Role of terminal complement pathway in the heterologous phase of antiglomerular basement membrane nephritis

Animal models of lupus nephritis: the past, present and a future outlook

Lupus nephritis (LN) is the most severe end-organ pathology in Systemic Lupus Erythematosus (SLE). Research has enhanced our understanding of immune effectors and inflammatory pathways in LN. However, even with the best available therapy, the rate of complete remission for proliferative LN remains below 50%. A deeper understanding of the resistance or susceptibility of renal cells to injury during the progression of SLE is critical for identifying new targets and developing effective long-term therapies. The complex and heterogeneous nature of LN, combined with the limitations of clinical research, make it challenging to investigate the aetiology of this disease directly in patients. Hence, multiple murine models resembling SLE-driven nephritis are utilised to dissect LN's cellular and genetic mechanisms, identify therapeutic targets, and screen novel compounds. This review discusses commonly used spontaneous and inducible mouse models that have provided insights into pathogenic mechanisms and long-term maintenance therapies in LN.

Unleashing the power of complement activation: unraveling renal damage in human anti-glomerular basement membrane disease

Anti-glomerular basement membrane (GBM) disease is a rare but life-threatening autoimmune disorder characterized by rapidly progressive glomerulonephritis with or without pulmonary hemorrhage. Renal biopsies of anti-GBM patients predominantly show linear deposition of IgG and complement component 3 (C3), indicating a close association between antigen-antibody reactions and subsequent complement activation in the pathogenesis of the disease. All three major pathways of complement activation, including the classical, lectin, and alternative pathways, are involved in human anti-GBM disease. Several complement factors, such as C3, C5b-9, and factor B, show a positive correlation with the severity of the renal injury and act as risk factors for renal outcomes. Furthermore, compared to patients with single positivity for anti-GBM antibodies, individuals who are double-seropositive for anti-neutrophil cytoplasmic antibody (ANCA) and anti-GBM antibodies exhibit a unique clinical phenotype that lies between ANCA-associated vasculitis (AAV) and anti-GBM disease. Complement activation may serve as a potential "bridge" for triggering both AAV and anti-GBM conditions. The aim of this article is to provide a comprehensive review of the latest clinical evidence regarding the role of complement activation in anti-GBM disease. Furthermore, potential therapeutic strategies targeting complement components and associated precautions are discussed, to establish a theoretical basis for complement-targeted therapies.

Renal diseases and the role of complement: Linking complement to immune effector pathways and therapeutics

The complement system is an ancient and phylogenetically conserved key danger sensing system that is critical for host defense against pathogens. Activation of the complement system is a vital component of innate immunity required for the detection and removal of pathogens. It is also a central orchestrator of adaptive immune responses and a constituent of normal tissue homeostasis. Once complement activation occurs, this system deposits indiscriminately on any cell surface in the vicinity and has the potential to cause unwanted and excessive tissue injury. Deposition of complement components is recognized as a hallmark of a variety of kidney diseases, where it is indeed associated with damage to the self. The provenance and the pathophysiological role(s) played by complement in each kidney disease is not fully understood. However, in recent years there has been a renaissance in the study of complement, with greater appreciation of its intracellular roles as a cell-intrinsic system and its interplay with immune effector pathways. This has been paired with a profusion of novel therapeutic agents antagonizing complement components, including approved inhibitors against complement components (C)1, C3, C5 and C5aR1. A number of clinical trials have investigated the use of these more targeted approaches for the management of kidney diseases. In this review we present and summarize the evidence for the roles of complement in kidney diseases and discuss the available clinical evidence for complement inhibition.

Development of an anti-human complement C6 monoclonal antibody that inhibits the assembly of membrane attack complexes

Membrane attack complexes (MACs; C5b-9) assembled after complement activation can directly injure self-tissues, leading to various diseases. Eculizumab, a monoclonal antibody (mAb) against complement component C5, is being used in the clinic to treat diseases in which MAC-mediated tissue damage is a primary cause. However, C5 is not a selective target for MAC assembly inhibition, and some patients respond incompletely or not at all to the eculizumab treatment. Therefore, C6, the next essential component in the terminal pathway of complement activation, may be an alternative target for the selective inhibition of MAC formation. Surprisingly, few reports describe a functional blockade of C6 using a specific mAb. Here, we report the development of an anti-human C6 mAb (clone 1C9) that recognizes C6 both in free circulation and within C5b6 complexes. This mAb blocked C7 binding to C5b6 complexes and consequently inhibited MAC formation and protected affected paroxysmal nocturnal hemoglobinuria patient red blood cells from MAC-mediated damage in vitro. In addition, this mAb cross-reacts with rhesus monkey but not mouse complement C6. Finally, 1C9 significantly reduced human complement-mediated intravascular hemolysis in vivo in a mouse model. These results suggest that the anti-C6 mAb holds promise as a new therapeutic agent that selectively targets MAC for many complement-mediated pathological conditions.

Complement activation contributes to the injury and outcome of kidney in human anti-glomerular basement membrane disease

PURPOSE

Linear or granular deposition of complement 3 (C3) along glomerular basement membrane (GBM) is generally revealed in kidneys of human anti-GBM disease. However, the mechanism of complement activation and its association with clinical features and outcomes are less clear.

