## 11 Rodent Models of Stx Challenge  
Although the pathology of human HUS can be mimicked in nonhuman primates through the administration of Stx alone, a small animal model of HUS would provide many benefits and be accessible to more investigators. C67 BL/6J mice, when given only multiple sub-lethal doses of Stx2 and no pre-treatments, develop some but not all aspects of human HUS [113]. Significantly increased plasma BUN and creatinine levels and proteinuria reflect glomerular renal injury. Hemolysis, neutrophila and lymphocytopenia were observed. Histologically, the kidneys showed fibrin (ogen) deposition in the glomerular capillary loops and swollen subendothelial zones containing flocculent material [113]. Thrombopenia, however, was not observed and lack of this HUS symptom appears to be common to all Stx murine models unless co-treated with an accessory pro-inflammatory mediator. Modeling HUS using mice challenged with EHEC toxins has been described using co-administration of Gram negative endotoxin (a.k.a., lipopolysaccharide, LPS) and Stx2 [123], a model that has been used recently by multiple investigators. This approach was based on the hypothesis that LPS from the commensal flora of the gut could potentially contribute to the pathogenesis of HUS. However, LPS had either a synergistic or protective effect, depending on the timing of administration. [PERSON] _et al._ found that when C57 BL/6 mice were given low sublethal doses of both LPS and Stx2 concurrently, they exhibited all signs of clinical HUS [29]. Neutrophila, thrombocytopenia, red cell hemolysis and increases in serum creatinine and BUN were all observed. In the kidney, glomerular fibrin deposition, microthrombi formation and glomerular ultrastructure changes were demonstrated by histology and electron microscopy [29]. Priming of the Stx system in this way with LPS, TNF\(\alpha\), ADP or similar potentiates toxin chemotactic and inflammatory responses _in vitro_ and _in vivo_[29, 117], but the relevance to disease pathogenesis in patients is not clear. In experimental settings, addition of LPS priming to Stx2 challenge in mice will induce thrombocytopenia, but this etiology is a consumptive disseminated intravascular coagulation [124, 125] rather than HUS and they have different coagulation profiles. This differentiation was discussed by [PERSON] _et al._ decades ago [17]. Additionally, the notion that compromised intestinal barrier function allows bacterial translocation or LPS leakage from commensal or pathogenic bacteria is far from established. This theory has been discussed with variable levels of support since the 1940s with respect to bacterial sepsis mechanisms and is proven to be poorly related to the patient condition, particularly for endotoxemia [126]. Some patients with EHEC O157:H7 infection have antibodies (usually IgM or IgA) against the bacterial LPS serotype [127, 128] but detection in healthy control sera as well as cross-reactivity with other LPS serotypes adds complexity [129], and there is no prospective evidence showing these antibodies are protective or prognostic. In non-EHEC bacterial sepsis, circulating _E. coli_ LPS is detectable in patients, but targeted neutralization of Gram negative LPS has consistently failed to improve 28 day all-cause mortality [130, 131]. The current thinking is that the primary contribution of the intestine during these infections comes from host cellular injury with resultant host-derived mediators to propagate local and systemic inflammation effects [132, 133]. Rats were studied as a Shiga toxemia model by [PERSON] _et al._ in 2008 to evaluate their ability to mimic development of HUS.