A role for T cell CD4 in contact mediated T dependent B cell activation

Using a monoclonal antibody to CD4 we have shown that occupation of CD4 on T cells induces a strong dose dependent inhibition of in vitro IgM plaque forming cell (PFC) response of spleen cells to the T dependent antigen (Ag), sheep red blood cells (SRBC), in Mishell-Dutton cultures. This inhibitory effect is not due simply to nonspecific perturbation or Fc binding, since F(ab) fragments of anti-CD4 are as potent as the intact antibodies, whereas antibodies to class I molecules or T cell CD5 have no effect. The anti-CD4 antibody appears to block contact dependent interaction between T and B cells and this inhibitory effect cannot be overcome by cytokines. Anti-CD4 did not inhibit the PFC response to the T independent antigen, trinitrophenylated lipopolysaccharide. The anti-CD4 antibody prevented the interaction of preactivated fixed SRBC specific T helper cells with B cells, suggesting that CD4 had a role in contact mediated interactions between T cells and B cells. Surprisingly, antibodies to CD40L failed to inhibit the SRBC specific PFC response. Thus CD4 appears to be an important molecule required for cognate interactions between T and B cells that are needed to generate an Ag specific PFC response.