Molecular basis of subtotal complement C6 deficiency. A carboxy-terminally truncated but functionally active C6

The ouroboros of autoimmunity

Human autoimmunity against elements conferring protective immunity can be symbolized by the 'ouroboros', a snake eating its own tail. Underlying infection is autoimmunity against three immunological targets: neutrophils, complement and cytokines. Autoantibodies against neutrophils can cause peripheral neutropenia underlying mild pyogenic bacterial infections. The pathogenic contribution of autoantibodies against molecules of the complement system is often unclear, but autoantibodies specific for C3 convertase can enhance its activity, lowering complement levels and underlying severe bacterial infections. Autoantibodies neutralizing granulocyte-macrophage colony-stimulating factor impair alveolar macrophages, thereby underlying pulmonary proteinosis and airborne infections, type I interferon viral diseases, type II interferon intra-macrophagic infections, interleukin-6 pyogenic bacterial diseases and interleukin-17A/F mucocutaneous candidiasis. Each of these five cytokine autoantibodies underlies a specific range of infectious diseases, phenocopying infections that occur in patients with the corresponding inborn errors. In this Review, we analyze this ouroboros of immunity against immunity and posit that it should be considered as a factor in patients with unexplained infection.

Host genetic susceptibility underlying SARS-CoV-2-associated Multisystem Inflammatory Syndrome in Brazilian Children

BACKGROUND

Multisystem Inflammatory Syndrome in Children (MIS-C) is a life-threatening complication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, which manifests as a hyper inflammatory process with multiorgan involvement in predominantly healthy children in the weeks following mild or asymptomatic coronavirus disease 2019 (COVID-19). However, host monogenic predisposing factors to MIS-C remain elusive.

METHODS

Herein, we used whole exome sequencing (WES) on 16 MIS-C Brazilian patients to identify single nucleotide/InDels variants as predisposition factors associated with MIS-C.

RESULTS

We identified ten very rare variants in eight genes (FREM1, MPO, POLG, C6, C9, ABCA4, ABCC6, and BSCL2) as the most promising candidates to be related to a higher risk of MIS-C development. These variants may propitiate a less effective immune response to infection or trigger the inflammatory response or yet a delayed hyperimmune response to SARS-CoV-2. Protein-Protein Interactions (PPIs) among the products of the mutated genes revealed an integrated network, enriched for immune and inflammatory response mechanisms with some of the direct partners representing gene products previously associated with MIS-C and Kawasaki disease (KD). In addition, the PPIs direct partners are also enriched for COVID-19-related gene sets. HLA alleles prediction from WES data allowed the identification of at least one risk allele in 100% of the MIS-C patients.

CONCLUSIONS

This study is the first to explore host MIS-C-associated variants in a Latin American admixed population. Besides expanding the spectrum of MIS-C-associated variants, our findings highlight the relevance of using WES for characterising the genetic interindividual variability associated with COVID-19 complications and ratify the presence of overlapping/convergent mechanisms among MIS-C, KD and COVID-19, crucial for future therapeutic management.

Comprehensive Update and Revision of Nomenclature on Complement C6 and C7 Variants

Complement genes encompass a wide array of variants, giving rise to numerous protein isoforms that have often been shown to exhibit clinical significance. Given that these variants have been discovered over a span of 50 y, one challenging consequence is the inconsistency in the terminology used to classify them. This issue is prominently evident in the nomenclature used for complement C6 and C7 variants, for which we observed a great discrepancy between previously published works and variants described in current genome browsers. This report discusses the causes for the discrepancies in C6 and C7 nomenclature and seeks to establish a classification system that would unify existing and future variants. The inconsistency in the methods used to annotate amino acids and the modifications pinpointed in the C6 and C7 primers are some of the factors that contribute greatly to the discrepancy in the nomenclature. Several variants that were classified incorrectly are highlighted in this report, and we showcase first-hand how a unified classification system is important to match previous with current genetic information. Ultimately, we hope that the proposed classification system of nomenclature becomes an incentive for studies on complement variants and their physiological and/or pathological effects.

Identification of a novel mutation in the C6 gene of a Han Chinese C6SD child with meningococcal disease

Deficiency of the sixth complement component (C6D) is a genetic disease associated with increased susceptibility to Neisseria meningitides infection. Individuals with C6D usually present with recurrent meningococcal disease (MD). According to the patients' C6 levels, C6D is divided into complete genetic deficiency of C6 and subtotal deficiency of C6 (C6SD). The present study reported on a Han Chinese pediatric patient with MD, in whom further investigation revealed a C6SD genetic lesion. A heterozygote nonsense mutation (c.1062C>G/p.Y354*) in the C6 gene was identified by Sanger sequencing. The mutation alters the tyrosine codon at position 354 to a termination codon and results in a truncated protein. In conclusion, the genetic lesion of a pediatric patient with C6SD who was diagnosed due to having MD was investigated and a novel pathogenic mutation in the C6 gene was identified. The study confirmed the clinical diagnosis for this patient with C6SD and also expanded the spectrum of C6 mutations.

