Development and application of an enzyme-linked immunosorbent assay for the quantitation of alternative complement pathway activation in human serum

Complement and COVID-19: Three years on, what we know, what we don't know, and what we ought to know

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus was identified in China in 2019 as the causative agent of COVID-19, and quickly spread throughout the world, causing over 7 million deaths, of which 2 million occurred prior to the introduction of the first vaccine. In the following discussion, while recognising that complement is just one of many players in COVID-19, we focus on the relationship between complement and COVID-19 disease, with limited digression into directly-related areas such as the relationship between complement, kinin release, and coagulation. Prior to the 2019 COVID-19 outbreak, an important role for complement in coronavirus diseases had been established. Subsequently, multiple investigations of patients with COVID-19 confirmed that complement dysregulation is likely to be a major driver of disease pathology, in some, if not all, patients. These data fuelled evaluation of many complement-directed therapeutic agents in small patient cohorts, with claims of significant beneficial effect. As yet, these early results have not been reflected in larger clinical trials, posing questions such as who to treat, appropriate time to treat, duration of treatment, and optimal target for treatment. While significant control of the pandemic has been achieved through a global scientific and medical effort to comprehend the etiology of the disease, through extensive SARS-CoV-2 testing and quarantine measures, through vaccine development, and through improved therapy, possibly aided by attenuation of the dominant strains, it is not yet over. In this review, we summarise complement-relevant literature, emphasise its main conclusions, and formulate a hypothesis for complement involvement in COVID-19. Based on this we make suggestions as to how any future outbreak might be better managed in order to minimise impact on patients.

Complement in sickle cell disease and targeted therapy: I know one thing, that I know nothing

Sickle cell disease (SCD) is a common inherited clinical syndrome, characterized by the presence of hemoglobin S. Anemia, susceptibility to infections and episodes of vaso-occlusive crisis (VOC) are among its features. Since SCD complications (VOC or delayed hemolytic transfusion reaction/DHTR) lead to significant morbidity and mortality, a number of studies have addressed their pathophysiology Although SCD pathophysiology has been mainly attributed to the interaction between sickle cells and neutrophils, platelets or endothelial cells in small vessels leading to hemolysis, the role of complement activation has been increasingly investigated. Importantly, complement inhibition with eculizumab has shown beneficial effects in DHTR. Given the unmet clinical need of novel therapeutics in SCD, our review summarizes current understanding of (a) complement system for the clinician, (b) complement activation in SCD both in asymptomatic state and severe clinical manifestations, (c) probable underlying mechanisms of complement activation in SCD, and (d) new therapeutic perspective of complement inhibition.

Complement alternative pathway activation associated with pulmonary hypertension in lupus nephritis patients

Pulmonary hypertension occurs in systemic lupus erythematosus (SLE) for several reasons, such as vasculopathy. Previous studies have indicated that the excessive activation of the complement alternative pathway might be involved in the pathogenesis of lupus nephritis, especially in the absence of factor H or its functional impairment. However, the clinical and pathological significance of the alternative complement activation in lupus nephritis patients with pulmonary hypertension remains elusive. The data on patients with pulmonary hypertension and non-pulmonary hypertension lupus nephritis were retrospectively analyzed in our centre. Major plasma levels of complement components were evaluated. The depositions of Bb, C3d and C5b-9 in the lung specimens of pulmonary hypertension combined with SLE patients were detected by immunofluorescence staining. Among 352 lupus nephritis cases, 24 were diagnosed with pulmonary hypertension and 328 with non-pulmonary hypertension. Higher levels of Bb and lower levels of factor H were detected in the pulmonary hypertension group in comparison with the negative group (P = 0.049, P = 0.024, respectively). Pulmonary hypertension was a risk factor for renal outcome as deduced by the log-rank and Cox test for survival analysis. C3d, C5b-9 and Bb were found to be positive in lung specimens of lupus nephritis patients with pulmonary hypertension. We concluded that activation of the complement alternative pathway may be involved in the pathogenesis of pulmonary hypertension in lupus nephritis.

SLE and Serum Complement: Causative, Concomitant or Coincidental?

BACKGROUND

Systemic Lupus Erythematosus (SLE) is an incurable autoimmune disorder with complement activation playing a key role in the pathogenesis of immune-mediated tissue injury. While quantifying complement to monitor SLE disease activity has been the standard of care since the 1950s, decreased complement levels are not consistently associated with flares.

OBJECTIVE

We seek to clarify the SLE phenotype in which complement deficiency is causative, concomitant, or coincidental.

METHODS

A PUBMED literature review was conducted using key words 'complement,' 'SLE,' and 'SLE flares' in English-only journals from 1972-2017. Relevant clinical studies and review articles were found that examined the measurement of complement levels in SLE, and more specifically, interpretation of low serum complement levels regardless of disease activity.

CONCLUSION

Complement activation plays a key role in the pathophysiology of SLE and it is recommended to continue monitoring serum levels of C3 and C4 to assess for disease activity. However, it is important to note that decreased serum complement is not consistently associated with disease flares.It is clinically important to find novel ways to assess disease activity in SLE. Reduced serum levels of cell-bound complement activation products may more accurately reflect disease activity than conventional serum C3 and C4 monitoring.

