Analysis of the mechanism of abnormal B cell activation characteristic of various autoimmune diseases

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The current studies were undertaken to explore the mechanisms of abnormal B cell activation in autoimmune disease. For this purpose, special attention was paid to the regulation of cell cycle progression of human B cells and the influences of cytokines, including IL-10 and IL-2. First, we examined in detail the regulation of the survival of human peripheral blood B cells by IL-10 and its relevance to the Ig production. Highly purified B cells from healthy adult individuals were cultured with Staphylococcus aureus Cowan I (SA) in the presence or absense of IL-10. When IL-10 was present during the initial activation of B cells with SA,IL-10 faciliated the apoptosis of SA activated B cells, thus resulting in the very modest IgM production. IL-2 prevented the IL-10-mediated progression of the apoptosis of SA activated B cells during the initial activation, and thus restored the further differentiation of these B cells into Ig secreting cells. By contrast, IL-10 rather rescued SA activated … More B cells from apoptosis and thus supported the differentiation of these B cells without any influences of IL-2, when it was added after 72h of cultures. These results indicate that the effects of IL-10 are different depending on the state of activation of B cells after ligation of antigen receptors.Recent clinical trials have demonstrated the efficacy of mizoribine in rheumatoid arthritis and lupus nephritis, in which abnormalities of B cell functions are also involved. We therefore examined the effects of mizoribine on the in vitro function of human B cells. IgM production was induced from highly purified B cells obtained from healthy donors by stimulation with Staphylococcus aureus Cown I (SA) plus IL-2. Mizoribine suppressed the production of IgM at its pharmacologically attainable concentrations (0.3-3mug/ml) in a dose-dependent manner. Time kinetics study revealed that mizoribine was requied to be present within the first 120 hr after the initiation of cultures to exert its suppressive effects on B cell responses. Cell cycle analysis by staining with propidium iodide disclosed that mizoribine suppressed the G1-S transition of activated B cells. The suppressive effects of mizoribine on the IgM production was not reversed by repletion of GTP with supplementation of GMP.Althogh mizoribine did not suppress the expression of CD25 and cdc2 kinase in B cells, mizoribine markedly suppressed the expression of cyclin A in SA activated B cells at the mRNA levels. These results indicate that mizoribine suppresses the production of IgM by directly inhibiting B clls with out interfering with the initial phase of activation. Moreover, the data demonstrate that mizoribine inhibits the G1-S transition of activated B cells in the cell cycle by suppressing the expression of cyclin A by a mechanism distinct from guanine ribonucleotide depletion. Less