The role of the putative CDR1-beta segment in the selection of the peripheral and germline alpha-beta T cell repertoires

Abstract

The T cell receptor (TCR) recognizes a composite ligand that consists of the "presenting" MHC molecule and an antigenic peptide bound to it. Experimental work in several antigen response systems and computer modeling have suggested a Fab-like conformation for the antigen-binding portion of the TCR. In this structure, the three putative complementarity determining regions (CDR) of each TCR chain are the contact points for ligand interaction. An examination of the function of the {dollar}\beta{dollar} chain putative CDR1 (CDR1{dollar}\beta{dollar}) in the IE{dollar}\sp{lcub}\rm k{rcub}{dollar}-restricted response to pigeon cytochrome c (pcc) has shown that the single Val {dollar}\to{dollar} Phe allelic variation at position 31 corresponds to a profound alteration in the general response repertoire. The current work was undertaken to determine the role played by the putative CDR1{dollar}\beta{dollar} in the TCR:ligand recognition events that shape the antigen-specific and germline T cell repertoires. The receptor chosen for these studies was the V{dollar}\sb\alpha{dollar}11/V{dollar}\sb\beta{dollar}3 5C.C7 TCR a because it responds to the pcc antigen (Ag) in the context of IE{dollar}\sp{lcub}\rm k{rcub}{dollar} and to the allo-antigen (allo-Ag) IA{dollar}\sp{lcub}\rm s{rcub}{dollar} while also reacting to the superantigen (sAg) staphylococcal enterotoxin A (SEA). After changing the existing {dollar}\sp{lcub}31{rcub}{dollar}Val of the {dollar}\beta{dollar} chain into a Phe by site-directed mutagenesis, the original and mutated TCRs were expressed in stably transfected cell lines and in transgenic mice. The in vitro experiments demonstrated that the CDR1{dollar}\beta{dollar} mutation alters the TCR:ligand interaction to the extent that Ag- and allo-Ag recognitions are abrogated and sAg reactivity is mitigated. The in vivo experiments showed that the same polymorphism also interferes with the recognition of the self-MHC:self-peptide selecting complex and therefore prevents the positive selection of the mutated receptors. Together, these data verify the Fab-like model for the TCR, and they are consistent with the proposition that the putative CDR1{dollar}\beta{dollar} loop of the TCR is involved in direct MHC contact. These results also further advance the idea that allelic polymorphisms in the TCR CDR segments may account for some of the diversity of the T cell repertoire.