Protection against anti-glomerular basement membrane (GBM)-mediated nephritis in C3- and C4-deficient mice

Prevention of C5 activation ameliorates spontaneous and experimental glomerulonephritis in factor H-deficient mice

Membranoproliferative glomerulonephritis (MPGN) type II (dense deposit disease) is an inflammatory renal disease characterized by electron-dense deposits and complement C3 on the glomerular basement membrane. There is no effective therapy. We investigated the role of C5 activation in a model of MPGN that develops spontaneously in complement factor H-deficient mice (Cfh(-/-)). At 12 months there was a significant reduction in mortality, glomerular cellularity, neutrophil numbers, and serum creatinine levels in Cfh(-/-) mice deficient in C5. Excessive glomerular neutrophil numbers, frequently seen in patients with MPGN during disease flares, were also observed in Cfh(-/-) mice after the administration of an antiglomerular basement membrane antibody. This exaggerated injurious phenotype was absent in Cfh(-/-) mice deficient in C5 but not in Cfh(-/-) mice deficient in C6, indicating a key role for C5 activation in the induction of renal lesions. Importantly, the renal injury was completely reversed in Cfh(-/-) mice pretreated with an anti-murine C5 antibody. These results demonstrate an important role for C5 in both spontaneous MPGN and experimentally induced nephritis in factor H-deficient mice and provide preliminary evidence that C5 inhibition therapy might be useful in human MPGN type II.

Mast cell-mediated remodeling and fibrinolytic activity protect against fatal glomerulonephritis

Mast cells are detrimental in several inflammatory diseases; however, their physiological roles are also increasingly recognized. Recent data suggest that mast cells may also be involved in renal diseases. We therefore used congenitally mast cell-deficient W/W(v) mice and normal +/+ littermates to assess their role in anti-glomerular basement membrane-induced glomerulonephritis. Following administration of anti-glomerular basement membrane Abs, W/W(v) mice exhibited increased mortality as compared with +/+ mice owing to rapid deterioration of renal function. Reconstitution of the mast cell population in W/W(v) mice restored protection. This was independent of activating FcgammaR, as protection was also obtained using mast cells deficient in FcRgamma. Comparative histological analysis of kidneys showed that deterioration of renal function was caused by the presence of thick layers of subendothelial glomerular deposits in W/W(v) mice, while +/+ mice or mast cell-reconstituted W/W(v) mice showed significantly less. Deposits appeared during the early phase of disease and persisted thereafter, and were accompanied by enhanced macrophage recruitment. Immunohistochemical analysis revealed increased amounts of fibrin and type I collagen in W/W(v) mice, which were also unable to maintain high tissue plasminogen activator and urinary-type plasminogen activator activity in urine in the heterologous phase of disease. Our results indicate that mast cells by their ability to mediate remodeling and repair functions are protective in immune complex-mediated glomerulonephritis.

Clinical aspects and molecular basis of primary deficiencies of complement component C3 and its regulatory proteins factor I and factor H

The complement system participates in both innate and acquired immune responses. Deficiencies in any of the protein components of this system are generally uncommon and require specialized services for diagnosis. Consequently, complement deficiencies are clinically underscored and may be more common than is normally estimated. As C3 is the major complement component and participates in all three pathways of activation, it is fundamental to understand all the clinical consequences observed in patients for which this protein is below normal concentration or absent in the serum. C3 deficiencies are generally associated with higher susceptibility to severe infections and in some cases with autoimmune diseases such as systemic lupus erythematosus. Here, we review the main clinical aspects and the molecular basis of primary C3 deficiency as well as the mutations in the regulatory proteins factor I and factor H that result in secondary C3 deficiencies. We also discuss the use of animal models to study these deficiencies.