Genetic Association of the Porcine C9 Complement Component with Hemolytic Complement Activity

The complement system is a part of the natural immune regulation mechanism against invading pathogens. Complement activation from three different pathways (classical, lectin, and alternative) leads to the formation of C5-convertase, an enzyme for cleavage of C5 into C5a and C5b, followed by C6, C7, C8, and C9 in membrane attack complex. The C9 is the last complement component of the terminal lytic pathway, which plays an important role in lysis of the target cells depending on its self-polymerization to form transmembrane channels. To address the association of C9 with traits related to disease resistance, the complete porcine C9 cDNA was comparatively sequenced to detect single nucleotide polymorphisms (SNPs) in pigs of the breeds Hampshire (HS), Duroc (DU), Berlin miniature pig (BMP), German Landrace (LR), Pietrain (PIE), and Muong Khuong (Vietnamese potbelly pig). Genotyping was performed in 417 F2 animals of a resource population (DUMI: DU×BMP) that were vaccinated with Mycoplasma hyopneumoniae, Aujeszky diseases virus and porcine respiratory and reproductive syndrome virus at 6, 14 and 16 weeks of age, respectively. Two SNPs were detected within the third exon. One of them has an amino acid substitution. The European porcine breeds (LR and PIE) show higher allele frequency of these SNPs than Vietnamese porcine breed (MK). Association of the substitution SNP with hemolytic complement activity indicated statistically significant differences between genotypes in the classical pathway but not in the alternative pathway. The interactions between eight time points of measurement of complement activity before and after vaccinations and genotypes were significantly different. The difference in hemolytic complement activity in the both pathways depends on genotype, kind of vaccine, age and the interaction to the other complement components. These results promote the porcine C9 (pC9) as a candidate gene to improve general animal health in the future.

Molecular cloning and sequencing of porcine C5 gene and its association with immunological traits

The complement system helps in the lysis of invading pathogens and modulates the inflammatory as well as the humoral and cellular immune responses. C5 mediates many potent inflammatory and cytolytic events after proteolytic activation by complement convertase enzymes. Hence, to investigate the role of pig C5 ( pC5) as a candidate gene for disease resistance in pigs, the complete cDNA of pC5 was sequenced, screened for single nucleotide polymorphisms (SNPs), and an association analysis with various immunological parameters measured in F2 animals of a pig resource population based on a cross of Duroc and Berlin miniature pigs (DUMI) was carried out. In total, 5,422 bp of pC5 cDNA was sequenced, which codes for the 1,677-amino-acid precursor of C5. Four polymorphic sites were detected, one of which was segregating in the DUMI population in three genotypic patterns: AA, AC and CC. Classical (CH50) and alternative (AH50) complement activities, C3c levels, haptoglobin (HP) acute phase protein levels, and antibody titers against Mycoplasma (Mk) and Aujesky (ADV) vaccines were measured in the resource population. Association analysis between C5 and the immunological parameters was carried out using repeated measures mixed and general linear model analysis. The homozygote AA was found to be significantly different from the other two genotypes with respect to AH50 and CH50, whereas genotype CC was found to be significantly different from the other genotypes for C3c and HP levels. No significant difference could be seen between genotypes for antibody titers against vaccinations. Association of C5 with complement activity traits and acute phase proteins promotes pC5 as a candidate gene for innate disease resistance.

Molecular characterization of the pig C3 gene and its association with complement activity

The complement system catalyzes direct lysis of micro-organisms and modulates phagocytosis, inflammation, humoral and cellular immune responses. Since the complement protein C3 is the central component within all pathways of complement activation, C3 is a candidate gene for complement activity and also for improved protection against many pathogens. The pig C3 gene was sequenced, screened for polymorphisms, and analyzed for association with hemolytic complement activity of the alternative and classical pathway (AH(50), CH(50)). C3c serum levels and haptoglobin (HP) serum concentrations were measured before and after vaccination against Mycoplasma hyopneumoniae, Aujeszky virus, and porcine reproductive and respiratory syndrome virus in F2 animals of a pig resource population based on crossbreeding of Duroc and Berlin Miniature Pig. The genomic C3 sequence covers 444 bp of promoter region, 41 exons and 40 introns, as well as 881 bp of the 3'-flanking region. The cDNA codes for a 1,661-amino acid precursor C3. Five polymorphic sites were detected in the 5'-UTR, intron 13, exon 15, exon 30, and the 3'-UTR. Within the resource population two haplotypes were found to segregate. Analysis of variance applying a repeated measures model revealed a significant effect of the interaction of C3 genotype and time of measurement relative to immunization on CH(50), AH(50,)and C3c that is likely to be due to variation of C3 expression. In contrast, the time course of the HP acute-phase reaction is not associated with C3 genomic variation. The association of C3 with complement activity indicates the importance of C3 as a candidate gene for natural resistance to micro-organisms, although the causative polymorphism modulating the expression of C3 remains to be delineated.

Complement and systemic lupus erythematosus

Complement is implicated in the pathogenesis of systemic lupus erythematosus (SLE) in several ways and may act as both friend and foe. Homozygous deficiency of any of the proteins of the classical pathway is causally associated with susceptibility to the development of SLE, especially deficiency of the earliest proteins of the activation pathway. However, complement is also implicated in the effector inflammatory phase of the autoimmune response that characterizes the disease. Complement proteins are deposited in inflamed tissues and, in experimental models, inhibition of C5 ameliorates disease in a murine model. As a further twist to the associations between the complement system and SLE, autoantibodies to some complement proteins, especially to C1q, develop as part of the autoantibody response. The presence of anti-C1q autoantibodies is associated with severe illness, including glomerulonephritis. In this chapter the role of the complement system in SLE is reviewed and hypotheses are advanced to explain the complex relationships between complement and lupus.

