Hereditary deficiency of the sixth component of complement in man. I. Immunochemical, biologic, and family studies

Acquired decrease of the C3b/C4b receptor (CR1, CD35) and increased C4d deposits on erythrocytes from ICU COVID-19 patients

In order to study the mechanisms of COVID-19 damage following the complement activation phase occurring during the innate immune response to SARS-CoV-2, CR1 (the regulating complement activation factor, CD35, the C3b/C4b receptor), C4d deposits on Erythrocytes (E), and the products of complement activation C3b/C3bi, were assessed in 52 COVID-19 patients undergoing O2 therapy or assisted ventilation in ICU units in Rheims France. An acquired decrease of CR1 density on E from COVID-19 patients was observed (Mean = 418, SD = 162, N = 52) versus healthy individuals (Mean = 592, SD = 287, N = 400), Student's t-test p < 10-6, particularly among fatal cases, and in parallel with several parameters of clinical severity. Large deposits of C4d on E in patients were well above values observed in normal individuals, mostly without concomitant C3 deposits, in more than 80% of the patients. This finding is reminiscent of the increased C4d deposits on E previously observed to correlate with sub endothelial pericapillary deposits in organ transplant rejection, and with clinical SLE flares. Conversely, significant C3 deposits on E were only observed among ¼ of the patients. The decrease of CR1/E density, deposits of C4 fragments on E and previously reported detection of virus spikes or C3 on E among COVID-19 patients, suggest that the handling and clearance of immune complex or complement fragment coated cell debris may play an important role in the pathophysiology of SARS-CoV-2. Measurement of C4d deposits on E might represent a surrogate marker for assessing inflammation and complement activation occurring in organ capillaries and CR1/E decrease might represent a cumulative index of complement activation in COVID-19 patients. Taken together, these original findings highlight the participation of complement regulatory proteins and indicate that E are important in immune pathophysiology of COVID-19 patients. Besides a potential role for monitoring the course of disease, these observations suggest that novel therapies such as the use of CR1, or CR1-like molecules, in order to down regulate complement activation and inflammation, should be considered.

The human model: a genetic dissection of immunity to infection in natural conditions

Tremendous progress has been achieved in developmental, cellular and molecular immunology in the past 20 years, largely due to studies using the mouse as a model system and the arrival of molecular genetics. Immunology is now faced with a difficult challenge. What are the functions of the individual cells and molecules in achieving immunity to infection? Renewed interest in animal models of disease has provided considerable insight in this area, but such models of infection suffer from the inherent limitation of being experimental. In humans, the complex host-environment interaction occurs in natural, as opposed to experimental, conditions. The human model is therefore an indispensable complement to animal models, as it allows an observational genetic dissection of immunity to infection.

[Deficiency of the 6th component of complement. Recurrent meningococcal and streptococcal meningitis]

A 39-year old male patient with a history of four episodes of meningitis (two of them due to Neisseria meningitidis) since the age of 12 developed meningitis caused by an ungroupable streptococcus. Deficiency of the sixth component of complement was discovered, making this the ninth case of recurrent meningitis associated with C6 deficiency. Streptococcal meningitis had never been reported in such cases.

Deficiency of the sixth component of complement and susceptibility to Neisseria meningitidis infections: studies in 10 families and five isolated cases

Complement component C6 deficiency (C6D) was diagnosed in 15 patients who presented, independently, with recurrent meningococcal infection. This condition is thus not particularly rare in the Cape. Ten of the patients belonged to multiplex families, and family studies led to the diagnosis of another 12 C6D cases among the siblings. Segregation analysis showed that C6D occurred more frequently among the siblings of affected individuals than would be expected for co-dominant inheritance. The possible reasons for this are discussed. We also observed that the 12 non-proband C6D siblings included only four with a history suggestive of meningococcal infection, and thus C6D individuals apparently differ in susceptibility to Neisseria meningitidis infection. We confirmed previous observations that primary infection occurs later in C6D individuals than amongst susceptible complement-sufficient individuals. Among 123 patients presenting with primary meningitis, one case of C6D was diagnosed. The data show that C6D is an important factor associated with susceptibility to meningococcal infection in the Cape.

