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PV and AL contributed equally Multiple studies from the EBMT registry and others have shown excellent survival rates after allogeneic haematopoietic stem cell transplantation (HSCT)for Wiskott-Aldrich syndrome (WAS) patients (Ozsahin et al, Blood 2008). The importance of myeloid engraftment for full disease correction has also been demonstrated (Moratto et al, Blood 2011). However, the vast majority of HSCTs in these studies were performed with (oral) busulfan/cyclophosphamide-based conditioning and in the early 2000 years or before. In 2005, the inborn errors working party (IEWP) of EBMT and ESID first recommended busulfan/fludarabine (BuFlu) or treosulfan/fludarabine (TreoFlu) based conditioning for primary immunodeficiencies such as WAS, with some centers deciding to add thiotepa (TT) to the conditioning. We performed a retrospective analysis via the EBMT and SCETIDE registries of WAS patients transplanted between 01/01/20006 and 12/31/2016 with these two regimens. The primary objective was to compare the overall (OS) and event-free survival (EFS) after HSCT with either BuFlu±TT or TreoFlu±TT conditioning. Secondary objectives included the influence of either conditioning regimen on acute and chronic GVHD, the degree of donor chimerism, incidence of secondary procedures after HSCT (2nd HSCT, stem cell boost, DLI, gene therapy or splenectomy) and rates of disease-specific complications after HSCT. At the time of this interim analysis, 174 patients were included, 92 (53%) with BuFlu±TT and 82 (47%) with TreoFlu±TT conditioning with a median age of 1.57 years (range 0.21-29.96) at HSCT and a median follow-up of 32.9 months (range 1.5-128.9). The donor was an HLA-matched sibling (MSD) in 30, a matched related donor (MRD) in 5, a matched unrelated donor (MUD, 9/10 or 10/10) in 105, a mismatched unrelated donor (MMUD, 5 years had a worse OS as compared to those 5 years or younger at HSCT (74.9% vs. 90.8%; log-rank test p=0.005). The type of donor had no influence on OS: 96.4% for MSD/MFD, 86.8% for MUD/MMUD and 87.7% for MMFD (log-rank test p=0.4). Whole blood chimerism was complete (>90% donor) in 60/75 evaluable patients (80%) at last follow-up or before secondary procedure (if a patient had one), 39/40 (98%) in the BuFlu±TT group and 21/35 (60%) in the TreoFlu±TT group. Twenty-six patients required a secondary procedure: stem cell boost in 4 patients, donor lymphocyte infusion in 9, 2nd HSCT in 15 and splenectomy in 1. Twenty-two of these 26 (84.6%) are alive and 14 of 16 with available chimerism data have a complete donor chimerism (>90%donor) at last follow-up. The 2-year cumulative incidence (CI) of secondary procedures was higher at 33.9% in the TreoFlu±TT versus 12.8% in the BuFlu±TT group (Gray's test p=0.017), and 2-year EFS (secondary procedure or death as event) was 61.4% in the TreoFlu±TT and 75.0% in the BuFlu±TT group (log-rank test p=0.2). Grade II-IV acute GVHD had the same incidence in both groups (100 day CI 24.4% vs. 26.3%; Gray's test p=0.849) and chronic GVHD of any grade was borderline more frequent in the TreoFlu±TT group (2 year CI 17.2% vs 6.7%; Gray's test p=0.054). The cumulative incidence of disease-specific complications occurring more than 6 months post HSCT (severe infections, bleeding, autoimmunity) was not different between the two groups (6.5% vs. 6.4%; Gray's test p=0.92). There was no malignancy reported after HSCT except for one EBV-post-transplant lymphoproliferative disorder (PTLD) 2.7 months after HSCT. In summary, HSCT with either BuFlu±TT or TreoFlu±TT conditioning reliably cures almost 90% of patients with WAS regardless of donor type. WAS-related complications are very rare events more than 6 months post HSCT. More patients required secondary procedures after treosulfan-based than busulfan-based conditioning. These data confirm the feasibility and efficacy of the regimens currently recommended by the IEWP. Disclosures Slatter: Medac: Other: Travel assistance. Chiesa:Gilead: Consultancy; Bluebird Bio: Consultancy. Kalwak:Sanofi: Other: travel grants; medac: Other: travel grants. Locatelli:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy; 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. Sykora:Aventis-Behring: Research Funding; medac: Research Funding. Zecca:Chimerix: Honoraria. Veys:Pfizer: Honoraria; Servier: Research Funding; Novartis: Honoraria.