METHODS

We measured the plasma and urinary levels of complement components, C1q, mannose-binding lectin (MBL), factor B (Ba), C3, C3a, C4, C4a, C5, C5a and soluble C5b-9 (SC5b-9), using ELISA in 20 patients with renal biopsy proven anti-GBM disease.

RESULTS

The end product of complement activation, SC5b-9, was elevated both in plasma and urine. The levels of C3 and C4 were normal in plasma, while elevated in urine. The levels of C5a and SC5b-9 were increased in plasma from 15% and 30% patients respectively, while they were raised in urine from almost all patients (100% and 92%). The levels of plasma SC5b-9 and urinary C5a were positively correlated with the serum creatinine at presentation (r=0.56, P=0.01; r=0.68, P=0.02, respectively) and the percentage of crescents in glomeruli (r=0.60, P=0.005; r=0.75, P=0.005, respectively). The plasma level of SC5b-9 was further identified as the predictor for renal failure during follow up (HR, 1.46; 95% CI, 1.12-1.90; P=0.005).

CONCLUSION

Complement cascade goes to the end in human anti-GBM disease and resides mainly in kidney. It plays pathogenic role in renal injury, by the possible proinflammatory effect of C5a and/or cell lysis effect of C5b-9. C5a and C5b-9 may be useful in clinical monitoring and predicting.

Pathology and protection in nephrotoxic nephritis is determined by selective engagement of specific Fc receptors

Introduction of heterologous anti–glomerular basement membrane antiserum (nephrotoxic serum, NTS) into presensitized mice triggers the production of IgG anti-NTS antibodies that are predominantly IgG2b and the glomerular deposition of pathogenic immune complexes, leading to accelerated renal disease. The pathology observed in this model is determined by the effector cell activation threshold that is established by the coexpression on infiltrating macrophages of the IgG2a/2b restricted activation receptor FcγRIV and its inhibitory receptor counterpart, FcγRIIB. Blocking FcγRIV with a specific monoclonal antibody thereby preventing IgG2b engagement or treatment with high dose intravenous γ-globulin (IVIG) to down-regulate FcγRIV while up-regulating FcγRIIB, protects mice from fatal disease. In the absence of FcγRIIB, IVIG is not protective; this indicates that reduced FcγRIV expression alone is insufficient to protect animals from pathogenic IgG2b immune complexes. These results establish the significance of specific IgG subclasses and their cognate FcγRs in renal disease.

Respective Roles of Decay-Accelerating Factor and CD59 in Circumventing Glomerular Injury in Acute Nephrotoxic Serum Nephritis

Decay-accelerating factor (DAF or CD55) and CD59 are regulators that protect self cells from C3b deposition and C5b-9 assembly on their surfaces. Their relative roles in protecting glomeruli in immune-mediated renal diseases in vivo are unknown. We induced nephrotoxic serum (NTS) nephritis in Daf1(-/-), CD59a(-/-), Daf1(-/-)CD59a(-/-), and wild-type (WT) mice by administering NTS IgG. After 18 h, we assessed proteinuria, and performed histological, immunohistochemical, and electron microscopic analyses of kidneys. Twenty-four mice in each group were studied. Baseline albuminuria in the Daf1(-/-), CD59a(-/-), and Daf1(-/-)CD59a(-/-) mice was 82, 83, and 139 as compared with 92 microg/mg creatinine in the WT controls (p > 0.1). After NTS, albuminuria in CD59a(-/-) and WT mice (186 +/- 154 and 183 +/- 137 microg/mg creatinine, p > 0.1) was similar. In contrast, Daf1(-/-) mice developed severe albuminuria (378 +/- 520, p < 0.05) that was further exacerbated in Daf1(-/-)CD59a(-/-) mice (577 +/- 785 micro g/mg creatinine, p < 0.05). Glomerular histology showed essentially no infiltrating leukocytes in any group. In contrast, electron microscopy revealed prominent podocyte foot process effacement in Daf1(-/-) mice with more widespread and severe damage in the double knockouts compared with only mild focal changes in CD59a(-/-) or WT mice. In all animals, deposition of administered (sheep) NTS Ig was equivalent. This contrasted with marked deposition of both C3 and C9 in Daf1(-/-)CD59a(-/-) and Daf1(-/-) mice, which was evident as early as 2 h post-NTS injection. The results support the proposition that in autoantibody-mediated nephritis, DAF serves as the primary barrier to classical pathway-mediated injury, while CD59 limits consequent C5b-9-mediated cell damage.