Human genetics of meningococcal infections

Neisseria meningitidis is a leading cause of bacterial septicaemia and meningitis worldwide. Meningococcal disease is rare but can be life threatening with a tendency to affect children. Many studies have investigated the role of human genetics in predisposition to N. meningitidis infection. These have identified both rare single-gene mutations as well as more common polymorphisms associated with meningococcal disease susceptibility and severity. These findings provide clues to the pathogenesis of N. meningitidis, the basis of host susceptibility to infection and to the aetiology of severe disease. From the multiple discoveries of monogenic complement deficiencies to the associations of complement factor H and complement factor H-related three polymorphisms to meningococcal disease, the complement pathway is highlighted as being central to the genetic control of meningococcal disease. This review aims to summarise the current understanding of the host genetic basis of meningococcal disease with respect to the different stages of meningococcal infection.

Human inborn errors of immunity to infection affecting cells other than leukocytes: from the immune system to the whole organism

Studies of vertebrate immunity have traditionally focused on professional cells, including circulating and tissue-resident leukocytes. Evidence that non-professional cells are also intrinsically essential (i.e. not via their effect on leukocytes) for protective immunity in natural conditions of infection has emerged from three lines of research in human genetics. First, studies of Mendelian resistance to infection have revealed an essential role of DARC-expressing erythrocytes in protection against Plasmodium vivax infection, and an essential role of FUT2-expressing intestinal epithelial cells for protection against norovirus and rotavirus infections. Second, studies of inborn errors of non-hematopoietic cell-extrinsic immunity have shown that APOL1 and complement cascade components secreted by hepatocytes are essential for protective immunity to trypanosome and pyogenic bacteria, respectively. Third, studies of inborn errors of non-hematopoietic cell-intrinsic immunity have suggested that keratinocytes, pulmonary epithelial cells, and cortical neurons are essential for tissue-specific protective immunity to human papillomaviruses, influenza virus, and herpes simplex virus, respectively. Various other types of genetic resistance or predisposition to infection in human populations are not readily explained by inborn variants of genes operating in leukocytes and may, therefore, involve defects in other cells. The probing of this unchartered territory by human genetics is reshaping immunology, by scaling immunity to infection up from the immune system to the whole organism.

A versatile assay to determine bacterial and host factors contributing to opsonophagocytotic killing in hirudin-anticoagulated whole blood

Most bacteria entering the bloodstream will be eliminated through complement activation on the bacterial surface and opsonophagocytosis. However, when these protective innate immune systems do not work optimally, or when bacteria are equipped with immune evasion mechanisms that prevent killing, this can lead to serious infections such as bacteremia and meningitis, which is associated with high morbidity and mortality. In order to study the complement evasion mechanisms of bacteria and the capacity of human blood to opsonize and kill bacteria, we developed a versatile whole blood killing assay wherein both phagocyte function and complement activity can easily be monitored and modulated. In this assay we use a selective thrombin inhibitor hirudin to fully preserve complement activity of whole blood. This assay allows controlled analysis of the requirements for active complement by replacing or heat-inactivating plasma, phagocyte function and bacterial immune evasion mechanisms that contribute to survival in human blood.

Severe infectious diseases of childhood as monogenic inborn errors of immunity

This paper reviews the developments that have occurred in the field of human genetics of infectious diseases from the second half of the 20th century onward. In particular, it stresses and explains the importance of the recently described monogenic inborn errors of immunity underlying resistance or susceptibility to specific infections. The monogenic component of the genetic theory provides a plausible explanation for the occurrence of severe infectious diseases during primary infection. Over the last 20 y, increasing numbers of life-threatening infectious diseases striking otherwise healthy children, adolescents, and even young adults have been attributed to single-gene inborn errors of immunity. These studies were inspired by seminal but neglected findings in plant and animal infections. Infectious diseases typically manifest as sporadic traits because human genotypes often display incomplete penetrance (most genetically predisposed individuals remain healthy) and variable expressivity (different infections can be allelic at the same locus). Infectious diseases of childhood, once thought to be archetypal environmental diseases, actually may be among the most genetically determined conditions of mankind. This nascent and testable notion has interesting medical and biological implications.

