Human C4-binding protein. I. Isolation and characterization

Enhancing hemostasis potency in hemophilia with a small interfering ribonucleic acid targeting protein S

BACKGROUND

One hemophilia treatment concept focuses on rebalancing coagulation and anticoagulation to restore normal blood clotting. Targeting the coagulation regulator, protein S (PS), in hemophilia shows promise to increase the generation of thrombin, a critical enzyme in the clotting process.

OBJECTIVES

This study aimed to: (1) assess whether inhibiting PS increases thrombin generation (TG) in plasma from individuals with hemophilia A (HA) and B (HB); and (2) develop a hepatocyte-targeted PS small interfering RNA (siRNA) therapy using N-acetylgalactosamine conjugation to restore hemostasis in hemophilia without increasing thromboembolic risks.

METHODS

We assessed TG in plasma from patients with HA and HB. To target the liver specifically, we developed a PS-siRNA conjugated with N-acetylgalactosamine. This approach ensures that PS levels remain adequate in other cells, thereby minimizing the risk of thrombosis. Additionally, we evaluated the therapeutic potential of PS-siRNA in preclinical models.

RESULTS

Inhibiting PS with a polyclonal antibody in plasma resulted in a 3- to 5-fold increase in TG in HA and a 4- to 9-fold increase in HB plasma, with a 70% reduction in plasma PS. In preclinical models, subcutaneous PS-siRNA therapy in HA mice and nonhuman primates successfully lowered PS levels and improved clot formation. It also prevented bleeding in both saphenous vein puncture and knee injury models in HA mice. Notably, it enhanced clotting without triggering widespread clot formation.

CONCLUSION

Reducing PS levels enhances TG in hemophilia models, and PS-siRNA therapy shows promise in improving hemostasis. This approach warrants further clinical investigation as a potential treatment for hemophilia.

Complement and complement regulatory protein in allogeneic and xenogeneic kidney transplantation

Kidney transplantation is the most optimal treatment for patients with end-stage renal disease, offering significant improvements in patient outcomes over dialysis. However, the potential for immune rejection, where the recipient's immune system attacks the transplanted kidney, can compromise transplant success. The complement system, a key component of the immune response, plays a crucial role in both acute and chronic rejection, including T-cell- and antibody-mediated rejection. Understanding and controlling the complement system is essential for managing rejection and enhancing graft survival and overall success of kidney transplantation. In allogeneic transplantation, complement activation through various pathways contributes to graft damage and failure. Recent advancements in genetic engineering enable the development of transgenic pigs expressing human complement regulatory proteins, which display potential for reducing rejection in xenotransplantation. Despite these advances, the complex mechanisms of complement activation and regulation are not fully understood, necessitating further research. This review examines the role of the complement system in kidney transplantation, explores the latest developments in complement regulatory strategies, and discusses potential therapeutic approaches to improve transplant outcomes.

Circulating levels of endogenous complement inhibitors correlate inversely with complement consumption in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is marked by excessive complement activation, contributing to tissue damage. Complement activation can be detected in many organs including the skin, kidney, and brain. The involvement of the central nervous system is particularly relevant to understanding neuropsychiatric SLE (NPSLE), one of the poorest understood manifestations of SLE for which no biomarkers are available. We studied the levels of complement inhibitors in SLE in relation to disease activity and as possible biomarkers to identify NPSLE. Serum levels of complement inhibitors C1-inhibitor (C1-INH), C4b-binding protein (C4BP), Factor I, and Factor H were measured in 345 SLE patients (including 102 with NPSLE) and 108 healthy controls. Compared with controls, SLE patients had higher C1-INH and C4BP but lower Factor I and H levels. All inhibitors positively correlated with total C3 and C4 levels. While correlating with the SLE Disease Activity Index (SLEDAI), no distinction in inhibitor levels was found between SLE and NPSLE patients. Over time, C1-INH and Factor H levels normalized, but no significant changes were observed for C4BP and Factor I. In SLE the levels of circulating complement inhibitors are inversely correlated to complement consumption but do not serve as biomarkers for NPSLE.

Aqueous humor perturbations in chronic smokers: a proteomic study

The detrimental effects of smoking are multisystemic and its effects on the eye health are significant. Smoking is a strong risk factor for age-related nuclear cataract, age-related macular degeneration, glaucoma, delayed corneal epithelial healing and increased risk of cystoid macular edema in patients with intermediate uveitis among others. We aimed to characterize the aqueous humor (AH) proteome in chronic smokers to gain insight into its perturbations and to identify potential biomarkers for smoking-associated ocular pathologies. Compared to the control group, chronic smokers displayed 67 (37 upregulated, 30 downregulated) differentially expressed proteins (DEPs). Analysis of DEPs from the biological point of view revealed that they were proteins involved in complement activation, lymphocyte mediated immunity, innate immune response, cellular oxidant detoxification, bicarbonate transport and platelet degranulation. From the molecular function point of view, DEPs were involved in oxygen binding, oxygen carrier activity, hemoglobin binding, peptidase/endopeptidase/cysteine-type endopeptidase inhibitory activity. Several of the upregulated proteins were acute phase reactant proteins such as clusterin, alpha-2-HS-glycoprotein, fibrinogen, alpha-1-antitrypsin, C4b-binding protein and serum amyloid A-2. Further research should confirm if these proteins might serve as biomarkers or therapeutic target for smoking-associated ocular diseases.

Complement networks in gene-edited pig xenotransplantation: enhancing transplant success and addressing organ shortage

The shortage of organs for transplantation emphasizes the urgent need for alternative solutions. Xenotransplantation has emerged as a promising option due to the greater availability of donor organs. However, significant hurdles such as hyperacute rejection and organ ischemia-reperfusion injury pose major challenges, largely orchestrated by the complement system, and activated immune responses. The complement system, a pivotal component of innate immunity, acts as a natural barrier for xenotransplantation. To address the challenges of immune rejection, gene-edited pigs have become a focal point, aiming to shield donor organs from human immune responses and enhance the overall success of xenotransplantation. This comprehensive review aims to illuminate strategies for regulating complement networks to optimize the efficacy of gene-edited pig xenotransplantation. We begin by exploring the impact of the complement system on the effectiveness of xenotransplantation. Subsequently, we delve into the evaluation of key complement regulators specific to gene-edited pigs. To further understand the status of xenotransplantation, we discuss preclinical studies that utilize gene-edited pigs as a viable source of organs. These investigations provide valuable insights into the feasibility and potential success of xenotransplantation, offering a bridge between scientific advancements and clinical application.