CD59a deficiency exacerbates accelerated nephrotoxic nephritis in mice

CD59 is a complement regulatory protein that inhibits the terminal part of the complement system, the membrane attack complex (MAC), a mediator of renal injury. Mice deficient in the Cd59a gene (mCd59a-/-) were used to investigate the role of CD59 in experimentally induced accelerated nephrotoxic nephritis, a model of immune complex-mediated glomerulonephritis. After accelerated nephrotoxic nephritis was induced by administration of sheep nephrotoxic globulin, mCd59a-/- mice and strain-matched controls on two genetic backgrounds, 129/Sv x C57BL/6 and 129/Sv, were examined. For both, mCd59a-/- mice developed significantly greater glomerular cellularity than wild-type (WT) mice at day 5 after administration. At day 10 post-administration, mCd59a-/- mice exhibited more glomerular thrombosis than WT mice (thrombosis score, 1.8 [range, 1.4 to 4.0] versus 0.8 [range, 0.2 to 1.5] quadrants thrombosed per glomerulus, respectively; P = 0.0006). In the majority of experiments, mCd59a-/- mice also had significantly more proteinuria than controls; however, there was no difference in serum creatinine or albumin. Quantitative immunofluorescence of kidney sections revealed significantly more C9 (as a marker of MAC) deposition within glomeruli of mCd59a-/- mice than WT controls (P < 0.001). There was no difference in deposition of C3 and sheep IgG between the two experimental groups. The lack of CD59a, by allowing unregulated MAC deposition, exacerbates the renal injury in this model of immune complex-mediated glomerulonephritis.

Decay-accelerating factor confers protection against complement-mediated podocyte injury in acute nephrotoxic nephritis

SUMMARY

Decay-accelerating factor (DAF or CD55) is one of a set of regulators that function to protect self cells from deposition of autologous C3b on their surfaces. Its relative importance in vivo, however, is incompletely understood. As one approach to address this issue, we induced nephrotoxic serum (NTS) nephritis in wild-type mice and Daf1 gene-floxed mice devoid of renal DAF expression. For these experiments NTS IgG was administered at a dose (0.5 mg iv) that requires complement for glomerular injury. After 18 hours, renal injury was assessed by proteinuria and by histologic, immunohistochemical, and electron microscopic analyses of kidneys. Fifteen normal and 15 DAF-deficient mice were studied. Baseline albuminuria in the Daf1(-/-) mice was 115.9 +/- 41.4 microg/mg creatinine as compared with 85.7 +/- 32.3 microg/mg creatinine in their Daf1(+/+) littermates (p = 0.075). After administration of NTS IgG, albuminuria increased to 2001.7 +/- 688.7 microg/mg creatinine as compared with 799.7 +/- 340.5 microg/mg creatinine in the controls (p = 0.0003). Glomerular histology was similar in Daf1(-/-) and Daf1(+/+) mice, with essentially no infiltrating leukocytes. In contrast, electron microscopy revealed severe podocyte fusion in the Daf1(-/-) mice but only mild focal changes in the controls. Immunohistochemical staining showed equivalent deposition of the administered (sheep) NTS IgG in the Daf1(-/-) and Daf1(+/+) animals. This contrasted with marked deposition of autologous murine C3 in the former and minimal deposition in the latter. The results show that DAF is essential physiologically for protecting glomeruli against autologous complement attack initiated by the classical pathway.

Accelerated nephrotoxic nephritis is exacerbated in C1q-deficient mice

C1q deficiency strongly predisposes to the development of systemic lupus erythematosus in humans and mice. We used the model of accelerated nephrotoxic nephritis in C1q-deficient mice to explore the mechanisms behind these associations. C1q-deficient mice developed severe glomerular thrombosis within 4 days of induction of disease, whereas wild-type mice developed mild injury. These findings suggest that C1q protects from immune-mediated glomerular injury. This exacerbated thrombosis was also seen in mice triply deficient in C1q, factor B, and C2, excluding a major pathogenic role for the alternative pathway of complement in this phenomenon. However, these mice did not develop elevated creatinine levels. No exacerbation of accelerated nephrotoxic nephritis was observed in mice doubly deficient in factor B and C2, suggesting a protective role for C1q against renal inflammation that is proximal to C2 activation. There were increased murine IgG deposits, neutrophil numbers, and apoptotic cells in the glomeruli of C1q-deficient mice compared with wild-type mice. Renal expression of genes encoding procoagulant proteins was also enhanced in C1q-deficient mice. The increased IgG deposits and apoptotic cells in the glomeruli of C1q-deficient mice suggest that the exacerbation of disease may be due to a defect in the clearance of immune complexes and/or apoptotic cells from their kidneys.

Induction of glomerulonephritis in rats with staphylococcal phosphatase: new aspects in post-infectious ICGN

Staphylococcal neutral phosphatase (NPtase) is a highly cationic bacterial surface bound protein. It has significant affinity for human and rat immunoglobulins in vitro and an electrostatic interaction may be involved. Radioisotopic studies showed that NPtase had a high affinity for the polyanionic structures of the rat renal glomerulus. When the left kidneys of germ-free or naive (non-immune) Wistar rats were perfused with 80 micrograms of I125 NPtase, 21 micrograms of NPtase were found in the left kidneys and 11 micrograms in the isolated glomeruli 15 minutes after perfusion. Deposits of autologous immunoglobulin and C3 were seen in the glomeruli of rats immediately after perfusion with NPtase (15 min) and persisted throughout the 14-day observation period. Histologically, neutrophil influx into the glomerulus was seen at 15 minutes and increased until three hours; subepithelial electron-dense deposits were found after three days and were still visible on day 14. Proteinuria started within the first 24 hours despite the absence of an immune response at this time and was still present on day 14. Similar results were observed in immune deficient athymic nude rats in the early phase. Perfusion of heparin after NPtase inhibited the deposition of IgG and C3 and prevented proteinuria in naive but not in actively immunized rats. This result provides further evidence that specific antibodies to NPtase were not involved in the immune complex-like deposits seen in the early phase. NPtase is a novel molecule, as it reveals both high affinity for the GBM and binding of circulating immunoglobulins, by a non-antigen-antibody mechanism, to form IC-like deposits on the GBM. These deposits are capable of activating the complement system, thus triggering a series of events leading to glomerulonephritis. These results delineate an additional pathway for the pathogenesis of ICGN related to bacterial infection.

