CAMP-INDEPENDENT EFFECTS OF CHOLERA-TOXIN ON B-CELL ACTIVATION .3. 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 c ould influence B cell activation, B cells were incubated with the A su bunit of cholera toxin. Ionomycin acted synergistically to induce B ce ll proliferation with the A subunit of cholera toxin but not with cAMP -enhancing agents or with the B subunit of cholera toxin, suggesting t hat the synergistic effect of the A subunit was mediated via ADP-ribos ylation and not via cAMP elevations or ganglioside G(M1)f binding. Ind eed, inhibitors of ADP-ribosylation blocked the synergistic effect. Un like anti-Ig, B cell proliferation stimulated by LPS or by the combina tion of the A subunit and ionomycin was observed in protein kinase C ( PKC)-depleted B cells. However, neither the A subunit nor ionomycin en hanced B cell proliferation stimulated by low dose LPS, suggesting tha t the A subunit plus ionomycin stimulated an activation pathway distin ct from the LPS-stimulated pathway. Additionally, unlike LPS, the A su bunit plus ionomycin did not stimulate B cells in vitro to secrete Ig. IL-4 acted synergistically with the A subunit to induce B cell prolif eration 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 p ersisted in PKC-depleted B cells. Taken together, our data suggest tha t 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.