of re-hospitalization after initial discharge. After a median follow-up of 10 months (range 50 days-20 months), all patients are alive and disease-free. Details on expansion/persistence of BPX-501 cells, as well as on recovery of CD3+ cells are shown in Figure 1B and 1C, respectively. Ig serum levels of SCID patients at 6 and 12 months after transplant are depicted in Figure 1D. The median follow-up for WAS patients is 510 days (124-531 days) and their median platelet count at 1, 3 and 12 months after haplo-HSCT was 136 (range 48-168), 204 (range 98-280) and 143 (117-365) x109/L, respectively. One SCID patient who developed BCG bacterial dissemination after the allograft resolved this infection and 3 patients (2 with SCID and one with WAS) cleared cytomegalovirus (2 children) or Adenovirus (one child) infections pre-existing at time of conditioning regimen. Conclusions: These results indicate that haplo-HSCT, after depletion of α/β T cells and B cells followed by adoptive infusion of donor BPX-501 cells, is an effective alternative for those children with PIDs in need of an urgent allograft or lacking a suitable HLA-matched donor. BPX-501 cells expand in vivo and persist over time, contributing to fasten the recovery of adaptive T-cell immunity and to clear infections. In view of the absence of fatalities and of the low rate of mild chronic GvHD, we conclude that infusion of BPX-501 T cells with the iC9 cell-suicide system may potentially render haplo-HSCT a first-line option for children with PIDs. Table 1. Table 1. Figure 1. Figure 1. Disclosures Qasim: Cellectis: Research Funding; Autolus: Consultancy, Equity Ownership, Research Funding; Catapult: Research Funding; Calimmune: Research Funding. Slatter:Medac: Other: Travel grants. Moseley:Bellicum Pharmaceuticals: Employment, Membership on an entity's Board of Directors or advisory committees.

A new method of graft manipulation based on physical removal of αβ+ T cells and CD19+ B cells, leaving mature NK cells and γδ T cells in the graft, has been recently developed for HLA-haploidentical HSCT. We demonstrated that γδ T cells collected from transplanted patients are endowed with capacity of killing leukemia cells after ex vivo treatment with zoledronic acid (ZOL). Thus, we hypothesized that infusion of ZOL in patients receiving this type of graft, may boost γδ T cell cytotoxic activity against leukemia cells. Thirty-three patients were treated with ZOL every 28 days at least twice. γδ T cells before and after ZOL treatments were studied till at least 7 months after HSCT by high-resolution mass spectrometry, flow-cytometry, and degranulation assay. Proteomic analysis of γd T cells purified from patients showed that, starting from the first infusion, ZOL caused up-regulation of proteins involved in activation processes and immune response, paralleled by down-regulation of proteins involved in proliferation. These findings are consistent with an induction of Vδ2 cell differentiation, paralleled by increased cytotoxicity of both Vδ1 and Vδ2 cells against primary leukemia blasts. Furthermore, a proteomic signature was identified for each individual ZOL treatment. Patients given 3 or more ZOL infusions had a better probability of survival in comparison to those given 1 or 2 treatments. In conclusion,ZOL influences Vδ2 cell activity, determines a specific proteomic signature and enhances anti-leukemia activity, this potentially resulting into an increased anti-tumor effect. Disclosures No relevant conflicts of interest to declare.