CD59: its role in complement regulation and potential for therapeutic use

CD59 regulates complement activation cascade at the final step, inhibiting formation of membrane attack complex (MAC). This protein, being anchored to the cell membrane via glycosyl phosphatidyl inositol (GPI), is expressed ubiquitously on cells which are in contact with body fluids containing components. Recently, MAC formation has been reported to play an important role in pathogenesis of inflammatory diseases such as ischemia or autoimmune diseases. In this review, we describe the structure and biological activities of CD59, the pathogenic role of MAC formation, and discuss application of soluble molecules of CD59 for therapeutic use.

Effects of systemic complement activation on renal circulation of rats

In a variety of immunopathological diseases activation of the complement cascade occurs either systemically or localized in the kidney. To elucidate the functional impact of complement activation upon the renal microcirculation, we administered cobra venom factor of Naja naja kaouthia (CVF) i.v. into thiobarbital anaesthetized female rats. CVF is a potent activator of the alternative pathway of complement by forming the C3-convertase CVF, Bb which cannot be downregulated by the natural inhibitor factors H and I and thereby leads to generation of the anaphylatoxins C3a and C5a and formation of the membrane attack complex (MAC). We utilized creatinine clearance and flowmeter measurements in the normal kidney and intravital microscopy of the split hydronephrotic rat kidney model to observe the microvascular changes. Bolus injection of CVF (100 U kg-1) resulted in an immediate reduction of RBF (-68% after 10 min), which remained decreased during the entire experiment (90 min). Systemic blood pressure was significantly reduced only in the early period (-23% of control: 126 mmHg after 10 min). After an initial anuric phase of 30 min duration, the glomerular filtration rate was significantly diminished by 47%. White cell count was decreased by about 50% after the experiments. Application of the competitive thromboxane A2-antagonist, BM 13505, reversed all renal and systemic CVF-effects. Continuous infusion of the competitive leukotriene D4-antagonist, ICI 198615, attenuated the late renal CVF-effects (i.e. 30 min after injection of CVF). Depletion of polymorphonuclear cells (PMN) attenuated the CVF-effects similar to BM 13505. Intravenous administration of CVF in the hydronephrotic kidney model resulted in a massive constriction of the interlobar and arcuate artery, with a fall in glomerular blood flow comparable to the reduction of RBF in the normal kidney. Diameters of the afferent arterioles--most sensitive to many vasoconstricting agents--were not significantly altered. Our results suggest that injection of CVF and the liberation of high amounts of the anaphylatoxins, C3a and C5a, induces the release of TXA2, which contributes to the early renal effects and the formation of cysteinyl-leukotrienes which play an important role in the late phase of systemic complement activation. Utilizing the split hydronephrotic kidney model we demonstrated the predominant action of complement activation on the large preglomerular vessels for the first time. PMN are seemingly involved in the liberation of secondary mediators which appear to reduce renal blood flow and glomerular filtration.

Membrane signaling by complement C5b-9, the membrane attack complex

The terminal complement complexes C5b-7, C5b-8 and C5b-9 are able to generate nonlethal cell signals. One universal consequence of a cell being targeted by C5b-8 or C5b-9 is an influx of Ca2+. In addition, other second messengers, including cAMP, inositol phosphate intermediates and arachidonate metabolites, are generated by the terminal complement complexes in specific cell types. In vivo, terminal complement complexes have been found in a wide variety of inflammatory processes in humans and in experimental animal models. Some of these models of inflammation putatively induced by terminal complement complexes have been tested in complement-deficient animals, and indeed no inflammation results, which supports the critical role of the terminal complement complexes in the pathogenesis of the lesion.

IgA nephropathy in patients with congenital C9 deficiency

The clinical, histologic, and immunopathological findings of three young Japanese males with congenital C9 deficiency and primary IgA nephropathy are reported. The C9 deficiency was discovered either through mass complement screening, or when low hemolytic activity for CH50 and normal C3 levels were detected in plasma. Hematuria and proteinuria were detected at the age of 8 or 9 years as a result of annual urinary screening tests for school children. Renal biopsy showed focal and segmental mesangial proliferation with small epithelial crescents in one patient, and mild, diffuse mesangial proliferation in two. IgA and C3 were deposited predominantly in the mesangial area, and staining for C9 was negative in these patients. Electron microscopy revealed electron dense deposits predominantly in mesangial and paramesangial zones. Immunohistochemical staining in renal biopsy tissues from two patients showed mesangial staining for C5, C8, and S-protein, but staining for C5b-9 neoantigen was completely negative. These results show that the formation of C5b-9 complex is not essential for the induction of human IgA nephropathy, and also for the proliferation of mesangial and even parietal epithelial cells.

