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Allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative therapy for most children with juvenile myelomonocytic leukemia (JMML). Novel therapies controlling the disorder prior to HSCT are needed. We conducted a phase 2, multicenter, open-label study to evaluate the safety and antileukemic activity of azacitidine monotherapy prior to HSCT in newly diagnosed JMML patients. Eighteen patients enrolled from September 2015 to November 2017 were treated with azacitidine (75 mg/m2) administered IV once daily on days 1 to 7 of a 28-day cycle. The primary end point was the number of patients with clinical complete remission (cCR) or clinical partial remission (cPR) after 3 cycles of therapy. Pharmacokinetics, genome-wide DNA-methylation levels, and variant allele frequencies of leukemia-specific index mutations were also analyzed. Sixteen patients completed 3 cycles and 5 patients completed 6 cycles. After 3 cycles, 11 patients (61%) were in cPR and 7 (39%) had progressive disease. Six of 16 patients (38%) who needed platelet transfusions were transfusion-free after 3 cycles. All 7 patients with intermediate- or low-methylation signatures in genome-wide DNA-methylation studies achieved cPR. Seventeen patients received HSCT; 14 (82%) were leukemia-free at a median follow-up of 23.8 months (range, 7.0-39.3 months) after HSCT. Azacitidine was well tolerated and plasma concentration-–time profiles were similar to observed profiles in adults. In conclusion, azacitidine monotherapy is a suitable option for children with newly diagnosed JMML. Although long-term safety and efficacy remain to be fully elucidated in this population, these data demonstrate that azacitidine provides valuable clinical benefit to JMML patients prior to HSCT. This trial was registered at www.clinicaltrials.gov as #NCT02447666.

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.

Background: Pediatric patients younger than two years of age with acute myeloid leukemia (AML) commonly receive a chemotherapy-based myeloablative conditioning regimen before allogeneic hematopoietic stem cell transplantation (HSCT). The optimal choice of cytotoxic agents is still controversial. Methods: A retrospective EBMT-registry based study was conducted to investigate the impact of different chemotherapy-based conditionings on the outcomes in young children. Children younger than two years of age receiving a first HSCT of bone marrow (BM), peripheral blood stem cells (PBSC) or cord blood (CB) from matched siblings (MSD) or unrelated donors (UD) in first complete remission (CR1) between 2000 and 2019 were included. Busulfan/Cyclophosphamide (BuCy) and BuCy/Melphalan (BuCyMel) were the most frequent combinations on which this analysis focused. The primary endpoint was leukemia-free survival (LFS). Multivariate analysis adjusting for differences between the conditioning regimens and risk factors influencing outcome was performed using the Cox's proportional hazards regression model. Results: 289 patients (56% male) transplanted at a median age of 1.2 years (IQR 0.9-1.6) after BuCy (164, 57%) or BuCyMel (125, 43%) were included. 184 (64%) patients received BM, 71 (24%) CB and 34 (12%) PBSC from UD (201, 70%) and MSD (88, 30%). In-vivo T-cell-depletion (TCD) was performed in 160 (58%, missing data 14) of the HSCTs with anti-thymocyte-globulin (ATG, 153) or alemtuzumab (7). Ex-vivo TCD was performed in 13 (5%, missing data 3) of the HSCTs. Graft-versus-host-disease (GvHD)-prophylaxis was Cyclosporin-A-based in 90% of the HSCTs. Median follow-up (FU) was 4.9 years (95% CI 3.9-5.5). After a median FU of 4 years, 4-y-LFS after BuCyMel (74.3%, 95% CI 65.1-81.4) was significantly better compared to BuCy (59.7%, 95% CI 51.2-67.2), hazard ratio (HR) 0.56 (95% CI 0.35-0.90, P=0.02). Overall survival (4-y-OS) after BuCyMel (77.2%, 95% CI 68.1-84.0) was significantly better compared to BuCy (66.6%, 95% CI 