Complement factor H related proteins (CFHRs)

Factor H related proteins comprise a group of five plasma proteins: CFHR1, CFHR2, CFHR3, CFHR4 and CFHR5, and each member of this group binds to the central complement component C3b. Mutations, genetic deletions, duplications or rearrangements in the individual CFHR genes are associated with a number of diseases including atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathies (C3 glomerulonephritis (C3GN), dense deposit disease (DDD) and CFHR5 nephropathy), IgA nephropathy, age related macular degeneration (AMD) and systemic lupus erythematosus (SLE). Although complement regulatory functions were attributed to most of the members of the CFHR protein family, the precise role of each CFHR protein in complement activation and the exact contribution to disease pathology is still unclear. Recent publications show that CFHR proteins form homo- as well as heterodimers. Genetic abnormalities within the CFHR gene locus can result in hybrid proteins with affected dimerization or recognition domains which cause defective functions. Here we summarize the recent data about CFHR genes and proteins in order to better understand the role of CFHR proteins in complement activation and in complement associated diseases.

An engineered construct combining complement regulatory and surface-recognition domains represents a minimal-size functional factor H

Complement is an essential humoral component of innate immunity; however, its inappropriate activation leads to pathology. Polymorphisms, mutations, and autoantibodies affecting factor H (FH), a major regulator of the alternative complement pathway, are associated with various diseases, including age-related macular degeneration, atypical hemolytic uremic syndrome, and C3 glomerulopathies. Restoring FH function could be a treatment option for such pathologies. In this article, we report on an engineered FH construct that directly combines the two major functional regions of FH: the N-terminal complement regulatory domains and the C-terminal surface-recognition domains. This minimal-size FH (mini-FH) binds C3b and has complement regulatory functions similar to those of the full-length protein. In addition, we demonstrate that mini-FH binds to the FH ligands C-reactive protein, pentraxin 3, and malondialdehyde epitopes. Mini-FH was functionally active when bound to the extracellular matrix and endothelial cells in vitro, and it inhibited C3 deposition on the cells. Furthermore, mini-FH efficiently inhibited complement-mediated lysis of host-like cells caused by a disease-associated FH mutation or by anti-FH autoantibodies. Therefore, mini-FH could potentially be used as a complement inhibitor targeting host surfaces, as well as to replace compromised FH in diseases associated with FH dysfunction.

Identification of a major linear C1q epitope allows detection of systemic lupus erythematosus anti-C1q antibodies by a specific peptide-based enzyme-linked immunosorbent assay

OBJECTIVE

Autoantibodies against C1q strongly correlate with the occurrence of severe nephritis in patients with systemic lupus erythematosus (SLE). We undertook this study to determine whether identification of the C1q epitope(s) recognized by these autoantibodies might lead to a better diagnostic assay and help elucidate the putative role of C1q and anti-C1q in SLE.

METHODS

SLE patient-derived anti-C1q Fab were used in a microarray-based peptide scan to identify the peptide sequence recognized by anti-C1q. Anti-C1q Fab binding to the target peptide was further analyzed using real-time interaction measurements (surface plasmon resonance) and peptide-based enzyme-linked immunosorbent assays (ELISAs).

RESULTS

A peptide scan of the collagen-like region of C1q identified 2 regions, 1 on the A chain and 1 on the B chain, that were the targets of the anti-C1q Fab. Binding was confirmed by surface plasmon resonance and showed nanomolar affinity. The A chain-derived peptide could specifically be detected in a peptide-based ELISA by SLE patient sera. Competition experiments suggested that this peptide represented one of the major linear epitopes of C1q that is the target of anti-C1q in SLE. Serum antibodies from most SLE patients but not from healthy individuals specifically bound to this epitope. Binding to the peptide correlated with binding of the same sera to native C1q but was found to be more sensitive for the detection of lupus nephritis.

CONCLUSION

We identified a major linear epitope of C1q that is the target of anti-C1q in SLE. The ELISA using this peptide was more specific and more sensitive than a conventional anti-C1q assay for the detection of active nephritis in SLE patients.

