The role of antibody in the activation of the alternative complement pathway

Alternative pathway amplification and infections

The alternative pathway (AP) is the phylogenetically oldest arm of the complement system and may have evolved to mark pathogens for elimination by phagocytes. Studies using purified AP proteins or AP-specific serum showed that C3b amplification on bacteria commenced following a lag phase of about 5 min and was highly dependent on the concentration of complement. Most pathogens have evolved several elegant mechanisms to evade complement, including expressing proteases that degrade AP proteins and secreting proteins that block function of C3 convertases. In an example of convergent evolution, many microbes recruit the AP inhibitor factor H (FH) using molecular mechanisms that mimic FH interactions with host cells. In most instances, the AP serves to amplify C3b deposited on microbes by the classical pathway (CP). The role of properdin on microbes appears to be restricted to stabilization of C3 convertases; scant evidence exists for its role as an initiator of the AP on pathogens in the context of serum. Therapeutic complement inhibition carries with it an increased risk of infection. Antibody (Ab)-dependent AP activation may be critical for complement activation by vaccine-elicited Ab when the CP is blocked, and its molecular mechanism is discussed.

Complement interactions with the pathogenic Neisseriae: clinical features, deficiency states, and evasion mechanisms

Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhea, while Neisseria meningitidis is an important cause of bacterial meningitis and sepsis. Complement is a central arm of innate immune defenses and plays an important role in combating Neisserial infections. Persons with congenital and acquired defects in complement are at a significantly higher risk for invasive Neisserial infections such as invasive meningococcal disease and disseminated gonococcal infection compared to the general population. Of note, Neisseria gonorrhoeae and Neisseria meningitidis can only infect humans, which in part may be related to their ability to evade only human complement. This review summarizes the epidemiologic and clinical aspects of Neisserial infections in persons with defects in the complement system. Mechanisms used by these pathogens to subvert killing by complement and preclinical studies showing how these complement evasion strategies may be used to counteract the global threat of meningococcal and gonococcal infections are discussed.

Drug-induced Glomerulonephritis: The Spectre of Biotherapeutic and Antisense Oligonucleotide Immune Activation in the Kidney

Prevalence of immune-mediated glomerulonephritis has increased in preclinical toxicity studies, with more frequent use of biotherapeutic agents (especially antigenic humanized molecules) and antisense oligonucleotide (ASO) therapies. Immune complex disease affects a small number of study monkeys, often correlates with antidrug antibody (ADA) titers, and occurs at a dose that favors immune complex formation or impedes clearance. While preclinical glomerulonephritis often fails to correlate with evidence of glomerular or vascular injury in human clinical trials and is not considered predictive, additional animal investigative immunohistochemical work may be performed to substantiate evidence for immune complex pathogenesis. While ADA is most commonly encountered as a predisposing factor with biotherapeutic agents, complement activation may occur without circulating complexes, and other mechanisms of non-ADA immune-mediated glomerulonephritis have been observed including nonendogenous immune aggregates and immunoregulatory pharmacology. Although glomerulonephritis associated with oligonucleotide therapies has been noted occasionally in preclinical studies and more rarely with human patients, pathophysiologic mechanisms involved appear to be different between species and preclinical cases are not considered predictive for humans. ADA is not involved in oligonucleotide-associated cases, and complement fixation plays a more important role in monkeys. Recent screening of ASOs for proinflammatory activity appears to have decreased glomerulonephritis incidence preclinically.

Low pH impairs complement-dependent cytotoxicity against IgG-coated target cells

Local acidosis is a common feature of allergic, vascular, autoimmune, and cancer diseases. However, few studies have addressed the effect of extracellular pH on the immune response. Here, we analyzed whether low pH could modulate complement-dependent cytotoxicity (CDC) against IgG-coated cells. Using human serum as a complement source, we found that extracellular pH values of 5.5 and 6.0 strongly inhibit CDC against either B lymphoblast cell lines coated with the chimeric anti-CD20 mAb rituximab or PBMCs coated with the humanized anti-CD52 mAb alemtuzumab. Suppression of CDC by low pH was observed either in cells suspended in culture medium or in whole blood assays. Interestingly, not only CDC against IgG-coated cells, but also the activation of the complement system induced by the alternative and lectin pathways was prevented by low pH. Tumor-targeting mAbs represent one of the most successful tools for cancer therapy, however, the use of mAb monotherapy has only modest effects on solid tumors. Our present results suggest that severe acidosis, a hallmark of solid tumors, might impair complement-mediated tumor destruction directed by mAb.

Host Soluble Mediators: Defying the Immunological Inertness of Aspergillus fumigatus Conidia

Aspergillus fumigatus produce airborne spores (conidia), which are inhaled in abundant quantity. In an immunocompromised population, the host immune system fails to clear the inhaled conidia, which then germinate and invade, leading to pulmonary aspergillosis. In an immunocompetent population, the inhaled conidia are efficiently cleared by the host immune system. Soluble mediators of the innate immunity, that involve the complement system, acute-phase proteins, antimicrobial peptides and cytokines, are often considered to play a complementary role in the defense of the fungal pathogen. In fact, the soluble mediators are essential in achieving an efficient clearance of the dormant conidia, which is the morphotype of the fungus upon inhalation by the host. Importantly, harnessing the host soluble mediators challenges the immunological inertness of the dormant conidia due to the presence of the rodlet and melanin layers. In the review, we summarized the major soluble mediators in the lung that are involved in the recognition of the dormant conidia. This knowledge is essential in the complete understanding of the immune defense against A. fumigatus.

Immune responses following meningococcal serogroups A, C, Y and W polysaccharide vaccination in C2-deficient persons: evidence for increased levels of serum bactericidal antibodies

Complement C2 deficiency (C2D) is associated with immunological diseases and increased susceptibility to invasive infections caused by encapsulated bacteria such as Neisseria menigitidis. In this study we evaluate the immunogenicity of vaccination against N. menigitidis in C2D. C2D patients (n=22) and controls (n=52) were given a tetravalent meningococcal polysaccharide vaccine. Serum bactericidal antibody (SBA) titres (serogroups A, C, Y and W) were analysed using a rabbit complement source. Levels of IgG, IgM, and IgA, factor B, and factor H, polymorphisms of MBL and Fc-gamma receptors were determined. The C2D patients responded with an increased SBA titre to all four serogroups (p<0.001). The response rates define as SBA titres ≥8 were found to be between 85.7% and 92.5%. The post-vaccination titres for serogroups C, Y and W were equal to healthy controls. C2D patients with a history of invasive infection had a lower post-vaccination SBA titres both compared to healthy C2D persons (p=0.03) and compared to controls (p<0.0001). We found that the G2M*n/G2M*n genotype were associated with a higher SBA titres after immunization (p=0.03). None of the other investigated immunological factors appear to be important in influencing the vaccine responses. Autoimmune diseases in C2D did not affect the vaccine response. In general, vaccination against meningococci gave rise to antibody responses in the C2D patients that equal healthy controls. The response rate was lower to serogroup A and among C2D patients with history of invasive infections. The presence of G2M*n/G2M*n genotype was associated with higher SBA titres after immunization.

