The pathobiology of the terminal complement complexes

Promotion of the inflammatory response in mid colon of complement component 3 knockout mice

To determine whether complement component 3 (C3) deficiency affects its receptor downstream-mediated inflammatory response, the current study was undertaken to measure alterations in the inducible nitric oxide synthase (iNOS)‑mediated cyclooxygenase‑2 (COX‑2) induction pathway, inflammasome pathway, nuclear factor-κB (NF-κB) activation, and inflammatory cytokine expressions in the mid colon of C3 knockout (KO) mice. Significant enhancement was observed in expressions of key components of the iNOS‑mediated COX‑2 induction pathway, and in the phosphorylation of mitogen‑activated protein (MAP) kinase members. A similar pattern of increase was also observed in the expression levels of inflammasome proteins in C3 KO mice. Moreover, compared to WT mice, C3 KO mice showed remarkably enhanced phosphorylation of NF-κB and Inhibitor of κB-α (IκB-α), which was reflected in entirety as increased expressions of Tumor necrosis factor (TNF), IL-6 and IL-1α. However, the levels of E-cadherin, tight junction channels and ion channels expressions were lower in the C3 KO mice, although myeloperoxidase (MPO) activity for neutrophils was slightly increased. Taken together, results of the current study indicate that C3 deficiency promotes inflammatory responses in the mid colon of C3 KO mice through activation of the iNOS‑mediated COX‑2 induction pathway, Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)-inflammasome pathway and NF-κB signaling pathway, and the enhancement of inflammatory cytokine expressions.

Soluble Membrane Attack Complex: Biochemistry and Immunobiology

The soluble membrane attack complex (sMAC, a.k.a., sC5b-9 or TCC) is generated on activation of complement and contains the complement proteins C5b, C6, C7, C8, C9 together with the regulatory proteins clusterin and/or vitronectin. sMAC is a member of the MACPF/cholesterol-dependent-cytolysin superfamily of pore-forming molecules that insert into lipid bilayers and disrupt cellular integrity and function. sMAC is a unique complement activation macromolecule as it is comprised of several different subunits. To date no complement-mediated function has been identified for sMAC. sMAC is present in blood and other body fluids under homeostatic conditions and there is abundant evidence documenting changes in sMAC levels during infection, autoimmune disease and trauma. Despite decades of scientific interest in sMAC, the mechanisms regulating its formation in healthy individuals and its biological functions in both health and disease remain poorly understood. Here, we review the structural differences between sMAC and its membrane counterpart, MAC, and examine sMAC immunobiology with respect to its presence in body fluids in health and disease. Finally, we discuss the diagnostic potential of sMAC for diagnostic and prognostic applications and potential utility as a companion diagnostic.

Different mechanisms of serum complement activation in the plasma of common (Chelydra serpentina) and alligator (Macrochelys temminckii) snapping turtles

Reptiles are declining worldwide yet our understanding of their immune function lags far behind other taxa. The innate immune system is the primary mode of defense in reptiles, and the serum complement cascade is its major component. We assessed serum complement activity of plasma in two closely related aquatic turtle species, the common snapping turtle (CST; Chelydra serpentina) and alligator snapping turtle (AST; Macrochelys temminckii). We used a sheep red blood cell (SRBC) hemolysis assay to assess serum complement activity. Although the antibacterial activities of the plasma of these turtle species are similar, the hemolytic activity was much stronger in CST than AST. Treatment with inhibitors of the serum complement cascade indicated differences in the mechanisms of complement activation between the turtle species. We subjected plasma from both turtle species to mannan affinity chromatography and analyzed the eluate with SDS-PAGE, which revealed that plasma from the CSTs contained only small amounts of one C-type lectin protein while the AST plasma contained high concentrations of two C-type lectins (31.0 and 35.9 kDa). Edman degradation analyses confirmed that the two AST proteins contained identical N-terminal sequences. Thus, the CST appears to rely more heavily on the alternative mechanism of serum complement activation, while the AST appears to rely more on the lectin-mediated pathway, which is a pattern recognition response to prokaryotes not activated by the SRBCs. These results are unique in that the use of serum complement pathways are generally assumed to be conserved within clades.

