cAMP-independent effects of cholera toxin on B cell activation. III. Cholera toxin A subunit-mediated ADP-ribosylation acts synergistically with ionomycin or IL-4 to induce B cell proliferation.

To investigate whether ADP-ribosylation of proteins by cholera toxin could influence B cell activation, B cells were incubated with the A subunit of cholera toxin. Ionomycin acted synergistically to induce B cell proliferation with the A subunit of cholera toxin but not with cAMP-enhancing agents or with the B subunit of cholera toxin, suggesting that the synergistic effect of the A subunit was mediated via ADP-ribosylation and not via cAMP elevations or ganglioside GM1 binding. Indeed, inhibitors of ADP-ribosylation blocked the synergistic effect. Unlike anti-Ig, B cell proliferation stimulated by LPS or by the combination of the A subunit and ionomycin was observed in protein kinase C (PKC)-depleted B cells. However, neither the A subunit nor ionomycin enhanced B cell proliferation stimulated by low dose LPS, suggesting that the A subunit plus ionomycin stimulated an activation pathway distinct from the LPS-stimulated pathway. Additionally, unlike LPS, the A subunit plus ionomycin did not stimulate B cells in vitro to secrete Ig. IL-4 acted synergistically with the A subunit to induce B cell proliferation to the same extent as it did with anti-Ig; unlike the anti-Ig plus IL-4 synergy, however, the A subunit plus IL-4-mediated synergy persisted in PKC-depleted B cells. Taken together, our data suggest that cholera toxin A subunit-catalyzed ADP-ribosylation modifies a non-Gs protein involved in the activation of B cells, either through a novel pathway or at a point distal to the activation of PKC along the anti-Ig-stimulated pathway.