A complement C5 gene mutation, c.754G>A:p.A252T, is common in the Western Cape, South Africa and found to be homozygous in seven percent of Black African meningococcal disease cases

Patients with genetically determined deficiency of complement component 5 are usually diagnosed because of recurrent invasive Neisseria meningitidis infections. Approximately 40 individual cases have been diagnosed worldwide. Nevertheless, reports of the responsible genetic defects have been sporadic, and we know of no previous reports of C5 deficiency being associated with a number of independent meningococcal disease cases in particular communities. Here we describe C5 deficiency in seven unrelated Western Cape, South African families. Three different C5 mutations c.55C>T:p.Q19X, c.754G>A:p.A252T and c.4426C>T:p.R1476X were diagnosed in index cases from two families who had both presented with recurrent meningococcal disease. p.Q19X and p.R1476X have already been described in North American Black families and more recently p.Q19X in a Saudi family. However, p.A252T was only reported in SNP databases and was not associated with disease until the present study was undertaken in the Western Cape, South Africa. We tested for p.A252T in 140 patients presenting with meningococcal disease in the Cape Town area, and found seven individuals in five families who were homozygous for the mutation p.A252T. Very low serum C5 protein levels (0.1-4%) and correspondingly low in vitro functional activity were found in all homozygous individuals. Allele frequencies of p.A252T in the Black African and Cape Coloured communities were 3% and 0.66% and estimated homozygosities are 1/1100 and 1/22,500 respectively. In 2012 we reported association between p.A252T and meningococcal disease. Molecular modelling of p.A252T has indicated an area of molecular stress in the C5 molecule which may provide a mechanism for the very low level in the circulation. This report includes seven affected families indicating that C5D is not rare in South Africa.

Association of candidate gene polymorphisms and TGF-beta/IL-10 levels with malaria in three regions of Cameroon: a case-control study

Background

Plasmodium falciparum malaria is one of the most widespread and deadliest infectious diseases in children under five years in endemic areas. The disease has been a strong force for evolutionary selection in the human genome, and uncovering the critical host genetic factors that confer resistance to the disease would provide clues to the molecular basis of protective immunity and improve vaccine development initiatives.

Methods

The effect of single nucleotide polymorphisms (SNPs) and plasma transforming growth factor beta (TGF-β) and interleukin 10 (IL-10) levels on malaria pathology was investigated in a case–control study of 1862 individuals from two major ethnic groups in three regions with intense perennial P. falciparum transmission in Cameroon. Thirty-four malaria candidate polymorphisms, including the sickle cell trait (HbS), were assayed on the Sequenom iPLEX platform while plasma TGF-β and IL-10 levels were measured by sandwich ELISA.

Results

The study confirms the known protective effect of HbS against severe malaria and also reveals a protective effect of SNPs in the nitrogen oxide synthase 2 (NOS2) gene against malaria infection, anaemia and uncomplicated malaria. Furthermore, ADCY9 rs10775349 (additive G) and ABO rs8176746 AC individuals were associated with protection from hyperpyrexia and hyperparasitaemia, respectively. Meanwhile, individuals with the EMR1 rs373533 GT, EMR1 rs461645 CT and RTN3 rs542998 (additive C) genotypes were more susceptible to hyperpyrexia while both females and males with the rs1050828 and rs1050829 SNPs of G6PD, respectively, were more vulnerable to anaemia. Plasma TGF-β levels were strongly correlated with heterozygosity for the ADCY9 rs2230739 and HBB rs334 SNPs while individuals with the ABO rs8176746 AC genotype had lower IL-10 levels.

Conclusion

Taken together, this study suggests that some rare polymorphisms in candidate genes may have important implications for the susceptibility of Cameroonians to severe malaria. Moreover using the uncomplicated malaria phenotype may permit the identification of novel pathways in the early development of the disease.

Compound heterozygous mutations in the C6 gene of a child with recurrent infections

The complement system plays an important role in both the innate and adaptive immune system. Patients with inherited complement deficiencies have an increased risk of systemic bacterial infections. Deficiencies of the terminal complement pathway are especially associated with invasive meningococcal disease. Here, we report a case of a boy that presented with arthritis and recurrent bacterial and viral infections. Extensive analyses revealed decreased complement activity of both classical and alternative pathway, indicating a deficiency of C3 or one of the factors of the terminal complement pathway. Mutational analysis of the C6 gene identified two compound heterozygous mutations. An unknown missense aberration was found that involves the loss of a cysteine, possibly affecting the 3D structure of the protein. Furthermore, a known splice site variation was identified that results in a 14% shorter protein, due to transcription of amino acids that are normally intronic until a stop codon is reached (exon-intron boundary defect). It is known that the protein with this latter aberration is still functionally active when present with other C6 mutations and therefore, the consequences of the combination of the identified variations have been studied. Quantitative ELISAs showed that at least one allele produced a circulating C6 molecule that can be incorporated in the membrane attack complex, likely the truncated protein. In the present case we observed relapsing bacterial and viral infections, but no meningococcal disease. The reduced complement activity can be explained by the identified genetic variations in C6, as recombinant C6 supplementation corrected complement function in vitro.

Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC)

The complement membrane attack complex (MAC) is formed by the sequential assembly of C5b with four homologous proteins as follows: one copy each of C6, C7, and C8 and 12-14 copies of C9. Together these form a lytic pore in bacterial membranes. C6 through C9 comprise a MAC-perforin domain flanked by 4-9 "auxiliary" domains. Here, we report the crystal structure of C6, the first and longest of the pore proteins to be recruited by C5b. Comparisons with the structures of the C8αβγ heterodimer and perforin show that the central domain of C6 adopts a "closed" (perforin-like) state that is distinct from the "open" conformations in C8. We further show that C6, C8α, and C8β contain three homologous subdomains ("upper," "lower," and "regulatory") related by rotations about two hinge points. In C6, the regulatory segment includes four auxiliary domains that stabilize the closed conformation, inhibiting release of membrane-inserting elements. In C8β, rotation of the regulatory segment is linked to an opening of the central β-sheet of its clockwise partner, C8α. Based on these observations, we propose a model for initiation and unidirectional propagation of the MAC in which the auxiliary domains play key roles: in the assembly of the C5b-8 initiation complex; in driving and regulating the opening of the β-sheet of the MAC-performin domain of each new recruit as it adds to the growing pore; and in stabilizing the final pore. Our model of the assembled pore resembles those of the cholesterol-dependent cytolysins but is distinct from that recently proposed for perforin.

Complement deficiency states and associated infections

A major function of the immune system is to protect the host from microbial infections. The complement system plays important roles in both the innate and the adaptive immune defense and also acts as a bridge between these arms of immunity. This is obvious from complement deficiencies which in varying degree, depending on which factor is missing, are associated with increased infection susceptibility and also increased risk for other, mainly autoimmune diseases. Genetically determined deficiencies are described for almost all complement proteins but the consequences show a wide variation. Here the genetic defects and molecular abnormalities in complement deficient persons, related clinically relevant infections and the options for prevention and therapy are reviewed. The roles of complement in host defense against common infections are also discussed.

Infections of people with complement deficiencies and patients who have undergone splenectomy

The complement system comprises several fluid-phase and membrane-associated proteins. Under physiological conditions, activation of the fluid-phase components of complement is maintained under tight control and complement activation occurs primarily on surfaces recognized as "nonself" in an attempt to minimize damage to bystander host cells. Membrane complement components act to limit complement activation on host cells or to facilitate uptake of antigens or microbes "tagged" with complement fragments. While this review focuses on the role of complement in infectious diseases, work over the past couple of decades has defined several important functions of complement distinct from that of combating infections. Activation of complement in the fluid phase can occur through the classical, lectin, or alternative pathway. Deficiencies of components of the classical pathway lead to the development of autoimmune disorders and predispose individuals to recurrent respiratory infections and infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. While no individual with complete mannan-binding lectin (MBL) deficiency has been identified, low MBL levels have been linked to predisposition to, or severity of, several diseases. It appears that MBL may play an important role in children, who have a relatively immature adaptive immune response. C3 is the point at which all complement pathways converge, and complete deficiency of C3 invariably leads to severe infections, including those caused by meningococci and pneumococci. Deficiencies of the alternative and terminal complement pathways result in an almost exclusive predisposition to invasive meningococcal disease. The spleen plays an important role in antigen processing and the production of antibodies. Splenic macrophages are critical in clearing opsonized encapsulated bacteria (such as pneumococci, meningococci, and Escherichia coli) and intraerythrocytic parasites such as those causing malaria and babesiosis, which explains the fulminant nature of these infections in persons with anatomic or functional asplenia. Paramount to the management of patients with complement deficiencies and asplenia is educating patients about their predisposition to infection and the importance of preventive immunizations and seeking prompt medical attention.

