On defining the rules for interactions between the T cell receptor and its ligand: a critical role for a specific amino acid residue of the T cell receptor beta chain.

The specificity of T cell-mediated immune responses is primarily determined by the interaction between the T cell receptor (TCR) and the antigenic peptide presented by the major histocompatibility complex (MHC) molecules. To refine our understanding of interactions between the TCR and the antigenic peptide of vesicular stomatitis virus (VSV) presented by the class I MHC molecule H-2Kb, we constructed a TCR alpha chain transgenic mouse in a TCR alpha-deficient background to define specific structural features in the TCR beta chain that are important for the recognition of the VSV/H-2Kb complex. We found that for a given peptide, a peptide-specific, highly conserved amino acid could always be identified at position 98 of the complementarity-determining region 3 (CDR3) loop of TCR beta chains. Further, we demonstrated that substitutions at position 6, but not position 1, of the VSV peptide induced compensatory changes in the TCR in both the amino acid residue at position 98 and the length of the CDR3beta loop. We conclude that the amino acid residue at position 98 of the CDR3beta loop is a key residue that plays a critical role in determining the specificity of TCR-VSV/H-2Kb interactions and that a specific length of the CDR3beta loop is required to facilitate such interactions. Further, these findings suggest that the alpha and beta chains of TCRs interact with amino acid residue(s) toward the N and C termini of the VSV peptide, respectively, providing functional evidence for the orientation of a TCR with its peptide/MHC ligand as observed in the crystal structures of TCR/peptide/MHC complexes.