Evolutionary conservation of antigen recognition: the chicken T-cell receptor beta chain.

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Abstract
T cells play important regulatory roles in the immune responses of vertebrates. Antigen-specific T-cell activation involves T-cell receptor (TCR) recognition of a peptide antigen presented by a major histocompatibility complex molecule, and much has been learned about this antigen recognition process through structural and genetic studies of mammalian TCRs. Although previous studies have demonstrated that avian T cells express cell-surface molecules analogous to the mammalian TCR heterodimers, TCR genes have not been identified in nonmammalian species. We now report the cloning of a cDNA that encodes the .beta. chain of the chicken TCR. Southern blot analysis using this TCR.beta. cDNA probe demonstrated that the chicken TCR.beta. locus was clonally rearranged in chicken T-cell lines. TCR.beta. mRNA was expressed in cells isolated from the thymus but not in cells from the bursa of Fabricius where B cells are generated. Sequence analysis of six additional TCR.beta. cDNAs suggested the existence of at least two variable (V) region families, three joining (J) elements, and single diversity (D) and constant (C) elements. As in mammals, considerable nucleotide diversity was observed at the junctions of the variable, diversity, and joining elements in chicken TCR.beta. cDNAs. Genomic V.beta. and J.beta. elements were also cloned and sequenced. Both elements are flanked by classical hepatmer/nonamer recombination signal sequences. Although the chicken and mammalian TCR.beta. chains displayed only 31% overall amino acid sequence identity, a number of conserved structural features were observed. These data indicate that (i) the chicken TCR.beta. repertoire is generated by combinatorial and junctional diversity and (ii) despite divergent evolution at the level of nucleotide sequence, important structural features of the TCR.beta. polypeptide are conserved between avian and mammalian species.