T-cell antigen receptors in Atlantic cod (Gadus morhua l.): structure, organisation and expression of TCR alpha and beta genes.

By using short degenerate primers complementing conserved T-cell antigen receptor (TCR) variable and constant region segments for PCR, we were able to isolate putative TCRalpha and beta chain full length cDNAs in Atlantic cod. The Valpha and Vbeta domains have the canonical features of known teleost and mammalian TCR V domains, including conserved residues in the beginning of FR2 and at the end of FR3. The Jalpha and Jbeta region possess the conserved Phe-Gly-X-Gly motif found in nearly all TCR and immunoglobulin light chain J regions. Similar to other vertebrates, the Atlantic cod Calpha and Cbeta sequences exhibit distinct immunoglobulin, connecting peptide, transmembrane and cytoplasmic regions. The Atlantic cod Cbeta sequence lacks a cysteine in its connecting peptide region, but other motifs proposed to be important for dimerisation and cell surface expression are observed. Four different cod Cbeta sequences were identified, two of which share 3' untranslated regions different from one of the other two sequences, suggesting the existence of isotypic gene variants of Cbeta. Based on Southern blot analyses, the TCRalpha and beta gene loci appear to be arranged in translocon organisation (as opposed to multicluster) with multiple V gene segments, some (D) and J gene segments and a single or few C gene segments. Northern blot analyses show expression of the TCRalpha and beta chains in thymus, spleen and head kidney, expression of the TCRbeta chain was also detected in the ovary. Interestingly, no expression was detected in intestine even though the existence of T-cells in intestine has been proposed in other teleost species.