The interactive Factor H-atypical hemolytic uremic syndrome mutation database and website: update and integration of membrane cofactor protein and Factor I mutations with structural models

Structural biology of complement receptors

The complement system plays crucial roles in a wide breadth of immune and inflammatory processes and is frequently cited as an etiological or aggravating factor in many human diseases, from asthma to cancer. Complement receptors encompass at least eight proteins from four structural classes, orchestrating complement-mediated humoral and cellular effector responses and coordinating the complex cross-talk between innate and adaptive immunity. The progressive increase in understanding of the structural features of the main complement factors, activated proteolytic fragments, and their assemblies have spurred a renewed interest in deciphering their receptor complexes. In this review, we describe what is currently known about the structural biology of the complement receptors and their complexes with natural agonists and pharmacological antagonists. We highlight the fundamental concepts and the gray areas where issues and problems have been identified, including current research gaps. We seek to offer guidance into the structural biology of the complement system as structural information underlies fundamental and therapeutic research endeavors. Finally, we also indicate what we believe are potential developments in the field.

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.

Coagulation and complement: Key innate defense participants in a seamless web

In 1969, Dr. Oscar Ratnoff, a pioneer in delineating the mechanisms by which coagulation is activated and complement is regulated, wrote, “In the study of biological processes, the accumulation of information is often accelerated by a narrow point of view. The fastest way to investigate the body’s defenses against injury is to look individually at such isolated questions as how the blood clots or how complement works. We must constantly remind ourselves that such distinctions are man-made. In life, as in the legal cliché, the devices through which the body protects itself form a seamless web, unwrinkled by our artificialities.” Our aim in this review, is to highlight the critical molecular and cellular interactions between coagulation and complement, and how these two major component proteolytic pathways contribute to the seamless web of innate mechanisms that the body uses to protect itself from injury, invading pathogens and foreign surfaces.

Analysis of 272 Genetic Variants in the Upgraded Interactive FXI Web Database Reveals New Insights into FXI Deficiency

Coagulation Factor XI (FXI) is a plasma glycoprotein composed of four apple (Ap) domains and a serine protease (SP) domain. FXI circulates as a dimer and activates Factor IX (FIX), promoting thrombin production and preventing excess blood loss. Genetic variants that degrade FXI structure and function often lead to bleeding diatheses, commonly termed FXI deficiency. The first interactive FXI variant database underwent initial development in 2003 at https://www.factorxi.org . Here, based on a much improved FXI crystal structure, the upgraded FXI database contains information regarding 272 FXI variants (including 154 missense variants) found in 657 patients, this being a significant increase from the 183 variants identified in the 2009 update. Type I variants involve the simultaneous reduction of FXI coagulant activity (FXI:C) and FXI antigen levels (FXI:Ag), whereas Type II variants result in decreased FXI:C yet normal FXI:Ag. The database updates now highlight the predominance of Type I variants in FXI. Analysis in terms of a consensus Ap domain revealed the near-uniform distribution of 81 missense variants across the Ap domains. A further 66 missense variants were identified in the SP domain, showing that all regions of the FXI protein were important for function. The variants clarified the critical importance of changes in surface solvent accessibility, as well as those of cysteine residues and the dimer interface. Guidelines are provided below for clinicians who wish to use the database for diagnostic purposes. In conclusion, the updated database provides an easy-to-use web resource on FXI deficiency for clinicians.

Genetic justification of severe COVID-19 using a rigorous algorithm

Recent studies suggest excessive complement activation in severe coronavirus disease-19 (COVID-19). The latter shares common characteristics with complement-mediated thrombotic microangiopathy (TMA). We hypothesized that genetic susceptibility would be evident in patients with severe COVID-19 (similar to TMA) and associated with disease severity. We analyzed genetic and clinical data from 97 patients hospitalized for COVID-19. Through targeted next-generation-sequencing we found an ADAMTS13 variant in 49 patients, along with two risk factor variants (C3, 21 patients; CFH,34 patients). 31 (32%) patients had a combination of these, which was independently associated with ICU hospitalization (p = 0.022). Analysis of almost infinite variant combinations showed that patients with rs1042580 in thrombomodulin and without rs800292 in complement factor H did not require ICU hospitalization. We also observed gender differences in ADAMTS13 and complement-related variants. In light of encouraging results by complement inhibitors, our study highlights a patient population that might benefit from early initiation of specific treatment.

The interaction between the complement system and hemostatic factors

PURPOSE OF REVIEW

To discuss the crosstalk between the complement system and hemostatic factors (coagulation cascade, platelet, endothelium, and Von Willebrand Factor), and the consequences of this interaction under physiologic and pathologic conditions.

RECENT FINDINGS

The complement and coagulation systems are comprised of serine proteases and are genetically related. In addition to the common ancestral genes, the complement system and hemostasis interact directly, through protein-protein interactions, and indirectly, on the surface of platelets and endothelial cells. The close interaction between the complement system and hemostatic factors is manifested both in physiologic and pathologic conditions, such as in the inflammatory response to thrombosis, thrombosis at the inflamed area, and thrombotic complications of complement disorders.

SUMMARY

The interaction between the complement system and hemostasis is vital for homeostasis and the protective response of the host to tissue injury, but also results in the pathogenesis of several thrombotic and inflammatory disorders.

