β2-Glycoprotein I promotes the binding of anionic phospholipid vesicles by macrophages

beta(2)-Glycoprotein I is a single-chain 50-kDa protein that circulates in plasma at a concentration of approximate to 200 mu g/mL. Its physiological role remains uncertain, but an important clue is the frequent presence of antibodies to this protein in patients with recurrent thrombosis, We have isolated beta(2)-glycoprotein I and examined its effect on the binding of phosphatidylserine (PS) vesicles by human monocyte-derived macrophages and by phorbol ester-stimulated THP-1 cells. beta(2)-Glycoprotein I stimulated the binding of PS vesicles by these cells in a concentration-dependent manner. Vesicles containing other anionic phospholipids, such as cardiolipin, phosphatidic acid, or cardiolipin, inhibited the binding, whereas PC vesicles had no effect. Platelet-derived microvesicles, which contain anionic phospholipid on the outer leaflet of their phospholipid bilayer, also inhibited beta(2)-glycoprotein I-dependent binding of anionic phospholipid vesicles. The binding is associated with incorporation of phospholipid in the cell membrane and internalization of beta(2)-glycoprotein I. These findings suggest a physiological function for beta(2)-glycoprotein I in the clearance of procoagulant anionic phospholipid-containing cell surfaces from the circulation.