Complement in human diseases: Lessons from complement deficiencies

Complement deficient cases reported in the second half of the last century have been of great help in defining the role of complement in host defence. Surveys of the deficient individuals have been instrumental in the recognition of the clinical consequences of the deficiencies. This review focuses on the analysis of the diseases associated with the deficiencies of the various components and regulators of the complement system and their therapeutic implications. The diagnostic approach leading to the identification of the deficiency is discussed here as a multistep process that starts with the screening assays and proceeds in specialized laboratories with the characterization of the defect at the molecular level. The organization of a registry of complement deficiencies is presented as a means to collect the cases identified in and outside Europe with the aim to promote joint projects on treatment and prevention of diseases associated with defective complement function.

Hereditary complement C7 deficiency in nine families: Subtotal C7 deficiency revisited

Deficiencies in terminal complement components, including the component C7, are uncommon and associated with an increased risk of recurrent systemic neisserial infection. A total of 22 molecular defects have been reported in the C7 gene with both complete (C7Q0) and subtotal (C7SD) C7 deficiencies. In this study we report the molecular basis of nine new cases of C7 deficiencies that were characterized by exon-specific sequence analysis. Seven different C7 gene mutations were identified corresponding to small deletions (n=2), splice site changes (n=1) and single base pair substitutions leading to nonsense (n=1) or missense (n=3) mutations. Altogether, three changes of the C7 gene (G357R, R499S and 5' splice donor site of intron 16) account for half of the molecular defects which emphasize that a restricted number of molecular abnormalities are involved in this deficiency. We identified two patients with combined C7Q0/C7SD(R499S) and established the C7SD(R499S) frequency at about 1% in normal Caucasian population. We demonstrated that C7(R499S) mutant protein is retained in the endoplasmic reticulum whereas the wild-type C7 is located in the Golgi apparatus. Our results provide evidence that R499S represents a loss-of-function polymorphism of C7 due to a defective folding of the protein.

Prevalence of mutations leading to complete C6 deficiency (C6Q0) in the Western Cape, South Africa and detection of novel mutations leading to C6Q0 in an Irish family

Complement component C6 is one of five terminal complement components incorporated into the membrane attack complex. Complete deficiency of C6 (C6Q0) leads to an increased susceptibility to Neisseria meningitidis infections, and affected individuals typically present with recurrent meningococcal disease. There is a relatively high prevalence of C6Q0 in the Western Cape, South Africa and three frameshift mutations have previously been described to be responsible for C6Q0 in this area-879delG, 1195delC, and 1936delG (current nomenclature). We have now genotyped a further nine genetically independent individuals with C6Q0, confirming previous reports that the most common defect in the Western Cape is 879delG. Moreover, we report the first identification of the 878delA mutation within the Western Cape, which has previously only been reported in individuals of African descent living in the United States or Europe. We also investigated the genotype of an Irish C6Q0 individual and her sibling, and report two previously undescribed mutations. One mutation alters a tyrosine codon to a stop codon within exon 10. The second mutation is within the 5' donor splice site of intron 3, and would, in all probability, disrupt splicing. These two mutations were shown to segregate independently. We also discuss the nomenclature for reporting C6 and C7 gene mutations, as the current nomenclature does not follow the recognised guidelines.

Recombinant C345C and factor I modules of complement components C5 and C7 inhibit C7 incorporation into the complement membrane attack complex

Complement component C5 binds to components C6 and C7 in reversible reactions that are distinct from the essentially nonreversible associations that form during assembly of the complement membrane attack complex (MAC). We previously reported that the approximately 150-aa residue C345C domain (also known as NTR) of C5 mediates these reversible reactions, and that the corresponding recombinant module (rC5-C345C) binds directly to the tandem pair of approximately 75-residue factor I modules from C7 (C7-FIMs). We suggested from these and other observations that binding of the C345C module of C5 to the FIMs of C7, but not C6, is also essential for MAC assembly itself. The present report describes a novel method for assembling a complex that appears to closely resemble the MAC on the sensor chip of a surface plasmon resonance instrument using the complement-reactive lysis mechanism. This method provides the ability to monitor individually the incorporation of C7, C8, and C9 into the complex. Using this method, we found that C7 binds to surface-bound C5b,6 with a K(d) of approximately 3 pM, and that micromolar concentrations of either rC5-C345C or rC7-FIMs inhibit this early step in MAC formation. We also found that similar concentrations of either module inhibited complement-mediated erythrocyte lysis by both the reactive lysis and classical pathway mechanisms. These results demonstrate that the interaction between the C345C domain of C5 and the FIMs of C7, which mediates reversible binding of C5 to C7 in solution, also plays an essential role in MAC formation and complement lytic activity.