Introduction Relapse, graft-versus-host disease (GvHD) and associated non-relapse mortality are the main obstacles to successful hematopoietic stem cell transplantation in children with leukemia. αβ T cell depletion was developed to prevent GvHD and improve immune reconstitution in recipients of mismatched grafts. Most current protocols use rabbit anti-thymocyte globulin (ATG) as an essential component of preparative regimen to secure engraftment and GVHD control. In order to avoid damaging effects of circulating ATG on graft NK and gd T cells, we have replaced ATG with pharmacologic blockade of IL-6 and CD80/CD28 co-stimulation axis in our ongoing study. Patients and methods Major transplantation outcomes were compared between participants of the current prospective trial (ATG-) and a retrospective control group (ATG+). A total of 165 children with acute leukemia (67 AML, 98 ALL, 68 female, 97 male, median age 8,7 y) underwent allo HSCT between 01.11.2013 and 01.03.2018. Of them 134 - from haploidentical donor and 31 from unrelated donor. All pts were in complete remission (CR1=80, CR2=67, CR>2=18). Ninety-two pts received treosulfan-based conditioning, 73 - TBI-based (all ALL). Either melphalan (n=46) or thiophosphamide (n=98) or etoposide (n=21) were added as a second agent. Fludarabine was used in all pts. Two types of GVHD prophylaxis were used: type 1 (ATG+), (n=98): thymoglobulin 5mg/kg, rituximab 200mg/m2 with either bortezomib on days +2, +5 (n=72) or tacro (n=9) or without any additional agents (n=17); Type 2 (ATG-) (n=67): tocilizumab at 8 mg/kg on day -1, bortezomib on day +2, +5 with abatacept at 10 mg/kg on day -1, +7, +14, +28 (n=63) or without added agent (n=4). αβ T cell depletion with CliniMACS was used in all cases. The median dose of CD34+ cells was 9x106/kg, αβ T cells - 16 x103/kg. Modified (CD45RA-depleted) donor lymphocyte infusions (DLI) were administered to 113 pts. Twenty-five patients received DLI on day 0 and 88 pts received DLI after engraftment. Median time of follow-up for survivors was 2 years (range, 0,3 - 4,5). Results Three patients died before engraftment due to septic event. Primary engraftment was achieved in 161 of 162 evaluable pts (99,3%), the median time to neutrophil and platelet recovery was 16 and 15 days. Among the whole cohort the cumulative incidence of acute graft-versus-host disease (GvHD) grades II - IV and III - IV was 11,5% (95% CI: 7,5-17,6) and 4,8% (95% CI: 2,5-9,5) respectively. The cumulative incidence of cGvHD was 10 % (95% CI: 6,3-15,9). The incidence of aGvHD and cGvHD was not different between ATG (+) and ATG (-). Among the whole cohort 2-year pTRM was 8% (95%CI: 4,8-13,5). pTRM was significantly lower among ATG (-) group - 1,5% (95%CI:0,2-10,4) versus 12,2% (95%CI:7,2-20,8) among ATG (+) group, p = 0,015. The cumulative incidence of relapse at 2 years was 21% (95%CI: 15,5-29), 24% (95%CI: 16-35), among ATG (+) and 19% (95%CI: 11-32), among ATG (-), p = 0,8. Two-year pEFS was 70% (95%CI: 62-77), 2-year pOS - 78% (95%CI: 71-85). Among patients, who received ATG (-) regimen, pEFS was 76% (95%CI: 68-89), as compared to 65% (95%CI: 56-75) among ATG (+), p=0,1 and pOS was 89% (95%CI: 81-97) versus 72% (95%CI: 63-81), p=0,032, respectively. αβ T cell recovery at day +30 was associated with a trend to decreased incidence of relapse, CI of relapse was 32% (95% CI:22 - 47) in those with αβ-T cell count < median vs 18 % (95% CI: 11-32) in those with αβ-cell count >median, p=0,08. EFS among αβ T" high" was 81% (+/-10) vs 56% (+/-14) among αβ T"low", p=0,002. Discussion We confirm that the depletion of αβ T cells from the unrelated and haploidentical graft in combination with intensive conditioning regimen ensures high engraftment rate and low transplant-related mortality. Our analysis suggests that polyclonal ATG serotherapy is not an essential part of the transplant regimen in αβ T-depleted transplantation. Combined administration of tocilizumab and abatacept after αβ T cell-depleted grafting effectively prevents GVHD, does not compromise engraftment, appears to decrease non-relapse mortality and improve survival. Disclosures No relevant conflicts of interest to declare.

