Activation complement

Sharma A. Identification of three physically and functionally distinct binding sites for C3b in human complement factor H by deletion mutagenesis. Pangburn M. Host recognition and target differentiation by factor H, a regulator of the alternative pathway of complement. Liszewski M. Control of the complement system. Kim D. Membrane complement regulatory proteins. Clin Immunol. Conway E. Thrombomodulin and its role in inflammation. Semin Immunopathol. Delvaeye M. Mutations in thrombomodulin in hemolytic-uremic syndrome.

Bajzar L. Purification and characterization of TAFI, a thrombin-activable fibrinolysis inhibitor. Campbell W. Inactivation of C3a and C5a octapeptides by carboxypeptidase R and carboxypeptidase N. Microbiol Immunol. Podack E. The SC5b-7 complex: formation, isolation, properties, and subunit composition.

Tschopp J. Clusterin, the human apolipoprotein and complement inhibitor, binds to complement C7, C8 beta, and the b domain of C9. Heinen S. Goicoechea de Jorge E. Dimerization of complement factor H-related proteins modulates complement activation in vivo. Lalli P. Locally produced C5a binds to T cell-expressed C5aR to enhance effector T-cell expansion by limiting antigen-induced apoptosis.

Vieyra M. Novel aspects of complement in kidney injury. Kidney Int. Strainic M. Kwan W. Signaling through C5a receptor and C3a receptor diminishes function of murine natural regulatory T cells.

Gueler F. Complement 5a receptor inhibition improves renal allograft survival. J Am Soc Nephrol. Zhou W. Intrarenal synthesis of complement. Song D. Compartmental localization of complement component transcripts in the normal human kidney.

C1q and systemic lupus erythematosus. Homozygous C1q deficiency causes glomerulonephritis associated with multiple apoptotic bodies. Nat Genet. Brooimans R. Interleukin 2 mediates stimulation of complement C3 biosynthesis in human proximal tubular epithelial cells.

Sacks S. C3 and C4 gene expression and interferon-gamma-mediated regulation in human glomerular mesangial cells. Interferon-gamma regulation of C4 gene expression in cultured human glomerular epithelial cells.

Complement C3 gene expression and regulation in human glomerular epithelial cells. Sheerin N. TNF-alpha regulation of C3 gene expression and protein biosynthesis in rat glomerular endothelial cells. Kulics J. Counterregulatory effects of interferon-gamma and endotoxin on expression of the human C4 genes. Falus A. Constitutive and IL 1-regulated murine complement gene expression is strain and tissue specific.

New boundaries for complement in renal disease. Thurman J. Abe K. The membrane attack complex, C5b-9, up regulates collagen gene expression in renal tubular epithelial cells. The membrane attack complex of complement induces caspase activation and apoptosis. Nangaku M. Complement regulatory proteins in glomerular diseases. Endoh M.

Immunohistochemical demonstration of membrane cofactor protein MCP of complement in normal and diseased kidney tissues. Ichida S. Localization of the complement regulatory proteins in the normal human kidney. Lesher A. Review: complement and its regulatory proteins in kidney diseases. Nephrology Carlton ; 15 — Gerritsma J.

Interferon-gamma induces biosynthesis of complement components C2, C4 and factor H by human proximal tubular epithelial cells. Buelli S. Protein load impairs factor H binding promoting complement-dependent dysfunction of proximal tubular cells. Quigg R. Transgenic mice overexpressing the complement inhibitor crry as a soluble protein are protected from antibody-induced glomerular injury.

Turnberg D. Complement and glomerulonephritis: new insights. Curr Opin Nephrol Hypertens. Xiao H. Alternative complement pathway in the pathogenesis of disease mediated by anti-neutrophil cytoplasmic autoantibodies. Am J Pathol. Glassock R. The pathogenesis of membranous nephropathy: evolution and revolution. Beck L. M-type phospholipase A2 receptor as target antigen in idiopathic membranous nephropathy. Cybulsky A. Experimental membranous nephropathy redux. Am J Physiol Renal Physiol.

