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The complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology. It starts with a description of complement contribution to the normal physiology (homeostasis) of a healthy organism, including the silent clearance of apoptotic cells and maintenance of cell survival. In pathology, complement can be a friend or a foe. It acts as a friend in the defense against pathogens, by inducing opsonization and a direct killing by C5b–9 membrane attack complex and by triggering inflammatory responses with the anaphylatoxins C3a and C5a. Opsonization plays also a major role in the mounting of an adaptive immune response, involving antigen presenting cells, T-, and B-lymphocytes. Nevertheless, it can be also an enemy, when pathogens hijack complement regulators to protect themselves from the immune system. Inadequate complement activation becomes a disease cause, as in atypical hemolytic uremic syndrome, C3 glomerulopathies, and systemic lupus erythematosus. Age-related macular degeneration and cancer will be described as examples showing that complement contributes to a large variety of conditions, far exceeding the classical examples of diseases associated with complement deficiencies. Finally, we discuss complement as a therapeutic target.

Key Points• Heme activates complement alternative pathway in serum and on endothelial cell surfaces.• Heme-induced complement activation in the presence of complement mutations contributes as a secondary hit to the development of aHUS.Atypical hemolytic uremic syndrome (aHUS) is characterized by genetic and acquired abnormalities of the complement system leading to alternative pathway (AP) overactivation and by glomerular endothelial damage, thrombosis, and mechanical hemolysis. Mutations per se are not sufficient to induce aHUS, and nonspecific primary triggers are required for disease manifestation. We investigated whether hemolysis-derived heme contributes to aHUS pathogenesis. We confirmed that heme activates complement AP in normal human serum, releasing C3a, C5a, and sC5b9. We demonstrated that hemeexposed endothelial cells also activate the AP, resulting in cell-bound C3 and C5b9. This was exacerbated in aHUS by genetic abnormalities associated with AP overactivation. Heme interacted with C3 close to the thioester bond, induced homophilic C3 complexes, and promoted formation of an overactive C3/C5 convertase. Heme induced decreased membrane cofactor protein (MCP) and decay-accelerating factor (DAF) expression on endothelial cells, giving Factor H (FH) a major role in complement regulation. Finally, heme promoted a rapid exocytosis of Weibel-Palade bodies, with membrane expression of P-selectin known to bind C3b and trigger the AP, and the release of the prothrombotic von Willebrand factor. These results strongly suggest that hemolysis-derived heme represents a common secondary hit amplifying endothelial damage and thrombosis in aHUS. (Blood. 2013;122(2):282-292) IntroductionAtypical hemolytic uremic syndrome (aHUS) is a rare kidneypredominant thrombotic microangiopathy (TMA) associated with formation of fibrin-platelet clots in the glomerular microvasculature leading to mechanical hemolysis. 1 This disease is related to a dysregulation of the complement alternative pathway (AP), as shown by the identification of genetic or acquired abnormalities in AP regulators or activators in more than 60% of patients. 2 aHUS has an incomplete penetrance among mutation carriers, and a triggering event is presumably required for the disease manifestation. This primary hit can be an infection, pregnancy, drug, or other cell-activating event. Sera from aHUS patients carrying mutations deposit complement on endothelial cells activated by inflammatory cytokines, 3 suggesting continuous aggression on the endothelium. However, not all infections trigger aHUS in patients. It is frequently the second pregnancy postpartum period that is associated with the disease.4 Therefore, other factors appear to be necessary to exceed the threshold of tolerable endothelial stress leading to severe TMA lesions.In acute phase TMA, mechanical hemolysis induces the release of hemoglobin into the bloodstream. 5 This hemoglobin is readily oxidized to ferric hemoglobin (methemoglobin) which, in turn, liberates heme. In physiological conditions...

