Ganglioside (GM1) distinguishes the effects of CD4 on signal transduction through the TCR/CD3 complex in human lymphocytes

Ganglioside (GM1) modulation of CD4 off the surface of T lymphocytes defined functions of the CD4 molecule during signal transduction through the T cell receptor (TCR)/CD3 complex. Antibody cross-linking of CD3 alone (3 × 3) stimulated phospholiphase C (PLC) activity, rapid Ca²⁺ flux, and protein phosphorylations in freshly isolated human T lymphocytes. Antibody cross-linking of CD4 and CD3 (3 × 4) stimulated greater signaling than that caused by 3 × 3. Cross-linking CD4 alone did not stimulate these signaling processes. GM1-modulation of CD4 from the cell surface blocked all aspects of the augmented signaling imparted by CD4 co-modulation with CD3. In comparison, pretreatment with the protein tyrosine kinase inhibitor genistein inhibited 3 × 4-stimulated PLC activity and protein phosphorylation but not Ca²⁺ flux. Antibody cross-linking of the tyrosine phosphatase CD45 with 3 × 4 (3 × 4 × 45) also inhibited CD4-augmented phosphorylations and like genistein did not reduce Ca²⁺ levels. In conclusion, these data demonstrate that CD4 can augment signal transduction through the TCR/CD3 complex by its physical proximity to CD3. TCR/CD3-signaling augmentation by CD4 stimulated protein tyrosine kinases and PLC activities but stimulated intracellular Ca²⁺ flux through an independent mechanism(s).