Clusterin in renal tissue: preferential localization with the terminal complement complex and immunoglobulin deposits in glomeruli

The membrane attack complex (MAC) of complement is activated by immune and non-immune mechanisms in the kidney. MAC has been found associated with glomerular immune deposits, but also to cell remnants, particularly along tubules and in vessel walls. Clusterin and S-protein (vitronectin) bind to MAC, rendering it cytolytically inactive. Both have been found associated with MAC in renal tissue. Here we analysed the deposition of clusterin and S-protein in 118 renal biopsies relative to the localization of the MAC using MoAbs. Statistical analysis was performed comparing no or little versus evident or strong staining by immunofluorescence (IF). In glomeruli, out of the 92 biopsies where both MAC and immunoglobulins were evaluated, deposits of MAC were found in the presence (32 out of 41) but also in the absence of immunoglobulins (20/51). Clusterin and S-protein deposits were seen, respectively, in 25 out of 61 and 36 out of 61 biopsies containing glomerular MAC, and almost never in its absence (one out of 50 for both). The association of the two inhibitors with MAC was observed mainly in glomeruli containing immunoglobulin deposits (respectively, 21 out of 32 and 25 out of 32), but not when immunoglobulins were absent (three out of 20 and seven out of 20) (coefficient of concordance, K = 0.47 and 0.43). The localization of MAC along tubules and in vessels was easily identified in most biopsies (93 out of 118) and was accompanied by S-protein in most cases (tubules, 86 out of 93; vessels, 82 out of 93) (K = 0.58 and 0.57 respectively) but not by clusterin (28 out of 93 and 24 out of 93). These results suggest that clusterin does not co-localize with MAC whenever there is formation and fixation of the MAC. It seems that clusterin has a particular affinity for MAC which is associated with immunoglobulin. This observation should help to distinguish between the different forms of MAC, and might indicate that MAC associated with immunoglobulin is essentially in its cytolytically inactive form.

Membrane attack complex (MAC) deposits in skin are not always accompanied by S-protein and clusterin

Clusterin and S-protein bind to the membrane attack complex of complement (MAC) rendering it cytolytically inactive. Tissue necrosis as produced by pulsed tunable dye laser therapy (PTDL), and immune complex-related diseases such as lupus erythematosus, are accompanied by local accumulation of MAC. However, the mechanisms responsible for this accumulation might differ, and lead to deposition of MAC in different forms (cytolytically active or inactive). Biopsy specimens of lesional (22) and non-lesional (10) skin from 27 patients with a positive lupus band test (LBT) were studied using monoclonal antibodies against clusterin, S-protein, and MAC by immunofluorescence and immunoperoxidase. Identical studies were performed in normal and angiomatous skin specimens from three normal individuals before and after laser irradiation. MAC was present in 30 of 32 positive LBT skin biopsies. MAC was not only present in lesional (21 of 22) but also in non-lesional skin (nine of 10), although the intensity of staining appeared to be lower in the latter. Clusterin and S-protein co-localized with MAC, respectively, in 20 and 12 specimens, and were not found in the absence of MAC. In addition S-protein deposits were seen only in biopsies positive for clusterin. Deposits of clusterin and S-protein did not correlate with the presence or absence of lesions. After irradiation with PTDL, the immediate complement activation was accompanied by MAC deposits that were granular and clearly located on vascular endothelial cells. Clusterin and S-protein were not present on these cells. In summary, clusterin localizes with MAC along the skin dermal-epidermal junction in patients with a positive LBT, suggesting that it has a similar and possibly more important role than S-protein in regulating immune complex-mediated MAC formation. By contrast, clusterin and S-protein are not involved at the time of MAC formation in cells undergoing necrosis after PTDL therapy.

Localization of 20-kD homologous restriction factor (HRF20) in diseased human glomeruli. An immunofluorescence study

The 20-kD homologous restriction factor (HRF20), which is identical to CD59, is a membrane-associated protein which inhibits the reaction of C9 to form membrane attack complex (MAC) of homologous complements. In various human glomerular diseases deposition of complement components is frequently seen and MAC is reported to associate with immune deposits. Using a specific monoclonal antibody, 1F5, against HRF20, we attempted to study the localization of HRF20 in human glomerulonephritides and to compare the localization of HRF20 with those of immune deposits and MAC. The frozen sections of kidney specimens were fixed in acetone at room temperature before staining. In normal kidneys and kidney specimens from the patients with minimal change nephrotic syndrome, membranous nephropathy, and IgA nephropathy, HRF20 was strongly localized in the peritubular capillaries and along Bowman's capsules. A weaker but well-defined staining was obtained in the mesangial area and faint staining was seen along the glomerular capillary walls. In contrast, glomerular capillary walls were rather strongly stained in the cases with diffuse lupus nephritis which had subendothelial dense deposits. These data suggest that HRF20 (CD59) is present in the human glomeruli and its expression is enhanced under certain conditions such as lupus nephritis.

