Saccharomyces boulardii protease inhibits the effects of Clostridium difficile toxins A and B in human colonic mucosa

Saccharomyces boulardii is a nonpathogenic yeast used in the treatment of Clostridium difficile diarrhea and colitis. We have reported that S. boulardii inhibits C. difficile toxin A enteritis in rats by releasing a 54-kDa protease which digests the toxin A molecule and its brush border membrane (BBM) receptor (I, Castagliuolo, J. T. LaMont, S. T. Nikulasson, and C. Pothoulakis, Infect. Immun, 63:5225-5232, 1996). The aim of this study was to further evaluate the role of S., boulardii protease in preventing C. difficile toxin A enteritis in rat ileum and determine whether it protects human colonic mucosa from C, difficile toxins, A polyclonal rabbit antiserum raised against purified S, boulardii serine protease inhibited by 73% the proteolytic activity present in S, boulardii conditioned medium in vitro. The anti-protease immunoglobulin G (IgG) prevented the action of S, boulardii on toxin A-induced intestinal secretion and mucosal permeability to [H-3]mannitol in rat ileal loops, while control rabbit IgG had no effect. The anti-protease IgG also prevented the effects of S, boulardii protease on digestion of toxins A and B and on binding of [H-3] toxin A and [H-3] toxin B to purified human colonic BBM. Purified S, boulardii protease reversed toxin A- and toxin B-induced inhibition of protein synthesis in human colonic (HT-29) cells, Furthermore, toxin A- and B-induced drops in transepithelial resistance in human colonic mucosa mounted in Ussing chambers were reversed by 60 and 68%, respectively, by preexposing the toxins to S, boulardii protease, We conclude that the protective effects of S. boulardii on C, difficile-induced inflammatory diarrhea in humans are due, at least in part, to proteolytic digestion of toxin A and B molecules by a secreted protease.