Separate control of B lymphocyte early activation and proliferation in response to anti-IgM antibodies.

Resting B cells enter and progress through the G1 phase of the cell cycle in response to low concentrations (1 to 5 micrograms/ml) of anti-IgM antibodies. Commitment to enter S phase requires the presence of a fivefold to 50-fold higher concentration of anti-IgM. These and other results strongly suggest that two separately controlled events are involved in B cell activation. The current studies demonstrate that B cells incubated with high concentrations of anti-IgM from the initiation of culture become independent of additional anti-IgM approximately 10 hr before entry into S phase. We have designated this anti-IgM independent portion of the G1 phase of the cell cycle as G1 beta, whereas the earlier phase is referred to as G1 alpha. Furthermore, low concentrations of anti-IgM are sufficient for progress through early portions of G1 alpha, but high concentrations are required for the last 4 to 8 hr (G1 alpha') if the cells are to go through the rest of the cell cycle. Removal of anti-IgM at any time during G1 alpha causes prompt cessation of the size enlargement that accompanies progress through G1. Such cells retain their size and their relative place in G1 for periods of at least 17 hr and recommence movement through G1 alpha phase when anti-IgM is readded. Thus, B cells may exist in states of partial activation and must possess a mechanism to integrate the amount of stimulatory signal they have received; they enter a commitment period for S phase only when that signal passes some threshold value.