Background: We recently completed a prospective study (ClinicalTrial.gov identifier: NCT01810120) which showed that haplo-HSCT after depletion of α/β T cells is an effective option for those children in need of an allograft and lacking an immediately available HLA-identical related or unrelated donor. However, recovery of adaptive T-cell immunity remains suboptimal and some patients died due to viral infections in the early post-transplant period. Thus, strategies aimed at accelerating early recovery of adaptive T-cell immunity are desirable. Study design and patients: We designed a phase I/II trial aimed at testing the safety and the efficacy of post-transplant infusion of donor-derived T cells transduced with the new iC9 suicide gene (BPX-501) in children with malignant or non-malignant disorders (ClinicalTrials.gov identifier: NCT02065869); enrollment started in December 2014. Cells are administered within 14 + 4 days after haplo-HSCT. The phase I portion of the trial consists of a classical 3+3 design with 3 cohorts, receiving escalating doses of BPX-501 cells of 2.5 x 105, 5 x105, and 1x106 cells/kg, respectively. Patients included in the phase II portion received the highest dose identified during the phase I portion of the study for a maximum of 60 children in both phase I/II portions of the study. As of July 25th 2015, 25 children have been screened and included in the study: 23 have been infused with BPX-501 cells. The analysis refers to the 16 patients with a minimum follow-up of 90 days after transplantation; they had acute lymphoblastic leukemia (ALL, 6), acute myeloid leukemia (1), severe combined immune-deficiency (4), Wiskott-Aldrich syndrome (3) and Fanconi Anemia (2). All children with acute leukemia were transplanted in morphological complete remission (CR). Median age at haplo-HSCT was 3.5 years (range, 03-17.8); 7 patients (44%) were females. All children received >10x106 CD34+ cells/Kg and Results: BPX-501 cells were infused at a median time of 16 days (range 13-18); median cell viability post-thaw was 91% (range 65-97). Treatment was well tolerated and no infusion-related side effects were recorded. The recommended dose identified during the phase I of the trial to be used for the phase II portion was 1x106 cells/kg. Four children developed grade I-II skin only acute graft-versus-host disease (GvHD) at 16, 20, 22 and 34 days after haplo-HSCT, respectively, which resolved with topical steroids; no patient had either gut or liver acute GvHD. The 100-day cumulative incidence (CI) of skin-only grade I-II acute GvHD was 25% (SE 3.6); it was 30% (SE 2.1) in the historical controls (Figure 1 - Panel A). No patient developed chronic GvHD. In 4 patients, mixed chimerism present at time of BPX-501 cell infusion completely reverted to full donor chimerism. None of the 16 patients included in the analysis had graft failure or died of transplant-related complications. Two patients, both with ALL transplanted in CR3, relapsed at 86 and 153 days after the allograft, respectively. Median time to discharge after haplo-HSCT was 28 days (range, 19-86) as compared to 38 days (range, 18-174) in the historical controls (p=0.08). Four patients were re-hospitalized due to: cytomegalovirus (CMV) infection (2), fever of unknown origin (1) and valganciclovir-induced neutropenia (1). BPX-501 cells progressively expanded over time and are still persisting, potentially contributing to the recovery of adaptive T-cell immunity. The mean number of both CD3+ and BPX-501 cells at the different time-points are reported in Figure 1 - Panel B, which also details the data of historical controls. Conclusions: Overall, these data indicate that the infusion of BPX-501 cells is safe and well tolerated. The 100-day CI of skin-only grade I-II acute GvHD observed in these patients is similar to that of children included in the previous trial of haplo-HSCT after depletion of α/β T cells. BPX-501 cells expand in vivo and persist over time, potentially contributing to accelerate the recovery of adaptive T-cell immunity, with improved clinical outcome. The iC9 cell-suicide system may increase the implementation of cellular therapy approaches aimed at optimizing immune recovery after transplantation. Figure 1. Figure 1. Disclosures Qasim: Cell Medica: Research Funding; Autolus Ltd: Consultancy, Equity Ownership, Research Funding; Miltenyi Biotec GmbH: Research Funding; Cellectis: Research Funding. Moseley:Bellicum Pharmaceuticals: Employment, Equity Ownership.

Background: Immune recovery is crucial for patients treated with allogeneic HSCT and in particular of those receiving a T-cell depleted haplo-HSCT. We recently developed a novel method of graft manipulation based on physical elimination of α/β T cells and B-lymphocytes for preventing graft-versus-host disease (GvHD) and EBV-related lymphoproliferative disorders, respectively. Thanks to this approach, we successfully conducted a prospective trial in children with malignant or non-malignant disorders (ClinicalTrial.gov identifier: NCT01810120). Although patients enrolled in this trial had faster immune recovery and lower incidence of infections than those given haplo-HSCT after infusion of positively selected CD34+ cells, reconstitution of adaptive T-cell immunity remains suboptimal. We therefore designed