Molecular Dissection of Interactions between Components of the Alternative Pathway of Complement and Decay Accelerating Factor (CD55)

Triple-fusion protein (TriFu): A potent, targeted, enzyme-like inhibitor of all three complement activation pathways

The introduction of a therapeutic anti-C5 antibody into clinical practice in 2007 inspired a surge into the development of complement-targeted therapies. This has led to the recent approval of a C3 inhibitory peptide, an antibody directed against C1s and a full pipeline of several complement inhibitors in preclinical and clinical development. However, no inhibitor is available that efficiently inhibits all three complement initiation pathways and targets host cell surface markers as well as complement opsonins. To overcome this, we engineered a novel fusion protein combining selected domains of the three natural complement regulatory proteins decay accelerating factor, factor H and complement receptor 1. Such a triple fusion complement inhibitor (TriFu) was recombinantly expressed and purified alongside multiple variants and its building blocks. We analyzed these proteins for ligand binding affinity and decay acceleration activity by surface plasmon resonance. Additionally, we tested complement inhibition in several in vitro/ex vivo assays using standard classical and alternative pathway restricted hemolysis assays next to hemolysis assays with paroxysmal nocturnal hemoglobinuria erythrocytes. A novel in vitro model of the alternative pathway disease C3 glomerulopathy was established to evaluate the potential of the inhibitors to stop C3 deposition on endothelial cells. Next to the novel engineered triple fusion variants which inactivate complement convertases in an enzyme-like fashion, stoichiometric complement inhibitors targeting C3, C5, factor B, and factor D were tested as comparators. The triple fusion approach yielded a potent complement inhibitor that efficiently inhibits all three complement initiation pathways while targeting to surface markers.

De-stabilizing innate immunity in COVID-19: effects of its own positive feedback and erratic viraemia on the alternative pathway of complement

Complement provides powerful, fast responses in the human circulation to SARS-CoV-2 (COVID-19 virus) infection of the lower respiratory tract. COVID-19 effects were investigated in a revised human in silico Mass Action model of complement's alternative pathway (AP) responses. Bursts of newly circulating virions increased the fission of Complement protein C3 into C3a and C3b via stimulation of the lectin pathway or inhibited complement factor H. Viral reproduction sub-models incorporated smoothly exponential or step-wise exponential growth. Starting complement protein concentrations were drawn randomly from published normal male or female ranges and each infection model run for 10 days. C3 and factor B (FB) syntheses driven by Lectin Pathway stimulation led to declining plasma C3 and increasing FB concentrations. The C3-convertase concentration, a driver of viral elimination, could match viral growth over three orders of magnitude but near-complete exhaustion of circulating C3 was more prevalent with step-wise than with 'smooth' increases in viral stimulation. C3 exhaustion could be prolonged. Type 2 Diabetes and hypertension led to greatly increased peak C3-convertase concentrations, as did short-term variability of COVID-19 viraemia, pulmonary capillary clotting and secondary acidosis. Positive feedback in the AP greatly extends its response range at the expense of stability.

SAR443809: a selective inhibitor of the complement alternative pathway, targeting complement factor Bb

Dysregulated activation of the complement system is implicated in the onset or progression of several diseases. Most clinical-stage complement inhibitors target the inactive complement proteins present at high concentrations in plasma, which increases target-mediated drug disposition and necessitates high drug levels to sustain therapeutic inhibition. Furthermore, many efforts are aimed at inhibiting only terminal pathway activity, which leaves opsonin-mediated effector functions intact. We describe the discovery of SAR443809, a specific inhibitor of the alternative pathway C3/C5 convertase (C3bBb). SAR443809 selectively binds to the activated form of factor B (factor Bb) and inhibits alternative pathway activity by blocking the cleavage of C3, leaving the initiation of classical and lectin complement pathways unaffected. Ex vivo experiments with patient-derived paroxysmal nocturnal hemoglobinuria erythrocytes show that, although terminal pathway inhibition via C5 blockade can effectively inhibit hemolysis, proximal complement inhibition with SAR443809 inhibits both hemolysis and C3b deposition, abrogating the propensity for extravascular hemolysis. Finally, intravenous and subcutaneous administration of the antibody in nonhuman primates demonstrated sustained inhibition of complement activity for several weeks after injection. Overall, SAR443809 shows strong potential for treatment of alternative pathway-mediated disorders.

A novel method for real-time analysis of the complement C3b:FH:FI complex reveals dominant negative CFI variants in age-related macular degeneration

Age-related macular degeneration (AMD) is linked to 2 main disparate genetic pathways: a chromosome 10 risk locus and the alternative pathway (AP) of complement. Rare genetic variants in complement factor H (CFH; FH) and factor I (CFI; FI) are associated with AMD. FH acts as a soluble cofactor to facilitate FI's cleavage and inactivation of the central molecule of the AP, C3b. For personalised treatment, sensitive assays are required to define the functional significance of individual AP genetic variants. Generation of recombinant FI for functional analysis has thus far been constrained by incomplete processing resulting in a preparation of active and inactive protein. Using an internal ribosomal entry site (IRES)-Furin-CFI expression vector, fully processed FI was generated with activity equivalent to serum purified FI. By generating FI with an inactivated serine protease domain (S525A FI), a real-time surface plasmon resonance assay of C3b:FH:FI complex formation for characterising variants in CFH and CFI was developed and correlated well with standard assays. Using these methods, we further demonstrate that patient-associated rare genetic variants lacking enzymatic activity (e.g. CFI I340T) may competitively inhibit the wild-type FI protein. The dominant negative effect identified in inactive factor I variants could impact on the pharmacological replacement of FI currently being investigated for the treatment of dry AMD.

Sialic acids on B cells are crucial for their survival and provide protection against apoptosis

Sialic acids (Sias) on the B cell membrane are involved in cell migration, in the control of the complement system and, as sialic acid-binding immunoglobulin-like lectin (Siglec) ligands, in the regulation of cellular signaling. We studied the role of sialoglycans on B cells in a mouse model with B cell-specific deletion of cytidine monophosphate sialic acid synthase (CMAS), the enzyme essential for the synthesis of sialoglycans. Surprisingly, these mice showed a severe B cell deficiency in secondary lymphoid organs. Additional depletion of the complement factor C3 rescued the phenotype only marginally, demonstrating a complement-independent mechanism. The B cell survival receptor BAFF receptor was not up-regulated, and levels of activated caspase 3 and processed caspase 8 were high in B cells of Cmas-deficient mice, indicating ongoing apoptosis. Overexpressed Bcl-2 could not rescue this phenotype, pointing to extrinsic apoptosis. These results show that sialoglycans on the B cell surface are crucial for B cell survival by counteracting several death-inducing pathways.

