Induction of anti-phospholipid autoantibodies by beta2-glycoprotein I bound to apoptotic thymocytes

The target of many anti-phospholipid autoantibodies (aPL) has been shown to be a complex between anionic phospholipid and the plasma protein beta2-glycoprotein I (beta2GPI) or the protein beta2GPI alone. As aPL binding studies have been performed almost exclusively in vitrothe identity of the natural target and/or immunogen for aPL in vivo remains undetermined. The anionic phospholipids of cell membranes represent an important potential target and immunogen for aPL. Although anionic phospholipids are normally absent from the extracellular surface of cell membranes, they redistribute from the inner to the outer leaflet during apoptosis. We have previously shown that beta2GPI binds selectively to the surface of apoptotic, but not viable, cells, and that binding of beta2GPI to the surface of apoptotic cells generates an epitope recognized by aPL from patients with primary aPL syndrome and systemic lupus erythematosus. We show here that immunization of non-autoimmune mice with beta2GPI combined with, or bound to, apoptotic cells induces aPL and lupus anticoagulant activity. Generation of aPL required heterologous beta2GPI, and occurred upon immunization with apoptotic cells and beta2GPI by three different routes of administration. Importantly, for intravenous immuniz-ations, generation of aPL occurred only when apoptotic cells and beta2GPI were injected together, but not when either was injected alone, suggesting that cell-bound beta2GPI is the true immunogen for production of aPL. Unlike other models of induced aPL, adjuvant was not an absolute requirement. Induced aPL reacted with murine, as well as bovine, beta2GPI, suggesting that heterologous beta2GPI bound to apoptotic cells can break tolerance and induce auto-antibodies reactive with autologous beta2GPI. Combined with our previous data, these results show that apoptotic cells can serve as both immunogens and natural targets for aPL.