Allogeneic hematopoietic cell transplantation with reduced-toxicity conditioning for X-linked hyper-IgM syndrome.

X-linked hyper-IgM syndrome (XHIGM) is an inborn error of immunity caused by variants in the CD40LG gene, leading to disrupted T-cell function and impaired immunoglobulin class-switch recombination. Supportive care alone is linked to unfavorable outcomes, necessitating allogeneic hematopoietic cell transplantation (HCT) as a curative option. Although myeloablative conditioning is recommended for HCT, concerns remain regarding its toxicity. We reviewed ten patients with XHIGM who underwent HCT using reduced-toxicity conditioning (RTC) at our institution between 2010 and 2024. All patients underwent HCT by 5 years of age (range: 2-5 years; median: 3 years). Donor sources included unrelated bone marrow, unrelated cord blood, and haploidentical-related donors. Conditioning primarily consisted of fludarabine and busulfan (target area under the curve: 60-65 mg/L·h), with low-dose total body irradiation (TBI) administered in five patients. Two patients experienced graft failure, requiring a second transplantation, and one died from post-transplant complications. The remaining nine patients survived. The 5-year overall survival rate was 88.9%; the 5-year graft-versus-host disease-free, relapse-free, and second transplantation-free survival rate was 67.5%. The addition of low-dose TBI to fludarabine/busulfan was associated with favorable long-term donor chimerism, although this finding should be considered hypothesis-generating. RTC may represent a feasible conditioning strategy for XHIGM.