Optimal donor selection is one of the key factors to enhance the success rate of allogeneic hematopoietic stem cell transplantation (HSCT). The effect of sex mismatch, especially the effect of Y chromosome mismatch in graft-versus-host disease (GVHD) direction (female donors to male recipients: denoted as FtoM mismatch) on overall survival (OS) has been controversial and not examined out of the patient population in Western countries. We retrospectively analyzed 225 cases of allogeneic HSCT and showed that FtoM mismatch confers a highly significant impact on OS (P < 0.005) in Japanese population. We demonstrated that this effect depends on the disease risk; for standard risk cases, this effect was significantly associated with poor outcome (for OS, P = 0.021), while for high risk cases, it had no effect on the results (for OS, P = 0.26). We further showed that FtoM mismatch was associated with nonrelapse mortality (P = 0.019) and most of them were GVHD-related in standard risk cases. In conclusion, FtoM mismatch has a significant impact on transplant outcome, especially in standard risk cases.
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