Overall neutralization of complement factor H by autoantibodies in the acute phase of the autoimmune form of atypical hemolytic uremic syndrome

Complement is a major innate immune surveillance system. One of its most important regulators is the plasma protein factor H (FH). FH inactivation by mutations or by autoantibodies is associated with a thrombotic microangiopathy disease, atypical hemolytic uremic syndrome. In this study, we report the characterization of blood samples from 19 anti-FH Ab-positive atypical hemolytic uremic syndrome patients collected at the acute phase of the disease. Analyses of the functional consequences and epitope mapping, using both fluid phase and solid phase approaches, were performed. The anti-FH Abs perturbed FH-mediated cell protection (100%), inhibited FH interaction with C3 (46%), and caused C3 consumption (47%). The Abs were directed against multiple FH epitopes located at the N and C termini. In all tested patients, high titers of FH-containing circulating immune complexes were detected. The circulating immune complex titers correlated with the disease stage better than did the Ab titers. Our results show that anti-FH autoantibodies induce neutralization of FH at acute phase of the disease, leading to an overall impairment of several functions of FH, extending the role of autoantibodies beyond the impairment of the direct cell surface protection.

Renal transplantation under prophylactic eculizumab in atypical hemolytic uremic syndrome with CFH/CFHR1 hybrid protein

We report the first observation of successful kidney transplantation under pre-emptive eculizumab treatment in a 7-year-old boy with atypical hemolytic uremic syndrome (aHUS) and a known hybrid CFH/CFHR1 gene, who was dependent on plasma therapy during the 3-year dialysis period. The hybrid CFH/CFHR1 protein has an altered C3b/C3d binding, is incapable to protect cells from complement attack and is directly implicated in aHUS pathogenesis. There was no evidence of recurrence during the first 16-month follow-up period. We conclude that eculizumab alone, without plasma therapy (plasma infusion and/or plasma exchange), is sufficient to prevent recurrence of aHUS and to maintain long-term graft function.

Acquired and genetic complement abnormalities play a critical role in dense deposit disease and other C3 glomerulopathies

Dense deposit disease and glomerulonephritis with isolated C3 deposits are glomerulopathies characterized by deposits of C3 within or along the glomerular basement membrane. Previous studies found a link between dysregulation of the complement alternative pathway and the pathogenesis of these diseases. We analyzed the role of acquired and genetic complement abnormalities in a cohort of 134 patients, of whom 29 have dense deposit disease, 56 have glomerulonephritis with isolated C3 deposits, and 49 have primary membranoproliferative glomerulonephritis type I, with adult and pediatric onset. A total of 53 patients presented with a low C3 level, and 65 were positive for C3 nephritic factor that was significantly more frequently detected in patients with dense deposit disease than in other histological types. Mutations in CFH and CFI genes were identified in 24 patients associated with a C3 nephritic factor in half the cases. We found evidence for complement alternative pathway dysregulation in 26 patients with membranoproliferative glomerulonephritis type I. The complement factor H Y402H variant was significantly increased in dense deposit disease. We identified one at-risk membrane cofactor protein (MCP) haplotype for glomerulonephritis with isolated C3 deposits and membranoproliferative glomerulonephritis type I. Thus, our results suggest a critical role of fluid-phase alternative pathway dysregulation in the pathogenesis of C3 glomerulopathies as well as in immune complex-mediated glomerular diseases. The localization of the C3 deposits may be under the influence of MCP expression.

Genetics of hemolytic uremic syndromes

Hemolytic uremic syndrome (HUS) is a very rare disease (two cases per year per 1 million population) but represents the most common cause of acute renal failure in young children that require dialysis. The majority of cases in childhood (90%) is caused by Shiga toxin producing Escherichia coli infection. This typical form of the disease does not relapse and has a good prognosis if the acute status can be managed successfully. Atypical HUS (aHUS) is a severe and frequently relapsing disorder with the same triad of thrombocytopenia, hemolysis and acute renal failure in the absence of Shiga toxin E. coli infection. More than 50% of patients with atypical HUS progress to chronic renal dysfunction and 10% die due to complications of the disease. Atypical HUS appears to have a genetic basis. Mutations in genes coding for components of the alternative complement pathway are found in about 60% of cases. The clinical presentation of aHUS overlaps with that of other thrombotic microangiopathies, rendering the diagnosis on clinical grounds alone extremely difficult. In recent years, genetic testing has opened the way for molecular diagnostics and helped establishing therapeutically and prognostically useful genotype-phenotype correlations. This review summarizes recent findings regarding the genetic basis of the HUS. The pathophysiology of the disease and the implication of genetic abnormalities in the complement system for the different types of HUS are discussed.

