New insights of an old defense system: structure, function, and clinical relevance of the complement system

Abstract

The complement system was discovered a century ago as a potent defense cascade of innate immunity. After its first description, continuous experimental and clinical research was performed, and three canonical pathways of activation were established. Upon activation by traumatic or surgical tissue damage, complement reveals beneficial functions of pathogen and danger defense by sensing and clearing injured cells. However, the latest research efforts have provided a more distinct insight into the complement system and its clinical subsequences. Complement has been shown to play a significant role in the pathogenesis of various inflammatory processes such as sepsis, multiorgan dysfunction, ischemia/reperfusion, cardiovascular diseases and many others. The three well-known activation pathways of the complement system have been challenged by newer findings that demonstrate direct production of central complement effectors (for example, C5a) by serine proteases of the coagulation cascade. In particular, thrombin is capable of producing C5a, which not only plays a decisive role on pathogens and infected/damaged tissues, but also acts systemically. In the case of uncontrolled complement activation, "friendly fire" is generated, resulting in the destruction of healthy host tissue. Therefore, the traditional research that focuses on a mainly positive-acting cascade has now shifted to the negative effects and how tissue damage originated by the activation of the complement can be contained. In a translational approach including structure-function relations of this ancient defense system, this review provides new insights of complement-mediated clinical relevant diseases and the development of complement modulation strategies and current research aspects.

Figure 1

Scheme of the similar structure of complement activation molecules C1q of the classical pathway, MBL of the lectin pathway and L-ficolin.

Figure 2

Scheme of MAC with its subdivisions and the two seven-transmembrane–spanning G-protein–coupled receptors C3aR and C5aR. The corresponding ligand (C3a/C5a) is displayed above each receptor.

Figure 3

The established pathways of complement activation associated with the new activation pathway and crosstalk between cells and the coagulation cascade with the complement system.

Figure 4

Scheme of the membrane bound and soluble complement regulators acting on different stages of the complement cascade.

Figure 5

Scheme of diseases in which complement has a major impact. ALS, amyotrophic lateral sclerosis; ALI, acute lung injury; ARDS, adult respiratory distress syndrome; MPGN, membranoproliferative glomerulonephritis; SLE, systemic lupus erythematosus; aHUS, atypical hemolytic uremic syndrome; MODS, multiple organ dysfunction syndrome. Adapted by permission from Macmillan Publishers Ltd: Ricklin D, Lambris JD. (2007) Complement-targeted therapeutics. Image reprinted with permission from Nat. Biotechnol. 25:1265–75.