Two types of C3 nephritic factor: properdin-dependent C3NeF and properdin-independent C3NeF

The IgG fraction of serum from patients with membranoproliferative glomerulonephritis (MPGN) types I and II or partial lipodystrophy (PLD) was found to contain C3 nephritic factor (C3NeF) which reacts with the alternative pathway C3 convertase C3bBb and stabilizes it. Two types of C3NeF were detected, of which one was heat sensitive (56 degrees C for 30 min) and properdin dependent (C3NeF:P) but the other was heat stable and properdin independent (C3NeF:nP). C3NeF:P was found in sera from patients with MPGN types I and II and it displayed the properties of properdin and IgG. C3NeF:P was observed in patients with reduced serum concentrations of C3 and terminal complement components (TCC), and the generation of SC5b-9 complex was increased in mixtures with normal human serum. C3NeF:nP was found in sera from patients with MPGN type II and PLD, whose sera revealed a selective decrease in C3 concentrations.

C5b-8 and C5b-9 modulate the collagen release of human glomerular epithelial cells

Aside from their lytic function the late complement components C5b-9 stimulate release of prostanoids, interleukin 1 and oxygen radicals from a number of cells. Since C5b-9 has also been connected to the development of sclerosis in animal models of glomerulonephritis, we addressed the question whether C5b-9 would affect the collagen synthesis. We used human glomerular epithelial cells (GEC) obtained as primary outgrowth cultures. The cells were cultivated in the presence of 14C-proline. Collagen synthesis was quantitated by counting the radioactivity associated with collagenase digestible material. Furthermore, collagen was analyzed by SDS-PAGE. GEC in culture produce spontaneously some collagen type IV. Addition of sublytic doses of highly purified C5b-9 increased the collagen synthesis considerably within 12 to 24 hours. In the absence of C9, C5b-8 stimulated collagen synthesis to a similar extent, whereas in the absence of C7 or C8, the collagen synthesis was not enhanced. Furthermore, fluid-phase-formed C5b-9 complexes did not stimulate the collagen synthesis, indicating that assembly of the complex on the target membrane was required. Since C5b-9 deposits are found in sclerotic areas, our data support the hypothesis that C5b-9, by stimulating collagen synthesis as well as release, might contribute to the development of chronic nephritis.

The role of mesangial cells in glomerular pathology

The glomerular mesangial cell has become increasingly recognized as a multifunctional cell capable of mediating glomerular disease. This article reviews recent findings regarding the biology of these cells, and the relevance that these findings may have for our understanding of glomerular pathology.

Localization of terminal complement components S-protein and SP-40,40 in renal biopsies

The terminal complement complex has been implicated in the development of glomerular injury in both experimental and, indirectly, in human glomerulonephritis. Recent data suggests that the terminal complement complex in human glomerulonephritis may be in the cytolytically inactive SC5b-9 form which also contains S-protein and a recently identified protein, SP-40,40. In this study renal biopsies were examined by immunofluorescence to determine the incidence and inter-relation of deposition of the SC5b-9 components C6, C9, S-protein and SP-40,40. All components of SC5b-9 were found in arteries and arterioles, along the tubular basement membrane and in areas of glomerulosclerosis in all biopsies. This deposition was sometimes associated with C3 but never immunoglobulin deposition and correlated with the degree of renal injury. In addition, in biopsies with glomerular deposition of immunoglobulin and C3, the SC5b-9, components co-localized with the immune deposits. Glomeruli without immune deposits or glomerulosclerosis contained none of the SC5b-9 components. The incidence and pattern of distribution of SP-40,40 was similar to that of S-protein, C6 and C9 in all of cases. These data confirm that the terminal complement complex in the kidney is, at least partly, in the SC5b-9 form both in the specific immune glomerular deposition and in the "non-specific" deposition in areas of renal injury. SP-40,40 is also found in the SC5b-9 complex in all forms of renal disease.

A properdin dependent nephritic factor slowly activating C3, C5, and C9 in membranoproliferative glomerulonephritis, types I and III

The IgG fraction of serum from patients with membranoproliferative glomerulonephritis (MPGN) types I and III was found to contain a nephritic factor (NFI/III) which differed from that usually present in MPGN type II and partial lipodystrophy (NFII) in that it converts C5 and C9 as well as C3, is dependent on properdin for its activity, and requires an incubation period of several hours rather than 30 min for its demonstration. C3 conversion occurred in the absence of an intact classical pathway. This nephritic factor was found in patients with reduced serum levels of terminal components and its activity, like that of the nephritic factor in MPGN type II, correlated with the serum C3 level indicating that these nephritic factors play a large role in producing hypocomplementemia. Although potentially nephritogenic because of its ability to activate the terminal pathway, the presence of this nephritic factor did not clearly correlate with clinical course.

Inherited deficiency of the seventh component of complement: studies of C7-consuming activity

Neither the hemolytic activity nor the protein level of the seventh component of serum complement (C7) was detectable in an 8-year-old girl with nephritis, but in her parents and her brother, they were about half of the normal level. The patient was a homozygote type with a complete deficiency of C7 while her parents and brother were all heterozygote type with a partial deficiency of C7. C7-consuming activity was demonstrated in the native serum of the patient with complete C7 deficiency, and it was found that large amounts of C56 were readily generated upon incubation of the patient's serum with zymosan. It is proposed that the C7-consuming activity in the native serum of this patient is due to small amounts of C56 generated during the activation of serum complement by some kind of infection such as a common cold.