The Complement System and C4b-Binding Protein: A Focus on the Promise of C4BPα as a Biomarker to Predict Clopidogrel Resistance

The complement system plays a dual role in the body, either as a first-line defense barrier when balanced between activation and inhibition or as a potential driver of complement-associated injury or diseases when unbalanced or over-activated. C4b-binding protein (C4BP) was the first circulating complement regulatory protein identified and it functions as an important complement inhibitor. C4BP can suppress the over-activation of complement components and prevent the complement system from attacking the host cells through the binding of complement cleavage products C4b and C3b, working in concert as a cofactor for factor I in the degradation of C4b and C3b, and consequently preventing or reducing the assembly of C3 convertase and C5 convertase, respectively. C4BP, particularly C4BP α-chain (C4BPα), exerts its unique inhibitory effects on complement activation and opsonization, systemic inflammation, and platelet activation and aggregation. It has long been acknowledged that crosstalk or interplay exists between the complement system and platelets. Our unpublished preliminary data suggest that circulating C4BPα exerts its antiplatelet effects through inhibition of both complement activity levels and complement-induced platelet reactivity. Plasma C4BPα levels appear to be significantly higher in patients sensitive to, rather than resistant to, clopidogrel, and we suggest that a plasma C4BPα measurement could be used to predict clopidogrel resistance in the clinical settings.

Diverse Functions of C4b-Binding Protein in Health and Disease

C4b-binding protein (C4BP) is a fluid-phase complement inhibitor that prevents uncontrolled activation of the classical and lectin complement pathways. As a complement inhibitor, C4BP also promotes apoptotic cell death and is hijacked by microbes and tumors for complement evasion. Although initially characterized for its role in complement inhibition, there is an emerging recognition that C4BP functions in a complement-independent manner to promote cell survival, protect against autoimmune damage, and modulate the virulence of microbial pathogens. In this Brief Review, we summarize the structure and functions of human C4BP, with a special focus on activities that extend beyond the canonical role of C4BP in complement inhibition.

C4BPA: A Novel Co-Regulator of Immunity and Fat Metabolism in the Bovine Mammary Epithelial Cells

The C4b binding protein alpha (C4BPA) chain primarily engages in critical inflammatory and coagulation processes. The previous transcriptomic analysis showed that C4BPA is a differentially expressed gene in lower and higher fat content mammary gland cell lines from Chinese Holstein. This study aimed to investigate the effects of C4BPA on the inflammation and milk fat synthesis in bMECs by C4BPA knockdown and overexpression. The results highlighted that knockdown of C4BPA in bMECs could suppress the mRNA and protein expression of IL-6, IL-8, IL-12, and the TLR-4/NF-κB pathway-related genes and promote the expression of complement and coagulation cascade pathways related genes as well as TNF-α. Moreover, knockdown of C4BPA expression in bMECs reduced the content of triglyceride (TG) and cholesterol (CHOL) in bMECs, increased NEFA content, reduced mRNA and protein expression of ACSL1 and PPARA, and increased the mRNA and protein expression of ELOVL6, FADS1, and LPL. The bMECs, with the overexpression of C4BPA, showed the enhanced expression of TLR-4/NF-κB linked genes, IL-6, IL-8, IL-12, and mRNA and protein level while reduced mRNA expression of TNF-α, compliment, and coagulation cascade related genes was observed. In bMECs, overexpression of C4BPA enhanced the content of TG and CHOL while reducing NEFA and stimulated the mRNA and protein expression of ACSL1, PPARA, and PPARG genes while inhibiting the mRNA and protein expression of FADS1 and LPL genes. Our results show that C4BPA not only regulates the lipid metabolism through the PPAR signaling pathway in bMECs but also contributes to the inflammatory response through TLR-4/NF-κB and the complement and coagulation cascade pathways. This study, for the first time, provides the primary basis for understanding the role of C4BPA in immunity and fat metabolism, which enables the researchers for innovative direction to investigate genes associated with fat metabolism and immunity. This study also advocates that the breeders must pay attention to such type of genes with multiple functions during animal breeding.

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.

Mapping Keratoconus Molecular Substrates by Multiplexed High-Resolution Proteomics of Unpooled Corneas

Keratoconus (KCN) is a leading cause for cornea grafting worldwide. Keratoconus is a multifactorial disease that causes progressive thinning of the cornea and whose etiology is poorly understood. Several studies have used proteomics on patient tear fluids to identify potential biomarkers. However, proteome of the cornea itself has not been investigated fully. We report here new findings from a case-control study using multiplexed mass spectrometry (MS) on individual (unpooled) corneas to gain deeper insights into proteins and biomarkers relevant to keratoconus. We employed a high-pressure approach to extract total protein from individual corneas from five cases and five controls, followed by trypsin digestion and tandem mass tag (TMT) labeling. The MS-derived data were searched using the Human NCBI RefSeq protein database v92, with peptides and proteins filtered at 1% false discovery rate. A total of 3132 proteins were detected, of which 627 were altered significantly (p ≤ 0.05) in keratoconus corneas. The increases were overwhelmingly in the mTOR/PI3/AKT signal-mediated regulations of cell survival and proliferation, nonsense-mediated decay of transcripts, and proteasomal pathways. The decreases were in several extracellular matrix proteins and in many members of the complement system. Importantly, this multiplexed proteomic study of keratoconus corneas identified, to our knowledge, the largest number of corneal proteins. The novel findings include changes in pathways that regulate transcript stability, proteasomal degradation, and the complement system in corneas with keratoconus. These observations offer new prospects toward future discovery of novel molecular targets for diagnostic and therapeutic innovations for patients with keratoconus.