Essential role for the lectin pathway in collagen antibody-induced arthritis revealed through use of adenovirus programming complement inhibitor MAp44 expression

Previous studies using mannose-binding lectin (MBL) and complement C4-deficient mice have suggested that the lectin pathway (LP) is not required for the development of inflammatory arthritis in the collagen Ab-induced arthritis (CAIA) model. MBL, ficolins and collectin-11 are key LP pattern recognition molecules that associate with three serine proteases-MASP-1, MASP-2, and MASP-3-and with two MBL-associated proteins designated sMAP and MBL-associated protein of 44kDA (MAp44). Recent studies have shown that MAp44, an alternatively spliced product of the MASP-1/3 gene, is a competitive inhibitor of the binding of the recognition molecules to all three MASPs. In these studies, we examined the effect of treatment of mice with adenovirus (Ad) programmed to express human MAp44 (AdhMAp44) on the development of CAIA. AdhMAp44 and Ad programming GFP (AdGFP) expression were injected i.p. in C57BL/6 wild type mice prior to the induction of CAIA. AdhMAp44 significantly reduced the clinical disease activity (CDA) score by 81% compared with mice injected with AdGFP. Similarly, histopathologic injury scores for inflammation, pannus, cartilage and bone damage, as well as C3 deposition in the cartilage and synovium, were significantly reduced by AdhMAp44 pretreatment. Mice treated with AdmMAp44, programming expression of mouse MAp44, also showed significantly decreased CDA score and histopathologic injury scores. In addition, administration of AdhMAp44 significantly diminished the severity of Ross River virus-induced arthritis, an LP-dependent model. Our study provides conclusive evidence that an intact complement LP is essential to initiate CAIA, and that MAp44 may be an appropriate treatment for inflammatory arthritis.

Antibody-mediated rejection despite inhibition of terminal complement

Terminal complement blockade has been shown to decrease the incidence of early acute antibody-mediated rejection (eAMR) in the first month after positive cross-match kidney transplant recipients, yet some patients still develop eAMR. The current study investigated possible mechanisms of eAMR despite eculizumab treatment. Of the 26 patients treated with eculizumab, two developed clinical eAMR and another patient developed histologic signs of eAMR without graft dysfunction ('subclinical eAMR'). Twenty-three did not have histologic injury on early surveillance biopsies. All 26 patients had therapeutic levels of eculizumab and showed complete blockade of complement in hemolytic assays. High levels of donor-specific alloantibody (DSA) including total IgG, IgG3, and C1q+ DSA were present in patients with and without eAMR, and none correlated well with eAMR. In contrast, IgM DSA was present in only four patients after transplantation: the two patients with clinical eAMR, one patient with subclinical AMR, and one patient without eAMR (P = 0.006 correlation with eAMR). Both clinical eAMR episodes were easily treated with plasma exchange which removed IgM more completely and rapidly than IgG, resulting in normalization of function and histology. These data suggest a possible role of antidonor IgM DSA in the pathogenesis of eAMR in patients treated with terminal complement blockade (ClinicalTrials.gov Identifier: NCT00670774).

Formation, Clearance, Deposition, Pathogenicity, and Identification of Biopharmaceutical-related Immune Complexes

Vascular inflammation, infusion reactions, glomerulopathies, and other potentially adverse effects may be observed in laboratory animals, including monkeys, on toxicity studies of therapeutic monoclonal antibodies and recombinant human protein drugs. Histopathologic and immunohistochemical (IHC) evaluation suggests these effects may be mediated by deposition of immune complexes (ICs) containing the drug, endogenous immunoglobulin, and/or complement components in the affected tissues. ICs may be observed in glomerulus, blood vessels, synovium, lung, liver, skin, eye, choroid plexus, or other tissues or bound to neutrophils, monocytes/macrophages, or platelets. IC deposition may activate complement, kinin, and/or coagulation/fibrinolytic pathways and result in a systemic proinflammatory response. IC clearance is biphasic in humans and monkeys (first from plasma to liver and/or spleen, second from liver or spleen). IC deposition/clearance is affected by IC composition, immunomodulation, and/or complement activation. Case studies are presented from toxicity study monkeys or rats and indicate IHC-IC deposition patterns similar to those predicted by experimental studies of IC-mediated reactions to heterologous protein administration to monkeys and other species. The IHC-staining patterns are consistent with findings associated with generalized and localized IC-associated pathology in humans. However, manifestations of immunogenicity in preclinical species are generally not considered predictive to humans.

Fibrinogen-specific antibody induces abdominal aortic aneurysm in mice through complement lectin pathway activation

Abdominal aortic aneurysm (AAA) is a common vascular disease associated with high mortality rate due to progressive enlargement and eventual rupture. There is currently no established therapy known to alter the rate of aneurysmal expansion. Thus, understanding the processes that initiate and sustain aneurysmal growth is pivotal for the development of medical therapies aimed at halting disease progression. Using an elastase-induced AAA mouse model that recapitulates key features of human AAA, we previously reported that a natural IgG antibody directs alternative pathway complement activation and initiates the inflammatory process that culminates in aneurysmal development. The target of this natural antibody, however, was unknown. Herein we identify a natural IgG that binds to fibrinogen deposited in elastase-perfused aortic tissues, activates the complement lectin pathway (LP), and induces AAA. Moreover, we establish that alterations in the glycosylation patterns of this antibody critically affect its ability to activate the LP in vivo. We find that LP activation precedes the alternative pathway and absence of the LP complement protein mannan-binding lectin abrogates elastase-induced AAA. In human AAA tissues the mouse anti-fibrinogen antibody recognizes epitopes that localize to the same areas that stain positively for mannan-binding lectin, which suggests that the complement LP is engaged in humans as well. Lastly, we demonstrate that circulating antibodies in a subset of AAA patients react against fibrinogen or fibrinogen-associated epitopes in human aneurysmal tissues. Our findings support the concept that an autoimmune process directed at aortic wall self-antigens may play a central role in the immunopathogenesis of AAA.

Meningococcal disease and the complement system

Despite considerable advances in the understanding of the pathogenesis of meningococcal disease, this infection remains a major cause of morbidity and mortality globally. The role of the complement system in innate immune defenses against invasive meningococcal disease is well established. Individuals deficient in components of the alternative and terminal complement pathways are highly predisposed to invasive, often recurrent meningococcal infections. Genome-wide analysis studies also point to a central role for complement in disease pathogenesis. Here we review the pathophysiologic events pertinent to the complement system that accompany meningococcal sepsis in humans. Meningococci use several often redundant mechanisms to evade killing by human complement. Capsular polysaccharide and lipooligosaccharide glycan composition play critical roles in complement evasion. Some of the newly described protein vaccine antigens interact with complement components and have sparked considerable research interest.