Correlation of alternative pathway (AP) lytic activity and AP-dependent neutrophil phagocytosis with factor B levels and consumption in serum

This work investigated the correlation between serum levels of factor B, AP-lytic activity, ratio of factor B activation by zymosan, and AP-dependent neutrophil phagocytosis in samples of normal human serum (NHS). In addition, since the antithyroid drug propylthiouracil (PTU) induces increased levels of AP lytic activity in rats, groups of these animals were treated with this drug in order to increase AP titers and to evaluate those parameters also in this condition. The results showed no correlation between factor B concentration and AP lytic activity in 18 samples of NHS or between factor B concentration and proportion of consumption by zymosan. Interestingly, this consumption was also not correlated with phagocytosis as measured by the chemiluminescence (CL) response of neutrophils to the opsonized particles. The two biological properties of phagocytosis and lytic activity, dependent of AP, were not correlated to each other in the NHS samples. In the samples of rat serum with increased AP lytic levels a different result was observed. A positive correlation between CL response and lytic activity occurred in serum of animals receiving a low PTU dose, but not in serum of animals receiving a high dose, where CL responses were lower than those of controls. The results are compared to literature data and discussed in terms of individual differences in resistance or susceptibility to infections and or diseases involving the complement system.

A novel ELISA for the assessment of classical pathway of complement activation in vivo by measurement of C4-C3 complexes

Measurements of complement split products by enzyme-linked immunosorbent assays (ELISA) are well established for the assessment of in vivo complement activation. We have combined two monoclonal antibodies (mAb) with specificities for C3b/iC3b/C3dg (mAb I3/15) and C4/C4b/C4d (mAb M4d2), respectively, in a sandwich ELISA to quantitate C4-C3 complexes as an indicator of complement activation. Serum incubated with heat aggregated IgG (HAG) was used as a standard and the C4-C3 levels expressed as microgram equivalent HAG/ml (microgram HAG-equ/ml). Normal values of C4-C3 complexes in plasma (EDTA) of healthy probands (n = 11) were 6.3 micrograms HAG-equ/ml +/- 1.5 (mean +/- 1 standard deviation (SD), with a range from 3.6 to 9.1). In patients with systemic lupus erythematosus (SLE, n = 23) C4-C3 values were clearly elevated (48.8 micrograms HAG-equ/ml +/- 52.9, range 7.5-184.7) as compared to samples from patients with idiopathic hypertension (IDH, n = 10) (6.5 micrograms HAG-equ/ml +/- 1.7, range 4.1-9.4). For SLE patients C4-C3 levels significantly correlated with values for C3b/iC3b/C3d (r = 0.69, p < 0.001) and C3 containing immune complexes (r = 0.68, p < 0.001), but not with the C4d fragment (r = 0.26). C4-C3 levels of 96% of the studied SLE patients were increased more than 2 SD above the normal mean as compared to 74% of C4d and activated C3 values, respectively. Serum treated with zymosan as an activator of the alternative pathway of complement did not exhibit higher C4-C3 values. These results demonstrate that the quantitation of in vivo generated C4-C3 complexes by ELISA provide a novel, sensitive parameter for classical pathway of complement activation.

Activation of human complement by totally human monoclonal antibodies

A uniquely developed series of totally human monoclonal antibodies (mAbs) were examined for their complement fixing properties in comparison to human myeloma preparations and to commercially available human polyclonal immunoglobulins. C3b and C4b deposition was measured using a kinetic ELISA technique. When the IgG myeloma proteins were tested for classical pathway activation, our findings were similar to those previously described, where IgG1 and IgG3 were more potent activators of the classical pathway than IgG2 and IgG4. However, those same studies determined that IgG2 was the best activator of the alternative pathway followed by IgG1 and IgG3 while IgG4 does not activate complement via either pathway. In our studies of alternative pathway activation, the IgG2 myeloma exhibited strong activation of the alternative pathway, but, at levels lower than the other three IgG subtypes. Using this test system, we examined the complement activating potential of four totally human mAbs that were constructed from the peripheral blood lymphocytes of a colon carcinoma patient in long term remission. We found that our uniquely constructed totally human IgG2 mAbs (A3, E1, F6 and F8) were able to activate complement by both the classical and alternative pathways to varying degrees. In addition, we found that the complement activating ability of the human mAbs was greater than that of the human IgG2 myeloma immunoglobulins or normal human IgG2 preparations. This study represents the first report of complement activation by totally human mAbs and confirms more recent findings which indicate that levels of complement activation by human IgG immunoglobulins cannot be predicted based solely on their subclass identity.

Covalent linkage of C3 to properdin during complement activation

Activation of the alternative complement pathway of serum produces complexes of properdin (P) and C3 as measured in a double antibody enzyme-linked immunosorbent assay. When purified from serum, these complexes decrease factor B hemolytic activity in serum and do not restore the alternative pathway hemolytic activity of serum deficient in P. Sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of activated serum containing biotinylated P, followed by blotting to nitrocellulose and development with streptavidin-alkaline phosphatase revealed a band at 53 kDa for monomeric P and an additional band at 160 kDa. P samples eluted from zymosan and purified from activated serum revealed a band at 116 kDa for C3 alpha and 74 kDa for C3 beta, and an additional band at 160 kDa when analyzed by SDS-PAGE, Western blotting and development with antibody to C3. The appearance of a 160 kDa band containing P and C3 indicates that these proteins are contained in a complex formed during activation of the alternative pathway. Activation of a purified reagent mixture containing factors B, D, and H, and 125I-labeled P or 125I-labeled C3, followed by SDS-PAGE and autoradiography confirmed the presence of a 160-kDa band which disappeared following hydroxylamine treatment of the sample. These data are consistent with a covalent linkage of C3 to P via the C3 alpha chain, producing the 160-kDa complex.