Noris M. Nat Rev Nephrol. Atypical hemolytic-uremic syndrome. Bresin E. Combined complement gene mutations in atypical hemolytic uremic syndrome influence clinical phenotype.

Servais A. Acquired and genetic complement abnormalities play a critical role in dense deposit disease and other C3 glomerulopathies. Abbate M. How does proteinuria cause progressive renal damage? Complement-mediated dysfunction of glomerular filtration barrier accelerates progressive renal injury. Park P. Uncontrolled activation from failure of these regulation mechanisms leads to complement-mediated direct cellular injury and thrombosis. Complement regulation and dysregulation.

Complement is activated by three pathways: 1 the lectin pathway, 2 the classical pathway, and 3 the alternative pathway. Importantly, the alternative pathway of complement APC also serves as an amplification loop for the lectin and classical pathways, accounting for the majority of complement activation products. The magnitude of APC activation depends on the amplification green and the degradation pathways red. C3 tickover allows C3b to covalently bind to nearby cell surfaces. C3bBb cleaves C3 to generate even more C3b setting off the amplification loop.

In the normal state, these pathways are in homeostasis. Dysregulation, for example from a loss of function mutation in CFH that reduces degradation red loop , leads to an increase in the amplification loop because degradation of the C3 convertase C3b. Bb is blocked, resulting in increased production of complement effectors C3a, C5a and C5b-9 the membrane attack complex. Additionally, complement amplifying triggers such as infection, surgery, or pregnancy can enhance the activation loop green and increase complement activation over the threshold required for clinical disease.

Figure was created using biorender. Complement and coagulation were traditionally considered distinct, nonoverlapping pathways. However, there is considerable crosstalk between these evolutionarily related proteolytic cascades.

Complement activation leads to the generation of the anaphyl-atoxins, C3a and C5a, and the membrane attack complex C5b C3a and C5a cause release of pro-inflammatory and procoagulant cytokines such as tumor necrosis factor and interleukin-6 from monocytes and endothelial cells, and these induce tissue factor and adhesion molecule expression on the cell surface. This was first shown in the context of antithymocyte globulin.

Complementopathies are disorders in which complement dysregulation drives disease pathogenesis, and complement inhibition has the potential to abate the disease course. Identifying the triggering event, if any, is critical because it may eventually help tailor the duration of complement inhibition therapy. These disorders are associated with inherited mutations in complement regulation genes, which confer a predisposition to disease. In addition to tissue injury from direct effects of complement, thrombosis is a hallmark of complementopathies, and the thrombotic risk is mitigated on treatment with terminal complement inhibitors.

C3-deficient mice were also resistant to aPL-mediated fetal loss. However, factor B is necessary for aPL mediated fetal loss and its inhibition ameliorates these effects, supporting a role of the alternative pathway in amplifying complement activation.

Fischetti et al showed that passive transfer of aPL IgG from patients with APS to mice primed with endotoxin led to thrombosis, whereas administration of control IgG did not. Intravascular microscopy showed thrombosis in mesenteric vessels, and IgG and C3 colocalized in the vessel wall.

In these murine models of thrombotic APS, C9 is deposited on the vascular endothelium indicating the presence of the membrane attack complex. This is supported by studies that show elevated levels of inflammatory cytokines in patients with APS. Nearly 3 decades ago, Davis et al demonstrated higher levels of C5b-9 in sera of patients with aPL and stroke compared with non-APS-related stroke. In a recent prospective study, we evaluated complement activation in sera of patients with APS by measuring complement dependent cell killing in the modified Ham assay and cell-surface deposition of C5b-9 by flow cytometry.

Complement activation was more likely to be detected near the time of a thrombotic event and In addition to complement activation via an environmental stressor, patients with complement-mediated disease are thought to be predisposed to dysregulation of the immune system because of the presence of genetic alterations. Incomplete penetrance is the rule rather than the exception, and the risk of developing disease for any given variant is difficult to predict.