Accumulation of immune complexes in glomerular disease is independent of locally synthesized c3

Although complement activation can make immune complex glomerulonephritis worse, the third complement component also can solubilize immune complexes and thus reduce the severity of disease. How C3 that is produced within the kidney contributes to this balance is unknown. This study therefore investigated the relative roles of systemic and local C3 production in a model of glomerular immune complex disease. Injection of sheep anti-glomerular basement membrane antibody into preimmunized mice resulted in accumulation of immune complexes and progressive loss of function over 14 d that was much more marked in C3-deficient (C3-/-) mice. In C3-sufficient mice that received a transplant of a C3-/- mouse kidney and in C3-/- mice with C3-sufficient mouse kidney transplants, the severity and the pattern of injury went with the complement status of the recipient. That is, mice with deficient circulating C3 developed severe glomerular immune complex disease, whereas those with a high level of circulating C3 had well-preserved glomerular structure and function. It is concluded that circulating C3 is a critical factor in reducing the glomerular accumulation of immune complexes. Local synthesis of C3 did not have a major influence on this aspect of glomerular disease.

Innate Stimuli Accentuate End-Organ Damage by Nephrotoxic Antibodies via Fc Receptor and TLR Stimulation and IL-1/TNF-α Production

Innate stimuli are well recognized as adjuvants of the systemic immune response. However, their role in driving end-organ disease is less well understood. Whereas the passive transfer of glomerular-targeting Abs alone elicited minimal renal disease, the concomitant delivery of innate stimuli triggered severe nephritis, characterized by proliferative glomerulonephritis with crescent formation, and tubulointerstitial disease. Specifically, stimulating TLR2, TLR3, TLR4, and TLR5 by using peptidoglycan, poly(I:C), LPS, and flagellin, respectively, all could facilitate anti-glomerular Ab-elicited nephritis. In this model, innate and immune triggers synergistically activated several cytokines and chemokines, including IL-1, IL-6, TNF-alpha, and MCP-1, some of which were demonstrated to be absolutely essential for the development of renal disease. Genetic studies revealed that, whereas the innate trigger is dependent on TLR/IL-1R-associated kinase-mediated signaling, the immune component was contingent on FcR-mediated signals. Importantly, infiltrating leukocytes as well as intrinsic glomerular cells may both serve to integrate these diverse signals. Extrapolating to spontaneous immune-mediated nephritis, although the adaptive immune system may be important in generating end-organ targeting Abs, the extent of damage inflicted by these Abs may be heavily dependent on cues from the innate immune system.

Minireview: functions of the renal tract epithelium in coordinating the innate immune response to infection

Infection of the urinary tract remains one of the most common infections affecting mankind. Renal epithelial cells, being one of the first cells to come into contact with invading organisms, are in a key position to coordinate host defense. The epithelium not only provides a physical barrier to infection, but can also augment the immune response via the production of a number of inflammatory mediators and antimicrobial proteins. Recent work has demonstrated that cells of the innate immune system, including epithelial cells, express toll-like receptors (TLRs), with the capacity to recognize bacterial components. Although the exact mechanisms remain unclear, engagement of TLRs can lead to epithelial cell activation and the production of inflammatory mediators. These include complement proteins, other bactericidal peptides, and chemotactic cytokines. The resulting inflammatory infiltrate serves to aid bacterial clearance, but can also lead to renal damage. In this review, we describe how renal epithelial cells contribute to the innate immune response to ascending urinary tract infection. We specifically relate previous work to more recent developments in this field. An improved understanding of the mechanisms involved may highlight potential therapeutic avenues to aid bacterial clearance and prevent the renal scarring associated with infection.

Anti-C1q autoantibodies deposit in glomeruli but are only pathogenic in combination with glomerular C1q-containing immune complexes