Functional complement C1q abnormality leads to impaired immune complexes and apoptotic cell clearance

C1q plays a key role in apoptotic cell and immune complex removal. Its absence contributes to the loss of tolerance toward self structures and development of autoimmunity. C1q deficiencies are extremely rare and are associated with complete lack of C1q or with secretion of surrogate C1q fragments. To our knowledge, we report the first case of a functional C1q abnormality, associated with the presence of a normal C1q molecule. Homozygous GlyB63Ser mutation was found in a patient suffering from lupus with neurologic manifestations and multiple infections. The GlyB63Ser C1q bound to Igs, pentraxins, LPSs, and apoptotic cells, similarly to C1q from healthy donors. However, the interaction of C1r(2)C1s(2) and C1 complex formation was abolished, preventing further complement activation and opsonization by C3. The mutation is located between LysB(61) and LysB(65) of C1q, suggested to form the C1r binding site. Our data infer that the binding of C1q to apoptotic cells in humans is insufficient to assure self-tolerance. The opsonization capacity of C4 and C3 fragments has to be intact to fight infections and to prevent autoimmunity.

Heme interacts with c1q and inhibits the classical complement pathway

C1q is the recognition subunit of the first component of the classical complement pathway. It participates in clearance of immune complexes and apoptotic cells as well as in defense against pathogens. Inappropriate activation of the complement contributes to cellular and tissue damage in different pathologies, urging the need for the development of therapeutic agents that are able to inhibit the complement system. In this study, we report heme as an inhibitor of C1q. Exposure of C1q to heme significantly reduced the activation of the classical complement pathway, mediated by C-reactive protein (CRP) and IgG. Interaction analyses revealed that heme reduces the binding of C1q to CRP and IgG. Furthermore, we demonstrated that the inhibition of C1q interactions results from a direct binding of heme to C1q. Formation of complex of heme with C1q caused changes in the mechanism of recognition of IgG and CRP. Taken together, our data suggest that heme is a natural negative regulator of the classical complement pathway at the level of C1q. Heme may play a role at sites of excessive tissue damage and hemolysis where large amounts of free heme are released.

Alternative complement pathway assessment in patients with atypical HUS

The atypical Hemolytic Uremic Syndrome (aHUS) is a rare thrombotic microangiopathy leading to end stage renal disease in approximately 60% of patients. Over the last decade, a clear link has been demonstrated between this disease and defective complement regulation. The hallmark of the aHUS is the association with mutations in complement alternative pathway genes. Endothelial damage is related to complement dysregulation, but the exact mechanism is just starting to be elucidated. Screening for and characterization of mutations in the components of the C3 convertase (C3 and FB) or its regulators (FH, FI, MCP, and Thrombomodulin) or anti-FH antibodies has become an indispensable part of the disease's diagnostic. This review will initially summarize current knowledge on the understanding of complement activation and regulation, followed by a description on the genetic analysis as well as the methods used for complement protein quantification. Another part of this review will focus on the mechanisms of action of aHUS-associated mutations. We will emphasize on when and why some mutations lead to protein deficiency, while others result in - to dysfunctional but normally expressed proteins. Finally, we will discuss how the therapy of aHUS patients can be modified according to the functional consequences of each particular genetic defect.

A human FVIII inhibitor modulates FVIII surface electrostatics at a VWF-binding site distant from its epitope

SUMMARY BACKGROUND

BO2C11 is a human monoclonal factor (F) VIII inhibitor. When bound to the C2 domain of FVIII, the Fab fragment of BO2C11 (Fab(BO2C11)) buries a surface of C2 that contains residues participating in a binding site for von Willebrand factor (VWF). BO2C11 has thus been proposed to neutralize FVIII by steric hindrance.

OBJECTIVES

The BO2C11 epitope on C2 overlaps with residues located at the periphery of the putative VWF binding site; hence, most of the residues that constitute the VWF binding site on C2 and a3 remain accessible for VWF interaction following BO2C11/FVIII complex formation. We thus investigated the contribution of alternative molecular mechanisms to FVIII inactivation by BO2C11.

METHODS

Continuum electrostatic calculations were applied to the crystal structure of C2, free or Fab(BO2C11)-complexed. In silico predictions were confirmed by site-directed mutagenesis and VWF-binding assays of the mutated FVIII.

