CD45 cross-linking regulates phospholipase C activation and tyrosine phosphorylation of specific substrates in CD3/Ti-stimulated T cells

In lymphocytes, CD45 regulates the increase in cytoplasmic calcium concentration that occurs after receptor cross-linking. Here we show that T cell receptor complex (CD3/Ti)-mediated inositol phosphate production was inhibited by CD45 ligation in Jurkat cells. CD3/Ti signaling in normal T cells was also inhibited by CD45 ligation, but coupling of CD4 with CD3/Ti gave augmented calcium signals that were entirely resistant to the inhibitory effect of CD45. In contrast, CD3-induced T cell proliferation was suppressed by immobilized CD45 mAb even in the presence of CD4 mAb. The effect of CD45 and CD4 ligation on tyrosine phosphorylation during T cell activation was directly examined by immunoblotting with anti-phosphotyrosine. Using immobilized mAb, CD45 ligation suppressed the tyrosine phosphorylation of specific substrates induced by CD3/Ti stimulation, including almost complete suppression of 150-, 36-, and 35-kDa proteins and partial suppression of 76- and 80-kDa proteins. Other tyrosine-phosphorylated proteins induced by CD3/Ti stimulation, including 135- and 21-kDa proteins, were not suppressed by simultaneous ligation of CD3/Ti and CD45. Simultaneous ligation of CD3 and CD4 enhanced tyrosine phosphorylation of all substrates, but did not overcome the CD45-mediated suppression of tyrosine phosphorylation of the 35- and 36-kDa proteins. The CD45-mediated suppression of phospholipase C activation is therefore modulated by association with CD4 without altering the specific inhibition of tyrosine phosphorylation and T cell proliferation after co-ligation of CD45 and CD3/Ti.