The T4 surface antigen is involved in the induction of helper function.

The OKT4 monoclonal antibody reacts with a 62K m.w. glycoprotein present on a subset of human T cells with the capacity to help or induce B cell differentiation. The present studies were undertaken to determine whether the T4 molecule itself plays any role in the helper function mediated by T4+ cells. Using a series of monoclonal antibodies (OKT4, OKT4A-E), which react with distinct noncompeting epitopes of the T4 molecule, we found that OKT4A and OKT4E, but not OKT4, antibodies inhibited the induction of B cell differentiation by T4+ cells. This inhibition was apparent when using nonirradiated or irradiated T4+ cells, and was noted over a wide range of concentrations. Importantly, inhibition occurred only if the antibody was present during the first 24 hr of cell culture, and the presence of antibody did not alter the kinetics of induction of B cell differentiation. Thus, these data suggest that the T4 molecule plays an early, critical role in cellular interactions required for helper cell function.