Because the role of complement in pathogenesis of disease has expanded beyond aHUS, so has the search for predisposing genetic alterations. We previously found a high incidence of germline variants in complement genes in patients with HELLP, providing evidence for complement in the etiology if this disorder. This study was done using a broad gene panel that resulted in an increased frequency of variants identified in control patients.

The functional consequences of the germline variants identified in individuals with CAPS remains an active area of investigation. However, other studies have suggested that the CFHR proteins have a direct role in complement regulation by competing for CFH binding sites or inhibiting C5 convertase activity. This may also be the case for complement receptor 1 CR1 , which has known roles in both the alternative and classical pathways.

Genetic testing of larger cohorts of patients with CAPS, and functional analysis of germline variants, is required to determine pathogenicity. Long term-anticoagulation with a vitamin K antagonist remains the standard of care for thrombotic APS, and a combination of aspirin with low molecular weight heparin is the mainstay of therapy for obstetric APS. Complement inhibitors such as eculizumab and ravulizumab are widely used to treat aHUS and PNH, and their safety and efficacy are well established.

A small phase 2 trial of eculizumab to allow renal transplantation in patients with CAPS enrolled three patients with APS two with prior CAPS who received eculizumab starting on the day before renal transplant surgery and continuing indefinitely post-transplant. All three patients underwent successful renal transplantation without recurrence of thrombosis or CAPS, and had functioning grafts at follow up of 4 months to 4 years.

It is likely that complement inhibitor therapy could be safely stopped once the thrombotic microangiopathy associated with CAPS has resolved, organ function has improved or plateaued at a new baseline, and the complement amplifying trigger is no longer active.

This is also consistent with emerging reports that eculizumab can be safely stopped in the majority of patients with aHUS who meet similar criteria. Women with APS are at particularly high risk for pregnancy complications and fetal loss, as well as a first or recurrent thrombosis during pregnancy. They are also at higher risk of hypertensive disorders of pregnancy such as preeclampsia and the HELLP syndrome, which clinically resembles a thrombotic microangiopathy, 84 and may be complement mediated at least in a subset of patients.

Eculizumab crosses the placenta minimally and has no adverse effects on the fetus. Complement inhibition as a therapeutic strategy for catastrophic, thrombotic, or obstetric APS needs to be tested in well-designed, prospective, clinical trials.

However, this presents several challenges. Enrolling adequate numbers of patients with a rare disease is challenging, and this will be especially challenging for CAPS, and if patients at highest risk of recurrent VTE who are also most likely to benefit from complement inhibition are selected. Any adequately powered trial in APS will need to recruit from multiple centers, and possibly multiple countries. The second challenge will be to identify the patient population most likely to benefit from adjunctive therapies in addition to anticoagulation.

For example, patients at high risk of thrombosis triple-positive aPL profile or those with aPL and prior pregnancy loss despite aspirin and low molecular weight heparin may be the population of interest for thrombotic and obstetric APS, respectively. Ideally, complement-related biomarkers would be able to identify patients who are more likely to be refractory to standard therapy and those who would benefit from complement inhibition as an adjunct to anticoagulation and antiplatelet therapy.

Unfortunately, standard serum complement assays such as C3, C4, C5b-9, or CH50 have not proved to be reliable biomarkers of disease activity for complement-mediated disorders such as aHUS, and have not been yet been shown to correlate with or predict the development of thrombosis.

Although functional assays such as the modified Ham assay appear to correlate with disease activity and thrombotic risk, these are not available clinically and their prognostic utility in monitoring still needs to be established in larger cohorts. The third challenge is selecting appropriate endpoints. Although recurrent thrombosis may require 1 or more years of follow up, this is the most clinically meaningful endpoint.

Surrogate endpoints such as suppression of thrombin generation and complement specific biomarker studies may be informative but cannot replace clinical outcomes.

Catastrophic APS presents the greatest opportunity for improving outcomes; however, recruiting patients with CAPS is exceptionally challenging because of the rarity of the condition and challenges with diagnosis.

