Alternative donor hematopoietic stem cell transplantation for sickle cell disease in Europe

Allogeneic hematopoietic stem cell transplantation (HSCT) is, to date, the only curative treatment for sickle cell disease (SCD). Because an 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. To date, few series of SCD patients transplanted with an unrelated donor, cord blood, and haploidentical related donor have been reported, but the high rates of rejection and chronic graft versus host disease have limited their widespread application. We describe the outcomes of a retrospective, registry-based, survey on 144 alternative donor HSCT performed for SCD in 30 European Society for Blood and Marrow Transplantation centers between 1999 and 2017. Data on 70 unrelated adult donors (49%), six cord blood (4%), and 68 haploidentical donors (47%; including post-HSCT Cy, ex vivo T-cell depleted, and other haplo-HSCTs) were reported and missing information was updated by the centers. Overall, 16% patients experienced graft failure, Grade II-IV acute GVHD at 100 days was 24%, whereas Grade III-IV was 10%. Chronic GVHD was observed in 24% (limited for 13 patients and extensive for 18 patients). Overall, the 3-year overall survival (OS) was 86% ± 3% and 3-year event-free survival (EFS; considering death and graft failure as events) was 72% ± 4%. We therefore conclude that alternative donor HSCT for SCD can be feasible but efforts in decreasing relapse and GVHD should be promoted to increase its safe and successful utilization. Moreover, a better knowledge of HLA matching and the tailoring of conditioning could help improve EFS and OS.