Anti-C1q autoantibodies are present in sera of patients with several autoimmune diseases, including systemic lupus erythematosus (SLE). Strikingly, in SLE the presence of anti-C1q is associated with the occurrence of nephritis. We have generated mouse anti-mouse C1q mAb's and used murine models to investigate whether anti-C1q autoantibodies actually contribute to renal pathology in glomerular immune complex disease. Administration of anti-C1q mAb JL-1, which recognizes the collagen-like region of C1q, resulted in glomerular deposition of C1q and anti-C1q autoantibodies and mild granulocyte influx, but no overt renal damage. However, combination of JL-1 with a subnephritogenic dose of C1q-fixing anti-glomerular basement membrane (anti-GBM) antibodies enhanced renal damage characterized by persistently increased levels of infiltrating granulocytes, major histological changes, and increased albuminuria. This was not observed when a non-C1q-fixing anti-GBM preparation was used. Experiments with different knockout mice showed that renal damage was dependent not only on glomerular C1q and complement activation but also on Fcgamma receptors. In conclusion, anti-C1q autoantibodies deposit in glomeruli together with C1q but induce overt renal disease only in the context of glomerular immune complex disease. This provides an explanation why anti-C1q antibodies are especially pathogenic in patients with SLE.

Anti-C1q autoantibodies deposit in glomeruli but are only pathogenic in combination with glomerular C1q-containing immune complexes

Anti-C1q autoantibodies are present in sera of patients with several autoimmune diseases, including systemic lupus erythematosus (SLE). Strikingly, in SLE the presence of anti-C1q is associated with the occurrence of nephritis. We have generated mouse anti-mouse C1q mAb's and used murine models to investigate whether anti-C1q autoantibodies actually contribute to renal pathology in glomerular immune complex disease. Administration of anti-C1q mAb JL-1, which recognizes the collagen-like region of C1q, resulted in glomerular deposition of C1q and anti-C1q autoantibodies and mild granulocyte influx, but no overt renal damage. However, combination of JL-1 with a subnephritogenic dose of C1q-fixing anti-glomerular basement membrane (anti-GBM) antibodies enhanced renal damage characterized by persistently increased levels of infiltrating granulocytes, major histological changes, and increased albuminuria. This was not observed when a non-C1q-fixing anti-GBM preparation was used. Experiments with different knockout mice showed that renal damage was dependent not only on glomerular C1q and complement activation but also on Fcgamma receptors. In conclusion, anti-C1q autoantibodies deposit in glomeruli together with C1q but induce overt renal disease only in the context of glomerular immune complex disease. This provides an explanation why anti-C1q antibodies are especially pathogenic in patients with SLE.

Implication of the complement system in the induction of anti-glomerular basement membrane glomerulonephritis in WKY rats

In WKY rats, administration of a small dose of anti-glomerular basement membrane antibody induces severe crescentic glomerulonephritis, which is characterized by infiltration of CD8-positive lymphocytes and monocytes/macrophages into the glomeruli with crescent formation. In this study, the involvement of the complement system was examined in the induction of this model. To deplete complement, cobra venom factor (CVF) was used. By a single injection with CVF at 24 h prior to the induction of this model, plasma C3 level from days 0-2 was less than 10% compared with pre-injection of CVF. Complement was almost depleted in the early phase of this model. No significant differences were observed in proteinuria and the frequency of glomeruli associated with the extracapillary crescentic lesions between the CVF-treated group and the control group. In addition, the number of monocytes/macrophages and CD8-positive lymphocyte infiltration into the glomeruli showed no significant differences between both groups. These results indicate that the possibility of complement system involvement is considered low in the induction of this model.

The membrane attack complex, C5b-9, up regulates collagen gene expression in renal tubular epithelial cells

Evidence suggesting a direct role for proteinuria in the pathogenesis of renal tubulointerstitial fibrosis is accumulating. However the mechanism by which proteinuria leads to injury is unknown. In proteinuric states complement proteins are filtered through the glomerulus and could contribute to the tubular damage. The aim of this study was to investigate the role of complement activation in the progression of interstitial fibrosis. To determine whether complement activation may be responsible for the pro-fibrotic response that occurs in the tubulointerstitial compartment we stimulated primary cultures of proximal tubular epithelial cells with membrane attack complex, C5b-9. This led to increased mRNA concentrations of both collagen type IV and its intracellular chaperone, Heat Shock Protein 47 (HSP47). To determine whether this occurred in vivo Adriamycin was used to induce proteinuria in female Balb/c mice. The expression of collagen type IV and HSP47 was increased in proteinuric mice compared to control mice. In proteinuric mouse kidney, C3 was deposited at sites of tubulointerstitial injury and there was a relationship between C3 deposition and immunochemical staining for collagen type IV and HSP47. In situ hybridization suggested that the renal tubular epithelium was actively expressing HSP47 mRNA and, by implication, excess collagen. These observations support the hypothesis that complement activation on tubular epithelial cells can directly increase the pro-fibrotic process associated with tubulointerstitial damage.

