BACKGROUND: The introduction of posttransplant cyclophosphamide (PTCy) for prophylaxis against graft-versus-host disease (GVHD) has led to an increase in the number of transplants from haploidentical donors. Accordingly, we aimed to understand the impact of HLA epitope mismatch on the outcomes of haploidentical hematopoietic stem cell transplantation (HSCT) with prophylaxis against GVHD. METHODS: This retrospective study included 1037 patients who underwent their first HSCT for hematologic malignancies from haploidentical peripheral blood donors in a Japanese registry between 2011 and 2019. In total, 542 patients received PTCy and 495 received antithymocyte globulin-based GVHD prophylaxis. RESULTS: In patients with high-risk disease who received PTCy, higher class I Predicted Indirectly ReCognizable HLA Epitopes (PIRCHE-I) scores were associated with a significantly lower risk of relapse, leading to a higher overall survival (OS: high PIRCHE-I patients compared with low PIRCHE-I patients: relapse: hazard ratio [HR], 0.67
95% confidence interval [CI], 0.46-0.98
P = 0.040
mortality: HR, 0.69
95% CI, 0.46-0.99
P = 0.042). In patients with standard-risk disease who received antithymocyte globulin, a significant association between class II PIRCHE (PIRCHE-II) and a lower incidence of nonrelapse mortality (NRM) leading to higher OS was observed (high PIRCHE-II patients compared with low PIRCHE-II patients, NRM: HR, 0.41
95% CI, 0.19-0.86
P = 0.019
OS: HR, 0.55
95% CI, 0.32-0.94
P = 0.030). CONCLUSIONS: These findings suggest the differential effects of T-cell epitope matching based on GVHD prophylaxis after haploidentical HSCT. Pretransplant disease status may also be important for understanding the graft-versus-leukemia effect of mismatched HLA in haploidentical HSCT using PTCy.