The Molecular Basis of Human IgG-Mediated Enhancement of C4b-Binding Protein Recruitment to Group A Streptococcus

Streptococcus pyogenes infects over 700 million people worldwide annually. Immune evasion strategies employed by the bacteria include binding of the complement inhibitors, C4b-binding protein (C4BP) and Factor H in a human-specific manner. We recently showed that human IgG increased C4BP binding to the bacterial surface, which promoted streptococcal immune evasion and increased mortality in mice. We sought to identify how IgG promotes C4BP binding to Protein H, a member of the M protein family. Dimerization of Protein H is pivotal for enhanced binding to human C4BP. First, we illustrated that Protein H, IgG, and C4BP formed a tripartite complex. Second, surface plasmon resonance revealed that Protein H binds IgG solely through Fc, but not Fab domains, and with high affinity (IgG-Protein H: K_D = 0.4 nM; IgG-Fc-Protein H: K_D ≤ 1.6 nM). Each IgG binds two Protein H molecules, while up to six molecules of Protein H bind one C4BP molecule. Third, interrupting Protein H dimerization either by raising temperature to 41°C or with a synthetic peptide prevented IgG-Protein H interactions. IgG-Fc fragments or monoclonal human IgG permitted maximal C4BP binding when used at concentrations from 0.1 to 10 mg/ml. In contrast, pooled human IgG enhanced C4BP binding at concentrations up to 1 mg/ml; decreased C4BP binding at 10 mg/ml occurred probably because of Fab-streptococcal interactions at these high IgG concentrations. Taken together, our data show how S. pyogenes exploits human IgG to evade complement and enhance its virulence. Elucidation of this mechanism could aid design of new therapeutics against S. pyogenes.

The Pneumococcal Surface Proteins PspA and PspC Sequester Host C4-Binding Protein To Inactivate Complement C4b on the Bacterial Surface

Complement is a critical component of antimicrobial immunity. Various complement regulatory proteins prevent host cells from being attacked. Many pathogens have acquired the ability to sequester complement regulators from host plasma to evade complement attack. We describe here how Streptococcus pneumoniae adopts a strategy to prevent the formation of the C3 convertase C4bC2a by the rapid conversion of surface bound C4b and iC4b into C4dg, which remains bound to the bacterial surface but no longer forms a convertase complex. Noncapsular virulence factors on the pneumococcus are thought to facilitate this process by sequestering C4b-binding protein (C4BP) from host plasma. When S. pneumoniae D39 was opsonized with human serum, the larger C4 activation products C4b and iC4b were undetectable, but the bacteria were liberally decorated with C4dg and C4BP. With targeted deletions of either PspA or PspC, C4BP deposition was markedly reduced, and there was a corresponding reduction in C4dg and an increase in the deposition of C4b and iC4b. The effect was greatest when PspA and PspC were both knocked out. Infection experiments in mice indicated that the deletion of PspA and/or PspC resulted in the loss of bacterial pathogenicity. Recombinant PspA and PspC both bound serum C4BP, and both led to increased C4b and reduced C4dg deposition on S. pneumoniae D39. We conclude that PspA and PspC help the pneumococcus to evade complement attack by binding C4BP and so inactivating C4b.

Chimeric Proteins Containing MAP-1 and Functional Domains of C4b-Binding Protein Reveal Strong Complement Inhibitory Capacities

The complement system is a tightly regulated network of proteins involved in defense against pathogens, inflammatory processes, and coordination of the innate and adaptive immune responses. Dysregulation of the complement cascade is associated with many inflammatory disorders. Thus, inhibition of the complement system has emerged as an option for treatment of a range of different inflammatory diseases. MAP-1 is a pattern recognition molecule (PRM)-associated inhibitor of the lectin pathway of the complement system, whereas C4b-binding protein (C4BP) regulates both the classical and lectin pathways. In this study we generated chimeric proteins consisting of MAP-1 and the first five domains of human C4BP (C4BP¹⁻⁵) in order to develop a targeted inhibitor acting at different levels of the complement cascade. Two different constructs were designed and expressed in CHO cells where MAP-1 was fused with C4BP¹⁻⁵ in either the C- or N-terminus. The functionality of the chimeric proteins was assessed using different in vitro complement activation assays. Both chimeric proteins displayed the characteristic Ca²⁺-dependent dimerization and binding to PRMs of native MAP-1, as well as the co-factor activity of native C4BP. In ELISA-based complement activation assays they could effectively inhibit the lectin and classical pathways. Notably, MAP-1:C4BP¹⁻⁵ was five times more effective than rMAP-1 and rC4BP¹⁻⁵ applied at the same time, emphasizing the advantage of a single inhibitor containing both functional domains. The MAP-1/C4BP chimeras exert unique complement inhibitory properties and represent a novel therapeutic approach targeting both upstream and central complement activation.

Functional Characterization of Alternative and Classical Pathway C3/C5 Convertase Activity and Inhibition Using Purified Models

Complement is essential for the protection against infections; however, dysregulation of complement activation can cause onset and progression of numerous inflammatory diseases. Convertase enzymes play a central role in complement activation and produce the key mediators of complement: C3 convertases cleave C3 to generate chemoattractant C3a and label target cells with C3b, which promotes phagocytosis; C5 convertases cleave C5 into chemoattractant C5a, and C5b, which drives formation of the membrane attack complex. Since convertases mediate nearly all complement effector functions, they are ideal targets for therapeutic complement inhibition. A unique feature of convertases is their covalent attachment to target cells, which effectively confines complement activation to the cell surface. However, surface localization precludes detailed analysis of convertase activation and inhibition. In our previous work, we developed a model system to form purified alternative pathway (AP) C5 convertases on C3b-coated beads and quantify C5 conversion via functional analysis of released C5a. Here, we developed a C3aR cell reporter system that enables functional discrimination between C3 and C5 convertases. By regulating the C3b density on the bead surface, we observe that high C3b densities are important for conversion of C5, but not C3, by AP convertases. Screening of well-characterized complement-binding molecules revealed that differential inhibition of AP C3 convertases (C3bBb) and C5 convertases [C3bBb(C3b)_n] is possible. Although both convertases contain C3b, the C3b-binding molecules Efb-C/Ecb and FHR5 specifically inhibit C5 conversion. Furthermore, using a new classical pathway convertase model, we show that these C3b-binding proteins not only block AP C3/C5 convertases but also inhibit formation of a functional classical pathway C5 convertase under well-defined conditions. Our models enable functional characterization of purified convertase enzymes and provide a platform for the identification and development of specific convertase inhibitors for treatment of complement-mediated disorders.