Strain Distribution Pattern of Susceptibility to Immune-Mediated Nephritis

The genetic basis of immune-mediated nephritis is poorly understood. Recent studies have demonstrated that the NZW mouse strain is more prone to immune-mediated nephritis compared with C57BL/6 and BALB/c strains. The present study extends these findings by challenging 12 additional inbred strains of mice with rabbit anti-mouse glomerular basement membrane (GBM) reactive sera. Compared with control sera-injected mice and anti-GBM-injected A/J, AKR/J, C3H/HeJ, DBA/2J, MRL/MpJ, NOD/LtJ, P/J, SJL/J, and SWR/J mice, the anti-GBM-injected BUB/BnJ, DBA/1J, and 129/svJ mice developed severe proteinuria and azotemia. Their kidneys exhibited pronounced glomerulonephritis, with crescent formation, as well as tubulointerstitial disease, with these phenotypes being particularly profound in 129/svJ mice. However, these strains did not appear to differ in the nature of their xenogeneic immune response to the administered rabbit sera, either quantitatively or qualitatively. Collectively, these findings allude to the presence of genetic elements in the BUB/BnJ, DBA/1J, and 129/svJ genomes that may potentially confer susceptibility to immune-mediated nephritis. Detailed studies to dissect out the immunological and genetic basis of renal disease in these three strains are clearly warranted.

Antibody-mediated glomerulonephritis in mice: the role of endotoxin, complement and genetic background

Antibody-mediated glomerulonephritis in man may be exacerbated by infection and this effect may be mediated by bacterial endotoxin. There is evidence supporting a role for endotoxin in heterologous nephrotoxic nephritis in rats, but the role of endotoxin in this model in mice has not previously been explored. Previous data in mice on the role of complement in this model are conflicting and this may be due to the mixed genetic background of mice used in these studies. We used the model of heterologous nephrotoxic nephritis in mice and explored the role of endotoxin, complement and genetic background. In this study we show a synergy between antibody and endotoxin in causing a neutrophil influx. We also show that C1q-deficient mice have an increased susceptibility to glomerular inflammation but this is seen only on a mixed 129/Sv x C57BL/6 genetic background. On a C57BL/6 background we did not find any differences in disease susceptibility when wildtype, C1q, factor B or factor B/C2 deficient mice were compared. We also demonstrate that C57BL/6 mice are more susceptible to glomerular inflammation than 129/Sv mice. These results show that endotoxin is required in this model in mice, and that complement does not play a major role in glomerular inflammation in C57BL/6 mice. C1q may play a protective role in mixed-strain 129/Sv x C57BL/6 mice, but the data may also be explained by systematic bias in background genes, as there is a large difference in disease susceptibility between C57BL/6 and 129/Sv mice.

Transferrin up-regulates chemokine synthesis by human proximal tubular epithelial cells: implication on mechanism of tubuloglomerular communication in glomerulopathic proteinura

BACKGROUND

The pathogenesis of glomerulosclerosis and tubulointerstitial fibrosis in proteinuric renal disease is obscure. We recently showed that transferrin, a key proteinuric component, mediates proximal tubular epithelial cell (PTEC) C3 synthesis. To further examine whether proteinuric tubular injury may induce glomerular inflammation and to characterize the role of transferrin in activating PTEC, glomerular mesangial cells (MC) were exposed to transferrin-activated PTEC culture supernatant and their proliferative and profibrotic responses analyzed.

