The relative desirability of an unrelated donor with a bidirectional 1-locus mismatch (1MM-Bi), a 1-locus mismatch only in the graft-versus-host direction (1MM-GVH), or a 1-locus mismatch only in the host-versus-graft direction (1MM-HVG) is not yet clear. We analyzed adult patients with leukemia or myelodysplastic syndrome who received a first allogeneic stem cell transplant from an HLA-A, -B, -C, and -DRB1 matched or 1-allele mismatched unrelated donor in Japan. The effects of 1MM-Bi (n= 1020), 1MM-GVH (n= 83), and 1MM-HVG (n= 83) compared with a zero mismatch (0MM) (n= 2570) were analyzed after adjusting for other significant variables. The risk of grades III to IV acute graft-versus-host disease (GVHD) was higher with marginal significance in the 1MM-GVH group than in the 0MM group (hazard ratio, 1.85; P=.014). However, there was no significant difference in overall or nonrelapse mortality between the 1MM-GVH and 0MM groups. There was no significant difference in acute GVHD or overall or nonrelapse mortality between the 1MM-HVG and 0MM groups. The risks of acute GVHD and overall mortality were significantly higher in the 1MM-Bi group than in the 0MM group. These findings indicate that unrelated donors with 1MM-GVH and 1MM-HVG are both good candidates for patients without an HLA-matched unrelated donor in a Japanese cohort.
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