Another Piece in the Antiphospholipid Antibody Syndrome Puzzle

Most patients with the pro-thrombotic condition known as antiphospholipid antibody (APLA) syndrome have circulating autoantibodies reactive with the phospholipid binding plasma protein β₂-glycoprotein I (β₂-GPI). Although β₂-GPI circulates at very high concentrations (3μM) and has been studied for many years, its biologic function remains obscure. In this manuscript, a team from the Netherlands has uncovered a potential role for β₂ -GPI in regulating primary hemostasis. Using two well-established ex vivo assays for platelet glycoprotein Ib complex-dependent binding to von Willebrand Factor (vWF) (ristocetin-induced platelet aggregation and shear stress induced adhesion of resting platelets to immobilized vWF), they found that adding increasing concentrations of β₂-GPI inhibited binding to a moderate, but statistically significant, degree. They then used in vitro binding assays to demonstrate that purified β₂-GPI binds directly to vWF with affinity well within the range of typical plasma levels. Importantly, β₂-GPI did not bind to "native" vWF, but only to the form rendered competent to bind GPIb induced when vWF is subjected to shear stress or incubated with ristocetin. Recombinant vWF proteins lacking the GPIB-binding A1 domain did not bind β₂-GPI while a mutant A1 domain found in patients with type IIb von Willebrand Disease (vWD) that is constitutively competent to bind GPIb bound β₂-GPI even in the absence of shear or ristocetin. To relate their findings to the APLA syndrome, the authors showed that anti-β₂-GPI antibodies, either monoclonal or isolated from patients’ sera, blocked binding of β₂-GPI to "active" vWF and blocked the inhibitory effect of β₂-GPI on platelet function. Furthermore, antibodies to β₂-GPI actually enhanced ristocetin-induced aggregation of platelet-rich plasma. Finally, the authors detected the "active" form of vWF in the circulation of patients with APLA syndrome with levels modestly, but significantly, higher in patients with history of thrombosis.

The APLA syndrome is a common and important acquired pro-thrombotic state unusual among the thrombophilias in that it is associated with both arterial and venous thrombosis and with spontaneous pregnancy loss. The syndrome is misnamed in that the autoantibodies circulating in patients do not react with phospholipids per se, but rather with various phospholipid-binding proteins, including β₂-GPI and prothrombin. By far the most commonly found reactivity is with β₂-GPI. Studies with animal models and human clinical studies linking antibody titer with thrombosis risk show that the pathophysiology of APLA syndrome is mediated by the circulating autoantibodies, but the precise molecular mechanisms by which these antibodies induce thrombosis is not clear. In fact, multiple potential mechanisms have been described. Some antibodies may have a direct prothrombotic effect by binding to and activating endothelial cells, leukocytes, and/or trophoblast cells. Some may have an indirect effect by preferentially inhibiting phospholipid-dependent anticoagulant pathways (for example, protein C) or by activating complement. In this new work, the authors propose a novel indirect effect related to the ability of β₂-GPI to act as a circulating antagonist of the GPIb complex, the platelet vWF receptor, and thereby modulate platelet adhesion to exposed subendothelial matrix at the site of a vascular injury. APLA antibodies, by blocking this natural "brake" on the system, create a prothrombotic state, perhaps akin to type IIB vWD or even TTP. (The unprocessed ultralarge vWF multimers seen in TTP also bind GPIb in the absence of ristocetin or shear.)