Ultrastructural localization of the membrane attack complex of complement in human renal tissues

The Complement Pathway: New Insights into Immunometabolic Signaling in Diabetic Kidney Disease

Significance: The metabolic disorder, diabetes mellitus, results in microvascular complications, including diabetic kidney disease (DKD), which is partly believe to involve disrupted energy generation in the kidney, leading to injury that is characterized by inflammation and fibrosis. An increasing body of evidence indicates that the innate immune complement system is involved in the pathogenesis of DKD; however, the precise mechanisms remain unclear. Recent Advances: Complement, traditionally thought of as the prime line of defense against microbial intrusion, has recently been recognized to regulate immunometabolism. Studies have shown that the complement activation products, Complement C5a and C3a, which are potent pro-inflammatory mediators, can mediate an array of metabolic responses in the kidney in the diabetic setting, including altered fuel utilization, disrupted mitochondrial respiratory function, and reactive oxygen species generation. In diabetes, the lectin pathway is activated via autoreactivity toward altered self-surfaces known as danger-associated molecular patterns, or via sensing altered carbohydrate and acetylation signatures. In addition, endogenous complement inhibitors can be glycated, whereas diet-derived glycated proteins can themselves promote complement activation, worsening DKD, and lending support for environmental influences as an additional avenue for propagating complement-induced inflammation and kidney injury. Critical Issues: Recent evidence indicates that conventional renoprotective agents used in DKD do not target the complement, leaving this web of inflammatory stimuli intact. Future Directions: Future studies should focus on the development of novel pharmacological agents that target the complement pathway to alleviate inflammation, oxidative stress, and kidney fibrosis, thereby reducing the burden of microvascular diseases in diabetes. Antioxid. Redox Signal. 37, 781-801.

Solidified glomerulosclerosis, identified using single glomerular proteomics, predicts end-stage renal disease in Chinese patients with type 2 diabetes

Few histological prognostic indicators for end-stage renal disease (ESRD) have been validated in diabetic patients. This biopsy-based study aimed to identify nephropathological risk factors for ESRD in Chinese patients with type 2 diabetes. Histological features of 322 Chinese type 2 diabetic patients with biopsy-confirmed diabetic nephropathy (DN) were retrospectively analysed. Cox proportional hazards analysis was used to estimate the hazard ratio (HR) for ESRD. Single glomerular proteomics and immunohistochemistry were used to identify differentially expressed proteins and enriched pathways in glomeruli. During the median follow-up period of 24 months, 144 (45%) patients progressed to ESRD. In multivariable models, the Renal Pathology Society classification failed to predict ESRD, although the solidified glomerulosclerosis (score 1: HR 1.65, 95% confidence interval [CI] 1.04-2.60; score 2: HR 2.48, 95% CI 1.40-4.37) and extracapillary hypercellularity (HR 2.68, 95% CI 1.55-4.62) were identified as independent risk factors. Additionally, single glomerular proteomics, combined with immunohistochemistry, revealed that complement C9 and apolipoprotein E were highly expressed in solidified glomerulosclerosis. Therefore, solidified glomerulosclerosis and extracapillary hypercellularity predict diabetic ESRD in Chinese patients. Single glomerular proteomics identified solidified glomerulosclerosis as a unique pathological change that may be associated with complement overactivation and abnormal lipid metabolism.

