## 2 Distinguishing the Thrombic Microangiopathy of EHEC-Related HUS  
Hemolytic uremic syndrome (HUS) is a clinical syndrome comprising the triad of thrombocytopenia, thrombotic microangiopathy, and hemolytic anemia, and is frequently associated with acute kidney injury. Pathogenic challenge that results in endothelial injury or dysfunction shifts the hemostatic balance toward a pro-thrombotic environment, resulting in thrombotic microangiopathy with inappropriate deposition of clots in microvascular beds causing tissue ischemia. The kidney is a particular target during EHEC infection in part because the toxin receptors are expressed at higher density on glomerular endothelial cells [26] and toxin activities induce endothelial expression of adhesion molecules that support interactions with activated platelets and leukocytes [27] which contribute significantly to clot formation [28]. Development of a thrombotic microangiopathy can arise as a complication of bacterial infections like EHEC, from genetic mutations in complement regulatory pathways (atypical HUS, aHUS), from deficiencies of regulatory ADAMTS13 enzyme (thrombotic thrombocytopenia purura, TTP) or even secondarily as a consequence of autoimmune diseases such as systemic lupus erythematosus. These pathways to HUS have different etiologies but frequently overlapping clinical presentation (Figure 1). Thrombolic microangiopathy can result from different molecular pathways. Hemolytic uremic syndrome (HUS), atypical HUS (aHUS) and thrombotic thrombocytopenia pur pur purura (TTP) have shared clinical manifestations, but differing molecular etiologies. EHEC-related HUS initiated by bacterial Stx injures endothelial cells by inducing endoplasmic reticulum (ER) stress responses and transcription events which include generation of inflammatory cytokines and chemokines. Endothelial injury and a pro-thrombotic environment in aHUS results from genetic mutations in complement pathway members and aberrant activation (complement factors H, IB, 3: CFH, CFI, CFB, C3). Coagulopathy during TTP results from inherited or immune-acquired deficiency in a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13), needed to cleave von Willebrand Factor (VWF) released from endothelial cells to prevent accumulation of prothrombotic ultra-large VWF (UL-VWF) oligomers. Thrombolic microangiopathy is a shared consequence and is related to, but distinct from, disseminated intravascular coagulation (DIC), a consumptive coagulopathy. The coagulopathy of HUS is distinguished from DIC by several parameters, including a normal or elevated fibrinogen level and normal or slightly elevated clotting times because coagulation factors are not consumed (Figure 2). Markers of fibrinolysis (fibrin degradation products, D-dimer) are only modestly increased in HUS relative to DIC, but measures of microangiopathic hemolytic anemia (MAHA) such as schistocytes are markedly increased. Additionally, the renal failure of DIC is attributable to acute tubular necrosis, whereas that associated with HUS is predominately a thrombotic microangiopathy of glomerular vasculature, with or without tubular necrosis. Figure 1: Thrombic Microangiopathy can result from different molecular pathways. Hemolytic uremic syndrome (HUS), atypical HUS (aHUS) and thrombotic thrombocytopenia pur purura (TTP) have shared clinical manifestations, but differing molecular etiologies. EHEC-related HUS initiated by bacterial Stx injures endothelial cells by inducing endoplasmic reticulum (ER) stress responses and transcription events which include generation of inflammatory cytokines and chemokines. Endothelial injury and a pro-thrombotic environment in aHUS results from genetic mutations in complement pathway members and aberrant activation (complement factors H, IB, 3: CFH, CFI, CFB, C3).