Rapidly progressive glomerulonephritis: classification, pathogenetic mechanisms, and therapy

Immunopathologic studies over the past two decades have demonstrated that rapidly progressive glomerulonephritis (RPGN) can result from glomerular deposition of anti-GBM antibody, immune complexes, or from some as yet undefined mechanism that does not involve glomerular antibody deposition. The latter process may be cell mediated and resembles a small vessel vasculitis. Most cases of idiopathic RPGN are not accompanied by pathogenic glomerular immunoglobulin deposition. Recent experimental studies of immune mechanisms of glomerular injury have identified several new processes that can induce damage to the capillary wall sufficient to result in crescentic glomerulonephritis (GN). These include direct effects of anti-GBM antibody alone and of the complement C5b-9 (membrane attack) complex, nephritogenic effects of inflammatory effector cells that involve reactive oxygen species and glomerular halogenation, and injury mediated by sensitized lymphocytes independently of antibody deposition. Macrophages have been shown to participate in both intracapillary and extracapillary fibrin deposition and crescent formation as well as to mediate capillary wall damage. The role of resident glomerular cells and cell-cell interactions in glomerulonephritis is still under active investigation. Despite these several advances in understanding immune injury to the glomerulus, therapy for RPGN remains largely empiric. Although the prognosis in RPGN has clearly improved over time, no form of disease-specific therapy has been clearly shown yet to be beneficial in a controlled study. Interpretation of the existing literature on therapy is complicated by the availability of only historical rather than concurrent controls, lack of attention to several variables known to affect disease outcome, and uncertainty regarding bias in favor of reporting positive results. Available data suggests that optimal outcomes may be achieved in anti-GBM nephritis by treatment with steroids, immunosuppression and plasma exchange, particularly when therapy is directed at patients with mild but rapidly progressive disease before oliguria or severe azotemia develop. Pulse steroids are probably the most cost-effective therapy for the idiopathic form of RPGN, but treatment with cytotoxic agents should be considered if clinical or histologic evidence of vasculitis is present.

Mechanism of prostaglandin E2 inhibition of acute changes in vascular permeability

In order to determine the mechanism of antiinflammatory activity, prostaglandin E2 (PGE2) or diluent was administered to rats 2 h prior to intradermal injections of various mediators of inflammatory vascular permeability changes. Vascular permeability was measured as the accumulation of [125I]rat serum albumin at the site of mediator injunction. PGE2 at 500 micrograms significantly inhibited protein leakage produced by histamine, platelet activating factor, zymosan, and zymosan-activated plasma. Pretreatment with PGE2 had no effect on protein leakage induced by injection of lysosomal enzymes, glucose oxidase, or xanthine oxidase. The accumulation of polymorphonuclear leukocytes (PMNs) at the site of injection of zymosan or zymosan-activated plasma was not altered by PGE2 administration. In separate experiments, the ability of PGE2 to alter phagocytosis and oxygen radical production by PMN was examined. PGE2 significantly inhibited phagocytosis at 2 h, but this returned to normal by 6 h. Production of hydrogen peroxide by PMN was not affected by PGE2. These results suggest that PGE2 prevents acute changes in vascular protein leakage by preventing endothelial cell contraction and by inhibiting specific PMN functions.

An enzyme-linked immunoabsorbent assay for the quantitation of the terminal complement complex from cell membranes or in activated human sera

A sensitive and simple enzyme-linked immunoabsorbent assay (ELISA) has been developed to measure the terminal complement complex (TCC) in solution. Commercially available antibodies to the native complement (C) components C5 and C9 were used in a double antibody sandwich technique sensitive enough to detect 0.3 microgram/ml of purified TCC. The TCC was not detected in normal human serum (NHS) nor was it generated when sera from patients with a genetic deficiency of functional C5, C7, C8 beta or C9 were activated with cobra venom factor (CVF). If the C8 beta deficient serum was reconstituted with the C8 beta chain and incubated with CVF, TCC were formed and detected by the assay. In in vitro experiments, the TCC was detected in NHS activated by either the classical or alternative pathway even when there was no measurable consumption of C5, C8 or C9. In addition, adaptation of a detergent extraction procedure permitted the quantitation by the assay, of TCC which were generated on sensitized sheep erythrocyte membranes. Experiments to test sample handling conditions showed no generation of TCC in NHS after four freeze/thaw cycles and spontaneous formation only if NHS had been incubated at 37 degrees C for 48 h. The TCC in zymosan-activated NHS were stable at 37 degrees C for 1 week. Patients with C activation associated diseases such as SLE and rheumatoid arthritis had increased levels of TCC that correlated with positive clinical tests for inflammation, even though C levels were normal when measured by routine techniques. These results suggest that this ELISA will provide a valuable tool for studying the role of C in the pathogenesis of C-mediated diseases and in examining the mechanism of tissue injury in in vitro experimental systems.