58.0-73.8), HR=0.58 (95% CI 0.35-0.97, P=0.04). No significant differences were found in the probability of relapse (4-y-RI (whole cohort) 26.2% (95% CI 21.0-31.7), HR of BuCyMel 0.59 (95% CI 0.34-1.02), P=0.06), non-relapse mortality (4-y-NRM (whole cohort) 7.8% (95% CI 5.0-11.4), HR of BuCyMel 0.49 (95% CI 0.19-1.24), P=0.13) and incidence of acute grade II-IV GvHD at day 100 (day-100-aGvHD II-IV (whole cohort) 36.8% (95% CI 31.2-42.5), HR of BuCyMel 0.59 (95% CI 0.35-1.01), P=0.06). Incidence of chronic GvHD (4-y-cGvHD (whole cohort)) was 9.8% (95%-CI 6.3-14.2). The donor type had no significant influence on the outcome. Conclusion: Bu-based conditionings of HSCT for infants with AML at high risk of relapse offer a high probability of cure. Conditioning with three alkylators (BuCyMel) resulted in better LFS and OS compared with two alkylators (BuCy) without significantly increasing the risk of both NRM and aGvHD. Future trials will evaluate the impact of the more recently introduced alkylator Treosulfan within the conditioning of HSCT in pediatric AML. Disclosures Peters: Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Travel grants. Locatelli: Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Miltenyi: Speakers Bureau; Medac: Speakers Bureau; Jazz Pharamceutical: Speakers Bureau; Takeda: Speakers Bureau. Moraleda: Pfizer: Other: Educational Grants, Research Funding; Sanofi: Other: Educational Grants, Research Funding; MSD: Other: Educational Grants, Research Funding; ROCHE: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Takeda: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Sandoz: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Gilead: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Jazz Pharmaceuticals: Consultancy, Honoraria, Other: Educational Grants, Research Funding; NovoNordisk: Other: Educational Grants, Research Funding; Janssen: Other: Educational Grants, Research Funding; Celgene: Other: Educational Grants, Research Funding; Amgen: Other: Educational Grants, Research Funding. Biffi: BlueBirdBio: Consultancy, Other: Advisory Board. Corbacioglu: Gentium/Jazz Pharmaceuticals: Consultancy, Honoraria.

Traditionally, hematopoietic stem cell transplantation (HSCT) from both HLA-matched related and unrelated donors (UD) has been used for treating children with acute leukemia (AL) in need of an allograft. Recently, HLA-haploidentical HSCT after αβ T-cell/B-cell depletion (αβhaplo-HSCT) was shown to be effective in single-center studies. Here, we report the first multicenter retrospective analysis of 127 matched UD (MUD), 118 mismatched UD (MMUD), and 98 αβhaplo-HSCT recipients, transplanted between 2010 and 2015, in 13 Italian centers. All these AL children were transplanted in morphological remission after a myeloablative conditioning regimen. Graft failure occurred in 2% each of UD-HSCT and αβhaplo-HSCT groups. In MUD vs MMUD-HSCT recipients, the cumulative incidence of grade II to IV and grade III to IV acute graft-versus-host disease (GVHD) was 35% vs 44% and 6% vs 18%, respectively, compared with 16% and 0% in αβhaplo-HSCT recipients (P < .001). Children treated with αβhaplo-HSCT also had a significantly lower incidence of overall and extensive chronic GVHD (P < .01). Eight (6%) MUD, 32 (28%) MMUD, and 9 (9%) αβhaplo-HSCT patients died of transplant-related complications. With a median follow-up of 3.3 years, the 5-year probability of leukemia-free survival in the 3 groups was 67%, 55%, and 62%, respectively. In the 3 groups, chronic GVHD-free/relapse-free (GRFS) probability of survival was 61%, 34%, and 58%, respectively (P < .001). When compared with patients given MMUD-HSCT, αβhaplo-HSCT recipients had a lower cumulative incidence of nonrelapse mortality and a better GRFS (P < .001). These data indicate that αβhaplo-HSCT is a suitable therapeutic option for children with AL in need of transplantation, especially when an allele-matched UD is not available.

