Human B cell activation and cell cycle progression: stimulation with anti-μ andStaphylococcus aureus Cowan strain I

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Abstract
The responses of resting human B lymphocytes to a variety of activation signals were studied. Human tonsillar B lymphocytes were separated according to size by countercurrent elutriation. The small B lymphocytes were then stimulated in vitro with various concentrations of anti-μ antibody in the presence or absence of B cell growth factor (BCGF) or with Staphylococcus aureus Cowan strain I (SAC). Cellular volume changes and RNA synthesis were measured over the first 24 h of stimulation and were similar with either 15 μg/ml of anti-μ, 100 μg/ml of anti-μ, or SAC. In the subsequent 24 h, however, substantial increases occurred in the amount of RNA synthesis and cell enlargement only in those cultures stimulated with 100 μg/ml of anti-μ or SAC, but not in the cultures stimulated with 15 μg/ml of anti-μ. The addition of BCGF to those cultures stimulated with 15 μg/ml of anti-μ did not alter the increases in cellular volume and RNA synthesis found 24 h after stimulation with anti-μ alone. However, over the subsequent 24 h, the presence of BCGF in culture enhanced both B cell volume changes and RNA synthesis, when compared to cultures stimulated with 15 μg/ml of anti-μ alone. In addition, BCGF enhanced DNA synthesis in cultures stimulated with low and high concentrations of anti-μ. DNA content changes following stimulation with anti-μ, anti-μ plus BCGF, and SAC were also measured using propidium iodide staining and flow cytometry. Optimal concentrations of anti-μ induced 20% of the resting B cells to enter S phase, while optimal concentrations of anti-μ plus BCGF or SAC induced approximately 40%. Finally, prestimulation of resting B cells for 24 h with a low concentration of anti-μ, sufficient for cell enlargement but not S phase progression, allowed for rapid entrance of the prestimulated B cells into S phase when a high concentration of anti-μ or SAC was added. These findings suggest the existence of a control point in the progression of human B cells through the cell cycle. This control point is located in the G1 phase of the cycle and is reached 24 to 36 h after a surface immunoglobulin-mediated stimulus.
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