Disparate functions of I-A and I-E molecules on B cells as evidenced by the inhibition with anti-I-A and anti-I-E antibodies of polyclonal B cell activation.

Polyclonal differentiation of unprimed B cells into IgM-producing cells induced by lipopolysaccharide (LPS) or T cell-derived lymphokine B151-TRF2 has been shown to contain a process of I-A-restricted B-B cell interaction, so that the B cell responses are inhibited by monoclonal antibodies (mAb) specific for I-A molecules. On the other hand, the B cell responses are also inhibited by anti-I-E mAb, although I-E molecules are not involved in such B-B cell interaction. In this study, we examined the mechanism underlying the anti-I-E-mediated inhibition of the B cell responses. The B cell responses induced by LPS or B151-TRF2 were inhibited by either anti-I-A or anti-I-E mAb added on day 0 over a 5-day culture period, whereas when added on day 3 the responses were inhibited only by anti-I-E mAb and not by anti-I-A mAb. To gain insight into the mechanism underlying the anti-I-E-mediated inhibition, we prepared monovalent Fab and divalent F(ab')2 fragments of anti-I-A and anti-I-E mAb and examined their effects on the B cell responses. We found that the B cell responses were inhibited by the F(ab')2 but not Fab fragment of anti-I-E mAb, whereas the Fab fragment of anti-I-A mAb still gave effective inhibition. The F(ab')2 but not Fab fragment of anti-I-E mAb induced increases in cyclic AMP (cAMP) levels in B cells, whereas the undigested anti-I-A mAb did not induce such increases. Furthermore, adenylate cyclase inhibitors, which inhibit cellular cAMP accumulation, circumvented the B cell responses inhibited by anti-I-E but not anti-I-A mAb. Thus, these results indicate that the anti-I-E-mediated inhibition of the B cell responses requires increases in intracellular cAMP levels induced by cross-linking of I-E molecules. In contrast, anti-I-A mAb inhibits the B cell responses without cross-linking of I-A molecules and cAMP accumulation. These results reinforce a unique function of I-A molecules as restriction elements in the Ia-restricted B-B cell interaction.