Immunological control mechanism against cholera toxin: interference with toxin binding to intestinal receptors

The immunological control mechanism against cholera toxin (CT) in the small intestine of rats was studied in vivo. CT binding to intestinal receptors was determined by injected radiolabeled CT into the loops of rat small intestine and subsequently separating purified microvillus membranes from mucosal scrapings of those loops. substantial radioactivity (10(5) cpm/mg of microvillus protein) was present in microvillus fractions of small intestine exposed to 125I-labeled CT compared to radioactivity (10(2) cpm/mg) in fractions from intestine exposed to radiolabeled bovine serum albumin (BSA) used as a control. CT binding to intestinal receptors was significantly inhibited (P less than 0.02) in rats immunized with crude toxin by a combined intraperitoneal and oral method compared to CT binding in animals immunized with BSA or controls, suggesting a specific relationship between intestinal antitoxin and inhibition of binding. Furthermore, ligated ileal loops from CT-immunized animals showed a significant decrease in fluid accumulation when exposed to CT compared to loops from control or BSA-immunized animals, suggesting that antitoxins also interfered with the biological action of CT under conditions of immunization. These studies provide direct evidence that intestinal antitoxins protect against CT-induced diarrhea by interfering with the attachment of the toxin to the intestinal microvillus surface.