| Abstract: | Mouse B lymphocytes respond by increased rates of DNA synthesis 14–24 hours after stimulation with the mitogen lipopolysaccharide. If, at this time, stimulated cells are treated for 12 hours with a “hot pulse” of thymidine, subsequent mitogen-induced maturation to high rate IgM-producing plaque-forming cells is abolished. Thus, B cells stimulated by mitogen to mature also incorporate thymidine into DNA and proliferate. DNA synthesis is inhibited to 99 % in 48-hour mitogen-stimulated B lymphocytes by 10?2 M hydroxyurea or 10?3 M cytosine arabinoside, while protein and IgM synthesis and secretion and the plaque-forming capacity of those cells are unaffected. When hydroxyurea is added to small, resting B cells at the time of initiation of mitogenic stimulation, these cells mature to 19 S IgM- secreting, plaque-forming cells in the absence of DNA synthesis and proliferation. Maturation in the absence of DNA synthesis is also manifested by an increase in ratios of rates of synthesis and secretion of IgM over those of all proteins in the cell and by the attachment of the “branch” sugars, galactoses and fucoses, to 19 S IgM secreted by the cells. This maturation of B cells in the absence of DNA synthesis occurs within 12 to 30 hours after stimulation and is mitogen dose dependent. The inhibition of B cells to synthesize DNA and to develop into clones of plaque-forming cells can be reversed by the removal of hydroxyurea for as long as 36 hours after mitogenic stimulation in the presence of the inhibitor. In the presence of hydroxyurea, B cells are stimulated to immature plasmablast-like cells containing surface-bound and little intracytoplasmic Ig within 16 to 24 hours of stimulation. Mature plasma cells containing no detectable surface-bound Ig, but abundant intracytoplasmic Ig are only developed in the uninhibited, but not in the hydroxyurea-inhibited mitogen-stimulated cells later in the response. Thus, two stages of B cell maturation and differentiation can be distinguished: the first stage of an immature plasmablast develops in the absence of DNA synthesis, while the later stage of the development of the mature plasma cell requires DNA synthesis. |
