Cloning and expression analysis of Atlantic halibut (Hippoglossus hippoglossus) CD3 genes

The CD3 complex is in higher vertebrates shown to be important for the activation of T-cells. The T-cell system in fish is believed to be similar to that in higher vertebrates, and the CD3 chains could therefore be an important marker for identification of T-cells in fish. Here, we report the cDNA and corresponding gene sequence of Atlantic halibut (Hippoglossus hippoglossus) CD3γδ, CD3ɛ, and CD3ζ chains, and the tissue-specific expression pattern of CD3 and T- cell receptor (TCR) genes. Important structural characteristics defining the CD3 genes seemed to be conserved in the halibut CD3 chains, such as a signal peptide, an extracellular region, a transmembrane helix having a negatively charged residue, and an ITAM bearing cytoplasmic tail. The extracellular domain of halibut CD3γδ and CD3ɛ included two cysteines presumably involved in Ig-fold stabilisation and the CxxCxE motif important for dimerization. A spliced variant of CD3ɛ was identified, lacking the Ig-fold, but with the CxxCxE motif intact. The real time RT-PCR analysis revealed a highly similar expression pattern of the CD3 genes and the TCRα and TCRβ genes, indicating that the functional relationship between the TCR and the CD3 genes are preserved in teleosts.