Phosphorylation of murine CD8 alpha is not essential for responses of T cell hybridomas to antigen

CD8 T cell differentiation antigens, expressed on class I-restricted T cells, have a key role in the control of recognition and response of these cells to antigen. It has been suggested that these molecules function as co-receptors together with antigen-specific T cell receptors to regulate T cell responses. We have addressed the question of whether cytoplasmic serine phosphorylation, which occurs on CD8 molecules after activation by antigen or phorbol esters, is relevant to its co-receptor function. By mutagenesis, we show that phorbol ester-induced phosphorylation occurs exclusively on CD8 alpha serine residue 216. However, inhibition of CD8 polypeptide phosphorylation does not appear to have a detrimental effect on several responses of CD8-dependent transfectants to antigen. This is in contrast to results reported with CD4 (N.Glaichenhaus, N.Shastri, D.R.Littmann and J.M.Turner. 1991. Cell, 64:511), suggesting that CD4 and CD8 molecules may play somewhat different roles in the control of T cell activation.