Phagocytosis of platelet microvesicles and β2-glycoprotein I

The majority of the antiphospholipid antibodies, present in patients with antiphospholipid syndrome, are directed against conformational epitopes in beta 2-glycoprotein I. beta 2-glycoprotein I is an anionic phospholipid-binding 50-kDa plasma protein whose physiological role is not clear. Here we investigate the role of beta 2-glycoprotein I in the phagocytosis of phosphatidylserine-expressing platelet microvesicles and the effect of autoantibodies to beta 2-glycoprotein I on this process. We labelled the glycans of beta 2-glycoprotein I with BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-hydrazide without affecting its phospholipid binding capacity. BODIPY-beta 2-glycoprotein I bound to platelet microvesicles in a concentration-dependent manner and promoted the phagocytosis of platelet microvesicles by THP-1 derived macrophages in vitro at physiological plasma concentrations with a half maximal effect at similar to 10 mu g/ml. beta 2-glycoprotein I-stimulated phagocytosis was inhibited by annexin A5 and the phosphatidylserine-binding C1C2 fragment of lactadherin. Furthermore, immunoaffinity purified beta 2-glycoprotein I-dependent antiphospholipid antibodies from five patients with antiphospholipid syndrome inhibited the phagocytosis in a concentration-dependent manner. These studies suggest that the binding of beta 2-glycoprotein I to phosphatidylserine-expressing procoagulant platelet microvesicles may promote their clearance by phagocytosis and autoantibodies to beta 2-glycoprotein I may inhibit this process to induce a procoagulant state.