A new map of glycosaminoglycan and C3b binding sites on factor H

The hypothetical molecular mechanism of the ethnic variations in the manifestation of age-related macular degeneration; focuses on the functions of the most significant susceptibility genes

Age-related macular degeneration (AMD) is the leading sight-threatening disease in developed countries. On the other hand, recent studies indicated an ethnic variation in the phenotype of AMD. For example, several reports demonstrated that the incidence of drusen in AMD patients is less in Asians compared to Caucasians though the reason has not been clarified yet. In the last decades, several genome association studies have disclosed many susceptible genes of AMD and revealed that the association strength of some genes was different among races and AMD phenotypes. In this review article, the essential findings of the clinical studies and genome association studies for the most significant genes CFH and ARMS2/HTRA1 in AMD of different races are summarized, and theoretical hypotheses about the molecular mechanisms underlying the ethnic variation in the AMD manifestation mainly focused on those genes between Caucasians and Asians are discussed.

The human factor H protein family - an update

Complement is an ancient and complex network of the immune system and, as such, it plays vital physiological roles, but it is also involved in numerous pathological processes. The proper regulation of the complement system is important to allow its sufficient and targeted activity without deleterious side-effects. Factor H is a major complement regulator, and together with its splice variant factor H-like protein 1 and the five human factor H-related (FHR) proteins, they have been linked to various diseases. The role of factor H in inhibiting complement activation is well studied, but the function of the FHRs is less characterized. Current evidence supports the main role of the FHRs as enhancers of complement activation and opsonization, i.e., counter-balancing the inhibitory effect of factor H. FHRs emerge as soluble pattern recognition molecules and positive regulators of the complement system. In addition, factor H and some of the FHR proteins were shown to modulate the activity of immune cells, a non-canonical function outside the complement cascade. Recent efforts have intensified to study factor H and the FHRs and develop new tools for the distinction, quantification and functional characterization of members of this protein family. Here, we provide an update and overview on the versatile roles of factor H family proteins, what we know about their biological functions in healthy conditions and in diseases.

Complement and coagulation crosstalk - Factor H in the spotlight

The immune complement and the coagulation systems are blood-based proteolytic cascades that are activated by pathway-specific triggers, based on protein-protein interactions and enzymatic cleavage reactions. Activation of these systems is finely balanced and controlled through specific regulatory mechanisms. The complement and coagulation systems are generally viewed as distinct, but have common evolutionary origins, and several interactions between these homologous systems have been reported. This complement and coagulation crosstalk can affect activation, amplification and regulatory functions in both systems. In this review, we summarize the literature on coagulation factors contributing to complement alternative pathway activation and regulation and highlight molecular interactions of the complement alternative pathway regulator factor H with several coagulation factors. We propose a mechanism where factor H interactions with coagulation factors may contribute to both complement and coagulation activation and regulation within the haemostatic system and fibrin clot microenvironment and introduce the emerging role of factor H as a modulator of coagulation. Finally, we discuss the potential impact of these protein interactions in diseases associated with factor H dysregulation or deficiency as well as evidence of coagulation dysfunction.

Complement factor I: Regulatory nexus, driver of immunopathology, and therapeutic

Complement factor I (FI) is the nexus for classical, lectin and alternative pathway complement regulation. FI is an 88 kDa plasma protein that circulates in an inactive configuration until it forms a trimolecular complex with its cofactor and substrate whereupon a structural reorganization allows the catalytic triad to cleave its substrates, C3b and C4b. In keeping with its role as the master complement regulatory enzyme, deficiency has been linked to immunopathology. In the setting of complete FI deficiency, a consumptive C3 deficiency results in recurrent infections with encapsulated microorganisms. Aseptic cerebral inflammation and vasculitic presentations are also less commonly observed. Heterozygous mutations in the factor I gene (CFI) have been demonstrated to be enriched in atypical haemolytic uraemic syndrome, albeit with a very low penetrance. Haploinsufficiency of CFI has also been associated with decreased retinal thickness and is a strong risk factor for the development of age-related macular degeneration. Supplementation of FI using plasma purified or recombinant protein has long been postulated, however, technical difficulties prevented progression into clinical trials. It is only using gene therapy that CFI supplementation has reached the clinic with GT005 in phase I/II clinical trials for geographic atrophy.

The loss of glycocalyx integrity impairs complement factor H binding and contributes to cyclosporine-induced endothelial cell injury

Background

Calcineurin inhibitors (CNIs) are associated with nephrotoxicity, endothelial cell dysfunction, and thrombotic microangiopathy (TMA). Evolving evidence suggests an important role for complement dysregulation in the pathogenesis of CNI-induced TMA. However, the exact mechanism(s) of CNI-induced TMA remain(s) unknown.

Methods

Using blood outgrowth endothelial cells (BOECs) from healthy donors, we evaluated the effects of cyclosporine on endothelial cell integrity. Specifically, we determined complement activation (C3c and C9) and regulation (CD46, CD55, CD59, and complement factor H [CFH] deposition) as these occurred on the endothelial cell surface membrane and glycocalyx.

Results

We found that exposing the endothelium to cyclosporine resulted in a dose- and time-dependent enhancement of complement deposition and cytotoxicity. We, therefore, employed flow cytometry, Western blotting/CFH cofactor assays, and immunofluorescence imaging to determine the expression of complement regulators and the functional activity and localization of CFH. Notably, while cyclosporine led to the upregulation of complement regulators CD46, CD55, and CD59 on the endothelial cell surface, it also diminished the endothelial cell glycocalyx through the shedding of heparan sulfate side chains. The weakened endothelial cell glycocalyx resulted in decreased CFH surface binding and surface cofactor activity.

Conclusion

Our findings confirm a role for complement in cyclosporine-induced endothelial injury and suggest that decreased glycocalyx density, induced by cyclosporine, is a mechanism that leads to complement alternative pathway dysregulation via decreased CFH surface binding and cofactor activity. This mechanism may apply to other secondary TMAs-in which a role for complement has so far not been recognized-and provide a potential therapeutic target and an important marker for patients on calcineurin inhibitors.

Complement-regulatory biomaterial coatings: Activity and selectivity profile of the factor H-binding peptide 5C6

The use of biomaterials in modern medicine has enabled advanced drug delivery strategies and led to reduced morbidity and mortality in a variety of interventions such as transplantation or hemodialysis. However, immune-mediated reactions still present a serious complication of these applications. One of the drivers of such reactions is the complement system, a central part of humoral innate immunity that acts as a first-in-line defense system in its own right but also coordinates other host defense responses. A major regulator of the complement system is the abundant plasma protein factor H (FH), which impairs the amplification of complement responses. Previously, we could show that it is possible to recruit FH to biomedical surfaces using the phage display-derived cyclic peptide 5C6 and, consequently, reduce deposition of C3b, an activation product of the complement system. However, the optimal orientation of 5C6 on surfaces, structural determinants within the peptide for the binding, and the exact binding region on FH remained unknown. Here, we show that the cyclic core and C-terminal region of 5C6 are essential for its interaction with FH and that coating through its N-terminus strongly increases FH recruitment and reduces C3-mediated opsonization in a microparticle-based assay. Furthermore, we could demonstrate that 5C6 selectively binds to FH but not to related proteins. The observation that 5C6 also binds murine FH raises the potential for translational evaluation in animal models. This work provides important insight for the future development of 5C6 as a probe or therapeutic entity to reduce complement activation on biomaterials. STATEMENT OF SIGNIFICANCE: Biomaterials have evolved into core technologies critical to biomedical and drug delivery applications alike, yet their safe and efficient use may be adversely impacted by immune responses to the foreign materials. Taking inspiration from microbial immune evasion strategies, our group developed a peptide-based surface coating that recruits factor H (FH), a host regulator of the complement system, from plasma to the material surface and prevents unwanted activation of this innate immunity pathway. In this study, we identified the molecular determinants that define the interaction between FH and the coated peptide, developed tethering strategies with largely enhanced binding capacity and provided important insight into the target selectivity and species specificity of the FH-binding peptide, thereby paving the way for preclinical development steps.