Despite advances in supportive therapy to prevent complications of sickle cell disease (SCD), access to care is not universal. Hematopoietic cell transplantation is, to date, the only curative therapy for SCD, but its application is limited by availability of a suitable HLA-matched donor and lack of awareness of the benefits of transplant. Included in this study are 1000 recipients of HLA-identical sibling transplants performed between 1986 and 2013 and reported to the European Society for Blood and Marrow Transplantation, Eurocord, and the Center for International Blood and Marrow Transplant Research. The primary endpoint was event-free survival, defined as being alive without graft failure; risk factors were studied using a Cox regression models. The median age at transplantation was 9 years, and the median follow-up was longer than 5 years. Most patients received a myeloablative conditioning regimen (n = 873; 87%); the remainder received reduced-intensity conditioning regimens (n = 125; 13%). Bone marrow was the predominant stem cell source (n = 839; 84%); peripheral blood and cord blood progenitors were used in 73 (7%) and 88 (9%) patients, respectively. The 5-year event-free survival and overall survival were 91.4% (95% confidence interval, 89.6%-93.3%) and 92.9% (95% confidence interval, 91.1%-94.6%), respectively. Event-free survival was lower with increasing age at transplantation (hazard ratio [HR], 1.09; P < .001) and higher for transplantations performed after 2006 (HR, 0.95; P = .013). Twenty-three patients experienced graft failure, and 70 patients (7%) died, with the most common cause of death being infection. The excellent outcome of a cohort transplanted over the course of 3 decades confirms the role of HLA-identical sibling transplantation for children and adults with SCD.

The outcome of high-risk (HR) Acute Lymphoblastic Leukemia (ALL) patients enrolled in AIEOP-BFM ALL 2000 study (NCT00613457) in Italy is described. Overall, 1999 Philadelphia negative ALL patients entered the study. HR criteria were: minimal residual disease (MRD) levels ≥10-3 at day 78 (HR-MRD), no complete remission (no-CR) at day 33, t(4;11) translocation, Prednisone Poor Response (PPR). Treatment (2 years) included protocol I, 3 polychemotherapy blocks, delayed intensification (protocol IIx2 or IIIx3), cranial radiotherapy, maintenance. 312 HR patients (15.6% of the total) had 5-year event-free survival (EFS) and overall survival (OS) of 58.9%(SE 2.8) and 68.9%(2.6). In hierarchical order, EFS was 45.9%(4.4) in 132 HR-MRD patients, 41.2%(11.9) in 17 patients no-CR at day 33, 36.4%(14.5) in 11 patients with t(4;11), 74.0%(3.6) in 152 HR patients only for PPR. No statistically significant difference was found for disease-free survival (DFS) in patients with very high risk features (HR-MRD, no-CR at day 33, t(4;11) translocation), given HSCT (n=66) or chemotherapy only (n=88), after adjusting for waiting time to hematopoietic stem cell transplantation (HSCT) (5.7 months). Patients at HR only for PPR have favorable outcome. High risk MRD is associated with poor outcome despite intensive treatment and/or HSCT and may qualify for innovative therapies. The study is registered at the US National Institutes of Health website http://clinicaltrials.gov as "Combination Chemotherapy Based on Risk of Relapse in Treating Young Patients With Acute Lymphoblastic Leukaemia" with the protocol identification number NCT00613457.

We previously reported that patients with fibrotic, chronic graft-versus-host disease (cGVHD) have antibodies activating the platelet-derived growth factor receptor pathway. Because this pathway can be inhibited by imatinib, we performed a pilot study including 19 patients with refractory cGVHD, given imatinib at a starting dose of 100 mg per day. All patients had active cGVHD with measurable involvement of skin or other districts and had previously failed at least 2 treatment lines. Patient median age was 29 years (range, 10-62 years), and median duration of cGvHD was 37 months (range, 4-107 months). The organs involved were skin (n = 17), lung (n = 11), and bowel (n = 5); 15 patients had sicca syndrome. Imatinib-related, grade 3 to 4 toxicity included fluid retention, infections, and anemia. Imatinib was discontinued in 8 patients: in 3 because of toxicity and in 5 because of lack of response (n = 3) or relapse of malignancy (n = 2). Overall response rate at 6 months was 79%, with 7 complete remissions (CRs) and 8 partial remissions (PRs). With a median follow-up of 17 months, 16 patients are alive, 14 still in CR or PR. The 18-month probability of overall survival is 84%. This study suggests that imatinib is a promising treatment for patients with refractory fibrotic cGVHD.