Mathematical Modeling of Complement Pathway Dynamics for Target Validation and Selection of Drug Modalities for Complement Therapies

Motivation: The complement pathway plays a critical role in innate immune defense against infections. Dysregulation between activation and regulation of the complement pathway is widely known to contribute to several diseases. Nevertheless, very few drugs that target complement proteins have made it to the final regulatory approval because of factors such as high concentrations and dosing requirements for complement proteins and serious side effects from complement inhibition.

Methods: A quantitative systems pharmacology (QSP) model of the complement pathway has been developed to evaluate potential drug targets to inhibit complement activation in autoimmune diseases. The model describes complement activation via the alternative and terminal pathways as well as the dynamics of several regulatory proteins. The QSP model has been used to evaluate the effect of inhibiting complement targets on reducing pathway activation caused by deficiency in factor H and CD59. The model also informed the feasibility of developing small-molecule or large-molecule antibody drugs by predicting the drug dosing and affinity requirements for potential complement targets.

Results: Inhibition of several complement proteins was predicted to lead to a significant reduction in complement activation and cell lysis. The complement proteins that are present in very high concentrations or have high turnover rates (C3, factor B, factor D, and C6) were predicted to be challenging to engage with feasible doses of large-molecule antibody compounds (≤20 mg/kg). Alternatively, complement fragments that have a short half-life (C3b, C3bB, and C3bBb) were predicted to be challenging or infeasible to engage with small-molecule compounds because of high drug affinity requirements (>1 nM) for the inhibition of downstream processes. The drug affinity requirements for disease severity reduction were predicted to differ more than one to two orders of magnitude than affinities needed for the conventional 90% target engagement (TE) for several proteins. Thus, the QSP model analyses indicate the importance for accounting for TE requirements for achieving reduction in disease severity endpoints during the lead optimization stage.

Complement component C3: A structural perspective and potential therapeutic implications

As the most abundant component of the complement system, C3 and its proteolytic derivatives serve essential roles in the function of all three complement pathways. Central to this is a network of protein-protein interactions made possible by the sequential proteolysis and far-reaching structural changes that accompany C3 activation. Beginning with the crystal structures of C3, C3b, and C3c nearly twenty years ago, the physical transformations underlying C3 function that had long been suspected were finally revealed. In the years that followed, a compendium of crystallographic information on C3 derivatives bound to various enzymes, regulators, receptors, and inhibitors generated new levels of insight into the structure and function of the C3 molecule. This Review provides a concise classification, summary, and interpretation of the more than 50 unique crystal structure determinations for human C3. It also highlights other salient features of C3 structure that were made possible through solution-based methods, including Hydrogen/Deuterium Exchange and Small Angle X-ray Scattering. At this pivotal time when the first C3-targeted therapeutics begin to see use in the clinic, some perspectives are also offered on how this continually growing body of structural information might be leveraged for future development of next-generation C3 inhibitors.

A structure-based approach for the development of a bicyclic peptide acting as a miniaturized anti-CD55 antibody

CD55 is a major regulator of the complement system, a complex network of proteins that cooperate to clear tissue and blood pathogens from the organism. Indeed, overexpression of CD55 is associated with many diseases and is connected to the resistance mechanisms exhibited by several cancers towards immunotherapy approaches. High level of CD55 expression on tumour cells renders it a good target for both imaging and immunotherapy. Indeed, a conceivable approach to tackle disease is to interfere with CD55-mediated complement regulation with the use of CD55-targeting antibodies. However, the large size and poor tissue penetration together with to the high costs of antibodies often limits their widespread therapeutic use. Here, we employed bioinformatic and chemical approaches to design and synthesize molecules of small dimensions able to mimic a CD55 blocking antibody. As a result, a bicyclic peptide, named as miniAB55, proved to bind CD55 with nanomolar affinity. This molecule represents an attracting chemical scaffold for CD55-directed diagnostic tools in diseases associated with CD55 overproduction. To further support the applicative potential of miniAB55, we prove that the miniAB55 binds CD55 on the same region involved in inactivation of the complement C3 and C5 convertases, thus opening promising scenarios for the development of complement-modulating tools.

Murine Factor H Co-Produced in Yeast With Protein Disulfide Isomerase Ameliorated C3 Dysregulation in Factor H-Deficient Mice

Recombinant human factor H (hFH) has potential for treating diseases linked to aberrant complement regulation including C3 glomerulopathy (C3G) and dry age-related macular degeneration. Murine FH (mFH), produced in the same host, is useful for pre-clinical investigations in mouse models of disease. An abundance of FH in plasma suggests high doses, and hence microbial production, will be needed. Previously, Pichia pastoris produced useful but modest quantities of hFH. Herein, a similar strategy yielded miniscule quantities of mFH. Since FH has 40 disulfide bonds, we created a P. pastoris strain containing a methanol-inducible codon-modified gene for protein-disulfide isomerase (PDI) and transformed this with codon-modified DNA encoding mFH under the same promoter. What had been barely detectable yields of mFH became multiple 10s of mg/L. Our PDI-overexpressing strain also boosted hFH overproduction, by about tenfold. These enhancements exceeded PDI-related production gains reported for other proteins, all of which contain fewer disulfide-stabilized domains. We optimized fermentation conditions, purified recombinant mFH, enzymatically trimmed down its (non-human) N-glycans, characterised its functions in vitro and administered it to mice. In FH-knockout mice, our de-glycosylated recombinant mFH had a shorter half-life and induced more anti-mFH antibodies than mouse serum-derived, natively glycosylated, mFH. Even sequential daily injections of recombinant mFH failed to restore wild-type levels of FH and C3 in mouse plasma beyond 24 hours after the first injection. Nevertheless, mFH functionality appeared to persist in the glomerular basement membrane because C3-fragment deposition here, a hallmark of C3G, remained significantly reduced throughout and beyond the ten-day dosing regimen.

Systems Biology Modeling of the Complement System Under Immune Susceptible Pathogens

The complement system is assembled from a network of proteins that function to bring about the first line of defense of the body against invading pathogens. However, complement deficiencies or invasive pathogens can hijack complement to subsequently increase susceptibility of the body to infections. Moreover, invasive pathogens are increasingly becoming resistant to the currently available therapies. Hence, it is important to gain insights into the highly dynamic interaction between complement and invading microbes in the frontlines of immunity. Here, we developed a mathematical model of the complement system composed of 670 ordinary differential equations with 328 kinetic parameters, which describes all three complement pathways (alternative, classical, and lectin) and includes description of mannose-binding lectin, collectins, ficolins, factor H-related proteins, immunoglobulin M, and pentraxins. Additionally, we incorporate two pathogens: (type 1) complement susceptible pathogen and (type 2) Neisseria meningitidis located in either nasopharynx or bloodstream. In both cases, we generate time profiles of the pathogen surface occupied by complement components and the membrane attack complex (MAC). Our model shows both pathogen types in bloodstream are saturated by complement proteins, whereas MACs occupy <<1.0% of the pathogen surface. Conversely, the MAC production in nasopharynx occupies about 1.5-10% of the total N. meningitidis surface, thus making nasal MAC levels at least about eight orders of magnitude higher. Altogether, we predict complement-imbalance, favoring overactivation, is associated with nasopharynx homeostasis. Conversely, orientating toward complement-balance may cause disruption to the nasopharynx homeostasis. Thus, for sporadic meningococcal disease, our model predicts rising nasal levels of complement regulators as early infection biomarkers.

