β2-glycoprotein I (β2-Gp1) is a cardiolipin-binding protein first identified in 1961. It is a 50 kDa glycoprotein with a plasma concentration of 1−4 µM. The physiological function of β2-Gp1 is not known even though it is conserved evolutionarily from invertebrates to humans. Anticardiolipin antibodies are associated with arterial, venous, microvascular thrombosis or with recurrent midtrimester abortion due to placental vascular insufficiency. These antibodies are not specific towards cardiolipin, but to conformational epitopes on proteins bound to cardiolipin. The most common protein antigen is β2-Gp1. High titers of these antibodies are associated with thrombosis and recurrent midtrimester abortion due to placental thrombosis. While a large number of hypotheses have been put forward, the precise mechanism of thrombosis is not known because the physiological function of β2-Gp1 is not known. Mitochondria are organelles that maintain energy supply in cells. Cardiolipin is a universal component of mitochondrial membrane. Its unique physicochemical properties play an important role in the structural organization and function. Recent investigations have shown mitochondria are released during tissue injury and are also present in the extracellular space as an intact organelle or enclosed in vesicle. Extracellular mitochondrial DNA resembles bacterial DNA and present as damage-associated molecular patterns (DAMP), which provoke a proinflammatory response. Our studies propose a novel function for evolutionarily conserved β2-Gp1 as a regulator of extracellular mitochondria levels in circulation and how antiβ2-Gp1 antibodies may lead to a procoagulant state. We show that β2-Gp1 binds to circulating cardiolipin-expressing extracellular mitochondria in a concentration-dependent manner. In vitro, β2-Gp1 promotes mitochondrial uptake and clearance by macrophages and endothelial cells. In vivo, β2-Gp1-deficient mice have increased circulating extracellular mitochondria. We also demonstrate defective clearance due to impaired phagocytosis of infused mitochondria from the circulation in β2-Gp1-deficient mice. We posit that the evolutionarily conserved β2-Gp1 mediates the clearance of circulating cardiolipin-expressing extracellular mitochondria. Our goals in the current proposal are to expand these studies, delving further into the role of β2-Gp1 in the clearance of circulating mitochondria, and to investigate how β2-Gp1 antibodies predispose to thrombosis. These specific aims of this proposal are (1) To further define the role of β2-Gp1 in the clearance of extracellular mitochondria. We will test the hypothesis that β2-Gp1 promotes clearance of cardiolipin-expressing mitochondria by macrophages and/or endothelial cells. (2) To determine the effect of engulfed exogenous mitochondria on macrophage and endothelial functions. (3) To determine the role of antiβ2-Gp1 antibodies in thrombosis seen in antiphospholipid syndrome. We will test the hypothesis that the up...