Activation and proliferation signals in mouse B cells. III. Intact (IGG) anti-immunoglobulin antibodies activate B cells but inhibit induction of DNA synthesis.

Intact (IgG) rabbit anti-immunoglobulin antibodies are generally not mitogenic for mouse B cells but, on the contrary, inhibit proliferation induced by either F(ab')2 anti-Ig or by lipopolysaccharide. We show here, however, that IgG anti-Ig activates mouse B cells, since it causes B cells to depolarize, to enlarge and to express increased levels of Ia antigens. In the continuing presence of IgG anti-Ig, B cells do not synthesise DNA. However, if cells cultured with IgG antibody are then washed, they start to proliferate earlier in response to F(ab')2 anti-Ig, i.e. they have become primed. We therefore conclude that IgG anti-Ig is an example of a 'step-one activator' for mouse B cells, which drives resting B cells out of Go, but actively prevents the cells from progressing into S. The latter effect appears to result from cross-linking of surface Ig and Fc receptors on B cells, although the mechanism is, as yet, unknown.