Mathematical Modelling of Alternative Pathway of Complement System

The complement system (CS) is an integral part of innate immunity and can be activated via three different pathways. The alternative pathway (AP) has a central role in the function of the CS. The AP of complement system is implicated in several human disease pathologies. In the absence of triggers, the AP exists in a time-invariant resting state (physiological steady state). It is capable of rapid, potent and transient activation response upon challenge with a trigger. Previous models of AP have focused on the activation response. In order to understand the molecular machinery necessary for AP activation and regulation of a physiological steady state, we built parsimonious AP models using experimentally supported kinetic parameters. The models further allowed us to test quantitative roles played by negative and positive regulators of the pathway in order to test hypotheses regarding their mechanisms of action, thus providing more insight into the complex regulation of AP.

Combining SPR with atomic-force microscopy enables single-molecule insights into activation and suppression of the complement cascade

Activation and suppression of the complement system compete on every serum-exposed surface, host or foreign. Potentially harmful outcomes of this competition depend on surface molecules through mechanisms that remain incompletely understood. Combining surface plasmon resonance (SPR) with atomic force microscopy (AFM), here we studied two complement system proteins at the single-molecule level: C3b, the proteolytically activated form of C3, and factor H (FH), the surface-sensing C3b-binding complement regulator. We used SPR to monitor complement initiation occurring through a positive-feedback loop wherein surface-deposited C3b participates in convertases that cleave C3, thereby depositing more C3b. Over multiple cycles of flowing factor B, factor D, and C3 over the SPR chip, we amplified C3b from ∼20 to ∼220 molecules·μm⁻². AFM revealed C3b clusters of up to 20 molecules and solitary C3b molecules deposited up to 200 nm away from the clusters. A force of 0.17 ± 0.02 nanonewtons was needed to pull a single FH molecule, anchored to the AFM probe, from its complex with surface-attached C3b. The extent to which FH molecules stretched before detachment varied widely among complexes. Performing force-distance measurements with FH(D1119G), a variant lacking one of the C3b-binding sites and causing atypical hemolytic uremic syndrome, we found that it detached more uniformly and easily. In further SPR experiments, K_D values between FH and C3b on a custom-made chip surface were 5-fold tighter than on commercial chips and similar to those on erythrocytes. These results suggest that the chemistry at the surface on which FH acts drives conformational adjustments that are functionally critical.

Molecular engineering of an efficient four-domain DAF-MCP chimera reveals the presence of functional modularity in RCA proteins

The complement system is highly efficient in targeting pathogens, but lack of its apposite regulation results in host-cell damage, which is linked to diseases. Thus, complement activation is tightly regulated by a series of proteins, which primarily belong to the regulators of complement activation (RCA) family. Structurally, these proteins are composed of repeating complement control protein (CCP) domains where two to four successive domains contribute to the regulatory functions termed decay-accelerating activity (DAA) and cofactor activity (CFA). However, the precise constitution of the functional units and whether these units can be joined to form a larger composition with dual function have not been demonstrated. Herein, we have parsed the functional units for DAA and CFA by constructing chimeras of the decay-accelerating factor (DAF) that exhibits DAA and membrane cofactor protein (MCP) that exhibits CFA. We show that in a four-CCP framework, a functional unit for each of the regulatory activities is formed by only two successive CCPs wherein each participates in the function, albeit CCP2 has a bipartite function. Additionally, optimal activity requires C-terminal domains that enhance the avidity of the molecule for C3b/C4b. Furthermore, by composing a four-CCP DAF-MCP chimera with robust CFA (for C3b and C4b) and DAA (for classical and alternative pathway C3 convertases), named decay cofactor protein, we show that CCP functional units can be linked to design a dual-activity regulator. These data indicate that the regulatory determinants for these two biological processes are distinct and modular in nature.

Nephritic Factors: An Overview of Classification, Diagnostic Tools and Clinical Associations

Nephritic factors comprise a heterogeneous group of autoantibodies against neoepitopes generated in the C3 and C5 convertases of the complement system, causing its dysregulation. Classification of these autoantibodies can be clustered according to their stabilization of different convertases either from the classical or alternative pathway. The first nephritic factor described with the capacity to stabilize C3 convertase of the alternative pathway was C3 nephritic factor (C3NeF). Another nephritic factor has been characterized by the ability to stabilize C5 convertase of the alternative pathway (C5NeF). In addition, there are autoantibodies against assembled C3/C5 convertase of the classical and lectin pathways (C4NeF). These autoantibodies have been mainly associated with kidney diseases, like C3 glomerulopathy and immune complex-associated-membranoproliferative glomerulonephritis. Other clinical situations where these autoantibodies have been observed include infections and autoimmune disorders such as systemic lupus erythematosus and acquired partial lipodystrophy. C3 hypocomplementemia is a common finding in all patients with nephritic factors. The methods to measure nephritic factors are not standardized, technically complex, and lack of an appropriate quality control. This review will be focused in the description of the mechanism of action of the three known nephritic factors (C3NeF, C4NeF, and C5NeF), and their association with human diseases. Moreover, we present an overview regarding the diagnostic tools for its detection, and the main therapeutic approach for the patients with nephritic factors.

Recombinant Production of MFHR1, A Novel Synthetic Multitarget Complement Inhibitor, in Moss Bioreactors

The human complement system is an important part of the immune system responsible for lysis and elimination of invading microorganisms and apoptotic body cells. Improper activation of the system due to deficiency, mutations, or autoantibodies of complement regulators, mainly factor H (FH) and FH-related proteins (FHRs), causes severe kidney and eye diseases. However, there is no recombinant FH therapeutic available on the market. The first successful recombinant production of FH was accomplished with the moss bioreactor, Physcomitrella patens. Recently, a synthetic regulator, MFHR1, was designed to generate a multitarget complement inhibitor that combines the activities of FH and the FH-related protein 1 (FHR1). The potential of MFHR1 was demonstrated in a proof-of-concept study with transiently transfected insect cells. Here, we present the stable production of recombinant glyco-engineered MFHR1 in the moss bioreactor. The key features of this system are precise genome engineering via homologous recombination, Good Manufacturing Practice-compliant production in photobioreactors, high batch-to-batch reproducibility, and product stability. Several potential biopharmaceuticals are being produced in this system. In some cases, these are even biobetters, i.e., the recombinant proteins produced in moss have a superior quality compared to their counterparts from mammalian systems as for example moss-made aGal, which successfully passed phase I clinical trials. Via mass spectrometry-based analysis of moss-produced MFHR1, we now prove the correct synthesis and modification of this glycoprotein with predominantly complex-type N-glycan attachment. Moss-produced MFHR1 exhibits cofactor and decay acceleration activities comparable to FH, and its mechanism of action on multiple levels within the alternative pathway of complement activation led to a strong inhibitory activity on the whole alternative pathway, which was higher than with the physiological regulator FH.