Deposition of the Membrane Attack Complex in Healthy and Diseased Human Kidneys

The membrane attack complex-also known as C5b-9-is the end-product of the classical, lectin, and alternative complement pathways. It is thought to play an important role in the pathogenesis of various kidney diseases by causing cellular injury and tissue inflammation, resulting in sclerosis and fibrosis. These deleterious effects are, consequently, targeted in the development of novel therapies that inhibit the formation of C5b-9, such as eculizumab. To clarify how C5b-9 contributes to kidney disease and to predict which patients benefit from such therapy, knowledge on deposition of C5b-9 in the kidney is essential. Because immunohistochemical staining of C5b-9 has not been routinely conducted and never been compared across studies, we provide a review of studies on deposition of C5b-9 in healthy and diseased human kidneys. We describe techniques to stain deposits and compare the occurrence of deposits in healthy kidneys and in a wide spectrum of kidney diseases, including hypertensive nephropathy, diabetic nephropathy, membranous nephropathy, IgA nephropathy, lupus nephritis, C3 glomerulopathy, and thrombotic microangiopathies such as the atypical hemolytic uremic syndrome, vasculitis, interstitial nephritis, acute tubular necrosis, kidney tumors, and rejection of kidney transplants. We summarize how these deposits are related with other histological lesions and clinical characteristics. We evaluate the prognostic relevance of these deposits in the light of possible treatment with complement inhibitors.

Complement C7 is Specifically Expressed in Mesangial Cells and is a Potential Diagnostic Biomarker for Diabetic Nephropathy and is Regulated by miR-494-3p and miR-574-5p

BACKGROUND

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease, but it remains relatively underdiagnosed.

OBJECTIVE

In this study, we aimed to explore the key regulatory pathways and potential biomarkers related to DN using integrated bioinformatics analysis and validation.

METHODS

First, the microarray data of the GSE30528 and GSE96804 datasets were downloaded from the Gene Expression Omnibus (GEO) database, and differentially expressed genes (DEGs) were screened. Then, weighted gene coexpression network analysis (WGCNA), gene ontology (GO) annotation, gene set enrichment analysis (GSEA) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed to identify key pathways and genes. qRT-PCR and receiver operating characteristic (ROC) curves were used to validate our results. Furthermore, single-cell RNA sequencing (scRNA-seq) data were reanalyzed to investigate the expression specificity of C7 in DN cells. An online database search and luciferase reporter assay identified the target relationship between miRNAs and C7.

RESULTS

The "complement and coagulation cascades" were significantly enriched, and complement C3 and C7 were candidate markers. The receiver operating characteristic (ROC) curve revealed that C7 had significant diagnostic value (AUC=0.865) in DN. Through scRNA-seq reanalysis, we found that C7 was specifically elevated in mesangial (MES) cells of DN. Moreover, we found that the expression of C7 was regulated by miR-494-3p and miR-574-5p.

CONCLUSION

This is the first study to reveal that C7 is specifically expressed in mesangial cells, is a potential diagnostic biomarker for diabetic nephropathy, and is regulated by miR-494-3p and miR-574-5p.

Urine Complement Proteins and the Risk of Kidney Disease Progression and Mortality in Type 2 Diabetes

OBJECTIVE

We examined the association of urine complement proteins with progression to end-stage renal disease (ESRD) or death in people with type 2 diabetes and proteinuric diabetic kidney disease (DKD).

RESEARCH DESIGN AND METHODS

Using targeted mass spectrometry, we quantified urinary abundance of 12 complement proteins in a predominantly Mexican American cohort with type 2 diabetes and proteinuric DKD (n = 141). The association of urine complement proteins with progression to ESRD or death was evaluated using time-to-event analyses.

RESULTS

At baseline, median estimated glomerular filtration rate (eGFR) was 54 mL/min/1.73 m² and urine protein-to-creatinine ratio 2.6 g/g. Sixty-seven participants developed ESRD or died, of whom 39 progressed to ESRD over a median of 3.1 years and 40 died over a median 3.6 years. Higher urine CD59, an inhibitor of terminal complement complex formation, was associated with a lower risk of ESRD (hazard ratio [HR] [95% CI per doubling] 0.50 [0.29-0.87]) and death (HR [95% CI] 0.56 [0.34-0.93]), after adjustment for demographic and clinical covariates, including baseline eGFR and proteinuria. Higher urine complement components 4 and 8 were associated with lower risk of death (HR [95% CI] 0.57 [0.41-0.79] and 0.66 [0.44-0.97], respectively); higher urine factor H-related protein 2, a positive regulator of the alternative complement pathway, was associated with greater risk of death (HR [95% CI] 1.61 [1.05-2.48]) in fully adjusted models.

