Conserved allomorphs of MR1 drive specificity of MR1-restricted TCRs
Major histocompatibility complex class-1-related protein (MR1), unlike human leukocyte antigen (HLA) class-1, has until recently been reported to be monomorphic. Tumor cell-specific MR1 restricted T cell receptors (TCRs) have been described, offering potential therapeutic application for cancer treatment. We show that human T cells expressing a TCR derived from an MR1-restricted T cell clone, termed MC.7.G5 (7G5.TCRT), retain MR1-directed cytotoxicity. However, activity is not pan-cancer, as initially reported with the clone MC.7.G5. Recognition is restricted by an allelic variant of MR1 (MR1*04) which is present at approximately 1% of the population at the heterozygote level. The 7G5 TCR is not cancer specific, as 7G5.TCRT and 7G5.TCRT-like TCRs react to both cancer and healthy cells expressing MR1*04 alleles. These data demonstrate that healthy individuals can harbor T cells reactive to an MR1 variant displaying self-ligands expressed in cancer and benign tissues. Targeting MR1 in cancer will require identification of cancer-specific presented ligands, and careful confirmation of cancer specificity of TCRs. MR1*04 may behave as an alloantigen warranting further study.