a phase I/II trial aimed at testing the effect on post-transplant immune recovery of adoptive infusion (within 14 + 4 days after transplantation) of BPX-501 cells in children given haplo-HSCT after depletion of α/β T and B cells (ClinicalTrials.gov identifier: NCT02065869). Patients and methods: As of July 25th 2015, 23 children have been infused with BPX-501 cells. The 9 children included in the phase I portion of the study were given 2.5x105, 5x105, and 1x106 BPX-501 cells/kg, respectively, while the 14 included in the phase II received 1x106 BPX-501 cells/kg. This analysis refers to the 16 patients with a minimum follow-up of 90 days; 7 children had acute leukemia and 9 non-malignant disorders. Basic phenotype of circulating lymphocytes was assessed by flow cytometry on fresh heparinized peripheral blood samples at 10, 20, 30, 60, 90, 120 and 150 days post haplo-HSCT, respectively. The following antibodies were used: anti-TCRαβ FITC/anti-TCRγδ PE/anti-CD3 PerCP-Cy™5.5 (WT31, 11F2, SK7), anti-CD4 APC Cy7 (RPA-T4), anti-CD19 BV 510 (SJ25C1), anti-CD3 BV 421 (UCHT1), anti-CD56 PeCy7 (B159), anti-CD16 APC (B73.1), anti-CD8 APC (RPA-T8) from BD Biosciences (San Diego, CA, USA). Antigen-driven activation of peripheral mononuclear cells was evaluated by standard lymphoproliferation assay (LPA) with 3H-thymidine pulsing on day 4 and harvesting 18 hours later. Antigens included PHA or CMV, EBV and AdV whole viral lysate. Results were scored positive with stimulation indexes (SI) >10 for PHA and >3 for viral antigens. Results: None of the patients died from transplant-related complications. Chimerism analysis investigated through short tandem repeats showed that in all but 4 patients, cells were of donor origin before the infusion of BPX-501 cells. In the 4 patients, there was a reversion to complete donor chimerism after infusion of BPX-501 cells. At early time points after haplo-HSCT, gδ T cells predominated over αβ T lymphocytes; subsequently, this latter population became the more largely represented. The number of both CD3+ T lymphocytes and of BPX-501 cells is shown in Panel A of Figure 1, reconstitution of whole T cells in historical children given haplo-HSCT after depletion of α/β T cells is also shown. The number of CD3+ T lymphocytes reached greater than 0.5x109/L 2 months after infusion of BPX-501 cells. Remarkably, while usually immune recovery after transplantation is characterized by prevalence of CD8+ cells, in our patients the physiological predominance of CD4+ lymphocytes was maintained (Panel B of Figure 1. Reconstitution of natural killer cells (NK) is shown in Panel C of Figure 1. As compared to patients receiving CD34+ selected cell haplo-HSCT, children included in this study had a faster reconstitution of mature KIR+/NKG2A- NK cells. Serum levels of IgA and IgM over time are shown in Panel D of Figure 1: there was a recovery of newly synthetized Ig at 3 months. The analysis of the function of T cells showed that the proliferative response to a polyclonal mitogen or to CMV lysate was comparable to that of a healthy control in 50% of patients as early as day + 60 after haplo-HSCT and BPX-501; on day +150, all patients reached a normal SI. Response to both EBV and AdV antigens was slightly delayed, but progressively improved over time (see also Figure 2). Conclusions: Overall, these data indicate that infusion of BPX-501 cells is able to accelerate the recovery of adaptive T-cell immunity since these cells, once infused, expand in vivo and persist over time, potentially contributing to protect patients from infections. Figure 1. Figure 1. Figure 2. Figure 2. Disclosures Moseley: Bellicum Pharmaceuticals: Employment, Equity Ownership.

Background: Allogeneic HSCT is a widely used treatment for children with acute leukemia (AL) either relapsed or at high risk of treatment failure. However, an HLA-identical sibling is available for only 20-25% of patients and an UD can be located in a suitable time only for a portion of the remaining population. HSCT from an HLA-haploidentical relative (haplo-HSCT) is now considered an alternative option, especially in view of the recent insights in graft manipulation. We recently developed a novel method of more selective T-cell depletion based on physical elimination of α/β T cells (ClinicalTrial.gov identifier: [NCT01810120][1]), shown to be effective for both preventing graft-versus-host disease (GvHD) and for conferring improved protection against infections in comparison to haplo-HSCT performed through the infusion of positively selected CD34+ cells. The initial results on 40 patients with AL were reported at the ASH Meeting in 2013 (Bertaina et al). We now present the comparison of the outcome of 80 children with AL given haplo-HSCT after α/β T-cell depletion (group 1) with that of patients transplanted from an HLA-identical sibling (group 2) or an UD (group 3) in the same time period. Patients and methods: All patients with AL were transplanted at the Bambino Gesu Children's Hospital in Rome, Italy, between December 2010 and September 2014; 80 patients were included in group 1, 41 in group 2 and 51 in group 3. Patients were offered α/β T-cell-depleted haplo-HSCT in the absence of suitable conventional donor (HLA identical sibling or 10/10 UD evaluated using high resolution typing) or