Mizoribine has been used to prevent rejection of organ allografts in humans and in animal models. Recent clinical trials have demonstrated its efficacy 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 Cowan I (SA) plus IL-2. Mizoribine suppressed the production of IgM at its pharmacologically attainable concentrations (1 to 3 micrograms/ml) in a dose-dependent manner. Mizoribine had to be present within the first 96 h after the initiation of cultures to exert its suppressive effects on B cell responses. Cell cycle analysis disclosed that mizoribine significantly decreased the numbers of B cells in S + G2 + M phases. Mizoribine did not decrease GTP levels in SA-stimulated B cells, whereas mizoribine led to a decrease in GTP levels in activated T cells, which was reversed by addition of GMP. Consistently, the suppressive effects of mizoribine on the IgM production of SA-stimulated B cells was not reversed by the addition of GMP as much as 40 microM, which overcame the inhibitory effects of mizoribine on the proliferation of anti-CD3-stimulated T cells. Although mizoribine did not suppress the expression of CD25 and cdc2 kinase, mizoribine markedly suppressed the expression of cyclin A in SA-activated B cells. These results indicate that mizoribine directly suppresses the function of human B cells without interfering with the initial phase of activation. Moreover, the data demonstrate that mizoribine interferes with the cell cycle progression of activated B cells by suppressing the expression of cyclin A by a mechanism distinct from guanine ribonucleotide depletion.

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