Complement-induced glomerular epithelial cell injury. Role of the membrane attack complex in rat membranous nephropathy

In passive Heymann nephritis (PHN) in rats, antibody (anti-Fx1A) reacts in situ with a glomerular epithelial antigen and induces complement (C)-mediated cell-independent proteinuria. To assess the role of the membrane attack complex (MAC), we determined the need for C8 in the pathogenesis of proteinuria in an autologous-phase model of PHN. Isolated rat kidneys, containing nonnephritogenic, non-C-fixing gamma 2 sheep anti-Fx1A (planted antigen), when perfused in vitro with C-fixing guinea pig anti-sheep IgG and a source of C (fresh human plasma 50% vol/vol in buffer containing bovine serum albumin), developed marked proteinuria after 20 min (0.58 +/- 0.08 mg/min X g, n = 8) that increased further to 3.20 +/- 0.93 mg/min X g after 80 min. In contrast, identical kidneys perfused with antibody and heat-inactivated or C8-deficient human plasma and normal kidneys perfused with antibody and fresh plasma excreted only 0.27 +/- 0.03 (n = 6), 0.27 +/- 0.04 (n = 5), and 0.40 +/- 0.05 mg/min X g (n = 6) after 20 min, and 0.13 +/- 0.02, 0.22 +/- 0.03, and 0.32 +/- 0.05 mg/min X g after 80 min, respectively. When C8-deficient plasma was reconstituted with sources of C8 (n = 3), proteinuria was restored to the level observed with fresh normal plasma. Differences in protein excretion could not be explained by quantitative differences in glomerular antigen or antibody content. Extensive ultrastructural damage to glomerular visceral epithelial cells was exclusively seen in antigen-containing kidneys perfused with antibody and C8-replete plasma. Thus, glomerular injury in this model results from an antigen-specific, antibody-directed, C8-dependent reaction involving assembly of the MAC. The ultrastructural findings argue in favor of MAC-induced cytotoxicity of the glomerular visceral epithelial cells.

Complement membrane attack complex stimulates production of reactive oxygen metabolites by cultured rat mesangial cells

To explore possible mechanisms by which complement membrane attack complexes (MAC) that are deposited in the glomerular mesangium might be pathogenic, we stimulated rat glomerular mesangial cells grown in vitro with nascent MACs formed from the purified human complement components C5b6 and normal human serum and measured production of superoxide ion (O2-) and hydrogen peroxide (H2O2). Mesangial cells incubated with C5b6 + serum, which results in cell membrane interaction with the MAC, produce 0.9 +/- 0.15 nmol O2-/10(5) cells per 30 min, which was significantly greater than the amount produced by cells incubated with C5b6 alone, serum alone, or decayed MACs that can no longer interact with the cell membrane (0.3 +/- 0.2, 0.4 +/- 0.1, 0.3 +/- 0.2 nmol O2-/10(5) cells per 30 min, respectively; P less than 0.02). Production of O2- after stimulation with MACs increased during the first 20 min of incubation but then plateaued. Cells exposed to decayed MACs produced small amounts of O2-, which did not increase from 20 to 60 min. Production of H2O2 was also observed after stimulation with MACs, and continued to increase during 60 min of incubation (1.22 +/- 0.16 nmol H2O2/10(5) cells per 60 min), whereas H2O2 production could not be detected after exposure to decayed MACs. Cell viability was not adversely affected by exposure to nascent MACs as determined by trypan blue exclusion or chromium-51 release. These results demonstrate that glomerular mesangial cell membrane interaction with the MAC stimulates the production of the toxic oxygen metabolites O- and H2O2. Activation of the terminal complement pathway by mesangial immune deposits in vivo might lead to tissue injury by stimulation of local production of toxic oxygen-free radicals.

Complement membrane attack complex stimulates production of reactive oxygen metabolites by cultured rat mesangial cells

To explore possible mechanisms by which complement membrane attack complexes (MAC) that are deposited in the glomerular mesangium might be pathogenic, we stimulated rat glomerular mesangial cells grown in vitro with nascent MACs formed from the purified human complement components C5b6 and normal human serum and measured production of superoxide ion (O2-) and hydrogen peroxide (H2O2). Mesangial cells incubated with C5b6 + serum, which results in cell membrane interaction with the MAC, produce 0.9 +/- 0.15 nmol O2-/10(5) cells per 30 min, which was significantly greater than the amount produced by cells incubated with C5b6 alone, serum alone, or decayed MACs that can no longer interact with the cell membrane (0.3 +/- 0.2, 0.4 +/- 0.1, 0.3 +/- 0.2 nmol O2-/10(5) cells per 30 min, respectively; P less than 0.02). Production of O2- after stimulation with MACs increased during the first 20 min of incubation but then plateaued. Cells exposed to decayed MACs produced small amounts of O2-, which did not increase from 20 to 60 min. Production of H2O2 was also observed after stimulation with MACs, and continued to increase during 60 min of incubation (1.22 +/- 0.16 nmol H2O2/10(5) cells per 60 min), whereas H2O2 production could not be detected after exposure to decayed MACs. Cell viability was not adversely affected by exposure to nascent MACs as determined by trypan blue exclusion or chromium-51 release. These results demonstrate that glomerular mesangial cell membrane interaction with the MAC stimulates the production of the toxic oxygen metabolites O- and H2O2. Activation of the terminal complement pathway by mesangial immune deposits in vivo might lead to tissue injury by stimulation of local production of toxic oxygen-free radicals.