MHC class I antigens regulate CD3-induced tyrosine phosphorylation of proteins in T cells

The MHC class I antigens enhance the T cell response to various mitogenic stimuli. Class I antigens co-cap and associate with the CD3 structure on these cells. The present work shows that co-aggregation of these MHC antigens with CD3 induces a sustained elevated Ca2+ response in T cells. The duration required for a 50% decline in peak response is five to 10 times longer when class I antigens are cross-linked with CD3 as compared to that when cells are stimulated through CD3 alone. Further analysis reveals that class I antigens prolong the duration of CD3-induced tyrosine phosphorylation of several proteins. No protein tyrosine kinase activity is found associated with these MHC antigens which could explain their influence on CD3 activation. Similarly, class I antigens do not depend on the membrane-bound protein tyrosine phosphatase CD45, since they elevate the degree of CD3-induced Ca2+ response in a CD45-deficient Jurkat cell line. On the contrary, in a CD45- HPB cell line defective in CD3 signaling, co-aggregation of class I antigens with CD3 does not induce Ca2+ flux. Therefore, the effect of class I antigens on CD3 function depends on the ability of CD3 to transduce a signal. Furthermore, cytofluorometric studies show that cross-linking of class I antigens with CD3 inhibits the internalization of the latter. Thus, class I antigens seem to prolong the duration of signal transduction through CD3 by retarding its down-regulation.