[Recurrent meningitis in familial deficiency of the 8th component of the complement system]

An 18-year-old man suffered from recurrent bacterial meningitis. Investigation of the complement system revealed deficiency of the 8th complement component (C8) in the patient and his sister. Genetic defects of the terminal complement components C5 to C8 predispose to Neisseria infections, probably due to a lack in bacteriolytic activity. It is to be noted that 1 year ago the patient had been hospitalized for a culture-proved pneumococcal meningitis.

Inherited deficiencies of complement components in man

Isolated inherited deficiency states of almost every complement protein have been recognized. Almost all are autosomal recessive traits. Deficiency of the early-acting components C1, C4 and C2 is associated with increased risk of immune complex disease, particularly systemic lupus erythematosus. Patients with deficiency of C3, factor I or factor H have increased susceptibility to infection by pyogenic bacteria, whereas those with deficiencies of properdin, C5, C6, C7 or C8 are prone to systemic neisserial infection. Inherited deficiency of C1 inhibitor is transmitted as an autosomal dominant trait, is genetically heterogeneous, and is associated with attacks of angioedema and consumption of C4 and C2. There is evidence that a plasmin-modified fragment of C2 is responsible for the angioedema in this disorder. Administration of androgens tends to correct the biochemical abnormalities of hereditary angioedema and to prevent attacks.

Inherited disorders of complement

Isolated complement component deficiencies are uncommon. Deficiencies of all eleven components and two inhibitors of the classical pathway have been described. Complete absence of the components of the alternative pathway has not been described. The consequences of a single defect in complement are often predictable from an understanding of the biologic activities associated with activation of the complement system. Deficiency of C1 esterase inhibitor gives rise to the disease, hereditary angioedema; deficiency of the early components of the classical pathway are associated with lupus erythematosus; C3 and C3 inactivator deficiencies with pyogenic infections; C5 dysfunction with Leiner's disease; deficiencies of the terminal components with recurrent Neisseria bacteremia; and C9 deficiency with normal health. The complement system and its associated biologic activities are reviewed. The present knowledge of the inherited complement deficiencies and associated diseases, with particular emphasis on the dermatologic manifestations, genetics, and diagnosis, is summarized.

Combined hereditary deficiency of the sixth component of complement and factor VIII coagulant activity in a Dutch family

Prompted by previous observations of defective blood clotting in rabbits deficient in the sixth component of complement (C6), and the discovery of a patient with both C6 and factor VIII deficiency, an evaluation was made of the haemostatic functions in this individual and his family members. The family contained three members homozygous for C6 deficiency (C6D); two of them were deficient also in factor VIII. In addition, one other member of the family was only deficient in factor VIII. The only C6D member without haemophilia A had a normal recalcification time without clinical symptoms of a bleeding disorder. Reconstitution of factor VIII and C6 deficient plasma from the various members of the family in this study with purified human C6 did not result in a change in the recalcification time. The results obtained from this study also indicate that there is no linkage between the inheritance of C6 and factor VIII.

An analysis of the lymphocytotoxic activity found in sera from patients with hypogammaglobulinaemia

Sera from 59 patients with hypogammaglobulinaemia (hypogamma) were assessed for lymphocytotoxic activity. The activity was compared with that found in sera from patients with systemic lupus erythematosus (SLE) in titre, class of antibody, target cell, and complement requirements. Sera from patients with hypogamma showed cytotoxic activity when compared with sera from normal donors; however, the activity was far less than that in sera from SLE patients, The cytotoxic titre of both hypogamma and SLE sera was greater against T cells than non-T cells. Sera showing activity could be used to sensitize lymphocytes and render them susceptible to subsequent lysis, but IgM was not detectable on the surface of the sensitized cells. Following sucrose density gradient fractionation of the sera the lymphocytotoxic activity was recovered in those fractions that contained IgM. Lymphocytotoxic activity was dependent on classical complement pathway activity. In all the experiments the hypogamma sera showed the same pattern of activity as the SLE sera, but the activity was always far weaker than that found in SLE sera. The evidence does not suggest that lymphocytotoxicity is likely to be more important in the pathogenesis of hypogamma.