Severe hepatic VOD/SOS is a potentially life-threatening complication of HCT conditioning that may also develop after high-dose chemotherapy. The most severe form of VOD/SOS is often accompanied by multi-organ failure (MOF) and is associated with a mortality rate of >80% when managed with supportive care alone. As part of the marketing authorization in Europe, there was an obligation to set up a disease registry of patients with severe VOD/SOS post-HCT who were treated with defibrotide. The goal of this registry was to collect safety and outcome data and assess patterns of defibrotide utilization in the post-approval setting. This multicenter, multinational, prospective observational study (NCT03032016) was performed by the European Society for Blood and Marrow Transplantation (EBMT). The study included patients with severe VOD/SOS post-HCT who were treated with defibrotide and enrolled from April 2015 to July 2018. Participating centers were members of the EBMT. Physicians registered patients diagnosed with severe VOD/SOS, as assessed by the investigator using classical/standard criteria (including but not limited to hyperbilirubinemia, hepatomegaly, ascites, and weight gain >5%), who consented to participate in the study. In addition, patients who were prescribed defibrotide for purposes other than the approved indication (eg, VOD/SOS prophylaxis, treatment of non-severe VOD/SOS or thrombotic microangiopathy) and consented to participate were registered and information collected. There were no specific exclusion criteria; however, treating physicians were alerted to contraindications, special warnings, and precautions detailed in the defibrotide summary of product characteristics. After inclusion, patient information was collected from participating centers at 100 days, 6 months, and 12 months post-HCT. The primary objective was to assess the incidence of specific serious adverse events (SAEs) of interest, which were hemorrhage and site of bleeding, hypotension, coagulopathy, allergic or hypersensitivity reactions, injection-site reaction, infection and septicemia, and thromboembolic events. Secondary endpoints included Day 100 survival, and overall rate of VOD/SOS (and MOF, if present) resolution (based on standard criteria). Summary statistics were calculated for baseline data and safety variables; outcome analyses are descriptive. Here we report an analysis of data with a cutoff of June 18, 2019. Database lock is planned for October 2019, and the presentation will be updated to include the final data. A total of 61 patients with severe VOD/SOS were included; MOF was diagnosed at registration in 34 (56%) patients. The median age of patients with severe VOD/SOS was 14.4 (range: 0-68) years, 34 (56%) aged An SAE of interest occurred in 19 (31%; 95% confidence interval [CI]: 20%-43%) patients with severe VOD/SOS. The most common SAEs of interest by category were infection (n = 13 [21%; 95% CI: 11%-32%]) and bleeding events (n = 8 [13%; 95% CI: 5%-22%]). The most common individual SAEs of interest (≥5% of patients) were pneumonia (8%), gastrointestinal bleeding (8%), and sepsis (5%). Death occurred in 30 (49%) patients within 1 year, with VOD/SOS indicated as a cause of death in 13/30 (43%) patients. The Kaplan-Meier-estimated survival rate at Day 100 for patients diagnosed with severe VOD/SOS post-HCT who were treated with defibrotide was 74% (95% CI: 61%-83%). At latest follow-up, the survival rate was 51%, with median Kaplan-Meier-estimated survival post-HCT not yet reached. VOD/SOS resolved in 46 (75%) patients; the cumulative rate of VOD/SOS resolution at Day 100 was 87% (95% CI: 72%-94%). Resolution of MOF was achieved in 19/34 (56%) patients who had MOF at VOD/SOS diagnosis. In conclusion, among patients with severe VOD/SOS post-HCT (with/without MOF), the incidence of SAEs of interest was consistent with that observed in previous defibrotide clinical trials. Treatment with defibrotide resulted in high rates of Day 100 survival and VOD/SOS resolution. Disclosures Mohty: Jazz Pharmaceuticals: Honoraria, Research Funding. Blaise:Sanofi: Honoraria; Molmed: Consultancy, Honoraria; Jazz Pharmaceuticals: Honoraria; Pierre Fabre medicaments: Honoraria. Labopin:Jazz Pharmaceuticals: Honoraria. Hanvesakul:Jazz Pharmaceuticals: Employment, Equity Ownership. Ryan:Jazz Pharmaceuticals: Employment, Equity Ownership. Lawson:Jazz Pharmaceuticals: Consultancy, 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.

