Complement factor I and cofactors in control of complement system convertase enzymes

A surface lipoprotein on Pasteurella multocida binds complement factor I to promote immune evasion

Pasteurella multocida is the leading cause of wound infections in humans following animals' bites or scratches. This bacterium is also commonly found in the respiratory tract of many mammals and can cause serious diseases resulting in the brutal rapid death of infected animals, especially cattle. To prevent these infections in cattle, a subunit-based vaccine utilizing the surface lipoprotein PmSLP was developed and showed remarkable protection with a single dose administration. Here, we report that PmSLP binds host complement factor I (FI) and facilitates cleavage of complement components C3b and C4b independently of any cofactors (e.g FH, C4BP), thereby allowing the pathogen to evade host defence. Cryo-EM structure of PmSLP bound to FI reveals that PmSLP stimulates FI enzymatic activity by stabilizing the catalytic domain. This is the first time that a bacterial protein has been shown to directly activate FI independent of complement cofactors and target all arms of the complement cascade.

The SCR-17 and SCR-18 glycans in human complement factor H enhance its regulatory function

Human complement factor H (CFH) plays a central role in regulating activated C3b to protect host cells. CFH contain 20 short complement regulator (SCR) domains and eight N-glycosylation sites. The N-terminal SCR domains mediate C3b degradation while the C-terminal CFH domains bind to host cell surfaces to protect these. Our earlier study of Pichia-generated CFH fragments indicated a self-association site at SCR-17/18 that comprises a dimerization site for human factor H. Two N-linked glycans are located on SCR-17 and SCR-18. Here, when we expressed SCR-17/18 without glycans in an Escherichia coli system, analytical ultracentrifugation showed that no dimers were now formed. To investigate this novel finding, full-length CFH and its C-terminal fragments were purified from human plasma and Pichia pastoris respectively, and their glycans were enzymatically removed using PNGase F. Using size-exclusion chromatography, mass spectrometry, and analytical ultracentrifugation, SCR-17/18 from Pichia showed notably less dimer formation without its glycans, confirming that the glycans are necessary for the formation of SCR-17/18 dimers. By surface plasmon resonance, affinity analyses interaction showed decreased binding of deglycosylated full-length CFH to immobilized C3b, showing that CFH glycosylation enhances the key CFH regulation of C3b. We conclude that our study revealed a significant new aspect of CFH regulation based on its glycosylation and its resulting dimerization.

Complement-Coagulation Cross-talk: Factor H-mediated regulation of the Complement Classical Pathway activation by fibrin clots

The classical pathway of the complement system is activated by the binding of C1q in the C1 complex to the target activator, including immune complexes. Factor H is regarded as the key downregulatory protein of the complement alternative pathway. However, both C1q and factor H bind to target surfaces via charge distribution patterns. For a few targets, C1q and factor H compete for binding to common or overlapping sites. Factor H, therefore, can effectively regulate the classical pathway activation through such targets, in addition to its previously characterized role in the alternative pathway. Both C1q and factor H are known to recognize foreign or altered-self materials, e.g., bacteria, viruses, and apoptotic/necrotic cells. Clots, formed by the coagulation system, are an example of altered self. Factor H is present abundantly in platelets and is a well-known substrate for FXIIIa. Here, we investigated whether clots activate the complement classical pathway and whether this is regulated by factor H. We show here that both C1q and factor H bind to the fibrin formed in microtiter plates and the fibrin clots formed under in vitro physiological conditions. Both C1q and factor H become covalently bound to fibrin clots, and this is mediated via FXIIIa. We also show that fibrin clots activate the classical pathway of complement, as demonstrated by C4 consumption and membrane attack complex detection assays. Thus, factor H downregulates the activation of the classical pathway induced by fibrin clots. These results elucidate the intricate molecular mechanisms through which the complement and coagulation pathways intersect and have regulatory consequences.

Identifying Circulating Urotensin II and Urotensin II-Related Peptide-Generating Enzymes in the Human Plasma Fraction Cohn IV-4

Urotensin II (UII) and UII-related peptide (URP) are vasoactive peptide hormones causing strong vasoconstriction or vasodilation, depending on the type of blood vessel. In humans, the active forms are resulting from proteolytic cleavage of their inactive precursor protein. In blood plasma, a defined protease converting the inactive UII and URP precursors into their active forms has not been identified yet. Using mass spectrometry-based enzyme screening for detecting UII- and URP-converting enzymes, the human plasma fraction Cohn IV-4 was chromatographed, and the resulting fractions were screened for UII- or URP-generating activity. Plasma kallikrein (PK) as a UII- and URP-generating protease was identified. URP generation was also found for the serine protease factor XIa, plasmin, thrombin, and, to a smaller extent, factor XIIa. It was demonstrated that in the Cohn IV-4 fraction, PK accounts for a significant amount of UII- and URP-generating activity.

Lipoteichoic Acid from Staphylococcus aureus Activates the Complement System via C3 Induction and CD55 Inhibition

Staphylococcus aureus inhibits complement activity by secreting a variety of toxins. However, the underlying mechanism of complement component regulation by lipoteichoic acid (LTA), a cell wall component of S. aureus, has not been elucidated. In this study, we observed that aLTA (LTA of S. aureus) increased C3 expression in THP-1 cells. The mechanism of aLTA-mediated C3 induction includes an aLTA-toll-like receptor (TLR) 2 interaction, interleukin 1 receptor associated kinase (IRAK) 2 recruitment, and nuclear factor kappa B (NF-kB) activation. In HepG2 cells, C3 protein production begins to increase from 3 h and increases steadily until 48 h. On the other hand, CD55 levels increased up to 6 h after aLTA treatment and started to decrease after 24 h and levels were decreased at 48 h by more than 50% compared to untreated cells. The expression of CD55 in HepG2 cells was shown to be regulated by IRAK-M induced by aLTA. Serum C3 levels increased in mice injected with aLTA, which resulted in an increase in the amount and activity of the membrane attack complex (MAC). We also observed that CD55 mRNA was increased in the liver 24 h after aLTA injection, but was decreased 48 h after injection. These results suggest that aLTA increases complement levels via induction of C3 and inhibition of CD55, which may cause associated MAC-mediated liver damage.