CONCLUSIONS

In a largely Mexican American cohort with type 2 diabetes and proteinuric DKD, urine abundance of several complement and complement regulatory proteins was strongly associated with progression to ESRD and death.

Complement 7 Is Up-Regulated in Human Early Diabetic Kidney Disease

There is a temporal window from the time diabetes is diagnosed to the appearance of overt kidney disease during which time the disease progresses quietly without detection. Currently, there is no way to detect early diabetic nephropathy (EDN). Herein, we performed an unbiased assessment of gene-expression analysis of postmortem human kidneys to identify candidate genes that may contribute to EDN. We then studied one of the most promising differentially expressed genes in both kidney tissue and blood samples. Differential transcriptome analysis of EDN kidneys and matched nondiabetic controls showed alterations in five canonical pathways, and among them the complement pathway was the most significantly altered. One specific complement pathway gene, complement 7 (C7), was significantly elevated in EDN kidney. Real-time PCR confirmed more than a twofold increase of C7 expression in EDN kidneys compared with controls. Changes in C7 gene product level were confirmed by immunohistochemistry. C7 protein levels were elevated in proximal tubules of EDN kidneys. Serum C7 protein levels were also measured in EDN and control donors. C7 levels were significantly higher in EDN serum than control serum. This latter finding was independently confirmed in a second set of blood samples from a previously collected data set. Together, our data suggest that C7 is associated with EDN, and can be used as a molecular target for detection and/or treatment of EDN.

Complement C3 and Risk of Diabetic Microvascular Disease: A Cohort Study of 95202 Individuals from the General Population

BACKGROUND

Whether the complement system is involved in the development of diabetic microvascular disease is unknown. We tested the hypothesis that high concentrations of complement C3 are associated with increased risk of diabetic retinopathy, nephropathy, and neuropathy in individuals from the general population.

METHODS

We studied 95202 individuals from the general population with baseline measurements of complement C3, genotyped for rs1065489, rs429608, and rs448260 determining concentrations of complement C3, and enrolled in the Copenhagen General Population Study from 2003 through 2013, following them until April 10, 2013. Rs1065489, rs429608, and rs448260 were identified with genome-wide association scans in 3752 individuals from the Copenhagen City Heart Study.

RESULTS

The cumulative incidence was increased from the lowest tertile to the highest tertile of complement C3 for diabetic retinopathy (log-rank trend, P = 1 × 10^-20), nephropathy (P = 7 × 10^-15), and neuropathy (P = 5 × 10^-10). Multifactorially adjusted hazard ratios for a 1 SD higher concentration of complement C3 were 1.87 (95% CI, 1.61-2.18) for diabetic retinopathy, 1.90 (1.62-2.23) for diabetic nephropathy, and 1.56 (1.29-1.89) for diabetic neuropathy. The multifactorially adjusted hazard ratio for individuals with the highest vs lowest tertile of complement C3 was 3.29 (1.78-6.07) for retinopathy, 2.71 (1.42-5.16) for nephropathy, and 2.40 (1.26-4.54) for neuropathy.

CONCLUSIONS

High baseline concentrations of complement C3 were associated with increased risk of diabetic retinopathy, nephropathy, and neuropathy in individuals from the general population. These epidemiological findings were substantiated by a Mendelian randomization approach, potentially indicating causality.

