The Endocytosis and Signaling of the γδ T Cell Coreceptor WC1 Are Regulated by a Dileucine Motif

WC1 proteins, which are specifically expressed by bovine γδ T cells from a gene array containing 13 members, are part of the scavenger receptor cysteine-rich family. WC1 cytoplasmic domains contains multiple tyrosines, one of which is required to be phosphorylated for TCR coreceptor activity, and a dileucine endocytosis motif. Like the TCR coreceptor CD4, WC1 is endocytosed in response to PMA. Because WC1 endocytosis may play a role in the activation of γδ T cells, we examined WC1 endocytosis in the adherent cell 293T and Jurkat T cell lines using a fusion protein of extracellular CD4 and the transmembrane and cytoplasmic domain of WC1. Individual mutation of the two leucine residues of the endocytic dileucine motif in the WC1 cytoplasmic domain significantly reduced PMA-induced endocytosis in both cell types and enhanced IL-2 production stimulated by cocross-linking of CD3/TCR and CD4/WC1 in Jurkat cells, suggesting that the sustained membrane coligation of CD3/TCR with WC1 caused by a decrease in endocytosis increases T cell activation. Mutation of two serines upstream of the endocytic dileucine motif affected endocytosis only in adherent 293T cells. Although the two upstream serines were not required for WC1 endocytosis in Jurkat cells, the pan-protein kinase C inhibitor Gö6983 blocked endocytosis of CD4/WC1, and mutation of the upstream serines in WC1 inhibited IL-2 production stimulated by cocross-linking of CD3/TCR and CD4/WC1. These studies provide insights into the signaling of WC1 gene arrays that are present in most mammals and play critical roles in γδ T cell responses to bacterial pathogens.