BETA-2-GLYCOPROTEIN-I IS A MAJOR PROTEIN ASSOCIATED WITH VERY RAPIDLYCLEARED LIPOSOMES IN-VIVO, SUGGESTING A SIGNIFICANT ROLE IN THE IMMUNE CLEARANCE OF NON-SELF PARTICLES

Liposomes recovered from the blood of liposome-treated CD1 mice were p reviously reported to have a complex protein profile associated with t heir mem branes (Chonn, A., Semple, S. C., and Cullis, P. R. (1992) J. Biol. Chem. 267, 18759-18765). In this study, we have further charact erized and identified the major proteins associated with very rapidly cleared large unilamellar vesicles. These liposomes contained phosphat idylcholine, cholesterol, and anionic phospholipids (phosphatidylserin e, phosphatidic acid, or cardiolipin) that dramatically enhance the cl earance rate of Liposomes hom the circulation. These anionic phospholi pids are normally found exclusively in the interior of cells but becom e expressed when cells undergo apoptosis or programmed cell death, and thus, they are believed to be markers of cell senescence, Analysis of the proteins associated with these liposomes by SDS-polyacrylamide ge l electrophoresis revealed that two of the major proteins associated w ith the liposome membranes are proteins with electrophoretic mobilitie s corresponding to M(r) of 66,000 and 50,000-55,000. The 66-kDa protei n was identified to be serum albumin by immunoblot analysis. Using var ious biochemical and immunological methods, we have identified the 50- 55-kDa protein as the murine equivalent of human beta(2)-glycoprotein I. beta 2-glycoprotein I has a strong affinity for phosphatidylserine, phosphatidic acid, and cardiolipin inasmuch as the levels of beta 2-g lycoprotein I associated with these anionic liposomes approach or even exceed those of serum albumin, which is present in serum at a concent ration 200-fold greater than beta 2-glycoprotein I. Further, we demons trate that the amount of beta 2-glycoprotein I associated with liposom es, as quantitated by an enzyme-linked immunosorbent assay, is correla ted with their clearance rates; moreover, the circulation residency ti me of cardiolipin-containing liposomes is extended in mice pretreated with anti-beta 2-glycoprotein I antibodies. These findings strongly su ggest that beta 2-glycoprotein I plays a primary role in mediating the clearance of liposomes and, by extension, senescent cells and foreign particles.