Apoptotic Cells, Anti-Phospholipid Antibodies, and Anti-Chromatin Antibodies in Autoimmunity

Abstract

Antiphospholipid antibodies (APAs) are detected in various autoimmune diseases, such as antiphospholipid syndrome (APS) and systemic lupus erythematosus. In addition to their binding to negatively charged phospholipids, APAs often cross-react with other molecules. Their potential biological effects are not fully understood. Apoptotic cells are a potential source of auto-antigens during systemic autoimmunity. Inefficient clearance of apoptotic cells results in the development of autoimmune manifestations and intracellular antigens such as nucleosomes become accessible during apoptosis. We examined a panel of monoclonal APAs generated from NZW/BXSB F1, a strain which spontaneously develops autoimmune symptoms reminiscent of APS. These APAs did not bind to live cells, but reacted strongly with different structures within apoptotic cells. Further analysis with various inhibitors indicated that the binding of APAs to apoptotic cells depends on specific caspase activities and on the modification of auto-antigens by reactive oxygen species (ROS). Therefore, apoptotic cells provide a potential source of APA antigens that may not be limited to phospholipids. Our data also indicate that physical accessibility and apoptosis-specific modification of auto-antigens by caspases or ROS are crucial factors for APA-antigen interactions. Various auto-antibodies such as APAs and anti-chromatin antibodies are pathogenic outcome of chronic autoimmune diseases. Their binding to auto-antigens, presumably exposed on apoptotic cells, elicits subsequent amplification of inflammatory responses, thus worsening disease progression. However, the precise immunological functions of auto-antibodies and the mechanism behind are not fully comprehended yet. We investigated immune responses generated by four different auto-immune complexes (auto-ICs) composed of auto-antibodies and apoptotic cells. In the presence of TLR ligation, the presence of auto-antibodies in auto-ICs amplified immune responses generated by apoptotic cells. In most cases, almost all the auto-ICs tested suppressed IL12, TNFa, while increasing IL10 production from macrophages. Further studies with various anti-Fc?R antibodies implied the essential role of various Fc?Rs in elevation of IL10 by auto-ICs. Studies with Mer-/- macrophages indicated that Mer is also crucial in auto-IC mediated augmentation of IL10 production. However, Mer was dispensable for the suppression of IL12. Taken together, auto-antibodies, by forming immune complexes with apoptotic cells, perform strong immunomodulatory functions. Particular importance is in the role of Fc?Rs and Mer in anti-inflammatory responses generated by auto-ICs. Paradoxical, but indispensible contribution of TLR ligation, especially TLR4, in anti-inflammatory responses generated by auto-ICs suggests that auto-antibodies may work as another layer of defense against endogenous danger signals.