Roles of adipocytes and fibroblasts in activation of the alternative pathway of complement in inflammatory arthritis in mice

The complement system is involved in mediation of joint damage in rheumatoid arthritis, with evidence suggesting activation of both the classical and alternative pathway (AP). The AP is both necessary and sufficient to mediate collagen Ab-induced arthritis, an experimental animal model of immune complex-induced joint disease. The AP in mice is dependent on MASP-1/3 cleavage of pro-factor D (pro-FD) into mature factor D (FD). The objectives of the current study were to determine the cells synthesizing MASP-1/3 and pro-FD in synovial tissue. Collagen Ab-induced arthritis was studied in wild-type C57BL/6 mice, and the localization of mRNA and protein for FD and MASP-1/3 in synovial adipose tissue (SAT) and fibroblast-like synoviocytes (FLS) was determined using various techniques, including laser capture microdissection. SAT was the sole source of mRNA for pro-FD. Cultured differentiated 3T3 adipocytes, a surrogate for SAT, produced pro-FD but no mature FD. FLS were the main source of MASP-1/3 mRNA and protein. Using cartilage microparticles (CMPs) coated with anti-collagen mAb and serum from MASP-1/3(-/-) mice as a source of factor B, pro-FD in 3T3 supernatants was cleaved into mature FD by MASP-1/3 in FLS supernatants. The mature FD was eluted from the CMP, and was not present in the supernatants from the incubation with CMP, indicating that cleavage of pro-FD into mature FD by MASP-1 occurred on the CMP. These results demonstrate that pathogenic activation of the AP can occur in the joint through immune complexes adherent to cartilage and the local production of necessary AP proteins by adipocytes and FLS.

Essential role of surface-bound complement factor H in controlling immune complex-induced arthritis

Factor H (fH) is an endogenous negative regulator of the alternative pathway (AP) that binds polyanions as well as complement activation fragments C3b and C3d. The AP is both necessary and sufficient to develop collagen Ab-induced arthritis (CAIA) in mice; the mechanisms whereby normal control of the AP is overcome and injury develops are unknown. Although primarily a soluble circulating protein, fH can also bind to tissues in a manner dependent on the carboxyl-terminal domain containing short consensus repeats 19 and 20. We examined the role of fH in CAIA by blocking its binding to tissues through administration of a recombinant negative inhibitor containing short consensus repeats 19 and 20 (rfH19-20), which impairs fH function and amplifies surface AP activation in vitro. Administration of rfH19-20, but not control rfH3-5, significantly worsened clinical disease activity, histopathologic injury, and C3 deposition in the synovium and cartilage in wild-type and fH(+/-) mice. In vitro studies demonstrated that rfH19-20 increased complement activation on cartilage extracts and injured fibroblast-like synoviocytes, two major targets of complement deposition in the joint. We conclude that endogenous fH makes a significant contribution to inhibition of the AP in CAIA through binding to sites of immune complex formation and complement activation.

Local release of properdin in the cellular microenvironment: role in pattern recognition and amplification of the alternative pathway of complement

Properdin, the only positive regulatory protein of the complement system, acts as both a stabilizer of the alternative pathway (AP) convertases and as a selective pattern recognition molecule of certain microorganisms and host cells (i.e., apoptotic/necrotic cells) by serving as a platform for de novo C3b,Bb assembly. Properdin, a highly positively charged protein, normally exists as cyclic dimers (P(2)), trimers (P(3)), and tetramers (P(4)) of head-to-tail associations of monomeric 53 kDa subunits. While most complement proteins are produced mainly in the liver, properdin is synthesized primarily by various cell types, including neutrophils, monocytes, primary T cells, and shear-stressed endothelial cells resulting in properdin serum levels of 4-25 μg/ml. Multiple inflammatory agonists stimulate the release of properdin from stimulated leukocytes into the cellular microenvironment. Concentrated, focused increases in properdin levels may lead to stabilization and initiation of AP convertases, thus greatly amplifying the complement response to a local stimulus. This review highlights current knowledge related to these properties and discusses the implications of properdin production in a pro-inflammatory microenvironment.

How immune complexes from certain IgG NAbs and any F(ab')₂ can mediate excessive complement activation

In sepsis death follows an excessive inflammatory response involving cytokines and complement that is activated primarily via the amplifying C3/C5 convertase. Excessive stimulation of complement amplification requires IgG-containing or F(ab')₂-containing immune complexes (IC) that capture dimeric C3b on one of their heavy chains or heavy chain fragments. The ability of IgG-IC to capture dimeric C3b by the Fab portion is dependent on an affinity for C3 within the Fab portion, but outside the antigen-binding region. This property is rare among IgG NAbs. In contrast to this, the lack of the Fc portion renders the Fab regions of any F(ab')(2)-IC accessible to nascent C3b, but dimeric C3b deposits only if F(ab')₂-IC form secondary IC with anti-hinge NAbs that rigidify the complex and thereby promote deposition of dimeric C3b. Both types of complexes, C3b₂-IgG-IC and C3b₂-F(ab')₂-IC/anti-hinge NAbs, are potent precursors of alternative C3 convertases and stimulate complement amplification along with properdin up to 750 times more effectively than C3b and properdin. F(ab')₂ fragments are not normally generated, but are formed from NAbs by enzymes from pathogens and neutrophils in sepsis. Unlike IgG-IC F(ab')₂-IC are not cleared by Fc-receptor dependent processes and circulate long enough to form secondary IC with anti-hinge NAbs that rigidify the complexes such that they capture dimeric C3b and gain the potency to stimulate complement amplification.

Antibody directs properdin-dependent activation of the complement alternative pathway in a mouse model of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a complex inflammatory vascular disease. There are currently limited treatment options for AAA when surgery is inapplicable. Therefore, insights into molecular mechanisms underlying AAA pathogenesis may reveal therapeutic targets that could be manipulated pharmacologically or biologically to halt disease progression. Using an elastase-induced AAA mouse model, we previously established that the complement alternative pathway (AP) plays a critical role in the development of AAA. However, the mechanism by which complement AP is initiated remains undefined. The complement protein properdin, traditionally viewed as a positive regulator of the AP, may also initiate complement activation by binding directly to target surfaces. In this study, we sought to determine whether properdin serves as a focal point for the initiation of the AP complement activation in AAA. Using a properdin loss of function mutation in mice and a mutant form of the complement factor B protein that produces a stable, properdin-free AP C3 convertase, we show that properdin is required for the development of elastase-induced AAA in its primary role as a convertase stabilizer. Unexpectedly, we find that, in AAA, natural IgG antibodies direct AP-mediated complement activation. The absence of IgG abrogates C3 deposition in elastase-perfused aortic wall and protects animals from AAA development. We also determine that blockade of properdin activity prevents aneurysm formation. These results indicate that an innate immune response to self-antigens activates the complement system and initiates the inflammatory cascade in AAA. Moreover, the study suggests that properdin-targeting strategies may halt aneurysmal growth.

Initiation of the alternative pathway of murine complement by immune complexes is dependent on N-glycans in IgG antibodies

OBJECTIVE

Collagen antibody-induced arthritis in mice exhibits a requirement for amplification by the alternative pathway of complement. Although the alternative pathway is activated by spontaneous hydrolysis, it is not known whether this pathway can also be initiated directly by IgG antibodies in immune complexes (ICs). IgG lacking terminal sialic acid and galactose (G0 IgG) can activate the lectin pathway of complement, but it is not known if G0 IgG can also activate the classical or alternative pathway. The purpose of this study was to examine the mechanism of initiation of the alternative pathway of complement by ICs.

METHODS

We used adherent ICs containing bovine type II collagen (CII) and 4 monoclonal antibodies (mAb) to CII (adCII-IC). C3 activation was measured in the presence of sera from wild-type C57BL/6 mice or from mice deficient in informative complement components. The mAb were used intact or after enzyme digestion to create G0 IgG or to completely remove the N-glycan.