Fulminant meningococcemia in a child with hereditary deficiency of the seventh component of complement and proteinuria

A previously healthy 14-year-old boy presented with fulminant meningococcemia. He was found to have a total deficiency of C7. His serum totally lacked bactericidal activity against Neisseria meningitidis. Addition of purified C7 restored the serum hemolytic and bactericidal activity. Susceptibility to disseminated Neisseria infections has previously been reported in 3 patients with C7 deficiency, as well as in a few patients with deficiency of C5, C6 and C8. These findings emphasize the importance of intact complement mediated bactericidal activity in host defense against disseminated Neisseria infections. Evaluation of the complement system in individuals with Neisseria infections appears mandatory.

Recurrent bacterial meningitis in patients with genetic defects of terminal complement components

Isolated genetic deficiencies of complement components in man are rare. We describe two kindreds with inborn deficiencies of either C5 or C6 in which both propositi presented with recurrent bacterial meningitis. Neisseria meningitidis was isolated from the cerebrospinal fluid of the C5-deficient patient and bactericidal activity against his autologous meningococcus was absent from whole fresh patients' serum despite a rising titre of complement-fixing antibody. The stimulated movement of normal leucocytes was impaired in the presence of C5-deficient serum but not in the presence of C6-deficient serum; neither deficiency reduced significantly the complement-dependent opsonization of Saccharomyces cerevisiae. HLA typing and complement component phenotyping showed no segregation with the complement defect in either the C5- or C6-deficient families. Normal individuals and apparent heterozygotes with approximately half the normal levels of the relevant component were found in both families, in keeping with an autosomal codominant inheritance of the defects.

Use of C6- and C7-deficient human sera in quantitative hemolytic assays for C6 and C7

Quantitative hemolytic assays for C6 and C7 using as R reagents sera from patients deficient in these components are described. The assays gave linear results. Normal range for serum C6 was found to be 21,400--41,700 C6 hemolytic units/ml; for serum C7 the normal range was 5540--9860 C7 hemolytic units/ml.

Deficiency of the fifth component of complement in human subjects. Clinical, genetic and immunologic studies in a large kindred

The discovery of a large kindred with a heritable deficiency of the fifth component of complement (C5) has permitted the accumulation of new clinical, genetic and immunologic data concerning the role of C5 in human subjects. The proband, who has had nine episodes of disseminated gonococcal infection, has a hemolytic C5 level of approximately 0.5 per cent of normal. No C5 protein was detectable, but low levels of functional C5 activity could be found using a sensitive bactericidal assay. The proband's twin as well as another sister also had extremely low levels of hemolytic C5(approximately 0.5 per cent normal), but both these subjects have been healthy. Hemolytic complement and bacteriolytic activity could be restored by the addition of purified C5. No chemotactic activity for polymorphonuclear leukocytes could be generated in the C5-deficient serums upon activation of either the classic or alternative pathways, again demonstrating the importance of C5 in human subjects for the production of chemotactic factors. The chemotactic responsiveness of the patients' polymorphonuclear leukocytes and monocytes to preformed chemotactic factors was not depressed. Twenty-two of 32 other family members from three generations had depressed whole hemolytic complement levels. In 19 of 30 family members, levels of hemolytic C5 ranged from 13 to 64 per cent of normal. No linkage for C5 deficiency and the A or B loci of the major histocompatibility complex could be found. These data suggest an autosomal codominant mode of inheritance of C5 deficiency. Deficiency of C5 is compatible with good health, but it can be associated with repeated disseminated gonococcal infection.