Use of Readily Accessible Inflammatory Markers to Predict Diabetic Kidney Disease

Diabetic kidney disease is a common complication of type 1 and type 2 diabetes and is the primary cause of end-stage renal disease in developed countries. Early detection of diabetic kidney disease will facilitate early intervention aimed at reducing the rate of progression to end-stage renal disease. Diabetic kidney disease has been traditionally classified based on the presence of albuminuria. More recently estimated glomerular filtration rate has also been incorporated into the staging of diabetic kidney disease. While albuminuric diabetic kidney disease is well described, the phenotype of non-albuminuric diabetic kidney disease is now widely accepted. An association between markers of inflammation and diabetic kidney disease has previously been demonstrated. Effector molecules of the innate immune system including C-reactive protein, interleukin-6, and tumor necrosis factor-α are increased in patients with diabetic kidney disease. Furthermore, renal infiltration of neutrophils, macrophages, and lymphocytes are observed in renal biopsies of patients with diabetic kidney disease. Similarly high serum neutrophil and low serum lymphocyte counts have been shown to be associated with diabetic kidney disease. The neutrophil-lymphocyte ratio is considered a robust measure of systemic inflammation and is associated with the presence of inflammatory conditions including the metabolic syndrome and insulin resistance. Cross-sectional studies have demonstrated a link between high levels of the above inflammatory biomarkers and diabetic kidney disease. Further longitudinal studies will be required to determine if these readily available inflammatory biomarkers can accurately predict the presence and prognosis of diabetic kidney disease, above and beyond albuminuria, and estimated glomerular filtration rate.

The role of complement system in adipose tissue-related inflammation

As the common factor linking adipose tissue to the metabolic context of obesity, insulin resistance and atherosclerosis are associated with a low-grade chronic inflammatory status, to which the complement system is an important contributor. Adipose tissue synthesizes complement proteins and is a target of complement activation. C3a-desArg/acylation-stimulating protein stimulates lipogenesis and affects lipid metabolism. The C3a receptor and C5aR are involved in the development of adipocytes' insulin resistance through macrophage infiltration and the activation of adipose tissue. The terminal complement pathway has been found to be instrumental in promoting hyperglycemia-associated tissue damage, which is characteristic of the major vascular complications of diabetes mellitus and diabetic ketoacidosis. As a mediator of the effects of the terminal complement complex C5b-9, RGC-32 has an impact on energy expenditure as well as lipid and glucose metabolic homeostasis. All of this evidence, taken together, indicates an important role for complement activation in metabolic diseases.

The lectin-like domain of thrombomodulin ameliorates diabetic glomerulopathy via complement inhibition

Coagulation and complement regulators belong to two interactive systems constituting emerging mechanisms of diabetic nephropathy. Thrombomodulin (TM) regulates both coagulation and complement activation, in part through discrete domains. TM’s lectin like domain dampens complement activation, while its EGF-like domains independently enhance activation of the anticoagulant and cytoprotective serine protease protein C (PC). A protective effect of activated PC in diabetic nephropathy is established. We hypothesised that TM controls diabetic nephropathy independent of PC through its lectin-like domain by regulating complement. Diabetic nephropathy was analysed in mice lacking TM’s lectin-like domain (TMLeD/LeD) and controls (TMwt/wt). Albuminuria (290 μg/mg vs. 166 μg/mg, p=0.03) and other indices of experimental diabetic nephropathy were aggravated in diabetic TMLeD/LeDmice. Complement deposition (C3 and C5b-9) was markedly increased in glomeruli of diabetic TMLeD/LeDmice. Complement inhibition with enoxaparin ameliorated diabetic nephropathy in TMLeD/LeDmice (e.g. albuminuria 85 μg/mg vs. 290 μg/mg, p <0.001). In vitroTM’s lectin-like domain cell-autonomously prevented glucose-induced complement activation on endothelial cells and –notably –on podocytes. Podocyte injury, which was enhanced in diabetic TMLeD/LeDmice, was reduced following complement inhibition with enoxaparin. The current study identifies a novel mechanism regulating complement activation in diabetic nephropathy. TM’s lectin-like domain constrains glucose-induced complement activation on endothelial cells and podocytes and ameliorates albuminuria and glomerular damage in mice.

