B-cell-stimulatory factor 1 reverses Fc receptor-mediated inhibition of B-lymphocyte activation.

Intact (IgG class) rabbit anti-immunoglobulin antibodies are not mitogenic for mouse B cells but inhibit proliferation induced by F(ab')2 anti-Fab fragment antibodies (anti-Ig). In addition, cross-linkage of Fc and surface immunoglobulin receptors on B cells by intact anti-Ig inhibits inositol phospholipid breakdown (but not Ca2+ flux) resulting from ligation of antigen receptors. This system, therefore, provides a polyclonal model for B-cell inactivation by antigen-antibody complexes. T-cell-derived B-cell-stimulatory factor 1 acts synergistically with submitogenic concentrations of F(ab')2 anti-Ig to induce B-cell proliferation. We show here that B-cell-stimulatory factor 1 and intact anti-Ig also induce B cells to synthesize DNA. However, B-cell-stimulatory factor 1 does not induce inositol phospholipid breakdown, does not mobilize Ca2+ in B cells, nor does it influence the magnitude of these responses provoked by intact anti-Ig.