Complement Proteins as Soluble Pattern Recognition Receptors for Pathogenic Viruses

The complement system represents a crucial part of innate immunity. It contains a diverse range of soluble activators, membrane-bound receptors, and regulators. Its principal function is to eliminate pathogens via activation of three distinct pathways: classical, alternative, and lectin. In the case of viruses, the complement activation results in effector functions such as virion opsonisation by complement components, phagocytosis induction, virolysis by the membrane attack complex, and promotion of immune responses through anaphylatoxins and chemotactic factors. Recent studies have shown that the addition of individual complement components can neutralise viruses without requiring the activation of the complement cascade. While the complement-mediated effector functions can neutralise a diverse range of viruses, numerous viruses have evolved mechanisms to subvert complement recognition/activation by encoding several proteins that inhibit the complement system, contributing to viral survival and pathogenesis. This review focuses on these complement-dependent and -independent interactions of complement components (especially C1q, C4b-binding protein, properdin, factor H, Mannose-binding lectin, and Ficolins) with several viruses and their consequences.

Proteomic Profiling of Gastric Signet Ring Cell Carcinoma Tissues Reveals Characteristic Changes of the Complement Cascade Pathway

Signet ring cell carcinoma (SRCC) is a histological subtype of gastric cancer with distinct features in multiple aspects compared with adenocarcinomas (ACs). The lack of a systematic molecular overview of this disease has led to slow progress in its clinical practice. In the present proteomics study, gastric tissues were collected from tumors and adjacent tissues, including 14 SRCCs and 34 ACs, and laser capture microdissection (LCM) was employed to eradicate the cellular heterogeneity of the tissues. The proteomes of tissues were profiled by data-independent acquisition (DIA) mass spectrometry (MS). Based on the over 6000 proteins quantified, univariate analysis and pathway enrichment revealed that some proteins and pathways demonstrated differences between SRCC and ACs. Importantly, the upregulation of a majority of complement-related proteins was notable for SRCC but not for ACs. A hypothesis, based on the proteomics evidence, was proposed that the complement cascade was evoked in the SRCC microenvironment upon infiltration, and the SRCC cells survived the complement cytotoxicity by secreting endogenous negative regulators. Moreover, an attempt was made to establish appropriate cell models for gastric SRCC through proteomic comparison of the 15 gastric cell lines and gastric tumors. The predictions of a supervised classifier suggested that none of these gastric cell lines qualified to mimic SRCC. This study discovered that the complement cascade is activated at a higher level in gastric SRCC than in ACs.

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LCM-DIA extracted unprecedented proteomic details of gastric in different subtypes.

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Complement cascade was found to be an SRCC-specific pathway for the first time.

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Gastric cell lines were evaluated based on proteomic features for the first time.

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Re-analyzable DIA data collected provide rich opportunity for future research.

Prognostic Value of Complement Properdin in Cancer

The complement system is readily triggered by the presence of damage-associated molecular patterns on the surface of tumor cells. The complement alternative pathway provides rapid amplification of the molecular stress signal, leading to complement cascade activation to deal with pathogens or malignant cells. Properdin is the only known positive regulator of the alternative pathway. In addition, properdin promotes the phagocytic uptake of apoptotic T cells by macrophages and dendritic cells without activating the complement system, thus, establishing its ability to recognize "altered-self". Dysregulation of properdin has been implicated in substantial tissue damage in the host, and in some cases, chronic unresolved inflammation. A corollary of this may be the development of cancer. Hence, to establish a correlation between properdin presence/levels in normal and cancer tissues, we performed bioinformatics analysis, using Oncomine and UALCAN. Survival analyses were performed using UALCAN and PROGgeneV2 to assess if properdin can serve as a potential prognostic marker for human lung adenocarcinoma (LUAD), liver hepatocellular carcinoma (LIHC), cervical squamous cell carcinoma (CESC), and pancreatic adenocarcinoma (PAAD). We also analyzed levels of tumor-infiltrating immune cells using TIMER, a tool for characterizing immune cell composition in cancers. We found that in LUAD and LIHC, there was a lower expression of properdin in the tumors compared to normal tissues, while no significant difference was observed in CESC and PAAD. Survival analysis demonstrated a positive association between properdin mRNA expression and overall survival in all 4 types of cancers. TIMER analysis revealed that properdin expression correlated negatively with tumor purity and positively with levels of infiltrating B cells, cytotoxic CD8+ T cells, CD4+ helper T cells, macrophages, neutrophils and dendritic cells in LUAD, CESC and PAAD, and with levels of B cells, CD8+ T cells and dendritic cells in LIHC. Immunohistochemical analysis revealed that infiltrating immune cells were the most likely source of properdin in the tumor microenvironment. Thus, complement protein properdin shows promise as a prognostic marker in cancer and warrants further study.

Innate Immune Pattern Recognition Receptors of Mycobacterium tuberculosis: Nature and Consequences for Pathogenesis of Tuberculosis

Innate immunity against Mycobacterium tuberculosis is a critical early response to prevent the establishment of the infection. Despite recent advances in understanding the host-pathogen dialogue in the early stages of tuberculosis (TB), much has yet to be learnt. The nature and consequences of this dialogue ultimately determine the path of infection: namely, either early clearance of M. tuberculosis, or establishment of M. tuberculosis infection leading to active TB disease and/or latent TB infection. On the frontline in innate immunity are pattern recognition receptors (PRRs), with soluble factors (e.g. collectins and complement) and cell surface factors (e.g. Toll-like receptors and other C-type lectin receptors (Dectin 1/2, Nod-like receptors, DC-SIGN, Mincle, mannose receptor, and MCL) that play a central role in recognising M. tuberculosis and facilitating its clearance. However, in a 'double-edged sword' scenario, these factors can also be involved in enhancement of pathogenesis as well. Furthermore, innate immunity is also a critical bridge in establishing the subsequent adaptive immune response, which is also responsible for granuloma formation that cordons off M. tuberculosis infection, establishing latency and acting as a reservoir for bacterial persistence and dissemination of future disease. This chapter discusses the current understanding of pattern recognition of M. tuberculosis by innate immunity and the role this plays in the pathogenesis and protection against TB.

Complement-Independent Modulation of Influenza A Virus Infection by Factor H

The complement system is an ancient innate immune defense mechanism that can recognize molecular patterns on the invading pathogens. Factor H, as an inhibitor of the alternative pathway, down-regulates complement activation on the host cell surface. Locally synthesized factor H at the site of infection/injury, including lungs, can act as a pattern recognition molecule without involving complement activation. Here, we report that factor H, a sialic acid binder, interacts with influenza A virus (IAV) and modulates IAV entry, as evident from down-regulation of matrix protein 1 (M1) in H1N1 subtype-infected cells and up-regulation of M1 expression in H3N2-infected A549 cells. Far-western blot revealed that factor H binds hemagglutinin (HA, ~70 kDa), neuraminidase (NA, ~60 kDa), and M1 (~25 kDa). IAV-induced transcriptional levels of IFN-α, TNF-α, IL-12, IL-6, IFN-α, and RANTES were reduced following factor H treatment for the H1N1 subtype at 6 h post-infection. However, for the H3N2 subtype, mRNA levels of these pro-inflammatory cytokines were enhanced. A recombinant form of vaccinia virus complement control protein (VCP), which like factor H, contains CCP modules and has complement-regulatory activity, mirrored the results obtained with factor H. Both factor H (25%), and VCP (45%) were found to reduce luciferase reporter activity in MDCK cells transduced with H1N1 pseudotyped lentiviral particles. Factor H (50%) and VCP (30%) enhanced the luciferase reporter activity for H3N2, suggesting an entry inhibitory role of factor H and VCP against H1N1, but not H3N2. Thus, factor H can modulate IAV infection and inflammatory responses, independent of its complement-related functions.