RESULTS

Both the classical and alternative pathways, but not the lectin pathway, mediated C3 activation induced by the adCII-IC. Mannose inhibited the alternative pathway-mediated C3 activation but had no effect on the classical pathway, and N-glycans in IgG were required by the alternative pathway but not the classical pathway. Both the classical and alternative pathways mediated C3 activation induced by G0 IgG. Mannose-binding lectin bound avidly to G0 IgG, but lectin pathway-mediated C3 activation was only slightly increased by G0 IgG.

CONCLUSION

The alternative pathway of complement is capable of initiating C3 activation induced by adCII-IC and requires the presence of N-glycans on the IgG. G0 IgG activates both the classical and alternative pathways more strongly than the lectin pathway.

Pathogenic complement activation in collagen antibody-induced arthritis in mice requires amplification by the alternative pathway

Immune complex-induced inflammation can be mediated by the classical pathway of complement. However, using mice genetically deficient in factor B or C4, we have shown that the collagen Ab-induced model of arthritis requires the alternative pathway of complement and is not dependent on the classical pathway. We now demonstrate that collagen Ab-induced arthritis is not altered in mice genetically deficient in either C1q or mannose-binding lectins A and C, or in both C1q and mannose-binding lectins. These in vivo results prove the ability of the alternative pathway to carry out pathologic complement activation in the combined absence of intact classical and lectin pathways. C3 activation was also examined in vitro by adherent collagen-anti-collagen immune complexes using sera from normal or complement-deficient mice. These results confirm the ability of the alternative pathway to mediate immune complex-induced C3 activation when C4 or C1q, or both C1q and mannose-binding lectins, are absent. However, when all three activation pathways of complement are intact, initiation by immune complexes occurs primarily by the classical pathway. These results indicate that the alternative pathway amplification loop, with its ability to greatly enhance C3 activation, is necessary to mediate inflammatory arthritis induced by adherent immune complexes.

Alternative Complement Pathway Activation Is Essential for Inflammation and Joint Destruction in the Passive Transfer Model of Collagen-Induced Arthritis

Activation of each complement initiation pathway (classical, alternative, and lectin) can lead to the generation of bioactive fragments with resulting inflammation in target organs. The objective of the current study was to determine the role of specific complement activation pathways in the pathogenesis of experimental anti-type II collagen mAb-passive transfer arthritis. C57BL/6 mice were used that were genetically deficient in either the alternative pathway protein factor B (Bf(-/-)) or in the classical pathway component C4 (C4(-/-)). Clinical disease activity was markedly decreased in Bf(-/-) compared with wild-type (WT) mice (0.5 +/- 0.22 (n = 6) in Bf(-/-) vs 8.83 +/- 0.41 (n = 6) in WT mice (p < 0.0001)). Disease activity scores were not different between C4(-/-) and WT mice. Analyses of joints showed that C3 deposition, inflammation, pannus, cartilage, and bone damage scores were all significantly less in Bf(-/-) as compared with WT mice. There were significant decreases in mRNA levels of C3, C4, CR2, CR3, C3aR, and C5aR in the knees of Bf(-/-) as compared with C4(-/-) and WT mice with arthritis; mRNA levels for complement regulatory proteins did not differ between the three strains. These results indicate that the alternative pathway is absolutely required for the induction of arthritis following injection of anti-collagen Abs. The mechanisms by which these target organ-specific mAbs bypass the requirements for engagement of the classical pathway remain to be defined but do not appear to involve a lack of alternative pathway regulatory proteins.

Factor B of the alternative complement pathway regulates development of airway hyperresponsiveness and inflammation

Exposure to inhaled allergens leads to increases in airway hyperresponsiveness (AHR) and inflammation, associated with increased levels of biologically active fragments derived from the complement C3 and C5 family of proteins. Further, complement activation during allergen challenge in sensitized animals is necessary for the development of AHR and airway inflammation. To define the complement pathway involved, we studied mice deficient in complement factor 4 (C4-/-), a critical component of the classical pathway, or factor B (fB-/-), an essential protein in the alternative complement pathway. WT, C4-/-, and fB-/- mice were sensitized to ovalbumin and subsequently exposed to nebulized ovalbumin (1% in saline) on 3 consecutive days. After allergen sensitization and challenge, fB-/- mice demonstrated significantly lower airway responsiveness to methacholine and less airway inflammation. In contrast, C4-/- mice showed no reduction in AHR and airway inflammation compared with WT mice. Tissue inflammation, goblet cell hyperplasia, and IL-4, IL-5, and IL-13 levels in BAL fluid were significantly reduced in fB-/- mice compared with C4-/- and WT mice. The development of AHR and airway inflammation in sensitized fB-/- mice could be restored after intranasal administration of purified factor B before the airway challenge. In addition, administration of a neutralizing anti-factor B mAb to sensitized mice before airway challenge reduced the development of AHR and airway inflammation. These results demonstrate that in sensitized hosts complement activation through the alternative pathway after allergen exposure is critical to the development of AHR and airway inflammation.

The central role of the alternative complement pathway in human disease

The complement system is increasingly recognized as important in the pathogenesis of tissue injury in vivo following immune, ischemic, or infectious insults. Within the complement system, three pathways are capable of initiating the processes that result in C3 activation: classical, alternative, and lectin. Although the roles that proinflammatory peptides and complexes generated during complement activation play in mediating disease processes have been studied extensively, the relative contributions of the three activating pathways is less well understood. Herein we examine recent evidence that the alternative complement pathway plays a key and, in most instances, obligate role in generating proinflammatory complement activation products in vivo. In addition, we discuss new concepts regarding the mechanisms by which the alternative pathway is activated in vivo, as recent clinical findings and experimental results have provided evidence that continuous active control of this pathway is necessary to prevent unintended targeting and injury to self tissues.

Critical role for complement receptor 3 (CD11b/CD18), but not for Fc receptors, in killing of Streptococcus pyogenes by neutrophils in human immune serum

During phagocytosis, surface receptors on neutrophils interact with pathogens opsonized with complement factor C3b/iC3b and in some cases with antibodies. In human immune sera antibodies directed against surface-bound M proteins mediated killing of Streptococcus pyogenes by neutrophils. Surprisingly, blocking of the Fc receptors had little effect on the killing. In contrast, inhibition of C3b/iC3b generation, or blocking of the major neutrophil iC3b receptor CD11b/CD18, enabled S. pyogenes to grow efficiently in immune sera. Inhibition of CD11b/CD18, but not of CD32, the major neutrophil signaling Fc receptor, prevented Streptococcus-induced NADPH oxidase-dependent respiratory burst, and blocking of C3b/iC3b formation inhibited Streptococcus-induced activation of Cdc42, a small GTPase critically involved in transmitting pro-inflammatory signals to the cytoskeleton. Consequently, ligation of CD11b/CD18 by bacteria-bound iC3b is necessary for inducing a neutrophil response leading to elimination of S. pyogenes in immune human serum.

