A previous Japanese study revealed that a human leucocyte antigen (HLA)-A or -B allele mismatch was associated with higher overall mortality, whereas an HLA-C or -DRB1 allele mismatch did not affect mortality after serologically matched unrelated bone marrow transplantation (BMT). This study reanalysed 3003 adult patients who underwent unrelated BMT from a serologically HLA-A, -B, or -DR matched unrelated donor between 1993 and 2009 using the latest database, that included 1966 HLA-matched unrelated BMT and 187, 31, 524, and 295 unrelated BMT with a single HLA-A, -B, -C, or -DRB1 allele mismatch, respectively. As opposed to our previous findings, HLA-C and -DRB1 mismatches had a significant negative impact [hazard ratio (HR) 1·35, P<0·001, and HR 1·45, P<0·001] on survival in the period 2000-2009. The negative impact of each single HLA allele mismatch was not significantly different among the HLA-A, -B, -C, and -DRB1 mismatches (P=0·79). An interaction test revealed that the effects of single HLA-C and -DRB1 allele mismatches significantly differed over the two time periods (P=0·032 and P=0·0072, respectively). In conclusion, the impact of a single HLA allele mismatch changed over time. In the recent cohort, the negative impact of HLA-DRB1 and -C mismatches became apparent.
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