A novel C3d-containing oligomeric vaccine provides insight into the viability of testing human C3d-based vaccines in mice

The use of C3d, the final degradation product of complement protein C3, as a "natural" adjuvant has been widely examined since the initial documentation of its immunogenicity-enhancing properties as a consequence of binding to complement receptor 2. Subsequently it was demonstrated that these effects are most evident when oligomeric, rather than when monomeric forms of C3d, are linked to various test protein antigens. In this study, we examined the feasibility of enhancing the adjuvant properties of human C3d further by utilizing C4b-binding protein (C4BP) to provide an oligomeric arrayed scaffold fused to the model antigen, tetanus toxin C fragment (TTCF). High molecular weight, C3d-containing oligomeric vaccines were successfully expressed, purified from mammalian cells and used to immunize groups of mice. Surprisingly, anti-TTCF antibody responses measured in these mice were poor. Subsequently we established by in vitro and in vivo analysis that, in the presence of mouse C3, human C3d does not interact with either mouse or even human complement receptor 2. These data confirm the requirement to develop murine versions of C3d based adjuvant compounds to test in mice or that mice would need to be developed that express both human C3 and human CR2 to allow the testing of human C3d based adjuvants in mouse in any capacity.

Complement in removal of the dead - balancing inflammation

Recognition and removal of apoptotic and necrotic cells must be efficient and highly controlled to avoid excessive inflammation and autoimmune responses to self. The complement system, a crucial part of innate immunity, plays an important role in this process. Thus, apoptotic and necrotic cells are recognized by complement initiators such as C1q, mannose binding lectin, ficolins, and properdin. This triggers complement activation and opsonization of cells with fragments of C3b, which enhances phagocytosis and thus ensures silent removal. Importantly, the process is tightly controlled by the binding of complement inhibitors C4b-binding protein and factor H, which attenuates late steps of complement activation and inflammation. Furthermore, factor H becomes actively internalized by apoptotic cells, where it catalyzes the cleavage of intracellular C3 to C3b. The intracellularly derived C3b additionally opsonizes the cell surface further supporting safe and fast clearance and thereby aids to prevent autoimmunity. Internalized factor H also binds nucleosomes and directs monocytes into production of anti-inflammatory cytokines upon phagocytosis of such complexes. Disturbances in the complement-mediated clearance of dying cells result in persistence of autoantigens and development of autoimmune diseases like systemic lupus erythematosus, and may also be involved in development of age-related macula degeneration.

Association of single nucleotide polymorphisms in the 5' upstream region of the C4BPA gene with essential hypertension in a northeastern Han Chinese population

A previous study of the authors using microarray analysis indicated that the expression of complement component 4 binding protein (C4BP)A is upregulated in essential hypertension (EH) patients, but the association between C4BPA variations and EH has not yet been clearly demonstrated. Since the 5′ upstream region is known to serve important roles in the gene expression regulation, the present study aimed to identify and analyze the association of single nucleotide polymorphisms (SNPs) in the 5′ upstream region between the C4BPA gene with EH in a case-control study among a northeastern Han Chinese population through direct sequencing as well as genotype detection. A total of 822 unrelated participants were included. The higher expression level of C4BPA in the peripheral blood of patients with EH was verified through reverse transcription-quantitative polymerase chain reaction and ELISA. A total of four SNPs, rs73079108, rs74148971, rs77660718 and rs11120211 were identified in the 5′ upstream region of C4BPA. Association analysis demonstrated that the genotypic frequencies of rs73079108 were significantly different between EH and the control groups (P=0.011), and A allelic frequency was lower in EH (P<0.001). Logistic regression analysis indicated that the rs73079108 polymorphism was closely associated with EH (AA:GA:GG genetic model: P=0.007, odds ratio (OR)=0.604, 95% confidence interval (CI) [0.418–0.873]; AA+GA:GG genetic model: P=0.005, OR=0.806, 95% CI[0.382–0.841]), and the A allele may be a protective factor. Subgroup analysis by sex and BMI presented concordant conclusions in female and non-obese samples. Further analysis indicated that rs73079108 was associated with systolic blood pressure (P<0.001), diastolic blood pressure (P=0.001) and fast blood glucose (FBG) (P=0.021). In addition, rs73079108 GA and GG carriers reported a significant increase in the level of the protein encoded by C4BPA than those of AA carriers. The rs73079108 polymorphism in the 5′ upstream region of C4BPA was associated with EH, and rs73079108-A may be an independent predictor.

Complement Evasion by Pathogenic Leptospira

Leptospirosis is a neglected infectious disease caused by spirochetes from the genus Leptospira. Pathogenic microorganisms, notably those which reach the blood circulation such as Leptospira, have evolved multiple strategies to escape the host complement system, which is important for innate and acquired immunity. Leptospira avoid complement-mediated killing through: (i) recruitment of host complement regulators; (ii) acquisition of host proteases that cleave complement proteins on the bacterial surface; and, (iii) secretion of proteases that inactivate complement proteins in the Leptospira surroundings. The recruitment of host soluble complement regulatory proteins includes the acquisition of Factor H (FH) and FH-like-1 (alternative pathway), C4b-binding protein (C4BP) (classical and lectin pathways), and vitronectin (Vn) (terminal pathway). Once bound to the leptospiral surface, FH and C4BP retain cofactor activity of Factor I in the cleavage of C3b and C4b, respectively. Vn acquisition by leptospires may result in terminal pathway inhibition by blocking C9 polymerization. The second evasion mechanism lies in plasminogen (PLG) binding to the leptospiral surface. In the presence of host activators, PLG is converted to enzymatically active plasmin, which is able to degrade C3b, C4b, and C5 at the surface of the pathogen. A third strategy used by leptospires to escape from complement system is the active secretion of proteases. Pathogenic, but not saprophytic leptospires, are able to secrete metalloproteases that cleave C3 (central complement molecule), Factor B (alternative pathway), and C4 and C2 (classical and lectin pathways). The purpose of this review is to fully explore these complement evasion mechanisms, which act together to favor Leptospira survival and multiplication in the host.