Protein therapeutics and their lessons: Expect the unexpected when inhibiting the multi-protein cascade of the complement system

Over a century after the discovery of the complement system, the first complement therapeutic was approved for the treatment of paroxysmal nocturnal hemoglobinuria (PNH). It was a long-acting monoclonal antibody (aka 5G1-1, 5G1.1, h5G1.1, and now known as eculizumab) that targets C5, specifically preventing the generation of C5a, a potent anaphylatoxin, and C5b, the first step in the eventual formation of membrane attack complex. The enormous clinical and financial success of eculizumab across four diseases (PNH, atypical hemolytic uremic syndrome (aHUS), myasthenia gravis (MG), and anti-aquaporin-4 (AQP4) antibody-positive neuromyelitis optica spectrum disorder (NMOSD)) has fueled a surge in complement therapeutics, especially targeting diseases with an underlying complement pathophysiology for which anti-C5 therapy is ineffective. Intensive research has also uncovered challenges that arise from C5 blockade. For example, PNH patients can still face extravascular hemolysis or pharmacodynamic breakthrough of complement suppression during complement-amplifying conditions. These "side" effects of a stoichiometric inhibitor like eculizumab were unexpected and are incompatible with some of our accepted knowledge of the complement cascade. And they are not unique to C5 inhibition. Indeed, "exceptions" to the rules of complement biology abound and have led to unprecedented and surprising insights. In this review, we will describe initial, present and future aspects of protein inhibitors of the complement cascade, highlighting unexpected findings that are redefining some of the mechanistic foundations upon which the complement cascade is organized.

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.

Age-Related Maculopathy Susceptibility 2 and Complement Factor H Polymorphism and Intraocular Complement Activation in Neovascular Age-Related Macular Degeneration

Purpose

To investigate the association of risk alleles in complement factor H (CFH) and age-related maculopathy susceptibility 2 (ARMS2) with complement activation products in the aqueous humor in eyes with neovascular age-related macular degeneration (nAMD) including polypoidal choroidal vasculopathy (PCV), retinal angiomatous proliferation (RAP), and pachychoroid neovasculopathy (PNV).

Design

Prospective, comparative, observational study.

Participants

Treatment-naïve patients with nAMD and cataract patients as controls.

Methods

The study included 236 eyes of 236 patients with nAMD and 49 control eyes of 49 patients. Aqueous humor samples were collected from 67 eyes with drusen-associated nAMD, 72 eyes with PCV, 26 eyes with RAP, and 71 eyes with PNV before intravitreal anti-VEGF injection and cataract surgery in the 49 control eyes. Clinical samples were measured for complement component 3a (C3a), C4a, and C5a using a bead-based immunoassay. Genotyping of the ARMS2 A69S (rs10490924), CFH I62V (rs800292), and CFH Y402H (rs1061170) was performed using TaqMan genotyping.

Main Outcome Measures

The levels of complement activation products (C3a, C4a, and C5a) in the aqueous humor in each genotype of ARMS2 and CFH.

Results

The C3a level in the aqueous humor was significantly elevated (P = 0.006) in patients with nAMD and the ARMS2 A69S risk allele, whereas the levels of the complement activation products were not associated with CFH I62V and Y402H genotypes. Among the control eyes, no significant differences were seen in any complement activation products for all genetic polymorphisms. The levels of the complement activation products in the aqueous humor of eyes with the nAMD subtypes for each genetic polymorphism did not show significant differences.

Conclusions

The C3a concentration in the aqueous humor was significantly higher in Japanese nAMD patients with the ARMS2 A69S risk allele, whereas it was not elevated in the patients with CFH I62V. Age-related maculopathy susceptibility 2 A69S polymorphism is strongly associated with local complement activation in nAMD patients.

Antibody recognition of complement Factor H reveals a flexible loop involved in Atypical Hemolytic Uremic Syndrome pathogenesis

Atypical hemolytic uremic syndrome (aHUS) is a disease associated with dysregulation of the immune complement system, especially of the alternative pathway (AP). Complement factor H (CFH), consisting of 20 domains called CCP1-20, downregulates the AP as a cofactor for mediating C3 inactivation by complement factor I (CFI). However, anomalies related to CFH are known to cause excessive complement activation and cytotoxicity. In aHUS, mutations and the presence of anti-CFH autoantibodies (AAbs) have been reported as plausible causes of CFH dysfunction, and it is known that CFH-related aHUS carries a high probability of end-stage renal disease. Elucidating the detailed functions of CFH at the molecular level will help to understand aHUS pathogenesis. Herein, we used biophysical data to reveal that a heavy-chain antibody fragment, termed VHH4, recognized CFH with high affinity. Hemolytic assays also indicated that VHH4 disrupted the protective function of CFH on sheep erythrocytes. Furthermore, X-ray crystallography revealed that VHH4 recognized the Leu1181-Leu1189_CCP20 loop, a known anti-CFH AAbs epitope. We next analyzed the dynamics of the C-terminal region of CFH, and showed that the epitopes recognized by anti-CFH AAbs and VHH4 were the most flexible regions in CCP18-20. Finally, we conducted mutation analyses to elucidate the mechanism of VHH4 recognition of CFH, and revealed that VHH4 inserts Trp1183_CCP20 residue of CFH into the pocket formed by the complementary determining region 3 loop. These results suggested that anti-CFH AAbs may adopt a similar molecular mechanism to recognize the flexible loop of Leu1181-Leu1189_CCP20, leading to aHUS pathogenesis.