METHODS

Human PTEC and MC were obtained by primary culture. Confluent, transferrin-stimulated PTEC were grown in serum-free medium to produce a "conditioned" medium that was incubated with quiescent MC. The proliferative response of MC was then assessed by thymidine uptake, and the expression of fibrogenic factors measured by reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). The chemokine profile in PTEC after transferrin treatment was examined by RT-PCR and ELISA.

RESULTS

"Conditioned" supernatant from PTEC, which contained the highest amounts of platelet-derived growth factor (PDGF), stimulated MC proliferation compared with serum-free (P = 0.03) or transferrin-containing (P = 0.009) control media. This proliferative response was partially abrogated by treating MC with anti-PDGF. MC expression of PDGF, but not transforming growth factor-beta or intercellular cell adhesion molecule-1, was up-regulated by conditioned PTEC medium. Transferrin up-regulated monocyte chemoattractant peptide-1, interleukin-8, and macrophage migration inhibitory factor expression in a time- and dose-dependent fashion, but had no effect on RANTES expression by PTEC.

CONCLUSIONS

These results provide experimental evidence suggesting that there is a tubuloglomerular "cross-talk" mechanism in the proteinuric state. PTEC-secreted PDGF, which further induces mesangial PDGF, could partially account for the mesangial proliferation frequently observed in proteinuric renal disease. Transferrin is one of the culprit nephrotic proteins leading to tubular overexpression of various proinflammatory chemokines, which may explain the interstitial changes observed in proteinuric states.

Increased susceptibility of decay-accelerating factor deficient mice to anti-glomerular basement membrane glomerulonephritis

To prevent complement-mediated autologous tissue damage, host cells express a number of membrane-bound complement inhibitors. Decay-accelerating factor (DAF, CD55) is a GPI-linked membrane complement regulator that is widely expressed in mammalian tissues including the kidney. DAF inhibits the C3 convertase of both the classical and alternative pathways. Although DAF deficiency contributes to the human hematological syndrome paroxysmal nocturnal hemoglobinuria, the relevance of DAF in autoimmune tissue damage such as immune glomerulonephritis remains to be determined. In this study, we have investigated the susceptibility of knockout mice that are deficient in GPI-anchored DAF to nephrotoxic serum nephritis. Injection of a subnephritogenic dose of rabbit anti-mouse glomerular basement membrane serum induced glomerular disease in DAF knockout mice but not in wild-type controls. When examined at 8 days after anti-glomerular basement membrane treatment, DAF knockout mice had a much higher percentage of diseased glomeruli than wild-type mice (68.8 +/- 25.0 vs 10.0 +/- 3.5%; p < 0.01). Morphologically, DAF knockout mice displayed increased glomerular volume (516 +/- 68 vs 325 +/- 18 x 10(3) microm(3) per glomerulus; p < 0.0001) and cellularity (47.1 +/- 8.9 vs 32.0 +/- 3.1 cells per glomerulus; p < 0.01). Although the blood urea nitrogen level showed no difference between the two groups, proteinuria was observed in the knockout mice but not in the wild-type mice (1.4 +/- 0.7 vs 0.02 +/- 0.01 mg/24 h albumin excretion). The morphological and functional abnormalities in the knockout mouse kidney were associated with evidence of increased complement activation in the glomeruli. These results support the conclusion that membrane C3 convertase inhibitors like DAF play a protective role in complement-mediated immune glomerular damage in vivo.

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.

Epithelial secretion of C3 promotes colonization of the upper urinary tract by Escherichia coli

To assess the role of complement in renal infection, we studied a model of Escherichia coli-induced pyelonephritis in mice deficient in complement components C3 and C4. Renal infection occurred less frequently in C3- and C4-deficient mice compared with wild-type mice. In vitro, renal epithelial cells internalized fewer bacteria in the absence of C3 or in the presence of blockade of C3 bound to the bacteria. Moreover, upregulation of epithelial C3 production by stimulation with lipopolysaccharide enhanced bacterial internalization. Here we provide evidence that uropathogenic E. coli might use host C3 to invade the renal epithelium and that local complement production is sufficient for the bacteria to achieve this effect.

