The Src tyrosine kinase Lck binds to CD2, CD4-1, and CD4-2 T cell co-receptors in channel catfish, Ictalurus punctatus

The binding of the lymphocyte specific protein tyrosine kinase (Lck) to T cell co-receptors is required for T cell development and activation. In mammals, Lck initiates signal transduction by binding to CD4 and CD8 co-receptors and phosphorylating ITAMs in the cytoplasmic tail of the CD3 molecules and the ζ chains. In addition, Lck can also bind to the adhesion molecule CD2 and trigger T cell activation. In this study, Lck and CD2 homologs were identified and characterized in channel catfish, Ictalurus punctatus. Lck and CD2 mRNAs were specifically expressed by clonal T cell lines, including both CD4(+) and CD4(-)CD8(-) CTL lines, and in mixed lymphocyte cultures (MLC). Western blot analyses using anti-trout Lck and anti-human p-Lck antibodies demonstrated that Lck protein is expressed in catfish clonal CTL and is phosphorylated at a conserved tyrosine residue. Because of the lack of CD8(+) CTL lines as well as the absence of CD8 message in MLC, we performed magnetic bead binding assays to correlate CD2, CD4, and CD8 co-receptor expression with Lck binding ability. Recombinant Lck reproducibly bound to CD2, CD4-1, and CD4-2, but not to CD8α or CD8β. These data provide one possible explanation for the apparent low numbers of CD8(+) CTL and the presence of CD4(+) and CD4(-)CD8(-)CD2(+) CTL in catfish.