The clinical presentation of CAPS is very similar to, and may be indistinguishable from, other thrombotic microangiopathies or sepsis and disseminated intravascular coagulation unless the patient has a known diagnosis of APS and a high index of suspicion is maintained. Finally, in the mannose-binding lectin pathway, the first enzymes to be activated are known as the m annan-binding lectin- a ssociated s erine p roteases MASP-1 and MASP-2, after which the pathway is essentially the same as the classical pathway.

Activated complement components are often designated by a horizontal line, for example, ; however, we will not use this convention. It is also useful to be aware that the large active fragment of C2 was originally designated C2a, and is still called that in some texts and research papers. Here, for consistency, we will call all large fragments of complement b, so the large active fragment of C2 will be designated C2b. The formation of C3 convertase activity is pivotal in complement activation, leading to the production of the principal effector molecules, and initiating the late events.

In the classical and MB-lectin pathways, the C3 convertase is formed from membrane-bound C4b complexed with C2b. In the alternative pathway , a homologous C3 convertase is formed from membrane-bound C3b complexed with Bb. The alternative pathway can act as an amplification loop for all three pathways, as it is initiated by the binding of C3b.

It is clear that a pathway leading to such potent inflammatory and destructive effects, and which, moreover, has a series of built-in amplification steps, is potentially dangerous and must be subject to tight regulation. One important safeguard is that key activated complement components are rapidly inactivated unless they bind to the pathogen surface on which their activation was initiated.

There are also several points in the pathway at which regulatory proteins act on complement components to prevent the inadvertent activation of complement on host cell surfaces, hence protecting them from accidental damage. We will return to these regulatory mechanisms later. We have now introduced all the relevant components of complement and are ready for a more detailed account of their functions.

To help distinguish the different components according to their functions, we will use a color code in the figures in this part of the chapter. This is introduced in Fig. The classical pathway plays a role in both innate and adaptive immunity. As we will see in Chapter 9, the first component of this pathway, C1q, links the adaptive humoral immune response to the complement system by binding to antibodies complexed with antigens. C1q can, however, also bind directly to the surface of certain pathogens and thus trigger complement activation in the absence of antibody.

C1q is part of the C1 complex , which comprises a single C1q molecule bound to two molecules each of the zymogens C1r and C1s. C1q is a calcium-dependent sugar-binding protein, a lectin, belonging to the collectin family of proteins, which contains both collagen-like and lectin domains hence the name collectin.

It has six globular heads, linked together by a collagen-like tail, which surround the C1r:C1s 2 complex Fig. Binding of more than one of the C1q heads to a pathogen surface causes a conformational change in the C1r:C1s 2 complex, which leads to activation of an autocatalytic enzymatic activity in C1r; the active form of C1r then cleaves its associated C1s to generate an active serine protease. The first protein in the classical pathway of complement activation is C1, which is a complex of C1q, C1r, and C1s.

C1q is composed of six identical subunits with globular heads and long collagen-like tails. The tails combine to bind to two molecules each more Once activated, the C1s enzyme acts on the next two components of the classical pathway , cleaving C4 and then C2 to generate two large fragments, C4b and C2b, which together form the C3 convertase of the classical pathway. In the first step, C1s cleaves C4 to produce C4b, which binds covalently to the surface of the pathogen.

The covalently attached C4b then binds one molecule of C2, making it susceptible, in turn, to cleavage by C1s. C1s cleaves C2 to produce the large fragment C2b, which is itself a serine protease. The complex of C4b with the active serine protease C2b remains on the surface of the pathogen as the C3 convertase of the classical pathway.

Its most important activity is to cleave large numbers of C3 molecules to produce C3b molecules that coat the pathogen surface. At the same time, the other cleavage product, C3a, initiates a local inflammatory response.

These reactions, which comprise the classical pathway of complement activation, are shown in schematic form in Fig.

The classical pathway of complement activation generates a C3 convertase that deposits large numbers of C3b molecules on the pathogen surface. The steps in the reaction are outlined here and detailed in the text. The cleavage of C4 by C1s exposes a reactive more The proteins of the classical pathway of complement activation.