Membrane complement regulatory proteins: insight from animal studies and relevance to human diseases

The complement system plays an important role in host defense. However, if not properly regulated, activated complement can also cause significant damage to host tissues. To prevent complement-mediated autologous tissue damage, host cells express a number of membrane-bound complement regulatory proteins. These include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46) and CD59. Recent studies of membrane complement regulatory proteins from various animal species have revealed similarities as well as significant differences from the corresponding human proteins. In this review, we summarize recent advances in this area and contrast the structure, function and tissue distribution of membrane complement regulatory proteins in human and nonprimate mammalian species. We also discuss how the characterization of the animal proteins has provided important clues and might continue to show relevance to the pathogenesis and therapeutics of a number of human diseases.

Complement component C3 is not required for full expression of immune complex glomerulonephritis in MRL/lpr mice

Complement activation and tissue deposition of complement fragments occur during disease progression in lupus nephritis. Genetic deficiency of some complement components (e.g., Factor B) and infusion of complement inhibitors (e.g., Crry, anti-C5 Ab) protect against inflammatory renal disease. Paradoxically, genetic deficiencies of early components of the classical complement pathway (e.g., C1q, C4, and C2) are associated with an increased incidence of lupus in humans and lupus-like disease in murine knockout strains. Complement protein C3 is the converging point for activation of all three complement pathways and thus plays a critical role in biologic processes mediated by complement activation. To define the role of C3 in lupus nephritis, mice rendered C3 deficient by targeted deletion were backcrossed for eight generations to MRL/lpr mice, a mouse strain that spontaneously develops lupus-like disease. We derived homozygous knockout (C3(-/-)), heterozygous (C3(+/-)), and C3 wild-type (C3(+/+)) MRL/lpr mice. Serum levels of autoantibodies and circulating immune complexes were similar among the three groups. However, there was earlier and significantly greater albuminuria in the C3(-/-) mice compared with the other two groups. Glomerular IgG deposition was also significantly greater in the C3(-/-) mice than in the other two groups, although overall pathologic renal scores were similar. These results indicate that C3 and/or activation of C3 is not required for full expression of immune complex renal disease in MRL/lpr mice and may in fact play a beneficial role via clearance of immune complexes.

Protection and injury: the differing roles of complement in the development of glomerular injury

The role of complement in autoimmune glomerulonephritis (as in other autoimmune diseases) is paradoxical, in that complement activation mediates acute inflammatory injury, yet inherited deficiency of complement may predispose to immune complex disease in particular immune complex glomerulonephritis. We have investigated the role of complement in experimentally induced glomerulonephritis in C3-deficient mice, using antibodies against the mouse glomerular basement membrane (GBM). In the acute phase of the disease, which is initiated by binding of heterologous antibody to the GBM, we confirmed that the inflammatory injury was positively complement dependent, with C3-deficient mice developing less severe injury. In contrast, in the autologous phase of the disease, mediated by the immune response against the heterologous antibody fixed in the GBM, the disease was negatively complement dependent. That is, by 14 days after disease induction the C3-deficient mice had heavier proteinuria and more severe uremia (p < 0.001) compared to the complement sufficient mice. The C3-deficient mice also showed a greater accumulation of electron-dense deposits in the GBM. These findings were reproduced in an accelerated model of this disease in which C3-deficient mice also develop more severe functional disturbance and demonstrate a higher rate of immune complex deposition. These data illustrate the potential for the net effect of complement to switch from a detrimental to a protective mode at different stages of autoimmune injury.

Intrarenal synthesis of complement

During the past decade, research has shown that the kidney has the capacity to synthesize most of the activation pathway components of the complement cascade. As well as implying physiological roles in local clearance of immune complexes and defense against invasive organisms, an increasing amount of evidence indicates that the intrarenal synthesis of complement makes an important contribution in the pathogenesis of renal injury. Here we review this evidence and present a case for more definitive investigation of these functions.

