Diversity, Molecular Characterization and Expression of T Cell Receptor γ in a Teleost Fish, the Sea Bass (<em>Dicentrarchus labrax,</em> L)

<div><p>Two lineages of T cells, expressing either the αβ T cell receptor (TR) or the γδ TR, exist in Gnathostomes. The latter type of T cells account for 1–10 % of T cells in blood and up to 30 % in the small intestine. They may recognize unconventional antigens (phosphorylated microbial metabolites, lipid antigens) without the need of major histocompatibility class I (MH1) or class II (MH2) presentation. In this work we have described cloning and structural characterization of TR -chain (TRG) from the teleost <em>Dicentrarchus labrax</em>. Further, by means of quantitative PCR analysis, we analyzed TRG expression levels both in poly I:C stimulated leukocytes <em>in vitro</em>, and following infection with betanodavirus <em>in vivo</em>. Two full length cDNAs relative to TRG, with the highest peptide and nucleotide identity with Japanese flounder, were identified. A multiple alignment analysis showed the conservation of peptides fundamental for TRG biological functions, and of the FGXG motif in the FR4 region, typical of most TR and immunoglobulin light chains. A 3D structure consisting of two domains mainly folded as beta strands with a sandwich architecture for each domain was also reported. TRG CDR3 of 8–18 AA in length and diversity in the TRG rearrangements expressed in thymus and intestine for a given V/C combination were evidenced by junction length spectratyping. TRG mRNA expression levels were high in basal conditions both in thymus and intestine, while in kidney and gut leukocytes they were up-regulated after <em>in vitro</em> stimulation by poly I:C. Finally, in juveniles the TRG expression levels were up-regulated in the head kidney and down-regulated in intestine after <em>in vivo</em> infection with betanodavirus. Overall, in this study the involvement of TRG-bearing T cells during viral stimulation was described for the first time, leading to new insights for the identification of T cell subsets in fish.</p>