Complement: A nostalgic journey

Coagulation and complement: Key innate defense participants in a seamless web

In 1969, Dr. Oscar Ratnoff, a pioneer in delineating the mechanisms by which coagulation is activated and complement is regulated, wrote, “In the study of biological processes, the accumulation of information is often accelerated by a narrow point of view. The fastest way to investigate the body’s defenses against injury is to look individually at such isolated questions as how the blood clots or how complement works. We must constantly remind ourselves that such distinctions are man-made. In life, as in the legal cliché, the devices through which the body protects itself form a seamless web, unwrinkled by our artificialities.” Our aim in this review, is to highlight the critical molecular and cellular interactions between coagulation and complement, and how these two major component proteolytic pathways contribute to the seamless web of innate mechanisms that the body uses to protect itself from injury, invading pathogens and foreign surfaces.

The utility of complement assays in clinical immunology: A comprehensive review

The multi-tasking organ liver, which is the major synthesis site of most serum proteins, supplies humoral components of the innate, - including proteins of the complement system; and, less intensely, also of the acquired immune system. In addition to hepatocyte origins, C1q, factor D, C3, C7 and other protein components of the complement system are produced at various body locations by monocytes/macrophages, lymphocytes, adipocytes, endometrium, enterocytes, keratinocytes and epithelial cells; but the contribution of these alternate sites to the total serum concentrations is slight. The two major exceptions are factor D, which cleaves factor B of the alternative pathway derived largely from adipocytes, and C7, derived largely from polymorphonuclear leukocytes and monocytes/macrophages. Whereas the functional meaning of the extrahepatic synthesis of factor D remains to be elucidated, the local contribution of C7 may up- or downregulate the complement attack. The liver, however, is not classified as part of the immune system but is rather seen as victim of autoimmune diseases, a point that needs apology. Recent histological and cell marker technologies now turn the hands to also conceive the liver as proactive autoimmune disease catalyst. Hosting non-hepatocytic cells, e.g. NK cells, macrophages, dendritic cells as well as T and B lymphocytes, the liver outreaches multiple sites of the immune system. Immunopharmacological follow up of liver transplant recipients teaches us on liver-based presence of ABH-glycan HLA phenotypes and complement mediated ischemia/regeneration processes. In clinical context, the adverse reactions of the complement system can now be curbed by specific drug therapy. This review extends on the involvement of the complement system in liver autoimmune diseases and should allow to direct therapeutic opportunities.

Complement deficiency and systemic lupus erythematosus: consensus and dilemma

The involvement of the complement system in the pathogenesis of autoimmune diseases is a matter of debate. However, the link between complement abnormalities and systemic lupus erythematosus (SLE) is well established and widely described. Homozygous and/or heterozygous complement-component deficiencies of the classical pathway (C1q, C1r, C1s, C4A, C4B and C2) are causally associated with susceptibility to the development of SLE. Although the severity of the disease and the strength of the association are heterogeneous for deficiencies of these proteins, they commonly cause peculiar SLE syndromes with an early age of onset, a susceptibility to bacterial infections and negative anti-dsDNA antibodies. In this review, we highlight the available data on complement deficiency and SLE with a focus on deficiencies in classical complement pathway components. We also discuss the paradox of the link between complement deficiency and lupus. The complement system acts as a 'friend' through the clearance of immune complexes and apoptotic cells, which explains the close association between complement deficiency and lupus. It also acts as an 'enemy' by participating in the effector inflammatory phase of the autoimmune response. Understanding the importance of complement deficiencies should provide novel targets for therapeutic interventions in the modulation of the immune response.