Quantification of complement C5b-9 binding to cells by flow cytometry

Interaction of the complement system, directly or indirectly (e.g., via antibodies), with cells activates the early and late complement components and culminates in the deposition of a membrane-spanning C5b-9 complex on the cell surface. At a high copy number, this C5b-9 will activate cell death, whereas at a low copy number, it will transmit various signals into cells. Quantification of C5b-9 deposition is useful for assessments of the capacity of cells and antibodies to activate complement. By using an antibody that identifies a novel antigen of the C5b-9 complex, the amount of C5b-9 complexes on cells can be quantified by flow cytometry. The detailed protocol is described in this chapter.

Control of NF-κB and inflammation by the unfolded protein response

Under inflammatory situations, endoplasmic reticulum (ER) stress occurs at local sites and modulates inflammatory processes. NF-κB is a key regulator for immune and inflammatory responses, and its activity is influenced by ER stress positively or negatively. Recent investigation suggested that ER stress induces activation of NF-κB in the early phase, whereas in the later phase, consequent unfolded protein response (UPR) inhibits NF-κB. This review summarizes current knowledge on potential mechanisms underlying the biphasic, bidirectional regulation of NF-κB by the UPR and possible roles for ER stress in the regulation of inflammation.

New insights of an old defense system: structure, function, and clinical relevance of the complement system

The complement system was discovered a century ago as a potent defense cascade of innate immunity. After its first description, continuous experimental and clinical research was performed, and three canonical pathways of activation were established. Upon activation by traumatic or surgical tissue damage, complement reveals beneficial functions of pathogen and danger defense by sensing and clearing injured cells. However, the latest research efforts have provided a more distinct insight into the complement system and its clinical subsequences. Complement has been shown to play a significant role in the pathogenesis of various inflammatory processes such as sepsis, multiorgan dysfunction, ischemia/reperfusion, cardiovascular diseases and many others. The three well-known activation pathways of the complement system have been challenged by newer findings that demonstrate direct production of central complement effectors (for example, C5a) by serine proteases of the coagulation cascade. In particular, thrombin is capable of producing C5a, which not only plays a decisive role on pathogens and infected/damaged tissues, but also acts systemically. In the case of uncontrolled complement activation, “friendly fire” is generated, resulting in the destruction of healthy host tissue. Therefore, the traditional research that focuses on a mainly positive-acting cascade has now shifted to the negative effects and how tissue damage originated by the activation of the complement can be contained. In a translational approach including structure-function relations of this ancient defense system, this review provides new insights of complement-mediated clinical relevant diseases and the development of complement modulation strategies and current research aspects.

Biphasic, bidirectional regulation of NF-kappaB by endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress induces an adaptive program called the unfolded protein response (UPR), which affects activity of an array of kinases and transcription factors. Previous reports provided evidence for activation of nuclear factor-kappaB (NF-kappaB), the major transcription factor regulating inflammatory processes, by ER stress. However, recent investigation also suggested that preceding ER stress suppresses activation of NF-kappaB by subsequent exposure to inflammatory stimuli. Although ER stress induces activation of NF-kappaB in the early phase, consequent UPR may inhibit NF-kappaB-dependent cellular activation in the later phase. This article summarizes current knowledge on potential mechanisms underlying the biphasic, bidirectional regulation of NF-kappaB by ER stress.

Identification and characterization of serum complement activity in the American crocodile (Crocodylus acutus)

Incubation of unsensitized sheep red blood cells with serum from the American crocodile (Crocodylus acutus) resulted in a concentration-dependent hemolysis. The hemolytic activity was heat-sensitive, and inhibited by EDTA in a concentration-dependent manner. The EDTA-inhibited SRBC hemolysis could be restored by the addition of excess Ca2+ or Mg2+, but not Ba2+ or Cu2+, revealing the specificity of this activity for these two divalent cations. The hemolytic activity of crocodile serum was titer-dependent, with 329 microL producing 50% of maximal SRBC hemolysis. The complement activity was also temperature-dependent, with decreased activity at lower temperatures (5-15 degrees C) and maximal activity occurred at 30-40 degrees C. The hemolysis occurred relatively slowly, with near zero activity after 10 min, 40% of activity observed within 15 min of exposure to SRBCs, and maximal activity at 30 min.