Utilization of Staphylococcal Immune Evasion Protein Sbi as a Novel Vaccine Adjuvant

Co-ligation of the B cell antigen receptor with complement receptor 2 on B-cells via a C3d-opsonised antigen complex significantly lowers the threshold required for B cell activation. Consequently, fusions of antigens with C3d polymers have shown great potential in vaccine design. However, these linear arrays of C3d multimers do not mimic the natural opsonisation of antigens with C3d. Here we investigate the potential of using the unique complement activating characteristics of Staphylococcal immune-evasion protein Sbi to develop a pro-vaccine approach that spontaneously coats antigens with C3 degradation products in a natural way. We show that Sbi rapidly triggers the alternative complement pathway through recruitment of complement regulators, forming tripartite complexes that act as competitive antagonists of factor H, resulting in enhanced complement consumption. These functional results are corroborated by the structure of the complement activating Sbi-III-IV:C3d:FHR-1 complex. Finally, we demonstrate that Sbi, fused with Mycobacterium tuberculosis antigen Ag85b, causes efficient opsonisation with C3 fragments, thereby enhancing the immune response significantly beyond that of Ag85b alone, providing proof of concept for our pro-vaccine approach.

Complement Component C3 Binds to the A3 Domain of von Willebrand Factor

von Willebrand factor (VWF) is a multimeric protein composed of monomeric subunits (∼280 kD) linked by disulfide bonds. During hemostasis and thrombosis, ultralarge (UL) VWF (ULVWF) multimers initiate platelet adhesion. In vitro, human C3 binds to ULVWF multimeric strings secreted by and anchored to human endothelial cell to promote the assembly and activation of C3 convertase (C3bBb) and C5 convertase (C3bBbC3b) of the alternative complement pathway (AP). The purified and soluble C3 avidly binds to recombinant human VWF A1A2A3, as well as the recombinant isolated human VWF A3 domain. Notably, the binding of soluble human ULVWF multimers to purified human C3 was blocked by addition of a monovalent Fab fragment antibody to the VWF A3 domain. We conclude that the A3 domain in VWF/ULVWF contains a docking site for C3. In contrast, purified human C4, an essential component of the classical and lectin complement pathways, binds to soluble, isolated A1, but not to ULVWF strings secreted by and anchored to endothelial cells. Our findings should facilitate the design of new therapeutic agents to suppress the initiation of the AP on ULVWF multimeric strings during thrombotic and inflammatory disorders.

Complement System and Age-Related Macular Degeneration: Implications of Gene-Environment Interaction for Preventive and Personalized Medicine

Age-related macular degeneration (AMD) is the most common cause of visual loss in developed countries, with a significant economic and social burden on public health. Although genome-wide and gene-candidate studies have been enabled to identify genetic variants in the complement system associated with AMD pathogenesis, the effect of gene-environment interaction is still under debate. In this review we provide an overview of the role of complement system and its genetic variants in AMD, summarizing the consequences of the interaction between genetic and environmental risk factors on AMD onset, progression, and therapeutic response. Finally, we discuss the perspectives of current evidence in the field of genomics driven personalized medicine and public health.

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.

An Engineered Complement Factor H Construct for Treatment of C3 Glomerulopathy

Background C3 glomerulopathy (C3G) is associated with dysregulation of the alternative pathway of complement activation, and treatment options for C3G remain limited. Complement factor H (FH) is a potent regulator of the alternative pathway and might offer a solution, but the mass and complexity of FH makes generation of full-length FH far from trivial. We previously generated a mini-FH construct, with FH short consensus repeats 1-5 linked to repeats 18-20 (FH1-5^18-20), that was effective in experimental C3G. However, the serum t1/2 of FH1-5^18-20 was significantly shorter than that of serum-purified FH.Methods We introduced the oligomerization domain of human FH-related protein 1 (denoted by R1-2) at the carboxy or amino terminus of human FH1-5^18-20 to generate two homodimeric mini-FH constructs (FHR1-2^1-5^18-20 and FH1-5^18-20^R1-2, respectively) in Chinese hamster ovary cells and tested these constructs using binding, fluid-phase, and erythrocyte lysis assays, followed by experiments in FH-deficient Cfh-/- mice.Results FHR1-2^1-5^18-20 and FH1-5^18-20^R1-2 homodimerized in solution and displayed avid binding profiles on clustered C3b surfaces, particularly FHR1-2^1-5^18-20 Each construct was >10-fold more effective than FH at inhibiting cell surface complement activity in vitro and restricted glomerular basement membrane C3 deposition in vivo significantly better than FH or FH1-5^18-20 FH1-5^18-20^R1-2 had a C3 breakdown fragment binding profile similar to that of FH, a >5-fold increase in serum t1/2 compared with that of FH1-5^18-20, and significantly better retention in the kidney than FH or FH1-5^18-20Conclusions FH1-5^18-20^R1-2 may have utility as a treatment option for C3G or other complement-mediated diseases.

A common CD55 rs2564978 variant is associated with the susceptibility of non-small cell lung cancer

CD55, as one of key membrane-bound complement-regulatory proteins (mCRPs), is crucial for the progression of various cancers. This study aims to investigate the role of CD55 variants in the development of non-small cell lung cancer (NSCLC). A case-control study, including 706 lung cancer cases and 706 health controls, was conducted in a Chinese population. The odds ratio (OR) and 95% confidence interval (95% CI) were estimated by unconditional logistic regression. We found that significantly higher lung cancer risk was linked with CD55 rs2564978 CC genotype (OR = 1.52, 95% CI = 1.11−2.07) or CT genotypes (OR = 1.34, 95% CI = 1.05−1.71), compared to the TT genotype. Stratified analysis showed that rs2564978 CC was associated with NSCLC risk among males (OR = 1.69, 95% CI = 1.14−2.49) and older subjects (OR = 1.75, 95% CI = 1.08−2.82). When stratified by smoking status, the risk effect of rs2564978 CC was more evident among smokers (OR = 2.01, 95% CI = 1.18−3.43) than non-smokers (OR = 1.30, 95% CI = 0.88−1.90). We also conducted the stratified analysis by NSCLC histological types and found that CD55 rs2564978 CC increased the risk of adenocarcinoma with OR (95% CI) of 1.35 (1.01−1.80). The reporter gene expression driven by rs2564978T-containing CD55 promoter was respectively 1.48-fold, 1.96-fold and 1.93-fold higher than those driven by the rs2564978C-containing CD55 promoter in A549, NCI-H2030 and NCI-H23 cells (P = 0.045, 0.010 and < 0.001). These findings indicate that CD55 rs2564978 polymorphism may contribute to an increased risk of NSCLC in Chinese population.