Acquisition of negative complement regulators by the saprophyte Leptospira biflexa expressing LigA or LigB confers enhanced survival in human serum

Leptospiral immunoglobulin-like (Lig) proteins are surface exposed molecules present in pathogenic but not in saprophytic Leptospira species. We have previously shown that Lig proteins interact with the soluble complement regulators Factor H (FH), FH like-1 (FHL-1), FH related-1 (FHR-1) and C4b Binding Protein (C4BP). In this study, we used the saprophyte L. biflexa serovar Patoc as a surrogate host to address the specific role of LigA and LigB proteins in leptospiral complement evasion. L. biflexa expressing LigA or LigB was able to acquire FH and C4BP. Bound complement regulators retained their cofactor activities of FI in the proteolytic cleavage of C3b and C4b. Moreover, heterologous expression of ligA and ligB genes in the saprophyte L. biflexa enhanced bacterial survival in human serum. Complement deposition on lig-transformed L. biflexa was assessed by flow cytometry analysis. With regard to MAC deposition, L. biflexa expressing LigA or LigB presented an intermediate profile: MAC deposition levels were greater than those found in the pathogenic L. interrogans, but lower than those observed for L. biflexa wildtype. In conclusion, Lig proteins contribute to in vitro control of complement activation on the leptospiral surface, promoting an increased bacterial survival in human serum.

Exploitation of complement regulatory proteins by Borrelia and Francisella

Pathogens have developed sophisticated mechanisms of complement evasion such as binding to the host complement regulatory proteins (CRPs) on their surface or expression of CRP mimicking molecules. The ability of pathogens to evade the complement system has been correlated with pathogenesis and host selectivity. Hitherto, little work has been undertaken to determine whether Borrelia and Francisella exploit various CRPs to block complement attack. Seventeen Borrelia (twelve species) and six Francisella (three subspecies) strains were used to assess their ability to bind human, sheep and cattle CRPs or mimic membrane associated complement regulators. A series of experiments including affinity ligand binding experiments, pull-down assays and mass spectrometry based protein identification, revealed an array of CRP binding proteins of Borrelia and Francisella. Unlike Francisella, Borrelia strains were able to bind multiple human CRPs. Three strains of Borrelia (SKT-4, SKT-2 and HO14) showed the presence of a human CD46-homologous motif, indicating their ability to possess putative human CD46 mimicking molecules. Similarly, five strains of Borrelia and two strains of Francisella may have surface proteins with human CD59-homologous motifs. Among ovine and bovine CRPs, the only CRP bound by Francisella (LVS, Tul4 strain) was vitronectin, while ovine C4BP, ovine factor H and bovine factor H were bound to Borrelia strains SKT-2, DN127 and Co53. This study presents an array of proteins of Borrelia and Francisella that bind CRPs or may mimic membrane-CRPs, thus enabling multiphasic complement evasion strategies of these pathogens.

Preliminary genomic survey and sequence analysis of the complement system in non-eutherian mammals

The complement system is a major mediator of the vertebrate immune system, which functions in both innate and specific immune responses. It comprises more than 30 proteins working to remove foreign cells by way of anaphylatoxins, opsonins or the membrane attack complex. Over the last few years, whole genome sequences of non-eutherian mammals (marsupials and a monotreme), the gray short-tailed opossum (Monodelphis domestica), tammar wallaby (Macropus eugenii), Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus) and platypus (Ornithorhynchus anatinus), have become publicly available. Using these sequences, we have identified an array of complement components in non-eutherians using online search tools and algorithms. Of 57 complement and complement-related genes investigated, we identified 46 in the gray short-tailed opossum genome, 27 in the tammar wallaby genome, 44 in the Tasmanian devil genome, 47 in the koala genome and 40 in the platypus genome. The results of this study confirm the presence of key complement components in the immune repertoire of non-eutherian mammals and provide a platform for future studies on immune protection in young marsupials.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Platelet mimicry: The emperor's new clothes?

Here we critically examine whether coating of nanoparticles with platelet membranes can truly disguise them against recognition by elements of the innate immune system. We further assess whether the "cloaking technology" can sufficiently equip nanoparticles with platelet-mimicking functionalities to include in vivo targeting of damaged blood vessels and binding to platelet-adhering opportunistic pathogens. We present views for improved, and pharmaceutically viable nanoparticle design strategies.

Purification and functional characterization of C4b-binding protein (C4BP)

C4b-binding protein (C4BP) is a soluble, 570 kDa large glycoprotein, present in plasma at a concentration of approximately 200 mg/L. C4BP is the main inhibitor of the classical and lectin pathways of complement, where it controls C4b-mediated reactions. Here, we describe a method for purification of C4BP from human plasma, which is based on barium chloride precipitation, anion exchange chromatography, and gel filtration. We also describe a functional assay, in which C4BP's cofactor activity to factor I, in the degradation of C4b, can be assessed.

Binding of complement inhibitor C4b-binding protein to a highly virulent Streptococcus pyogenes M1 strain is mediated by protein H and enhances adhesion to and invasion of endothelial cells

Streptococcus pyogenes AP1, a strain of the highly virulent M1 serotype, uses exclusively protein H to bind the complement inhibitor C4b-binding protein (C4BP). We found a strong correlation between the ability of AP1 and its isogenic mutants lacking protein H to inhibit opsonization with complement C3b and binding of C4BP. C4BP bound to immobilized protein H or AP1 bacteria retained its cofactor activity for degradation of (125)I-C4b. Furthermore, C4b deposited from serum onto AP1 bacterial surfaces was processed into C4c/C4d fragments, which did not occur on strains unable to bind C4BP. Recombinant C4BP mutants, which (i) lack certain CCP domains or (ii) have mutations in single aa as well as (iii) mutants with additional aa between different CCP domains were used to determine that the binding is mainly mediated by a patch of positively charged amino acid residues at the interface of domains CCP1 and CCP2. Using recombinant protein H fragments, we narrowed down the binding site to the N-terminal domain A. With a peptide microarray, we identified one single 18-amino acid-long peptide comprising residues 92-109, which specifically bound C4BP. Biacore was used to determine KD = 6 × 10(-7) M between protein H and a single subunit of C4BP. C4BP binding also correlated with elevated levels of adhesion and invasion to endothelial cells. Taken together, we identified the molecular basis of C4BP-protein H interaction and found that it is not only important for decreased opsonization but also for invasion of endothelial cells by S. pyogenes.

