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 d efined functions of the CD4 molecule during signal transduction throug h the T cell receptor (TCR)/CD3 complex. Antibody cross-linking of CD3 alone (3 x 3) stimulated phospholipase C (PLC) activity, rapid Ca2+ f lux, and protein phosphorylations in freshly isolated human T lymphocy tes. Antibody cross-linking of CD4 and CD3 (3 x 4) stimulated greater signaling than that caused by 3 x 3. Cross-linking CD4 alone did not s timulate these signaling processes. GM1-modulation of CD4 from the cel l surface blocked all aspects of the augmented signaling imparted by C D4 co-modulation with CD3. In comparison, pretreatment with the protei n tyrosine kinase inhibitor genistein inhibited 3 x 4-stimulated PLC a ctivity and protein phosphorylation but not Ca2+ flux. Antibody cross- linking of the tyrosine phosphatase CD45 with 3 x 4 (3 x 4 x 45) also inhibited CD4-augmented phosphorylations and like genistein did not re duce Ca2+ levels. In conclusion, these data demonstrate that CD4 can a ugment signal transduction through the TCR/CD3 complex by its physical proximity to CD3. TCR/CD3-signaling augmentation by CD4 stimulated pr otein tyrosine kinases and PLC activities but stimulated intracellular Ca2+ flux through an independent mechanism(s).