DAF in diabetic patients is subject to glycation/inactivation at its active site residues

Decay accelerating factor (DAF or CD55) is a cell associated C3 and C5 convertase regulator originally described in terms of protection of self-cells from systemic complement but now known to modulate adaptive T cell responses. It is expressed on all cell types. We investigated whether nonenzymatic glycation could impair its function and potentially be relevant to complications of diabetes mellitus and other conditions that result in nonenzymatic glycation including cancer, Alzheimer's disease, and aging. Immunoblots of affinity-purified DAF from erythrocytes of patients with diabetes showed pentosidine, glyoxal-AGEs, carboxymethyllysine, and argpyrimidine. HPLC/MS analyses of glucose modified DAF localized the sites of AGE modifications to K¹²⁵ adjacent to K¹²⁶, K¹²⁷ at the junction of CCPs2-3 and spatially near R⁹⁶, and R¹⁰⁰, all identified as being critical for DAF's function. Functional analyses of glucose or ribose treated DAF protein showed profound loss of its regulatory activity. The data argue that de-regulated activation of systemic complement and de-regulated activation of T cells and leukocytes could result from non-enzymatic glycation of DAF.

Immobilization Strategies for Functional Complement Convertase Assembly at Lipid Membrane Interfaces

The self-assembly formation of complement convertases-essential biomacromolecular complexes that amplify innate immune responses-is triggered by protein adsorption. Herein, a supported lipid bilayer platform was utilized to investigate the effects of covalent and noncovalent tethering strategies on the self-assembly of alternative pathway C3 convertase components, starting with C3b protein adsorption followed bythe addition of factors B and D. Quartz crystal microbalance-dissipation (QCM-D) experiments measured the real-time kinetics of convertase assembly onto supported lipid bilayers. The results demonstrate that the nature of C3b immobilization onto supported lipid bilayers is a key factor governing convertase assembly. The covalent attachment of C3b to maleimide-functionalized supported lipid bilayers promoted the self-assembly of functional C3 convertase in the membrane-associated state and further enabled successful evaluation of a clinically relevant complement inhibitor, compstatin. By contrast, noncovalent attachment of C3b to negatively charged supported lipid bilayers also permitted C3b protein uptake, albeit membrane-associated C3b did not support convertase assembly in this case. Taken together, the findings in this work demonstrate that the attachment scheme for immobilizing C3b protein at lipid membrane interfaces is critical for downstream C3 convertase assembly, thereby offering guidance for the design and evaluation of membrane-associated biomacromolecular complexes.

Disease-linked mutations in factor H reveal pivotal role of cofactor activity in self-surface-selective regulation of complement activation

Spontaneous activation enables the complement system to respond very rapidly to diverse threats. This activation is efficiently suppressed by complement factor H (CFH) on self-surfaces but not on foreign surfaces. The surface selectivity of CFH, a soluble protein containing 20 complement-control protein modules (CCPs 1-20), may be compromised by disease-linked mutations. However, which of the several functions of CFH drives this self-surface selectivity remains unknown. To address this, we expressed human CFH mutants in Pichia pastoris We found that recombinant I62-CFH (protective against age-related macular degeneration) and V62-CFH functioned equivalently, matching or outperforming plasma-derived CFH, whereas R53H-CFH, linked to atypical hemolytic uremic syndrome (aHUS), was defective in C3bBb decay-accelerating activity (DAA) and factor I cofactor activity (CA). The aHUS-linked CCP 19 mutant D1119G-CFH had virtually no CA on (self-like) sheep erythrocytes (E_S) but retained DAA. The aHUS-linked CCP 20 mutant S1191L/V1197A-CFH (LA-CFH) had dramatically reduced CA on E_S but was less compromised in DAA. D1119G-CFH and LA-CFH both performed poorly at preventing complement-mediated hemolysis of E_S PspCN, a CFH-binding Streptococcus pneumoniae protein domain, binds CFH tightly and increases accessibility of CCPs 19 and 20. PspCN did not improve the DAA of any CFH variant on E_S Conversely, PspCN boosted the CA, on E_S, of I62-CFH, R53H-CFH, and LA-CFH and also enhanced hemolysis protection by I62-CFH and LA-CFH. We conclude that CCPs 19 and 20 are critical for efficient CA on self-surfaces but less important for DAA. Exposing CCPs 19 and 20 with PspCN and thus enhancing CA on self-surfaces may reverse deficiencies of some CFH variants.

Hijacking Complement Regulatory Proteins for Bacterial Immune Evasion

The human complement system plays an important role in the defense against invading pathogens, inflammation and homeostasis. Invading microbes, such as bacteria, directly activate the complement system resulting in the formation of chemoattractants and in effective labeling of the bacteria for phagocytosis. In addition, formation of the membrane attack complex is responsible for direct killing of Gram-negative bacteria. In turn, bacteria have evolved several ways to evade complement activation on their surface in order to be able to colonize and invade the human host. One important mechanism of bacterial escape is attraction of complement regulatory proteins to the microbial surface. These molecules are present in the human body for tight regulation of the complement system to prevent damage to host self-surfaces. Therefore, recruitment of complement regulatory proteins to the bacterial surface results in decreased complement activation on the microbial surface which favors bacterial survival. This review will discuss recent advances in understanding the binding of complement regulatory proteins to the bacterial surface at the molecular level. This includes, new insights that have become available concerning specific conserved motives on complement regulatory proteins that are favorable for microbial binding. Finally, complement evasion molecules are of high importance for vaccine development due to their dominant role in bacterial survival, high immunogenicity and homology as well as their presence on the bacterial surface. Here, the use of complement evasion molecules for vaccine development will be discussed.

Solution Structures of Complement C2 and Its C4 Complexes Propose Pathway-specific Mechanisms for Control and Activation of the Complement Proconvertases

The lectin (LP) and classical (CP) pathways are two of the three main activation cascades of the complement system. These pathways start with recognition of different pathogen- or danger-associated molecular patterns and include identical steps of proteolytic activation of complement component C4, formation of the C3 proconvertase C4b2, followed by cleavage of complement component C2 within C4b2 resulting in the C3 convertase C4b2a. Here, we describe the solution structures of the two central complexes of the pathways, C3 proconvertase and C3 convertase, as well as the unbound zymogen C2 obtained by small angle x-ray scattering analysis. We analyzed both native and enzymatically deglycosylated C4b2 and C2 and showed that the resulting structural models were independent of the glycans. The small angle x-ray scattering-derived models suggest a different activation mode for the CP/LP C3 proconvertase as compared with that established for the alternative pathway proconvertase C3bB. This is likely due to the rather different structural and functional properties of the proteases activating the proconvertases. The solution structure of a stabilized form of the active CP/LP C3 convertase C4b2a is strikingly similar to the crystal structure of the alternative pathway C3 convertase C3bBb, which is in accordance with their identical functions in cleaving the complement proteins C3 and C5.

