Structure, diversity, and evolution of the T-cell receptor VB gene repertoire in primates

AB T-cell receptors (TCR) that recognize major histocompatibility complex (MHC)/peptide antigen complexes regulate humoral and cellular arms of the adaptive immune response. Antigen binding sites of MHC and immunoglobulin heavy chain variable regions (Igh-V) are subject to diversity enhancing selection. We sought to establish whether positive Darwinian selection has driven diversity of TCRBV chains in the primate lineage by sequencing rearranged TCR from rhesus monkeys and chimpanzees and comparing them with those of humans. Rates of synonymous (silent) and nonsynonymous (replacement) substitutions indicate selection against amino acid replacements in TCRBV frameworks, and relaxation of these constraints in putative MHC/peptide contact sites. The lack of positive selection for variability in likely ligand contact sites suggests that mechanisms generating somatic diversity in TCR junctional regions have relaxed the pressure for selection of variability in the TCR V region encoded in the germline.