## 3 Hemolytic Uremic Syndrome (EHEC and Stx)  
Over 90% of HUS cases are associated with EHEC infection, also referred to as D + HUS, to indicate that there was diarrhea before the development of HUS. HUS from infection with other pathogens, usually _Streptococcus pneumoniae_ (SP-HUS) which exposes the Thomsen-Freidenreich antigen with autoimmune consequences, results in longer hospitalization and long-term kidney injury [31, 32]. A diarral prodrome can be observed in non-EHEC HUS cases, albeit rarely [33]. Patients usually develop a prodromal hemorrhagic colitis after EHEC contaminated food or water has been ingested [34], with HUS developing after approximately 7-10 days [35]. It is widely acknowledged that the Shiga-like toxins produced by the enterohemorrhagic _E. coli_ are the main virulence factors driving the organ damage in patients and animal models. EHEC secrete Shiga-like toxin-1 (Stx1) and/or Shiga-like toxin 2 (Stx2), and there are multiple Stx2 variants [36]. The two major toxins are structurally similar with a shared AB\({}_{5}\) domain structure, but only 56% amino acid homology. Stx are ribosome-inactivating toxins, similar to Shiga toxin from _Shigella dysenteriae_ serotype-1 [37] and ricin from castor beans. The enzymatic A subunit and a cell binding B subunit that organizes into pentamers recognize a globotriaosylceramide (Gb\({}_{3}\)) membrane receptor on cells, particularly glomerular endothelial cells [38]. Internalized toxin-receptor complexes undergo retrograde transport to the endoplasmic reticulum via the Golgi apparatus where the A subunit _N_-glycosidase activity removes an adenine from 28S ribosomal RNA to inhibit protein synthesis [39]. A recent study shows Stx1 intracellular trafficking is mediated by Golgi protein GPP130 and disruption of this interaction leads to lysosomal degradation of Stx1 and protection of mice from lethal toxemia [40]. The toxins also induce inflammation in patients [41] and animal models [42], and endoplasmic reticulum stress-induced transcriptional events are stimulated in susceptible cells [43, 44]. The Shiga toxins target Gb\({}_{3}\)-rich tubular epithelium, as well as the glomerular endothelial cells, with cellular apoptosis, necrosis and thrombotic microangiopathy contributing to the acute kidney failure seen in patients [45]. In patients, the renal pathology as a consequence of EHEC infection includes cortical necrosis, glomerular thromboses and congestion with widened subendothelial space, endothelial cell swelling, neutrophil, and occasional mesangialysis [46, 47]. In the baboon model of Stx challenge which presents with all manifestations of HUS [48], glomerular injury after Stx1 and Stx2 challenge featured prominent HUS-like capillary wall changes, including thickening of the glomerular basement membrane and double contouring with prominent thrombus formation, and severe endothelial injury with diffuse cell swelling and focal endothelial denudation (Figure 3). Because this is an animal model allowing comparisons between toxins, some differences emerged with glomerular endothelial injury predominating after Stx1, whereas mesangialysis and eosinophilic infiltration accompanied the glomerular injury in the Stx2-challenged animals [49]. Endothelial activation by the Shiga-like toxins induces an inflammatory milieu that is a strong contributor to the thrombi formation in the microvasculature during development of HUS. Stx2 upregulates chemokines monocyte chemotactic protein-1 (MCP-1, CCL2) and IL-8 (CCL8) [27, 42] and Stx1 increases expression of cellular adhesion molecules ICAM-1, VCAM-1, and E-selectin on endothelial cells [50]. In human monocytes, EHEC infection and Shiga toxins induces inflammatory cytokines and chemokines including IL-1\(\beta\), IL-6, IL-8, and TNF\(\alpha\)[51]. In the GI of patients, the colon is severely affected, showing edema and hemorrhage, consistent with the hemorrhagic colitis that isoften seen preceding HUS [52].