CD3⁺TCRαβ/CD19⁺-Depleted Mismatched Family or Unrelated Donor Salvage Stem Cell Transplantation for Graft Dysfunction in Inborn Errors of Immunity

A minority of children experience significant graft dysfunction after undergoing allogeneic hematopoietic stem cell transplantation (HSCT) for inborn errors of immunity (IEI). The optimal approach to salvage HSCT in this scenario is unclear with respect to conditioning regimen and stem cell source. This single-center retrospective case series reports the outcomes of salvage CD3⁺TCRαβ/CD19-depleted mismatched family or unrelated donor stem cell transplantation (TCRαβ-SCT) between 2013 and 2022 for graft dysfunction in 12 children with IEI. Outcomes of interest were overall survival (OS), event-free survival (EFS), graft-versus-host disease (GVHD)-free and event-free survival (GEFS), toxicities, GVHD, viremia and long-term graft function. In this retrospective audit of patients who underwent second CD3⁺TCRαβ/CD19-depleted mismatched donor HSCT using treosulfan-based reduced-toxicity myeloablative conditioning, the median age at first HSCT was 8.76 months (range, 2.5 months to 6 years), and that at second TCRαβ-SCT was 3.6 years (range, 1.2 to 11 years). The median interval between first and second HSCTs was 1.7 years (range, 3 months to 9 years). The primary diagnoses were severe combined immunodeficiency (SCID) (n = 5) and non-SCID IEI (n = 7). Indications for second HSCT were primary aplasia (n = 1), secondary autologous reconstitution (n = 6), refractory acute GVHD (aGVHD) (n = 3), and secondary leukemia (n = 1). Donors were either haploidentical parental donors (n = 10) or mismatched unrelated donors (n = 2). All patients received TCRαβ/CD19-depleted peripheral blood stem cell (PBSC) grafts with a median CD34⁺ cell dose of 9.3 × 10⁶/kg (range, 2.8 to 32.3 × 10⁶/kg) and a median TCRαβ⁺ cell dose of 4 × 10⁴/kg (range, 1.3 to 19.2 × 10⁴/kg). All patients engrafted, with a median time neutrophil and platelet recovery of 15 days (range, 12 to 24 days) and 12 days (range, 9 to 19 days). One patient developed secondary aplasia, and 1 had secondary autologous reconstitution; both underwent a successful third HSCT. Four (33%) had grade II aGVHD, and none had grade III-IV aGVHD. No patients had chronic GVHD (cGVHD), but 1 patient developed extensive cutaneous cGVHD after their third HSCT using PBSCs and antithymocyte globulin. Nine (75%) had at least 1episode of blood viremia with human herpesvirus 6 (n = 6; 50%), adenovirus (n = 6; 50%), Epstein-Barr virus (n = 3; 25%), or cytomegalovirus (n = 3; 25%). The median duration of follow-up was 2.3 years (range, .5 to 10 years), and the 2-year OS, EFS, and GEFS were 100% (95% confidence interval [CI], 0 to 100%), 73% (95% CI, 37% to 90%), and 73% (95% CI, 37% to 90%), respectively. TCRαβ-SCT from mismatched family or unrelated donors, using a chemotherapy-only conditioning regimen, is a safe alternative donor salvage transplantation strategy for second HSCT in patients without a suitably matched donor.