Genetic variation in MHC proteins is associated with T cell receptor expression biases

Within each individual, a highly diverse T cell receptor (TCR) repertoire interacts with peptides presented by major histocompatibility complex (MHC) molecules. Despite extensive research, it remains controversial whether germline-encoded TCR-MHC contacts promote TCR-MHC specificity and if so, whether there exist differences in TCR V-gene compatibilities with different MHC alleles. We applied eQTL mapping to test for associations between genetic variation and TCR V-gene usage in a large human cohort. We report strong trans associations between variation in the MHC locus and TCR V-gene usage. Fine mapping of the association signals reveals specific amino acids in MHC genes that bias V-gene usage, many of which contact or are spatially proximal to the TCR or peptide. Hence, these MHC variants, several of which are linked to autoimmune diseases, can directly affect TCR-MHC interaction. These results provide the first examples of trans-QTLs mediated by protein-protein interactions, and are consistent with intrinsic TCR-MHC specificity.