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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.
Highlights
Research on guinea pigs demonstrated that the bactericidal activity of blood depended on the already described heat-labile alexin, and on a heat-stable bactericidal factor
Matsushita et al detected the proteolytic activity of the MBLassociated serine proteases (MASP-1 and MASP-2), leading to the formation of the classical C3 convertase [8,9,10,11]
Of the C5 convertase initiates the last phase of the complement cascade, which is identical for all three pathways
Summary
Established Pathways Complement activation can occur through three major amplification pathways. The final step toward formation of a stable transmembrane pore with a diameter of 10 angstrom is the binding of 10–15 C9 proteins, which generate a cylindrical structure (Figure 2) Of such a pore may lead to osmotic imbalance through the constant flow of ions, small molecules and water along their concentration gradient, resulting in the lysis of the target cell [26]. C1-INH controls the activity of the classical pathway by binding to the C1 complex and initiating the diffusion of the fragments C1r and C1s. This process leads to an irreversible inactivation of the initiating serine protease. Clinical complications of these reactions can be progressive sepsis and
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