B lymphocyte activation by insoluble anti-immunoglobulin: Induction of immunoglobulin secretion by a T cell-dependent soluble factor

Purified rabbit antibodies to mouse kappa chain or Fab become strongly mitogenic for mouse splenic B lymphocytes when covalently attached to the surface of polyacrylamide beads. The same antibody preparations are weakly if at all mitogenic in soluble form. The proliferative response to these anti-Ig beads is not dependent on T lymphocytes, adherent cells, serum supplement, or Fc receptor interactions. The response to anti-Ig beads can be blocked by soluble F(ab')₂ anti-Ig at concentrations that modulate surface Ig, or by excess soluble mouse IgG. Hence, the response results from direct interaction of the beads with B cells via surface Ig. In the presence of a supernatant of Con A-treated spleen cells (Con A supernatant), B lymphocytes respond to anti-Ig beads by sustained proliferation and differentiation to polyclonal IgM and IgG secretion. The response is comparable in magnitude to the lipopolysaccharide response. Ig secretion is completely dependent on the Con A supernatant, which by itself has only a small, primarily differentiative effect on isolated B lymphocytes. It can be added 22 hr after the start of culture without changing the kinetics of the response; addition at later times up to 66 hr delays the appearance of secreting cells, but not the size of the peak response. Cells from 10-day-old mice and adult male (CBA/Nfemale x BALB/cmale)F₁ mice fail to respond. These experiments suggest that redistribution of surface Ig by the anti-Ig beads delivers a limited proliferative signal to a subset of B lymphocytes, but that sustained cell division and differentiation to Ig secretion by anti-Ig activated B lymphocytes require soluble substances produced by activated T lymphocytes and/or T lymphocyte-activated accessory cells.