TRANSLATIONAL IMPACT

Abstract

Enterohemorrhagic Escherichia coli (EHEC) is an emerging zoonotic pathogen that causes acute gastroenteritis and hemorrhagic colitis in humans. It also causes disease in newborn calves and asymptomatically colonizes the gut mucosa of adult bovines, therefore constituting the main reservoir for food and environmental contamination. The hallmark gut lesion induced by EHEC involves bacterial attachment to the gut epithelium and effacing of gut microvilli, but innate immune responses and inflammation are conspicuously limited. Given that EHEC is strictly a human and calf pathogen that does not infect mice, it has been a challenge to develop simple models that permit detailed investigation of virulence mechanisms. Tissue tropism cannot be mimicked in vitro because EHEC is neither species- nor cell-type-specific in culture. Although human in vitro organ cultures have been used, this model has a short lifespan, does not accurately mimic normal gut architecture and lacks an intact immune system. Therefore, a better model is needed to study the molecular bases for the human and calf specificity of EHEC, its tissue tropism and the innate immune response induced upon infection.To address this need, the authors developed an in vivo model of EHEC infection by xenotransplantation of human or bovine fetal intestinal tissue under the skin of SCID mice (which do not reject the foreign tissue owing to lack of an adaptive immune system). Xenografts of human and bovine tissue originally isolated from different gut regions develop into mature tissues and faithfully mimic the tissue of human and calf intestine. Infection of xenografts with EHEC induces the formation of typical attaching and effacing lesions and tissue damage resembling hemorrhagic colitis. By contrast, xenografts infected with an EHEC mutant deficient in the type III secretion system (a major EHEC virulence mechanism) remained undamaged. Notably, EHEC does not attach to or damage the epithelium of small intestinal tissue.This xenograft model of EHEC infection faithfully recapitulates several aspects of the natural human and calf disease, including host and tissue tropism, typical histopathology, and attenuation of innate immune responses. Therefore, this relatively simple and economical xenograft model should be effective for resolving molecular mechanisms of host and tissue tropism, as well as the immunomodulatory effects of EHEC in intact tissue. A challenging step to further developing this model would be to connect the xenografted segments within the intestines of mice in order to establish a system that is fully reconstituted with normal microflora as well as the human or bovine immune systems.