Genetic control of the eighth component of complement

Using isoelectric focusing in polyacrylamide gel and a hemolytic assay for development of patterns, extensive, structural polymorphism in human C8 has been delineated. Two alleles, C8A and C8B, have been identified in orientals, with gene frequencies of 0.655 and 0.345. In blacks, what appears to be a third common allele was found, so that frequencies were 0.692, 0.259, and 0.049 for C8A, C8B, and C8A1. In whites, C8A1 was rare with a frequency of 0.003, and frequencies for C8A and C8B were 0.649 and 0.349. Inheritance was autosomal codominant in family studies and the distribution of types in random unrelated populations fit the Hardy-Weinberg equilibrium in all groups. C8 allotypes have been determined for two previously studied families, each with a homozygous C8-deficient propositus. This study suggests that C8 deficiency is a silent or null allele of the C8 structural locus, and that half normal levels of C8 cannot be used as a single criterion for the establishment of heterozygous C8 deficiency. C8 allotypes, as well as 18 other autosomal markers, were also determined for 24 families. The C8 structural locus is not closely linked to these markers, including the human histocompatibility loci complex.

Host defense against Neisseria meningitidis requires a complement-dependent bactericidal activity

Some individuals, with severe or recurrent infection with Neisseria species, have been identified as lacking a component in the terminal attack sequence of complement (complement components 5 to 9). The relevance of the terminal attack sequence to various phases of host defense was tested with the use of the C-11 strain of meningococci and human serum genetically deficient in complement component 8 (C8-D). The C8-D serum was comparable to normal serum in supporting ingestion and intracellular killing by leukocytes but was not bactericidal in the fluid phase unless reconstituted with C8. Thus, serum complement-dependent bactericidal activity may be especially critical for the host's defense against invasive Neisseria species.

Further evidence for independent segregation of the HLA system and a structural gene for the sixth component of complement (C6)

Previous studies have shown conflicting results with respect to the relationship between C6 deficiency and the HLA system. The present investigation of two kinships, in which C6 deficiency was associated with the expression of an amorphic (or silent) C6 allele, has provided further evidence for the genetic independence of a structural C6 locus and the HLA system.

Neisseria meningitidis bacteremia in association with deficiency of the sixth component of complement

The serum of a 26-year-old black man with a recent episode of meningococcemia complicated by meningitis and arthritis was found to lack hemolytic complement activity. The sixth component of complement was not detected by functional or immunochemical assays whereas other components were normal by hemolytic assay. His fresh acute-phase serum lacked complement-mediated bactericidal activity against the homologous strain of Neisseria meningitidis, but the addition of fresh normal serum or purified C6 restored bactericidal activity as well as hemolytic activity. The absence of C6 activity could not be accounted for on the basis of an inhibitor. Opsonization and chemotaxis functioned normally. Histocompatibility typing of family members did not demonstrate evidence for genetic linkage of C6 deficiency with the major histocompatibility loci. This report represents the first published case of C6 deficiency associated with bacteremic Neisseria infections in which antimeningococcal bactericidal antibodies have been definitively demonstrated against the homologous strain in the acute phase of the illness.

Simultaneous occurrence of hereditary C6 and C2 deficiency in a French-Canadian family

The sera of four sisters were found to lack the sixth component of complement (C6) and the serum of one was also partially deficient in the second component (C2). Two other blood relatives were found to be heterozygous for both deficiencies, while only one sibling had normal values. The father of these eight siblings was heterozygous for C2D and C6D and in the third generation, six children were heterozygous for C6 deficiency was treated for chronic active brucel-transmitted; the C6 deficiency was not linked to the HLA system, while the C2-deficiency segregated with the haplotype A10,B18. The proband, homozygous for C6 deficiency was treated for chronic active Brucellosis and in another sibling with C6 deficiency, toxoplasmosis was diagnosed. Neither bleeding disorders nor a tendency to collagen diseases have been observed and the opsonic activity was normal in the sera of all family members.