The solution structure of the complement deregulator FHR5 reveals a compact dimer and provides new insights into CFHR5 nephropathy

The human complement Factor H-related 5 protein (FHR5) antagonizes the main circulating complement regulator Factor H, resulting in the deregulation of complement activation. FHR5 normally contains nine short complement regulator (SCR) domains, but a FHR5 mutant has been identified with a duplicated N-terminal SCR-1/2 domain pair that causes CFHR5 nephropathy. To understand how this duplication causes disease, we characterized the solution structure of native FHR5 by analytical ultracentrifugation and small-angle X-ray scattering. Sedimentation velocity and X-ray scattering indicated that FHR5 was dimeric, with a radius of gyration (Rg ) of 5.5 ± 0.2 nm and a maximum protein length of 20 nm for its 18 domains. This result indicated that FHR5 was even more compact than the main regulator Factor H, which showed an overall length of 26-29 nm for its 20 SCR domains. Atomistic modeling for FHR5 generated a library of 250,000 physically realistic trial arrangements of SCR domains for scattering curve fits. Only compact domain structures in this library fit well to the scattering data, and these structures readily accommodated the extra SCR-1/2 domain pair present in CFHR5 nephropathy. This model indicated that mutant FHR5 can form oligomers that possess additional binding sites for C3b in FHR5. We conclude that the deregulation of complement regulation by the FHR5 mutant can be rationalized by the enhanced binding of FHR5 oligomers to C3b deposited on host cell surfaces. Our FHR5 structures thus explained key features of the mechanism and pathology of CFHR5 nephropathy.

SiMPLOD, a Structure-Integrated Database of Collagen Lysyl Hydroxylase (LH/PLOD) Enzyme Variants

PLOD genes encode for procollagen lysyl hydroxylase enzymes (LH/PLOD), a family of proteins essential for collagen biosynthesis. Several mutations affect these genes, causing severe disorders, such as Ehlers-Danlos and Bruck syndrome, as well a connective tissue disease with phenotype resembling osteogenesis imperfecta caused by lack of LH3 functions. The recently determined three-dimensional (3D) structures of the full-length human LH3/PLOD3 isoform, together with the structure of a fragment of a viral LH/PLOD homolog, are now allowing molecular mapping of the numerous disease-causing mutations, providing insights often suitable for the interpretation of the resulting disease phenotypes. However, the added value of molecular structure interpretation is affected by the limited accessibility of complex molecular data to scientific communities lacking direct expertise in structural biology. In this work, we present a Structurally-integrated database for Mutations of PLOD genes (SiMPLOD), a publicly-available manually-curated online database with an embedded molecular viewer interface for the visualization and interpretation of LH/PLOD mutations on available molecular models. Each SiMPLOD entry is accompanied by manual annotations extrapolated from literature references and comments about the localization of the amino acid variants on the molecular structure. Additional links to the appropriate online resources for clinically-relevant as well as biochemical data are also provided in a standardized format. The web application is available at http://fornerislab.unipv.it/SiMPLOD. © 2019 American Society for Bone and Mineral Research.

Molecular basis of complement factor I deficiency in Tunisian atypical haemolytic and uraemic syndrome patients

AIM

The aim of the present study was to characterize the molecular basis of complement factor I deficiency in Tunisian atypical haemolytic and uremic syndrome patients with low factor I levels.

METHODS

Six adults and seven children were enrolled in this study. Complement factor I levels were assessed by a homemade sandwich ELISA and ranged between 12.5% and 60%. Genomic DNA was amplified by way of a polymerase chain reaction using intronic primers flanking the 13 coding exons. Sequencing of amplified products was carried out by the dye terminator sequencing method. Molecular study was performed on parental samples for three dead paediatric patients. The control group consisted of 100 healthy Tunisian donors.

RESULTS

We identified a total of 13 substitutions and one insertion: seven in introns, four in exons and three in UTR. The new mutations were c.-132G > C, c.71 + 181 T > A in 5'UTR and intron 1, respectively. Three intronic polymorphisms were predicted to have impact on splicing events: c.482 + 6C > T, c.884-42_884-41insTTAAA (rs34422850) and c.1429 + 33 A > G (rs9998151). They were three missense mutations leading to a p.Ile 357Met, p.Ile416Leu and p.GLu548Gln. p.Ile 357Met was found in two patients and one relative. Half of the patients had associated mutation and/or polymorphisms.

CONCLUSION

This is the first genetic study in Tunisian and Maghrebin atypical haemolytic and uraemic syndrome patients. The high occurrence of Ile357Met mutation may reflect a founding effect. Functional impact of the two new mutations c.-132G > C and c.71 + 181A > T have to be studied. Association of simultaneous genetic abnormalities may explain the variability of atypical haemolytic and uraemic syndrome, penetrance and disease phenotype.

Streptococcus pneumoniae From Patients With Hemolytic Uremic Syndrome Binds Human Plasminogen via the Surface Protein PspC and Uses Plasmin to Damage Human Endothelial Cells

Pneumococcal hemolytic uremic syndrome (HUS) in children is caused by infections with Streptococcus pneumoniae. Because endothelial cell damage is a hallmark of HUS, we studied how HUS-inducing pneumococci derived from infant HUS patients during the acute phase disrupt the endothelial layer. HUS pneumococci efficiently bound human plasminogen. These clinical isolates of HUS pneumococci efficiently bound human plasminogen via the bacterial surface proteins Tuf and PspC. When activated to plasmin at the bacterial surface, the active protease degraded fibrinogen and cleaved C3b. Here, we show that PspC is a pneumococcal plasminogen receptor and that plasmin generated on the surface of HUS pneumococci damages endothelial cells, causing endothelial retraction and exposure of the underlying matrix. Thus, HUS pneumococci damage endothelial cells in the blood vessels and disturb local complement homeostasis. Thereby, HUS pneumococci promote a thrombogenic state that drives HUS pathology.

