Activation and Cell Cycle Control of Murine B Lymphocytes

Abstract

SUMMARY The cell cycle of activated murine B lymphocytes (B cells) is controlled by the occupancy of surface membrane-bound immunoglobulin (Ig) and by two types of growth factors, called α and β factors. These growth factors are produced in an endocrine fashion by the interaction of helper T lymphocytes (T cells) with antigen-presenting macrophages (A cells). Antigen is taken up, processed and presented on the surface of A cells in the context of class II major histocompatibility complex (MHC) glycoproteins. Helper T cells recognize this association of antigen and class II MHC molecules. A cells produce α factors and T cells produce β factors. The molecular nature of these factors and of the corresponding receptors on B cells has yet to be elucidated, although it can be shown that the complement component C3d replaces α factor action. Resting, G0 phase B cells are refractory to the action of α and β factors. They have to be excited, i.e. rendered susceptible to the action of these factors. This can be achieved by the interaction with helper T cells that recognize antigen, bound by surface membrane Ig, in the context of class II MHC glycoproteins on the surface of resting G0 B cells. Excitation can also occur in a polyclonal fashion by cross-linking of suface Ig with immobilized, Ig-specific antibodies, or by the interaction with polyclonal activators of B cells, such as lipopolysaccharides. Entry into the cell cycle is asynchronous. Activated, cycling B cells can be synchronized by size separation, using velocity sedimentation. Synchronized B cells will retain their synchrony for several divisions, when they are stimulated by immobilized Ig-specific antibodies, α and β factors. They divide every 20 h at 37°C. Omission of either of the three stimuli arrests B cells, though at different points in the cell cycle. Three restriction points are found: the first occurs immediately after mitosis and is controlled by the binding of immobilized Ig-specific antibodies to surface membrane-bound Ig. The second is observed in the G1 phase, around 6–8 h after mitosis and 2–4h before entry into S phase. It is controlled by α factors. The third is found 2–4 h before mitosis, in G2 phase, and is controlled by β factors.