Regulators of complement activity mediate inhibitory mechanisms through a common C3b-binding mode

Regulators of complement activation (RCA) inhibit complement‐induced immune responses on healthy host tissues. We present crystal structures of human RCA (MCP, DAF, and CR1) and a smallpox virus homolog (SPICE) bound to complement component C3b. Our structural data reveal that up to four consecutive homologous CCP domains (i–iv), responsible for inhibition, bind in the same orientation and extended arrangement at a shared binding platform on C3b. Large sequence variations in CCP domains explain the diverse C3b‐binding patterns, with limited or no contribution of some individual domains, while all regulators show extensive contacts with C3b for the domains at the third site. A variation of ~100° rotation around the longitudinal axis is observed for domains binding at the fourth site on C3b, without affecting the overall binding mode. The data suggest a common evolutionary origin for both inhibitory mechanisms, called decay acceleration and cofactor activity, with variable C3b binding through domains at sites ii, iii, and iv, and provide a framework for understanding RCA disease‐related mutations and immune evasion.

Insights into the Effects of Complement Factor H on the Assembly and Decay of the Alternative Pathway C3 Proconvertase and C3 Convertase

The activated fragment of C3 (C3b) and factor B form the C3 proconvertase (C3bB), which is cleaved by factor D to C3 convertase (C3bBb). Older studies (Conrad, D. H., Carlo, J. R., and Ruddy, S. (1978)J. Exp. Med.147, 1792-1805; Pangburn, M. K., and Müller-Eberhard, H. J. (1978)Proc. Natl. Acad. Sci. U.S.A.75, 2416-2420; Kazatchkine, M. D., Fearon, D. T., and Austen, K. F. (1979)J. Immunol.122, 75-81) indicated that the complement alternative pathway regulator factor H (FH) competes with factor B for C3b binding; however, the capability of FH to prevent C3bB assembly has not been formally investigated. Moreover, in the few published studies FH did not favor C3bB dissociation. Whether FH may affect C3bBb formation from C3bB is unknown. We set up user-friendly assays based on combined microplate/Western blotting techniques that specifically detect either C3bB or C3bBb, with the aim of investigating the effect of FH on C3bB assembly and decay and C3bBb formation and decay. We document that FH does not affect C3bB assembly, indicating that FH does not efficiently compete with factor B for C3b binding. We also found that FH does not dissociate C3bB. FH showed a strong C3bBb decay-accelerating activity, as reported previously, and also exerted an apparent inhibitory effect on C3bBb formation. The latter effect was not fully attributable to a rapid FH-mediated dissociation of C3bBb complexes, because blocking decay with properdin and C3 nephritic factor did not restore C3bBb formation. FH almost completely prevented release of the smaller cleavage subunit of FB (Ba), without modifying the amount of C3bB complexes, suggesting that FH inhibits the conversion of C3bB to C3bBb. Thus, the inhibitory effect of FH on C3bBb formation is likely the sum of inhibition of C3bB conversion to C3bBb and of C3bBb decay acceleration. Further studies are required to confirm these findings in physiological cell-based settings.

Complement System Part I - Molecular Mechanisms of Activation and Regulation

Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage. Here, we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. This article will review the mechanisms of activation of alternative, classical, and lectin pathways, the formation of C3 and C5 convertases, the action of anaphylatoxins, and the membrane-attack-complex. We will also discuss the importance of structure-function relationships using the example of atypical hemolytic uremic syndrome. Lastly, we will discuss the development and benefits of therapies using complement inhibitors.

Viral regulators of complement activation: structure, function and evolution

The complement system surveillance in the host is effective in controlling viral propagation. Consequently, to subvert this effector mechanism, viruses have developed a series of adaptations. One among these is encoding mimics of host regulators of complement activation (RCA) which help viruses to avoid being labeled as 'foreign' and protect them from complement-mediated neutralization and complement-enhanced antiviral adaptive immunity. In this review, we provide an overview on the structure, function and evolution of viral RCA proteins.

Pancreatic ductal adenocarcinoma: a review of immunologic aspects

With the continued failures of both early diagnosis and treatment options for pancreatic cancer, it is now time to comprehensively evaluate the role of the immune system on the development and progression of pancreatic cancer. It is important to develop strategies that harness the molecules and cells of the immune system to treat this disease. This review will focus primarily on the role of immune cells in the development and progression of pancreatic ductal adenocarcinoma and to evaluate what is known about the interaction of immune cells with the tumor microenvironment and their role in tumor growth and metastasis. We will conclude with a brief discussion of therapy for pancreatic cancer and the potential role for immunotherapy. We hypothesize that the role of the immune system in tumor development and progression is tissue specific. Our hope is that better understanding of this process will lead to better treatments for this devastating disease.

Investigating the structure of the factor B vWF-A domain/CD55 protein-protein complex using DEER spectroscopy: successes and pitfalls

The electron paramagnetic resonance technique of double electron-electron resonance (DEER) was used to measure nanometre-scale distances between nitroxide spin labels attached to the complement regulatory protein CD55 (also known as decay accelerating factor) and the von Willebrand factor A (vWF-A) domain of factor B. Following a thorough assessment of the quality of the data, distances obtained from good-quality measurements are compared to predicted distances from a previously hypothesised model for the complex and are found to be incompatible. The success of using these distances as restraints in multi-body docking routines is presented critically.

Rational engineering of a minimized immune inhibitor with unique triple-targeting properties

Inadequate control of the complement system is the underlying or aggravating factor in many human diseases. Whereas treatment options that specifically target the alternative pathway (AP) of complement activation are considered highly desirable, no such option is available in the clinic. In this study, we present a successful example of protein engineering, guided by structural insight on the complement regulator factor H (FH), yielding a novel complement-targeted therapeutic (mini-FH) with clinical potential. Despite a 70% reduction in size, mini-FH retained and in some respects exceeded the regulatory activity and cell surface-recognition properties of its parent protein FH, including the recently described recognition of sites of oxidative stress. Importantly, the chosen design extended the functional spectrum of the inhibitor, as mini-FH showed increased binding to the surface-bound opsonins iC3b and C3dg when compared with FH. Thus, mini-FH is equipped with a unique and clinically valuable triple-targeting profile toward diseased host cells, through its binding to sites of ongoing complement activation, markers of oxidative damage, and host surface-specific polyanions. When assessed in a clinically relevant AP-mediated disease model of paroxysmal nocturnal hemoglobinuria, mini-FH largely outperformed FH and indicated advantages over clinically evaluated AP inhibitors. Thus, the rational engineering of a streamlined FH construct not only provided insight into the function of a key complement regulator, but also yielded a novel inhibitor that combines a triple-targeting approach with high AP-specific inhibitory activity (IC50 ~ 40 nM), which may pave the way toward new options for the treatment of complement-mediated diseases.

