IntroductionThe antiphospholipid syndrome (APS) is characterized by arterial or venous thrombosis and/or recurrent fetal loss in the presence of antiphospholipid antibodies (APLAs). [1][2][3] It is now widely accepted that the majority of pathologic antibodies in patients with this disorder are actually directed against phospholipid-binding proteins, the most common of which is  2 -glycoprotein I ( 2 GPI).The pathogenesis of APS-associated thrombosis is multifactorial, and a number of mechanisms have been proposed. 4 These include inhibition of protein C activation and activity, 5,6 inhibition of annexin V assembly on exposed phospholipid surfaces, 7 and prevention of appropriate interactions of antithrombin with glycosaminoglycans, 8 among others. 4,9 Studies from our laboratory and others suggest that  2 GPI-dependent activation of vascular cells by APLA/anti- 2 GPI antibodies plays a central role in disease pathogenesis 10,11 and may initiate the cascade of events that leads to thrombus development. For example,  2 GPI binds to endothelial cell annexin A2, and subsequent cross-linking of annexin A2-bound  2 GPI initiates endothelial cell activation through a pathway that may involve Toll-like receptor 4 (TLR-4) [11][12][13] and leads to activation of NF-B. 14 A similar pathway may be functional in monocytes, activation of which also contributes to the development of thrombosis in patients with APLA. 15 In addition, APLA may promote platelet activation in the presence of subthreshold concentrations of agonists, 16 although whether this is a receptor-mediated process, and if so, its relationship to platelet  2 GPI binding sites, such as GP1b 17 and apoER2, 18 requires further study.In endothelial cells, activation of NF-B stimulates an inflammatory and procoagulant response 19 and plays a critical role in the ability of APLA to promote thrombosis. 20 Thus, modulation of NF-B activity may provide an opportunity to reverse the pathologic vascular response in APS. The Krüppel-like factors (KLFs), 21 particularly KLF2 and KLF4, inhibit inflammatory cytokinemediated responses in endothelial cells, 22,23 at least in part through inhibition of NF-B activity. 23 However, expression of KLF2 itself may be inhibited by inflammatory cytokines and/or vascular injury, [23][24][25] although the expression of KLF4 appears to be increased under these conditions. 26 In considering the importance of NF-B in endothelial cell activation mediated by APLA/anti- 2 GPI antibodies 14 and the potentially opposing effects of KLF2 and KLF4, we hypothesized that changes in expression of these transcription factors might influence the endothelial cell response to APLA/anti- 2 GPI antibodies. Here, we report that, unlike responses to inflammatory cytokines, the expression of both KLF2 and KLF4 is decreased in response to APLA/anti- 2 GPI antibodies. Moreover, restoring the expression of these KLFs blocks endothelial cell activation in response to APLA/anti- 2 GPI antibodies. These activities result from inhibition of NF-B transcrip...