Complement inhibitors: a resurgent concept in anti-inflammatory therapeutics

In addition to its essential role in immune defense, the complement system contributes to tissue damage in many clinical conditions. Thus, there is a pressing need to develop therapeutically effective complement inhibitors to prevent these adverse effects. This concept, though old, received little scientific attention until recently. Data from animal models of diseases that have been produced using complement-deficient, knockout, and transgenic animals, as well as data demonstrating that complement proteins are produced in many important tissue sites (including the brain) have attracted the interest of many basic research scientists and applied scientists from the biotechnology field and larger pharmaceutical firms. This resurgence of interest has generated a wealth of new information in the field of complement inhibition. In this article, we comprehensively review up-to-date information in the field of complement inhibitors.

Modulation of renal disease in MRL/lpr mice genetically deficient in the alternative complement pathway factor B

In systemic lupus erythematosus, the renal deposition of complement-containing immune complexes initiates an inflammatory cascade resulting in glomerulonephritis. Activation of the classical complement pathway with deposition of C3 is pathogenic in lupus nephritis. Although the alternative complement pathway is activated in lupus nephritis, its role in disease pathogenesis is unknown. To determine the role of the alternative pathway in lupus nephritis, complement factor B-deficient mice were backcrossed to MRL/lpr mice. MRL/lpr mice develop a spontaneous lupus-like disease characterized by immune complex glomerulonephritis. We derived complement factor B wild-type (B+/+), homozygous knockout (B-/-), and heterozygous (B+/-) MRL/lpr mice. Compared with B+/- or B+/+ mice, MRL/lpr B-/- mice developed significantly less proteinuria, less glomerular IgG deposition, and decreased renal scores as well as lower IgG3 cryoglobulin production and vasculitis. Serum C3 levels were normal in the B-/- mice compared with significantly decreased levels in the other two groups. These results suggest that: 1) factor B plays an important role in the pathogenesis of glomerulonephritis and vasculitis in MRL/lpr mice; and 2) activation of the alternative pathway, either by the amplification loop or by IgA immune complexes, has a prominent effect on serum C3 levels in this lupus model.

Targeting the complement system

Interest has blossomed in the development of complement inhibitors, in parallel with a growth in our understanding of the biology of the complement cascade. The first generation of designed inhibitors was based on naturally occurring complement receptors and regulatory molecules. These agents provided useful tools for exploring the role of complement in experimental models of disease, but may have limited therapeutic application in humans because of their short half-lives, limited bioavailability and possible antigenicity. More recently, humanized antibodies and synthetic molecules that block the activation of complement have been developed, which look as though they may overcome some of these difficulties. The possibility for precision inhibition of a limited part of the complement cascade, or for inhibition confined to a single organ, may offer effective therapeutic results, while avoiding the disadvantages of nonselective complement blockade. This review examines the recent evidence that complement inhibition will reduce tissue damage resulting from organ transplantation, ischaemia-reperfusion injury, cancer, glomerulonephritis and the use of extracorporeal circuits.

Generalized lupus erythematosus profundus in a patient with genetic partial deficiency of C4

Lupus erythematosus profundus (LEP) is an unusual variant of cutaneous lupus erythematosus (CLE)that is characterized by chronic, recurrent inflammation of the subcutaneous tissue leading to fibrosis. It is found in the settings of both discoid and systemic lupus erythematosus. Generalized forms are extremely rare. We present a case of generalized LEP associated with genetic partial C4-deficiency.

Renal, central nervous system and pancreatic overexpression of recombinant soluble Crry in transgenic mice. A novel means of protection from complement-mediated injury