Characterization of serum complement activity of saltwater (Crocodylus porosus) and freshwater (Crocodylus johnstoni) crocodiles

We employed a spectroscopic assay, based on the hemolysis of sheep red blood cells (SRBCs), to assess the innate immune function of saltwater and freshwater crocodiles in vitro. Incubation of serum from freshwater and saltwater crocodiles with SRBCs resulted in concentration-dependent increases in SRBC hemolysis. The hemolytic activity occurred rapidly, with detectable activity within 2 min and maximum activity at 20 min. These activities, in both crocodilian species, were heat sensitive, unaffected by 20 mM methylamine, and completely inhibited by low concentrations of EDTA, suggesting that the alternative serum complement cascade is responsible for the observed effects. The hemolytic activities of the sera were inhibited by other chelators of divalent metal ions, such as phosphate and citrate. The inhibition of SRBC hemolysis by EDTA could be completely restored by the addition of 10 mM Ca2+ or Mg2+, but not Ba2+, Cu2+ or Fe2+, indicating specificity for these metal ions. The serum complement activities of both crocodilians were temperature-dependent, with peak activities occurring at 25-30 degrees C and reduced activities below 25 degrees C and above 35 degrees C.

Identification of alternative pathway serum complement activity in the blood of the American alligator (Alligator mississippiensis)

Incubation of different dilutions of alligator serum with sheep red blood cells (SRBCs) that had not been sensitized with antibodies resulted in concentration-dependent hemolytic activity. This hemolytic activity was not affected by the presence of ammonium hydroxide and methylamine, known inactivators of the classical complement cascade. However, the hemolytic activities were inhibited by EDTA and salicylaldoxime, indicating that the alternate pathway is primarily responsible for these activities. Immunofixation of electrophoretically-resolved alligator serum proteins with antihuman C3 polyclonal antibodies resulted in detection of a protein antigenically similar to human C3 in alligator serum. SDS-PAGE, followed by Western blot analysis, revealed the presence of two alligator serum proteins with nearly identical molecular weights as human C3alpha and C3beta. SRBC hemolysis and antibacterial activity by alligator serum was significantly reduced in the presence of antihuman C3 antibodies. The hemolytic effect of alligator serum was shown to occur rapidly, with significant activity within 5 min and maximal activity occurring at 15 min. SRBC hemolysis was also temperature-dependent, with reduced activity below 15 degrees C and above 30 degrees C. These data suggest that the antibiotic properties of alligator serum are partially due to the presence of a complement-facilitated humoral immune response analogous to that described in mammalian systems.

Role of divalent metal ions in serum complement activity of the American alligator (Alligator mississippiensis)

Treatment of alligator serum with different concentrations of EDTA resulted in a concentration-dependent inhibition of serum-mediated sheep red blood cell (SRBC) hemolysis. This inhibition of serum-dependent hemolysis was observed for other chelators of divalent metal ions, such as phosphate and citrate. Treatment of alligator serum with 5 mM EDTA completely inhibited SRBC hemolysis, which could be totally restored by the addition of 5 mM Ca(2+) or Mg(2+), but not Cu(2+) or Ba(2+). These data indicate a specific need for Ca(2+) and/or Mg(2+) in the serum-mediated hemolysis of SRBCs. Kinetic analyses revealed that the addition of 30 mM EDTA 1 min after incubation of SRBCs with serum resulted in only 30% inhibition of hemolytic activity. However, addition of EDTA as early as 3 min post-incubation resulted in complete SRBC hemolysis. Pretreatment of serum with EDTA inhibited the hemolytic activity, but the activity could be restored in a time-dependent manner by the addition of Ca(2+)or Mg(2+). These data indicate that, as in human serum, the need for divalent metal ions occurs early in the alligator serum complement cascade.

