The epitopes for some antiphospholipid antibodies are adducts of oxidized phospholipid and β₂glycoprotein 1 (and other proteins)

Abstract

Circulating autoantibodies to phospholipids (aPLs), such as cardiolipin (CL), are found in patients with antiphospholipid antibody syndrome (APS). We recently demonstrated that many aPLs bound to CL only after it had been oxidized (OxCL), but not to a reduced CL analogue that could not undergo oxidation. We now show that the neoepitopes recognized by some aPLs consist of adducts formed between breakdown products of oxidized phospholipid and associated proteins, such as ␤ 2 glycoprotein 1 (␤ 2 GP1). Addition of human ␤ 2 GP1, polylysine, native low-density lipoprotein, or apolipoprotein AI to OxCL-coated wells increased the anticardiolipin antibody (aCL) binding from APS sera that first had been diluted so that no aCL binding to OxCL could be detected. No increase in aCL binding was observed when these proteins were added to wells coated with reduced CL. The ability of ␤ 2 GP1, polylysine, or low-density lipoprotein to be a ''cofactor'' for aCL binding to OxCL was greatly reduced when the proteins were methylated. Incubation of ␤ 2 GP1 with oxidized 1-palmitoyl-2-linoleyl-[1-14 C]-phosphatidylcholine (PC), but not with dipalmitoyl-[1-14 C]-PC, led to formation of covalent adducts with ␤ 2 GP1 recognized by APS sera. These data suggest that the reactive groups of OxCL, such as aldehydes generated during the decomposition of oxidized polyunsaturated fatty acids, form covalent adducts with ␤ 2 GP1 (and other proteins) and that these are epitopes for aCLs. Knowledge that the epitopes recognized by many aPLs are adducts of oxidized phospholipid and associated proteins, including ␤ 2 GP1, may give new insights into the pathogenic events underlying the clinical manifestations of APS.

Autoantibodies in systemic autoimmune diseases: specificity and pathogenicity

“…These receptors are listed in Table 1 and include DNA, RNA, citrullinated fibrinogen, cardiac myosin, cardiolipin, and oxidated phospholipids (51)(52)(53)(54)(55)(56).…”

Inhibition of APC anticoagulant activity on oxidized phospholipid by anti–β2-glycoprotein I monoclonal antibodies

“…29,32,38,39 This would imply that antibody binding would also stabilize the lipid binding conformer of ␤ 2 -GPI, leading to enhanced binding even in the absence of antibody divalency. To distinguish these possibilities, and because the membrane requirements for APC inhibition by APAs differ from those for LA activity, 3 Fab fragments of mAb no.…”

“…29,32 However, the impact of phospholipid oxidation on inhibition of APC activity by anti-␤ 2 -GPI antibodies has not been reported. We have now used mouse monoclonal antibodies (mAbs) raised to purified ␤ 2 -GPI as a model for antibodies likely to be present in thrombotic antiphospholipid syndrome patients.…”

Structurally similar oxidized phospholipids differentially regulate endothelial binding of monocytes and neutrophils

“…Furthermore, the presence of antibodies to oxidized phospholipids in patients with atherosclerosis, diabetes, hypertension, antiphospholipid syndrome, preeclampsia, and other chronic diseases underlines the potential importance of these molecules (9)(10)(11). We previously have identified two biologically active oxidized phospholipids present in MM-LDL and oxidized PAPC (OxPAPC) as 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphor ylcholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphorylcholine (PGPC) that activate endothelial cells to bind monocytes (12).…”