Complement Factor H-Related Proteins FHR1 and FHR5 Interact With Extracellular Matrix Ligands, Reduce Factor H Regulatory Activity and Enhance Complement Activation

Components of the extracellular matrix (ECM), when exposed to body fluids may promote local complement activation and inflammation. Pathologic complement activation at the glomerular basement membrane and at the Bruch's membrane is implicated in renal and eye diseases, respectively. Binding of soluble complement inhibitors to the ECM, including factor H (FH), is important to prevent excessive complement activation. Since the FH-related (FHR) proteins FHR1 and FHR5 are also implicated in these diseases, our aim was to study whether these FHRs can also bind to ECM components and affect local FH activity and complement activation. Both FH and the FHRs showed variable binding to ECM components. We identified laminin, fibromodulin, osteoadherin and PRELP as ligands of FHR1 and FHR5, and found that FHR1 bound to these ECM components through its C-terminal complement control protein (CCP) domains 4-5, whereas FHR5 bound via its middle region, CCPs 3-7. Aggrecan, biglycan and decorin did not bind FH, FHR1 and FHR5. FHR5 also bound to immobilized C3b, a model of surface-deposited C3b, via CCPs 3-7. By contrast, soluble C3, C3(H₂O), and the C3 fragments C3b, iC3b and C3d bound to CCPs 8-9 of FHR5. Properdin, which was previously described to bind via CCPs 1-2 to FHR5, did not bind in its physiologically occurring serum forms in our assays. FHR1 and FHR5 inhibited the binding of FH to the identified ECM proteins in a dose-dependent manner, which resulted in reduced FH cofactor activity. Moreover, both FHR1 and FHR5 enhanced alternative complement pathway activation on immobilized ECM proteins when exposed to human serum, resulting in the increased deposition of C3-fragments, factor B and C5b-9. Thus, our results identify novel ECM ligands of FH family proteins and indicate that FHR1 and FHR5 are competitive inhibitors of FH on ECM and, when bound to these ligands, they may enhance local complement activation and promote inflammation under pathological conditions.

A synthetic protein as efficient multitarget regulator against complement over-activation

The complement system constitutes the innate defense against pathogens. Its dysregulation leads to diseases and is a critical determinant in many viral infections, e.g., COVID-19. Factor H (FH) is the main regulator of the alternative pathway of complement activation and could be a therapy to restore homeostasis. However, recombinant FH is not available. Engineered FH versions may be alternative therapeutics. Here, we designed a synthetic protein, MFHR13, as a multitarget complement regulator. It combines the dimerization and C5-regulatory domains of human FH-related protein 1 (FHR1) with the C3-regulatory and cell surface recognition domains of human FH, including SCR 13. In summary, the fusion protein MFHR13 comprises SCRs FHR1_1-2:FH_1-4:FH₁₃:FH_19-20. It protects sheep erythrocytes from complement attack exhibiting 26 and 4-fold the regulatory activity of eculizumab and human FH, respectively. Furthermore, we demonstrate that MFHR13 and FHR1 bind to all proteins forming the membrane attack complex, which contributes to the mechanistic understanding of FHR1. We consider MFHR13 a promising candidate as therapeutic for complement-associated diseases.

Ruiz-Molina et al. design a synthetic protein, MFHR13, as a multi-target complement regulator. They demonstrate that it protects sheep erythrocytes from complement attack exhibiting 26 and 4-fold the regulatory activity of eculizumab and human Factor H, respectively, which highlights its therapeutic potential.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

Dysregulation of complement factor H in juvenile-onset systemic lupus erythematosus patients

Objectives

This study aims to evaluate the expression pattern of factor H in peripheral blood and the frequency of factor H autoantibodies in plasma of juvenile-onset systemic lupus erythematosus (jSLE) patients compared to healthy controls.

Patients and methods

Between March 2019 and October 2019, a total of 30 healthy individuals (3 males, 27 females; mean age: 26±7.4 years; range, 18 to 40 years) and 65 jSLE patients (age of onset ≤16 years) (2 males, 63 females; mean age: 23.4±7 years; range, 15 to 38 years) were included. Factor H expression pattern was examined in blood of all subjects using quantitative real-time polymerase chain reaction and the frequency of factor H autoantibodies was estimated in plasma using enzyme-linked immunosorbent assay.

Results

Factor H expression was significantly downregulated in jSLE patients compared to healthy controls (p<0.01). A significant underexpression of factor H was observed in jSLE patients with nephritis compared to those without nephritis (p<0.03), while there was no association of factor H expression levels with any of the other clinical and serological features, disease activity or disease damage index of patients. Only 5% of jSLE patients were positive for factor H autoantibodies without any correlations with the clinical data or disease activity of patients.

Conclusion

Our study results suggest that factor H expression can be dysregulated in jSLE patients.

Selective Binding of Heparin/Heparan Sulfate Oligosaccharides to Factor H and Factor H-Related Proteins: Therapeutic Potential for C3 Glomerulopathies

Complement dysregulation is characteristic of the renal diseases atypical hemolytic uremic syndrome (aHUS) and complement component 3 glomerulopathy (C3G). Complement regulatory protein Factor H (FH) inhibits complement activity, whereas FH-related proteins (FHRs) lack a complement regulatory domain. FH and FHRs compete for binding to host cell glycans, in particular heparan sulfates (HS). HS is a glycosaminoglycan with an immense structural variability, where distinct sulfation patterns mediate specific binding of proteins. Mutations in FH, FHRs, or an altered glomerular HS structure may disturb the FH : FHRs balance on glomerular endothelial cells, thereby leading to complement activation and the subsequent development of aHUS/C3G. In this study, we aimed to identify specific HS structures that could specifically compete off FHRs from HS glycocalyx (HS_Glx), without interfering with FH binding. FH/FHR binding to human conditionally immortalized glomerular endothelial cells (ciGEnCs) and HS_Glx purified from ciGEnC glycocalyx was assessed. HS modifications important for FH/FHR binding to HS_Glx were analyzed using selectively desulfated heparins in competition with purified HS_Glx. We further assessed effects of heparinoids on FHR1- and FHR5-mediated C3b deposition on ciGEnCs. In the presence of C3b, binding of FH, FHR1 and FHR5 to ciGEnCs was significantly increased, whereas binding of FHR2 was minimal. FHR1 and 5 competitively inhibited FH binding to HS_Glx, leading to alternative pathway dysregulation. FHR1 and FHR5 binding was primarily mediated by N-sulfation while FH binding depended on N-, 2-O- and 6-O-sulfation. Addition of 2-O-desulfated heparin significantly reduced FHR1- and FHR5-mediated C3b deposition on ciGEnCs. We identify 2-O-desulfated heparin derivatives as potential therapeutics for C3G and other diseases with dysregulated complement.

The complement cascade in the regulation of neuroinflammation, nociceptive sensitization, and pain

The complement cascade is a key component of the innate immune system that is rapidly recruited through a cascade of enzymatic reactions to enable the recognition and clearance of pathogens and promote tissue repair. Despite its well-understood role in immunology, recent studies have highlighted new and unexpected roles of the complement cascade in neuroimmune interaction and in the regulation of neuronal processes during development, aging, and in disease states. Complement signaling is particularly important in directing neuronal responses to tissue injury, neurotrauma, and nerve lesions. Under physiological conditions, complement-dependent changes in neuronal excitability, synaptic strength, and neurite remodeling promote nerve regeneration, tissue repair, and healing. However, in a variety of pathologies, dysregulation of the complement cascade leads to chronic inflammation, persistent pain, and neural dysfunction. This review describes recent advances in our understanding of the multifaceted cross-communication that takes place between the complement system and neurons. In particular, we focus on the molecular and cellular mechanisms through which complement signaling regulates neuronal excitability and synaptic plasticity in the nociceptive pathways involved in pain processing in both health and disease. Finally, we discuss the future of this rapidly growing field and what we believe to be the significant knowledge gaps that need to be addressed.

