Complement and apoptosis.
Apoptosis and necrosis are two forms of cell death characterized by distinct morphologies. Until recently, complement-mediated cell lysis has been presented as a classical example of necrotic cell death. However, recent reports on apoptogenic effects of complement have shaken this dogma. The field has become even more confusing with descriptions of anti-apoptotic effects of complement. Necrosis has been associated traditionally with inflammation, whereas apoptosis has been regarded as noninflammatory. Therefore, first descriptions of the capacity of the complement system to identify apoptotic cells and to be activated by them, led to the development of the concept that complement opsonizes apoptotic cells for fast clearance by phagocytic cells. In the absence of such opsonization, (eg in C1q or C4 deficiency), apoptotic cells may remain longer in the body and may stimulate autoantibody production or undergo pro-inflammatory secondary necrosis. This has been associated in man and mouse with the development of an autoimmune disease like systemic lupus erythematosus. However, complement may also interfere with the programmed intention of apoptosis to avoid triggering of inflammation. Our recent results show that, under specific conditions, early apoptotic cells are not only opsonized by complement but may also be lysed, raising the possibility that under certain conditions apoptosis will be associated with an inflammatory reaction. The review describes and discusses the reports covering the various aspects of the interface between complement and apoptosis and its possible relevance to autoimmune diseases and inflammation and raises the following questions: 1. Can activated complement proteins induce apoptotic cell death? 2. Can complement protect cells from apoptosis? 3. Do apoptotic cells activate complement? and 4. What role is complement playing in clearance of apoptotic cells and which complement receptors are involved? Clearly, this research field is highly complex and still at its formation stage, yet interesting and important mechanisms are bound to emerge from these studies.