Methods for assessing complement activation in the clinical immunology laboratory

Complement activation is a key component of the pathogenesis of immune-mediated tissue damage in many diseases. Assessment of complement activation in current practice is largely based on the measurement of intact C3 and C4 or the determination of complement haemolytic function. These parameters reflect activation only indirectly, are insensitive and open to influence by factors other than complement conversion. New approaches to evaluate complement activation directly using sensitive techniques have been developed, and several could be adopted easily in most laboratories. These concentrate on the detection of activation fragments, neoantigens or complexes that only arise as a direct result of complement activation. The wide application of these techniques in research and clinical practice would enhance our understanding of the pathogenesis of a range of inflammatory and infectious diseases.

Assessment of complement activation in vivo

Hemolytic assays that measure the functional integrity of the complement system and the quantitation of individual components by immunochemical techniques have been widely used in the past for the assessment of in vivo complement activation. However, the complement system comprises a large number of interacting serum proteins which are subject to independent synthetic and catabolic processes. The fact that complement proteins are rapidly metabolized under in vivo conditions adds to the complexity of complement analysis. Assays that are based on monoclonal antibodies with specificities for activation-dependent neoepitopes now allow the direct determination of complement fragments in plasma. These methods are superior to the quantitation of native proteins. Several parameters that differentially affect the generation or the catabolism of individual complement activation products still have to be taken into account when elevated plasma levels of complement fragments suggest in vivo complement activation. These factors include the binding to complement fragment receptors, the degradation by serum proteases and renal or hepatic clearance. An accurate estimation of complement activation in vivo requires the simultaneous determination of both the native components and the activation products.

Dissociation of primary antigen-antibody bonds is essential for complement mediated solubilization of immune precipitates

The role of dissociation of primary antigen-antibody bonds in the solubilization of immune complexes (IC) has been investigated using photo-affinity crosslinked IC comprising NAP15-BSA and murine monoclonal anti-DNP antibodies. Non-covalently linked IC were solubilized rapidly when incubated with normal human serum (NHS), whereas covalently-linked IC were solubilized poorly or not at all. The rate and extent of complement activation produced by incubating covalently-linked and non-covalently linked IC with NHS was similar as assessed by the production of the C1s:C1-inhibitor, C3:properdin and C5b-9 complexes and the anaphylatoxins C4a and C3a. Thus, the inability of serum to solubilize photo-affinity crosslinked IC must be due to failure of dissociation of primary antigen-antibody bonds.

Measurement of complement activation products in patients with chronic rheumatic diseases

Measurement of the complement activation products C1s:C1-inh, C3bP and C5b-9 by ELISA in plasma samples from normals, rheumatoid arthritis (RA) and systemic lupus erythematosis (SLE) patients showed significantly elevated levels in the two patient groups (P less than 0.0001 for C1s:C1-inh, C3bP and C5b-9) compared to normals. In seropositive RA patients there were significant correlations between the levels of the three complement activation complexes and IgM-RF, IgG-RF and IgA-RF. However, IgM-RF did not interfere with any of the ELISA systems. Mean levels of C1s:C1-inh, C3bP and C5b-9 were the same in paired plasma and synovial fluids; however, C3bP levels in the paired samples did not correlate with one another by rank. Our conclusions are that: (a) elevated plasma levels of these complement activation products are detectable in rheumatic diseases; (b) plasma levels of these complement activation products are related to Rheumatoid factor (RF) levels in seropositive RA patients; and (c) IgM-RF does not influence these solid-phase ELISA procedures.

Mechanism of action of an inhibitor of complement-mediated prevention of immune precipitation

Glycoprotein 60 (gp60) is a normal plasma protein (mean concentration in normal serum 34 micrograms/ml) that is present in increased levels (mean concentration 97 micrograms/ml) in the sera of patients with rheumatoid arthritis (RA). Purified gp60 binds to IgG but not to IgM, and competitively inhibits the binding of C1q. In fluid-phase studies, purified gp60 was shown to reduce immune complex-mediated complement activation in a dose-dependent manner. The addition of Fab anti-gp60 to normal serum was associated with (i) increased levels of complement-mediated prevention of immune precipitation (PIP); (ii) increased total haemolytic complement activity when EAIgG, but not when EAIgM, were used as targets; and (iii) increased immune complex-mediated complement activation. Thus gp60 appears to regulate immune complex-mediated classical pathway activation. The findings that Fab anti-gp60 (i) only partly restored PIP in RA sera showing reduced PIP levels and (ii) only partly reduced inhibition of PIP by RA sera, show that gp60 is not entirely responsible for these abnormalities.

