Investigating the contributions of leukocyte responses and kidney cell stress on Shiga- toxin pathogenesis
Parello, Caitlin Suzanne Leibowitz
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BACKGROUND: Shiga toxin (Stx)-producing enterohemorrhagic Escherichia coli (EHEC) are emerging food- and water- borne pathogens and a leading cause of acute renal failure in otherwise healthy children. Ribotoxic Shiga toxins are the primary virulence factors and are responsible for the potentially lethal EHEC complication of hemolytic uremic syndrome (HUS). HUS, defined clinically by microangiopathic hemolytic anemia, thrombocytopenia and thrombotic microangiopathy which contribute to acute kidney injury or renal failure, is associated with significant patient morbidity. No pathogen- or toxin- specific therapeutic exists, and antibiotic use is contraindicated. Understanding the molecular mechanisms of Stx toxicity could lead to the development of Stx specific therapies. HYPOTHESIS: Experimental evidence suggests a role for leukocytes in systemic Stx2 trafficking and in Stx2 mediated kidney pathology. Cell stress responses, such as the ER stress response and ribosomal stress response, are hypothesized to induce apoptosis, and ultimately cell death, contributing to kidney injury; however these processes have only been described in vitro. If leukocyte and kidney cell stress responses are playing significant roles in in vivo Stx2 kidney injury, then down-regulation of these processes may provide therapeutic benefit. RESULTS: Mice injected with Stx2 or infected with Stx2-producing bacteria developed lethal kidney injury as judged by biomarkers and histopathology. Experimentally induced leukopenia did not alter kidney injury in either model, but did cause striking increases in the intestinal bacterial colonization which was dependent on the presence of Stx2. No Stx binding capacity was observed for either murine or human leukocytes ex vivo. Transcriptional evidence of kidney ER stress and apoptotic biomarkers were observed in both models of Stx2-mediated kidney injury, but down-regulation of these processes did not yield therapeutic benefit. CONCLUSIONS: Contrary to the current disease paradigm, no major role for leukocytes in systemic Stx2 trafficking or kidney injury was observed in vivo, but a novel role for host immune responses to Stx2 in the control of intestinal colonization by Stx2-producing bacteria was identified. Cell stress and apoptosis is induced by Stx2 in vivo but prevention of these is not sufficient to appreciably alter organ injury or survival in the murine models.