Escape of leukemia blasts from HLA-specific CTL pressure in a recipient of HLA one locus-mismatched bone marrow transplantation

► HLA-B locus was selectively down-regulated in post-transplant leukemia blasts. ► All isolated cytotoxic T lymphocyte (CTL) clones recognized the HLA-B∗51:01 molecule. ► Pretransplant leukemia blasts were lysed by CTLs, but post-transplant blasts were not. ► B∗51:01-reactive CTLs were detected in patient blood just before relapse. ► Our data indicate leukemia escape from immunological pressure by HLA-specific CTLs.