IntroductionA variety of injuries to microvascular endothelial cells (MVECs) may precipitate episodes of thrombotic thrombocytopenic purpura (TTP) in the setting of deficiency of ADAMTS13 von Willebrand factor (VWF)-cleaving protease activity. 1 However, the specific factors initiating MVEC injury and the mechanism of their lineage specificity are unknown. Autopsy and biopsy studies support the argument that EC lesions characteristic of TTP are primary events, not secondary to microthrombi formation. 2 This injury appears to be apoptotic and restricted to certain lineages of ECs; large vessels are never involved, nor are pulmonary MVECs. 3,4 These pathologic distinctions can be reproduced in vitro by exposure of primary human ECs to plasmas from patients with acute TTP. 4 Cytokines linked to MVEC apoptosis and procoagulant phenotype in vitro are dysregulated during acute TTP in vivo. These include tumor necrosis factor (TNF)-␣ and interleukin (IL)-1. 5 TNF-␣ and IL-1 also induce release of ultralarge VWF multimers from MVECs,6,7 another hallmark of TTP. 1 Plasma levels of TNF-␣ and IL-1 are increased at the onset of disease, with normalization following TTP treatment by plasma exchange. 6,8 In other types of thrombotic microangiopathy, including diarrhea/shigatoxin-associated (D ϩ ) hemolytic uremic syndrome (HUS), significant alterations in these cytokines have not been documented. 9,10 However, levels of IL-1 or TNF-␣ required to induce MVEC apoptosis in vitro are 2-to 3-log-fold greater than those present in TTP plasma.TNF-␣, a related cytokine, TNF-related apoptosis-inducing ligand (TRAIL), and another soluble factor up-regulated during the acute phase of TTP, interferon (IFN)-␥, 8 can interact to elicit apoptosis in several cell types. 11,12 Although the levels required still far exceed those present in TTP, such synergy studies offer insight into modulation of cell-specific cytoprotective pathways that might underlie the differential sensitivity of divergent ECs to injury in TTP.For example, EC viability depends upon several interdependent pathways involving proangiogenic, integrin, and VE-cadherinassociated molecules. 13 These pathways are regulated by NF-B, Akt, and other signaling mechanisms acting through cellular FLICE-like inhibitory protein (c-FLIP), inhibitors of apoptosis (IAP), and Bcl-2. 13,14 Some of these systems may be involved in the EC injury of D ϩ HUS, for which the shiga-like toxins are etiologic agents. Shigatoxins down-regulate Bcl-2 15 and c-FLIP, 16 sensitizing ECs to lipopolysaccharide-induced apoptosis. 15 TNF-␣ and IFN-␥ can facilitate the EC apoptosis-inducing activity of shigatoxin in vitro by up-regulation of shigatoxin receptor CD77 17 and dephosphorylation and subsequent acceleration of ubiquitindependent degradation of Bcl-2 in the proteasome. 18 Yet, levels of TNF-␣ and IFN-␥ required to effect those changes-100 ng/mL TNF-␣ and 200 to 1000 U/mL (20-100 ng/mL) IFN-␥ 15,18 -still far exceed those found in D ϩ HUS or TTP.We had explored the involvement of Bcl-2, Fas, and TNF-␣...