Decreased CD4 expression by polarized T helper 2 cells contributes to suboptimal TCR-induced phosphorylation and reduced Ca2+ signaling.

Polarized Th1 and Th2 cells expressing the same TCR produce distinct biochemical responses to ligand engagement. Compared to Th1 cells, Th2 cells show altered substrate tyrosine phosphorylation and a diminished or transient Ca2+ response. Here we demonstrate that agonist stimulation of Th1 cells leads to the predominant appearance of fully phosphorylated (p23) TCR zeta, substantial phosphorylation of zeta-associated protein 70 (ZAP-70), and strong elevation of intracellular Ca2+, whereas agonist stimulation of Th2 cells expressing an identical TCR results in an elevated p21:p23 TCR zeta ratio, little or no detectable ZAP-70 phosphorylation, and a more limited elevation in intracellular Ca2+. Th2 cells consistently had twofold lower surface CD4 expression as compared to Th1 cells with the same TCR. When CD4 levels in Th2 cells were raised to Th1 levels using retroviral gene transfer, the transduced cells showed greater generation of p23 phospho-zeta, measurable phosphorylation of ZAP-70, and increased Ca2+ responses. These findings suggest that the apparent qualitative differences in TCR signaling characterizing Th1 versus Th2 cells are largely the result of modest quantitative variation in CD4 expression, with decreased CD4 expression playing a significant role in attenuating the proximal signaling responsiveness of Th2 cells to TCR ligands.