{beta}2 Glycoprotein I ({beta}2GPI) binds platelet factor 4 (PF4): implications for the pathogenesis of antiphospholipid syndrome.

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 beta2 glycoprotein I (beta2GPI). To investigate potential platelet ligands of beta2GPI, platelet membrane proteins from healthy persons and patients with APS were passed through a beta2GPI-affinity column. By using mass spectrometry, platelet factor 4 (PF4) appeared as the dominant beta2GPI binding protein. PF4 could bind in vitro, with high-affinity, recombinant beta2GPI, and the binding was abrogated by soluble beta2GPI. Coprecipitation experiments further confirmed this interaction. In silico molecular docking showed that PF4 tetramers can bind 2 beta2GPI molecules simultaneously. Size exclusion chromatography confirmed that anti-beta2GPI antibodies selectively interact with complexes composed of (beta2GPI)(2)-(PF4)(4). In addition, as shown by the beta2GPI antigenicity evaluation, the reactivity of APS sera was higher against PF4-beta2GPI complex than against beta2GPI alone. On complex formation, anti-beta2GPI-beta2GPI-PF4 significantly induced platelet p38MAPK phosphorylation and TXB2 production, mainly through F(ab')(2) fragments of antibodies. In summary, this study makes evident that beta2GPI forms stable complexes with PF4, leading to the stabilization of beta2GPI dimeric structure that facilitates the antibody recognition. This interaction can probably be involved in the procoagulant tendency of APS.