Introduction Allogeneic Hematopoietic Cell Transplantation (allo HCT) is currently the only curative therapy for high-risk hematologic malignancies due to the immune response of the donor cells against the malignant cells (graft versus tumor effect; GVT), but with the cost of Graft Versus Host Disease (GVHD). Despite extensive research, very few predictors of GVHD and GVT have been identified to date. Additionally, clinical GVHD diagnosis can be challenging due to chemotherapy-related or infection-related organ toxicity manifestations, which further complicate prediction and treatment stratification algorithms. In order to study the mechanisms of GVHD and GVT and to identify potential GVHD markers we apply a novel approach, called Transcriptome Fingerprint Assay (TFA), relying on high frequency sampling and blood transcript profiling. The TFA is a multiplex microfluidics q-PCR based assay linked with a computational model for modular functional transcriptome analyses, uniquely tailored to answer complex questions on immune perturbations through frequent profiling of gene expression signatures from < 1 ml of blood (Chaussabel and Baldwin. Nat Rev Immunol 2014, Speake et al. Clin Exp Immunol 2017). This approach has been successfully applied to stratify patients' prognosis in autoimmune and infectious diseases (Banchereau R et al. Cell 2016, Dunning et al. Nat Immunol 2018). In our study we use the TFA to capture longitudinal immune signatures as dynamic "snapshots" of the patient's immune system after HCT. Hypotheses Fluctuations over-time in gene expression of allo HCT patients' immune system reflect the pathologic/disease control programs (GVHD/GVT) and may be used to identify diagnostic and predictive biomarkers. GVHD/GVT control immune programs depend on the "inner" interface between the donor immune-system and the recipient, and are influenced by external variables, as infections or drugs. These variables can affect the immune system-related gene expression and can be measured. Objectives To systematically measure gene expression signatures in immune perturbations post-allo HCT, in order to: Identify GVHD-related immune signatures consistent with clinical diagnosis of GVHD.Predict and stratify therapy-resistant GVHD and severe chronic GVHD, according to immune signatures.Identify links (causative and consequential) between GVHD, GVT, relapse, and other post-transplant immune perturbations (e.g. infections). Methods Enroll 250 allo HCT patients to populate a "GVHD cohort" and a "non-GVHD cohort" of 50 patients each, and 50 donors (healthy controls cohort) . Patients donate micro-quantities of blood (50 to 600 microliters), weekly until day 100 post-transplant and every 2 weeks thereafter until 2 years after transplant. Detailed clinical, laboratory and therapy annotations are captured during the follow-up. Gene expression of 264 immune-related genes for each sample are measured through Fluidigm BioMark high throughput qPCR system, and normalized to the geometric mean Ct of 8 housekeeping genes. Data interpretation is performed through TFA modular analyses and correlated with the clinical annotations. Results Results of three series of patients' samples are shown to exemplify the potential of TFA as a method to study the mechanisms of GVHD and GVT. All three patients underwent myeloablative peripheral blood stem cell transplant from an HLA identical sibling donor. Two patients developed steroid responsive-acute GVHD (patient #1: GVHD stage I was diagnosed on day 38 post HCT, patient #4: GVHD stage III was diagnosed on day 21 post HCT). One patient (Patient #6) did not develop clinical GVHD, but routine skin biopsy on day 80 revealed apoptotic cells consistent with subclinical skin GVHD. Principal Component Analysis (PCA) of the three patients' series is shown in Figure 1, the dynamic transcriptomes according to TFA modules of patients #1 and #6 are shown in Figure 2, and representative TFA modular fingerprints are shown in Figure 3. Conclusion We anticipate that using the TFA approach will help to fill knowledge gaps instrumental to solve clinical dilemmas related to allo HCT complications, and to improve the clinical outcomes of allo HCT patients. Disclosures No relevant conflicts of interest to declare.

