STRUCTURE AND DIVERSITY OF THE T-CELL ANTIGEN RECEPTOR BETA-CHAIN IN A TELEOST FISH

Cell-mediated immunity (e.g., allograft rejection) is found in all ver tebrates, and these reactions are known to depend on thymus-derived ce lls in amphibian, avian, and mammalian species. The participation of p eripheral T cell-like lymphocytes subpopulations to fish immunity is n ow well documented, but the developmental origin, migration, and perip heral tissue distribution of these cells remain practically unknown. T his is mainly due to the difficulty of efficiently thymectomizing fish at an early stage of development and to the lack of Ab strictly speci fic for thymocytes and T cell surface Ag. One strategy for analyzing T cell biology in fish would be to characterize the genes encoding poly peptides homologous to the TCR molecules. This report describes cDNA c lones from the rainbow trout (Oncorhynchus mykiss) that have sequences very similar to amphibian, avian, and mammalian TCR beta-chains. Thre e complete trout V beta segments belonging to different families were analyzed; one of them had limited amino acid sequence similarity to th e human V beta 20 family. The 10 trout beta-chain-joining segments all retain the invariant mammalian J beta residues, and comparison of 66 V beta-J beta junctions led to the identification of a D beta-like seq uence (GGACAGGG) that is shorter than but very similar to the chicken D beta and mammalian D beta 1 sequences. There is considerable diversi ty at the V beta-D beta and D beta-J beta junctions, suggesting the pr esence of N-nucleotides. The trout C beta extracellular domain is shor ter than mammalian C beta, and the hinge region has no cysteine residu e. The transmembrane C beta domain contains a lysine residue that in m ammals is thought to be involved in charged interactions with members of the CD3 complex.