RESULTS

Binding of Fab(BO2C11) to C2 induced perturbations in the electrostatic potential of C2 and in the local electrostatic parameters of 18 charged residues in C2, which are distant from the BO2C11 epitope. Nine of the predicted electrostatic hotspots clustered on the VWF-binding site of C2. Mutation of some of the predicted electrostatic hotspots has been associated with hemophilia A and reduced VWF binding in vitro.

CONCLUSIONS

Inhibitors may neutralize FVIII by alteration of protein surface electrostatics at a long distance from their epitope. Perturbation of the electrostatic environment of C2, either upon binding by anti-FVIII antibodies or consecutive to missense mutations in the F8 gene, may lead to hampered VWF binding and reduced FVIII residence time in circulation.

Interaction of the globular domain of human C1q with Salmonella typhimurium lipopolysaccharide

Gram-negative bacteria can bind complement protein C1q in an antibody-independent manner and activate classical pathway via their lipopolysaccharides (LPS). Earlier studies have implicated the collagen-like region of human C1q in binding LPS. In recent years, a number of C1q target molecules, previously considered to interact with collagen-like region of C1q, have been shown to bind via the globular domain (gC1q). Here we report, using recombinant forms of the globular head regions of C1q A, B and C chains, that LPS derived from Salmonella typhimurium interact specifically with the B-chain of the gC1q domain in a calcium-dependent manner. LPS and IgG-binding sites on the gC1q domain appear to be overlapping and this interaction can be inhibited by a synthetic C1q inhibitor, suggesting common interacting mechanisms.

Antibodies Use Heme as a Cofactor to Extend Their Pathogen Elimination Activity and to Acquire New Effector Functions

Various pathological processes are accompanied by release of high amounts of free heme into the circulation. We demonstrated by kinetic, thermodynamic, and spectroscopic analyses that antibodies have an intrinsic ability to bind heme. This binding resulted in a decrease in the conformational freedom of the antibody paratopes and in a change in the nature of the noncovalent forces responsible for the antigen binding. The antibodies use the molecular imprint of the heme molecule to interact with an enlarged panel of structurally unrelated epitopes. Upon heme binding, monoclonal as well as pooled immunoglobulin G gained an ability to interact with previously unrecognized bacterial antigens and intact bacteria. IgG-heme complexes had an enhanced ability to trigger complement-mediated bacterial killing. It was also shown that heme, bound to immunoglobulins, acted as a cofactor in redox reactions. The potentiation of the antibacterial activity of IgG after contact with heme may represent a novel and inducible innate-type defense mechanism against invading pathogens.

C1q and its growing family

C1q is the target recognition protein of the classical complement pathway and a major connecting link between innate and acquired immunity. As a charge pattern recognition molecule of innate immunity, C1q can engage a broad range of self and non-self ligands via its heterotrimeric globular (gC1q) domain and thus trigger the classical pathway. The trimeric gC1q signature domain has been identified in a variety of non-complement proteins that can be grouped together as a C1q family. The X-ray crystal structures of the gC1q domain of a few members of the C1q family reveal a compact jelly-roll beta-sandwich fold similar to that of the multifunctional tumor necrosis factor (TNF) ligand family, hence the C1q and TNF superfamily. This review is an update on the structural and functional aspects of the gC1q domain of human C1q. We also mention the diverse range of proteins that utilize a gC1q domain in order to reflect on its importance as a versatile scaffold to support a variety of functions.

Iron Ions and Haeme Modulate the Binding Properties of Complement Subcomponent C1q and of Immunoglobulins

The complement system and circulating antibodies play a major role in the defence against infection. They act at the sites of inflammation, where the harsh microenvironment and the oxidative stress lead to the release of free iron ions and haeme. The aim of this study was to analyse the consequences of the exposure of C1q and immunoglobulins to iron ions or haeme. The changes in target recognition by C1q and in the rheumatoid factor activity of the immunoglobulins were investigated. The exposure of C1q to ferrous ions increased its binding to IgG and to IgM. In contrast, haeme inhibited C1q binding to all studied targets, especially to IgG1 and C-reactive protein. Thus, the haeme released as a result of tissue damage and oxidative stress may act as a negative feedback regulator of an inappropriate complement triggering as seen in ischaemia-reperfusion tissue injury. The results also show that iron ions and haeme were able to reveal rheumatoid factor activity of IgG. The modulation of the C1q-target binding as well as the revealing of rheumatoid factor activity of IgG by exposure to redox-active agents released at the sites of inflammation may have important consequences for the understanding of the immunopathological mechanisms of inflammatory and autoimmune diseases.

