CHOLERA-TOXIN AND CHOLERA-TOXIN B-SUBUNIT INDUCE IGA SWITCHING THROUGH THE ACTION OF TGF-BETA-1

Cholera toxin (CT) and its B subunit (CTB) are potent immunogens and a djuvants that, either alone or linked to protein Ags can stimulate muc osal immune responses, modulate the induction of oral tolerance, and s timulate IgA isotype switching. The present studies addressed the mech anisms by which CT and CTB promote IgA switching. CT and rCTB, in the presence of IL-2, significantly increased IgA isotype sn itching at th e clonal level in populations of purified and LPS-activated murine sur face IgA(-) spleen B cells, as determined by ELISA. enzyme linked immu nospot assays, and limiting dilution analysis. The IgA stimulatory eff ects of CT and CTB were independent of the A subunit of CT, CTB and CT did not increase the secretory. rate of IgA-producing cells or the cl onal burst size of IgA clones, and did inhibit B cell growth. Because TGF-beta 1 also inhibits B cell growth and promotes IgA switching, fur ther studies tested whether the activity of CTB and CT on IgA isotype switching was mediated through TGF-beta 1. Anti-TGF-beta Ab and solubl e TGF-beta 1 type IIR inhibited CTB- and CT-stimulated IgA isotype swi tching. Furthermore, increased TGF-beta 1 mRNA levels and bioactire TG F-beta 1, within a range shown to induce IgA isotype switching. were d etected in cultures of surface IgA(-) B cells stimulated with CT or CT B and IL-2, These data indicate that CTB- and CT-stimulated IgA isotyp e switching are mediated through TGF-beta 1, The finding that CTB up-r egulates TGF-beta 1 activity has important implications for understand ing the mechanisms by which CTB promotes both IgA mucosal immunity and oral tolerance.