Impacts of Posttransplant Cyclophosphamide Dose on Graft-versus-Leukemia Effects via HLA-B Leader in HLA-Haploidentical Peripheral Blood Stem Cell Transplantation

Introduction: The graft-versus-leukemia effect of HLA-B leader dimorphism, i.e., methionine (M) or threonine (T) at position −21 of the leader sequence, has been observed in HLA-haploidentical hematopoietic cell transplantation with posttransplant cyclophosphamide (PTCy-haplo). However, the biological mechanism has been unclear, and the contributions of HLA-B leader genotype to risk reduction of relapse might be dependent on posttransplant cyclophosphamide (PTCy) doses. Methods: To investigate whether the effect of HLA-B leader dimorphism was modified by the PTCy dose, we retrospectively analyzed 99 patients who received PTCy-haplo. Results: In the low-dose PTCy group, the patient M+ HLA-B leader genotype did not significantly affect the cumulative incidence of relapse (CIR) but negatively impacted the overall survival (OS) compared to the M− genotype. In contrast, in the high-dose PTCy group, patients with the M+ genotype had a decreased CIR, but no significant difference in the OS was observed between patients with the M+ and M− genotypes. Regardless of PTCy doses, the patient M+ genotype had detrimental effects on nonrelapse mortality. Conclusion: Our findings suggest that the effect of the patient HLA-B leader genotype is modified by the PTCy dose, providing immunological insight into the PTCy dosage and supporting further studies to investigate the underlying mechanisms.