title: Interaction of β2-glycoprotein I with lipopolysaccharide leads to Toll-like receptor 4 (TLR4)-dependent activation of macrophages.
abstract: β(2)-Glycoprotein I (β(2)GPI) 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(2)GPI 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(2)GPI antibodies activate endothelial cells and monocytes by signaling through TLR4, it is unclear how anti-β(2)GPI antibodies and/or β(2)GPI 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(2)GPI interacts specifically with LPS and that this interaction is responsible for apparent TLR4 activation by β(2)GPI. Here, we show that both LPS and TLR4 are required for β(2)GPI to bind to and activate macrophages. Untreated β(2)GPI stimulated TNF-α production in TLR4-sufficient (but not TLR4-deficient) macrophages. In contrast, neither polymyxin B-treated nor delipidated β(2)GPI stimulated TNF-α production. Furthermore, β(2)GPI bound to LPS in a specific and dose-dependent manner. Finally, untreated β(2)GPI bound to the surface of TLR4-sufficient (but not TLR4-deficient) macrophages. Polymyxin B treatment of β(2)GPI abolished macrophage binding. Our findings suggest a potential new biological activity for β(2)GPI 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.
authors.affiliation: Division of Rheumatology, Department of Medicine, Research Institute of the McGill University Health Centre, Montreal, Quebec H3G 1A4, Canada.
authors.forename: Patrick
authors.initials: P
authors.lastname: Laplante
authors.affiliation: Division of Rheumatology, Department of Medicine, Research Institute of the McGill University Health Centre, Montreal, Quebec H3G 1A4, Canada.
authors.forename: Pascal
authors.initials: P
authors.lastname: Amireault
authors.affiliation: Division of Rheumatology, Department of Medicine, Research Institute of the McGill University Health Centre, Montreal, Quebec H3G 1A4, Canada.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Subang
authors.affiliation: Division of Rheumatology, Department of Medicine, Research Institute of the McGill University Health Centre, Montreal, Quebec H3G 1A4, Canada.
authors.forename: Mélanie
authors.initials: M
authors.lastname: Dieudé
authors.affiliation: Section of Nephrology, Department of Medicine, University of Illinois at Chicago, Chicago, Illinois 60680; Section of Nephrology, Department of Medicine, Jesse Brown Veterans Affairs Medical Center, Chicago, Illinois 60612.
authors.forename: Jerrold S
authors.initials: JS
authors.lastname: Levine
authors.affiliation: Division of Rheumatology, Department of Medicine, Research Institute of the McGill University Health Centre, Montreal, Quebec H3G 1A4, Canada. Electronic address: joyce.rauch@mcgill.ca.
authors.forename: Joyce
authors.initials: J
authors.lastname: Rauch
mesh_terms: D000818:Animals; D004731:Endotoxins / Q000378:metabolism; D005260:Female; D005786:Gene Expression Regulation; D007113:Immunity, Innate; D008024:Ligands; D008055:Lipids / Q000737:chemistry; D008070:Lipopolysaccharides / Q000737:chemistry / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008856:Microscopy, Fluorescence / Q000379:methods; D011112:Polymyxin B / Q000494:pharmacology; D051197:Toll-Like Receptor 4 / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism; D053482:beta 2-Glycoprotein I / Q000378:metabolism*
chemical_list: D004731:Endotoxins; D008024:Ligands; D008055:Lipids; D008070:Lipopolysaccharides; D051197:Toll-Like Receptor 4; D014409:Tumor Necrosis Factor-alpha; D053482:beta 2-Glycoprotein I; D011112:Polymyxin B