Separate signals for human B cell proliferation and differentiation in response to Staphylococcus aureus: evidence for a two-signal model of B cell activation.

Abstract We have investigated the role of T cells in the mitogenic response of human peripheral blood B lymphocytes to killed Cowan strain Staphylococcus aureus (SAC). We found that mononuclear cells exhaustively depleted of T cells by removal of cells rosetting with AET-treated SRBC, or by cytotoxic treatment with the monoclonal antibody anti-Leu-1 and complement, proliferate in response to SAC, but do not differentiate into immunoglobulin- (Ig) secreting cells (ISC). This held true not only for B cells secreting polyclonal Ig detected in a reverse hemolytic plaque-forming cell assay or in ELISA assays for total IgM and IgG, but also for the B cells secreting specific antibody against KLH or tetanus toxoid in response to SAC 2 wk after booster immunization in vivo. We found no evidence for a B cell subset that does not require T help for continued Ig secretion in response to SAC. The differentiative response of B cells to SAC could be restored by the addition of autologous T cells to the T-depleted population. SAC was shown not to stimulate T cells itself; the simultaneous stimulation of T cells with PWM and of B cells with SAC resulted in the ability of very small numbers of T cells to provide the signal required for B cell maturation into ISC. Similarly, supernatants of stimulated T cells efficiently substituted for T cells in this process. Mixed lymphocyte culture supernatants consistently resulted in increased numbers of ISC when added to B cells undergoing stimulation by SAC, but not when added to unstimulated B cells. Inasmuch as SAC is known to stimulate human B cells by interaction of its cell wall protein A with B cell surface Ig, we propose that the two signals required for B cell stimulation by SAC are identical to those required for responses to specific antigens, and that the present system allows an assay for T cell-derived differentiation factors acting on B cells that have undergone direct antigenic stimulation or its equivalent.