The trypsin-like serine protease domain of Paralichthys olivaceus complement factor I regulates complement activation and inhibits bacterial growth

In mammals, complement factor I (CFI) is a serine protease in serum and plays a pivotal role in the regulation of complement activation. In the presence of cofactor, CFI cleaves C3b to iC3b, and further degrades iC3b to C3c and C3d. In teleost, the function of CFI is poorly understood. In this study, we examined the immunological property of CFI from Japanese flounder (Paralichthys olivaceus) (PoCFI), a teleost species with important economic value. PoCFI is composed of 597 amino acid residues and possesses a trypsin-like serine protease (Tryp) domain. We found that PoCFI expressions occurred in nine different tissues and were upregulated by bacterial challenge. Recombinant PoCFI-Tryp (rPoCFI-Tryp) inhibited complement activation and degraded C3b in serum. rPoCFI-Tryp exhibited apparent binding capacities to a board-spectrum of bacteria and inhibited bacterial growth. These results provide the first evidence to indicate that CFI in teleost negatively regulates complement activation via degradation C3b, and probably plays a role in host immune defense against bacterial infection.

Cell-Type-Specific Complement Expression in the Healthy and Diseased Retina

Complement dysregulation is a feature of many retinal diseases, yet mechanistic understanding at the cellular level is limited. Given this knowledge gap about which retinal cells express complement, we performed single-cell RNA sequencing on ~92,000 mouse retinal cells and validated our results in five major purified retinal cell types. We found evidence for a distributed cell-type-specific complement expression across 11 cell types. Notably, Müller cells are the major contributor of complement activators c1s, c3, c4, and cfb. Retinal pigment epithelium (RPE) mainly expresses cfh and the terminal complement components, whereas cfi and cfp transcripts are most abundant in neurons. Aging enhances c1s, cfb, cfp, and cfi expression, while cfh expression decreases. Transient retinal ischemia increases complement expression in microglia, Müller cells, and RPE. In summary, we report a unique complement expression signature for murine retinal cell types suggesting a well-orchestrated regulation of local complement expression in the retinal microenvironment.

Overshooting complement activity contributes to retinal degeneration. Pauly et al. demonstrate a distinct complement expression profile of retinal cell types that changes with aging and during retinal degeneration. This prompts the intriguing concept of a local retinal complement activation possibly independent of the systemic components typically produced by the liver.

The story of complement factor I

Factor I was first discovered in 1966. Its importance became apparent with the description of the original Factor I deficient patient in Boston in 1967. This patient presented with a hyperactive alternative complement pathway resulting in secondary complement deficiency due to continuous complement consumption. On the basis of these findings, the mechanism of the alternative pathway was worked out. In 1975, the surprise finding was made that elevating levels of Factor I in plasma down-regulated the alternative pathway. Attempts to exploit this finding for clinical use had a long and frustrating history and it was not until 2019 that the first patient was treated with the gene therapy vector for age related macular degeneration by Professor Sir Robert MacLaren in Oxford. This review follows the long and contorted course from initial observations to clinical use of complement Factor I.

Two distinct conformations of factor H regulate discrete complement-binding functions in the fluid phase and at cell surfaces

Factor H (FH) is the major regulator of C3b in the alternative pathway of the complement system in immunity. FH comprises 20 short complement regulator (SCR) domains, including eight glycans, and its Y402H polymorphism predisposes those who carry it to age-related macular degeneration. To better understand FH complement binding and self-association, we have studied the solution structures of both the His-402 and Tyr-402 FH allotypes. Analytical ultracentrifugation revealed that up to 12% of both FH allotypes self-associate, and this was confirmed by small-angle X-ray scattering (SAXS), MS, and surface plasmon resonance analyses. SAXS showed that monomeric FH has a radius of gyration (Rg ) of 7.2-7.8 nm and a length of 25 nm. Starting from known structures for the SCR domains and glycans, the SAXS data were fitted using Monte Carlo methods to determine atomistic structures of monomeric FH. The analysis of 29,715 physically realistic but randomized FH conformations resulted in 100 similar best-fit FH structures for each allotype. Two distinct molecular structures resulted that showed either an extended N-terminal domain arrangement with a folded-back C terminus or an extended C terminus and a folded-back N terminus. These two structures are the most accurate to date for glycosylated full-length FH. To clarify FH functional roles in host protection, crystal structures for the FH complexes with C3b and C3dg revealed that the extended N-terminal conformation accounted for C3b fluid-phase regulation, the extended C-terminal conformation accounted for C3d binding, and both conformations accounted for bivalent FH binding to glycosaminoglycans on the target cell surface.

Overview of C3 Glomerulopathy

C3 glomerulopathy is an umbrella term, which includes several rare forms of glomerulonephritis (GN) with underlying defects in the alternate complement cascade. A common histological feature noted in all these GN is dominant C3 deposition in the glomerulus. In this review, we will provide an overview of the complement system as well as mediators, with an introduction to pharmaceutical agents that can alter the pathway.

A novel factor I activity in Nipah virus inhibits human complement pathways through cleavage of C3b

Complement is an innate immune system that most animal viruses must face during natural infections. Given that replication and dissemination of the highly pathogenic Nipah virus (NiV) include exposure to environments rich in complement factors, we tested the in vitro sensitivity of NiV to complement-mediated neutralization. Here we show that NiV was completely resistant to in vitro neutralization by normal human serum (NHS). Treatment of purified NiV with NHS activated complement pathways, but there was very little C3 deposition on virus particles. In in vitro reconstitution experiments, NiV particles provided time- and dose-dependent factor I-like protease activity capable of cleaving C3b into inactive C3b (iC3b). NiV-dependent inactivation of C3b only occurred with the cofactors factor H and soluble CR1 but not with CD46. Purified NiV particles did not support C4b cleavage. Electron microscopy of purified NiV particles showed immunogold labeling with anti-factor I antibodies. Our results suggest a novel mechanism by which NiV evades the human complement system through a unique factor I-like activity.