Introduction The outcome hematopoietic stem cell transplantation (HSCT) in a cohort of children with chemorefractory leukemia is poor. The incidence of relapse exceeds 50% and survival varies from 10 to 40%. Additional attempts at remission induction with various combinations of chemotherapy are unlikely to improve the outcome and will contribute to toxicity. We hypothesized that personalized targeted therapy combined with high-dose chemotherapy may improve the outcome of allogeneic hematopoietic stem cell transplantation in a cohort of pediatric patients with refractory leukemia. Bcl-2 and CD38 were chosen as potential targets due to frequent expression in pediatric acute leukemias, availability of marketed targeted therapies, venetoclax and daratumumab, and expected non-overlapping toxicity profile of these agents and the conditioning regimen. Materials and methods A total of 16 pts with chemorefractory disease (T-ALL - 2, AML - 8, JMML - 6, 12 male, 4 female, median age 5,7 years), underwent HSCT between November 2017 and June 2018, median follow-up - 3 months (1,6-7). All pts were transplanted from haploidentical donors, had active disease (AD) at the moment of SCT, for 12 (75%) pts it was the first allogenic HSCT, for 4 pts it was the second HSCT. Median bone marrow leukemia burden before cytoreduction was 22% (3-75). Bcl-2 expression on the tumor cells was detected in all pts (100%) with the median expression of 69% (0,7-100), CD38 expression was detected in 10 pts (AML=7, ALL=2, JMML-1) with the median expression of 96% (71-100). Ten pts received treosulfan-based conditioning, 3 - busulfan-based and 3 -TBI-based. GVHD prophylaxis included tocilizumab at 8 mg/kg on day -1, post-transplant bortezomib and abatacept at 10 mg/kg on day -1, +7, +14, +28. Three pts received thymoglobulin 5mg\kg. According to the expression of Bcl-2 and CD38 on tumor cells, 9 patients (56%) received Daratumumab (anti-CD38 monoclonal antibody) on day -6, 15 patients (94%) received venetoclax at 300 mg/m2/day on days -7 to -2. TCRαβ+/CD19+ depletion of PBSC with CliniMACS technology was implemented in all cases. The median dose of CD34+ cells in transplant was 11 x106/kg (range 7-18), α/β T cells - 40x103/kg (range 11- 139). Modified (CD45RA-depleted) donor lymphocyte infusions (DLI) were administered to 15 pts, 9 pts received modified DLI on day 0. Result Primary engraftment was achieved in 13 (81%) of 16 pts. The median time to ANC and platelets recovery was 14 days (11-22). Engraftment was 100% (10 of 10) among patients with acute leukemia and 50% (3 of 6) among patients with JMML. Three patients with JMML had early disease progression. There were no significant toxic effects after HSCT and no cases of transplant-related mortality. The median NK- cells count by the day +30 was 0,185 x 106/ml (range 0,019- 0,472), the median levels of αβ T cells and gd T cells were 0,045 x 106 /ml (range 0 - 0,364) and 0,07 x 106 /ml (range 0 - 0,349, respectively. Acute GVHD grade 1-2 was developed in 2 pts (15%), none of them required systemic immunosuppressive therapy. There were no cases of chronic GVHD. One (7,6%) patient with AML relapsed on day +61. Three pts (1 with AML and 2 with JMML) died from disease progression, 1 patient with JMML died from complications after the second HSCT. At the moment of reporting 12 pts (9 of 10 with acute leukemia and 3 of 6 with JMML) are alive, in complete remission with a median follow up of 3 months (1,5-7m). Conclusion We suggest that addition of venetoclax and datatumumab to the backbone of myeloablative haploidentical HSCT with αβ T cell depletion is not associated with increased toxicity and may lead to improved early outcomes in a cohort of pediatric patients with chemorefractory acute leukemia. This approach can be further tested in a prospective trial with the goal to increase the anti-leukemic efficacy of HSCT. Disclosures No relevant conflicts of interest to declare.