A distinctive histidine residue is essential for in vivo glycation-inactivation of human CD59 transgenically expressed in mice erythrocytes: Implications for human diabetes complications

Clinical and experimental evidences support a link between the complement system and the pathogenesis of diabetes complications. CD59, an extracellular cell membrane-anchored protein, inhibits formation of the membrane attack complex (MAC), the main effector of complement-mediated tissue damage. This complement regulatory activity of human CD59 (hCD59) is inhibited by hyperglycemia-induced ɛ-amino glycation of Lys⁴¹ . Biochemical and structural analyses of glycated proteins with known three-dimensional structure revealed that glycation of ɛ-amino lysyl residues occurs predominantly at "glycation motives" that include lysyl/lysyl pairs or proximity of a histidyl residue, in which the imidazolyl moiety is ≈ 5Å from the ɛ-amino group. hCD59 contains a distinctive Lys⁴¹ /His⁴⁴ putative glycation motif within its active site. In a model of transgenic diabetic mice expressing in erythrocytes either the wild type or a H44Q mutant form of hCD59, we demonstrate in vivo that the His⁴⁴ is required for Lys⁴¹ glycation and consequent functional inactivation of hCD59, as evidenced using a mouse erythrocytes hemolytic assay. Since (1) the His⁴⁴ residue is not present in CD59 from other animal species and (2) humans are particularly prone to develop complications of diabetes, our results indicate that the Lys⁴¹ /His⁴⁴ glycation motif in human CD59 may confer humans a higher risk of developing vascular disease in response to hyperglycemia.

The role of complement inhibitors beyond controlling inflammation

The complement system is an arm of innate immunity that aids in the removal of pathogens and dying cells. Due to its harmful, pro-inflammatory potential, complement is controlled by several soluble and membrane-bound inhibitors. This family of complement regulators has been recently extended by the discovery of several new members, and it is becoming apparent that these proteins harbour additional functions. In this review, the current state of knowledge of the physiological functions of four complement regulators will be described: cartilage oligomeric matrix protein (COMP), CUB and sushi multiple domains 1 (CSMD1), sushi domain-containing protein 4 (SUSD4) and CD59. Complement activation is involved in both the development of and defence against cancer. COMP expression is pro-oncogenic, whereas CSMD1 and SUSD4 act as tumour suppressors. These effects may be related in part to the complex influence of complement on cancer but also depend on unrelated functions such as the protection of cells from endoplasmic reticulum stress conveyed by intracellular COMP. CD59 is the main inhibitor of the membrane attack complex, and its deficiency leads to complement attack on erythrocytes and severe haemolytic anaemia, which is now amenable to treatment with an inhibitor of C5 cleavage. Unexpectedly, the intracellular pool of CD59 is crucial for insulin secretion from pancreatic β-cells. This finding is one of several relating to the intracellular functions of complement proteins, which until recently were only considered to be present in the extracellular space. Understanding the alternative functions of complement inhibitors may unravel unexpected links between complement and other physiological systems, but is also important for better design of therapeutic complement inhibition.

Incident microalbuminuria and complement factor mannan-binding lectin-associated protein 19 in people with newly diagnosed type 1 diabetes

BACKGROUND

Evidence links the lectin pathway of complement activation to diabetic kidney disease. Upon carbohydrate-recognition by pattern-recognition molecules, eg, mannan-binding lectin (MBL), the MBL-associated serine protease (MASP-2) is activated and initiates the complement cascade. The MASP2 gene encodes MASP-2 and the alternative splice product MBL-associated protein 19 (MAp19). Both MAp19 and MASP-2 circulate in complex with MBL. We tested the hypothesis that MAp19 and MASP-2 concentrations predict the risk of incident microalbuminuria.

METHODS

Baseline MAp19 and MASP-2 were measured in 270 persons with newly diagnosed type 1 diabetes tracked for incidence of persistent microalbuminuria in a prospective observational 18-year-follow-up study.