The quantitative role of alternative pathway amplification in classical pathway induced terminal complement activation

Complement activation with formation of biologically potent mediators like C5a and the terminal C5b-9 complex (TCC) contributes essentially to development of inflammation and tissue damage in a number of autoimmune and inflammatory conditions. A particular role for complement in the ischaemia/reperfusion injury of the heart, skeletal muscle, central nervous system, intestine and kidney has been suggested from animal studies. Previous experiments in C3 and C4 knockout mice suggested an important role of the classical or lectin pathway in initiation of complement activation during intestinal ischaemia/reperfusion injury while later use of factor D knockout mice showed the alternative pathway to be critically involved. We hypothesized that alternative pathway amplification might play a more critical role in classical pathway-induced C5 activation than previously recognized and used pathway-selective inhibitory mAbs to further elucidate the role of the alternative pathway. Here we demonstrate that selective blockade of the alternative pathway by neutralizing factor D in human serum diluted 1 : 2 with mAb 166-32 inhibited more than 80% of C5a and TCC formation induced by solid phase IgM and solid- and fluid-phase human aggregated IgG via the classical pathway. The findings emphasize the influence of alternative pathway amplification on the effect of initial classical pathway activation and the therapeutic potential of inhibiting the alternative pathway in clinical conditions with excessive and uncontrolled complement activation.

Hereditary C2 deficiency in Sweden: frequent occurrence of invasive infection, atherosclerosis, and rheumatic disease

Although frequently asymptomatic, homozygous C2 deficiency (C2D) is known to be associated with severe infections and rheumatic disease. We describe the clinical findings in 40 persons with C2D from 33 families identified in Sweden over 25 years. Medical records covering 96% of the accumulated person-years were reviewed, giving a mean observation time of 39 years (range, 1-77 yr). Severe infection was the predominant clinical manifestation in the cohort: 23 patients had a past history of invasive infections, mainly septicemia or meningitis caused by Streptococcus pneumoniae, and 12 patients had repeated infections of this kind. Nineteen patients had at least 1 episode of pneumonia, and recurrent pneumonia was documented in 10 patients. Repeated infections occurred mainly during infancy and childhood. Systemic lupus erythematosus was found in 10 patients. Another 7 patients had undifferentiated connective tissue disease (n = 4) or vasculitis (n = 3). We found no correlation between susceptibility to invasive infection and rheumatologic disease. Cardiovascular disease occurred at a high rate, with a total of 10 acute myocardial infarctions and 5 cerebrovascular episodes in 6 patients. Causes of death among the C2D patients were infection (n = 5), acute myocardial infarction (n = 3), and cancer (n = 1). We suggest that severe infection may be the principal clinical manifestation of C2D. We also provide novel evidence for a possible role of C2D in the development of atherosclerosis consistent with findings in mannan-binding deficiency and experimental C3 deficiency. In addition, we confirm the well-known association between C2D and systemic lupus erythematosus.

Innate autoimmunity

The adaptive immune system has evolved highly specific pattern recognition proteins and receptors that, when triggered, provide a first line of host defense against pathogens. Studies reveal that these innate recognition proteins are also self-reactive and can initiate inflammation against self-tissues in a similar manner as with pathogens. This specific event is referred to as "innate autoimmunity." In this review, we describe two classes of autoimmune responses, that is, reperfusion injury and fetal loss syndrome, in which the recognition and injury are mediated by innate immunity. Both disorders are common and are clinically important. Reperfusion injury (RI) represents an acute inflammatory response after a reversible ischemic event and subsequent restoration of blood flow. Findings that injury is IgM and complement dependent and that a single natural antibody prepared from a panel of B-1 cell hybridomas can restore injury in antibody-deficient mice suggest that RI is an autoimmune-type disorder. Fetal loss syndrome is also an antibody- and complement-dependent disorder. Although both immune and natural antibodies are likely involved in recognition of phospholipid self-antigens, inhibition of the complement pathway in rodent models can block fetal loss. As new innate recognition proteins and receptors are identified, it is likely that innate responses to self represent frequent events and possibly underlie many of the known chronic autoimmune disorders normally attributable to dysregulation of adaptive immunity.

Complement system on the attack in autoimmunity

The antiphospholipid syndrome is characterized clinically by fetal loss and thrombosis and serologically by the presence of autoantibodies to lipid-binding proteins. In a model of this procoagulant condition in which these antibodies are injected into pregnant mice, fetal loss was prevented by blocking of complement activation. Specifically, interaction of complement component 5a (C5a) with its receptor is necessary for thrombosis of placental vasculature. Inhibition of complement activation may have a therapeutic role in this disease.

Phylogenetic aspects of the complement system

During evolution two general systems of immunity have emerged: innate or, natural immunity and adaptive (acquired), or specific immunity. The innate system is phylogenetically older and is found in some form in all multicellular organisms, whereas the adaptive system appeared about 450 million years ago and is found in all vertebrates except jawless fish. The complement system in higher vertebrates plays an important role as an effector of both the innate and the acquired immune response, and also participates in various immunoregulatory processes. In lower vertebrates complement is activated by the alternative and lectin pathways and is primarily involved in the opsonization of foreign material. The Agnatha (the most primitive vertebrate species) possess the alternative and lectin pathways while cartilaginous fish are the first species in which the classical pathway appears following the emergence of immunoglobulins. The rest of the poikilothermic species, ranging from teleosts to reptilians, appear to contain a well-developed complement system resembling that of the homeothermic vertebrates. It seems that most of the complement components have appeared after the duplication of primordial genes encoding C3/C4/C5, fB/C2, C1s/C1r/MASP-1/MASP-2, and C6/C7/C8/C9 molecules, in a process that led to the formation of distinct activation pathways. However, unlike homeotherms, several species of poikilotherms (e.g. trout) have recently been shown to possess multiple forms of complement components (C3, factor B) that are structurally and functionally more diverse than those of higher vertebrates. We hypothesize that this remarkable diversity has allowed these animals to expand their innate capacity for immune recognition and response. Recent studies have also indicated the possible presence of complement receptors in protochordates and lower vertebrates. In conclusion, there is considerable evidence suggesting that the complement system is present in the entire lineage of deuterostomes, and regulatory complement components have been identified in all species beyond the protochordates, indicating that the mechanisms of complement activation and regulation have developed in parallel.

Vaccination responses to capsular polysaccharides of Neisseria meningitidis and Haemophilus influenzae type b in two C2-deficient sisters: alternative pathway-mediated bacterial killing and evidence for a novel type of blocking IgG

Meningitis caused by Neisseria meningitidis serogroup W-135 was diagnosed in a 14-year-old girl with a history of neonatal septicemia and meningitis caused by group B streptococci type III. C2 deficiency type I was found in the patient and her healthy sister. Both sisters were vaccinated with tetravalent meningococcal vaccine and a conjugate Haemophilus influenzae type b vaccine. Three main points emerged from the analysis. First, vaccination resulted in serum bactericidal responses demonstrating anticapsular antibody-mediated recruitment of the alternative pathway. Second, addition of C2 to prevaccination sera produced bactericidal activity in the absence of anticapsular antibodies, which suggested that the bactericidal action of antibodies to subcapsular antigens detected in the sera might strictly depend on the classical pathway. A third point concerned a previously unrecognized type of blocking activity. Thus, postvaccination sera of the healthy sister contained IgG that inhibited killing of serogroup W-135 in C2-deficient serum, and the deposition of C3 on enzyme-linked immunosorbent assay plates coated with purified W-135 polysaccharide. Our findings suggested blocking to be serogroup-specific and dependent on early classical pathway components. Retained opsonic activity probably supported post-vaccination immunity despite blocking of the bactericidal activity. The demonstration of functional vaccination responses with recruitment of alternative pathway-mediated defense should encourage further trial of capsular vaccines in classical pathway deficiency states.

