The Role of β2-Glycoprotein I in the Clearance of Platelet Microvesicles.

Abstract 145

The physiological function of β2-glycoprotein I is unclear and several studies suggest a role in the clearance of anionic phospholipid containing membranes. Anionic phospholipid containing liposomes are cleared rapidly from the circulation by the reticuloendothelial cells. In rats, uptake of liposomes by Kupffer cells requires that the liposomes bind to plasma proteins. In mice, the clearance of liposomes from the circulation is related to their ability to interact with plasma proteins. β2-glycoprotein I was identified as a major protein associated with rapid clearance of liposomes and pretreating the mice with antiβ2- glycoprotein I antibodies was found to significantly increase the half-life of the liposome. In vitro, β2-glycoprotein I was also shown to promote the phagocytosis of phosphatidylserine containing liposomes and apoptotic tumor cells. In conditions associated with increased microvesicles generation such as disseminated intravascular coagulation, plasma levels of β;2-glycoprotein I was reduced presumably due to its consumption.

Antibodies to β2 glycoprotein I are frequently seen in patients with systemic lupus erythematosus and at times, in otherwise normal individuals. A subset of these antibodies prevents the assembly of the prothrombinase and the tenase complexes on phospholipid membrane, leading to the lupus anticoagulant effect. The presence of these antibodies is clinically very significant, as individuals harboring these antibodies are at risk for thromboembolic manifestations. We studied the role of β-glycoprotein I in the clearance of procoagulant platelet microvesicles and the effect of the auto antibodies in the phagocytosis of platelet microvesicles.

We labeled β2-glycoprotein I with BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-hydrazide and β2-glycoprotein I incorporated 1.8 mole of BODIPY /mole. Labeling of β2-glycoprotein I with BODIPY did not change the binding efficacy of β2-glycoprotein I to cardiolipin as determined by Elisa assay. Binding of BODIPY-β2-glycoprotein I to platelet microvesicles was analyzed by flow cytometry. BODIPY- β2-glycoprotein I bound to phosphatidylserine-expressing platelet microvesicles in a concentration-dependent manner. Binding was inhibited by 50 fold molar excess of unlabeled β2-glycoprotein I, annexin A5 and the phosphatidylserine-binding C1C2 fragment of lactadherin. β2-glycoprotein I also promoted the phagocytosis of platelet microvesicles by THP-1 derived macrophages in vitro at physiological plasma concentrations with a half maximal effect at ∼10 ug/ml. β2-glycoprotein I-mediated phagocytosis was inhibited by annexin V and the C1C2 fragment of lactadherin. Furthermore, immunoaffinity purified β2-glycoprotein I-dependent antiphospholipid antibodies from 5 patients inhibited the phagocytosis in a concentration dependent manner.

These studies suggest β2-glycoprotein I binding to phosphatidylserine-expressing procoagulant platelet microvesicles promotes their clearance by macrophages and autoantibodies to β2-glycoprotein I inhibit the process. The predictive value of antiβ-2 glycoprotein I for thrombosis is highly variable but the correlation is stronger in patients with lupus. In lupus, there is impaired clearance of procoagulant apoptotic cells. β2-glycoprotein I may have a significant role in their clearance and antibodies to β2-glycoprotein I may causally related to the thrombosis in these patients by inhibiting the clearance.