Donor selection for patients with HLA-homozygous haplotypes in allogeneic hematopoietic stem cell transplantation

HLA homozygous haplotypes occur worldwide, but outcomes after allogeneic hematopoietic stem cell transplantation using alternative donor sources remain uncertain. We retrospectively analyzed the Japanese national transplantation registry to compare outcomes after first allogeneic hematopoietic stem cell transplantation in patients with HLA homozygous haplotypes. Donors were classified as homo-to-homo, defined as HLA-matched, or hetero-to-homo, defined as allele-level mismatches at HLA-A, -B, -C, and/or -DRB1 restricted to the host-versus-graft direction. The unrelated donor homo-to-homo group served as the reference. We included 691 patients: related donor homo-to-homo (n = 121), related donor hetero-to-homo (n = 76), unrelated donor homo-to-homo (n = 374), unrelated donor hetero-to-homo (n = 22), cord blood homo-to-homo (n = 40), and cord blood hetero-to-homo (n = 58). Compared with the unrelated donor homo-to-homo group, overall survival was inferior in the cord blood homo-to-homo group (adjusted hazard ratio [HR], 1.71; 95% confidence interval [CI], 1.11–2.64; P = 0.015), whereas the unrelated donor hetero-to-homo group showed a nonsignificant trend toward inferior overall survival (adjusted HR, 1.77; 95% CI, 0.97–3.22; P = 0.061). In this Japanese cohort, cord blood homo-to-homo transplantation was associated with inferior overall survival, whereas related donor hetero-to-homo and cord blood hetero-to-homo transplantation were not. These findings should be interpreted cautiously given the retrospective design and long study period, and require validation in contemporary, ethnically diverse cohorts.