Carboxymethyl benzylamide sulfonate dextrans (CMDBS), a family of biospecific polymers endowed with numerous biological properties: a review

The functionalized dextrans termed carboxymethyl benzylamide sulfonate dextran (CMDBS) represent a family encompassing a wide range of polymers. These soluble macromolecular compounds, which are substituted with specific chemical functional groups, are designed to interact with living systems. By analogy with glycosaminoglycan heparin, a natural highly charged anionic polysaccharide that exerts a variety of biological effects, we postulated that CMDBS compounds also possess binding sites capable of specific interactions with biological constituents, depending on the overall composition of the polymer. The synthesis and heparin-like properties of these CMDBS have been extensively investigated. Thus, it appears that dextran derivatives can mimic the action of heparin in regard to its interactions with antithrombin and serine proteases involved in blood coagulation. Other derivatives interact with various components of the immune system or with adhesive proteins such as fibronectin in modulating the proliferation of Staphylococcus aureus. Because they are able to stimulate wound healing in various in vivo models, these polysaccharides may also constitute a family of tissue repair agents because of their protecting and potentiating effects with heparin binding growth factors. Moreover, dextran derivatives in contact with cells such as endothelial cells, smooth muscle cells, or tumoral cells can affect both cell proliferation and metabolism. It appears that these bioactive polymers are also efficient tools to investigate the precise mechanism of action of individual biological activities by contrasting their mode of action to that of heparin. In addition to their numerous biological properties and biospecificity, functionalized dextrans are relatively simple to manufacture and exempt of donor contaminant, which make them attractive in a variety of clinical applications.

Effect of complement consumption by cobra venom factor on the course of primary infection with simian immunodeficiency virus in rhesus monkeys

Cobra venom factor (CVF)-induced consumption of complement proteins was used to investigate the role of complement in vivo in the immunopathogenesis of simian immunodeficiency virus of macaques (SIVmac) infection in rhesus monkeys. Repeated administration of CVF was shown to deplete complement to <5% of baseline hemolytic activity of serum complement for 10 days in a normal monkey. Three groups of SIVmac-infected animals were then evaluated: monkeys treated with CVF resulting in complement depletion from days -1 to 10 postinfection, monkeys treated with CVF resulting in complement depletion from days 10 to 21 postinfection, and control monkeys that received no CVF. CD8+ SIVmac-specific cytotoxic T lymphocyte (CTL) generation and CD4+ T lymphocyte depletion during primary infection were not affected by CVF treatment. Viral load, assessed by measurements of plasma p27gag antigen and viral RNA, was transiently higher during the first 4 weeks following infection in the CVF-treated monkeys and the subsequent clinical course in these treated animals was accelerated. These results suggest that complement proteins may participate in immune defense mechanisms that decrease virus replication following the initial burst of intense viremia during primary SIVmac infection. However, we cannot rule out that the observed increased virus replication was induced by immune activation resulting from the administration of a foreign antigen to these monkeys.

Alternative pathway activation of the complement system in preterm infants with early onset infection

The increased incidence of infection in preterm neonates has been related in part to their relative deficiency of most complement components, because complement is known to participate in the defense against bacterial and viral infections. In a prospective study, complement activation products were determined in 52 preterm infants. Twenty preterm infants suffered from proven early onset infection, 11 infants were presumed to suffer from infection, which could not be confirmed. Twenty-one preterm infants without infection or perinatal asphyxia formed the control group. EDTA plasma was obtained within the first 6 h after birth, and follow-up examinations were done in 15 patients with proven infection during the next 24 h. The complement activation products C3a-desArg, C3bBbP, and sC5b-9 were measured with enzyme immunoassay systems. In preterm neonates with early onset infection, a significant elevation of C3a-desArg was found in the very early course of the disease. C3a-desArg generation resulted from alternative pathway activation as shown by a concurrent increase of C3bBbP concentration. In addition, significantly higher concentrations of sC5b-9 predicted infection in the first few hours after birth. Thus, despite very low levels of native complement proteins, preterm babies are able to generate remarkable amounts of activation products of the complement cascade. The elevation of these activation products preceded by hours significant changes of routine laboratory markers of infection, such as leukocyte count, differential blood count, and C-reactive protein. Thus they might help to identify preterm neonates with severe systemic infection earlier than other laboratory parameters.

