Clearance of dying cells and systemic lupus erythematosus: the role of C1q and the complement system.

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease of unknown etiology characterized by the presence of pathogenic high-titer autoantibodies to a diverse group of autoantigens. In 88% of patients, autoantibodies are present an average of 3.3 years before diagnosis. Antinuclear, anti-Ro, anti-La, and anti-phospholipid antibodies appear first, followed by anti-DNA, anti-Smith and anti-ribonucleoprotein. These autoantibodies have features of an antigen-driven, T-cell-dependent immune response. Once present, the course of SLE is characterized by disease flares and autoimmune dysregulation. Programmed cell death (PCD), an essential developmental and homeostatic mechanism, is the preferred physiological death processes for cells as well as an important immune response regulator. Appropriate clearance of apoptotic material completes the PCD process, and is essential for regulating of inflammation and maintaining self-tolerance. Early complement proteins are important in protecting humans against the development of SLE and the protective role of C1q and complement in SLE is mainly related to their role in clearance of dying cells. However, the complement system is also an important ingredient in inflammation, which mediates SLE pathogenesis. Thus, the question remains whether complement factors have either a protective or a destructive role, or a combination of both.