Polyphosphates and Complement Activation

To sustain life in environments that are fraught with risks of life-threatening injury, organisms have developed innate protective strategies such that the response to wounds is rapid and localized, with the simultaneous recruitment of molecular, biochemical, and cellular pathways that limit bleeding and eliminate pathogens and damaged host cells, while promoting effective healing. These pathways are both coordinated and tightly regulated, as their over- or under-activation may lead to inadequate healing, disease, and/or demise of the host. Recent advances in our understanding of coagulation and complement, a key component of innate immunity, have revealed an intriguing linkage of the two systems. Cell-secreted polyphosphate promotes coagulation, while dampening complement activation, discoveries that are providing insights into disease mechanisms and suggesting novel therapeutic strategies.

Loss-of-Function Mutations in the CFH Gene Affecting Alternatively Encoded Factor H-like 1 Protein Cause Dominant Early-Onset Macular Drusen

Purpose

To characterize the molecular mechanism underpinning early-onset macular drusen (EOMD), a phenotypically severe subtype of age-related macular degeneration (AMD), in a subgroup of patients.

Design

Multicenter case series, in vitro experimentation, and retrospective analysis of previously reported variants.

Participants

Seven families with apparently autosomal dominant EOMD.

Methods

Patients underwent a comprehensive ophthalmic assessment. Affected individuals from families A, B, and E underwent whole exome sequencing. The probands from families C, D, F, and G underwent Sanger sequencing analysis of the complement factor H (CFH) gene. Mutant recombinant factor H like-1 (FHL-1) proteins were expressed in HEK293 cells to assess the impact on FHL-1 expression and function. Previously reported EOMD-causing variants in CFH were reviewed.

Main Outcome Measures

Detailed clinical phenotypes, genomic findings, in vitro characterization of mutation effect on protein function, and postulation of the pathomechanism underpinning EOMD.

Results

All affected participants demonstrated bilateral drusen. The earliest reported age of onset was 16 years (median, 46 years). Ultra-rare (minor allele frequency [MAF], ≤0.0001) CFH variants were identified as the cause of disease in each family: CFH c.1243del, p.(Ala415ProfsTer39) het; c.350+1G→T het; c.619+1G→A het, c.380G→A, p.(Arg127His) het; c.694C→T p.(Arg232Ter) het (identified in 2 unrelated families in this cohort); and c.1291T→A, p.(Cys431Ser). All mutations affect complement control protein domains 2 through 7, and thus are predicted to impact both FHL-1, the predominant isoform in Bruch’s membrane (BrM) of the macula, and factor H (FH). In vitro analysis of recombinant proteins FHL-1_R127H, FHL-1_A415f/s, and FHL-1_C431S demonstrated that they are not secreted, and thus are loss-of-function proteins. Review of 29 previously reported EOMD-causing mutations found that 75.8% (22/29) impact FHL-1 and FH. In total, 86.2% (25/29) of EOMD-associated variants cause haploinsufficiency of FH or FHL-1.

Conclusions

Early-onset macular drusen is an underrecognized, phenotypically severe subtype of AMD. We propose that haploinsufficiency of FHL-1, the main regulator of the complement pathway in BrM, where drusen develop, is an important mechanism underpinning the development of EOMD in a number of cases. Understanding the molecular basis of EOMD will shed light on AMD pathogenesis given their pathologic similarities.

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

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

Atypical Hemolytic-Uremic Syndrome: An Update on Pathophysiology, Diagnosis, and Treatment

Atypical hemolytic uremic syndrome (aHUS), a rare variant of thrombotic microangiopathy, is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal impairment. The condition is associated with poor clinical outcomes with high morbidity and mortality. Atypical HUS predominantly affects the kidneys but has the potential to cause multi-organ system dysfunction. This uncommon disorder is caused by a genetic abnormality in the complement alternative pathway resulting in over-activation of the complement system and formation of microvascular thrombi. Abnormalities of the complement pathway may be in the form of mutations in key complement genes or autoantibodies against specific complement factors. We discuss the pathophysiology, clinical manifestations, diagnosis, complications, and management of aHUS. We also review the efficacy and safety of the novel therapeutic agent, eculizumab, in aHUS, pregnancy-associated aHUS, and aHUS in renal transplant patients.

Diagnostic Utility of Complement Serology for Atypical Hemolytic Uremic Syndrome

OBJECTIVE

To investigate the clinical utility of a 9-analyte complement serology panel (COMS) covering complement function (CH50 and AH50), components (C3, C4), factor B (CFB), factor H, and activation markers (C4d, Bb, and soluble membrane attack complex) for the diagnosis of atypical hemolytic uremic syndrome (aHUS).

METHODS

Physician orders for COMS from January 19, 2015, through November 4, 2016, were reviewed. Demographic characteristics, patient diagnosis, and laboratory parameters were recorded.