The α7β0 isoform of the complement regulator C4b-binding protein induces a semimature, anti-inflammatory state in dendritic cells

The classical pathway complement regulator C4b-binding protein (C4BP) is composed of two polypeptides (α- and β-chains), which form three plasma oligomers with different subunit compositions (α7β1, α7β0, and α6β1). We show in this article that the C4BP α7β0 isoform (hereafter called C4BP[β(-)] [C4BP lacking the β-chain]), overexpressed under acute-phase conditions, induces a semimature, tolerogenic state on human monocyte-derived dendritic cells (DCs) activated by a proinflammatory stimulus. C4BP isoforms containing β-chain (α7β1 and α6β1; C4BP[β(+)]) neither interfered with the normal maturation of DCs nor competed with C4BP(β(-)) activity on these cells. Immature DCs (iDCs) treated with C4BP(β(-)) retained high endocytic activity, but, upon LPS treatment, they did not upregulate surface expression of CD83, CD80, and CD86. Transcriptional profiling of these semimature DCs revealed that treatment with C4BP(β(-)) prevented the induction of IDO and BIC-1, whereas TGF-β1 expression was maintained to the level of iDCs. C4BP(β(-))-treated DCs were also unable to release proinflammatory Th1 cytokines (IL-12, TNF-α, IFN-γ, IL-6, IL-8) and, conversely, increased IL-10 secretion. They prevented surface CCR7 overexpression and, accordingly, displayed reduced chemotaxis, being morphologically indistinguishable from iDCs. Moreover, C4BP(β(-))-treated DCs failed to enhance allogeneic T cell proliferation, impairing IFN-γ production in these cells and, conversely, promoting CD4(+)CD127(low/neg)CD25(high)Foxp3(+) T cells. Deletion mutant analysis revealed that the complement control protein-6 domain of the α-chain is necessary for the tolerogenic activity of C4BP(β(-)). Our data demonstrate a novel anti-inflammatory and immunomodulatory function of the complement regulator C4BP, suggesting a relevant role of the acute-phase C4BP(β(-)) isoform in a number of pathophysiological conditions and potential applications in autoimmunity and transplantation.

Complement activation and cardiac surgery: a novel target for improving outcomes

Complement activation and the resulting inflammatory response is an important potential mechanism for multisystem organ injury in cardiac surgery. Novel therapeutic strategies using complement inhibitors may hold promise for improving outcomes for cardiac surgical patients by attenuating complement activation or its biologically active effector molecules. Recent clinical trials evaluating complement inhibitors have provided important data to further delineate the impact of complement activation and its inhibition on clinical outcomes. In this review we examine the role of complement activation and its inhibition as a therapeutic approach in cardiac surgery.

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.

Complement in cancer and cancer immunotherapy

Recently, there has been an increase of interest in the use of biological or immune-based therapies for patients with malignancies. This has been informed by the deeper understanding of the crosstalk between the host immune system and malignant tumours, as well as the potential advantages of immunotherapy-high specificity and less toxicity compared to standard approaches. The particular emphasis of this article is on the role of the complement system in tumour growth and antibody-based cancer immunotherapy. The functional consequences from overexpression of complement regulators by tumours and the development of strategies for overcoming this are discussed in detail. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of cancer.

A computational and experimental study of the regulatory mechanisms of the complement system

The complement system is key to innate immunity and its activation is necessary for the clearance of bacteria and apoptotic cells. However, insufficient or excessive complement activation will lead to immune-related diseases. It is so far unknown how the complement activity is up- or down- regulated and what the associated pathophysiological mechanisms are. To quantitatively understand the modulatory mechanisms of the complement system, we built a computational model involving the enhancement and suppression mechanisms that regulate complement activity. Our model consists of a large system of Ordinary Differential Equations (ODEs) accompanied by a dynamic Bayesian network as a probabilistic approximation of the ODE dynamics. Applying Bayesian inference techniques, this approximation was used to perform parameter estimation and sensitivity analysis. Our combined computational and experimental study showed that the antimicrobial response is sensitive to changes in pH and calcium levels, which determines the strength of the crosstalk between CRP and L-ficolin. Our study also revealed differential regulatory effects of C4BP. While C4BP delays but does not decrease the classical complement activation, it attenuates but does not significantly delay the lectin pathway activation. We also found that the major inhibitory role of C4BP is to facilitate the decay of C3 convertase. In summary, the present work elucidates the regulatory mechanisms of the complement system and demonstrates how the bio-pathway machinery maintains the balance between activation and inhibition. The insights we have gained could contribute to the development of therapies targeting the complement system.

The complement system, which is the frontline immune defense, constitutes proteins that flow freely in the blood. It quickly detects invading microbes and alerts the host by sending signals into immune responsive cells to eliminate the hostile substances. Inadequate or excessive complement activities harm the host and may lead to immune-related diseases. Thus, it is crucial to understand how the host boosts the complement activity to protect itself and simultaneously establishes tight surveillance to attain homeostasis. Towards this goal, we developed a detailed computational model of the human complement system. To overcome the challenges resulting from the large model size, we applied probabilistic approximation and inference techniques to train the model on experimental data and explored the key network features of the model. Our model-based study highlights the importance of infection-mediated microenvironmental perturbations, which alter the pH and calcium levels. It also reveals that the inhibitor, C4BP induces differential inhibition on the classical and lectin complement pathways and acts mainly by facilitating the decay of the C3 convertase. These predictions were validated empirically. Thus, our results help to elucidate the regulatory mechanisms of the complement system and potentially contribute to the development of complement-based immunomodulation therapies.

The pH-regulated antigen 1 of Candida albicans binds the human complement inhibitor C4b-binding protein and mediates fungal complement evasion

Candida albicans binds and utilizes human complement inhibitors, such as C4b-binding protein (C4BP), Factor H, and FHL-1 for immune evasion. Here, we identify Candida pH-regulated antigen 1 (Pra1) as the first fungal C4BP-binding protein. Recombinant Pra1 binds C4BP, as shown by ELISA and isothermal titration calorimetry, and the Pra1-C4BP interaction is ionic in nature. The Pra1 binding domains within C4BP were localized to the complement control protein domain 4 (CCP4), CCP7, and CCP8. C4BP bound to Pra1 maintains complement-inhibitory activity. C4BP and Factor H bind simultaneously to Candida Pra1 and do not compete for binding at physiological levels. A Pra1-overexpressing C. albicans strain, which had about 2-fold Pra1 levels at the surface acquired also about 2-fold C4BP to the surface, compared with the wild type strain CAI4. A Pra1 knock-out strain showed ∼22% reduced C4BP binding. C4BP captured by C. albicans from human serum inhibits C4b and C3b surface deposition and also maintains cofactor activity. In summary, Candida Pra1 represents the first fungal C4BP-binding surface protein. Pra1, via binding to C4BP, mediates human complement control, thereby favoring the immune and complement evasion of C. albicans.