Sensitive and specific assays for C3 nephritic factors clarify mechanisms underlying complement dysregulation

C3 nephritic factors are autoantibodies that prolong the half-life or prevent regulation of the alternative pathway C3 convertase, resulting in uncontrolled complement activation. They are strongly associated with renal disease but their role in pathogenesis remains controversial. Here we optimized and compared a panel of assays to identify and interrogate nephritic factor activities. Of 101 patients with histologic or clinically evident disease, 48 were positive in some or all assays. In the presence of properdin, binding of autoantibody was detected in 39 samples and convertase stabilization was detected in 36. Forty-two of 48 nephritic factors tested prevented convertase decay by factor H, and most of these by decay accelerating factor (28) and complement receptor 1 (34). Representative properdin-independent nephritic factors had no effect on C5 cleavage and terminal pathway activity, while properdin-dependent nephritic factors enhanced activity. Biacore analysis of four purified IgG samples confirmed resistance to decay and showed that properdin-independent nephritic factors increased convertase half-life over 50-fold, whereas properdin-dependent nephritic factors increased the half-life 10- to 20-fold and also increased activity of the C3 convertase up to 10-fold. Thus, our study provides a rational approach to detect and characterize nephritic factors in patients.

Pancreatic ductal adenocarcinoma: a review of immunologic aspects

With the continued failures of both early diagnosis and treatment options for pancreatic cancer, it is now time to comprehensively evaluate the role of the immune system on the development and progression of pancreatic cancer. It is important to develop strategies that harness the molecules and cells of the immune system to treat this disease. This review will focus primarily on the role of immune cells in the development and progression of pancreatic ductal adenocarcinoma and to evaluate what is known about the interaction of immune cells with the tumor microenvironment and their role in tumor growth and metastasis. We will conclude with a brief discussion of therapy for pancreatic cancer and the potential role for immunotherapy. We hypothesize that the role of the immune system in tumor development and progression is tissue specific. Our hope is that better understanding of this process will lead to better treatments for this devastating disease.

A novel hybrid CFH/CFHR3 gene generated by a microhomology-mediated deletion in familial atypical hemolytic uremic syndrome

Genomic disorders affecting the genes encoding factor H (fH) and the 5 factor H related proteins have been described in association with atypical hemolytic uremic syndrome. These include deletions of CFHR3, CFHR1, and CFHR4 in association with fH autoantibodies and the formation of a hybrid CFH/CFHR1 gene. These occur through nonallelic homologous recombination secondary to the presence of large segmental duplications (macrohomology) in this region. Using multiplex ligation-dependent probe amplification to screen for such genomic disorders, we have identified a large atypical hemolytic uremic syndrome family where a deletion has occurred through microhomology-mediated end joining rather than nonallelic homologous recombination. In the 3 affected persons of this family, we have shown that the deletion results in formation of a CFH/CFHR3 gene. We have shown that the protein product of this is a 24 SCR protein that is secreted with normal fluid-phase activity but marked loss of complement regulation at cell surfaces despite increased heparin binding. In this study, we have therefore shown that microhomology in this area of chromosome 1 predisposes to disease associated genomic disorders and that the complement regulatory function of fH at the cell surface is critically dependent on the structural integrity of the whole molecule.

Access to the complement factor B scissile bond is facilitated by association of factor B with C3b protein

Factor B is a zymogen that carries the catalytic site of the complement alternative pathway C3 convertase. During convertase assembly, factor B associates with C3b and Mg(2+) forming a pro-convertase C3bB(Mg(2+)) that is cleaved at a single factor B site by factor D. In free factor B, a pair of salt bridges binds the Arg(234) side chain to Glu(446) and to Glu(207), forming a double latch structure that sequesters the scissile bond (between Arg(234) and Lys(235)) and minimizes its unproductive cleavage. It is unknown how the double latch is released in the pro-convertase. Here, we introduce single amino acid substitutions into factor B that preclude one or both of the Arg(234) salt bridges, and we examine their impact on several different pro-convertase complexes. Our results indicate that loss of the Arg(234)-Glu(446) salt bridge partially stabilizes C3bB(Mg(2+)). Loss of the Arg(234)-Glu(207) salt bridge has lesser effects. We propose that when factor B first associates with C3b, it bears two intact Arg(234) salt bridges. The complex rapidly dissociates unless the Arg(234)-Glu(446) salt bridge is released whereupon conformational changes occur that activate the metal ion-dependent adhesion site and partially stabilize the complex. The remaining salt bridge is then released, exposing the scissile bond and permitting factor D cleavage.

Common polymorphisms in C3, factor B, and factor H collaborate to determine systemic complement activity and disease risk

Common polymorphisms in complement alternative pathway (AP) proteins C3 (C3(R102G)), factor B (fB(R32Q)), and factor H (fH(V62I)) are associated with age-related macular degeneration (AMD) and other pathologies. Our published work showed that fB(R32Q) influences C3 convertase formation, whereas fH(V62I) affects factor I cofactor activity. Here we show how C3(R102G) (C3S/F) influences AP activity. In hemolysis assays, C3(102G) activated AP more efficiently (EC(50) C3(102G): 157 nM; C3(102R): 191 nM; P < 0.0001). fB binding kinetics and convertase stability were identical, but native and recombinant fH bound more strongly to C3b(102R) (K(D) C3b(102R): 1.0 μM; C3b(102G): 1.4 μM; P < 0.0001). Accelerated decay was unaltered, but fH cofactor activity was reduced for C3b(102G), favoring AP amplification. Combining disease "risk" variants (C3(102G), fB(32R), and fH(62V)) in add-back assays yielded sixfold higher hemolytic activity compared with "protective" variants (C3(102R), fB(32Q), and fH(62I); P < 0.0001). These data introduce the concept of a functional complotype (combination of polymorphisms) defining complement activity in an individual, thereby influencing susceptibility to AP-driven disease.