Existence of different but overlapping IgG- and IgM-binding sites on the globular domain of human C1q

C1q is the first subcomponent of the classical complement pathway that binds antigen-bound IgG or IgM and initiates complement activation via association of serine proteases C1r and C1s. The globular domain of C1q (gC1q), which is the ligand-recognition domain, is a heterotrimeric structure composed of the C-terminal regions of A (ghA), B (ghB), and C (ghC) chains. The expression and functional characterization of ghA, ghB, and ghC modules have revealed that each chain has some structural and functional autonomy. Although a number of studies have tried to identify IgG-binding sites on the gC1q domain, no such attempt has been made to localize IgM-binding site. On the basis of the information available via the gC1q crystal structure, molecular modeling, mutational studies, and bioinformatics, we have generated a series of substitution mutants of ghA, ghB, and ghC and examined their interactions with IgM. The comparative analysis of IgM- and IgG-binding abilities of the mutants suggests that the IgG- and IgM-binding sites within the gC1q domain are different but may overlap. Whereas Arg(B108), Arg (B109), and Tyr(B175) mainly constitute the IgM-binding site, the residues Arg(B114), Arg(B129), Arg(B163), and His(B117) that have been shown to be central to IgG binding are not important for the C1q-IgM interaction. Given the location of Arg(B108), Arg (B109), and Tyr(B175) in the gC1q crystal structure, it is likely that C1q interacts with IgM via the top of the gC1q domain.

Interaction of C1q with IgG1, C-reactive protein and pentraxin 3: mutational studies using recombinant globular head modules of human C1q A, B, and C chains

C1q is the first subcomponent of the classical complement pathway that can interact with a range of biochemically and structurally diverse self and nonself ligands. The globular domain of C1q (gC1q), which is the ligand-recognition domain, is a heterotrimeric structure composed of the C-terminal regions of A (ghA), B (ghB), and C (ghC) chains. The expression and functional characterization of ghA, ghB, and ghC modules have revealed that each chain has specific and differential binding properties toward C1q ligands. It is largely considered that C1q-ligand interactions are ionic in nature; however, the complementary ligand-binding sites on C1q and the mechanisms of interactions are still unclear. To identify the residues on the gC1q domain that are likely to be involved in ligand recognition, we have generated a number of substitution mutants of ghA, ghB, and ghC modules and examined their interactions with three selected ligands: IgG1, C-reactive protein (CRP), and pentraxin 3 (PTX3). Our results suggest that charged residues belonging to the apex of the gC1q heterotrimer (with participation of all three chains) as well as the side of the ghB are crucial for C1q binding to these ligands, and their contribution to each interaction is different. It is likely that a set of charged residues from the gC1q surface participate via different ionic and hydrogen bonds with corresponding residues from the ligand, instead of forming separate binding sites. Thus, a recently proposed model suggesting the rotation of the gC1q domain upon ligand recognition may be extended to C1q interaction with CRP and PTX3 in addition to IgG1.

Role of Ca2+ in the electrostatic stability and the functional activity of the globular domain of human C1q

C1q is the recognition subunit of the classical pathway of the complement system and a major connecting link between classical pathway-driven innate immunity and IgG- or IgM-mediated acquired immunity. The basic structural subunit of C1q is composed of an N-terminal triple-helical collagen-like region and a C-terminal heterotrimeric globular head domain (gC1q) that is made up of individual A, B, and C chains. Recent crystallographic studies have revealed that the gC1q domain, which is the main target-binding region of C1q, has a compact and spherical heterotrimeric assembly, held together by both electrostatic and nonpolar interactions, with quasi-3-fold symmetry. A characteristic feature of the gC1q domain is the presence of a exposed Ca(2+) located near the apex. We have investigated, using theoretical and experimental approaches, the role of Ca(2+) in the electrostatic stability and target-binding properties of the native C1q as well as recombinant monomeric forms of the C-terminal regions of the A, B, and C chains. Here, we report that Ca(2+) primarily influences the target recognition properties of C1q toward IgG, IgM, C-reactive protein, and pentraxin 3. At pH 7.4, the loss of Ca(2+) leads to changes in the direction of electric moment from coaxial (where the putative C-reactive protein-binding site is located) to perpendicular to the molecular axis (toward the most likely IgG-binding site), which appears important for target recognition by C1q and subsequent complement activation.