Human Serum Albumin is an Essential Component of the Host Defense Mechanism Against Clostridium difficile Intoxication.

The pathogenic effects of Clostridium difficile are primarily attributable to the production of the large protein toxins A (TcdA) and B (TcdB). These toxins monoglucosylate Rho GTPases in the cytosol of host cells, causing destruction of the actin cytoskeleton with cytotoxic effects. Low human serum albumin (HSA) levels indicate a higher risk of acquiring and developing a severe C. difficile infection (CDI) and are associated with recurrent and fatal disease. Our results show that HSA at physiological concentrations rescues human epithelial colorectal adenocarcinoma cells and protects stem cell-derived human intestinal organoids from intoxication by a TcdA:TcdB mixture. Moreover, zebrafish embryos injected with HSA and subsequently treated with TcdB show a higher survival rate, less blood vessel damage in the tail region and less intracranial hemorrhaging than embryos treated with TcdB alone. According to docking and biochemical analyses, HSA specifically binds via its domain II to TcdA and TcdB and thereby induces their autoproteolytic cleavage at physiological concentrations. This process impairs toxin internalization into the host cell and reduces the toxin-dependent glucosylation of Rho proteins. Our data provide evidence for a specific HSA-dependent self-defense mechanism against C. difficile toxins and provide an explanation for the clinical correlation between CDI severity and hypoalbuminemia.