Differential sensitivity to mutations in a single peptide by two TCRs having identical beta-chains and closely related alpha-chains

The TCR on CD4 T cells binds to and recognizes MHC class II:antigenic pepti de complexes through molecular contacts with the peptide amino acid residue s that face up and out of the peptide-binding groove. This interaction prim arily involves the complementarity-determining regions (CDR) of the TCR alp ha- and beta -chains contacting up to five residues of the peptide. We have used two TCRs that recognize the same antigenic peptide and have identical V beta8.2 chains, but differ in all three CDR of their related V alpha2 ch ains, to examine the fine specificity of the TCR:peptide contacts that lead to activation. By generating a peptide library containing all 20 aa residu es in the five potential TCR contact sites, we were able to demonstrate tha t the two similar TCRs responded differentially when agonist, nonagonist, a nd antagonist peptide functions were examined. Dual substituted peptides co ntaining an agonist residue at the N terminus, which interacts with CDR2 al pha, and an antagonist residue at the C terminus, which interacts with the CDR3 beta, were used to show that the nature of the overall signal through the TCR is determined by a combination of the type of signal received throu gh both the TCR alpha- and beta -chains.