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.

SPRD: a surface plasmon resonance database of common factors for better experimental planning

Background

Surface plasmon resonance is a label-free biophysical technique that is widely used in investigating biomolecular interactions, including protein-protein, protein-DNA, and protein-small molecule binding. Surface plasmon resonance is a very powerful tool in different stages of small molecule drug development and antibody characterization. Both academic institutions and pharmaceutical industry extensively utilize this method for screening and validation studies involving direct molecular interactions. In most applications of the surface plasmon resonance technology, one of the studied molecules is immobilized on a microchip, while the second molecule is delivered through a microfluidic system over the immobilized molecules. Changes in total mass on the chip surface is recorded in real time as an indicator of the molecular interactions.

Main body

Quality and accuracy of the surface plasmon resonance data depend on experimental variables, including buffer composition, type of sensor chip, coupling chemistry of molecules on the sensor surface, and surface regeneration conditions. These technical details are generally included in materials and methods sections of published manuscripts and are not easily accessible using the common internet browser search engines or PubMed. Herein, we introduce a surface plasmon resonance database, www.sprdatabase.info that contains technical details extracted from 5140 publications with surface plasmon resonance data. We also provide an analysis of experimental conditions preferred by different laboratories. These experimental variables can be searched within the database and help future users of this technology to design better experiments.

Conclusion

Amine coupling and CM5 chips were the most common methods used for immobilizing proteins in surface plasmon resonance experiments. However, number of different chips, capture methods and buffer conditions were used by multiple investigators. We predict that the database will significantly help the scientific community using this technology and hope that users will provide feedback to improve and expand the database indefinitely. Publicly available information in the database can save a great amount of time and resources by assisting initial optimization and troubleshooting of surface plasmon resonance experiments.

Functional Characterization of Rare Genetic Variants in the N-Terminus of Complement Factor H in aHUS, C3G, and AMD

Membranoproliferative glomerulonephritis (MPGN), C3 glomerulopathy (C3G), atypical haemolytic uraemic syndrome (aHUS) and age-related macular degeneration (AMD) have all been strongly linked with dysfunction of the alternative pathway (AP) of complement. A significant proportion of individuals with MPGN, C3G, aHUS and AMD carry rare genetic variants in the CFH gene that cause functional or quantitative deficiencies in the factor H (FH) protein, an important regulator of the AP. In silico analysis of the deleteriousness of rare genetic variants in CFH is not reliable and careful biochemical assessment remains the gold standard. Six N-terminal variants of uncertain significance in CFH were identified in patients with these diseases of the AP and selected for analysis. The variants were produced in Pichia Pastoris in the setting of FH CCPs 1–4, purified by nickel affinity chromatography and size exclusion and characterized by surface plasmon resonance and haemolytic assays as well as by cofactor assays in the fluid phase. A single variant, Q81P demonstrated a profound loss of binding to C3b with consequent loss of cofactor and decay accelerating activity. A further 2 variants, G69E and D130N, demonstrated only subtle defects which could conceivably over time lead to disease progression of more chronic AP diseases such as C3G and AMD. In the variants S159N, A161S, and M162V any functional defect was below the capacity of the experimental assays to reliably detect. This study further underlines the importance of careful biochemical assessment when assigning functional consequences to rare genetic variants that may alter clinical decisions for patients.

Factor H Autoantibodies and Complement-Mediated Diseases

Factor H (FH), a member of the regulators-of-complement-activation (RCA) family of proteins, circulates in human plasma at concentrations of 180–420 mg/L where it controls the alternative pathway (AP) of complement in the fluid phase and on cell surfaces. When the regulatory function of FH is impaired, complement-mediated tissue injury and inflammation occur, leading to diseases such as atypical hemolytic uremic syndrome (a thrombotic microangiopathy or TMA), C3 glomerulopathy (C3G) and monoclonal gammopathy of renal significance (MGRS). A pathophysiological cause of compromised FH function is the development of autoantibodies to various domains of the FH protein. FH autoantibodies (FHAAs) are identified in 10.9% of patients with aHUS, 3.2% of patients with C3G, and rarely in patients with MGRS. The phenotypic variability of FHAA-mediated disease reflects both the complexity of FH and the epitope specificity of FHAA for select regions of the native protein. In this paper, we have characterized FHAA epitopes in a large cohort of patients diagnosed with TMA, C3G or MGRS. We explore the epitopes recognized by FHAAs in these diseases and the association of FHAAs with the genetic deletion of both copies of the CFHR1 gene to show how these disease phenotypes are associated with this diverse spectrum of autoantibodies.

Revisiting the role of factor H in age-related macular degeneration: Insights from complement-mediated renal disease and rare genetic variants

Ophthalmologists are long familiar with the eye showing signs of systemic disease, but the association between age-related macular degeneration and abnormal complement activation, common to several renal disorders, has only recently been elucidated. Although complement activation products were identified in drusen almost three decades ago, it was not until the early 21st century that a single-nucleotide polymorphism in the complement factor H gene was identified as a major heritable determinant of age-related macular degeneration, galvanizing global efforts to unravel the pathogenesis of this common disease. Advances in proteomic analyses and familial aggregation studies have revealed distinctive clinical phenotypes segregated by the functional effects of common and rare genetic variants on the mature protein and its splice variant, factor H-like protein 1. The predominance of loss-of-function, N-terminal mutations implicate age-related macular degeneration as a disease of general complement dysregulation, offering several therapeutic avenues for its modulation. Here, we explore the molecular impact of these mutations/polymorphisms on the ability of variant factor H/factor H-like protein 1 to localize to polyanions, pentraxins, proinflammatory triggers, and cell surfaces across ocular and renal tissues and exert its multimodal regulatory functions and their clinical implications. Finally, we critically evaluate key therapeutic and diagnostic efforts in this rapidly evolving field.

Tuning the Functionality by Splicing: Factor H and Its Alternative Splice Variant FHL-1 Share a Gene but Not All Functions

The alternative pathway regulator Factor H-like protein 1 (FHL-1) is composed of the first 7 N-terminal complement control protein domains of Factor H (FH) and protects host surfaces from uncontrolled complement attack. Although FHL-1 shares the N-terminal regulatory domains with FH, it was thought to be a weaker regulator. Recently, the regulatory activity of FHL-1 was shown to be comparable to FH. Nonetheless, the question remained whether FHL-1 is an indispensable, unique regulator. The discovery that FHL-1 is the predominant regulator on Bruch’s membrane, a critical site for the onset and progression of age-related-macular degeneration (AMD), showed that FHL-1 is essential for complement regulation. A common single nucleotide polymorphism in FH/FHL-1 that predisposes for AMD underlines the important role of FHL-1 in this context. Reports that some cancer tissues specifically upregulate FHL-1 expression, thereby evading immune surveillance, suggests a pronounced regulatory activity of the splice variant. Several microorganisms specifically recruit FHL-1 to evade complement attack. From a phylogenetic point of view, FHL-1 appears much later than other complement regulators, which could imply a specific role that is possibly not systemic but rather tissue specific. This review focuses on the current knowledge of FHL-1 and its physiological and pathophysiological roles.