IMPORTANCE

Viruses have evolved mechanisms to limit complement-mediated neutralization, some of which involve hijacking cellular proteins involved in control of inappropriate complement activation. Here we report a previously unknown mechanism whereby NiV provides a novel protease activity capable of in vitro cleavage and inactivation of C3b, a key component of the complement cascade. These data help to explain how an enveloped virus such as NiV can infect and disseminate through body fluids that are rich in complement activity. Disruption of the ability of NiV to recruit complement inhibitors could form the basis for the development of effective therapies and safer vaccines to combat these highly pathogenic emerging viruses.

The role of complement in tumor growth

Complement is a central part of the immune system that has developed as a first defense against non-self cells. Neoplastic transformation is accompanied by an increased capacity of the malignant cells to activate complement. In fact, clinical data demonstrate complement activation in cancer patients. On the basis of the use of protective mechanisms by malignant cells, complement activation has traditionally been considered part of the body's immunosurveillance against cancer. Inhibitory mechanisms of complement activation allow cancer cells to escape from complement-mediated elimination and hamper the clinical efficacy of monoclonal antibody-based cancer immunotherapies. To overcome this limitation, many strategies have been developed with the goal of improving complement-mediated effector mechanisms. However, significant work in recent years has identified new and surprising roles for complement activation within the tumor microenvironment. Recent reports suggest that complement elements can promote tumor growth in the context of chronic inflammation. This chapter reviews the data describing the role of complement activation in cancer immunity, which offers insights that may aid the development of more effective therapeutic approaches to control cancer.

Purification, quantification, and functional analysis of Complement Factor H

Complement Factor H (FH) is an abundant, non-enzymic plasma/serum glycoprotein, which has a major role in regulating activation of the complement system. It can be purified from human plasma/serum by affinity chromatography, using a monoclonal anti-FH antibody as ligand. Other affinity chromatography ligands, including cardiolipin and trinitrophenyl-bovine serum albumin (TNP-BSA), can be used to purify human FH and also FH from a wide range of vertebrates, including mammals, birds, bony fish. Human FH protein concentration can be quantified by sandwich ELISA. The activity of FH is generally measured by assays which detect the cleavage, by complement factor I, of the complement protein C3b to form iC3b. Cleavage occurs only in the presence of a cofactor, and FH is one of a small number of cofactors for this reaction.

Affinity purification of human factor H on polypeptides derived from streptococcal m protein: enrichment of the Y402 variant

Recent studies indicate that defective activity of complement factor H (FH) is associated with several human diseases, suggesting that pure FH may be used for therapy. Here, we describe a simple method to isolate human FH, based on the specific interaction between FH and the hypervariable region (HVR) of certain Streptococcus pyogenes M proteins. Special interest was focused on the FH polymorphism Y402H, which is associated with the common eye disease age-related macular degeneration (AMD) and has also been implicated in the binding to M protein. Using a fusion protein containing two copies of the M5-HVR, we found that the Y402 and H402 variants of FH could be efficiently purified by single-step affinity chromatography from human serum containing the corresponding protein. Different M proteins vary in their binding properties, and the M6 and M5 proteins, but not the M18 protein, showed selective binding of the FH Y402 variant. Accordingly, chromatography on a fusion protein derived from the M6-HVR allowed enrichment of the Y402 protein from serum containing both variants. Thus, the exquisite binding specificity of a bacterial protein can be exploited to develop a simple and robust procedure to purify FH and to enrich for the FH variant that protects against AMD.

Genetic influences on plasma CFH and CFHR1 concentrations and their role in susceptibility to age-related macular degeneration

It is a longstanding puzzle why non-coding variants in the complement factor H (CFH) gene are more strongly associated with age-related macular degeneration (AMD) than functional coding variants that directly influence the alternative complement pathway. The situation is complicated by tight genetic associations across the region, including the adjacent CFH-related genes CFHR3 and CFHR1, which may themselves influence the alternative complement pathway and are contained within a common deletion (CNP147) which is associated with protection against AMD. It is unclear whether this association is mediated through a protective effect of low plasma CFHR1 concentrations, high plasma CFH or both. We examined the triangular relationships of CFH/CFHR3/CFHR1 genotype, plasma CFH or CFHR1 concentrations and AMD susceptibility in combined case–control (1256 cases, 1020 controls) and cross-sectional population (n = 1004) studies and carried out genome-wide association studies of plasma CFH and CFHR1 concentrations. A non-coding CFH SNP (rs6677604) and the CNP147 deletion were strongly correlated both with each other and with plasma CFH and CFHR1 concentrations. The plasma CFH-raising rs6677604 allele and raised plasma CFH concentration were each associated with AMD protection. In contrast, the protective association of the CNP147 deletion with AMD was not mediated by low plasma CFHR1, since AMD-free controls showed increased plasma CFHR1 compared with cases, but it may be mediated by the association of CNP147 with raised plasma CFH concentration. The results are most consistent with a regulatory locus within a 32 kb region of the CFH gene, with a major effect on plasma CFH concentration and AMD susceptibility.

Properdin and factor h: opposing players on the alternative complement pathway "see-saw"

Properdin and factor H are two key regulatory proteins having opposite functions in the alternative complement pathway. Properdin up-regulates the alternative pathway by stabilizing the C3bBb complex, whereas factor H downregulates the pathway by promoting proteolytic degradation of C3b. While factor H is mainly produced in the liver, there are several extrahepatic sources. In addition to the liver, factor H is also synthesized in fetal tubuli, keratinocytes, skin fibroblasts, ocular tissue, adipose tissue, brain, lungs, heart, spleen, pancreas, kidney, muscle, and placenta. Neutrophils are the major source of properdin, and it is also produced by monocytes, T cells and bone marrow progenitor cell line. Properdin is released by neutrophils from intracellular stores following stimulation by N-formyl-methionine-leucine-phenylalanine (fMLP) and tumor necrosis factor alpha (TNF-α). The HEP G2 cells derived from human liver has been found to produce functional properdin. Endothelial cells also produce properdin when induced by shear stress, thus is a physiological source for plasma properdin. The diverse range of extrahepatic sites for synthesis of these two complement regulators suggests the importance and need for local availability of the proteins. Here, we discuss the significance of the local synthesis of properdin and factor H. This assumes greater importance in view of recently identified unexpected and novel roles of properdin and factor H that are potentially independent of their involvement in complement regulation.