Production of biologically active complement factor H in therapeutically useful quantities

Human complement factor H (FH), an abundant 155-kDa plasma glycoprotein with 40 disulphide bonds, regulates the alternative-pathway complement cascade. Mutations and single nucleotide polymorphisms in the FH gene predispose to development of age-related macular degeneration, atypical haemolytic uraemic syndrome and dense deposit disease. Supplementation with FH variants protective against disease is an enticing therapeutic prospect. Current sources of therapeutic FH are restricted to human blood plasma highlighting a need for recombinant material. Previously FH expression in cultured plant, mammalian or insect cells yielded protein amounts inadequate for full characterisation, and orders of magnitude below therapeutic usefulness. Here, the V62,Y402 variant of FH has been produced recombinantly (rFH) in Pichia pastoris cells. Codon-optimisation proved essential whilst exploitation of the yeast mating α-factor peptide ensured secretion. We thereby produced multiple 10s-of-milligram of rFH. Following endoglycosidase H digestion of N-linked glycans, rFH (with eight residual N-acetylglucosamine moieties) was purified on heparin-affinity resin and anion-exchange chromatography. Full-length rFH was verified by mass spectrometry and Western blot using monoclonal antibodies to the C-terminus. Recombinant FH is a single non-aggregated species (by dynamic light scattering) and fully functional in biochemical and biological assays. An additional version of rFH was produced in which eight N-glycosylation sequons were ablated by Asn-Gln substitutions resulting in a glycan-devoid product. Successful production of rFH in this potentially very highly expressing system makes production of therapeutically useful quantities economically viable. Furthermore, ease of genetic manipulation in P. pastoris would allow production of engineered FH versions with enhanced pharmacokinetic and pharmacodynamic properties.

Allosteric inhibition of complement function by a staphylococcal immune evasion protein

The complement system is a major target of immune evasion by Staphylococcus aureus. Although many evasion proteins have been described, little is known about their molecular mechanisms of action. Here we demonstrate that the extracellular fibrinogen-binding protein (Efb) from S. aureus acts as an allosteric inhibitor by inducing conformational changes in complement fragment C3b that propagate across several domains and influence functional regions far distant from the Efb binding site. Most notably, the inhibitor impaired the interaction of C3b with complement factor B and, consequently, formation of the active C3 convertase. As this enzyme complex is critical for both activation and amplification of the complement response, its allosteric inhibition likely represents a fundamental contribution to the overall immune evasion strategy of S. aureus.

Lessons from functional and structural analyses of disease-associated genetic variants in the complement alternative pathway

Complement is an essential component of innate immunity and a major trigger of inflammatory responses. A critical step in complement activation is the formation of the C3 convertase of the alternative pathway (AP), a labile bimolecular complex formed by activated fragments of the C3 and factor B components that is fundamental to provide exponential amplification of the initial complement trigger. Regulation of the AP C3 convertase is essential to maintain complement homeostasis in plasma and to protect host cells and tissues from damage by complement. During the last decade, several studies have associated genetic variations in components and regulators of the AP C3 convertase with a number of chronic inflammatory diseases and susceptibility to infection. The functional characterization of these protein variants has helped to decipher the critical pathogenic mechanisms involved in some of these complement related disorders. In addition, these functional data together with recent 3D structures of the AP C3 convertase have provided fundamental insights into the assembly, activation and regulation of the AP C3 convertase.

Human C3 mutation reveals a mechanism of dense deposit disease pathogenesis and provides insights into complement activation and regulation

Dense deposit disease (DDD) is a severe renal disease characterized by accumulation of electron-dense material in the mesangium and glomerular basement membrane. Previously, DDD has been associated with deficiency of factor H (fH), a plasma regulator of the alternative pathway (AP) of complement activation, and studies in animal models have linked pathogenesis to the massive complement factor 3 (C3) activation caused by this deficiency. Here, we identified a unique DDD pedigree that associates disease with a mutation in the C3 gene. Mutant C(3923ΔDG), which lacks 2 amino acids, could not be cleaved to C3b by the AP C3-convertase and was therefore the predominant circulating C3 protein in the patients. However, upon activation to C3b by proteases, or to C3(H₂O) by spontaneous thioester hydrolysis, C(3923ΔDG) generated an active AP C3-convertase that was regulated normally by decay accelerating factor (DAF) but was resistant to decay by fH. Moreover, activated C(3b923ΔDG) and C3(H₂O)(923ΔDG) were resistant to proteolysis by factor I (fI) in the presence of fH, but were efficiently inactivated in the presence of membrane cofactor protein (MCP). These characteristics cause a fluid phase-restricted AP dysregulation in the patients that continuously activated and consumed C3 produced by the normal C3 allele. These findings expose structural requirements in C3 that are critical for recognition of the substrate C3 by the AP C3-convertase and for the regulatory activities of fH, DAF, and MCP, all of which have implications for therapeutic developments.

The role of CD97 in regulating adaptive T-cell responses

CD97 was identified as an early activation marker on T cells, having low expression on naive T cells. This is a common feature of molecules that have a role in T-cell function. It was subsequently identified as a ligand for CD55, which has been previously identified as an innate regulator of complement. The interaction of this receptor-ligand pair has been shown to provide a potent costimulatory signal to human T cells, despite their modest affinity. Though both CD97 and CD55 are expressed on T cells as well as antigen presenting cells (APCs), their interaction is significant when CD97 on APCs interacts with CD55 on T cells. The converse interaction is poorly defined and may be less significant. A unique aspect of the interaction of CD97 with CD55 is the stimulation of naive T cells, leading to the induction of IL-10 producing cells that behave like Trl regulatory cells. This raises a number of questions regarding the dual functions of CD55; regulating complement and stimulating T cells via CD97 interaction and any potential overlap in the consequences of these dual roles.

Decay-accelerating factor attenuates C-reactive protein-potentiated tissue injury after mesenteric ischemia/reperfusion

BACKGROUND

C-reactive protein (CRP) is an acute pro-inflammatory mediator that has been demonstrated to enhance ischemia/reperfusion (IR) injury by virtue of activating the complement system. CRP is able to interact with complement proteins such as C1q, complement factor H, and C4b-binding protein. Since complement activation is central in the expression of tissue injury following IR, we have investigated the effects of human decay-accelerating factor (DAF), a complement inhibitor, on CRP-potentiated complement activation and tissue injury in mice subjected to mesenteric IR.

MATERIALS AND METHODS

Male C57B1/6 mice were allocated into eight groups: (1) Sham-operated group without IR injury; (2) CRP+Sham group; (3) IR group; (4) CRP+IR group; (5) DAF group; (6) CRP+DAF group; (7) IR+DAF group, and (8) CRP+IR+DAF group. Intestinal and lung injury, neutrophil infiltration, myeloperoxidase (MPO) expression, complement component deposition, and interleukin-6 (IL-6) production were assessed for each treatment group of mice.

RESULTS

We report that administration of DAF significantly attenuates the CRP-enhanced intestinal injury as well as remote lung damages following acute mesenteric IR in mice, while DAF inhibits complement activation, suppresses neutrophil infiltration, and reduces IL-6 production.

CONCLUSIONS

Our study suggests that inhibition complement activation with DAF may prove useful for the treatment of post-ischemic inflammatory injuries associated with an increased production of CRP.