ω-carboxyl variants of 7-ketocholesteryl esters are ligands for β₂-glycoprotein I and mediate antibody-dependent uptake of oxidized LDL by macrophages

β₂-Glycoprotein I (β₂-GPI) is a major antigen for anticardiolipin antibodies (aCL, Abs) present in patients with antiphospholipid syndrome. We recently reported that β₂-GPI specifically binds to oxidized LDL (oxLDL) and that the β₂-GPI's major ligand, oxLig-1 is 7-ketocholesteryl-9-carboxynonanoate (Kobayashi, K., E. Matsuura, Q. P. Liu, J. Furukawa, K. Kaihara, J. Inagaki, T. Atsumi, N. Sakairi, T. Yasuda, D. R. Voelker, and T. Koike. 2001. A specific ligand for β₂-glycoprotein I mediates autoantibody-dependent uptake of oxidized low density lipoprotein by macrophages. J. Lipid Res. 42: 697-709). In the present study, we demonstrate that ω-carboxylated 7-ketocholesteryl esters are critical for β₂-GPI binding. A positive ion mass spectrum of a novel ligand, designated oxLig-2, showed fragmented ions at m/z 383 and 441 in the presence of acetone, which share features of oxLig-1 and 7-ketocholesterol. In the negative ion mode, ions at m/z 627, 625, and 243 were observed. oxLig-2 was most likely 7-ketocholesteryl-12-carboxy (keto) dodecanoate. These ligands were recognized by β₂-GPI. Liposome binding to macrophages was significantly increased depending on the ligand's concentration, in the presence of β₂-GPI and an anti-β₂-GPI Ab. Synthesized variant, 7-ketocholesteryl-13-carboxytridecanoate (13-COOH-7KC), also showed a significant interaction with β₂-GPI and a similar binding profile with macrophages. Methylation of the carboxyl function diminished all of the specific ligand interactions with β₂-GPI. Thus, ω-carboxyl variants of 7-ketocholesteryl esters can mediate anti-β₂-GPI Ab-dependent uptake of oxLDL by macrophages, and autoimmune atherogenesis linked to β₂-GPI interaction with oxLDL.