Phospholipid-Bound β2-Glycoprotein I Induces the Production of Anti-Phospholipid Antibodies

Abstract

Apoptotic-cell-bound β2-glycoprotein I (β2GPI), but not apoptotic cells or β2GPI alone, can induce the production of anti-phospholipid (anti-PL) antibodies (Ab) in normal mice. Although it is presumed that β2GPI binds to anionic phospholipid (PL) exposed on the apoptotic cell membrane, the precise nature of this complex and its immunogenicity is unclear. To address these issues, we investigated the structure and immunogenicity of human β2GPI in the presence of different PL that may be expressed on the surface of apoptotic cells. BALB/c mice were immunized intravenously (iv) with β2GPI in the presence of cardiolipin (CL), phosphatidylglycerol (PG), phosphatidylserine (PS), phosphatidylcholine (PC), or PS/PC (25%/75%) vesicles. Cardiolipin+β2GPI induced the highest levels of anti-β2GPI and anti-CL IgG Ab and lupus anticoagulant (LA) activity, while β2GPI with PC or PS/PC vesicles produced no significant anti-PL Ab. PS+β2GPI was somewhat immunogenic, but less so than PG+β2GPI. β2GPI was immunogenic in the presence of native (CLN), but not hydrogenated (CLH), CL. Circular dichroism analysis demonstrated that the structure of β2GPI was altered specifically by interaction with CLN, but not other anionic PL, including CLH. Similarly, the structure of CLNwas affected by interaction with β2GPI, as detected by31P nuclear magnetic resonance. These findings demonstrate that β2GPI complexed with CLNis structurally altered, highly immunogenic, and induces the production of IgG anti-PL Ab. Furthermore, the structural modification and the generation of immunogenic epitopes on β2GPI upon interaction with CLNrequire the presence of unsaturated fatty acid chains, suggesting a role for oxidation in this process.