The MB-lectin pathway uses a protein very similar to C1q to trigger the complement cascade. This protein, called the mannan-binding lectin MBL , is a collectin, like C1q. Mannan-binding lectin binds specifically to mannose residues, and to certain other sugars, which are accessible and arranged in a pattern that allows binding on many pathogens. On vertebrate cells, however, these are covered by other sugar groups, especially sialic acid. Thus, mannan-binding lectin is able to initiate complement activation by binding to pathogen surfaces.

It is present at low concentrations in normal plasma of most individuals, and, as we will see in the last part of this chapter, its production by the liver is increased during the acute-phase reaction of the innate immune response. Thus the MB-lectin pathway initiates complement activation in the same way as the classical pathway , forming a C3 convertase from C2b bound to C4b.

People deficient in mannan-binding lectin experience a substantial increase in infections during early childhood, indicating the importance of the MB-lectin pathway for host defense. Mannan-binding lectin forms a complex with serine proteases that resembles the complement C1 complex. MBL forms clusters of two to six carbohydrate-binding heads around a central collagen-like stalk. This structure, easily discernible under the electron more We have seen that the classical and MB-lectin pathways of complement activation are initiated by proteins that bind to pathogen surfaces.

During the triggered-enzyme cascade that follows, it is important that activating events are confined to this same site, so that C3 activation also occurs on the surface of the pathogen, and not in the plasma or on host cell surfaces. This is achieved principally by the covalent binding of C4b to the pathogen surface. Cleavage of C4 exposes a highly reactive thioester bond on the C4b molecule that allows it to bind covalently to molecules in the immediate vicinity of its site of activation.

In innate immunity , C4 cleavage is catalyzed by a C1 or MBL complex bound to the pathogen surface, and C4b can bind adjacent proteins or carbohydrates on the pathogen surface. If C4b does not rapidly form this bond, the thioester bond is cleaved by reaction with water and this hydrolysis reaction irreversibly inactivates C4b Fig. This helps to prevent C4b from diffusing from its site of activation on the microbial surface and becoming coupled to host cells.

Cleavage of C4 exposes an active thioester bond that causes the large fragment, C4b, to bind covalently to nearby molecules on the bacterial cell surface. C2 becomes susceptible to cleavage by C1s only when it is bound by C4b, and the C2b serine protease is thereby also confined to the pathogen surface, where it remains associated with C4b, forming a C3 convertase. The activation of C3 molecules thus also occurs at the surface of the pathogen.

Furthermore, the C3b cleavage product is also rapidly inactivated unless it binds covalently by the same mechanism as C4b, and it therefore opsonizes only the surface on which complement activation has taken place.

This pathway can proceed on many microbial surfaces in the absence of specific antibody , and it leads to the generation of a distinct C3 convertase designated C3b , Bb. In contrast to the classical and MB-lectin pathways of complement activation, the alternative pathway does not depend on a pathogen-binding protein for its initiation; instead it is initiated through the spontaneous hydrolysis of C3, as shown in the top three panels of Fig.

The distinctive components of the pathway are listed in Fig. A number of mechanisms ensure that the activation pathway will only proceed on the surface of a pathogen. Complement activated by the alternative pathway attacks pathogens while sparing host cells, which are protected by complement regulatory proteins. The complement component C3 is cleaved spontaneously in plasma to give C3 H 2 O , which binds factor B and more The proteins of the alternative pathway of complement activation.

This occurs through the spontaneous hydrolysis of the thioester bond in C3 to form C3 H 2 O which has an altered conformation, allowing binding of the plasma protein factor B. This complex is a fluid-phase C3 convertase , and although it is only formed in small amounts it can cleave many molecules of C3 to C3a and C3b.

Much of this C3b is inactivated by hydrolysis, but some attaches covalently, through its reactive thioester group, to the surfaces of host cells or to pathogens. C3b bound in this way is able to bind factor B, allowing its cleavage by factor D to yield the small fragment Ba and the active protease Bb. This results in formation of the alternative pathway C3 convertase, C3b,Bb see Fig.

