Antigenic basis of diversity in the gammadelta T cell co-receptor WC1 family

WC1 co-receptors are transmembrane glycoproteins with 11 extracellular scavenger receptor cysteine rich (SRCR) domains. They are related to the CD163 family but are uniquely expressed by gammadelta T cells. We recently showed that at least 13 members comprise the WC1 gene family in cattle, a model animal species for studies of gammadelta T cell biology. Since WC1 co-receptors participate in directing functional responses by gammadelta T cells either through the ligands they bind or the signals they transduce, availability of reagents to identify the expression of individual WC1 molecules of this diverse family would be valuable in further elucidating mechanisms of gammadelta T cell responsiveness. Although monoclonal antibodies (mAbs) have been widely used to identify WC1 co-receptors on gammadelta T cells, the locations of the antigenic epitopes recognized are unknown. Here, we mapped the epitopes to particular SRCR domains and evaluated their distribution among WC1 molecules. To do this, cDNA representing the extracellular domains of seven different WC1 genes was expressed in mammalian cells and analyzed for reactivity with anti-WC1 mAbs using ELISA and Western blotting. The study included mAbs that are broadly reactive with WC1(+) gammadelta T cells and those that divide WC1(+) gammadelta T cells into functionally distinct subpopulations. We found that mAb CC15 is a pan-reactive anti-WC1 mAb recognizing an epitope in the closely related SRCR domains 2 and 7 and that this epitope is present in at least domain 2 or 7 of all seven WC1 molecules evaluated here. Five other anti-WC1 mAbs, typified by mAb IL-A29, were found to be broadly reactive, recognizing epitopes in the related SRCR domains 4 and 9 but each having a unique pattern of reactivity with the seven WC1 molecules. Finally, the subpopulation-specific anti-WC1 mAbs, including those that recognize either the archetypal WC1.1 or WC1.2 molecule, were found to react with epitopes in the most variable WC1 domain, i.e. domain 1, of a restricted number of WC1 co-receptors.