Bivalent and co-operative binding of complement factor H to heparan sulfate and heparin

FH (Factor H) with 20 SCR (short complement regulator) domains is a major serum regulator of complement, and genetic defects in this are associated with inflammatory diseases. Heparan sulfate is a cell-surface glycosaminoglycan composed of sulfated S-domains and unsulfated NA-domains. To elucidate the molecular mechanism of binding of FH to glycosaminoglycans, we performed ultracentrifugation, X-ray scattering and surface plasmon resonance with FH and glycosaminoglycan fragments. Ultracentrifugation showed that FH formed up to 63% of well-defined oligomers with purified heparin fragments (equivalent to S-domains), and indicated a dissociation constant K(d) of approximately 0.5 μM. Unchanged FH structures that are bivalently cross-linked at SCR-7 and SCR-20 with heparin explained the sedimentation coefficients of the FH-heparin oligomers. The X-ray radius of gyration, R(G), of FH in the presence of heparin fragments 18-36 monosaccharide units long increased significantly from 10.4 to 11.7 nm, and the maximum lengths of FH increased from 35 to 40 nm, confirming that large compact oligomers had formed. Surface plasmon resonance of immobilized heparin with full-length FH gave K(d) values of 1-3 μM, and similar but weaker K(d) values of 4-20 μM for the SCR-6/8 and SCR-16/20 fragments, confirming co-operativity between the two binding sites. The use of minimally-sulfated heparan sulfate fragments that correspond largely to NA-domains showed much weaker binding, proving the importance of S-domains for this interaction. This bivalent and co-operative model of FH binding to heparan sulfate provides novel insights on the immune function of FH at host cell surfaces.

The complement system of the goat: haemolytic assays and isolation of major proteins

BACKGROUND

The aim of the present study was to develop a haemolytic assay for the study of the complement system in dairy goats (Capra aegagrus hircus) and to characterize the major goat complement system proteins.

RESULTS

The commonly used sheep erythrocyte sensitized with rabbit antibodies were not sensitive to lysis by goat serum, but the combination of human red blood cells (RBC) plus rabbit antibodies was the best option found for goat complement assay. A buffer based on HEPES instead of the classical veronal (barbitone) was developed. Three proteins were isolated: factor H, C1q and C3 and these were compared with the corresponding human proteins. A novel affinity chromatography technique was developed for isolation of factor H.

CONCLUSIONS

Human RBC plus rabbit antibodies were a suitable option for haemolytic assays. The isolated proteins are similar to the human counterparts.

Complement activation: an emerging player in the pathogenesis of cardiovascular disease

A wealth of evidence indicates a fundamental role for inflammation in the pathogenesis of cardiovascular disease (CVD), contributing to the development and progression of atherosclerotic lesion formation, plaque rupture, and thrombosis. An increasing body of evidence supports a functional role for complement activation in the pathogenesis of CVD through pleiotropic effects on endothelial and haematopoietic cell function and haemostasis. Prospective and case control studies have reported strong relationships between several complement components and cardiovascular outcomes, and in vitro studies and animal models support a functional effect. Complement activation, in particular, generation of C5a and C5b-9, influences many processes involved in the development and progression of atherosclerosis, including promotion of endothelial cell activation, leukocyte infiltration into the extracellular matrix, stimulation of cytokine release from vascular smooth muscle cells, and promotion of plaque rupture. Complement activation also influences thrombosis, involving components of the mannose-binding lectin pathway, and C5b-9 in particular, through activation of platelets, promotion of fibrin formation, and impairment of fibrinolysis. The participation of the complement system in inflammation and thrombosis is consistent with the physiological role of the complement system as a rapid effector system conferring protection following vessel injury. However, in the context of CVD, these same processes contribute to development of atherosclerosis, plaque rupture, and thrombosis.

High complement factor I activity in the plasma of children with autism spectrum disorders

Autism spectrum disorders (ASDs) are neurodevelopmental and behavioural syndromes affecting social orientation, behaviour, and communication that can be classified as developmental disorders. ASD is also associated with immune system abnormality. Immune system abnormalities may be caused partly by complement system factor I deficiency. Complement factor I is a serine protease present in human plasma that is involved in the degradation of complement protein C3b, which is a major opsonin of the complement system. Deficiency in factor I activity is associated with an increased incidence of infections in humans. In this paper, we show that the mean level of factor I activity in the ASD group is significantly higher than in the control group of typically developed and healthy children, suggesting that high activity of complement factor I might have an impact on the development of ASD.

Human complement Factor H modulates C1q-mediated phagocytosis of apoptotic cells

Complement is implicated in the clearance of apoptotic cells by phagocytes. Deficiencies in early complement components, particularly C1q, are associated with an increased risk of the development of systemic lupus erythematosus. C1q is considered to be important in this process through interaction with apoptotic cells and phagocytes. In the present study, we confirm that apoptotic cells are recognized not only by C1q but also by the complement regulatory protein Factor H. Both C1q and Factor H bind to apoptotic cells in a dose-dependent and saturable manner. We further examined the role of C1q and Factor H in the clearance of apoptotic cells by monocytes. C1q enhanced uptake/adhesion of apoptotic cells by monocytes whereas Factor H alone had no effect on this process. However, when both C1q and Factor H were present on the apoptotic cell surface, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced. This effect of Factor H also occurred if monocytes were pre-treated with Factor H, and then exposed to C1q-coated apoptotic cells. The results were consistent with Factor H interacting with monocytes through the integrin CD11b/CD18. We conclude that under physiological conditions, Factor H may be important in controlling the inflammation which might arise from C1q deposition on apoptotic cells.

Complement in health and disease

The complement system consists of about 35-40 proteins and glycoproteins present in blood plasma or on cell surfaces. Its main biological function is to recognise "foreign" particles and macromolecules, and to promote their elimination either by opsonisation or lysis. Although historically complement has been studied as a system for immune defence against bacteria, it has an important homeostatic role in which it recognises damaged or altered "self" components. Thus complement has major roles in both immune defence against microorganisms, and in clearance of damaged or "used" host components. Since complement proteins opsonise or lyse cells, complement can damage healthy host cells and tissues. The system is regulated by many endogenous regulatory proteins. Regulation is sometimes imperfect and both too much and too little complement activation is associated with many diseases. Excessive or inappropriate activation can cause tissue damage in diseases such as rheumatoid arthritis, age-related macular degeneration (AMD), multiple sclerosis, ischemia-reperfusion injury (e.g. ischemic stroke). Insufficient complement activity is associated with susceptibility to infection (mainly bacterial) and development of autoimmune disease, like SLE (systemic lupus erythematosus).