if affected by rapidly progressive disease not permitting time to identify an UD. Clinical characteristics of patients assigned to the 3 groups and those of their donor are shown in Table1. All children were given a fully myeloablative regimen. No group 1 patient was given any post-transplantation GvHD prophylaxis, while patients of group 2 and 3 were given Cyclosporine-A and short-term methotrexate. Group 1 and 3 patients received ATG Fresenius® (4 mg/Kg/day) from day -5 to -3 for preventing both graft rejection and GvHD. Results: All group 2 and 3 patients had sustained engraftment of donor cells, while 1 of the 80 patients included in group 1 experienced primary graft failure and was rescued by haplo-HSCT from the other parent. The cumulative incidence (CI) of acute GvHD was 30%, 41% and 42%, respectively. Remarkably, all children of the group 1 who developed acute GvHD had a skin-only involvement, while 17% and 16.3% of those of group 2 and 3 had either gut or liver involvement (p

Background: T-cell depleted haplo-HSCT is an established treatment for children with primary immune deficiencies (PID). However, children given this type of allograft are exposed to the risk of fatal events due to viral infections because of the prolonged impairment of adaptive immunity. We recently developed a novel method of selective T-cell depletion based on physical elimination of α/β T cells (ClinicalTrial.gov identifier: NCT01810120), which was shown to be safe and more effective than transplantation of positively-selected CD34+ cells for preventing life-threatening infections. However, we recorded some severe and even fatal viral infections, which prompted us to explore innovative approaches to accelerate the recovery of adaptive immunity. For this purpose, we designed an ongoing phase I/II trial aimed at testing the safety and the efficacy of post-transplant infusion of BPX-501 cells in children with malignant or non-malignant disorders (ClinicalTrials.gov identifier: NCT02065869). We report 3 cases of children with either severe combined immune deficiency (SCID) or Wiskott-Aldrich syndrome (WAS), who were enrolled in the dose escalation phase of the study and who cleared cytomegalovirus (CMV) or Adenovirus (AdV) infections likely due to the contribution of the BPX-501 cells. Patients and methods: Patient #1, affected by SCID, was transplanted from the HLA-haploidentical father. Before transplantation she had CMV-DNAemia which was treated with ganciclovir until donor stem cell infusion. She was given 2.5 x 105/kg BPX-501 cells on day 17 after transplantation. Patient #2, also affected by SCID, was transplanted from the HLA-haploidentical mother. Before transplantation she had AdV-DNAemia and high load of the virus in stools. She was given 5 x 105/kg BPX-501 cells on day 15 after transplantation. Patient #3 was affected by WAS and referred to the transplant unit; in the months preceding haplo-HSCT the child had developed CMV retinitis and hepatitis with high levels of CMV-DNAemia. This patient was transplanted from the father and received 1 x 106/kg BPX-501 cells on day 15 after haplo-HSCT. Basic phenotype of circulating lymphocytes was assessed by flow cytometry on fresh heparinized peripheral blood samples at 10, 20, 30, 60, 90, 120 and 150 days post haplo-HSCT, respectively. Since BPX-501 cells are CD3+/CD19+, it was easy to track the presence of these genetically modified cells. CMV specific reconstitution was also monitored through the INF gamma ELISPOT assay. In particular, peripheral blood mononuclear cells were stimulated for 16hrs in the presence of peptide libraries derived from pp65, IE1 and IE2 CMV-specific antigens. Results: The increase in the number of both CD3+ T lymphocytes and BPX-501 cells over time after transplantation together with the modifications of DNAemia of both CMV and AdV in the 3 patients are reported in Panel A, B and C, respectively, of Figure 1. In all of these patients, the pre-existing viral infection was progressively cleared once the BPX-501 cells were infused. These cells expanded in vivo and are still persisting, contributing to the recovery of adoptive immunity. The median time to reach an absolute number of α/β CD3+ cells greater than 0.5x109/L was 90, 90 and 30 days, respectively. None of these patients experienced either acute or chronic Graft-versus-Host Disease (GvHD) and no organ inflammatory-related toxicity was recorded. All children are alive and disease free, without infections, at day +200, +180 and +160, respectively. The 2 patients with CMV infection showed a specific response for at least one CMV-derived antigen; indeed, one patient showed a prevalence in pp65 response, whereas in the second one, we observed a specific anti-CMV response against all three tested antigens (Figure 1 - Panel D). Conclusions: Infusion of BPX-501 cells is able to accelerate the recovery of adaptive T-cell immunity in children with PID given haplo-HSCT after depletion of α/β T cells, thus rendering the procedure safer even in children with active infections at time of transplantation. These cells, once infused, expand in vivo and persist over time, contributing to the clearance of viral infections, without inducing GvHD. Figure 1. Figure 1. Disclosures Moseley: Bellicum Pharmaceuticals: Employment, Equity Ownership.