Sequential requirements for cell cycle progression of resting human B cells after activation by anti-Ig

We have previously demonstrated that in vivo activated normal human B cells reflected differential sensitivity on the basis of cell size to either an activation signal delivered by anti-mu or a proliferative signal delivered by a monoclonal B cell growth factor (BCGF) produced by a human T-T hybridoma. In this manuscript, we have shown that by using an in vitro co-culture system of stimulation with anti-mu and BCGF, purified small (resting) B cells could be induced by anti-mu stimulation to express functionally active BCGF acceptor sites concomitant with RNA synthesis and cell enlargement. We have demonstrated that purified small B cells could be activated to increase both cell size and RNA synthesis within 8 hr of anti-mu stimulation. DNA synthesis by anti-mu-stimulated B cells began at 36 to 44 hr from initial stimulation only when BCGF was added to the culture. In addition, large B cells that had been incubated with anti-mu for 24 to 48 hr manifested significant incorporation of [3H]thymidine if BCGF was added to cultures. These data strongly suggest that anti-mu may induce the G0 phase B cells into the G1 phase and that BCGF may then push at least a subset of these G1 phase-B cells into the S phase. This experimental system may provide a useful model of normal human B cell activation and proliferation and may allow a more precise delineation of each phase of this cascade in the B cell repertoire.