RESULTS

There were 177 COMS orders for 147 patients. The median patient age was 44.9 years (range, 0.9-88.0 years). Common reasons for ordering COMS included monitoring and diagnosis of C3 glomerulopathy and renal dysfunction and differentiation of aHUS from other thrombotic microangiopathies (TMAs). Forty-four patients had COMS ordered for TMAs: 8 had aHUS and all had 1 or more abnormalities within the alternative pathway of complement. Although the sensitivity of this finding for the diagnosis of aHUS is 100%, the specificity is only 28%, with a positive likelihood ratio of 1.39. Patients with aHUS had lower CH50, C3, and CFB than did those with secondary non-aHUS TMA (all P<.01). A combined CFB of 20.9 mg/dL or less and CH50 of 56% or less led to sensitivity of 75% with increased specificity of 88.9% and a diagnostic odds ratio of 24.

CONCLUSION

A COMS abnormality should not be interpreted in isolation. In conjunction with clinical presentation, a decrease in both CFB and CH50 may be an important clue to support the diagnosis of aHUS.

Complement-coagulation connections

: Complement and coagulation are evolutionarily related proteolytic cascades in the blood that are critical for effecting an appropriate innate response to injury that limits bleeding and infection, while promoting healing. Although often viewed as distinct, it has long been recognized that cross-talk likely exists between these pathways. Only recently have molecular links been established. These are providing insights that are revealing opportunities for the development of novel therapeutic strategies to better treat a wide range of thrombotic, inflammatory, immune, infectious, and malignant diseases. In this brief review, the complex relationship between complement and coagulation is highlighted, underlining some of the newly uncovered interactions, in the hopes of stimulating innovative research that will yield improvements in patient outcomes.

Targeted exome sequencing in anti-factor H antibody negative HUS reveals multiple variations

BACKGROUND

Genetic susceptibility to atypical hemolytic uremic syndrome (aHUS) may lie within genes regulating or activating the alternate complement and related pathways converging on endothelial cell activation.

METHODS

We tested 32 Indian patients of aHUS negative for antibodies to complement factor H for genetic variations in a panel of 15 genes, i.e., CFH, CFHR1-5, CFI, CFB, C3, CD46, MASP2, DGKE, ADAMTS13, THBD and PLG using next-generation DNA sequencing and for copy number variation in CFHR1-3.

RESULTS

Despite absence of a public database of exome variations in the Indian population and limited functional studies, we could establish a genetic diagnosis in 6 (18.8%) patients using a stringent scheme of prioritization. One patient carried a likely pathogenic variation. The number of patients carrying possibly pathogenic variation was as follows: 1 variation: 5 patients, 2 variations: 9 patients, 3 variations: 5 patients, 4 variations: 9 patients, 5 variations: 2 patients and 6 variations: 2 patients. Homozygous deletion of CFHR1-3 was present in five patients; none of these carried a diagnostic genetic variation. Patients with or without diagnostic variation did not differ significantly in terms of enrichment of genetic variations that were rare/novel or predicted deleterious, or for possible environmental triggers.

CONCLUSION

We conclude that genetic testing for multiple genes in patients with aHUS negative for anti-FH antibodies reveals multiple candidate variations that require prioritization. Population data on variation frequency of the Indian population and supportive functional studies are likely to improve diagnostic yield.

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.

Complement-mediated thrombotic microangiopathy secondary to sepsis-induced disseminated intravascular coagulation successfully treated with eculizumab: A case report

Secondary thrombotic microangiopathies (TMAs) are induced by several underlying conditions; most are resolved by treating background disease. Eculizumab is a human monoclonal antibody that blocks the final stage of the complement system and effectively treats atypical hemolytic uremic syndrome (aHUS). In this report, we present a patient with TMA secondary to sepsis- induced coagulopathy, who was successfully treated with eculizumab.A 44-year-old woman, who had no special medical history or familial history of TMAs, was admitted on suspicion of septic shock. Physical examination revealed gangrene on her soles. Blood tests revealed a decreased platelet count, disseminated intravascular coagulation (DIC), renal dysfunction, hemolysis, and infection. Although the coagulation disorder improved with intensive care, the low platelet count, elevated lactate dehydrogenase levels, and renal dysfunction persisted. Our investigations subsequently excluded thrombotic thrombocytopenic purpura and Shiga toxin-producing Escherichia coli-induced HUS. Plasma exchange only improved lactate dehydrogenase levels. We clinically diagnosed this case as atypical HUS and started eculizumab treatment. The patient's platelet count increased, her renal dysfunction improved, and the gangrene on her feet was ameliorated. The patient was discharged without maintenance dialysis therapy after approximately 3 months. Subsequent tests revealed elevated serum levels of soluble C5b-9, and genetic testing revealed compound heterozygous c.184G > A (Val62Ile) and c.1204T > C (Tyr402His) single-nucleotide polymorphisms in complement factor H.We encountered a case of complement-mediated TMA accompanied by DIC, which was successfully treated with eculizumab. Further studies are necessary to support the optimal use of eculizumab for TMA with background diseases.

Complement Dysregulation and Disease: Insights from Contemporary Genetics

The vertebrate complement system consists of sequentially interacting proteins that provide for a rapid and powerful host defense. Nearly 60 proteins comprise three activation pathways (classical, alternative, and lectin) and a terminal cytolytic pathway common to all. Attesting to its potency, nearly half of the system's components are engaged in its regulation. An emerging theme over the past decade is that variations in these inhibitors predispose to two scourges of modern humans. One, occurring most often in childhood, is a rare but deadly thrombomicroangiopathy called atypical hemolytic uremic syndrome. The other, age-related macular degeneration, is the most common form of blindness in the elderly. Their seemingly unrelated clinical presentations and pathologies share the common theme of overactivity of the complement system's alternative pathway. This review summarizes insights gained from contemporary genetics for understanding how dysregulation of this powerful innate immune system leads to these human diseases.