RESULTS

Seventy-five participants (28%) developed microalbuminuria during follow-up. MBL-associated protein 19 concentrations were higher in participants that later progressed to microalbuminuria as compared with those with persistent normoalbuminuria (268 ng/mL [95% CI, 243-293] vs 236 ng/mL [95% CI, 223-250], P = .02). Participants with MAp19 concentration within the highest quartile of the cohort had an increased risk of microalbuminuria as compared with participants with MAp19 concentration within the combined lower 3 quartiles in unadjusted Cox analysis, hazard ratio 1.86 ([95% CI, 1.17-2.96], P = .009). This remained significant in adjusted models, eg, adjusting for age, sex, HbA_1c , systolic blood pressure, urinary albumin excretion, smoking, serum creatinine, and serum cholesterol. MBL-associated serine protease concentration was not associated with incidence of microalbuminuria.

CONCLUSIONS

In conclusion, the results show an association between baseline MAp19 concentration and the incidence of microalbuminuria in an 18-year-follow-up study on persons with newly diagnosed type 1 diabetes.

Immunity and inflammation in diabetic kidney disease: translating mechanisms to biomarkers and treatment targets

Increasing incidences of obesity and diabetes have made diabetic kidney disease (DKD) the leading cause of chronic kidney disease and end-stage renal disease worldwide. Despite current pharmacological treatments, including strategies for optimizing glycemic control and inhibitors of the renin-angiotensin system, DKD still makes up almost one-half of all cases of end-stage renal disease in the United States. Compelling and mounting evidence has clearly demonstrated that immunity and inflammation play a paramount role in the pathogenesis of DKD. This article reviews the involvement of the immune system in DKD and identifies important roles of key immune and inflammatory mediators. One of the most recently identified biomarkers is serum amyloid A, which appears to be relatively specific for DKD. Novel and evolving treatment approaches target protein kinases, transcription factors, chemokines, adhesion molecules, growth factors, advanced glycation end-products, and other inflammatory molecules. This is the beginning of a new era in the understanding and treatment of DKD, and we may have finally reached a tipping point in our fight against the growing burden of DKD.

The role of the complement system in diabetic nephropathy

The development of type 1 and type 2 diabetes mellitus has a substantial negative impact on morbidity and mortality and is responsible for substantial individual and socioeconomic costs worldwide. One of the most serious consequences of diabetes mellitus is the development of diabetic angiopathy, which manifests clinically as microvascular and macrovascular complications. One microvascular complication, diabetic nephropathy, is the most common cause of end-stage renal disease in developed countries. Although several available therapeutic interventions can delay the onset and progression of diabetic nephropathy, morbidity associated with this disease remains high and new therapeutic approaches are needed. In addition, not all patients with diabetes mellitus will develop diabetic nephropathy and thus new biomarkers are needed to identify individuals who will develop this life-threatening disease. An increasing body of evidence points toward a role of the complement system in the pathogenesis of diabetic nephropathy. For example, circulating levels of mannose-binding lectin (MBL), a pattern recognition molecule of the innate immune system, have emerged as a robust biomarker for the development and progression of this disease, and evidence suggests that MBL, H-ficolin, complement component C3 and the membrane attack complex might contribute to renal injury in the hyperglycaemic mileu. New approaches to modulate the complement system might lead to the development of new agents to prevent or slow the progression of diabetic nephropathy.

Deficiency of the complement regulatory protein CD59 accelerates the development of diabetes-induced atherosclerosis in mice

AIMS

Clinical and experimental evidence supports a strong link between the complement system, complement regulatory proteins and the pathogenesis of diabetes vascular complications. We previously reported that the complement regulatory protein CD59 is inactivated by glycation in humans with diabetes. Our objective for this study is to assess experimentally how the deficiency of CD59 impacts the development of diabetic atherosclerosis in vivo.