Unraveling the Effect of a Potentiating Anti-Factor H Antibody on Atypical Hemolytic Uremic Syndrome-Associated Factor H Variants

The complement system plays an important role in our innate immune system. Complement activation results in clearance of pathogens, immune complex, and apoptotic cells. The host is protected from complement-mediated damage by several complement regulators. Factor H (FH) is the most important fluid-phase regulator of the alternative pathway of the complement system. Heterozygous mutations in FH are associated with complement-related diseases such as atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration. We recently described an agonistic anti-FH mAb that can potentiate the regulatory function of FH. This Ab could serve as a potential new drug for aHUS patients and alternative to C5 blockade by eculizumab. However, it is unclear whether this Ab can potentiate FH mutant variants in addition to wild-type (WT) FH. In this study, the functionality and potential of the agonistic Ab in the context of pathogenic aHUS-related FH mutant proteins was investigated. The binding affinity of recombinant WT FH and the FH variants, W1183L, V1197A, R1210C, and G1194D to C3b was increased upon addition of the potentiating Ab and similarly, the decay-accelerating activity of all mutants is increased. The potentiating anti-FH Ab is able to restore the surface regulatory function of most of the tested FH mutants to WT FH levels on a human HAP-1 cell line and on sheep erythrocytes. In conclusion, our potentiating anti-FH is broadly active and able to enhance both WT FH function as well as most aHUS-associated FH variants tested in this study.

Characterization of Binding Properties of Individual Functional Sites of Human Complement Factor H

Factor H exists as a 155,000 dalton, extended protein composed of twenty small domains which is flexible enough that it folds back on itself. Factor H regulates complement activation through its interactions with C3b and polyanions. Three binding sites for C3b and multiple polyanion binding sites have been identified on Factor H. In intact Factor H these sites appear to act synergistically making their individual contributions difficult to distinguish. Recombinantly expressed fragments of human Factor H were examined using surface plasmon resonance (SPR) for interactions with C3, C3b, iC3b, C3c, and C3d. Eleven recombinant proteins of lengths from one to twenty domains were used to show that the three C3b-binding sites exhibit 100-fold different affinities for C3b. The N-terminal site [complement control protein (CCP) domains 1-6] bound C3b with a K_d of 0.08 μM and this interaction was not influenced by the presence or absence of domains 7 and 8. Full length Factor H similarly exhibited a K_d for C3b of 0.1 μM. Unexpectedly, the N-terminal site (CCP 1-6) bound native C3 with a K_d of 0.4 μM. The C-terminal domains (CCP 19-20) exhibited a K_d of 1.7 μM for C3b. We localized a weak third C3b binding site in the CCP 13-15 region with a K_d estimated to be ~15 μM. The C-terminal site (CCP 19-20) bound C3b, iC3b, and C3d equally well with a K_d of 1 to 2 μM. In order to identify and compare regions of Factor H that interact with polyanions a family of 18 overlapping three domain recombinant proteins spanning the entire length of Factor H were expressed and purified. Immobilized heparin was used as a model polyanion and SPR confirmed the presence of heparin binding sites in CCP 6-8 (K_d 1.2 μM) and in CCP 19-20 (4.9 μM) and suggested the existence of a weak third polyanion binding site in the center of Factor H (CCP 11-13). Our results unveil the relative contributions of different regions of Factor H to its regulation of complement, and may contribute to the understanding of how defects in certain Factor H domains lead to disease.

Anti-complement factor H autoantibodies may be protective in lupus nephritis

BACKGROUND

This study aimed to investigate the role of anti-CFH autoantibodies in lupus nephritis based on a well-defined cohort.

METHODS

One hundred twenty patients with biopsy-proven active lupus nephritis were collected as the discovery cohort, sixty patients served as the validation cohort, thirty-four patients with SLE without renal involvement (NR-SLE) were as disease controls, and thirty healthy donors were also included. The anti-CFH autoantibodies and IgG subclasses were detected by ELISA, and epitopes were evaluated by western blot. Anti-CFH autoantibodies were purified by affinity chromatography column, and the interference on the biofunctions of CFH was further studied by the C3b binding assay and cofactor activity assay in vitro.

RESULTS

The prevalence of anti-CFH autoantibodies in lupus nephritis was significantly higher than that in healthy controls (8.3% (10/120) vs. 0% (0/30), P = 0.017), and no significant difference was found between the discovery and the validation group (8.3% (10/120) vs. 11.7% (7/60), P = 0.268) or the discovery and the NR-SLE group (8.3% (10/120) vs. 11.8% (4/34), P = 0.231). The subclass was mainly IgG2 (7/10), and major epitopes were in the middle (8/10 in SCRs 11-14) and N-terminal (7/10 in SCRs 1-4) regions of CFH. Patients with anti-CFH autoantibodies had a significantly lower prevalence of acute kidney injury (0% (0/10) vs. 40.0%(4/10), P = 0.025), lower serum creatinine levels (0.76 (0.40, 1.06) vs. 1.43 (0.46, 11.15), mg/dL, P = 0.023), and higher hemoglobin levels (113.8 ± 24.63 vs. 90.0 ± 22.53, g/L, P = 0.037) than those who were negative after further stratified analysis. A functional study showed that anti-CFH autoantibodies purified from patients with lupus nephritis could improve the binding between CFH and C3b, and also enhance the cofactor activity of CFH in vitro.

CONCLUSIONS

Anti-CFH autoantibodies were detected in patients with lupus nephritis in approximately 10% of patients with polyepitopes and IgG2 subclass predominance. Patients with anti-CFH autoantibodies presented with milder renal damage, and the purified autoantibodies could enhance the C3b binding and CFI cofactor activity of CFH in vitro, which suggested a protective role in the lupus nephritis.

Complement inhibitor factor H expressed by breast cancer cells differentiates CD14+ human monocytes into immunosuppressive macrophages

Macrophages are a major immune cell type in the tumor microenvironment, where they display a tumor-supporting phenotype. Factor H (FH) is a complement inhibitor that also plays a role in several cellular functions. To date, the phenotype of monocytes stimulated with FH has been unexplored. We discovered that FH is a survival factor for CD14⁺ primary human monocytes, promoting their differentiation into macrophages in serum-free medium. This activity was localized to the C-terminal domains of FH and it was inhibited in plasma, indicating that the phenomenon may be most relevant in tissues. FH-induced macrophages display characteristics of immunosuppressive cells including expression of CD163 and CD206, release of the anti-inflammatory cytokine IL-10 and changes in metabolism. Furthermore, FH-induced macrophages express low levels of HLA-DR but high levels of co-inhibitory molecule programmed death-ligand 1 (PD-L1), and accordingly, a reduced capacity for T-cell activation. Finally, we show that FH is expressed by human breast cancer cells and that this correlates with the presence of immunosuppressive macrophages, breast cancer recurrence and severity of the disease. We propose that the expression of FH by tumor cells and the promotion of an immunosuppressive cancer microenvironment by this protein should be taken into account when considering the effectiveness of immunotherapies against breast cancer.

