Arterial thrombosis in the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is a non-inflammatory autoimmune disease that mainly affects young women. The syndrome is characterized by recurrent thrombosis or pregnancy morbidity in association with the persistent serological presence of antiphospholipid antibodies. Antiphospholipid antibodies are heterogeneous and recognize a wide variety of protein ligands, all of which have phospholipid binding properties. The plasma protein beta2-glycoprotein I (beta2-GPI) is considered the most relevant antigen in the syndrome. Although the consensus is that antiphospholipid antibodies increase the risk of a thrombotic event, the extent of this increase in risk is unclear. We therefore assessed the antiphospholipid antibody-related risk of myocardial infarction or ischemic stroke in women below age 50. Although several antiphospholipid antibody subpopulations are reported to be associated with an increased risk of thrombosis, only the lupus anticoagulant subpopulation was associated with an increased risk of myocardial infarction (5-fold increased risk) or ischemic stroke (43-fold increased risk). The presence of additional risk factors such as smoking or oral contraceptive use increased the risk of both myocardial infarction or ischemic stroke further. While lupus anticoagulant appeared to be a strong risk factor for myocardial infarction in young women, we could not confirm these results in men below age 70. Our understanding of the molecular mechanisms through which antiphospholipid antibodies exert their prothrombotic influence has increased over the years. Antiphospholipid antibodies are known to induce endothelial cell activation, cause increased platelet activation and influence the coagulation system. Several cellular receptors are postulated to mediate the prothrombotic effects of antiphospholipid antibodies. One of these receptors is Apolipoprotein E receptor 2’ (ApoER2’), a member of the low density lipoprotein-receptor family. Previous work has shown antiphospholipid antibody-beta2-GPI complexes bind to both ApoER2’ and the platelet adhesive receptor glycoprotein Ibalpha (gpIbalpha) with high affinity, which mediates increased thrombus formation in in-vitro flow models. We further investigated the role of both gpIbalpha and ApoER2’ in antiphospholipid antibody induced platelet activation. Upon stimulation of platelets with antiphospholipid antibodies or a recombinant dimer of beta2GPI that mimics the function of antiphospholipid antibodies, intracellular signalling that originated from both gpIbalpha and ApoER2’ was observed. Inhibition of the interaction of dimers of beta2-GPI with either receptor completely abrogated platelet activation in an in-vitro flow model, despite continued signalling through the uninhibited receptor. These results suggest both receptors are crucial in the mechanism behind platelet activation in the antiphospholipid syndrome. The role of ApoER2 in the thrombotic manifestations of the antiphospholipid syndrome was further investigated in a murine thrombosis model. Intraperitoneal administration of dimers of beta2-GPI or patient-derived antiphospholipid antibodies to wild-type mice resulted in increased thrombus formation upon induction of vascular injury, increased vascular tissue factor activity and increased monocytic tissue factor activity compared with mice injected with buffer. Administration of dimers of beta2-GPI or purified antiphospholipid antibodies to ApoER2-deficient mice caused normal thrombus formation after vascular injury, normal vascular tissue factor activity and normal monocyte activation, which confirms the importance of ApoER2 in the chain of events leading to thrombosis in the antiphospholipid syndrome