Complement in acute and chronic arthritides: assessment of C3c, C9, and protectin (CD59) in synovial membrane

OBJECTIVES

To investigate the role of complement cascade induced damage and protection against it in acute arthritides compared to rheumatoid arthritis and other chronic joint derangements.

METHODS

C3c, C9, and protectin (CD59) were examined by avidin-biotin-peroxidase complex staining.

RESULTS

Marked deposits of C3c and C9 were found in synovial vasculature and intercellular matrix of the lining in rheumatoid arthritis and in acute arthritides (including bacterial, reactive, and osteoarthritis flare up). Furthermore, protectin was not visible in synovial lining cells and was relatively weakly expressed in stromal and endothelial cells in rheumatoid arthritis; also in acute arthritides protectin expression was weak. In contrast, C3c and C9 deposits were not found in chronic conditions associated with degenerative diseases (osteoarthritis and osteochondritis dissecans) or mechanical causes (patellar luxation and a ruptured meniscus), in which also the protectin expression was prominent in synovial lining, endothelial and some stromal cells.

CONCLUSIONS

Activation of the complement in rheumatoid arthritis and in acute arthritides seems to be associated with a decreased protection of synovial cells against cellular effects and lysis mediated by membrane attack complex.

Sulfonated dextran inhibits complement activation and complement-dependent cytotoxicity in an in vitro model of hyperacute xenograft rejection

In the present study, we demonstrate that a substituted soluble dextran derivative bearing 73% carboxylic groups and 15% benzylamide sulfonate groups, termed CMDBS25, inhibits complement activation and complement-mediated damage in an in vitro model of xenogeneic rejection. Incubation of porcine aortic endothelial cells with normal human serum resulted in time-dependent complement consumption as assessed by C3a generation in the fluid phase and deposition of activated complement fragments C3, C5 and of C5b-9 on target cells. The presence of C5b-9 membrane attack complex was associated with 51Cr release from prelabelled endothelial cells. The addition of 5-25 mg of CMDBS25/ml under the experimental conditions used, inhibited complement activation and C3a generation in a dose-dependent fashion. CMDBS25 (25 mg/ml) totally suppressed iC3b, C5 and C5b-9 cytolytic complex deposition on cells and inhibits by 42% lysis of target endothelial cells. Native dextran had no effect. Our observations document the anti-complementary properties of sulfonated dextran derivatives and their potential as therapeutic agents for the prevention of complement-dependent hyperacute xenograft rejection.

Effect of substituted dextran derivative on complement activation in vivo

A soluble dextran derivatized with carboxylic groups (73%) and benzylamide sulphonate groups (15%), termed CMDBS 25, exhibited significant anticomplementary activity in the absence of anticoagulant activity. The polysaccharide inhibited both classical and alternative pathway-dependent complement activation in human and rat serum in vitro. Simultaneous administration of CMDBS 25 (100 mg) and crushed Sephadex G25 (20 mg) into normal Lewis rats suppressed systematic complement consumption that was induced by Sephadex in the animals by 98% for 1 h. Two consecutive injections of 100 mg of CMDBS at 1 h interval resulted in total suppression of systemic complement activation for 2 h and in 50% suppression for an additional 2 h. Infusion of CMDBS alone was well tolerated and had no effect on CH50 in serum in vivo. Our results demonstrate that CMDBS 25 exhibits anticomplementary properties in vivo and suggest that the polymer represents a potential therapeutic agent for pathological conditions associated with complement activation.

