β2 glycoprotein I (β2GPI) binds platelet factor 4 (PF4): Implications for the pathogenesis of antiphospholipid syndrome

Abstract

Antiphospholipid syndrome (APS) is an autoimmune thrombophilia characterized by arterial/venous thrombosis and/or pregnancy morbidity in the presence of antiphospholipid antibodies that mainly recognize β2 glycoprotein I (β2GPI). To investigate potential platelet ligands of β2GPI, platelet membrane proteins from healthy persons and patients with APS were passed through a β2GPI-affinity column. By using mass spectrometry, platelet factor 4 (PF4) appeared as the dominant β2GPI binding protein. PF4 could bind in vitro, with high-affinity, recombinant β2GPI, and the binding was abrogated by soluble β2GPI. Coprecipitation experiments further confirmed this interaction. In silico molecular docking showed that PF4 tetramers can bind 2 β2GPI molecules simultaneously. Size exclusion chromatography confirmed that anti-β2GPI antibodies selectively interact with complexes composed of (β2GPI)2-(PF4)4. In addition, as shown by the β2GPI antigenicity evaluation, the reactivity of APS sera was higher against PF4-β2GPI complex than against β2GPI alone. On complex formation, anti-β2GPI-β2GPI-PF4 significantly induced platelet p38MAPK phosphorylation and TXB2 production, mainly through F(ab′)2 fragments of antibodies. In summary, this study makes evident that β2GPI forms stable complexes with PF4, leading to the stabilization of β2GPI dimeric structure that facilitates the antibody recognition. This interaction can probably be involved in the procoagulant tendency of APS. © 2010 by The American Society of Hematology.