Structure and diversity of the T cell antigen receptor β-chain in a teleost fish

Abstract

Cell-mediated immunity (e.g., allograft rejection) is found in all vertebrates, and these reactions are known to depend on thymus-derived cells in amphibian, avian, and mammalian species. The participation of peripheral T cell-like lymphocytes subpopulations to fish immunity is now well documented, but the developmental origin, migration, and peripheral tissue distribution of these cells remain practically unknown. This is mainly due to the difficulty of efficiently thymectomizing fish at an early stage of development and to the lack of Ab strictly specific for thymocytes and T cell surface Ag. One strategy for analyzing T cell biology in fish would be to characterize the genes encoding polypeptides homologous to the TCR molecules. This report describes cDNA clones from the rainbow trout (Oncorhynchus mykiss) that have sequences very similar to amphibian, avian, and mammalian TCR β-chains. Three complete trout V β segments belonging to different families were analyzed; one of them had limited amino acid sequence similarity to the human V β 20 family. The 10 trout β-chain-joining segments all retain the invariant mammalian J β residues, and comparison of 66 V β-J β junctions led to the identification of a D β-like sequence (GGACAGGG) that is shorter than but very similar to the chicken D β and mammalian D β 1 sequences. There is considerable diversity at the V β-D β and D β-J β junctions, suggesting the presence of N-nucleotides. The trout C β extracellular domain is shorter than mammalian C β, and the hinge region has no cysteine residue. The transmembrane C β domain contains a lysine residue that in mammals is thought to be involved in charged interactions with members of the CD3 complex.