Growth control of activated, synchronized murine B cells by the C3d fragment of human complement

Three restriction points control the cell cycle of activated B lymphocytes. The first occurs directly after mitosis and is controlled by the occupancy of surface-bound immunoglobulin. The second is observed approximately 4 h after mitosis in the G1 phase of the cycle, that is, before DNA replication, and is controlled by growth factors that are produced by macrophages which we have previously classified as alpha-type factors. The third restriction point occurs in the G2 phase, 2-4 h before mitosis, and is controlled by beta-type growth factors probably produced by helper T lymphocytes. The third component of complement, C3, has long been implicated in the control of B-cell responses. C3 is secreted by monocytes and macrophages. We have found recently that crosslinked, but not soluble, human C3 stimulates activated, but not resting, murine B cells to thymidine uptake. Here we investigate the role of C3b and C3d in the progression of the cell cycle of activated, synchronized murine B cells. We find that crosslinked C3d replaces the action of alpha-factors within the cell cycle of these cells and allows entry into S phase. In contrast, soluble C3d inhibits the action of alpha-factors. This implies that a C3d-specific receptor, probably the murine analogue to the human complement receptor CR2, is a growth factor receptor on activated B cells that will give the cell a growth-positive signal when it is crosslinked, while occupancy by the soluble form of C3d will result in inhibition of the action of alpha-factors or of crosslinked C3b or C3d. A stretch of weak homology between the cDNA sequence of murine C3d and those of murine growth factors indicates that an insulin-like growth factor could be the active principle of C3d that controls the cell cycle of activated B cells.