Complement alternative pathway plays an important, but not clearly understood, role in neutrophil-mediated diseases. We here show that neutrophils themselves activate complement when stimulated by cytokines or coagulation-derived factors. In whole blood, tumor necrosis factor/ formyl-methionyl-leucyl-phenylalanine or phorbol myristate acetate resulted in C3 fragments binding on neutrophils and monocytes, but not on T cells. Neutrophils, stimulated by tumor necrosis factor, triggered the alternative pathway on their surface in normal and C2-depleted, but not in factor B-depleted serum and on incubation with purified C3, factors B and D. This occurred independently of neutrophil proteases, oxidants, or apoptosis. Neutrophil-secreted properdin was detected on the cell surface and could focus "in situ" the alternative pathway activation. Importantly, complement, in turn, led to further activation of neutrophils, with enhanced CD11b expression and oxidative burst. Complement-induced neutrophil activation involved mostly C5a and possibly C5b-9 complexes, detected on tumor necrosis factor-and serum-acti- IntroductionNeutrophils and the complement alternative pathway (AP) are major effectors of cell-mediated and humoral innate immunity. The analysis of complement knockout mice, in experimental inflammatory diseases, shed new light on the participation of complement AP in neutrophil-mediated diseases, such as rheumatoid arthritis, 1 membranoproliferative glomerulonephritis, 2 ischemia-reperfusion injury, 3 and, more recently, antineutrophil cytoplasmic autoantibody (ANCA)-associated small-vessel vasculitis. 4 In an experimental model of ANCA-associated small-vessel vasculitis, the unexpected observation that factor B-deficient mice were protected from the disease, whereas C4-deficient mice were not, 4 revealed the alternative complement pathway as a new partner of this neutrophilmediated disease. It was thus reasonable to propose that neutrophils could participate in the activation of complement AP.If the activation of neutrophils by complement fragments, such as C3a or C5a, is well known, data on complement triggering by neutrophils are scarce. Neutrophil proteases and oxidants have been reported to activate complement in cell-free systems, 5-7 and supernatants of ANCA-activated neutrophils were shown to release unknown complement activating factors. 4 We investigated the ability of the neutrophil surface to activate complement and deposit active complement fragments on their plasma membrane.Complement activation on blood cells is kept under control by fluid phase regulators (ie, plasma factor H and C4b-binding protein) and by cell membrane regulators (ie, decay accelerating factor DAF [CD55] and membrane cofactor protein MCP [CD46]). These regulators prevent the formation and accelerate the decay of C3bBb and C4b2a alternative and classic C3-convertases and act as cofactors for factor I-mediated C3b and C4b proteolysis. Neutrophils also express CR1 (CD35), which is the only cofactor allowing factor I to fully degrade ...

Our results suggest that the mTORC pathway is involved in the vascular lesions associated with the antiphospholipid syndrome. (Funded by INSERM and others.).

Complement is a major innate immune defense against pathogens, tightly regulated to prevent host tissue damage. Atypical hemolytic uremic syndrome (aHUS) is characterized by endothelial damage leading to renal failure and is highly associated with abnormal alternative pathway regulation. We characterized the functional consequences of

Atypical hemolytic uremic syndrome (aHUS) is a rare renal thrombotic microangiopathy commonly associated with rare genetic variants in complement system genes, unique to each patient/family. Here, we report 14 sporadic aHUS patients carrying the same mutation, R139W, in the complement C3 gene. The clinical presentation was with a rapid progression to end-stage renal disease (6 of 14) and an unusually high frequency of cardiac (8 of 14) and/or neurologic (5 of 14) events. Although resting glomerular endothelial cells (GEnCs) remained unaffected by R139W-C3 sera, the incubation of those sera with GEnC preactivated with proinflammatory stimuli led to increased C3 deposition, C5a release, and procoagulant tissue-factor expression. This functional consequence of R139W-C3 resulted from the formation of a hyperactive C3 convertase. Mutant C3 showed an increased affinity for factor B and a reduced binding to membrane cofactor protein (MCP; CD46), but a normal regulation by factor H (FH). In addition, the frequency of at-risk FH and MCP haplotypes was significantly higher in the R139W-aHUS patients, compared with normal donors or to healthy carriers. These genetic background differences could explain the R139W-aHUS incomplete penetrance. These results demonstrate that this C3 mutation, especially when associated with an at-risk FH and/or MCP haplotypes, becomes pathogenic following an inflammatory endothelium-damaging event. (Blood. 2012;119(18):4182-4191)

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