Inherited deficiency of the seventh component of complement associated with nephritis. Propensity to formation of C56 and related C7-consuming activity

A 46-yr-old female with chronic pyelonephritis was found to lack complement (C) activity by the use of hemolytic screen assays in agarose gels. These assays also revealed a propensity of patient serum to form an activated complex of the fifth and sixth components of C, C56. Each of the C component hemolytic activities was present in normal or elevated amounts with the exception of C7, which was undetectable; addition of purified C7 led to the restoration of hemolytic activity. C-dependent phagocytosis, immune adherence, and neutrophil chemotaxis were normal. Family studies demonstrated that the defect was transmitted as an autosomal codominant apparently not linked with alleles at the HLA-A or HLA-B loci. Persisting C56 was readily formed in this as compared to normal serum upon incubation with multiple C activators including zymosan, inulin, immune complexes, heat-aggregated human gamma globulin, endotoxin, and agarose. A heat-stable (56 degrees C, 30 min) activity which consumed C7 with time-and temperature-dependent kinetics was detected in plasma and serum, and seemed to be similar to a "C7 inactivator" previously described in another C7-deficient individual. However, this activity was found to have properties identical to those of C56 during low ionic strength precipitation and chromatography on Sephadex G-200, to be specifically removed upon passage through an anti-C5 immunoadsorbent column, and to be associated with a small amount of C56, suggesting that it represents an expression of small amounts of C56 rather than a new C-inhibitory activity. Thus, an individual with chronic nephritis lacking C7 is reported; the utility of a hemolytic screen assay in agarose plates for the detection of such patients is emphasized; persisting C56 is shown readily to be formed in this serum; and the presence of C7-consuming activity which is associated with and in all likelihood attributable to C56 is shown.

Genetics of complement deficiencies associated with lupus-like syndromes

There appears to be an association between deficiencies of a number of complement components, particularly of the second component (C2), and rheumatic diseases, especially lupus. The meaning of this association is not clear, but the linkage of deficiency of C2 with HLA, especially HLA-A10, B18, Dw2, as well as with BfS, suggests a possible linkage to immune response genes.

The "Hageman" connection: interrelationships of blood coagulation, fibrino(geno)lysis, kinin generation, and complement activation

The activation pathways for the generation of enzymes involved in blood clotting, clot lysis, complement activation, and kinin generation are briefly reviewed. The interrelationship of the four systems is illustrated by the multiple functions of four key enzymes: Factor XIIa, kallikrein, plasmin, and C1 esterase. The pivotal role of Factor XIIa in establishing this connection is elucidated.

Disorders of leukocyte chemotaxis

The rapid accumulation of inflammatory cells at sites of microbial invasion or neoplastic transformation is a central event in immunologically-mediated host defense. The availability of methodology to accurately quantify leukocyte migration in vitro has allowed the disclosure of previously unrecognized clinical disorders, namely leukocyte dysmotility syndromes. Although this area of clinical investigation is in its infancy, one can identify several processes associated with abnormal leukocyte accumulation. Abnormalities of immune recognition, chemotactic factor production, cellular motility or inhibitors of chemotaxis have been identified in different human diseases. In the upcoming years, pharmacological intervention directed at correcting specific causes of leukocyte dysmotility may well enhance our ability to treat certain infectious, inflammatory, and neoplastic diseases.