The molecular identification of factor H and factor I molecules in rainbow trout provides insights into complement C3 regulation

The complement system in vertebrates plays a crucial role in the elimination of pathogens. To regulate complement on self-tissue and to prevent spontaneous activation and systemic depletion, complement is controlled by both fluid-phase and membrane-bound inhibitors. One such inhibitor, complement factor I (CFI) regulates complement by proteolytic cleavage of components C3b and C4b in the presence of specific cofactors. Complement factor H (CFH), the main cofactor for CFI, regulates the alternative pathway of complement activation by acting in the breakdown of C3b to iC3b. To gain further insight into the origin of C3 regulation in bony fish we have cloned and characterized the CFI and CFH1 cDNAs in the rainbow trout (Oncorhynchus mykiss). In this study we report the primary sequence, the tissue expression profile, the polypeptide domain architecture and the phylogenetic analysis of trout CFI and CFH1 genes. The deduced amino acid sequences of trout CFI and CFH1 polypeptides exhibit 42% and 32% identity with human orthologs, respectively. RNA expression analysis showed that CFI is expressed differentially in trout tissues, while liver is the main source of CFH1 expression. Our data indicate that factor H and I genes have emerged during evolution as early as the divergence of teleost fish.

Structural basis for complement factor I control and its disease-associated sequence polymorphisms

The complement system is a key component of innate and adaptive immune responses. Complement regulation is critical for prevention and control of disease. We have determined the crystal structure of the complement regulatory enzyme human factor I (fI). FI is in a proteolytically inactive form, demonstrating that it circulates in a zymogen-like state despite being fully processed to the mature sequence. Mapping of functional data from mutants of fI onto the structure suggests that this inactive form is maintained by the noncatalytic heavy-chain allosterically modulating activity of the light chain. Once the ternary complex of fI, a cofactor and a substrate is formed, the allosteric inhibition is released, and fI is oriented for cleavage. In addition to explaining how circulating fI is limited to cleaving only C3b/C4b, our model explains the molecular basis of disease-associated polymorphisms in fI and its cofactors.

Factor H as a regulator of the classical pathway activation

C1q, the first subcomponent of the classical pathway, is a charge pattern recognition molecule that binds a diverse range of self, non-self and altered self ligands, leading to pro-inflammatory complement activation. Although complement is required for tissue homeostasis as well as defence against pathogens, exaggerated complement activation can be damaging to the tissue. Therefore, a fine balance between complement activation and inhibition is necessary. We have recently found that factor H, a polyanion recognition molecule and soluble regulator of alternative pathway activation in blood and on cell surfaces, can directly compete with C1q in binding to anionic phospholipids (cardiolipin), lipid A and Escherichia coli (three known activators of the classical pathway) and acts as a direct down regulator of the complement classical pathway. This ability of factor H to dampen classical pathway activation is distinct from its role as an alternative pathway down-regulator. Thus, by directly competing for specific C1q ligands (exogenous as well as endogenous), factor H is likely to be involved in fine-tuning and balancing the C1q-driven inflammatory processes in autoimmunity and infection. However, in the case of apoptotic cells, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced by factor H. Thus, factor H may be important in controlling the inflammation, which might arise from C1q deposition on apoptotic cells.

Chemical labelling of active serum thioester proteins for quantification

The complement serum proteins C3 and C4 and the protease inhibitor α-2 macroglobulin are all members of the C3/α-2M thioester protein family, an evolutionarily ancient and conserved family that contains an intrachain thioester bond. The chemistry of the thioester bond is a key to the function of the thioester proteins. All these proteins function by covalently linking to their target by acyl transfer of the protein via the thioester moiety. We show that the signature thioester bond can be targeted with nucleophiles linked to a bioreporter molecule, site-specifically modifying the whole, intact thioester protein. Conditions were optimised to label selectively and efficiently pull-down unprocessed thioester-containing proteins from serum. We demonstrated pull-down of full-length C3, α-2M and C4 from sera in high salt, using a biotinylated nucleophile and streptavidin-coated resin, confirmed by MALDI-TOF MS identification of the gel bands. The potential for the development of a quantitative method for measuring active C3 in serum was investigated in patient sera pre and post operation. Quantifying active C3 in clinical assays using current methods is difficult. Methods based on antibody detection (e.g. nephelometry) do not distinguish between active C3 and inactive breakdown products. C3-specific haemolytic assays can be used, but these require use of relatively unstable reagents. The current work represents a promising robust, enzyme- and antibody-free chemical method for detecting active thioester proteins in blood, plasma or serum.

Interactions of complement proteins C1q and factor H with lipid A and Escherichia coli: further evidence that factor H regulates the classical complement pathway

Proteins of the complement system are known to interact with many charged substances. We recently characterized binding of C1q and factor H to immobilized and liposomal anionic phospholipids. Factor H inhibited C1q binding to anionic phospholipids, suggesting a role for factor H in regulating activation of the complement classical pathway by anionic phospholipids. To extend this finding, we examined interactions of C1q and factor H with lipid A, a well-characterized activator of the classical pathway. We report that C1q and factor H both bind to immobilized lipid A, lipid A liposomes and intact Escherichia coli TG1. Factor H competes with C1q for binding to these targets. Furthermore, increasing the factor H: C1q molar ratio in serum diminished C4b fixation, indicating that factor H diminishes classical pathway activation. The recombinant forms of the Cterminal, globular heads of C1q A, B and C chains bound to lipid A and E. coli in a manner qualitatively similar to native C1q, confirming that C1q interacts with these targets via its globular head region. These observations reinforce our proposal that factor H has an additional complement regulatory role of down-regulating classical pathway activation in response to certain targets. This is distinct from its role as an alternative pathway down-regulator. We suggest that under physiological conditions, factor H may serve as a downregulator of bacterially-driven inflammatory responses, thereby fine-tuning and balancing the inflammatory response in infections with Gram-negative bacteria.