Potentiation of complement regulator factor H protects human endothelial cells from complement attack in aHUS sera

Mutations in the gene encoding for complement regulator factor H (FH) severely disrupt its normal function to protect human cells from unwanted complement activation, resulting in diseases such as atypical hemolytic uremic syndrome (aHUS). aHUS presents with severe hemolytic anemia, thrombocytopenia, and renal disease, leading to end-stage renal failure. Treatment of severe complement-mediated disease, such as aHUS, by inhibiting the terminal complement pathway, has proven to be successful but at the same time fails to preserve the protective role of complement against pathogens. To improve complement regulation on human cells without interfering with antimicrobial activity, we identified an anti-FH monoclonal antibody (mAb) that induced increased FH-mediated protection of primary human endothelial cells from complement, while preserving the complement-mediated killing of bacteria. Moreover, this FH-activating mAb restored complement regulation in sera from aHUS patients carrying various heterozygous mutations in FH known to impair FH function and dysregulate complement activation. Our data suggest that FH normally circulates in a less active conformation and can become more active, allowing enhanced complement regulation on human cells. Antibody-mediated potentiation of FH may serve as a highly effective approach to inhibit unwanted complement activation on human cells in a wide range of hematological diseases while preserving the protective role of complement against pathogens.

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.

Regulation of regulators: Role of the complement factor H-related proteins

The complement system, while being an essential and very efficient effector component of innate immunity, may cause damage to the host and result in various inflammatory, autoimmune and infectious diseases or cancer, when it is improperly activated or regulated. Factor H is a serum glycoprotein and the main regulator of the activity of the alternative complement pathway. Factor H, together with its splice variant factor H-like protein 1 (FHL-1), inhibits complement activation at the level of the central complement component C3 and beyond. In humans, there are also five factor H-related (FHR) proteins, whose function is poorly characterized. While data indicate complement inhibiting activity for some of the FHRs, there is increasing evidence that FHRs have an opposite role compared with factor H and FHL-1, namely, they enhance complement activation directly and also by competing with the regulators FH and FHL-1. This review summarizes the current stand and recent data on the roles of factor H family proteins in health and disease, with focus on the function of FHR proteins.

Immunological features and functional analysis of anti-CFH autoantibodies in patients with atypical hemolytic uremic syndrome

OBJECTIVE

Atypical hemolytic uremic syndrome (aHUS) is associated with defective complement regulation. Anti-complement factor H (CFH) antibodies were thought to participate in the pathogenesis of aHUS. The aim of this study was to address the functions and properties of CFH autoantibodies in a Chinese Han cohort of aHUS patients.

METHODS

Thirty-six anti-CFH antibody-positive aHUS patients at the acute phase of the disease were involved in this study. Clinical data of the patients were collected. Anti-CFH immunoglobulin G (IgG) subclasses and antibody isotypes were detected by ELISA. Epitope mapping was performed using recombinant CFH fragments (SCRs 1-4, SCR 7, SCRs 11-14, and SCRs 19-20). Purified IgG from plasma from seven patients were used for functional analyses.

RESULTS

All patients presented with the classic triad of HUS. The anti-CFH autoantibodies mostly bound to the SCRs 19-20 domains of CFH but not the SCRs 1-4 domains. CFI cofactor activity was not disturbed by the anti-CFH antibody in any of the seven patients. Purified IgG interfered with the binding of CFH to C3b and CFH-mediated sheep erythrocyte protection in all seven patients. IgG from 4/5 (80%) patients tested inhibited the binding of CFH to glomerular endothelial cells.

CONCLUSIONS

Our study suggests that the properties of CFH antibodies from patients with aHUS, including the recognition of SCRs and IgG subclasses, can influence and impair the biological role of CFH and therefore contribute to aHUS susceptibility.

Biophysical analysis of sialic acid recognition by the complement regulator Factor H

Complement factor H (FH), an elongated and substantially glycosylated 20-domain protein, is a soluble regulator of the complement alternative pathway (AP). It contains several glycan binding sites which mediate recognition of α2-3-linked sialic acid (FH domain 20) and glycosaminoglycans (domains 6–8 and 19–20). FH also binds the complement C3-activation product C3b, a powerful opsonin and focal point for the formation of C3-convertases of the AP feedback loop. In freely circulating FH the C3b binding site in domains 19–20 is occluded, a phenomenon that is not fully understood and could be mediated by an intramolecular interaction between FH’s intrinsic sialylated glycosylation and its own sialic acid binding site. In order to assess this possibility, we characterized FH’s sialylation with respect to glycosidic linkage type and searched for further potential, not yet characterized sialic acid binding sites in FH and its seven-domain spanning splice variant and fellow complement regulator FH like-1 (FHL-1). We also probed FH binding to the sialic acid variant Neu5Gc which is not expressed in humans but on heterologous erythrocytes that restrict the human AP and in FH transgenic mice. We find that FH contains mostly α2-6-linked sialic acid, making an intramolecular interaction with its α2-3-sialic acid specific binding site and an associated self-lock mechanism unlikely, substantiate that there is only a single sialic acid binding site in FH and none in FHL-1, and demonstrate direct binding of FH to the nonhuman sialic acid Neu5Gc, supporting the use of FH transgenic mouse models for studies of complement-related diseases.

Self-Damage Caused by Dysregulation of the Complement Alternative Pathway: Relevance of the Factor H Protein Family

The alternative pathway is a continuously active surveillance arm of the complement system, and it can also enhance complement activation initiated by the classical and the lectin pathways. Various membrane-bound and plasma regulatory proteins control the activation of the potentially deleterious complement system. Among the regulators, the plasma glycoprotein factor H (FH) is the main inhibitor of the alternative pathway and its powerful amplification loop. FH belongs to a protein family that also includes FH-like protein 1 and five factor H-related (FHR-1 to FHR-5) proteins. Genetic variants and abnormal rearrangements involving the FH protein family have been linked to numerous systemic and organ-specific diseases, including age-related macular degeneration, and the renal pathologies atypical hemolytic uremic syndrome, C3 glomerulopathies, and IgA nephropathy. This review covers the known and recently emerged ligands and interactions of the human FH family proteins associated with disease and discuss the very recent experimental data that suggest FH-antagonistic and complement-activating functions for the FHR proteins.

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 potent complement factor C3-specific nanobody inhibiting multiple functions in the alternative pathway of human and murine complement

The complement system is a complex, carefully regulated proteolytic cascade for which suppression of aberrant activation is of increasing clinical relevance, and inhibition of the complement alternative pathway is a subject of intense research. Here, we describe the nanobody hC3Nb1 that binds to multiple functional states of C3 with subnanomolar affinity. The nanobody causes a complete shutdown of alternative pathway activity in human and murine serum when present in concentrations comparable with that of C3, and hC3Nb1 is shown to prevent proconvertase assembly, as well as binding of the C3 substrate to C3 convertases. Our crystal structure of the C3b-hC3Nb1 complex and functional experiments demonstrate that proconvertase formation is blocked by steric hindrance between the nanobody and an Asn-linked glycan on complement factor B. In addition, hC3Nb1 is shown to prevent factor H binding to C3b, rationalizing its inhibition of factor I activity. Our results identify hC3Nb1 as a versatile, inexpensive, and powerful inhibitor of the alternative pathway in both human and murine in vitro model systems of complement activation.

