DeFranco A L, Raveche E S, Paul W E
J Immunol. 1985 Jul;135(1):87-94.
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.
静止的B细胞在低浓度（1至5微克/毫升）抗IgM抗体的作用下进入并通过细胞周期的G1期。进入S期需要五倍至五十倍高浓度的抗IgM。这些以及其他结果有力地表明，B细胞活化涉及两个分别受控的事件。目前的研究表明，从培养开始就用高浓度抗IgM孵育的B细胞在进入S期前约10小时变得不再依赖额外的抗IgM。我们将细胞周期G1期的这个抗IgM非依赖部分称为G1β，而较早的阶段称为G1α。此外，低浓度的抗IgM足以使细胞通过G1α的早期部分，但如果细胞要完成细胞周期的其余部分，则在最后4至8小时（G1α'）需要高浓度。在G1α的任何时候去除抗IgM都会导致伴随G1期进展的细胞大小增大立即停止。这样的细胞在至少17小时内保持其大小和在G1中的相对位置，并在重新添加抗IgM时重新开始通过G1α期移动。因此，B细胞可能处于部分活化状态，并且必须具有一种机制来整合它们接收到的刺激信号的量；只有当该信号超过某个阈值时，它们才进入S期的承诺期。
