Requirements for mitogenic stimulation of murine B cells by soluble anti-IgM antibodies

The requirements for the stimulation of proliferation in murine b cells by soluble anti-IgM antibodies were examined. For optimal stimulation, the presence of both 2-mercaptoethanol (2-ME) and fetal calf serum (FCS) was found necessary for both anti-IgM and LPS-stimulated proliferation. This requirement could be substituted by a stable and alkylation-insensitive cofactor produced in serum by exposure to 2-ME followed by dialysis. Filler thymocytes could also partially substitute for the 2-ME-FCS factor. In the presence of this cofactor, accessory cells were not necessary for the response of B cells to anti-IgM and LPS. The proliferative response was found in B cells from adult mice. However, B cells from neonatal mice did not respond to anti-IgM antibodies under any conditions. Whereas the proliferative response to LPS was maximal by 2 to 3 weeks of age, the response to anti-IgM was still not maximal at 2 months of age. Reactivity of the anti-Ig antibodies against the IgM Fc determinants was necessary for stimulation, whereas reactivity to Fab determinants had no effect. Bivalent F(ab')₂ anti-IgM antibody fragments were more stimulatory than intact antibodies, but monovalent fab fragments were totally nonstimulatory. These findings indicate both a requirement for receptor cross-linking and an inhibitory effect produced by the antibody Fc fragment. B cells had to be continuously exposed to both IgM and the 2-ME-FCS factor in order to proliferate; neither agent exerted its total effect in a short period of time.