Complement component C7 deficiency in two Spanish families

Different genetic mutations have been described in complement component C7 deficiency, a molecular defect clinically associated with an increased susceptibility to neisserial recurrent infections. In this work we report the genetic basis of C7 deficiency in two different Spanish families (family 1 and family 2). In family 1, of Gypsy ethnical background, exon-specific polymerase chain reaction and sequencing revealed a not previously described single base deletion of nucleotide 1309 (exon 10) in the patient, as well as in her father, leading to a stop codon that causes the premature truncation of the C7 protein (K416 X 419). Additionally, the patient and her mother displayed a missense mutation at position 1135 (exon 9) located in the first nucleotide of the codon GGG (CGG), resulting in a change of amino acid (G357R). This mutation was firstly described in individuals of Moroccan Sephardic Jewish ancestry and has been also reported among Spaniards. In family 2, another novel mutation was found in homozygosity in two siblings; a two base-pair deletion of nucleotides 1922 and 1923 in exon 14 leading to the generation of a downstream stop codon causing the truncation of the C7 protein product (S620 X 630). Our results provide more evidence for the heterogeneous molecular basis of C7 deficiency as well as for the subsequent susceptibility to meningococcal disease, since different families carry different molecular defects. On the other hand, certain C7 defects appear to be prevalent in individuals from certain populations or living in defined geographical areas.

Complement Components C5 and C7: Recombinant Factor I Modules of C7 Bind to the C345C Domain of C5

Studies reported over 30 years ago revealed that latent, nonactivated C5 binds specifically and reversibly to C6 and C7. These reversible reactions are distinct from the essentially nonreversible associations with activated C5b that occur during assembly of the membrane attack complex, but they likely involve some, perhaps many, of the same molecular contacts. We recently reported that these reversible reactions are mediated by the C345C (NTR) domain at the C terminus of the C5 alpha-chain. Earlier work by others localized the complementary binding sites to a tryptic fragment of C6 composed entirely of two adjacent factor I modules (FIMs), and to a larger fragment of C7 composed of its homologous FIMs as well as two adjoining short consensus repeat modules. In this work, we expressed the tandem FIMs from C7 in bacteria. The mobility on SDS-polyacrylamide gels, lack of free sulfhydryl groups, and atypical circular dichroism spectrum of the recombinant product rC7-FIMs were all consistent with a native structure. Using surface plasmon resonance, we found that rC7-FIMs binds specifically to both C5 and the rC5-C345C domain with K(D) approximately 50 nM, and competes with C7 for binding to C5, as expected for an active domain. These results indicate that, like C6, the FIMs alone in C7 mediate reversible binding to C5. Based on available evidence, we suggest a model for an irreversible membrane attack complex assembly in which the C7 FIMs, but not those in C6, are bound to the C345C domain of C5 within the fully assembled complex.

Expression and characterization of the C345C/NTR domains of complement components C3 and C5

Complement components C3, C4, and C5 are members of the thioester-containing alpha-macroglobulin protein superfamily. Within this superfamily, a unique feature of the complement proteins is a 150-residue-long C-terminal extension of their alpha-subunits that harbors three internal disulfide bonds. Previous reports have suggested that this is an independent structural module, homologous to modules found in other proteins, including netrins and tissue inhibitors of metalloproteinases. Because of its distribution, this putative module has been named both C345C and NTR. To assess the structures of these segments of the complement proteins, their relationships with other domains, and activities as independent structures, we expressed C345C from C3 and C5 in a bacterial strain that permits cytoplasmic disulfide bond formation. Affinity purification directly from cell lysates yielded recombinant C3- and C5-C345C with properties consistent with multiple intramolecular disulfide bonds and high beta-sheet contents. rC5-, but not rC3-C345C inhibited complement hemolytic activity, and surface plasmon resonance studies revealed that rC5-C345C binds to complement components C6 and C7 with dissociation constants of 10 and 3 nM, respectively. Our results provide strong evidence that this binding corresponds to the previously described reversible binding of C5 to C6 and C7, and taken together with earlier work, indicate that the C5-C345C module interacts directly with the factor I modules in C6 and C7. The high binding affinities suggest that complexes composed of C5 bound to C6 or C7 exist in plasma before activation and may facilitate assembly of the complement membrane attack complex.