Murine systemic thrombophilia and hemolytic uremic syndrome from a factor H point mutation

Complement plays a key role in host defense, but its dysregulation can cause autologous tissue injury. Complement activation is normally controlled by regulatory proteins, including factor H (FH) in plasma and membrane cofactor protein (MCP) on the cell surface. Mutations in FH and MCP are linked to atypical hemolytic uremic syndrome, a type of thrombotic microangiopathy (TMA) that causes renal failure. We describe here that disruption of FH function on the cell surface can also lead to disseminated complement-dependent macrovascular thrombosis. By gene targeting, we introduced a point mutation (W1206R) into murine FH that impaired its interaction with host cells but did not affect its plasma complement-regulating activity. Homozygous mutant mice carrying this mutation developed renal TMA as well as systemic thrombophilia involving large blood vessels in multiple organs, including liver, lung, spleen, and kidney. Approximately 30% of mutant mice displayed symptoms of stroke and ischemic retinopathy, and 48% died prematurely. Genetic deficiency of complement C3 and factor D prevented both the systemic thrombophilia and renal TMA phenotypes. These results demonstrate a causal relationship between complement dysregulation and systemic angiopathy and suggest that complement activation may contribute to various human thrombotic disorders involving both the micro- and macrovasculature.

Acute Systolic Heart Failure Associated with Complement-Mediated Hemolytic Uremic Syndrome

Complement-mediated hemolytic uremic syndrome (otherwise known as atypical HUS) is a rare disorder of uncontrolled complement activation that may be associated with heart failure. We report the case of a 49-year-old female with no history of heart disease who presented with microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. Given her normal ADAMSTS13 activity, evidence of increased complement activation, and renal biopsy showing evidence of thrombotic microangiopathy, she was diagnosed with complement-mediated HUS. She subsequently developed acute hypoxemic respiratory failure secondary to pulmonary edema requiring intubation and mechanical ventilation. A transthoracic echocardiogram showed evidence of a Takotsubo cardiomyopathy with an estimated left ventricular ejection fraction of 20%, though ischemic cardiomyopathy could not be ruled out. Treatment was initiated with eculizumab. After several failed attempts at extubation, she eventually underwent tracheotomy. She also required hemodialysis to improve her uremia and hypervolemia. After seven weeks of hospitalization and five doses of eculizumab, her renal function and respiratory status improved, and she was discharged in stable condition on room air and independent of hemodialysis. Our case illustrates a rare association between acute systolic heart failure and complement-mediated HUS and highlights the potential of eculizumab in stabilizing even the most critically-ill patients with complement-mediated disease.

Complement regulator CD46: genetic variants and disease associations

Membrane cofactor protein (MCP; CD46) is an ubiquitously expressed complement regulatory protein that protects host cells from injury by complement. This type-I membrane glycoprotein serves as a cofactor for the serine protease factor I to mediate inactivation of C3b and C4b deposited on host cells. More than 60 disease-associated mutations in MCP have now been identified. The majority of the mutations are linked to a rare thrombotic microangiopathic-based disease, atypical hemolytic uremic syndrome (aHUS), but new putative links to systemic lupus erythematosus, glomerulonephritis, and pregnancy-related disorders among others have also been identified. This review summarizes our current knowledge of disease-associated mutations in this complement inhibitor.

Reincarnation of ancient links between coagulation and complement

Throughout evolution, organisms have developed means to contain wounds by simultaneously limiting bleeding and eliminating pathogens and damaged host cells via the recruitment of innate defense mechanisms. Disease emerges when there is unchecked activation of innate immune and/or coagulation responses. A key component of innate immunity is the complement system. Concurrent excess activation of coagulation and complement - two major blood-borne proteolytic pathways - is evident in numerous diseases, including atherosclerosis, diabetes, venous thromboembolic disease, thrombotic microangiopathies, arthritis, cancer, and infectious diseases. Delineating the cross-talk between these two cascades will uncover novel therapeutic insights.

The local complement activation on vascular bed of patients with systemic sclerosis: a hypothesis-generating study

Objective

The role of complement system in the pathogenesis of systemic sclerosis (SSc) has been debated during the last decade but an evident implication in this disease has never been found. We carried out an explorative study on SSc patients to evaluate the expression of soluble and local C5b-9 complement complex and its relation with a complement regulator, the Membrane Cofactor Protein (MCP, CD46) on skin vascular bed as target distinctive of SSc disease. We also analyzed two polymorphic variants in the complement activation gene cluster involving the MCP region.

Methods

C5b-9 plasma levels of SSc patients and healthy subjects were analyzed by ELISA assay. Archival skin biopsies of SSc patients and controls were subjected to immunofluorescence analysis to detect C5b-9 and MCP on vascular endothelial cells. The expression of MCP was validated by immunoblot analysis with specific antibody. Polymorphic variants in the MCP gene promoter were tested by a quantitative PCR technique-based allelic discrimination method.

Results

Even though circulating levels of C5b-9 did not differ between SSc and controls, C5b-9 deposition was detected in skin biopsies of SSc patients but not in healthy subjects. MCP was significantly lower in skin vessels of SSc patients than in healthy controls and was associated with the over-expression of two polymorphic variants in the MCP gene promoter, which has been related to more aggressive phenotypes in other immune-mediated diseases.

Conclusions

Our results firsty document the local complement activation with an abnormal expression of MCP in skin vessels of SSc patients, suggesting that a subset of SSc patients might be exposed to more severe organ complications and clinical evolution due to abnormal local complement activation.

