CD2/LFA-3 LIGATION INDUCES PHOSPHOLIPASE-C-GAMMA-1 TYROSINE PHOSPHORYLATION AND REGULATES CD3 SIGNALING

Activation of T cells through the TCR/CD3 receptor complex with either specific Ag or antibody results in tyrosine phosphorylation of intracellular protein substrates and phosphatidylinositol-phospholipase C (PLC) signaling, leading to the generation of PI breakdown products and the mobilization of intracellular calcium. Stimulation of the T cell surface receptor CD2 similarly propagates early signals through phosphatidylinositol-PLC activation. Previous reports have shown that CD3 activation leads to tyrosine phosphorylation of the PLC isozyme PLC-gamma-1. In this report, we investigated the potential similarity between CD3-induced signaling through PLC-gamma-1 and that induced by CD2. We show that stimulation of CD2 receptors on T cells caused tyrosine phosphorylation of PLC-gamma-1. Cross-linking of CD2 with CD3 receptors augmented the phosphorylation of PLC-gamma-1 on tyrosine, whereas ligation of the CD45 tyrosine phosphatase with CD2 receptors prevented PLC-gamma-1 tyrosine phosphorylation. T cells stimulated by ligation of CD2 with its counter-receptor in the form of a soluble LFA-3/Ig fusion protein cross-linked on the cell surface, resulted in a low, but detectable level of PLC-gamma-1 phosphorylation with prolonged kinetics, whereas that induced by cross-linking with anti-CD2 was stronger but transient. Co-ligation of LFA-3/Ig with suboptimal concentrations of anti-CD3 resulted in profound augmentation of PLC-gamma-1 tyrosine phosphorylation, mobilization of intracellular calcium and T cell proliferation. To explore the relationship between CD3- and CD2-stimulated signaling, T cells were desensitized through 1 h incubation with anti-CD3. CD3 receptor modulation potently down-regulated CD2-induced PLC-gamma-1 tyrosine phosphorylation and calcium mobilization. In contrast, PMA or ionomycin treatment did not alter CD2-stimulated tyrosine phosphorylation of PLC-gamma-1, suggesting that tyrosine kinase inhibition by CD3 receptor modulation was not caused by signaling events downstream of PLC-gamma-1. Taken together, these results support the hypothesis that CD2 provides a potent co-stimulatory signal for CD3-induced T cell activation that is associated with tyrosine kinase(s) and PLC-gamma-1.