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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1999;19:2807.)
© 1999 American Heart Association, Inc.

Thrombosis

ß₂-Glycoprotein I Promotes the Binding of Anionic Phospholipid Vesicles by Macrophages

Perumal Thiagarajan; Anhquyen Le; Claude R. Benedict

From the Divisions of Hematology (P.T., A.L.) and Cardiology (C.R.B.), Department of Internal Medicine, University of Texas Health Sciences Center, Houston.

Correspondence to Perumal Thiagarajan, MD, University of Texas Health Sciences Center, 6431 Fannin St, MSB 5.284, Houston, TX 77030. E-mail perumal{at}heart.med.uth.tmc.edu

Abstract—ß₂-Glycoprotein I is a single-chain 50-kDa protein that circulates in plasma at a concentration of 200 µg/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 ß₂-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. ß₂-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 ß₂-glycoprotein I–dependent binding of anionic phospholipid vesicles. The binding is associated with incorporation of phospholipid in the cell membrane and internalization of ß₂-glycoprotein I. These findings suggest a physiological function for ß₂-glycoprotein I in the clearance of procoagulant anionic phospholipid-containing cell surfaces from the circulation.

Key Words: lupus anticoagulant • antiphospholipid antibody • ß₂-glycoprotein I • anionic phospholipid vesicles

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