## 3 Hemolytic Uremic Syndrome (EHEC and Stx)
Intestinal leukocytosis is more prominent with \(\mathrm{D}+\mathrm{HUS}\) compared to non-EHEC HUS [53], and with Stx in the intestinal epithelial cells [9], there is likely production of pro-inflammatory and chemotactic mediators (IL-1, IL-6, IL-8, TNF\(\alpha\)) from epithelial cells and resident macrophages [54, 55, 56]. Together, this group of chemokines and adhesion molecules serves to recruit and activate neutrophils, contributing to increased tissue damage. Normally endothelial cells present non-thrombogenic surfaces, and both Shiga toxins and inflammatory mediators can shift the endothelium to a pro-thrombotic state, contributing to thrombus formation [57]. For example, Stx2 causes about a 15% decrease in thrombomodulin surface antigen expression of human glomerular endothelial cells _in vitro_, which is further potentiated by TNF\(\alpha\)[58], although whether this amount of antigen loss significantly impacted function was not determined. Thromomodulin is an essential cofactor of the protein C anticoagulant pathway, an assembly of molecules on the endothelium that maintains blood fluidity [59, 60]. Loss of thrombomodulin  
Figure 3: Histopathology of renal injury in nonhuman primates after challenge with Stx1 or Stx2. Light microscopy evaluation of kidney sections from baboons challenged i.v. with different doses of Stx1 or Stx2 reveal dose-dependent kidney injury. (**A**–**C**) Comparison of Stx1 challenges reveals deteriorating glomerular structure, increasing tubular edema and increasing hemorrhage with increasing toxin dose. Clinically, the 10 ng/kg Stx1 dose induced mild and transient effects in the animals, with unremarkable pathology at the light microscopy level. In contrast, a 100 ng/kg Stx1 challenge was lethal within 3–4 days accompanied by fulminant HUS, acute renal failure and systemic inflammatory responses; (**D**) The 50 ng/kg Stx2 dose is comparatively severe and 100% lethal by day 5 post-challenge, also with HUS and renal failure. Both toxins elicited leukocyte infiltration and interstitial hemorrhage. Hematoxylin and eosin staining of paraffin embedded tissue sections; 40\(\times\) magnification. expression, or other members of this pathway, correlates with a pro-coagulant environment [61] and circulating soluble thrombomodulin levels are accepted measures of endothelial injury [62]. However, endothelial and leukocyte activation are probably only part of what induces the formation of microthrombi during D + HUS. Studies have also looked at whether platelet activation might contribute to microthrombi genesis. Plasma from patients with D + HUS has shown evidence of platelet activation and degranulation [63]. Stx1 can activate endothelial cells, especially ones that express more Gb\({}_{3}\) receptor due to TNF\(\alpha\) or LPS priming, to cause increased adhesion of platelets [64] and inhibition of platelet cluster degradation by the inhibition of ADAMTS13 [65]. Collectively, endothelial, platelet and neutrophil activation all contribute to the formation of microthrombi that then becomes a major driving force of D + HUS pathophysiology. Cytokines can disrupt the anti-thrombotic properties of the microvascular endothelial cells and increase cell-surface Gb\({}_{3}\) receptor expression [53]. [PERSON] _et al._ hypothesized previously that the sensitivity of endothelial cells to Shiga toxins depended greatly on their source and levels of Gb\({}_{3}\) expression, and found that while TNF\(\alpha\) and lipopolysaccharide-LPS could increase sensitivity of human umbilical vein endothelial cells to Shiga toxins, they had no effect on renal microvascular endothelial cell Gb\({}_{3}\) expression or Stx sensitivity [26]. It has been shown that the numbers of polymorphonuclear leukocytes present at the beginning of D + HUS is predictive of outcome, but not in cases of atypical HUS [66], a related but distinct syndrome (see below).