Does co-aggregation of the CD45 and CD3 antigens inhibit T cell antigen receptor complex-mediated activation of phospholipase C and protein kinase C?

The binding of agonistic monoclonal antibodies (mAb) to the CD3 antigen in T cells induces a rapid increase in tyrosine phosphorylation, inositive phosphate (IP) production, a rise in intracellular calcium and protein kinase C (PKC) activation. These intracellular signals have been implicated in the control of interleukin-2 and interleukin-2R receptor gene expression, thereby regulating T cell proliferation. Previous studies have shown that co-ligation of the CD45 and CD3 antigens inhibits CD3-induced tyrosine phosphorylation, IP production, calcium signals and T cell proliferation. It has therefore been suggested that the CD45 antigen uncouples the T cell receptor (TcR) from mitogenic signal pathways. In this study co-ligation of the CD3 and CD45 antigens with precisely constructed bispecific mAb did not inhibit CD3-induced T cell proliferation, IP production, calcium signals, diacylglycerol production or PKC activation. Furthermore, co-ligation of CD3 and CD45 antigens already cross-linked with IgM mAb did not lead to inhibition of CD3-induced calcium signals. Inhibitions of CD3-induced intracellular signals were observed following co-ligation of IgG CD45 and CD3 mAb with anti-IgG (F(ab')2 fragments. However, comparable inhibitions were also noted following co-ligation of CD3 with other abundant cell-surface antigens such as CD5 and LFA-1, and inhibitions were only observed when the CD3 mAb used required cross-linking to induce signals. These results suggested that the inhibitory effects of CD45 IgG mAb were not specific and were caused by the prevention of CD3-CD3 cross-linking following CD3 antigen co-ligation with other cell surface molecules. These findings are inconsistent with a specific inhibitory role for the CD45 phosphotyrosine phosphatase in uncoupling the TcR from mitogenic signal pathways.