Introduction: Allogeneic hematopoietic stem cell transplantation (HSCT) is, to date, the only curative treatment for sickle cell disease (SCD). Because a human leukocyte antigen (HLA) matched sibling donor is not always available, alternative stem cell sources such as unrelated or haploidentical related donors have been explored. The likelihood of finding a 10/10 (HLA-A, B, C, DRB1 and DQB1) matched donor varies among ethnic groups, with the lowest probability among individuals of African descent. To date, few series of SCD patients transplanted with an unrelated donor (UD) have been reported, but the high rates of rejection and chronic graft versus host disease (cGvHD) have limited its widespread application. Patients and methods: We report the results of a retrospective, registry based, survey on 70 UD HSCT performed in patients (pts) with SCD from UD in 22 European Society for Blood and Marrow Transplantation (EBMT) centers between 2005 and 2017. Data were collected from the EBMT database and missing information was updated by the centers. Median follow up was 38 (range 2-154) months. Most pts were HbSS (n=54; 78%), had positive serology for CMV (80%), and a Karnofsky score >80% (98%). Eighteen pts had a major ABO incompatibility. Recurrent vaso-occlusive crisis (n=58), cerebral vasculopathy (n=23) and acute chest syndrome (n=24) were the main indications for HSCT. Red blood cell (RBC) transfusions pre-HSCT were reported in 97% of pts of whom 53% received more than 20 transfusions; 14% of the transfused pts had RBC alloantibodies. Hydroxyurea pre-HSCT was used in 65% of pts. Median age at HSCT was 9.6 years (range 2-43) with 87% of pts being ≤ 16 years. Stem cell source was bone marrow (BM) in 55 pts (79%) and peripheral blood (PBSC) in 15 (21%). The median number of infused TNC /kg was 3.6 x 108 for BM and 7.1 x 108 for PBSC; the median number of infused CD34/kg was 4.4 x 106 for BM and 8.3 x 106 for PBSC. HLA matching at high resolution typing was 10/10 (HLA-A, B, C, DRB1 and DQB1) in 31, 9/10 in 17 and 8/10 in 4 of the patient-donor pairs; intermediate resolution typing was available for 10 (10/10 or 9/10) and the HLA information was missing for the remaining 8 patient-donor pairs. The most frequent conditioning regimens were fludarabine-thiotepa-treosulfan (64%) and busulfan- cyclophosphamide (12%). GvHD prophylaxis was cyclosporine plus methotrexate in 59%. Anti-thymocyte globulin was used in 90% and alemtuzumab in 9% of pts. Results: The cumulative incidence (CI) of neutrophil engraftment at 60 days was 93% (95% CI 76-100), with median time to engraftment of 18 days; platelet engraftment at 180 days was 90% (95% CI 83-98) with a median time of 20 days. Ten pts had graft failure (5 primary and 5 secondary) of whom 6 had a second transplant and were all alive at last FU (median 9.5 months after second HSCT). The CI of grade II-IV aGVHD at 100 days was 23% (95% CI 15-36), and 8 pts (11%) had grade III-IV. Acute GVHD was more frequent in patients who received PBSC (PBSC 42.9%, BM 18.2%, p=0.062). Three-year CI of cGVHD was 23% (95% CI 15-36), 7 pts (10%) had limited and 9 (13%) extensive cGvHD. Three-year overall survival (OS) was 90±4%; three-year event free survival (EFS) (considering death and graft failure as events) was 76±6%; HLA matching between donor and recipient was the most important factor for OS and EFS. Considering only pts-donor pairs with high resolution HLA typing available (n=52), 3-year OS was 96±4% in 10/10 group compared to 77±11% in 9/10 plus 8/10 group (p 0.065), 3-year EFS was 85±7% vs 62±12% (p 0.040), respectively. No significant differences between the groups were observed in CI of neutrophil engraftment, aGVHD and cGVHD. Conclusion: UD HSCT is a valid option for SCD patients who lack an HLA-identical sibling donor. Nevertheless, efforts are still needed to improve outcomes after UD HSCT. Our results indicate that using a 10/10 HLA matched UD improves both OS and EFS compared to donors with 1 or more mismatches; so, when such a matched unrelated donor is not found, using an haplo relative or an unrelated cord blood as donor source should be evaluated. A prospective trial is in preparation to evaluate the use of haploidentical donors for HSCT in SCD (EudraCT number: 2018-002652-33). Disclosures No relevant conflicts of interest to declare.