Zinc binding to the Tyr402 and His402 allotypes of complement factor H: possible implications for age-related macular degeneration

The Tyr402His polymorphism of complement factor H (FH) with 20 short complement regulator (SCR) domains is associated with age-related macular degeneration (AMD). How FH contributes to disease pathology is not clear. Both FH and high concentrations of zinc are found in drusen deposits, the key feature of AMD. Heterozygous FH is inhibited by zinc, which causes FH to aggregate. Here, zinc binding to homozygous FH was studied. By analytical ultracentrifugation, large amounts of oligomers were observed with both the native Tyr402 and the AMD-risk His402 homozygous allotypes of FH and both the recombinant SCR-6/8 allotypes with Tyr/His402. X-ray scattering also showed that both FH and SCR-6/8 allotypes strongly aggregated at >10 μM zinc. The SCR-1/5 and SCR-16/20 fragments were less likely to bind zinc. These observations were supported by bioinformatics predictions. Starting from known zinc binding sites in crystal structures, we predicted 202 putative partial surface zinc binding sites in FH, most of which were in SCR-6. Metal site prediction web servers also suggested that SCR-6 and other domains bind zinc. Predicted SCR-6/8 dimer structures showed that zinc binding sites could be formed at the protein-protein interface that would lead to daisy-chained oligomers. It was concluded that zinc binds weakly to FH at multiple surface locations, most probably within the functionally important SCR-6/8 domains, and this explains why zinc inhibits FH activity. Given the high pathophysiological levels of bioavailable zinc present in subretinal deposits, we discuss how zinc binding to FH may contribute to deposit formation and inflammation associated with AMD.

Complement activation by phospholipids: the interplay of factor H and C1q

Complement proteins in blood recognize charged particles. The anionic phospholipid (aPL) cardiolipin binds both complement proteins C1q and factor H. C1q is an activator of the complement classical pathway, while factor H is an inhibitor of the alternative pathway. To examine opposing effects of C1q and factor H on complement activation by aPL, we surveyed C1q and factor H binding, and complement activation by aPL, either coated on microtitre plates or in liposomes. Both C1q and factor H bound to all aPL tested, and competed directly with each other for binding. All the aPL activated the complement classical pathway, but negligibly the alternative pathway, consistent with accepted roles of C1q and factor H. However, in this system, factor H, by competing directly with C1q for binding to aPL, acts as a direct regulator of the complement classical pathway. This regulatory mechanism is distinct from its action on the alternative pathway. Regulation of classical pathway activation by factor H was confirmed by measuring C4 activation by aPL in human sera in which the C1q:factor H molar ratio was adjusted over a wide range. Thus factor H, which is regarded as a down-regulator only of the alternative pathway, has a distinct role in downregulating activation of the classical complement pathway by aPL. A factor H homologue, β2-glycoprotein-1, also strongly inhibits C1q binding to cardiolipin. Recombinant globular domains of C1q A, B and C chains bound aPL similarly to native C1q, confirming that C1q binds aPL via its globular heads.

Macrophage scavenger receptor A mediates adhesion to apolipoproteins A-I and E

Macrophage scavenger receptor A (SR-A) is a multifunctional, multiligand pattern recognition receptor with roles in innate immunity, apoptotic cell clearance, and age-related degenerative pathologies, such as atherosclerosis and Alzheimer's disease. Known endogenous SR-A ligands are polyanionic and include modified lipoproteins, advanced glycation end products, and extracellular matrix proteins. No native plasma ligands have been identified, but it is known that SR-A recognition of unidentified serum components mediates integrin-independent macrophage adhesion, which may drive chronic local inflammation. In this study, we used a high-throughput fractionation and screening method to identify novel endogenous SR-A ligands that may mediate macrophage adhesion. SR-A was found to recognize the exchangeable apolipoproteins A-I and E (apo A-I and apo E, respectively) in both lipid-free and lipid-associated form, suggesting the shared amphipathic alpha-helix as a potential recognition motif. Adhesion of RAW 264.7 macrophages to surfaces coated with apo A-I and apo E4 proved to be integrin-independent and could be blocked by anti-SR-A antibodies. The presence of apo A-I and apo E in pathological deposits, such as atherosclerotic lesions and neurotoxic Alzheimer's plaques, suggests a possible contribution of SR-A-dependent adhesion of macrophages to an inflammatory microenvironment.

Complement factor H binds at two independent sites to C-reactive protein in acute phase concentrations

Factor H (FH) regulates the activation of C3b in the alternative complement pathway, both in serum and at host cell surfaces. It is composed of 20 short complement regulator (SCR) domains. The Y402H polymorphism in FH is a risk factor for age-related macular degeneration. C-reactive protein (CRP) is an acute phase protein that binds Ca(2+). We established the FH-CRP interaction using improved analytical ultracentrifugation (AUC), surface plasmon resonance (SPR), and synchrotron x-ray scattering methods. Physiological FH and CRP concentrations were used in 137 mM NaCl and 2 mM Ca(2+), in which the occurrence of denatured CRP was avoided. In solution, AUC revealed FH-CRP binding. The FH-CRP interaction inhibited the formation of higher FH oligomers, indicating that CRP blocked FH dimerization sites at both SCR-6/8 and SCR-16/20. SPR confirmed the FH-CRP interaction and its NaCl concentration dependence upon using either immobilized FH or CRP. The SCR-1/5 fragment of FH did not bind to CRP. In order of increasing affinity, SCR-16/20, SCR-6/8 (His-402), and SCR-6/8 (Tyr-402) fragments bound to CRP. X-ray scattering showed that FH became more compact when binding to CRP, which is consistent with CRP binding at two different FH sites. We concluded that FH and CRP bind at elevated acute phase concentrations of CRP in physiological buffer. The SCR-16/20 site is novel and indicates the importance of the FH-CRP interaction for both age-related macular degeneration and atypical hemolytic uremic syndrome.

Complement and its role in innate and adaptive immune responses

The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges. As such, a study of its functions allows insight into the molecular underpinnings of host-pathogen interactions as well as the organization and orchestration of the host immune response. This review attempts to summarize the roles that complement plays in both innate and adaptive immune responses and the consequences of these interactions on host defense.

Multiple routes of complement activation by Mycobacterium bovis BCG

Mycobacterium tuberculosis is the leading cause of infectious disease in humans in the world. It evades the host immune system by being phagocytosed by macrophages and residing intracellularly. Complement-dependent opsonisation of extracellular mycobacteria may assist them to enter macrophages. This work examines in detail the mechanisms of complement activation by whole mycobacteria using Mycobacterium bovis BCG as a model organism. M. bovis BCG directly activates the classical, lectin and alternative pathways, resulting in fixation of C3b onto macromolecules of the mycobacterial surface. Investigation into the classical pathway has shown direct binding of human C1q to whole mycobacteria in the absence of antibodies. Most human sera contain IgG and IgM-anti-(M. bovis BCG), and pre-incubation with human immunoglobulin enhances C1q binding to the bacteria. Therefore classical pathway activation is both antibody-independent and dependent. The bacteria also activate the alternative pathway in an antibody-independent manner, but Factor H also binds, suggesting some regulation of amplification by this pathway. For the lectin pathway we have demonstrated direct binding of both MBL and L-ficolin from human serum to whole mycobacteria and subsequent MASP2 activation. H-ficolin binding was not observed. No M. bovis BCG cell surface or secreted protease appears likely to influence complement activation. Together, these data provide a more detailed analysis of the mechanisms by which M. bovis BCG interacts with the complement system.