New functional and structural insights from updated mutational databases for complement factor H, Factor I, membrane cofactor protein and C3

aHUS (atypical haemolytic uraemic syndrome), AMD (age-related macular degeneration) and other diseases are associated with defective AP (alternative pathway) regulation. CFH (complement factor H), CFI (complement factor I), MCP (membrane cofactor protein) and C3 exhibited the most disease-associated genetic alterations in the AP. Our interactive structural database for these was updated with a total of 324 genetic alterations. A consensus structure for the SCR (short complement regulator) domain showed that the majority (37%) of SCR mutations occurred at its hypervariable loop and its four conserved Cys residues. Mapping 113 missense mutations onto the CFH structure showed that over half occurred in the C-terminal domains SCR-15 to -20. In particular, SCR-20 with the highest total of affected residues is associated with binding to C3d and heparin-like oligosaccharides. No clustering of 49 missense mutations in CFI was seen. In MCP, SCR-3 was the most affected by 23 missense mutations. In C3, the neighbouring thioester and MG (macroglobulin) domains exhibited most of 47 missense mutations. The mutations in the regulators CFH, CFI and MCP involve loss-of-function, whereas those for C3 involve gain-of-function. This combined update emphasizes the importance of the complement AP in inflammatory disease, clarifies the functionally important regions in these proteins, and will facilitate diagnosis and therapy.

A new compilation of 324 mutations in four major proteins from the complement alternative pathway reveals mutational hotspots in factor H and complement C3, and less so in factor I and membrane cofactor protein. Their associations with function are discussed.

A complement factor B mutation in a large kindred with atypical hemolytic uremic syndrome

PURPOSE

Gain-of-function mutations in complement factor B (CFB) were recently identified in patients with atypical hemolytic uremic syndrome (aHUS), but are extremely rare. Our purpose is to describe a large kindred with aHUS associated with a CFB mutation and to further understand CFB-mutated aHUS patients.

METHODS AND RESULTS

We report a large kindred in which 3 members had aHUS. This kindred revealed that 9 of 12 members, including 2 affected patients, had persistent activation of the alternative pathway with low complement component 3 and that those 9 members showed a CFB mutation (c.1050G > C, p.Lys350Asn) in exon 8. This missense mutation was heterozygous in 8 of them and homozygous in only one. From structural studies, this mutation is shown to be located in close proximity to the Mg2-binding site within a von Willebrand factor type A domain of CFB, resulting in a gain-of-function effect of CFB and predisposition to aHUS. At present, 2 of the 3 members with aHUS have maintained normal renal function for a long-term period.

CONCLUSIONS

This kindred illustrates that a CFB mutation (c.1050G > C, p.Lys350Asn) can result in aHUS. In the future, phenotype-genotype correlations and outcome in CFB-mutated aHUS patients need to be further investigated by accumulation of a number of cases.

Molecular Interactions between Complement Factor H and Its Heparin and Heparan Sulfate Ligands

Complement factor H (CFH) is the major regulator of the central complement protein C3b in the alternative pathway of complement activation. A molecular view of the CFH interaction with native heparan sulfate (HS) is central for understanding the mechanism of how surface-bound CFH interacts with C3b bound to host cell surfaces. HS is composed of sulfated heparin-like S-regions that alternate with desulfated NA-regions. Solution structural studies of heparin (equivalent to the S-regions) and desulfated HS (the NA-regions) by scattering and ultracentrifugation showed that each structure was mostly extended and partially bent, but with greater bending and flexibility in the NA-regions compared to the S-regions. Their solution structures have been deposited in the Protein Data Bank. The largest HS oligosaccharides showed more bent and flexible structures than those for heparin. A folded-back domain structure for the solution structure of the 20 domains in CFH was determined likewise. CFH binds to the S-regions but less so to the NA-regions of HS. The bivalent interaction of CFH–heparin was observed by ultracentrifugation, and binding studies of CFH fragments with heparin-coated sensor chips. In common with other CFH interactions with its physiological and pathophysiological ligands, the CFH–heparin and CFH–C3b interactions have moderate micromolar dissociation constants K_D, meaning that these complexes do not fully form in vivo. The combination of the solution structures and binding studies indicated a two-site interaction model of CFH with heparin at cell surfaces. By this, the bivalent binding of CFH to a cell surface is co-operative. Defective interactions at either of the two independent CFH–heparin sites reduce the CFH interaction with surface-bound C3b and lead to immune disorders.

Detection of cell membrane-bound CD46 using flow cytometry

CD46 is an important regulator of the complement system by preventing unwanted deposition of the complement activation products and opsonins C3b/C4b onto self-tissue. Recently, intracellular signals mediated by CD46 activation on several distinct human cell types have demonstrated that CD46 also plays decisive roles in immuneregulation. The growing recognition of CD46 as key regulator in several vital biological processes, led to increased demand in sensitive methods for monitoring CD46 expression and changes thereof on cells and in tissues. Here we describe a method, which allows for studying CD46 expression on the surface of cells using specific antibodies in combination with fluorescence-activated cell sorting (FACS) analysis.

