Unrelated donor (URD) hematopoietic cell transplantation (HCT) can eradicate chronic myelogenous leukemia (CML). It has been postulated that greater donor:recipient-histoincompatibility can augment the graft vs. leukemia (GvL) effect. We previously reported similar, but not equivalent outcomes of URD vs. sibling donor HCT for CML using an older, less precise classification of HLA-matching.

Our recently refined HLA-matching classification, suitable for interpretation when complete allele-level typing is unavailable was used to reanalyze outcomes of previous HCT for CML.

We observed that new matching criteria identifies substantially more frequent mismatch than older, less precise “6 of 6 antigen” matched URD-HCT. Only 37% of those previously called “HLA-matched” were HLA-well-matched in new classification and 44% were partially-matched. The refined matching classification confirmed that partially matched or mismatched URD:recipient pairs have significantly greater risks of graft failure compared to either sibling or well-matched URD-HCT. Acute and chronic GVHD are significantly more frequent with all levels of recategorized URD HLA-matching. Importantly, survival and leukemia-free survival remain significantly worse following URD-HCT at any matching level. No augmented GvL effect accompanied URD HLA-mismatch. Compared to sibling donor transplants, we observed marginally increased, but not significantly different risks of relapse in either well-matched, partially-matched or mismatched URD-HCT.

These data confirm the utility of our revised HLA-matching schema as applicable for analysis of retrospective data sets when fully informative, allele level-typing is unavailable. In this analysis, greater histoincompatibility can augment GVHD but does not improve protection against relapse thus the best donor is still the most closely matched.