| Summary: | The commercial importance of Atlantic salmon (Salmo salar) in aquaculture has fuelled much research into fish health since outbreaks of infectious disease cause major financial losses. All aspects of immunology are therefore currently of great interest. The aim of this study was to extend the characterisation of T cell markers to enable antibody production and design of expression assays, to study the immune system and immune responses of salmon. Therefore, the genes and cDNAs for the T cell coreceptors CD3, CD8 and CD4 were cloned using a combination of synteny analysis and homology cloning. CD3 in Atlantic salmon consists of three different molecules; a γδ chain, the forerunner of separate γ and δ chains in mammals, an ε chain and a ζ chain (mammals also have an η chain, which is a splice variant of the ζ gene). The translated sequences have low identities to mammalian CD3 sequences (12-34%), but they exhibit similar characteristics with single immunoglobulin-like domains in the γδ and ε genes and immunoreceptor tyrosine-based activation motifs in all cytoplasmic domains. Two copies of the CD3ζ gene were cloned, but these are considered to be alleles. The CD3ε gene has a second copy, but it is a pseudogene containing frame-shifts and stop codons and was poorly expressed compared to the intact CD3ε gene. The CD3γδ gene also appeared to be duplicated and the variants are named CD3γδ-A and CD3γδ -B. Further evidence for the homology of CD3γδ and CD3ε genes was found in their genomic orientation where the pseudo-CD3ε gene is located tail to tail with a CD3γδ gene, similar to the gene organisation in higher vertebrates. There are two CD8 genes in Atlantic salmon, CD8α and CD8β. The synteny of these genes in humans is conserved with the publically available fugu genome sequence, allowing the subsequent identification of the CD8β gene in Atlantic salmon. There are two main transcripts and two alternative transcripts which could result in a putative truncated cytoplasmic domain in CD8α and a severely truncated version ...
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