Nephritogenic λ Light Chain Dimer: A Unique Human Miniautoantibody Against Complement Factor H

A unique monoclonal Ig λ light chain dimer (protein LOI) was isolated from the serum and urine of a patient with hypocomplementemic membranoproliferative glomerulonephritis. In vitro the λ light chain dimer efficiently activated the alternative pathway of complement (AP). When added to normal human serum, LOI temporarily enhanced AP hemolytic activity, but during a prolonged incubation the hemolytic activity was depleted. Protein LOI was found to bind to factor H, the main regulator molecule of AP. By binding to the short consensus repeat domain 3 of factor H, the dimer LOI blocked one of three interaction sites between H and C3b and thus inhibited the activity of H and induced an uncontrolled activation of the AP. Structural analysis showed that LOI belonged to the Vλ3a subgroup of λ light chains. The variable (V) region of LOI was most closely related to the predicted product of the Vλ3 germline gene Iglv3s2, although it contained several unique residues that in a tertiary homology model structure form an unusual ring of charged residues around a hydrophobic groove in the putative Ag binding site. This site fitted considerably well with a putative binding site in the molecular model of domain 3 of factor H containing a reciprocal ring of charged amino acids around a hydrophobic area. Apparently, functional blocking of factor H by the Ab fragment-like λ light chain dimer had initiated the development of a severe form of membranoproliferative glomerulonephritis. Thus, the λ light chain dimer LOI represents the first described pathogenic miniautoantibody in human disease.

Enhancement of lectin pathway haemolysis by immunoglobulins

We recently reported that indicator sheep erythrocytes (E) coated with mannan and sensitized with mannan-binding lectin (MBL) (E-M-MBL) are lysed by human serum in the absence of calcium via the lectin pathway of complement activation by a process which requires alternative pathway amplification and is associated with increased binding of and control by complement regulatory proteins C4 bp and factor H. In the present study, we investigated the effect of immunoglobulin (Ig) on this haemolysis. Co-sensitization of indicator E with anti-E haemolysin led to threefold enhancement of lectin pathway haemolysis in the absence of calcium, associated with increased binding of C3 and C5. Lysis was enhanced approximately twofold when E-M-MBL were chemically or immunologically coated with IgM or IgA, and fourfold when coated with IgG, prior to lysis in human serum-Mg-ethyleneglycol tetraacetic acid. The presence of haemolysin did not reduce the binding or inhibitory activity of C4 bp, and the enhancing activity of haemolysin was retained in serum depleted of C4 bp. By contrast, binding of factor H was greatly reduced in the presence of haemolysin, which had no enhancing effect in serum depleted of factor H. These experiments demonstrate the ability of IgG, IgM and IgA to enhance lectin pathway cytolysis, and that this enhancement occurs by neutralization of the inhibitory activity of factor H. Immunoglobulin enhancement of lectin pathway cytolysis represents another interaction between the innate and adaptive systems of immunity.

The influence of antibody functional affinity on the effector functions involved in the clearance of circulating immune complexes anti-BSA IgG/BSA

A systematic study was carried out to investigate the role of antibody functional affinity in the capacity of immune complexes (IC) to activate the complement system and to trigger subsequently the molecular events involved in the handling of IC by providing a clearance mechanism. For this purpose, two populations of polyclonal anti-BSA IgG antibodies of different affinities were prepared, with values of 1.89x10(8) M(-1) and 4.94x10(8) M(-1). First we studied the capacity of IC formed at equivalence with both antibodies to activate the classical and the alternative pathways of human complement and the ability of the complexes to bind to erythrocyte C3b-C4b receptors (CR1; CD35). The data showed that the highest affinity antibodies were more efficient in activating complement by both pathways. However, their binding to erythrocyte CR1 was significantly lower compared to the binding of the lowest affinity IgG. Second we compared these IC in terms of their ability to stimulate the respiratory burst of neutrophils (PMN) and to induce the release of PMN lysosomal enzymes. In general, both of these PMN functions were better stimulated by the IC prepared with the IgG antibodies having a highest affinity, although the effects were variable for different IC concentrations. The suggestion to be drawn from the data is that the antibody affinity has an influence on the formation of the immune complex lattice, modulating its three-dimensional structure and the arrangement of the antibody Fc fragments, interfering with complement activation and access to the neutrophil IgG receptors. The significance of these observations for the understanding of how affinity influences the precise biological mechanism that participates in the fate of IC is discussed.

Evolution and diversity of the complement system of poikilothermic vertebrates

In mammals the complement system plays an important role in innate and acquired host defense mechanisms against infection and in various immunoregulatory processes. The complement system is an ancient defense mechanism that is already present in the invertebrate deuterostomes. In these species as well as in agnathans (the most primitive vertebrate species), both the alternative and lectin pathway of complement activation are already present, and the complement system appears to be involved mainly in opsonization of foreign material. With the emergence of immunoglobulins in cartilaginous fish, the classical and lytic pathways first appear. The rest of the poikilothermic species, from teleosts to reptilians, appear to contain a well-developed complement system resembling that of homeothermic vertebrates. However, important differences remain. Unlike homeotherms, several species of poikilotherms have recently been shown to possess multiple forms of complement components (C3 and factor B) that are structurally and functionally more diverse than those of higher vertebrates. It is noteworthy that the multiple forms of C3 that have been characterized in several teleost fish are able to bind with varying efficiencies to various complement-activating surfaces. We hypothesize that this diversity has allowed these animals to expand their innate capacity for immune recognition.

Mannan-specific immunoglobulin G antibodies in normal human serum accelerate binding of C3 to Candida albicans via the alternative complement pathway

Candida albicans activates the classical and alternative complement pathways, leading to deposition of opsonic complement fragments on the cell surface. Our previous studies found that antimannan immunoglobulin G (IgG) in normal human serum (NHS) allows C. albicans to initiate the classical pathway. The purpose of this study was to determine whether antimannan IgG also plays a role in initiation of the alternative pathway. Pooled NHS was rendered free of classical pathway activity by chelation of serum Ca2+ with EGTA alone or in combination with immunoaffinity removal of antimannan antibodies. Kinetic analysis revealed a 6-min lag in detection of C3 binding to C. albicans incubated in EGTA-chelated NHS, compared to a 12-min lag in NHS that was both EGTA chelated and mannan absorbed. The 12-min lag was shortened to 6 min by addition of affinity-purified antimannan IgG. The accelerating effect of antimannan IgG on alternative pathway initiation was dose dependent and was reproduced in a complement binding reaction consisting of six purified proteins of the alternative pathway. Both Fab and F(ab')2 fragments of antimannan IgG facilitated alternative pathway initiation in a manner similar to that observed with intact antibody. Immunofluorescence analysis showed that addition of antimannan IgG to EGTA-chelated and mannan-absorbed serum promoted an early deposition of C3 molecules on the yeast cells but had little or no effect on distribution of the cellular sites for C3 activation. Thus, antimannan IgG antibodies play an important regulatory role in interactions between the host complement system and C. albicans.

