Experiments were performed to investigate the effect of cholera toxin (CT) on human B cell function. Highly purified (greater than 98% CD20+) human peripheral blood B cells were exposed to CT in the presence or absence of anti-mu antibody. Treatment of highly purified B cells with CT stimulated enhanced expression of surface DR molecules, whereas it did not enhance expression of other B cell surface activation markers including transferrin or IL-2R. Neither the A nor the B subunits of CT by themselves enhanced the expression of surface DR Ag. In addition, 8-bromo-cAMP alone or in combination with the B subunit did not increase the expression of human B cell surface DR Ag. These findings suggest that neither elevation of cAMP nor binding to GM1 ganglioside are sufficient to stimulate this activation parameter in B cells. Associated with CT-mediated enhanced expression of MHC class II molecules we found that CT-treated B cells also served as stronger stimulators, compared with control cells, of both autologous and allogeneic MLR responses in peripheral blood T cells. Although CT stimulated early events in B cell activation, it inhibited anti-mu antibody-induced B cell thymidine incorporation by 55 to 75%. Inhibitory effects of CT were observed even when CT was added to cultures as late as 36 h after the addition of the anti-mu antibody. These results suggest that CT has both a stimulatory and inhibitory effect on human B cells and that the stimulatory effect may be mediated via a cAMP-independent mechanism.

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