Introduction Hematopoietic stem cell transplantation from non-sibling donors remains the only curative option for severe aplastic anemia patients refractory to ATG/CsA immunosuppression. Although the results of MUD and haploidentical transplantation in SAA have improved significantly, graft-versus host disease (GVHD) remains a serious problem, associated with significant morbidity and mortality. We investigated the role of new method of graft processing - TCR alpha/beta depletion as a way to improve the results of MUD and haploidentical transplants in SAA. Patients and methods Forty two patients with SAA were treated since November 2012 till February 2016. Median age at HSCT was 11(3-22) years, 27 male/15 female. All pts. were refractory/relapsed (36/6) after at least two courses of ATG/CsA, 3 pts. had concurrent severe hemolytic PNH. Time from diagnosis to transplant was 17(143-8,6)/15(99-5,5) months. Donors were unrelated volunteers in 32 cases, haploidentical parents in 10 cases. Preparative regimen included cyclophosphamide 100-150 mg/kg, fludarabine 150mg/kg, ATG and 2-6Gy thoraco-abdominal irradiation, in haplo transplants patients additionaly received thiophosphamide at 10mg/kg. Two patients recieved alemtuzumab instead of ATG because of anaphylaxis. Post-transplant GVHD prophylaxis included Tacro and Mtx on days +1, +3, +6. PBSC grafts were depleted of TCR alpha/beta cells and CD19 cells with CliniMACS device as recommended by the manufacturer. Patients received a median of 10(6,0-23) x106 CD34 per kg, 8(1-39) x104 TCR alpha/beta per kg. Results All patients engrafted with a median of 15 days for WBC and 13,5 days for platelets. In 4 patients after MUD transplantation secondary graft failure (rejection) developed, two of them were successfully retransplanted. Cumulative incidence of aGVHD 2-3 was 9% (95% CI: 3-27%) and 40%(95% CI: 18-85%) in MUD and haplo respectively, 90% patients with aGVHD had only skin involvement. No grade 4 aGVHD detected. Cumulative incidence of grade 3 aGVHD was 3%(95% CI: 0,5-21%) and 10%(95% CI: 2-64%) in MUD and haplo, respectively. Cumulative incidence of cGVHD was 12%(95% CI: 0,5-58%) and 30%(95% CI: 11-77%) in MUD and haplo respectively. A median follow up is 2 years. Seven patients died of viral infection - CMV (2 pts.), viral infection - CMV plus GVHD (2 pts.), microangiopathy (1 pt), 2 pts. died after second transplantation (1 - disseminated toxoplasmosis, 1 - ADV and CMV and GVHD). Event and GVHD-free survival is 73%(95% CI: 57-89%) and 60% (95% CI: 30-90%) in MUD and haplo respectively (pic.1) Overall survival is 86%(95% CI: 74-99%) and 78%(95% CI: 50-100%) respectively for MUD and haploidentical (pic.2). Prolonged stable mixed chimerism in T-cells was detected in recipients of MUD grafts in contrast to haploidentical grafts (pic/3). Conclusion TCR alpha/beta depletion is a robust platform for allogeneic transplantation from MUD and haploidentical donors in severe aplastic anemia. Results should be further improved by additional measures to control viral infections and prevent rejection in MUD transplants. Figure 1 Figure 1. Figure 2 Figure 2. Figure 3 Figure 3. Disclosures No relevant conflicts of interest to declare.