Deposition of complement activation products on plastic-adsorbed immunoglobulins. A simple ELISA technique for the detection of defined complement deficiencies

The activation of complement components in human serum has been studied using immunoglobulins adsorbed to microtiter plates. The sequential deposition of complement fragments was detected by a series of mono- and polyclonal antibodies in an indirect enzyme-linked immunosorbent assay (ELISA). Antibodies against C1q, C1s, C4b/d, C3b/d, factor B, C5b-9 membrane attack complex (MAC), the regulatory complement proteins C4 binding protein (C4bp) and properdin were reactive. Several lines of evidence suggest that complement activation was via the classical pathway: (1) complement activation was highly isotype-restricted with regard to the adsorbed Igs (human IgG1 and IgG3 as well as mouse IgM, IgG2a and IgG2b isotypes are strong activators in contrast to human IgG2, IgG4, IgA and mouse IgG1); (2) Ca2+ depletion, heat treatment (56 degrees C for 45 min), incubation with 0.5 M KSCN or heat-aggregated immunoglobulins (aggIgG) abrogated serum activity; (3) complement deficient sera (C1q def', C2 def', C6 def' human sera; C2 def', C4 def' guinea pig sera) showed impaired deposition of the complement components that follow the missing component in the cascade of activation. In a clinical study sera from patients with systemic lupus erythematosus (SLE) were investigated in order to measure the effect of hypocomplementemia due to complement consumption. The results obtained suggest that this new and simple assay is well suited for (1) the detection of various inherited complement deficiencies, (2) the semiquantitative evaluation of sera with decreased complement levels, (3) a more detailed study of complement components bound to a solid phase.

Factor B activation products in patients with systemic lupus erythematosus. A marker of severe disease activity

Complement activation patterns were determined in a group of 51 patients with systemic lupus erythematosus (SLE), and the clinical outcomes of these patients at 2 years were correlated with the complement activation patterns. Activation of the classical pathway was monitored by analysis of C4a desArg levels and total C4 levels, and activation of the alternative pathway was monitored by isoelectric focusing/immunofixation and quantitative analyses of Factor B. Activation of C3 (a general measure of complement activation) was determined by an enzyme-linked immunosorbent assay for C3d and by quantitative analysis of C3. Patients were stratified into 3 groups: those with C4 but not C3 activation; those with C4 and C3 but not Factor B activation; and those with C4, C3, and Factor B activation. At the end of 2 years, there was a statistically significant difference in the morbidity and mortality rates of the third group of SLE patients compared with those in the other 2 groups. There was also a statistically significant association between the presence of Ba and cutaneous vasculitis. Unlike the patterns seen with in vitro-activated serum or with membrane-activated plasma, the Bb activation fragment was not present in the activated plasma samples from the SLE patients. These data suggest that activation of the alternative complement pathway may be a marker for severe SLE and that the Bb fragment may be playing a role in the development of this more serious pathologic condition.

A new method for the estimation of C3d. Affinity clearance of C-determinant-bearing C3 molecules and fragments followed by estimation of C3d by ELISA

A method is described to quantitate human complement fragment C3d. Test samples were treated with a predetermined excess of anti-C3c-Sepharose beads in the presence of EDTA to remove all the C-determinant-bearing C3 molecules or fragments. C3d left in the supernatant was then estimated by ELISA. Using this method, C3d could be estimated accurately in normal plasma samples. A good correlation (r = 0.93) was observed between C3d values obtained by this method and values obtained by the widely used method of Perrin and coworkers. The average C3d plasma concentration was 2.8 mg/l (SD = 0.7 mg/l, n = 21). The interassay coefficient of variation using a normal plasma pool (C3d 2.7 mg/l) was 8.3% and using normal plasma pools in which the C3d concentrations were raised to 10.3 and 17.4 mg/l by the addition of aged normal serum the levels were 8.0 and 7.5% respectively. Intra-assay coefficients of variation with these samples were 4.6, 3.0 and 2.8%, respectively. 16 patients with renal dysfunction had C3d levels in the range of 4.3-10.0 mg/l and 15 patients undergoing continued ambulant peritoneal dialysis had levels of 3.3-12.2 mg/l. The C3d content in peritoneal dialysate of patients undergoing dialysis varied from 9.3 to 383 micrograms/l.

Epstein-Barr virus regulates activation and processing of the third component of complement

Serum incubated with purified EBV was found to contain C3 cleavage fragments characteristic of C3c. Since the cofactors necessary for such cleavage of C3b by factor I are not normally present in serum, EBV was tested for factor I cofactor activity. Purified EBV from both human and marmoset EBV-producing cell lines was found to act as a cofactor for the factor I-mediated breakdown C3b to iC3b and iC3b to C3c and C3dg. EBV also acted as a cofactor for the factor I-mediated cleavage of C4b to iC4b and iC4b to C4c and C4d. EBV from both the human and marmoset cell lines accelerated the decay of the alternative pathway C3 convertase. The classical pathway C3 convertase was unaffected. Multiple lines of evidence eliminated the possibility that marmoset or human CR1 was responsible for the functional activities of EBV preparations. The spectrum of activities was different from CR1 in that EBV and EBV-expressing cell lines failed to rosette with C3b or particles bearing C3b, the primary functional assay for CR1, and EBV did not accelerate classical pathway C3 convertase decay, another property of CR1. In addition, CR1 could not be detected immunologically on marmoset or human EBV-expressing cells and mAbs to CR1 failed to alter EBV-produced decay acceleration and factor I cofactor activities, although the antibodies blocked the same CR1-dependent functional activities. The multiple complement regulatory activities exhibited by purified EBV derived from human and marmoset cells differ from those of any of the known C3 or C4 regulatory proteins. These various activities would be anticipated to provide survival value for the virus by subverting complement- and cell-dependent host defense mechanisms.

