Studies of the pathogenesis of Shiga toxin-producing Escherichia coli-induced hemolytic uremic syndrome

Abstract: Hemolytic uremic syndrome (HUS) is characterized by non-immune hemolytic anemia, thrombocytopenia and acute kidney failure. The most common form of HUS is associated with gastrointestinal Shiga toxin-producing enterohemorrhagic Escherichia coli (EHEC) infection. EHEC are non-invasive strains with Shiga toxin as the unique virulence factor. The aim of this study was to describe host-pathogen interactions during EHEC infection using patient samples, an established mouse model and blood cells in vitro. In the first study the mechanism by which hemolysis is induced was investigated. Red blood cells from HUS patients and from controls were utilized to demonstrate complement activation on red blood cells in patients and show that Shiga toxin could induce hemolysis mediated by complement activation. Shiga toxin also induced the release of complement-coated red blood cell-derived microvesicles. The toxin may thus contribute to the induction of hemolysis. After intestinal colonization Shiga toxin is released into the circulation but does not circulate in free form. In the second study the means by which toxin reaches the kidney was investigated showing that the toxin circulated within blood cell-derived microvesicles (in patients and in the mouse model). These toxin-containing microvesicles were internalized in renal endothelial and epithelial cells where they emptied their cargo. The study thereby identified a novel mechanism of bacterial virulence by which virulence factors evade the host response. In the third study the importance of the antimicrobial peptide cathelicidin in EHEC infection was investigated using cathelicidin-deficient mice. These mice exhibited a thinner colonic mucosa layer, increased intestinal colonization, severe symptoms accompanied by intestinal and renal damage, in comparison to the wild-type cathelicidin-producing mice, which remained asymptomatic. Thus cathelicidin was found to be protective in murine EHEC infection. In the fourth study the protective effect of complement blockade during murine EHEC infection was determined, showing that early blockade, or inherent deficiency, of the terminal complement complex was protective whereas late blockade was not. In summary, this thesis unraveled mechanisms of induction of hemolysis and of toxin transfer and as well as the protective role of the anti-microbial cathelicidin and the terminal complement cascade in EHEC-HUS.