Complement component C7 deficiency in a Spanish family

Different genetic mutations have been described in complement component C7 deficiency, a molecular defect which is clinically associated with an increased susceptibility to neisserial recurrent infections, although some cases remain asymptomatic. In this work we report the genetic bases of C7 deficiency in one Spanish family. Exon-specific PCR and sequencing revealed a novel point mutation at nucleotide 615 (exon 6) leading to a stop codon (UGG to UGA) in the patient, his mother, and sister. This transversion causes the premature truncation of the C7 protein (W183X). Additionally, we detected a missense mutation at position 1135 (exon 9) located in the first nucleotide of the codon GGG (CGG), resulting in an amino acid change (G357R) in the patient, his father, as well as in his sister. This latter mutation had been previously described in individuals from Moroccan Sephardic Jewish ancestry. Since both heterozygous mutations were found in the patient as well as in his asymptomatic sister, we analyse other meningococcal defence mechanisms such as polymorphisms of the opsonin receptors on polymorphonuclear cells. Results showed that the patient and his sister bore identical combinations of FcgammaRIIA-H/R131 and FcgammaRIIIB-NA1/2 allotypes. Our results provide further evidence that the molecular pathogenesis of C7 deficiency as well as susceptibility to meningococcal disease are heterogeneous, since different families carry different molecular defects, although many of the C7 defects appear to be homogeneous in individuals from certain geographical areas. The missense mutation G357R would make an interesting topic of analysis with regard to meningococcal disease susceptibility in the Spanish population.

Restricted genetic defects underlie human complement C6 deficiency

Complement C6 homozygous deficiency (C6D) has been rarely observed in Caucasians but was reported at higher prevalence among African-Americans. We report on the molecular basis of C6D in seven unrelated black individuals of North or Central Africa descent who live in France. These patients have presented Neisseria meningitidis infection (four cases), focal and segmental glomerulosclerosis with hyalinosis (one case), systemic lupus erythematosus (one case) or Still's disease (one case). All patients exhibited undetectable antigenic C6 by using a sensitive ELISA assay. An additional four cases of complete C6 deficiency with no associated disease have been characterized after family studies. Exons 6, 7 and 12 have been described recently as the location of molecular defects on the C6 gene in randomly chosen black Americans. Genomic DNA from the seven patients were subjected to direct polymerase chain reaction amplification of these three exons. Nucleotide sequencing analysis of the amplified DNA fragments revealed a homozygous single-base deletion (1936delG) in exon 12 in three cases and four compound heterozygous deletions for a single base in exon 7 (1195delC) or in exon 6 (878delA) associated with the same deletion in exon 12 (1936delG). Our observations further establish the restricted pattern of genetic defects associated with homozygous C6 complement deficiency in individuals of African descent.

Function of the factor I modules (FIMS) of human complement component C6

In order to elucidate the function of complement component C6, truncated C6 molecules were expressed recombinantly. These were either deleted of the factor I modules (FIMs) (C6des-748-913) or both complement control protein (CCP) modules and FIMs (C6des-611-913). C6des-748-913 exhibited approximately 60-70% of the hemolytic activity of full-length C6 when assayed for Alternative Pathway activity, but when measured for the Classical Pathway, C6des-748-914 was only 4-6% as effective as C6. The activity difference between C6 and C6des-748-913 for the two complement pathways can be explained by a greater stability of newly formed metastable C5b* when produced by the Alternative Pathway compared with that made by the Classical Pathway. The half-lives of metastable C5b* and the decay of (125)I-C5b measured from cells used to activate the Alternative Pathway were found to be about 5-12-fold longer than those same parameters derived from cells that had activated the Classical Pathway. (125)I-C5 binds reversibly to C6 in an ionic strength-dependent fashion, but (125)I-C5 binds only weakly to C6des-FIMs and not at all to C6des-CCP/FIMs. Therefore, although the FIMs are not required absolutely for C6 activity, these modules promote interaction of C6 with C5 enabling a more efficient bimolecular coupling ultimately leading to the formation of the C5b-6 complex.

Genetic Basis of Human Complement C8α-γ Deficiency

Deficiency of the α-γ subunit of the eighth component of complement (C8α-γD) is frequently associated with recurrent neisserial infections, especially meningitis caused by Neisseria meningitidis. We here report the molecular basis of C8α-γD in two unrelated Japanese subjects. Screening all 11 exons of the C8α gene and all 7 exons of the C8γ gene and their boundaries by exon-specific PCR/single-strand conformation polymorphism demonstrated aberrant single-stranded DNA fragments in exon 2 of C8α gene in case 1 and in exons 2 and 9 of C8α gene in case 2. Nucleotide sequencing of the amplified DNA fragments in case 1 revealed a homozygous single-point mutation at the second exon-intron boundary, inactivating the universally conserved 5′ splice site consensus sequence of the second intron (IVS2+1G→T). Case 2 was a compound heterozygote for the splice junction mutation, IVS2+1G→T, and a nonsense mutation at Arg394 (R394X). R394X was caused by a C to T transition at nucleotide 1407, the first nucleotide of the codon CGA for Arg394, leading to a stop codon TGA. No mutations were detected in the C8γ gene by our method. Our results indicate that the pathogenesis of C8α-γD might be caused by heterogeneous molecular defects in the C8α gene.