Immunoglobulin prevents complement-mediated hyperacute rejection in swine-to-primate xenotransplantation

Immunoglobulins regulate the complement system by activating complement on foreign surfaces and diverting reactive complement proteins away from autologous cell surfaces. Based on this model, we explored the ability of Ig to balance complement activation versus control in a pig-to-primate cardiac xenotransplantation model in which the binding of xenoreactive antibodies of the recipient to graft blood vessels and the activation of complement cause hyperacute rejection. Human IgG added to human serum caused a dose-dependent decrease in deposition of iC3b, cytotoxicity, and heparan sulfate release when the serum was incubated with porcine endothelial cells. This decrease was not caused by alteration in antibody binding or consumption of complement but presumably reflected decreased formation of C3 convertase on the endothelial cells. Infusion of purified human IgG into nonhuman primates prevented hyperacute rejection of porcine hearts transplanted into the primates. As expected, the transplants contained deposits of recipient Ig and C1q but not other complement components. The inhibition of complement on endothelial cell surfaces and in the xenotransplantation model supports the idea that IgG regulates the classical complement pathway and supports therapeutic use of that agent in humoral-mediated disease.

Xenoreactive natural antibodies in the world of natural antibodies: typical or unique?

Xenoreactive natural antibodies are thought to be responsible for initiating the hyperacute rejection of porcine organs transplanted into primates. Progress has been made in recent years in the characterization of the specificity and functions of these antibodies. Xenoreactive natural antibodies recognize Gal alpha 1-3Gal, a carbohydrate related to the blood group A and blood group B antigens. The presence of Gal alpha 1-3Gal may not be sufficient to allow the binding of xenoreactive natural antibodies under physiological conditions; rather, the clustering of Gal alpha 1-3Gal determinants may dictate the extent to which xenoreactive natural antibodies attach to surfaces on which Gal alpha 1-3Gal is expressed. The predominant role of xenoreactive natural antibodies in the pathogenesis of hyperacute rejection in porcine-to-primate xenotransplantation involves the activation of complement. Complement activation is mediated by IgM, not IgG xenoreactive natural antibodies. Based on functional avidity, thermal binding optima, thermal liability and homogeneity of binding interactions, xenoreactive natural IgM appear to be members of a family of natural antibodies which includes isohaemagglutinins. The classification of xenoreactive antibodies and isohaemagglutinins into a family of antibodies may provide further insight into the nature and physiological functions of xenoreactive natural antibodies.

Immunoglobulin G enhances C3 degradation on coagulase-negative staphylococci

Antibody and complement are essential to host defense against infection with coagulase-negative staphylococci in the neonate. To evaluate the influence of antibody on C3 deposition, we compared the C3 fragments deposited on coagulase-negative staphylococci after opsonization with normal human serum or with hypogammaglobulinemic serum. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (immunoblot) analysis, the degradation products of C3 were less apparent at 1 and 2 min after opsonization with hypo- and agammaglobulinemic serum than those from normal human serum. This finding suggested that antibody contained in normal human serum contributes to efficient C3 deposition in the early phases of opsonization. There was no clear difference in C3 deposition when slime-producing strains were compared with non-slime-producing strains. The addition of intravenous immunoglobulin to hypogammaglobulinemic serum and serum from premature neonates rendered C3 deposition comparable to that seen with normal human serum. The data from these experiments suggest that the addition of immunoglobulin G may improve host defense against coagulase-negative staphylococci in the hypogammaglobulinemic premature neonate by enhancing C3 deposition, thus promoting opsonophagocytosis of these bacteria.

Early complement components enhance neutralization of Chlamydia trachomatis infectivity by human sera

Immunoglobulin G in human serum neutralizes chlamydial infectivity in vitro. Complement-intact, C5-depleted, and C8-depleted human serum all have significantly more neutralizing activity than serum heated to inactivate early components of complement. Cobra venom factor, an analog of human C3b, enhances neutralization of antichlamydial immunoglobulin G in the absence of early complement components.

Vaccinia virus complement-control protein prevents antibody-dependent complement-enhanced neutralization of infectivity and contributes to virulence

The role of a viral gene product in evasion of the host immune response was investigated. The antibody-dependent complement-enhanced neutralization of vaccinia virus infectivity was prevented by the culture medium from vaccinia virus-infected cells. The vaccinia virus complement-control protein (VCP) was identified as the secreted product of vaccinia virus gene C21L and has homology to a group of eukaryotic genes encoding regulators of complement activation. Thus, the culture medium from cells infected with a C21L deletion mutant was VCP deficient and had little or no effect on antibody-dependent complement-enhanced neutralization. In addition, the anticomplement effect was associated with the C21L-encoded protein partially purified from the medium of cells infected with wild-type virus. Antibody-dependent, complement-enhanced neutralization of vaccinia virus occurred with a complement source that was deficient in the classical pathway complement component C4 and required the alternative pathway complement factor B. Furthermore, the presence of VCP abrogated the complement-enhanced neutralization in C4-deficient serum. Together with previous hemolysis data, the present result suggests that VCP can inhibit both the classical and alternative pathways of complement activation. Skin lesions caused by the C21L deletion mutant were smaller than those caused by wild-type virus, demonstrating an important role for VCP in virulence. The C21L deletion mutant also was attenuated in C4-deficient guinea pigs, consistent with in vitro studies. Vaccinia virus appears to have acquired the ability to regulate the complement cascade for the purpose of evading the host immune response.

Mesangial cell autoantigens in immunoglobulin A nephropathy and Henoch-Schönlein purpura

The autoantigen(s) that we have previously described in human glomeruli, recognized in IgA nephropathy, has (have) been identified as mesangial cell in origin. Cultured mesangial cells expressed 48- and 55-kD components binding IgG isotype autoantibodies (IgG-MESCA) present in sera of patients with both IgA nephropathy and Henoch-Schönlein purpura (HSP). IgG-MESCA were not detected in sera of normals, or patients with other autoimmune-mediated glomerulonephritides: anti-glomerular basement membrane disease, Wegener's granulomatosis, or in IgM-mesangial proliferative disease. Binding specificity was proven by F(ab')2 studies in enzyme-linked immunosorbent assay (ELISA) and Western blotting, and there was no significant affinity of IgA or IgM immunoglobulins. Fluorescein isothiocyanate-conjugated IgG from ELISA-positive sera localized to the mesangium and peripheral capillary loops of glomeruli, supporting the belief that the antigen is expressed in normal human renal tissue. However, only about one third of mesangial cells in culture showed affinity for IgG from ELISA-positive sera, suggesting variable expression of the antigen(s) in vitro. The only autoantigen(s) present in glomeruli, and extractable from whole normal glomeruli by the techniques employed, localized on the mesangial cell. In both IgA nephropathy and HSP, autoimmunity was intermittently present, with fluctuating levels of IgG-MESCA detectable in sera. There were positive correlations with the degree of glomerular injury assessed by erythrocyturia and proteinuria in IgA nephropathy, but significance was reached with only the degree of hematuria in HSP. These findings suggest a contributing role in the pathogenesis of the mesangial proliferative lesions and demonstrate autoimmunity common to both IgA nephropathy and HSP.