Activation of complement by human serum IgA, secretory IgA and IgA1 fragments

Activation of the complement (C) system by human IgA was studied. Both subclasses of IgA, IgA1 and IgA2, and secretory IgA were shown to activate C, as determined by deposition of C3 on glutaraldehyde-activated microwells coated with IgA. The activation of the C system occurred in the presence of MgEGTA and not in D-deficient serum. In addition to C3, deposition of properdin (P) but not of C4 was detected. These results indicate that C activation, as determined by measuring deposition of C3 and P, occurred by the alternative pathway (AP). The data further show that the major part of the hinge region, which is deleted in IgA2 as compared with IgA1 and which forms the major structural difference between the two subclasses, is not involved in C activation. Reduction and alkylation destroyed the ability of IgA to activate C, as has also been demonstrated for IgG. In order to define the C activating region of the IgA molecule, several fragments of IgA1 were tested. The four-chain molecules F(ab')2 and F(abc)2 were shown to activate the AP. No activation was observed with the two-chain fragments Fab and Fc. The Fc fragment of IgA also did not activate the CP, as does the Fc fragment of IgG. This indicates that activation of the AP of C by IgA is dependent on the presence of the F(ab')2 fragment.

IN CONCLUSION

human IgA does activate C by the AP. This activation requires an intact F(ab')2 fragment.

Complement activation occurs through both classical and alternative pathways prior to onset and resolution of adult respiratory distress syndrome

We have previously reported that plasma concentrations of the terminal complement (C) complex (TCC), C5b-9, increased significantly 2 days prior to onset of adult respiratory distress syndrome (ARDS) and also 1 day preceding its resolution. To determine the pathway of complement activation that preceded development and resolution of this acute inflammatory lung injury in septic patients, we quantified the C1rC1s-C1 inhibitor complex and the C3bP complex, which are generated following activation of classical and alternative complement pathways, respectively. Two days prior to diagnosis of ARDS, the plasma C1rC1s-C1 inhibitor complex and C3bP complex levels increased 22 and 14%, respectively. Furthermore, significant correlations were identified between concentrations of the TCC and C1rC1s-C1 inhibitor complex (r = 0.73, P = 0.003) and also with the levels of the TCC and C3bP complex (r = 0.81, P = 0.002) before onset of ARDS. Equally of interest, the C1rC1s-C1 inhibitor complex and C3bP complex concentrations increased 68 and 35%, respectively, 1 day before resolution of ARDS. Similarly, significant elevations of TCC concentrations preceding resolution of ARDS correlated with C1rC1s-C1 inhibitor complex (r = 0.66, P = 0.02) and also with C3bP complex (r = 0.72, P = 0.002) levels. Our results indicate that both the classical and alternative complement pathways are activated prior to onset of ARDS and also before its resolution in septic patients.

Detection of the terminal complement complex in patient plasma following acute myocardial infarction

The mechanisms of inflammation responsible for the myocardial tissue damage seen after an acute myocardial infarction (AMI) have not been clearly identified. Recent lines of evidence, demonstrating depressed sera levels of individual complement components in patients after myocardial infarction, have suggested involvement of the complement (C) system in micro- and macrovascular injury subsequent to AMI. The present study assessed the role of complement as a mediator of myocardial inflammation by quantifying products of complement activation including, the terminal complement complex (TCC) the cytolytic component of the complement system, C1rC1s-C1 inhibitor complex and C3bBbP complex, formed following activation of the classical and alternative pathway, respectively, and anaphylatoxins C3a and C5a in 41 patients following AMI. Plasma TCC and C1rC1s-C1 inhibitor complex concentrations increased up to 32-fold (P less than 0.001) and 8-fold (P less than 0.001), respectively, while the C3bBbP complex, C3a des-Arg and C5a des-Arg each increased over 2-fold (P less than 0.001) 16 h after AMI, and were only minimally detectable during non-inflammatory myocardial conditions. Furthermore, TCC concentrations increased over 150% (P less than 0.001) one day after patients reinfarcted, subsequent to hospitalization for a primary AMI. These results demonstrate activation of complement after AMI and suggest that inflammatory mediators of the complement system may contribute to myocardial tissue damage during the infarction process.

Determination of C4b.C4-bp complex formed by the activation of classical complement pathway using an enzyme-linked immunosorbent assay

We developed a quantitative enzyme-linked immunosorbent assay (ELISA) for the detection of C4b.C4-bp complex by incubating the sample on anti-C4-bp-coated plate and then developing with HRP-labeled anti-C4. The amount of C4b.C4-bp complex, generated in vivo by the interaction of purified C4b with C4-bp or normal human serum with aggregated human IgG, was measured by the ELISA. The complex, however, rapidly decreased in serum by the action of factor I. Six out of the 100 plasma samples from patients with various diseases were found positive in the ELISA. One plasma sample from a patient with SLE showed high level of C4b.C4-bp complex with decreased levels of factor I, C4, C4-bp and CH50. These results suggest that the detection of C4b.C4-bp complex is useful for monitoring the diseases in which the classical pathway activation is expected.

