Human α-Defensins Inhibit Clostridium difficile Toxin B

Abstract

Clostridium difficile toxins A and B are major virulence factors implicated in pseudomembranous colitis and antibiotic-associated diarrhea. The toxins are glucosyltransferases, which inactivate Rho proteins involved in cellular signaling. Human alpha-defensins as part of the innate immune system inactivate various microbial pathogens as well as specific bacterial exotoxins. Here, we studied the effects of alpha-defensins human neutrophil protein (HNP)-1, HNP-3, and enteric human defensin (HD)-5 on the activity of C difficile toxins A and B. Inactivation of C difficile toxins by alpha-defensins in vivo was monitored by microscopy, determination of the transepithelial resistance of CaCo-2 cell monolayers, and analysis of the glucosylation of Rac1 in toxin-treated cells. In vitro glucosylation was used to determine K(m) and median inhibitory concentration (IC(50)) values. Formation of defensin-toxin complexes was analyzed by precipitation and turbidity studies. Treatment of cells with human alpha-defensins caused loss of cytotoxicity of toxin B, but not of toxin A. Only alpha-defensins, but not beta-defensin-1 or cathelicidin LL-37, inhibited toxin B-catalyzed in vitro glucosylation of Rho guanosine triphosphatases in a competitive manner, increasing K(m) values for uridine 5'-diphosphate-glucose up to 10-fold. The IC(50) values for inhibition of toxin B-catalyzed glucosylation by the alpha-defensins were 0.6-1.5 micromol/L. At high concentrations, defensins (HNP-1 > or = 2 micromol/L) caused high-molecular-mass aggregates, comparable to Bacillus anthracis protective antigen and lethal factor. Our data indicate that toxin B interacts with high affinity with alpha-defensins and suggest that defensins may provide a defense mechanism against some types of clostridial glucosylating cytotoxins.