When C3b binds to host cells, a number of complement -regulatory proteins, present in the plasma and on host cell membranes combine to prevent complement activation from proceeding. These proteins interact with C3b and either prevent the convertase from forming, or promote its rapid dissociation see Fig. Thus, the complement receptor 1 CR1 and a membrane-attached protein known as decay-accelerating factor DAF or CD55 compete with factor B for binding to C3b on the cell surface, and can displace Bb from a convertase that has already formed.

Convertase formation can also be prevented by cleaving C3b to its inactive derivative iC3b. This is achieved by a plasma protease, factor I , in conjunction with C3b-binding proteins that can act as cofactors, such as CR1 and membrane cofactor of proteolysis MCP or CD46 , another host cell membrane protein.

Factor H is another complement-regulatory protein in plasma that binds C3b and, like CR1, it is able to compete with factor B and displace Bb from the convertase in addition to acting as a cofactor for factor I.

Moreover, we show that FB, a crucial component of the alternative pathway, is upregulated in HF patients and particularly in patients with ischemic cardiomyopathy. Recently we showed that the alternative pathway is dysregulated in HF as reflected by increased levels of factor D and properdin and decreased levels of factor H Yasuda et al.

However, patients with acute myocardial infarction had markedly increased values of the activation fragment Bb indicating alternative pathway activation This is in contrast to a study by Hertle et al. Lastly, we have previously described increased levels of FB and Bb in patients with aortic stenosis 22 , a common valvular disease and a possible underlying cause of HF.

However, none of these studies were focusing purely on patient with HF. To the best of our knowledge, our study is the first to show that FB is elevated in HF and particularly in patients with ischemic heart disease. However, FB was not associated with adverse clinical outcomes during follow-up. Despite having a relatively low number of controls, the values in these apparently healthy individuals were close to the reference range of C3bBbP, as given by Bergseth et al.

The positive correlation between C3bBbP levels and kidney function may seem surprisingly. However, the correlation was weak and should be interpreted with caution. Somewhat surprisingly, the HF patients in this cohort on average had normal TCC levels compared to controls, and, thus there was no evidence of increased terminal pathway activation.

This is contrary to our previous findings in other HF cohorts, where we reported increased levels of TCC 6 , At several points the previous and current two, not overlapping, populations differ; among others, our population is older and has a higher frequency of comorbidities such as diabetes and hypertension.

The discrepancy between these studies could potentially reflect that the previous HF cohort comprised of more end-stage HF e. Nonetheless, why TCC levels are normal in the presence of enhanced activation of the alternative pathway, is not clear. However, in some patients with nephritic factors, early-phase activation may not lead to activation of the terminal pathway Whereas complement activation in the circulation is clearly of interest, data on activation of the alternative complement pathway within the myocardium in HF patients are essential lacking.

There a few clinical studies on complement disposition within the myocardium and all of them on myocardial infarcted tissue. However, it is also stated that the absence of properdin does not completely rule out the involvement of the alternative pathway, since deposition can be hard to judge by immunofluorescence microscopy. The activation of these pathways in myocardial infarcted tissue is also confirmed by Yasojima et al.

In both publications the studied patients died from an acute myocardial infarction, which may not reflect the situation in patients with chronic HF as in the present study. There are some limitations that should be considered when interpreting the current study.

The number of controls as well as patients with severe HF was limited. Furthermore, although statistically significant, some of the correlation coefficients were rather low. Moreover, we lack data on complement deposition with the failing myocardium. In summary, our results show that circulating levels of FB and C3bBbP are elevated in patients with HF suggesting a role for activation of the alternative complement pathway in HF.

However, the pathophysiological consequences of these findings are unknown, since complement factors did not correlate with disease severity. The studies involving human participants were reviewed and approved by Regional Committee for Medical and Health Research Ethics.

MH organized the database. All authors contributed to manuscript revision, read, and approved the submitted version. NS is currently an employee of Johnson and Johnson; the current work was conducted when she was employed by Oslo University Hospital. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher. National Center for Biotechnology Information , U. Front Immunol. Published online Dec Margrethe Flesvig Holt , 1 , 2 Annika E.