Evolution of the complement system

The ancestral form of the alternative pathway of complement activation probably originated as a primitive independent immune system. Subsequent evolution of an adaptive immune response drove the specialization of the classical pathway to connect antibody-mediated nonself recognition to the complement-dependent effector mechanisms. In this article Timothy Farries and John Atkinson consider how the contemporary complexity arose by a succession of credible alterations at the genetic level, and the selective advantages provided at each step.

Infectious diseases associated with complement deficiencies

The complement system consists of both plasma and membrane proteins. The former influence the inflammatory response, immune modulation, and host defense. The latter are complement receptors, which mediate the cellular effects of complement activation, and regulatory proteins, which protect host cells from complement-mediated injury. Complement activation occurs via either the classical or the alternative pathway, which converge at the level of C3 and share a sequence of terminal components. Four aspects of the complement cascade are critical to its function and regulation: (i) activation of the classical pathway, (ii) activation of the alternative pathway, (iii) C3 convertase formation and C3 deposition, and (iv) membrane attack complex assembly and insertion. In general, mechanisms evolved by pathogenic microbes to resist the effects of complement are targeted to these four steps. Because individual complement proteins subserve unique functional activities and are activated in a sequential manner, complement deficiency states are associated with predictable defects in complement-dependent functions. These deficiency states can be grouped by which of the above four mechanisms they disrupt. They are distinguished by unique epidemiologic, clinical, and microbiologic features and are most prevalent in patients with certain rheumatologic and infectious diseases. Ethnic background and the incidence of infection are important cofactors determining this prevalence. Although complement undoubtedly plays a role in host defense against many microbial pathogens, it appears most important in protection against encapsulated bacteria, especially Neisseria meningitidis but also Streptococcus pneumoniae, Haemophilus influenzae, and, to a lesser extent, Neisseria gonorrhoeae. The availability of effective polysaccharide vaccines and antibiotics provides an immunologic and chemotherapeutic rationale for preventing and treating infection in patients with these deficiencies.

The effect of antibody isotype and antigenic epitope density on the complement-fixing activity of immune complexes: a systematic study using chimaeric anti-NIP antibodies with human Fc regions

A systematic study has been carried out to investigate the role of immunoglobulin isotype, epitope density, and antigen/antibody ratio on the capacity of immune complexes to activate the classical and alternative pathways of human complement and for the complexes subsequently to bind to erythrocyte C3b-C4b receptors (CRI). For this purpose, a series of chimaeric monoclonal anti-NIP antibodies was used, which all shared the same combining site but had different human constant domains. Antigen epitope density was varied by coupling different numbers of NIP hapten molecules to bovine serum albumin. All three parameters affect complement fixation. In general, complement activation is better in antibody excess and at equivalence than it is in antigen excess, and better at high epitope density than at low epitope density, although the effects are variable for different immunoglobulin isotypes and for the two pathways. It has been confirmed that IgG1 and IgG3 are good activators of the classical pathway and are tolerant to variations in both epitope density and antigen/antibody ratio. IgG4 and IgA do not activate the classical pathway in any circumstances. IgG2 activates the classical pathway only at high epitope density and at equivalence or antibody excess. IgM activates the classical pathway well only at the higher epitope densities and at equivalence or antibody excess but, in addition, shows an interesting and unexpected prozone phenomenon where immune complex in antibody excess inhibits complement activation by the classical pathway. The results of the alternative pathway activation are strikingly different. IgA is by far the best activator of the alternative pathway and is relatively tolerant to epitope density and to antigen/antibody ratio. IgM, IgG1 and IgG3 do not significantly activate the alternative pathway in any circumstances. IgG2 is the best IgG subclass for alternative pathway activation but requires high epitope density and equivalence or antibody excess. Binding to CR1 in general parallels the amount of complement fixed independent to the pathway by which it is fixed. However, IgG1 and IgG3 complexes in antigen excess activate complement well but bind poorly to CR1. Nascently formed complexes seem to bind complement in a way that is similar to that bound by preformed complexes, but are then less able to bind to red cell CR1. These observations help to explain the pathogenesis of complement activation in various autoimmune and immune complex diseases such as systemic lupus erythematosus, autoimmune thyroiditis and others.

Inherited complement deficiency states: implications for immunity and immunological disease

The study of complement deficiency states and their influence on immune function has generated new insights and still provides a challenge to continued investigation. The association of classical pathway deficiencies (C1, C4, C2 or C3) with immunological diseases such as SLE and glomerulonephritis has contributed to current knowledge concerning complement-dependent immune complex handling and elimination. Susceptibility to systemic infection with encapsulated bacteria is encountered in most forms of inherited complement deficiency. Recurrent neisserial infection is the only clinical manifestation clearly associated with defects of the membranolytic sequence C5-C9, while deficiency of properdin, a component of the alternative activation pathway, appears to predispose to nonrecurrent meningococcal disease. Inherited complement deficiency is rare, but the perspective is widened by the more common occurence of acquired defects in immunological diseases, and the apparent requirement for efficient complement recruitment in host defense. Another aspect is the possibility that complement deficiency might alleviate or prevent inflammatory symptoms. Notably, complement deficiency has not been reported in classical rheumatoid arthritis. Considerations of this kind would be refuted or modified by findings of complement deficiency in single patients.

Activation of the human complement system by phenolic glycolipid 1 of Mycobacterium leprae

The activation of the complement system by phenolic glycolipid 1 (PGL) from Mycobacterium leprae was studied. It was found that PGL consumed haemolytic complement through both the classical and the alternative pathways. This was further studied at the level of C3. Although the activation was independent of anti-PGL antibodies present in normal human serum, the addition of antibody augmented the activation of complement by PGL. The uptake of C3 through the classical pathway was enhanced predominantly by IgM antibody whereas, IgG antibody against PGL was responsible for the augmentation of the alternative pathway activation. Furthermore, it was found that both the disaccharide and trisaccharide components of PGL were able to activate the complement system.

Complement-fixing properties of human IgA antibodies. Alternative pathway complement activation by plastic-bound, but not specific antigen-bound, IgA

The complement-fixing properties of human IgA antibodies bound to specific antigen, or coated directly on plastic surfaces, were examined in comparison with those of IgG antibodies. Use was made of antigen-binding (anti-staphylococcal alpha-toxin) IgA and IgG monoclonal antibodies and normal polyclonal IgA and IgG, purified greater than 99.9% by avoidance of denaturing processes. Complement-fixation ELISA was used, with a high density of biotin-conjugated staphylococcal alpha-toxin bound to avidin-coated plates for the efficient capture of antibodies, and conditions were adjusted for the assessment of classical and alternative pathways of complement activation. Although IgA coated directly on plastic surfaces activated the alternative complement pathway in a dose-dependent manner, IgA antibodies bound to antigen failed to fix complement by either classical or alternative pathways. In contrast, IgG antibodies, either bound to antigen or coated directly on plastic, activated complement mainly by the classical pathway. It was concluded that the complexation of IgA antibodies with antigen is insufficient to elicit complement activation: rather a degree of denaturation seems to play a part in the expression of alternative complement pathway-activating properties by IgA.