Pathophysiology of Anti-PL Syndrome in Thrombosis.

Abstract SCI-45

The antiphospholipid (aPL) syndrome (APS) is an enigmatic autoimmune thrombotic disorder that was initially identified through astute clinical observations and the development of a quantitative test for aPL antibodies. The condition is marked by 2 types of assays: 1) immunoassays that were derived from the “biologic false positive syphilis” test and 2) coagulation assays that detect “lupus anticoagulants” (LAs), which are inhibitors of phospholipid-dependent coagulation reactions. The most recent consensus investigational criteria for diagnosing APS require that patients have evidence for thrombosis and/or pregnancy complications attributable to placental insufficiency and also laboratory evidence for persistent (aPL) antibodies, detected by high levels of IgG or IgM antibodies against cardiolipin and β2-glycoprotein I (β2GPI) and/or through abnormal LA assays. The thrombotic disorder requires long term anticoagulant treatment, which is accompanied by the risk of bleeding complications. Several mechanisms have been postulated and can be classified as involving: 1) aPL antibody-mediated inhibition of endogenous anticoagulant mechanisms; 2) aPL antibody-triggered signaling events on target cells (vascular endothelial cells, monocytes, platelets and trophoblasts) that promote proadhesive and prothrombotic phenotypes; and 3) aPL antibody-mediated complement activation. At present, it appears that the APS diagnostic entity actually includes several distinct subsets that reflect the actions of heterogeneous antibodies directed against different epitopes of phospholipid-binding proteins which then may yield different clinical sequelae. We have accumulated significant data indicating that a major one of these mechanisms involves aPL antibody-mediated disruption of annexin A5 (AnxA5) activity. AnxA5 is a potent anticoagulant protein whose anticoagulant properties are a consequence of its high affinity for anionic phospholipid. The protein forms 2-dimensional crystals on phospholipid surfaces that shield the phospholipids from availability for coagulation reactions. AnxA5 appears to play a thrombomodulatory role on the surfaces of cells lining the placental and systemic vasculatures. It is highly expressed on the apical membranes of placental syncytiotrophoblasts, the location where maternal blood interfaces with fetal cells. aPL antibody-antigen complexes disrupt the ordered crystallization of AnxA5, displace the protein from phospholipid membrane surfaces and thereby accelerate coagulation reactions. This effect of the antibodies has been demonstrated on artificial bilayers, on cultured trophoblasts and on endothelial cells and platelets. This disruption has been appears to be a consequence of aPL antibodies that recognize a specific epitope within domain I of β2GPI, the key antigen recognized by aPL antibodies. Based upon these data, we developed a novel clinical assay “the AnxA5 resistance (A5R) test” to identify patients who have antibodies that interfere with the anticoagulant activity of AnxA5. Initial studies indicate that a large proportion of APS patients have evidence for A5R. It therefore appears possible that measurement for A5R may provide a mechanistic assay for APS. We are also developing treatments to target this mechanism and protect AnxA5 from antibody-mediated disruption which may open novel nonanticoagulant approaches to treating APS.