• β2-glycoprotein I;
  • antiphospholipid syndrome;
  • apolipoprotein E receptor 2′ (apoER2′);
  • glycoprotein Ibα;
  • platelet adhesion

Summary. Background: The major antigen implicated in the antiphospholipid syndrome is beta2-glycoprotein I (β2GPI). Dimerized β2GPI binds to apolipoprotein E receptor 2′ (apoER2′) on platelets and increases platelet adhesion to collagen under conditions of flow. Aim: To investigate whether the interaction between dimerized β2GPI and platelets is sufficiently strong to resist shear stresses. Methods: We studied the interaction of platelets with immobilized dimerized β2GPI under conditions of flow, and further analyzed the interaction using surface plasmon resonance and solid phase immunoassays. Results: We found that dimerized β2GPI supports platelet adhesion and aggregate formation under venous flow conditions. Adhesion of platelets to dimerized β2GPI was completely inhibited by the addition of soluble forms of both apoER2′ and GPIbα, and the addition of receptor-associated protein and the removal of GPIbα from the platelet surface. GPIbα co-precipitated with apoER2′, suggesting the presence of complexes between GPIbα and apoER2′ on platelet membranes. The interaction between GPIbα and dimeric β2GPI was of intermediate affinity (Kd = 180 nm) and Zn2+, but not Ca2+-dependent. Deletion of domain V from dimeric β2GPI strongly reduced its binding to both GPIbα and apoER2′. Antibodies that inhibit the binding of thrombin to GPIbα inhibited platelet adhesion to dimeric β2GPI completely, while antibodies blocking the binding of von Willebrand factor to GPIbα had no effect. Dimeric β2GPI showed reduced binding to low-sulfated GPIbα compared to the fully sulfated form. Conclusion: We show that platelets adhere to dimeric β2GPI under both arterial and venous shear stresses. Platelets adhere via two receptors: GPIbα and apoER2′. These receptors are present in a complex on the platelet surface.