Three restriction points in the cell cycle of activated murine B lymphocytes

The cell cycle of activated B lymphocytes was found to be controlled by three restriction points. The first occurs immediately after mitosis and was found to be controlled by the binding of Sepharose-bound, immunoglobulin-specific antibodies to surface membrane-bound immunoglobulin. Exposure to this stimulus as short as 15 min or as long as 36 hr allowed B cells to move into the G1 phase up to the next restriction point. The second restriction point was observed to be approximately equal to 4 hr after mitosis, in the G1 phase of the cycle and 3-4 hr before the B cells entered S phase, and was found to be controlled by alpha-type B-cell growth factors produced by the P388D1 macrophage line. A third restriction point occurs in the G2 phase, 2-4 hr before mitosis, and is apparently controlled by beta-type B-cell growth factors that are likely to be produced by helper T lymphocytes.