Statistical Validation of Rare Complement Variants Provides Insights into the Molecular Basis of Atypical Hemolytic Uremic Syndrome and C3 Glomerulopathy

Atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G) are associated with dysregulation and overactivation of the complement alternative pathway. Typically, gene analysis for aHUS and C3G is undertaken in small patient numbers, yet it is unclear which genes most frequently predispose to aHUS or C3G. Accordingly, we performed a six-center analysis of 610 rare genetic variants in 13 mostly complement genes (CFH, CFI, CD46, C3, CFB, CFHR1, CFHR3, CFHR4, CFHR5, CFP, PLG, DGKE, and THBD) from >3500 patients with aHUS and C3G. We report 371 novel rare variants (RVs) for aHUS and 82 for C3G. Our new interactive Database of Complement Gene Variants was used to extract allele frequency data for these 13 genes using the Exome Aggregation Consortium server as the reference genome. For aHUS, significantly more protein-altering rare variation was found in five genes CFH, CFI, CD46, C3, and DGKE than in the Exome Aggregation Consortium (allele frequency < 0.01%), thus correlating these with aHUS. For C3G, an association was only found for RVs in C3 and the N-terminal C3b-binding or C-terminal nonsurface-associated regions of CFH In conclusion, the RV analyses showed nonrandom distributions over the affected proteins, and different distributions were observed between aHUS and C3G that clarify their phenotypes.

Diseases of complement dysregulation-an overview

Atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathy (C3G), and paroxysmal nocturnal hemoglobinuria (PNH) are prototypical disorders of complement dysregulation. Although complement overactivation is common to all, cell surface alternative pathway dysregulation (aHUS), fluid phase alternative pathway dysregulation (C3G), or terminal pathway dysregulation (PNH) predominates resulting in the very different phenotypes seen in these diseases. The mechanism underlying the dysregulation also varies with predominant acquired autoimmune (C3G), somatic mutations (PNH), or inherited germline mutations (aHUS) predisposing to disease. Eculizumab has revolutionized the treatment of PNH and aHUS although has been less successful in C3G. With the next generation of complement therapeutic in late stage development, these archetypal complement diseases will provide the initial targets.

Factor H C-Terminal Domains Are Critical for Regulation of Platelet/Granulocyte Aggregate Formation

Platelet/granulocyte aggregates (PGAs) increase thromboinflammation in the vasculature, and PGA formation is tightly controlled by the complement alternative pathway (AP) negative regulator, Factor H (FH). Mutations in FH are associated with the prothrombotic disease atypical hemolytic uremic syndrome (aHUS), yet it is unknown whether increased PGA formation contributes to the thrombosis seen in patients with aHUS. Here, flow cytometry assays were used to evaluate the effects of aHUS-related mutations on FH regulation of PGA formation and characterize the mechanism. Utilizing recombinant fragments of FH spanning the entire length of the protein, we mapped the regions of FH most critical for limiting AP activity on the surface of isolated human platelets and neutrophils, as well as the regions most critical for regulating PGA formation in human whole blood stimulated with thrombin receptor-activating peptide (TRAP). FH domains 19–20 were the most critical for limiting AP activity on platelets, neutrophils, and at the platelet/granulocyte interface. The role of FH in PGA formation was attributed to its ability to regulate AP-mediated C5a generation. AHUS-related mutations in domains 19–20 caused differential effects on control of PGA formation and AP activity on platelets and neutrophils. Our data indicate FH C-terminal domains are key for regulating PGA formation, thus increased FH protection may have a beneficial impact on diseases characterized by increased PGA formation, such as cardiovascular disease. Additionally, aHUS-related mutations in domains 19–20 have varying effects on control of TRAP-mediated PGA formation, suggesting that some, but not all, aHUS-related mutations may cause increased PGA formation that contributes to excessive thrombosis in patients with aHUS.

Complete functional characterization of disease-associated genetic variants in the complement factor H gene

Genetic analyses in atypical hemolytic uremic syndrome (aHUS) and C3-glomerulopathy (C3G) patients have provided an excellent understanding of the genetic component of the disease and informed genotype-phenotype correlations supporting an individualized approach to patient management and treatment. In this context, a correct categorization of the disease-associated gene variants is critical to avoid detrimental consequences for patients and their relatives. Here we describe a comprehensive procedure to measure levels and functional activity of complement regulator factor H (FH) encoded by CFH, the commonest genetic factor associated with aHUS and C3G, and present the results of the analysis of 28 uncharacterized, disease-associated, FH variants. Sixteen variants were not expressed in plasma and eight had significantly reduced functional activities that impact on complement regulation. In total, 24 of 28 CFH variants were unambiguously categorized as pathogenic and the nature of the pathogenicity fully documented for each. The data also reinforce the genotype-phenotype correlations that associate specific FH functional alterations with either aHUS or C3G and illustrate important drawbacks of the prediction algorithms dealing with variants located in FH functional regions. We also report that the novel aHUS-associated M823T variant is functionally impaired. This was unexpected and uncovered the important contribution of regions outside the N-terminal and C-terminal functional domains to FH regulatory activities on surfaces. Thus, our work significantly advances knowledge towards a complete functional understanding of the CFH genetic variability and highlights the importance of functional analysis of the disease-associated CFH variants.

Neisseria cinerea Expresses a Functional Factor H Binding Protein Which Is Recognized by Immune Responses Elicited by Meningococcal Vaccines

Neisseria meningitidis is a major cause of bacterial meningitis and sepsis worldwide. Capsular polysaccharide vaccines are available against meningococcal serogroups A, C, W, and Y. More recently two protein-based vaccines, Bexsero and Trumenba, against meningococcal serogroup B strains have been licensed; both vaccines contain meningococcal factor H binding protein (fHbp). fHbp is a surface-exposed lipoprotein that binds the negative complement regulator complement factor H (CFH), thereby inhibiting the alternative pathway of complement activation. Recent analysis of available genomes has indicated that some commensal Neisseria species also contain genes that potentially encode fHbp, although the functions of these genes and how immunization with fHbp-containing vaccines could affect the commensal flora have yet to be established. Here, we show that the commensal species Neisseria cinerea expresses functional fHbp on its surface and that it is responsible for recruitment of CFH by the bacterium. N. cinerea fHbp binds CFH with affinity similar to that of meningococcal fHbp and promotes survival of N. cinerea in human serum. We examined the potential impact of fHbp-containing vaccines on N. cinerea We found that immunization with Bexsero elicits serum bactericidal activity against N. cinerea, which is primarily directed against fHbp. The shared function of fHbp in N. cinerea and N. meningitidis and cross-reactive responses elicited by Bexsero suggest that the introduction of fHbp-containing vaccines has the potential to affect carriage of N. cinerea and other commensal species.

