The antiphospholipid syndrome is defined by the presence of antiphospholipid antibodies in blood of patients with thrombosis or fetal loss. There is ample evidence that ␤ 2 -glycoprotein I (␤ 2 GPI) is the major antigen for antiphospholipid antibodies. The autoantibodies recognize ␤ 2 GPI when bound to anionic surfaces and not in solution. We showed that ␤ 2 GPI can exist in at least 2 different conformations: a circular plasma conformation and an "activated" open conformation. We also showed that the closed, circular conformation is maintained by interaction between the first and fifth domain of ␤ 2 GPI. By changing pH and salt concentration, we were able to convert the conformation of ␤ IntroductionThe antiphospholipid syndrome (APS) is defined as the presence of antiphospholipid antibodies in the blood of patients with thrombosis or fetal loss. The APS is one of the most common causes of acquired thrombophilia, 1 especially at younger age. In 1990, it was shown that the so-called antiphospholipid antibodies do not recognize phospholipids directly but they interact with phospholipids via the plasma protein ␤ 2 -glycoprotein I (␤ 2 GPI). [2][3][4] However, the discovery of ␤ 2 GPI as target for the autoantibodies did not provide a more in-depth knowledge on the underlying cause of the syndrome. It was unclear which metabolic pathway was disturbed by the autoantibodies because no physiologic function has convincingly been ascribed to ␤ 2 GPI to date. Nevertheless, as antibodies against ␤ 2 GPI can induce thrombosis in animal models, 5-7 the protein ␤ 2 GPI must hold an important functional clue to our understanding of the syndrome.␤ 2 GPI is a highly abundant 43-kDa protein that circulates at a concentration of approximately 200 g/mL. ␤ 2 GPI consists of 326 amino acids arranged in 5 short consensus repeat domains. 8,9 The first 4 domains contain 60 amino acids each, whereas the fifth domain has a 6-residue insertion and an additional 19-amino acid C-terminal extension. The extra amino acids are responsible for the formation of a large positively charged patch within the fifth domain of ␤ 2 GPI 10 that forms the binding site for anionic phospholipids. The anti-␤ 2 GPI antibodies that recognize an epitope located in the first domain correlate better with the thrombotic complications than antibodies directed against other domains. 11-13 Antibodies directed against ␤ 2 GPI have become one of the serologic markers characterizing the APS. 14 After binding to anionic surfaces, ␤ 2 GPI exposes a cryptic epitope that is recognized by the autoantibodies present in the APS. 11,12,15,16 These antibodies only recognize ␤ 2 GPI when it is bound to a surface and do not recognize ␤ 2 GPI in solution. Moreover, no circulating immune complexes between antibodies and ␤ 2 GPI have been detected in patient plasmas. 17 This seems not to be the result of clearance of these complexes from plasma because plasma levels of ␤ 2 GPI in antiphospholipid patients are the same as plasma levels of ␤ 2 GPI in healthy persons. 18 The crystal st...

Background Generation of active procoagulant cofactor FVa and its subsequent association with the enzyme FXa to form the prothrombinase complex is a pivotal initial event in blood coagulation and has been the subject of investigative effort, speculation and controversy. The current paradigm assumes that FV activation is initiated by limited proteolysis by traces of (meizo) thrombin. Methods and Results Recombinant tick salivary protein TIX-5 was produced and anticoagulant properties were studied using plasma, whole blood and purified systems. Here we report that TIX-5 specifically inhibits FXa-mediated FV activation involving the B-domain of FV and show that FXa activation of FV is pivotal for plasma and blood clotting. In line, tick feeding is impaired on TIX-5 immune rabbits displaying the in vivo importance of TIX-5. Conclusions Our data elucidate a unique molecular mechanism by which ticks inhibit the host's coagulation system. Based on our data we propose a revised blood coagulation scheme wherein direct FXa-mediated FV activation occurs in the initiation phase during which thrombin-mediated FV activation is restrained by fibrinogen and inhibitors.