Control of proliferation and differentiation in B lymphocytes by anti-Ig antibodies and a serum-derived cofactor

The effects of various anti-Ig antibodies on different B lymphocyte functions were investigated. With the proper accessory cofactor(s) derived from serum, anti-IgM antibodies induced a vigorous proliferative response in normal adult murine B cells, while polyspecific anti-Ig and anti-IgD had no effect. Without the required cofactor, all three anti-Ig antibodies were inhibitory for mitogenic responses. All three anti-Ig antibodies were also inhibitory for mitogen-induced antibody responses with or without the cofactor. Even with the required cofactor, neonatal B cells as well as adult C3H/HeJ B cells were not triggered into proliferation by anti-IgM. Finally, the cofactor required for anti-IgM-triggered mitogenesis was shown to be generated from serum by 2-mercaptoethanol and to be approximately 65,000 in molecular weight. These results indicate that, for at least some responses, B lymphocyte surface IgM molecules are involved in both triggering and suppression, depending both on the developmental state of the B cell and the presence or absence of accessory influences. In these experiments, IgD gave evidence only of being suppressive.