A Non-Sense Mutation at Arg95 Is Predominant in Complement 9 Deficiency in Japanese

Deficiency of the ninth component of complement (C9D) is one of the most common genetic abnormalities in Japan, with an incidence of one homozygote in 1000. Although C9D individuals are usually healthy, it has been shown that they have an significantly increased risk of developing meningococcal meningitis. In the present study we report the molecular bases for C9D in 10 unrelated Japanese subjects. As a screening step for mutations, exons 2 to 11 of the C9 gene were analyzed using exon-specific PCR/single-strand conformation polymorphism analysis, which demonstrated aberrantly migrating DNA bands in exon 4 in all the C9D subjects. Subsequent direct sequencing of exon 4 of the C9D subjects revealed that eight of the 10 C9D subjects were homozygous for a C to T transition at nucleotide 343, the first nucleotide of the codon CGA for Arg95, leading to a TGA stop codon (R95X). R95X is a novel mutation different from those recently identified in a Swiss family with C9D. Cases 6 and 7 were heterozygous for the R95X mutation. Family study in case 10 confirmed the genetic nature of the defect. In case 6, the second mutation for C9D of the C9 gene was identified to be the substitution of Cys to Tyr at amino acid residue 507 (C507Y), while the genetic defect(s) in the other allele in case 7 remains unknown. Our results indicate that a novel mutation, R95X, is present in most cases of C9D in Japan.

Elucidation of the disulfide-bonding pattern in the factor I modules of the sixth component (C6) of human complement

Complement component C6 is known to contain two factor I modules in tandem at its C-terminus. To localize the disulfide bridges in those domains, native C6 was cleaved with trypsin, followed by subtilisin. The resulting digests were separated by reversed-phase HPLC, and all of the potential cystine-containing fragments were detected by a fluorescence assay and amino acid composition analyses. Final identification of the disulfide bonds was achieved by Edman degradation of the corresponding peptides. From the data gained a 1-3, 2-9, 4-7, 5-10, 6-8 pattern was determined (Cys752-Cys802, Cys763-Cys780, Cys765-Cys816, Cys772-Cys795, Cys841-Cys852, Cys846-Cys898, Cys859-Cys876, Cys861-Cys911, Cys867-Cys891). These findings are compared with the strongly related complement components C7 and factor I.

How partial C7 deficiency with chronic and recurrent bacterial infections can mimic total C7 deficiency: temporary restoration of host C7 levels following plasma transfusion

An apparently completely complement C7-deficient patient with refractory otitis media and two episodes of meningococcal disease was given therapeutic plasma transfusions in 1992 and 1994. Following these transfusions unexpected changes were found in C7 levels. Immediately after transfusion the serum C7 levels failed to rise to the expected levels but then rose to 5-10% of the normal mean during the next 5 days and remained at that level for more than 2 weeks before eventually returning to zero. The patient's DNA genotyped C7 M, and therefore C7 N donor plasma was selected for the second transfusion to allow identification of the source of the C7 circulating post-transfusion. This C7 phenotyped C7 M, demonstrating it to be of recipient origin. Therefore, the apparently completely C7-deficient patient was able to secrete some C7. By a combination of DNA typing and isoelectric focusing of the C7 appearing after transfusion, it was demonstrated that the patient was heterozygous for combined subtotal C6/C7 deficiency (inherited from his father) and a different, so far uncharacterized, subtotal C7 deficiency (inherited from his mother). The low amount of C7 secreted appeared to be constantly consumed, probably by generation of C5b6 as a result of his chronic infection. He had been shown to have circulating C5b6 most of the time, and thus only when sufficient exogenous C7 was given to consume the free C5b6 did his own C7 appear in circulation.

Importance of the third thrombospondin repeat of C6 for terminal complement complex assembly

The anti-C6 monoclonal antibody WU 6-4 was shown to be unequivocally native restricted since it neither binds to the terminal complement complex (TCC) nor to C5b6. In addition, it was shown to inhibit TCC formation by interfering with C5b6 generation. Using the pUEX expression system and C6 cDNA the WU 6-4 epitope was mapped to the third thrombospondin repeat of C6. This protein domain may therefore contribute to the C5 binding site of C6 and be involved in terminal complement complex assembly. The presence of the epitope in rabbit C6 indicates a useful model for studying inhibition of TCC formation in vivo.