Update on hemolytic uremic syndrome: Diagnostic and therapeutic recommendations

Hemolytic uremic syndrome (HUS) is a rare disease. In this work the authors review the recent findings on HUS, considering the different etiologic and pathogenetic classifications. New findings in genetics and, in particular, mutations of genes that encode the complement-regulatory proteins have improved our understanding of atypical HUS. Similarly, the complement proteins are clearly involved in all types of thrombotic microangiopathy: typical HUS, atypical HUS and thrombotic thrombocytopenic purpura (TTP). Furthermore, several secondary HUS appear to be related to abnormalities in complement genes in predisposed patients. The authors highlight the therapeutic aspects of this rare disease, examining both “traditional therapy” (including plasma therapy, kidney and kidney-liver transplantation) and “new therapies”. The latter include anti-Shiga-toxin antibodies and anti-C5 monoclonal antibody “eculizumab”. Eculizumab has been recently launched for the treatment of the atypical HUS, but it appears to be effective in the treatment of typical HUS and in TTP. Future therapies are in phases I and II. They include anti-C5 antibodies, which are more purified, less immunogenic and absorbed orally and, anti-C3 antibodies, which are more powerful, but potentially less safe. Additionally, infusions of recombinant complement-regulatory proteins are a potential future therapy.

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

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

Eculizumab in the treatment of atypical haemolytic uraemic syndrome and other complement-mediated renal diseases

PURPOSE OF REVIEW

This review considers the use of eculizumab in the treatment of atypical haemolytic uraemic syndrome (aHUS) as well as the other complement-mediated renal diseases, including dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). In addition, a brief discussion of the effectiveness of eculizumab for the prevention of antibody-mediated rejection (AMR) in the setting of renal transplant and the treatment of shiga toxin associated haemolytic uraemic syndrome (STEC HUS) is also provided.

RECENT FINDINGS

No randomized controlled trials exist to support the use of eculizumab in renal disease. The results of two unpublished, prospective adult and adolescent trials support its utility in aHUS, whereas retrospective data support the effectiveness in paediatric aHUS. These two data sets form the basis of the sole renal indication for eculizumab. One small, single-centre trial and a growing number of case reports support the use of eculizumab in C3 glomerulopathy (C3G). There are limited trial data in AMR and renal transplant. Finally, there are conflicting data for the use of eculizumab in STEC HUS.

SUMMARY

The cumulative published data establish the effectiveness of eculizumab in a select group of renal diseases that have at the centre of their disease either abnormal complement control or maladaptive complement activation.

Thrombocytopenia is not mandatory to diagnose haemolytic and uremic syndrome

BACKGROUND

Hemolytic and uremic syndrome (HUS) diagnosis involves association of non immune hemolytic anemia, thrombocytopenia, and renal failure. HUS without thrombocytopenia has been observed, we call it partial HUS. Its real frequency and outcome are unknown. The aim of this study was to determine the prevalence of patients with normal platelets count in two HUS cohorts and to compare their outcome to patients with thrombocytopenia.

METHODS

We retrospectively identified HUS diagnosis in two different cohorts. The first cohort was from a single center and consisted of all cases of HUS whatever the aetiology, the second was multicentric and consisted of atypical HUS patients. These cohorts were divided into two groups depending on the presence or absence of thrombocytopenia. Clinical and biological data were compared between thrombopenic and non thrombopenic group.

RESULTS

We identified 13% (20/150) of patients with normal platelets count: 10 episodes (18%) of HUS in six patients (14%) in the monocentric cohort and 14 patients (13%) with 17 episodes (12%) in the multicentric cohort of atypical HUS. Groups differed in platelets count and LDH level. In both cohorts, renal outcome was similar to patient presenting with thrombocytopenia.

CONCLUSION

HUS with normal platelets count is not infrequent. Relative to classical clinical presentation of HUS, partial HUS has similar characteristics and identical poor renal outcome and so must be treated in the same way.

Thrombocytopenia is not mandatory to diagnose haemolytic and uremic syndrome

BACKGROUND

Hemolytic and uremic syndrome (HUS) diagnosis involves association of non immune hemolytic anemia, thrombocytopenia, and renal failure. HUS without thrombocytopenia has been observed, we call it partial HUS. Its real frequency and outcome are unknown. The aim of this study was to determine the prevalence of patients with normal platelets count in two HUS cohorts and to compare their outcome to patients with thrombocytopenia.

METHODS

We retrospectively identified HUS diagnosis in two different cohorts. The first cohort was from a single center and consisted of all cases of HUS whatever the aetiology, the second was multicentric and consisted of atypical HUS patients. These cohorts were divided into two groups depending on the presence or absence of thrombocytopenia. Clinical and biological data were compared between thrombopenic and non thrombopenic group.

RESULTS

We identified 13% (20/150) of patients with normal platelets count: 10 episodes (18%) of HUS in six patients (14%) in the monocentric cohort and 14 patients (13%) with 17 episodes (12%) in the multicentric cohort of atypical HUS. Groups differed in platelets count and LDH level. In both cohorts, renal outcome was similar to patient presenting with thrombocytopenia.

CONCLUSION

HUS with normal platelets count is not infrequent. Relative to classical clinical presentation of HUS, partial HUS has similar characteristics and identical poor renal outcome and so must be treated in the same way.

Familial atypical hemolytic uremic syndrome: a review of its genetic and clinical aspects

Atypical hemolytic uremic syndrome (aHUS) is a rare renal disease (two per one million in the USA) characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Both sporadic (80% of cases) and familial (20% of cases) forms are recognized. The study of familial aHUS has implicated genetic variation in multiple genes in the complement system in disease pathogenesis, helping to define the mechanism whereby complement dysregulation at the cell surface level leads to both sporadic and familial disease. This understanding has culminated in the use of Eculizumab as first-line therapy in disease treatment, significantly changing the care and prognosis of affected patients. However, even with this bright outlook, major challenges remain to understand the complexity of aHUS at the genetic level. It is possible that a more detailed picture of aHUS can be translated to an improved understanding of disease penetrance, which is highly variable, and response to therapy, both in the short and long terms.

