Interaction of β 2-Glycoprotein I with lipopolysaccharide leads to toll-like receptor 4 (TLR4)-dependent activation of macrophages

Abstract

β 2-Glycoprotein I (β 2GPI) is an abundant plasma protein that binds to the surface of cells and particles expressing negatively charged lipids, but its physiological role remains unknown. Antibodies to β 2GPI are found in patients with anti-phospholipid syndrome, a systemic autoimmune disease associated with vascular thrombosis and pregnancy morbidity. Although it has been suggested that anti- β 2GPI antibodies activate endothelial cells and monocytes by signaling through TLR4, it is unclear how anti-β 2GPI antibodies and/or β 2GPI interact with TLR4. A number of mammalian proteins (termed "endogenous Toll-like receptor (TLR) ligands") have been reported to bind to TLR4, but, in most cases, subsequent studies have shown that LPS interaction with these proteins is responsible for TLR activation. We hypothesized that, like other endogenous TLR ligands, β 2GPI interacts specifically with LPS and that this interaction is responsible for apparent TLR4 activation by β 2GPI. Here, we show that both LPS and TLR4 are required for β 2GPI to bind to and activate macrophages. Untreated β 2GPI stimulated TNF-α production in TLR4-sufficient (but not TLR4-deficient) macrophages. In contrast, neither polymyxin B-treated nor delipidated β 2GPI stimulated TNF-α production. Furthermore, β 2GPI bound to LPS in a specific and dose-dependent manner. Finally, untreated β 2GPI bound to the surface of TLR4-sufficient (but not TLR4-deficient) macrophages. Polymyxin B treatment of β 2GPI abolished macrophage binding. Our findings suggest a potential new biological activity for β 2GPI as a protein that interacts specifically with LPS and point to the need to evaluate newly discovered endogenous TLR ligands for potential interactions with LPS.