Activation of the alternative pathway of complement by human serum IgA

In order to study the activation of complement by soluble aggregates of human polyclonal serum IgA, lysis of sheep erythrocytes (E) coated with several IgA preparations was used as a model. A complement nonactivating monoclonal mouse IgG1 against IgA was used to coat the cells. IgA, isolated from normal human serum, was aggregated by either N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP), glutaraldehyde, carbodiimide or heating. Depending on the size of the aggregates, and on the method of aggregation, E coated with aggregated IgA (E gamma 1.AIgA) could be lysed. The alternative pathway of complement appeared to mediate the lysis because the latter was observed in the presence of EGTA containing 5 mM Mg2+ (MgEGTA) and properdin (P) was deposited on the cells. Furthermore, no lysis was observed in C3-deficient serum. In the absence of AIgA the cells were not lysed, and no P deposition was observed. In another set of experiments E gamma 1.AIgA were first reacted with purified C3, B, D and P for 30 min at 30 degrees C, and subsequently in rat serum EDTA at 37 degrees C. Lysis occurred when E gamma 1.AIgA were prepared using SPDP-, glutaraldehyde- or carbodiimide-AIgA. Incubation of 100 micrograms/ml SPDP-AIgA with normal human serum for 30 min at 37 degrees C in the presence or absence of MgEGTA also induced consumption of total complement. The other soluble AIgA preparations were less effective in activating complement. These results suggest that polymeric serum IgA is capable of activating the alternative pathway of complement.

Hypocomplementemia with low C1s-C1 inhibitor complex in systemic lupus erythematosus

Ninety-three serum and plasma samples from 45 patients with systemic lupus erythematosus were analyzed for the complex formed by C1s and its inhibitor, as well as for C3, C4, C4a desarginine, and staphylococcal protein A-bound immune complexes. There were statistically significant correlations between C1s-C1 inhibitor complex and CH50, between C1s-C1 inhibitor complex and C4, and between C1s-C1 inhibitor complex and C4a desarginine. Serial studies were performed on 24 patients over a period of 6 months. Seven of 21 patients with hypocomplementemia had persistently normal levels of C1s-C1 inhibitor complex, 7 had transiently abnormal levels of C1s-C1 inhibitor complex, and 7 had sustained abnormal levels of C1s-C1 inhibitor complex. Two of 3 pregnant patients with normal levels of complement had abnormal levels of C1s-C1 inhibitor complex. Staphylococcal protein A-bound immune complexes demonstrated no correlation with any of the complement assays. Complement activation, as measured by C1s-C1 inhibitor complex, is often a transient phenomenon in systemic lupus erythematosus patients with persistent hypocomplementemia.

Assay of classical and alternative pathway activities of murine complement using antibody-sensitized rabbit erythrocytes

Methods for measurement of classical complement pathway activity (CH50) and alternative complement pathway activity (ACH50) in mouse serum using rabbit erythrocytes sensitized with guinea pig anti-rabbit erythrocyte antibody have been established. The assays measured CH50 values in mouse sera that could hardly be determined by the conventional method using antibody-sensitized sheep red blood cells. Mouse serum ACH50 values determined by the method were also 5-7 times higher than those obtained in conventional assays with rabbit erythrocytes. Both the CH50 and ACH50 values varied with the strain among the 25 different strains of mice studied. BALB/c (nu/nu, male), LT/SuJ and Jcl-ICR27 strains exhibited higher CH50 values, and NIH (nu/+), ICR (nu/nu), NOD (male) and AKR strains showed lower values. The ACH50 was higher in C3H/HeN (male), C57BL/6J (male), Jcl-ICR27 and BALB/c (nu/nu, male) mice, and lower in ICR (nu/nu), NOD (female) and AKR mice. Sera from 16 out of the 25 mouse strains showed ACH50 values comparable to or higher than those in man. As for CH50, however, even the highest value seen in BALB/c (nu/nu, male) mice corresponded to about three-fifths of an average value in man. It is concluded that the complement system of mice, especially the alternative pathway of complement activation, functions as actively as that in man. It was also found that male mice have higher CH50 and ACH50 values than female mice. The differences in these parameters between males and females were only slight at the age of 4 weeks and became conspicuous after 6 weeks at which time both the CH50 and ACH50 virtually reached their respective peak levels of activity.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.

Quantitation of activation of the human terminal complement pathway by ELISA

We have devised an enzyme-linked immunosorbent assay (ELISA) to quantitate fluid phase terminal complement pathway activation. Upon activation to form C5b-9, terminal complement components express neoantigens not present in the unassembled individual components. Expression of one of these neoantigens occurs at the step of C9 activation. C9 neoantigen is present in fluid phase SC5b-9 complexes, membrane-bound MC5b-9 complexes, and in in vitro polymerized C9. Under physiologic conditions, the presence of C9 neoantigen indicates that the terminal complement pathway is activated through the terminal component C9. In our assay for C9 neoantigen, we used rabbit antiserum to polymerized C9 rendered specific for C9 neoantigenic determinants by serial absorption with human serum, human C9, and other terminal complement components bound to Sepharose. Using the IgG from this antiserum, we devised a sandwich ELISA to bind SC5b-9 from solution onto polystyrene plates. The ELISA plates were developed with the use of goat antiserum to native C9 epitopes followed by a swine anti-goat IgG-alkaline phosphatase conjugate. Quantitation of SC5b-9 in solution was performed by comparing sample OD to a standard curve generated with human SC5b-9 that was purified from zymosan-activated serum. The assay was sensitive to as little as 100 ng of SC5b-9/ml and should be useful for screening plasma, serum, cerebrospinal fluid, or other biological fluids for the presence of terminal complement pathway activation.