Genetics and complement in atypical HUS

Central to the pathogenesis of atypical hemolytic uremic syndrome (aHUS) is over-activation of the alternative pathway of complement. Following the initial discovery of mutations in the complement regulatory protein, factor H, mutations have been described in factor I, membrane cofactor protein and thrombomodulin, which also result in decreased complement regulation. Autoantibodies to factor H have also been reported to impair complement regulation in aHUS. More recently, gain of function mutations in the complement components C3 and Factor B have been seen. This review focuses on the genetic causes of aHUS, their functional consequences, and clinical effect.

Human complement regulators C4b-binding protein and C1 esterase inhibitor interact with a novel outer surface protein of Borrelia recurrentis

The spirochete Borrelia recurrentis is the causal agent of louse-borne relapsing fever and is transmitted to humans by the infected body louse Pediculus humanus. We have recently demonstrated that the B. recurrentis surface receptor, HcpA, specifically binds factor H, the regulator of the alternative pathway of complement activation, thereby inhibiting complement mediated bacteriolysis. Here, we show that B. recurrentis spirochetes express another potential outer membrane lipoprotein, termed CihC, and acquire C4b-binding protein (C4bp) and human C1 esterase inhibitor (C1-Inh), the major inhibitors of the classical and lectin pathway of complement activation. A highly homologous receptor for C4bp was also found in the African tick-borne relapsing fever spirochete B. duttonii. Upon its binding to B. recurrentis or recombinant CihC, C4bp retains its functional potential, i.e. facilitating the factor I-mediated degradation of C4b. The additional finding that ectopic expression of CihC in serum sensitive B. burgdorferi significantly increased spirochetal resistance against human complement suggests this receptor to substantially contribute, together with other known strategies, to immune evasion of B. recurrentis.

Borrelia recurrentis, the causal agent of louse-borne relapsing fever is transmitted to humans via infected body lice. Infection with B. recurrentis has been achieved only in humans and is accompanied by a systemic inflammatory disease, multiple relapses of fever and massive spirochetemia. A key virulence factor of B. recurrentis is their potential to undergo antigenic variation. However, for survival in the blood during the early phase of infection and for persistence in human tissues, spirochetes must be endowed with robust tools to escape innate immunity. We have recently shown that B. recurrentis acquires the serum-derived regulator factor H, thereby blocking the alternative complement pathway. Here, we show that B. recurrentis expresses in addition a novel outer surface lipoprotein that selectively binds serum-derived C4b-binding protein and C1 esterase inhibitor, two endogenous regulators of the classical and lectin pathway of complement activation. The combined data underscore the versatility of B. recurrentis to effectively evade innate and adaptive immunity, including serum resistance. Thus, the present study elucidates a new mechanism of B. recurrentis important for its evasion from complement attack and will be helpful for the development of new drugs against this fatal infection.

Functional characterization of LcpA, a surface-exposed protein of Leptospira spp. that binds the human complement regulator C4BP

We have previously shown that pathogenic leptospiral strains are able to bind C4b binding protein (C4BP). Surface-bound C4BP retains its cofactor activity, indicating that acquisition of this complement regulator may contribute to leptospiral serum resistance. In the present study, the abilities of seven recombinant putative leptospiral outer membrane proteins to interact with C4BP were evaluated. The protein encoded by LIC11947 interacted with this human complement regulator in a dose-dependent manner. The cofactor activity of C4BP bound to immobilized recombinant LIC11947 (rLIC11947) was confirmed by detecting factor I-mediated cleavage of C4b. rLIC11947 was therefore named LcpA (for leptospiral complement regulator-acquiring protein A). LcpA was shown to be an outer membrane protein by using immunoelectron microscopy, cell surface proteolysis, and Triton X-114 fractionation. The gene coding for LcpA is conserved among pathogenic leptospiral strains. This is the first characterization of a Leptospira surface protein that binds to the human complement regulator C4BP in a manner that allows this important regulator to control complement system activation mediated either by the classical pathway or by the lectin pathway. This newly identified protein may play a role in immune evasion by Leptospira spp. and may therefore represent a target for the development of a human vaccine against leptospirosis.

C4BPB/C4BPA is a new susceptibility locus for venous thrombosis with unknown protein S-independent mechanism: results from genome-wide association and gene expression analyses followed by case-control studies

Through its binding with protein S (PS), a key element of the coagulation/fibrinolysis cascade, the C4b-binding protein (C4BP) has been hypothesized to be involved in the susceptibility to venous thrombosis (VT). To identify genetic factors that may influence the plasma levels of the 3 C4BP existing isoforms, alpha(7)beta(1), alpha(6)beta(1), and alpha(7)beta(0), we conducted a genome-wide association study by analyzing 283 437 single nucleotide polymorphisms (SNPs) in the Genetic Analysis of Idiopathic Thrombophilia (GAIT) study composed of 352 persons. Three SNPs at the C4BPB/C4BPA locus were found genome-wide significantly associated with alpha(7)beta(0) levels. One of these SNPs was further found to explain approximately 11% of the variability of mRNA C4BPA expression in the Gutenberg Heart Study composed of 1490 persons, with no effect on C4BPB mRNA expression. The allele associated with increased alpha(7)beta(0) plasma levels and increased C4BPA expression was further found associated with increased risk of VT (odds ratio [OR] = 1.24 [1.03-1.53]) in 2 independent case-control studies (MARseille THrombosis Association study [MARTHA] and FActeurs de RIsque et de récidives de la maladie thromboembolique VEineuse [FARIVE]) gathering 1706 cases and 1379 controls. This SNP was not associated with free PS or total PS. In conclusion, we observed strong evidence that the C4BPB/C4BPA locus is a new susceptibility locus for VT through a PS-independent mechanism that remains to be elucidated.

