Patients with prolonged clotting times caused by lupus anticoagulant (LAC) are at risk for thrombosis. This paradoxal association is not understood. LAC is frequently caused by anti-beta2-glycoprotein I (beta 2GPI) antibodies. Antibody-induced dimerization of beta 2GPI increases the affinity of beta 2GPI for phospholipids, explaining the observed prolonged clotting times. We constructed dimers of beta 2GPI that mimic effects of beta 2GPI-anti-beta 2GPI antibody complexes, and we studied their effects on platelet adhesion and thrombus formation in a flow system. Dimeric beta 2GPI increased platelet adhesion to collagen by 150% and increased the number of large aggregates. We also observed increased platelet adhesion to collagen when whole blood was spiked with patient-derived polyclonal anti-beta 2GPI or some, but not all, monoclonal anti-beta 2GPI antibodies with LAC activity. These effects could be abrogated by inhibition of thromboxane synthesis. A LAC-positive monoclonal anti-beta 2GPI antibody, which did not affect platelet adhesion, prevented the induced increase in platelet adhesion by beta 2GPI dimers. Furthermore, increased platelet adhesion disappeared after preincubation with receptor-associated protein, a universal inhibitor of interaction of ligands with members of the low density lipoprotein receptor family. Using co-immunoprecipitation, it was shown that dimeric beta 2GPI can interact with apolipoprotein E receptor 2 (apoER2'), a member of the low density lipoprotein receptor family present on platelets. These results demonstrate that dimeric beta 2GPI induces increased platelet adhesion and thrombus formation, which depends on activation via apoER2'.

To cite this article: de Groot PG, Lutters B, Derksen RHWM, Lisman T, Meijers JCM, Rosendaal FR. Lupus anticoagulants and the risk of a first episode of deep venous thrombosis. J Thromb Haemost 2005; 3: 1993-7.Summary. We have determined lupus anticoagulants, anti-b 2 glycoprotein I (b 2 GPI) and antiprothrombin antibodies in the Leiden Thrombophilia Study, a population-based casecontrol study designed to determine risk factors for deep venous thrombosis (DVT). Lupus anticoagulant (LAC) was measured in 473 patients and 472 control subjects. Four control subjects (0.9%) and 14 patients (3.1%) had a positive LAC, resulting in a 3.6-fold increased risk [odds ratio (OR) 3.6, 95% CI: 1.2-10.9]. Of the total population, 49 were positive for anti-b 2 GPI antibodies: 15 controls (3.4%) and 34 patients (7.5%), implying a 2.4-fold increased risk (95% CI: 1.3-4.2). Antiprothrombin antibodies were present in 114 subjects: 48 controls (11.0%) and 66 cases (14.6%) with an OR of 1.4 (95% CI: 1.0-2.1). When LAC was considered in the co-presence of antiprothrombin or anti-b 2 GPI antibodies the OR increased to 10.1 (95% CI: 1.3-79.8). A LAC without a positive anti-b 2 GPI or antiprothrombin test was not associated with a risk for DVT (OR 1.3,. This study demonstrates that the presence of LAC, anti-b 2 GPI antibodies and antiprothrombin antibodies are risk factors for DVT in a general population. The strongest association holds for the combination LAC and the presence of anti-b 2 GPI or antiprothrombin antibodies.

The antiphospholipid syndrome is caused by autoantibodies directed against ␤ 2 -glycoprotein I (␤ 2 GPI). Dimerization of ␤ 2 GPI results in an increased platelet deposition to collagen. We found that apolipoprotein E receptor 2 (apoER2 ), a member of the low density lipoprotein receptor family, is involved in activation of platelets by dimeric ␤ 2 GPI. To identify which domain of dimeric ␤ 2 GPI interacts with apoER2 , we have constructed domain deletion mutants of dimeric ␤ 2 GPI, lacking domain I (⌬I), II (⌬II), or V (⌬V), and a mutant with a W316S substitution in the phospholipid (PL)-insertion loop of domain V. ⌬I and ⌬II prolonged the clotting time, as did full-length dimeric ␤ 2 GPI; ⌬V had no effect on the clotting time. Second, ⌬I and ⌬II bound to anionic PL, comparable with full-length dimeric ␤ 2 GPI. ⌬V and the W316S mutant bound with decreased affinity to anionic PL. Platelet adhesion to collagen increased significantly when full-length dimeric ␤ 2 GPI, ⌬I, or ⌬II (mean increase 150%) were added to whole blood. No increase was found with plasma ␤ 2 GPI, ⌬V, or the W316S mutant. Immunoprecipitation indicated that full-length dimeric ␤ 2 GPI, ⌬I, ⌬II, and the W316S mutant can interact with apoER2 on platelets. ⌬V did not associate with apoER2 . We conclude that domain V is involved in both binding ␤ 2 GPI to anionic PL and in interaction with apoER2 and subsequent activation of platelets. The binding site in ␤ 2 GPI for interaction with apoER2 does not overlap with the hydrophobic insertion loop in domain V.The antiphospholipid syndrome is a non-inflammatory autoimmune disease associated with a wide variety of clinical symptoms. The main clinical features are arterial, venous, or small vessel thrombosis, both early and late pregnancy losses, and pre-eclampsia (1-4). The syndrome is diagnosed when one of the above clinical criteria is accompanied by the persistent presence of antiphospholipid antibodies (aPL) 3 (lupus anticoagulants and anticardiolipin antibodies) in the plasma of patients. These aPL are a heterogeneous group of antibodies directed to plasma proteins with affinity for anionic phospholipids (PL). We now know that the most important plasma protein, to which the aPL are directed, is ␤ 2 -glycoprotein I (␤ 2 GPI or apolipoprotein H) (5, 6).␤ 2 -Glycoprotein I is abundantly present in plasma (ϳ200 g/ml) and is mainly synthesized in the liver, although mRNA coding for ␤ 2 GPI has been found in a variety of cells such as trophoblasts, placental cells, endothelial cells, and neurons (7-9). The mature sequence of human ␤ 2 GPI consists of 326 (44 kDa) amino acids (aa) with four N-linked glycosylation sites. It is composed of five repeating units that belong to the complement control protein family. The first four domains have ϳ60 aa residues and 4 cysteines each, with potential disulfide bridges joining the first to third and the second to fourth cysteines to contribute to a "looped-back " structure, called Sushi domains. The fifth domain is aberrant, having 82 aa and three disulfide bridges. A...