Multimeric interactions between complement factor H and its C3d ligand provide new insight on complement regulation

Activation of C3 to C3b signals the start of the alternative complement pathway. The C-terminal short complement regulator (SCR)-20 domain of factor H (FH), the major serum regulator of C3b, possesses a binding site for C3d, a 35-kDa physiological fragment of C3b. Size distribution analyses of mixtures of SCR-16/20 or FH with C3d by analytical ultracentrifugation in 50 and 137 mM NaCl buffer revealed a range of discrete peaks, showing that multimeric complexes had formed at physiologically relevant concentrations. Surface plasmon resonance studies showed that native FH binds C3d in two stages. An equilibrium dissociation constant K(D)(1) of 2.6 microM in physiological buffer was determined for the first stage. Overlay experiments indicated that C3d formed multimeric complexes with FH. X-ray scattering showed that the maximum dimension of the C3d complexes with SCR-16/20 at 29 nm was not much longer than that of the unbound SCR-16/20 dimer. Molecular modelling suggested that the ultracentrifugation and scattering data are most simply explained in terms of associating dimers of each of SCR-16/20 and C3d. We conclude that the physiological interaction between FH and C3d is not a simple 1:1 binding stoichiometry between the two proteins that is often assumed. Because the multimers involve the C-terminus of FH, which is bound to host cell surfaces, our results provide new insight on FH regulation during excessive complement activation, both in the fluid phase and at host cell surfaces decorated by C3d.

Electrostatic interactions contribute to the folded-back conformation of wild type human factor H

Factor H (FH), a major serum regulator of C3b in the complement alternative pathway, is composed of 20 short complement regulator (SCR) domains. Earlier solution structures for FH showed that this has a folded-back domain arrangement and exists as oligomers. To clarify the molecular basis for this, analytical ultracentrifugation and X-ray scattering studies of native FH were performed as a function of NaCl concentration and pH. The sedimentation coefficient for the FH monomer decreased from 5.7 S to 5.3 S with increase in NaCl concentration, showing that weak electrostatic inter-domain interactions affect its folded-back structure. FH became more elongated at pH 9.4, showing the involvement of histidine residue(s) in its folded-back structure. Similar studies of partially deglycosylated FH suggested that oligosaccharides were not significant in determining the FH domain structure. The formation of FH oligomers decreased with increased NaCl concentration, indicating that electrostatic interactions also affect this. X-ray scattering showed that the maximum length of FH increased from 32 nm in low salt to 38 nm in high salt. Constrained X-ray scattering modelling was used to generate significantly improved FH molecular structures at medium resolution. In 50 mM NaCl, the modelled structures showed that inter-SCR domain contacts are likely, while these contacts are fewer in 250 mM NaCl. The results of this study show that the conformation of FH is affected by its local environment, and this may be important for its interactions with C3b and when bound to polyanionic cell surfaces.

Factor H facilitates the clearance of GBM bound iC3b by controlling C3 activation in fluid phase

Dense deposit disease (DDD) is strongly associated with the uncontrolled activation of the complement alternative pathway. Factor H (CFH)-deficient (Cfh^−/−) mice spontaneously develop C3 deposition along the glomerular basement membrane (GBM) with subsequent development of glomerulonephritis with features of DDD, a lesion dependent on C3 activation. In order to understand the role of CFH in preventing renal damage associated with the dysregulation of the alternative pathway we administered purified mouse CFH (mCFH) to Cfh^−/− mice. 24 h following the administration of mCFH we observed an increase in plasma C3 levels with presence of intact C3 in circulation showing that mCFH restored control of C3 activation in fluid phase. mCFH resulted in the reduction of iC3b deposition along the GBM. The exogenous mCFH was readily detectable in plasma but critically not in association with C3 along the GBM. Thus, the reduction in GBM C3 was dependent on the ability of mCFH to regulate C3 activation in plasma. Western blot analysis of glomeruli from Cfh^−/− mice demonstrated the presence of iC3b. Our data show that the C3 along the GBM in Cfh^−/− mice is the C3 fragment iC3b and that this is derived from plasma C3 activation. The implication is that successful therapy of DDD is likely to be achieved by therapies that inhibit C3 turnover in plasma.

Complement activation and inhibition: a delicate balance

Complement is part of the innate immune defence and not only recognizes microbes but also unwanted host molecules to enhance phagocytosis and clearance. This process of opsonisation must be tightly regulated to prevent immunopathology. Endogenous ligands such as dying cells, extracellular matrix proteins, pentraxins, amyloid deposits, prions and DNA, all bind the complement activator C1q, but also interact with complement inhibitors C4b-binding protein and factor H. This contrasts to the interaction between C1q and immune complexes, in which case no inhibitors bind, resulting in full complement activation. Disturbances to the complement regulation on endogenous ligands can lead to diseases such as age-related macular degeneration, neurological and rheumatic disorders. A thorough understanding of these processes might be crucial to developing new therapeutic strategies.

Uncontrolled zinc- and copper-induced oligomerisation of the human complement regulator factor H and its possible implications for function and disease

Polymorphisms in factor H (FH), a major regulator of complement activation, and the accumulation of high zinc concentrations in the outer retina are both associated with age-related macular degeneration. FH is inhibited by zinc, which causes FH to aggregate. To investigate this, we quantitatively studied zinc-induced FH self-association by X-ray scattering and analytical ultracentrifugation to demonstrate uncontrolled FH oligomerisation in conditions corresponding to physiological levels of FH and pathological levels of zinc in the outer retina. By scattering, FH at 2.8-7.0 microM was unaffected until [Zn] increased to 20 microM, whereupon the radius of gyration, RG, values increased from 9 to 15 nm at [Zn]=200 microM. The maximum dimension of FH increased from 32 to 50 nm, indicating that compact oligomers had formed. By ultracentrifugation, size-distribution analyses showed that monomeric FH at 5.57 S was the major species at [Zn] up to 60 microM. At [Zn] above 60 microM, a series of large oligomers were formed, ranging up to 100 S in size. Oligomerisation was reversed by ethylenediaminetetraacetic acid. Structurally distinct large oligomers were observed for Cu, while Ni, Cd and Fe showed low amounts of oligomers and Mg and Ca showed no change. Fluid-phase assays showed reduced FH activities that correlated with increased oligomer formation. The results were attributed to different degrees of stabilisation of weak self-dimerisation sites in FH by transition metals. The relevance of metal-induced FH oligomer formation to complement regulation and age-related macular degeneration is discussed.