The exotoxins TcdA and TcdB are the major virulence factors of Clostridium 
difficile. Circulating neutralizing antitoxin antibodies are protective in C. 
difficile infection (CDI), as demonstrated, in part, by the protective effects 
of actoxumab and bezlotoxumab, which bind to and neutralize TcdA and TcdB, 
respectively. The question of how systemic IgG antibodies neutralize toxins in 
the gut lumen remains unresolved, although it has been suggested that the Fc 
receptor FcRn may be involved in active antibody transport across the gut 
epithelium. In this study, we demonstrated that genetic ablation of FcRn and 
excess irrelevant human IgG have no impact on actoxumab-bezlotoxumab-mediated 
protection in murine and hamster models of CDI, suggesting that Fc-dependent 
transport of antibodies across the gut wall is not required for efficacy. Tissue 
distribution studies in hamsters suggest, rather, that the transport of 
antibodies depends on toxin-induced damage to the gut lining. In an in vitro 
two-dimensional culture system that mimics the architecture of the intestinal 
mucosal epithelium, toxins on the apical side of epithelial cell monolayers are 
neutralized by basolateral antibodies, and antibody transport across the cell 
layer is dramatically increased upon addition of toxin to the apical side. 
Similar data were obtained with F(ab')2 fragments, which lack an Fc domain, 
consistent with FcRn-independent paracellular, rather than transcellular, 
transport of antibodies. Kinetic studies show that initial damage caused by 
apical toxin is required for efficient neutralization by basolateral antibodies. 
These data may represent a general mechanism of humoral response-mediated 
protection against enteric pathogens.