Abstract
Recently, many immune-related genes have been extensively studied in ducks, but relatively little is known about their TCR genes. Here, we determined the germline and expressed repertoire of TCR genes in White Peking duck. The genomic organization of the duck TCRα/δ, TCRγ and unconventional TCRδ2 loci are highly conserved with their counterparts in mammals or chickens. By contrast, the duck TCRβ locus is organized in an unusual pattern, (Vβ)n-Dβ-(Jβ)2-Cβ1-(Jβ)4-Cβ2, which differs from the tandem-aligned clusters in mammals or the translocon organization in some teleosts. Excluding the first exon encoding the immunoglobulin domain, the subsequent exons of the two Cβ show significant diversity in nucleotide sequence and exon structure. Based on the nucleotide sequence identity, 49 Vα, 30 Vδ, 13 Vβ and 15 Vγ unique gene segments are classified into 3 Vα, 5 Vδ, 4 Vβ and 6 Vγ subgroups, respectively. Phylogenetic analyses revealed that most duck V subgroups, excluding Vβ1, Vγ5 and Vγ6, have closely related orthologues in chicken. The coding joints of all cDNA clones demonstrate conserved mechanisms that are used to increase junctional diversity. Collectively, these data provide insight into the evolution of TCRs in vertebrates and improve our understanding of the avian immune system.
Highlights
Conventional T cell receptors (TCRs) are disulfide-linked heterodimers comprising either αand βchains or γand δchains
An analysis of the two Bacterial artificial chromosome (BAC) sequences showed that the δlocus was located within the αlocus, resembling the genomic organization of the TCRα/δlocus in other tetrapods (Fig. 1a)
The duck Cαand Cδgenes were encoded by three exons that successively encoded the Ig domain, connecting peptide (Cp), and transmembrane-cytoplasmic (Tm-Ct) domain, all of which contained the three conserved cysteines required for intra- and inter-chain disulfide bond formation and the conserved lysine and arginine residues responsible for the interaction with other TCR dimers (Fig. 2a and b)
Summary
Conventional T cell receptors (TCRs) are disulfide-linked heterodimers comprising either αand βchains or γand δchains. The αβT cells mainly assist in immunoglobulin (Ig) production and cytolytic T cell responses Their αβTCR complexes bind to the peptide antigens presented by major histocompatibility complex (MHC) or MHC-like molecules. Unconventional TCR chains that use Ig-like V domains have been discovered in a few distantly related vertebrate species. These unconventional TCR chains include TCRδthat uses VHδand is found in amphibians, birds, and duckbill platypus, the NAR-TCR found in cartilaginous fish, and the TCRμ,which is only found in nonplacental mammals. We report the detailed genomic organization and repertoire diversity of all TCR loci in White Peking duck, including three conventional TCR loci (TCRα/δ, TCRβand TCRγ)and the recently discovered TCRδ2 locus, providing a theoretical basis for further understanding of the avian adaptive immune system as well as the evolutionary relationships of TCRs in vertebrates
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