Plasma gelsolin facilitates interaction between β2 glycoprotein I and α5β1 integrin

Antiphospholipid syndrome (APS) is characterized by thrombosis and the presence of antiphospholipid antibodies (aPL) that directly recognizes plasma β(2)-glycoprotein I (β(2) GPI). Tissue factor (TF), the major initiator of the extrinsic coagulation system, is induced on monocytes by aPL in vitro, explaining in part the pathophysiology in APS. We previously reported that the mitogen-activated protein kinase (MAPK) pathway plays an important role in aPL-induced TF expression on monocytes. In this study, we identified plasma gelsolin as a protein associated with β(2) GPI by using immunoaffinity chromatography and mass spectrometric analysis. An in vivo binding assay showed that endogenous β(2) GPI interacts with plasma gelsolin, which binds to integrin a(5) β(1) through fibronectin. The tethering of β(2) GPI to monoclonal anti-β(2) GPI autoantibody on the cell surface was enhanced in the presence of plasma gelsolin. Immunoblot analysis demonstrated that p38 MAPK protein was phosphorylated by monoclonal anti-β(2) GPI antibody treatment, and its phosphorylation was attenuated in the presence of anti-integrin a(5) β(1) antibody. Furthermore, focal adhesion kinase, a downstream molecule of the fibronectin-integrin signalling pathway, was phosphorylated by anti-β(2) GPI antibody treatment. These results indicate that molecules including gelsolin and integrin are involved in the anti-β(2) GPI antibody-induced MAPK pathway on monocytes and that integrin is a possible therapeutic target to modify a prothrombotic state in patients with APS.