Phospholipid-bound β2-glycoprotein I induces the production of anti-phospholipid antibodies

Apoptotic-cell-bound beta 2-glycoprotein I (beta 2GPI), but not apoptotic cells or beta 2GPI alone, can induce the production of anti-phospholipid (anti-PL) antibodies (Ab) in normal mice. Although it is presumed that beta 2GPI 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 beta 2GPI in the presence of different PL that may be expressed on the surface of apoptotic cells. BALB/c mice were immunized intravenously (iv) with beta 2GPI in the presence of cardiolipin (CL), phosphatidylglycerol (PG), phosphatidylserine (PS), phosphatidylcholine (PC), or PS/PC (25%/75%) vesicles. Cardiolipin+beta 2GPI induced the highest levels of anti-beta 2GPI and anti-CL IgG Ab and lupus anticoagulant (LA) activity, while beta 2GPI with PC or PS/PC vesicles produced no significant anti-PL Ab. PS+beta 2GPI was somewhat immunogenic, but less so than PG+beta 2GPI. beta 2GPI was immunogenic in the presence of native (CLN), but not hydrogenated (CLH), CL. Circular dichroism analysis demonstrated that the structure of beta 2GPI was altered specifically by interaction with CLN, but not other anionic PL, including CLH. Similarly, the structure of CLN was affected by interaction with beta 2GPI, as detected by P-31 nuclear magnetic resonance. These findings demonstrate that beta 2GPI complexed with CLN is structurally altered, highly immunogenic, and induces the production of IgG anti-PL Ab. Furthermore, the structural modification and the generation of immunogenic epitopes on beta 2GPI upon interaction with CLN require the presence of unsaturated fatty acid chains, suggesting a role for oxidation in this process. (C) 2000 Academic Press.