| Enterohemorrhagic Escherichia coli (EHEC), including serotype O157:H7, elaborate Shiga toxin 2 (Stx2), a potent cytotoxin. This toxin initiates the serious renal condition, hemolytic-uremic syndrome (HUS), that sometimes occurs as a result of EHEC infection. Stx2 is bound in human serum by serum amyloid P component (HuSAP). In this study, we demonstrated that HuSAP prevents Stx2 from engaging its cellular receptor, globotriaosylceramide, and neutralizes Stx2 in cell culture and mouse models. However, HuSAP does not appear to neutralize Stx2 in humans. We therefore investigated possible explanations for why it is unable to do so. Several reports suggest that, in EHEC-infected patients, Stx2 binds the surface of circulating neutrophils and is transported by these cells to the kidney, ultimately leading to HUS. Here, we showed that Stx2 is capable of binding to human, but not murine, neutrophils by a globotriaosylceramide-independent receptor and does so in the presence of HuSAP. We proposed that this dissimilarity would explain the differential protective role of HuSAP in mice and humans. In order to test this hypothesis we developed a humanized Shigatoxemia mouse model by transfusing human neutrophils into HuSAP-expressing transgenic mice. Inconsistent with our hypothesis, however, we found there was no effect on the sensitivity of HuSAP-transgenic mice to Stx2 when their neutrophils were replaced with human. Further, we failed to demonstrate any effect of leukopenia or human neutrophil supplementation on the trafficking of radiolabelled Stx2 to target organs in mice, which suggests that neither human nor murine neutrophils participate in Stx2 transport. We therefore investigated other explanations for why HuSAP does not prevent HUS in humans. In this regard, we demonstrated that the Stx2 mouse model in which HuSAP is protective does not fully reproduce the symptoms of HUS. However, injecting mice with Stx2 and lipopolysaccharide caused renal pathology that was comparable to that observed in HUS patients and reversed the protective effect of HuSAP. Because neutralization of the systemic immune response to lipopolysaccharide had no effect on HuSAP-transgenic mouse survival, we propose that the sensitization induced by lipopolysaccharide resulted from immune responses triggered specifically in the kidney. |