Protective role of complement in experimental Escherichia coli endocarditis

Fourteen strains of Escherichia coli were tested for ability to cause infective endocarditis in rabbits prepared by prior placement of an intracardiac catheter. Strains that were resistant to the bactericidal action of serum caused E. coli endocarditis in 91.4% of rabbits, whereas serum-sensitive strains usually failed to cause persisting infection (11.3% infected, P less than 0.001). Although serum-sensitive E. coli lodged on heart valves within 1 h after intravenous injection, they survived less than 24 h in most normal rabbits. In contrast to normals, all five C6-deficient rabbits injected with a serum-sensitive strain of E. coli developed infective endocarditis (P less than 0.005). No correlation was found between the presence of K1 antigen and the incidence of experimental E. coli endocarditis. Thus, the ability of strains of E. coli to establish persisting endocardial infection in rabbits appears to be directly associated with resistance to the complement-mediated serum bactericidal system. These findings may explain in part the rarity of gram-negative bacillary endocarditis in patients; they also indicate that in certain special circumstances the serum bactericidal system can play a decisive role in host defense.

Linkage between the gene (or genes) controlling synthesis of the fourth component of complement and the major histocompatibility complex

In an attempt to map the gene (or genes) controlling the synthesis fo the fourth component of complement (C4), we performed linkage studies in a family with hereditary C4 deficiency. The proband, a seven-year-old boy with lupus erythematosus, consistently lacked deteftable serum C4 by both functional and protein measurements. The complement defect was transmitted as an autosomal recessive disorder. Eight of 15 family members were considered to be heterozygotes, seven because of low C4 levels and one because of genetic data (obligate heterozygote). The gene (or genes) coding for C4 deficiency appeared to be linked to the major histocompatibility complex (A2,B12,DW2 on the maternal side and A2,BW15,LD108 on the paternal side) and to other markers known to be in close proximity to the histocompatibility complex on chromosome 6 (phosphoglucomutase-3, glyoxalase-1 and properdin factor B).

Gonococci causing disseminated gonococcal infection are resistant to the bactericidal action of normal human sera

The susceptibility of strains of Neisseria gonorrhoeae to the bactericidal action of normal human sera was determined for isolates from patients with disseminated gonococcal infection and uncomplicated gonorrhea. Serum susceptibility was correlated with penicillin susceptibility and auxotype. 38 of 39 strains (97%) of N. gonorrhoeae from Seattle patients with disseminated gonococcal infection were resistant to the complement-dependent bactericidal action of normal human sera. 36 of these were inhibited by less than or equal to mug/ml of penicillin G and required arginine, hypoxanthine, and uracil for growth on chemically defined medium (Arg-Hyx-Ura- auxotype). 12 of 43 isolates from patients with uncomplicated gonorrhea were also of the Arg-Hyx-Ura-auxotype, inhibited by less than or equal to 0.030 mug/ml of penicillin G, and serum resistant. Of the 31 remaining strains of other auxotypes isolated from patients with uncomplicated gonorrhea, 18 (58.1%) were sensitive to normal human sera in titers ranging from 2 to 2,048. The bactericidal action of normal human sera may prevent the dissemination of serum-sensitive gonococci. However, since only a small proportion of individuals infected by serum-resistant strains develop disseminated gonococcal infection, serum resistance appears to be a necessary but not a sufficient virulence factor for dissemination. Host factors such as menstruation and pharyngeal gonococcal infection may favor the dissemination of serum-resistant strains. Since serum-resistant Arg-Hyx-Ura strains are far more frequently isolated from patients with disseminated gonococcal infection than serum-resistant strains of other auxotypes, Arg-Hyx-Ura-strains may possess other virulence factors in addition to serum resistance.

Complement and the clinician

The estimation of complement in serum, and in other body fluids, particularly synovial fluid, now has an established place in the assessment, prognosis and response to treatment of those diseases associated with immunological phenomena. Some knowledge of the complement sequence itself, the patterns characteristic of various disease processes and the interacting factors which may affect the levels of the various components have been summarised. Disease states closely resembling lupus erythematosus have been associated with genetically determined deficiencies of classic pathway components and deficiencies of terminal sequence components may lead to severe recurrent infections.