The Murine Factor H-Related Protein FHR-B Promotes Complement Activation

Factor H-related (FHR) proteins consist of varying number of complement control protein domains that display various degrees of sequence identity to respective domains of the alternative pathway complement inhibitor factor H (FH). While such FHR proteins are described in several species, only human FHRs were functionally investigated. Their biological role is still poorly understood and in part controversial. Recent studies on some of the human FHRs strongly suggest a role for FHRs in enhancing complement activation via competing with FH for binding to certain ligands and surfaces. The aim of the current study was the functional characterization of a murine FHR, FHR-B. To this end, FHR-B was expressed in recombinant form. Recombinant FHR-B bound to human C3b and was able to compete with human FH for C3b binding. FHR-B supported the assembly of functionally active C3bBb alternative pathway C3 convertase via its interaction with C3b. This activity was confirmed by demonstrating C3 activation in murine serum. In addition, FHR-B bound to murine pentraxin 3 (PTX3), and this interaction resulted in murine C3 fragment deposition due to enhanced complement activation in mouse serum. FHR-B also induced C3 deposition on C-reactive protein, the extracellular matrix (ECM) extract Matrigel, and endothelial cell-derived ECM when exposed to mouse serum. Moreover, mouse C3 deposition was strongly enhanced on necrotic Jurkat T cells and the mouse B cell line A20 by FHR-B. FHR-B also induced lysis of sheep erythrocytes when incubated in mouse serum with FHR-B added in excess. Altogether, these data demonstrate that, similar to human FHR-1 and FHR-5, mouse FHR-B modulates complement activity by promoting complement activation via interaction with C3b and via competition with murine FH.

Complement factor H in AMD: Bridging genetic associations and pathobiology

Age-Related Macular Degeneration (AMD) is a complex multifactorial disease characterized in its early stages by lipoprotein accumulations in Bruch's Membrane (BrM), seen on fundoscopic exam as drusen, and in its late forms by neovascularization ("wet") or geographic atrophy of the Retinal Pigmented Epithelial (RPE) cell layer ("dry"). Genetic studies have strongly supported a relationship between the alternative complement cascade, in particular the common H402 variant in Complement Factor H (CFH) and development of AMD. However, the functional significance of the CFH Y402H polymorphism remains elusive. In this article, we critically review the literature surrounding the functional significance of this polymorphism. Furthermore, based on our group's studies we propose a model in which CFH H402 affects CFH binding to heparan sulfate proteoglycans leading to accelerated lipoprotein accumulation in BrM and drusen progression. We also review the literature on the role of other complement components in AMD pathobiologies, including C3a, C5a and the membrane attack complex (MAC), and on transgenic mouse models developed to interrogate in vivo the effects of the CFH Y402H polymorphism.

Novel Evasion Mechanisms of the Classical Complement Pathway

Complement is a network of soluble and cell surface-associated proteins that gives rise to a self-amplifying, yet tightly regulated system with fundamental roles in immune surveillance and clearance. Complement becomes activated on the surface of nonself cells by one of three initiating mechanisms known as the classical, lectin, and alternative pathways. Evasion of complement function is a hallmark of invasive pathogens and hematophagous organisms. Although many complement-inhibition strategies hinge on hijacking activities of endogenous complement regulatory proteins, an increasing number of uniquely evolved evasion molecules have been discovered over the past decade. In this review, we focus on several recent investigations that revealed mechanistically distinct inhibitors of the classical pathway. Because the classical pathway is an important and specific mediator of various autoimmune and inflammatory disorders, in-depth knowledge of novel evasion mechanisms could direct future development of therapeutic anti-inflammatory molecules.

Protection of host cells by complement regulators

The complement cascade is an ancient immune-surveillance system that not only provides protection from pathogen invasion but has also evolved to participate in physiological processes to maintain tissue homeostasis. The alternative pathway (AP) of complement activation is the evolutionarily oldest part of this innate immune cascade. It is unique in that it is continuously activated at a low level and arbitrarily probes foreign, modified-self, and also unaltered self-structures. This indiscriminate activation necessitates the presence of preformed regulators on autologous surfaces to spare self-cells from the undirected nature of AP activation. Although the other two canonical complement activation routes, the classical and lectin pathways, initiate the cascade more specifically through pattern recognition, their activity still needs to be tightly controlled to avoid excessive reactivity. It is the perpetual duty of complement regulators to protect the self from damage inflicted by inadequate complement activation. Here, we review the role of complement regulators as preformed mediators of defense, explain their common and specialized functions, and discuss selected cases in which alterations in complement regulators lead to disease. Finally, rational engineering approaches using natural complement inhibitors as potential therapeutics are highlighted.

Structural insight into proteolytic activation and regulation of the complement system

The complement system is a highly complex and carefully regulated proteolytic cascade activated through three different pathways depending on the activator recognized. The structural knowledge regarding the intricate proteolytic enzymes that activate and control complement has increased dramatically over the last decade. This development has been pivotal for understanding how mutations within complement proteins might contribute to pathogenesis and has spurred new strategies for development of complement therapeutics. Here we describe and discuss the complement system from a structural perspective and integrate the most recent findings obtained by crystallography, small-angle X-ray scattering, and electron microscopy. In particular, we focus on the proteolytic enzymes governing activation and their products carrying the biological effector functions. Additionally, we present the structural basis for some of the best known complement inhibitors. The large number of accumulated molecular structures enables us to visualize the relative size, position, and overall orientation of many of the most interesting complement proteins and assembled complexes on activator surfaces and in membranes.

Properdin: a tightly regulated critical inflammatory modulator

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non‐physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin‐centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long‐term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.

Human Tamm-Horsfall protein, a renal specific protein, serves as a cofactor in complement 3b degradation

Tamm-Horsfall protein (THP) is an abundant urinary protein of renal origin. We hypothesize that THP can act as an inhibitor of complement since THP binds complement 1q (C1q) of the classical complement pathway, inhibits activation of this pathway, and is important in decreasing renal ischemia-reperfusion injury (a complement-mediated condition). In this study, we began to investigate whether THP interacted with the alternate complement pathway via complement factor H (CFH). THP was shown to bind CFH using ligand blots and in an ELISA (K_D of 1 × 10⁻⁶ M). Next, the ability of THP to alter CFH’s normal action as it functioned as a cofactor in complement factor I (CFI)–mediated complement 3b (C3b) degradation was investigated. Unexpectedly, control experiments in these in vitro assays suggested that THP, without added CFH, could act as a cofactor in CFI-mediated C3b degradation. This cofactor activity was present equally in THP isolated from 10 different individuals. While an ELISA demonstrated small amounts of CFH contaminating THP samples, these CFH amounts were insufficient to explain the degree of cofactor activity present in THP. An ELISA demonstrated that THP directly bound C3b (K_D ~ 5 × 10⁻⁸m), a prerequisite for a protein acting as a C3b degradation cofactor. The cofactor activity of THP likely resides in the protein portion of THP since partially deglycosylated THP still retained cofactor activity. In conclusion, THP appears to participate directly in complement inactivation by its ability to act as a cofactor for C3b degradation, thus adding support to the hypothesis that THP might act as an endogenous urinary tract inhibitor of complement.

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.