Rapid phosphorylation and modulation of the T4 antigen on cloned helper T cells induced by phorbol myristate acetate or antigen.

A cell surface protein known as T4 (CD4, Leu3), Mr = 55,000, is expressed on the subset of human T lymphocytes which provides helper function for B cell and cytotoxic T cell activities. We show that T4 is constitutively phosphorylated and that phorbol myristate acetate (PMA) induces a rapid serine phosphorylation which is followed by a fast dephosphorylation. Within 5 min after PMA treatment, there is a 24% reduction of T4 on the cell surface, by 4 h the loss is nearly complete, and by 20 h T4 is re-expressed. Addition of antigen to a T4+ antigen-reactive T cell clone induces both phosphorylation and dephosphorylation with kinetics similar to that described for PMA. Antigen also causes reduction of cell surface T4, although to a lesser degree than stimulation with PMA. The rapid phosphorylation/dephosphorylation of T4 as well as its movement from the cell surface suggest that T4 functions as a receptor for an unknown ligand.