Complement inhibition by human vitronectin involves non-heparin binding domains

Vitronectin (complement S-protein), a multifunctional glycoprotein, inhibits complement-mediated cytolysis at two identified stages of terminal complement complex (TCC) formation: blocking of C5b-7 membrane binding, and prevention of C9 polymerization. However, the functional domain(s) of vitronectin involved in these reactions remains incompletely defined. In order to identify the complement inhibition site, a 12-kD heparin binding fragment and two other internal fragments (53 kD and 43 kD) of vitronectin were isolated after cyanogen bromide (CNBr) treatment of the native molecule. Potent inhibition of guinea pig erythrocyte (GPE) reactive lysis was demonstrated with native vitronectin, total CNBr digest and the 53-kD and 43-kD fragments, but only very poorly by the heparin binding 12-kD peptide. Similarly, the 43-kD fragment blocked the binding of C5b-7 to immobilized vitronectin, whereas the 12-kD fragment had no effect. These data localize the C5b-7 binding site to a 43-kD internal region. Further characterization of the fragments was carried out in an assay which detected C9 polymerization in the presence of C5b-8. Polymerized material was separated by PAGE, detected by autoradiography and quantified after excision from the gels. Results showed that polymerization did not occur in the presence of the 53-kD and 43-kD fragments. However, the 12-kD heparin binding fragment had no effect. It is proposed that prevention of C5b-8-induced C9 polymerization resides at a site in an internal region of the vitronectin molecule.

The kinetics and distribution of C9 and SC5b-9 in vivo: effects of complement activation

Many diseases associated with complement activation are characterized by tissue deposition of components of the terminal complement complex (TCC). The ninth component of complement (C9) plays an important role in the cytolytic effects, and may contribute to the non-lethal cell-regulating functions of the TCC. In this study we examined the behaviour of radiolabelled human C9 and its soluble complexed form SC5b-9 in vivo in order to determine the effects of complement activation on its turnover, distribution and molecular size. In normal rabbits the metabolic parameters of 125I-C9 (median and range) were: plasma half-life (t1/2) 25.9 (20.6-29.5) h, fractional catabolic rate (FCR) 5.7 (5.3-7.0)%/h, and extravascular/intravascular ratio (EV/IV) 0.7 (0.6-1.1). The distribution of radiolabelled C9 amongst body tissues was similar to that observed for rabbit serum albumin (RSA). Activation of the complement cascade with i.v. injection of cobra venom factor (CVF) resulted in rapid disappearance of C9 from the plasma and accumulation of protein-bound radiolabeled in the spleen (exceeding the plasma concentration) and the liver. RSA metabolism and distribution were unaffected by CVF. Fine performance liquid chromatography (FPLC) gel filtration of plasma samples suggested that monomeric C9 was the only major radiolabelled protein present during normal turnovers, whereas CVF administration was accompanied by the prompt appearance of a high mol. wt species consistent in size with SC5b-9. When injected directly, 125I-SC5b-9 disappeared rapidly from the plasma, falling by 50% in 0.7 (0.6-0.8) h, and less than 15% remaining after 4 h with accumulation of protein-bound label in the spleen and liver. These results demonstrate the complexity of C9 metabolism during complement activation.

Two-site ELISA for quantification of the terminal C5b-9 complement complex in plasma. Use of monoclonal and polyclonal antibodies against a neoantigen of the complex

A quantitative ELISA procedure using monoclonal and polyclonal antibodies against neoantigens of the terminal C5b-9 complement complex has been developed. The ELISA was demonstrated to be both sensitive and reproducible. The normal range for C5b-9 determinations, defined as 2.5-97.5% interval of the values obtained in 76 healthy blood donors, was 3.12-10.3 AU/ml. The presence of rheumatoid factor did not affect the determination of C5b-9 as demonstrated by immunoabsorption studies.