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

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

Hemolytic-uremic syndrome, malignant hypertension and IgA nephropathy: successful treatment with plasma exchange therapy

A young patient with hemolytic-uremic syndrome and malignant hypertension with serious deterioration of renal function is described whose biopsy specimen showed additional IgA mesangial deposits. The patient responded to steroid treatment and to plasma exchange therapy without the need of hemodialysis sessions. In the following years, he achieved clinical remission and his blood pressure was in normal ranges without any further complications. IgA glomerulonephritis is rarely associated to hemolytic-uremic syndrome and malignant hypertension, with only a few previously described cases. We present an overview of potential pathophysiological connections between these diseases.

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

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

The C5a receptor has a key role in immune complex glomerulonephritis in complement factor H-deficient mice

Chronic serum sickness leads to the formation of glomerular immune complexes; however, C57BL/6 mice do not develop glomerulonephritis unless complement factor H (CFH) is absent from the plasma. Here we studied the role for C5a receptor (R) in this setting. The exaggerated humoral immune response in CFH^−/− mice was normalized in CFH^−/−C5aR^−/− double knockout mice, highlighting the C5aR dependence. The CFH knockout mice developed proliferative glomerulonephritis with endocapillary F4/80⁺ macrophage infiltration, a process reduced in the double knockout mice. There was no interstitial inflammation by histologic criteria or flow cytometry for F4/80⁺Ly6C^hiCCR2^hi inflammatory macrophages. There were, however, more interstitial CD3⁺CD4⁺ T lymphocytes in CFH knockout mice with chronic serum sickness, while double knockout mice had greater than 5-fold more Ly6C^loCCR2^lo anti-inflammatory macrophages compared to the CFH knockout mice. Mice lacking C5aR were significantly protected from functional renal disease as assessed by blood urea nitrogen levels. Thus, IgG- and iC3b-containing immune complexes are not inflammatory in C57BL/6 mice. Yet when these mice lack CFH, sufficient C3b persists in glomeruli to generate C5a and activate C5aR.

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

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

Structural and functional characterization of the product of disease-related factor H gene conversion

Numerous complement factor H (FH) mutations predispose patients to atypical hemolytic uremic syndrome (aHUS) and other disorders arising from inadequately regulated complement activation. No unifying structural or mechanistic consequences have been ascribed to these mutants beyond impaired self-cell protection. The S1191L and V1197A mutations toward the C-terminus of FH, which occur in patients singly or together, arose from gene conversion between CFH encoding FH and CFHR1 encoding FH-related 1. We show that neither single nor double mutations structurally perturbed recombinant proteins consisting of the FH C-terminal modules, 19 and 20 (FH19-20), although all three FH19-20 mutants were poor, compared to wild-type FH19-20, at promoting hemolysis of C3b-coated erythrocytes through competition with full-length FH. Indeed, our new crystal structure of the S1191L mutant of FH19-20 complexed with an activation-specific complement fragment, C3d, was nearly identical to that of the wild-type FH19-20:C3d complex, consistent with mutants binding to C3b with wild-type-like affinity. The S1191L mutation enhanced thermal stability of module 20, whereas the V1197A mutation dramatically decreased it. Thus, although mutant proteins were folded at 37 °C, they differ in conformational rigidity. Neither single substitutions nor double substitutions increased measurably the extent of FH19-20 self-association, nor did these mutations significantly affect the affinity of FH19-20 for three glycosaminoglycans, despite critical roles of module 20 in recognizing polyanionic self-surface markers. Unexpectedly, FH19-20 mutants containing Leu1191 self-associated on a heparin-coated surface to a higher degree than on surfaces coated with dermatan or chondroitin sulfates. Thus, potentially disease-related functional distinctions between mutants, and between FH and FH-related 1, may manifest in the presence of specific glycosaminoglycans.

Factor h: a complement regulator in health and disease, and a mediator of cellular interactions

Complement is an essential part of innate immunity as it participates in host defense against infections, disposal of cellular debris and apoptotic cells, inflammatory processes and modulation of adaptive immune responses. Several soluble and membrane-bound regulators protect the host from the potentially deleterious effects of uncontrolled and misdirected complement activation. Factor H is a major soluble regulator of the alternative complement pathway, but it can also bind to host cells and tissues, protecting them from complement attack. Interactions of factor H with various endogenous ligands, such as pentraxins, extracellular matrix proteins and DNA are important in limiting local complement-mediated inflammation. Impaired regulatory as well as ligand and cell recognition functions of factor H, caused by mutations or autoantibodies, are associated with the kidney diseases: atypical hemolytic uremic syndrome and dense deposit disease and the eye disorder: age-related macular degeneration. In addition, factor H binds to receptors on host cells and is involved in adhesion, phagocytosis and modulation of cell activation. In this review we discuss current concepts on the physiological and pathophysiological roles of factor H in light of new data and recent developments in our understanding of the versatile roles of factor H as an inhibitor of complement activation and inflammation, as well as a mediator of cellular interactions. A detailed knowledge of the functions of factor H in health and disease is expected to unravel novel therapeutic intervention possibilities and to facilitate the development or improvement of therapies.