Increased activation of the alternative complement pathway in sickle cell disease

Complement proteins play an important role in host defenses against Streptococcus pneumoniae, a major cause of serious infections in sickle cell (SS) disease. Previous studies have suggested abnormalities of the alternative complement pathway in SS disease. We measured activation of the alternative pathway in sera from patients with SS disease utilizing an enzyme immunoassay which detects C3b,P complexes, derivative of the C3b,Bb,P alternative pathway convertase. In all, 89% of SS sera had elevated concentrations of C3b,P complexes, indicative of increased alternative pathway activation. Chronic activation of the alternative pathway may contribute to impaired host defense in SS patients.

Complement effector mechanisms in health and disease

Complement is an effector system able to mediate a number of biological activities in vitro and in vivo. Most familiar is the ability of the system to mediate the lytic destruction of numerous kinds of cells and pathogenic organisms including bacteria, viruses, and virus-infected cells. In addition, the complement system also activates neutrophils, monocytes, basophils, mast cells, and lymphocytes to perform specialized functions. While generally considered to be confined to the effector side of immune reactions, recent evidence indicates that the complement system also directly recognizes and is triggered by a number of bacteria and viruses as well as virus-infected cells in the absence of antibody. In such reactions, complement fulfills the recognition role normally associated with the antibody molecule or immune lymphocyte. The complement system may thus also function as a natural surveillance system operative prior to the induction of specific immunity. Involvement of the complement system in biological reactions has been ascertained by several techniques over the years. These include quantitation of individual complement components in human sera and demonstration of complement deposition in diseased tissues in human diseases and in experimental diseases in animals. Such techniques, however, have limitations in specificity and sensitivity. Assays which detect specific features of the complement activation process have become available in recent years. These tests detect the physical, chemical, or antigenic changes characteristic of the complement activation process. These assays are extremely specific and quantitative; furthermore, most are usable with samples from patients. Three general approaches have been utilized to develop such specific quantitative assays for complement activation. The first includes assays which quantitate activation-specific limited proteolysis of the complement components. The second type of assay includes tests which detect and quantitate new antigens or other activation-specific antigenic changes. The third category is represented by assays which detect and quantitate the protein-protein complexes characteristic of the activation process. Examples of tests presenting each of these approaches are given.(ABSTRACT TRUNCATED AT 400 WORDS)

Radioimmunoassay of the attack complex of complement in serum from patients with systemic lupus erythematosus

We developed a radioimmunoassay to measure the attack complex of complement (SC5b-9) in serum from patients with systemic lupus erythematosus. The radioimmunoassay used a monoclonal antibody to an antigen of C9, which is absent from native C9 but present on SC5b-9. SC5b-9 was detectable in 13 of 63 normal subjects, with a mean value of 0.5 unit (range, 0 to 4.8), and elevated in 13 of 14 patients with active lupus. Analysis of 108 samples from the patients with lupus revealed a mean value of 10.1 units (range, 0 to 35) when the disease was active and 1.0 unit (range, 0 to 433) when it was clinically stable. SC5b-9 was a more sensitive measure of disease activity than C3, C4, or CH50; its specificity was equivalent to that of C3 and C4. SC5b-9 was elevated in serum from 6 of 49 patients with other forms of glomerulonephritis. Our study documents the presence of SC5b-9 in abnormal serum and correlates its elevation with clinical disease activity in patients with active systemic lupus erythematosus.

The role of antibody and complement in the control of viral infections

Host defense against viral infection is extremely complex and includes both humoral and cellular immune mechanisms. This contribution examines the mechanisms by which antibody (Ab) and the complement (C) system, major constituents of the humoral immune system, inactivate viruses and block viral maturation in virus-infected cells in vitro. Ab and C may neutralize viruses by envelopment in a coating of protein, by aggregation by lysis, or by facilitating interactions with various effector cells. Ab and C molecules deposited on the surfaces of viruses may physically interfere with the ability of the virus to infect a potentially susceptible cell. This appears to be the most common mechanism by which Ab and C neutralize viruses. In rare instances, Ab and/or C may aggregate viruses; aggregation reduces the net number of infectious particles and thus is manifest as neutralization. C may lyse enveloped viruses, resulting in irreversible viral inactivation. However, this does not appear to be a major mechanism of viral neutralization. Finally, the Fc portions of bound Ab molecules as well as bound C molecules may interact with effector cells with specific receptors for these factors and thereby facilitate viral destruction. In regard to virus-infected cells, the deposition of Ab or C on the cell surface may prevent the maturation or release of viral particles and alter normal cellular functions. Ab and C may also lyse virus-infected cells, abruptly stopping further viral maturation. Such lytic events require only the F(ab')2 portion of the Ab molecule and proceed via activation of the alternative C pathway. Effector cells may also interact with Ab and/or C molecules deposited on virus-infected cells, leading to cytotoxic reactions and/or ingestion depending on the type of effector cell involved. The activated C system has the ability to produce an acute inflammatory response leading to alterations in vessel permeability, edema, changes in smooth-muscle contractility, and the influx of leukocytes. Such inflammatory responses occurring in tissues, including the skin, as a result of C activation not only retard the spread of the infection and facilitate the destruction of the infectious agent, but also in all likelihood damage normal tissues in the vicinity. In addition, C activation in tissues also has the ability to stimulate arachidonic acid metabolism and induce the release of histamine and other mediators as well as pyrogens from appropriate cell types. A number of the systemic symptoms characteristic of viral infections, such as headaches, myalgias, and fever, likely result from such processes.