Vitronectin-mediated inhibition of complement: evidence for different binding sites for C5b-7 and C9

In the activated complement system, vitronectin (complement S-protein) occupies the metastable membrane binding site of the nascent precursor complex C5b-7, so that the newly formed SC5b-7 is unable to insert into cell membranes. Some evidence also indicates that vitronectin limits on-going membrane-associated pore formation by inhibiting C9 polymerization. It has been assumed that these two stages of terminal complement complex (TCC) inhibition take place through charge interactions between the heparin-binding region of vitronectin and homologous cysteine-rich sequences of the late complement proteins C6, C7, C8 and C9. We examined SC5b-7 formation and inhibition of C9 binding in the TCC using separate haemolytic assays. The mode of action of vitronectin in these assays was compared with two 15mer peptides which span residues 348-379 of the heparin-binding region, and a heparin-affinity polypeptide, protamine sulphate. The results showed that vitronectin acts predominantly through SC5b-7 production with a lesser effect on the inhibition of C9 lytic pore formation. In contrast, protamine sulphate did not prevent C5b-7 membrane attachment, but was a potent inhibitor of C9-mediated lysis. The peptides did not inhibit C5b-7 membrane insertion and only one affected C9 binding. These data suggest that the two stages of TCC inhibition involve separate binding sites on the vitronectin molecule. The site for association with nascent C5b-7 is unknown, whereas inhibition of C9 binding and pore formation takes place through the heparin-binding region.

Immunological evaluation of late complement component-deficient individuals

The possible contribution of additional immunologic variables to the susceptibility of late complement component-deficient individuals to meningococcal disease has not been systematically examined in previous studies. Thus, we studied three groups of patients: (1) 24 healthy individuals, (2) 8 complement-sufficient individuals with a history of recurrent bacterial meningitis, and (3) 19 complement-deficient individuals with prior meningococcal infection. No statistical differences were noted among the three groups for the following parameters: the absolute number and the percentage of lymphocytes; CD3+, CD4+, CD8+, CD20+, and CD16+ cells; and the CD4+/CD8+ ratio. The concentration of C4 and circulating immune complexes was also similar among the groups. The concentrations of IgG, IgM, and IgA were slightly, but significantly, decreased in the complement-deficient individuals. Of interest, the coefficient of spontaneous and lipopolysaccharide-stimulated activation of neutrophils was significantly depressed in the deficient individuals. We hypothesize that the terminal complement components may participate in maximal neutrophil activation.

Urinary C3dg and C5b-9 indicate active immune disease in human membranous nephropathy

We have measured complement activation markers, C3dg and C5b-9 in plasma and urine from patients with idiopathic membranous nephropathy and IgA nephropathy. There was no significant difference in levels of plasma C5b-9 between the patient groups. However, high plasma concentrations of C3dg were associated significantly with IgA nephropathy with 45% of patients having levels over 25 U/ml (P less than 0.001). High concentrations of urinary C3dg and C5b-9 were associated significantly with membranous nephropathy (43% and 43% of the patient group, respectively) compared to patients with IgA nephropathy (10% and 0%, respectively, P less than 0.001). In a retrospective analysis of 31 patients with membranous nephropathy, 66% of patients with high initial urinary C5b-9 showed an unstable clinical course compared to 18% of patients with initially absent or low C5b-9 (P less than 0.001). We suggest that high urinary C5b-9 identifies those patients with a membranous lesion which retains an active immunological component contributing to the pathology of progressive glomerular damage.

Functionally active complement proteins C6 and C7 detected in C6- and C7-deficient individuals

Two sensitive sandwich ELISAs based on monoclonal antibodies directed to native C6 and C7 allowed the detection and quantitation of these complement proteins in 20 out of 37 serum samples from individuals who had previously been classified as deficient in these proteins as assessed by immunochemical and/or functional assays. Furthermore, serum from four C6-deficient and one combined C6-/C7-deficient individual showed an increase in the terminal complement complex (TCC) and a decrease in native C6 and C7 after complement activation as assayed by specific ELISAs. Despite their (incomplete) deficiencies, these individuals therefore possess functionally active terminal complement proteins with respect to their ability to generate the TCC. As these individuals have no history of a susceptibility to neisserial infections, even low concentrations of functionally active C6 and C7 may provide sufficient protection against those micro-organisms whose destruction requires TCC formation.