Performance of agarose IEF gels as the first dimension support for non-denaturing micro-2-DE in the separation of high-molecular-mass plasma proteins and protein complexes

Agarose micro-column gels (1% w/v agarose, diameter 1.4 mm and length 35 mm) were prepared as the first-dimension IEF support for non-denaturing 2-DE and the performance was compared with that of polyacrylamide gels (4.2% T and 4.8% C, the same gel size) using a human plasma sample. Sorbitol was not added in the agarose IEF gels, since its presence not only delayed the focusing of the proteins but also deteriorated the protein resolution. The optimum IEF time of the agarose gels for separation of 2 microL plasma sample (ca. 120 microg proteins) was decided to be 46 min, which is much shorter than that of the polyacrylamide gels (75 min). MALDI-MS and PMF assignment of the spots on the micro-2-DE gels at apparent molecular mass above ca. 5x10(2) kDa and pI from 4 to 8 revealed that when polyacrylamide IEF gels were used, many of the high-molecular-mass proteins resided at the sample loading edge or in basic pI regions as smear bands. When agarose IEF gels were used, most of the high-molecular-mass proteins moved to more acidic pI positions and were better focused, and their apparent pI values matched well with those previously reported for purified proteins. These results demonstrated the advantages of agarose-IEF/2-DE for the separation of high-molecular-mass proteins and protein complexes under non-denaturing conditions.

Binding of complement regulatory proteins to group A Streptococcus

Streptococcus pyogenes or Group A Streptococcus (GAS) is the etiologic agent of important human infections such as acute pharyngitis, impetigo, rheumatic fever and the streptococcal toxic shock syndrome. Binding of the complement regulatory proteins factor H, factor H-like protein 1 (FHL-1), C4b-binding protein (C4BP), or CD46 is a crucial step in the pathogenesis of these infections. M protein is the GAS protein that generally mediates these interactions. However, a detailed analysis of the reports that have investigated the binding of complement regulatory components to GAS indicates that this microorganism has evolved alternative mechanisms for the recruitment of complement regulatory proteins to the bacterial surface. This article summarizes these data to provide a starting point for future research aimed at the characterization of additional mechanisms developed by GAS to evade the immune system.

Lack of association between polymorphisms in C4b-binding protein and atypical haemolytic uraemic syndrome in the Spanish population

Dysregulation of the alternative pathway of complement activation, caused by mutations or polymorphisms in the genes encoding factor H, membrane co-factor protein, factor I or factor B, is associated strongly with predisposition to atypical haemolytic uraemic syndrome (aHUS). C4b-binding protein (C4BP), a major regulator of the classical pathway of complement activation, also has capacity to regulate the alternative pathway. Interestingly, the C4BP polymorphism p.Arg240His has been associated recently with predisposition to aHUS and the risk allele His240 showed decreased capacity to regulate the alternative pathway. Identification of novel aHUS predisposition factors has important implications for diagnosis and treatment in a significant number of aHUS patients; thus, we sought to replicate these association studies in an independent cohort of aHUS patients. In this study we show that the C4BP His240 allele corresponds to the C4BP*2 allele identified previously by isoelectric focusing in heterozygosis in 1.9-3.7% of unrelated Caucasians. Crucially, we found no differences between 102 unrelated Spanish aHUS patients and 128 healthy age-matched Spanish controls for the frequency of carriers of the His240 C4BP allele. This did not support an association between the p.Arg240His C4BP polymorphism and predisposition to aHUS in the Spanish population. In a similar study, we also failed to sustain an association between C4BP polymorphisms and predisposition to age-related macular degeneration, another disorder which is associated strongly with polymorphisms in factor H, and is thought to involve alternative pathway dysregulation.

C4b-binding protein (C4BP) inhibits development of experimental arthritis in mice

OBJECTIVES

To assess the human complement inhibitor C4b-binding protein (C4BP) for treatment of arthritis.

METHODS

We have used two mouse models of rheumatoid arthritis (RA) to assess the therapeutic effect of C4BP on different phases of arthritis, the collagen antibody-induced arthritis (CAIA), an acute antibody-induced disease and the collagen-induced arthritis (CIA), which carries the full complexity of arthritis.

RESULTS

Purified human C4BP injected intraperitoneally alleviated CAIA significantly in a manner similar to cobra venom factor that depletes complement due to massive activation. Furthermore, C4BP was injected before and after the disease development into CIA mice. In the former case, the disease onset was delayed and in the latter, the severity of the disease was reduced in animals treated with C4BP. However, C4BP did not affect the anti-CII antibody synthesis. C4BP present in mouse sera decreased activity of the classical but not the alternative pathway of the complement system when these were assessed in a fluid phase. However, C4BP was efficiently inhibiting the alternative pathway when present on the activating surface. Taken together, the disease ameliorating effect of C4BP appears to be related to inhibition of both pathways of complement.

CONCLUSIONS

Although human C4BP was cleared relatively fast from the circulation and was only moderately affecting complement activity, its effect on the disease severity was substantial, suggesting that minor alterations in complement activity can have significant therapeutic value in RA.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

Immune evasion of leptospira species by acquisition of human complement regulator C4BP

Leptospirosis is a spirochetal zoonotic disease of global distribution with a high incidence in tropical regions. In the last 15 years it has been recognized as an important emerging infectious disease due to the occurrence of large outbreaks in warm-climate countries and, occasionally, in temperate regions. Pathogenic leptospires efficiently colonize target organs after penetrating the host. Their invasiveness is attributed to the ability to multiply in blood, adhere to host cells, and penetrate into tissues. Therefore, they must be able to evade the innate host defense. The main purpose of the present study was to evaluate how several Leptospira strains evade the protective function of the complement system. The serum resistance of six Leptospira strains was analyzed. We demonstrate that the pathogenic strain isolated from infected hamsters avoids serum bactericidal activity more efficiently than the culture-attenuated or the nonpathogenic Leptospira strains. Moreover, both the alternative and the classical pathways of complement seem to be responsible for the killing of leptospires. Serum-resistant and serum-intermediate strains are able to bind C4BP, whereas the serum-sensitive strain Patoc I is not. Surface-bound C4BP promotes factor I-mediated cleavage of C4b. Accordingly, we found that pathogenic strains displayed reduced deposition of the late complement components C5 to C9 upon exposure to serum. We conclude that binding of C4BP contributes to leptospiral serum resistance against host complement.