Crry is a potent complement regulator that inhibits classical and alternative pathway C3 convertases in rodents. We have produced transgenic animals expressing Crry as a recombinant soluble protein driven by the broadly active metallothionein-I promoter. These animals have high serum and urinary levels of rsCrry leading to inhibition of complement activity. In nephrotoxic serum nephritis (NSN), injected antibodies bind to glomeruli, leading to complement activation and subsequent glomerular injury and albuminuria. We have shown that rsCrry can block such injury and reduce albuminuria by as much as 75%. Corresponding to the reduction in albuminuria was the complete absence of C3 staining in glomeruli by immunofluorescence microscopy in 17/20 transgene positive animals. Support for a local source of protective rsCrry in this model is provided by the demonstration of Crry transgene mRNA in the glomerulus and a very high fractional excretion of rsCrry in the urine. Therefore, rsCrry expression markedly ameliorates an antibody-induced disease model in vivo. In addition, local synthesis of Crry in other organs that are targets of immune injury has been found. For example, Crry transgene mRNA is present throughout the central nervous system and in pancreatic islets. Thus, continuous complement inhibition at the C3 convertase step appears to be feasible and is effective in complement-mediated injury states. A number of disease models affecting these target organs can be tested using these mice.

Glomerular inflammation: use of genetically deficient mice to elucidate the roles of leukocyte adhesion molecules and Fc-gamma receptors in vivo

Single gene knock-outs in mice have been used to define the biological role of leukocyte adhesion receptors, Fc-gamma receptors and complement in animal models of immune complex glomerulonephritis. These studies have shown important differences in the role of P-selectin in glomerular inflammation and inflammation at other sites, and have given a new appreciation of the dominant role played by Fc-gamma receptors in immune complex-induced glomerular injury.

Transgenic mice overexpressing the complement inhibitor crry as a soluble protein are protected from antibody-induced glomerular injury

Complement receptor 1-related gene/protein y (Crry) is a potent murine membrane complement regulator that inhibits classical and alternative pathway C3 convertases. In nephrotoxic serum (NTS) nephritis, injected antibodies (Abs) bind to glomeruli, leading to complement activation and subsequent glomerular injury and albuminuria. To study the phenotypic effects of continuous complement pathway blockade, transgenic mice were created that express recombinant soluble (rs) Crry directed by the broadly active and heavy metal-inducible metallothionein-I promoter. One transgenic line expressing high levels of rsCrry was propagated. Serum rsCrry levels were 18.7 +/- 2.7 microg/ml (n = 5) at basal level and increased to 118.1 +/- 20.6 microg/ml 4 d after addition of zinc to the drinking water. By reverse transcription polymerase chain reaction (RT-PCR), transgene messenger (m)RNA was present in liver, kidney, brain, lung, and spleen, but not in heart. By in situ RT-PCR analysis of kidneys, transgene mRNA was widely expressed both in renal glomeruli and tubules. Urinary excretion of rsCrry was 113.4 +/- 22.4 microg/ml with a fractional excretion relative to creatinine of 13.2 +/- 2.7%, consistent with local renal production of rsCrry and secretion into urine. The founder and all transgene positive adult animals have remained healthy with no mortality or apparent phenotypic abnormalities, including infection or immune complex disease. To determine whether rsCrry blocked complement-mediated injury, NTS nephritis was induced by injection of NTS immunoglobulin (Ig)G, followed by an 18-h urine collection to quantitate the excretion of albumin as a measure of glomerular injury. In transgene-negative littermates (n = 15), transgene-positive animals (n = 10), and transgene-positive animals fed zinc (n = 10), albuminuria was 4,393 +/- 948, 1,783 +/- 454, and 1,057 +/- 277 microg/mg creatinine, respectively (P < 0.01 by ANOVA). Glomerular C3 was evident by immunofluorescence staining in 12/15 transgene-negative animals, but in none of the transgene-positive animals fed zinc. Thus, we have produced the first transgenic animals that overexpress a soluble C3 convertase inhibitor. rsCrry expression markedly ameliorates an Ab-induced disease model in vivo. These results support the hypothesis that continuous complement inhibition at the C3 convertase step is feasible and effective in complement-mediated injury states.

Complement and complement inhibitors: their role in autoimmune and inflammatory diseases

The role of complement in the pathogenesis of renal injury is now being confirmed with the use of knockout technology and specific inhibitors. Chronic injury, particularly of the tubules and interstitium, and local complement synthesis are emerging as potential additional targets for complement-based therapy. Strategies are being developed to prevent complement-induced injury both in transplant and in native kidneys.