METHODS

We crossed mCD59 sufficient and deficient mice into the ApoE^-/- background to generate mCd59ab^+/+/ApoE^-/- and mCd59ab^-/-/ApoE^-/- mice, and induced diabetes by multiple low dose injections of streptozotocin. Atherosclerosis was detected by hematoxylin and eosin (H&E) and oil red-O staining. Membrane attack complex (MAC) deposition and macrophage infiltration were detected by immunostaining.

RESULTS

Diabetic mCD59 deficient (mCD59ab^-/-/ApoE^-/-) mice developed nearly 100% larger atherosclerotic lesion areas in the aorta (7.5%±0.6 vs 3.6%±0.7; p<0.005) and in the aortic roots (H&E: 26.2%±1.9 vs. 14.3%±1.1; p<0.005), in both cases associated with increased lipid (Oil red-O: 14.9%±1.1 vs. 7.8%±1.1; p<0.05) and MAC deposition (6.8%±0.8 vs. 3.0%±0.7; p<0.005) and macrophage infiltration (31.5%±3.7 vs. 16.4%±3.0; p<0.05) in the aortic roots as compared to their diabetic mCD59 sufficient (mCD59ab^+/+/ApoE^-/-) counterpart.

CONCLUSIONS

The deficiency of CD59 accelerates the development of diabetic atherosclerosis.

Role of complement and complement regulatory proteins in the complications of diabetes

It is well established that the organ damage that complicates human diabetes is caused by prolonged hyperglycemia, but the cellular and molecular mechanisms by which high levels of glucose cause tissue damage in humans are still not fully understood. The prevalent hypothesis explaining the mechanisms that may underlie the pathogenesis of diabetes complications includes overproduction of reactive oxygen species, increased flux through the polyol pathway, overactivity of the hexosamine pathway causing intracellular formation of advanced glycation end products, and activation of protein kinase C isoforms. In addition, experimental and clinical evidence reported in past decades supports a strong link between the complement system, complement regulatory proteins, and the pathogenesis of diabetes complications. In this article, we summarize the body of evidence that supports a role for the complement system and complement regulatory proteins in the pathogenesis of diabetic vascular complications, with specific emphasis on the role of the membrane attack complex (MAC) and of CD59, an extracellular cell membrane-anchored inhibitor of MAC formation that is inactivated by nonenzymatic glycation. We discuss a pathogenic model of human diabetic complications in which a combination of CD59 inactivation by glycation and hyperglycemia-induced complement activation increases MAC deposition, activates pathways of intracellular signaling, and induces the release of proinflammatory, prothrombotic cytokines and growth factors. Combined, complement-dependent and complement-independent mechanisms induced by high glucose promote inflammation, proliferation, and thrombosis as characteristically seen in the target organs of diabetes complications.

Increased complement activation in human type 1 diabetes pancreata

OBJECTIVE

Evidence supporting an association between complement (C) and type 1 diabetes (T1D) includes the identification of C-fixing islet cell autoantibodies in T1D sera and genetic associations with the major histocompatibility complex III C4 region on chromosome 6. Therefore, we investigated whether C activation was present in pancreata from those with or at increased risk (positive for T1D associated autoantibodies) for T1D.

RESEARCH DESIGN AND METHODS

Immunohistochemical techniques were used to measure the C degradation product C4d in organ donor pancreata from patients with T1D and type 2 diabetes and autoantibody-positive and autoantibody-negative subjects.

RESULTS

Median C4d antigen density differed across the groups (P < 0.0001) and was highest in patients with T1D. C4d immunostaining localized to the blood vessel endothelium and extracellular matrix surrounding blood vessels and exocrine ducts. Receiver operating characteristic analysis resulted in 81.8% sensitivity and 94.4% specificity for C4d staining.

CONCLUSIONS

These data suggest that C activation is occurring within pancreata from patients with T1D and C4d may be a biomarker for T1D.

Diabetes-induced changes in mannan-binding lectin levels and complement activation in a mouse model of type 1 diabetes

Circulating mannan-binding lectin (MBL) levels are elevated in type 1 diabetes. Further, high MBL levels are associated with the development of diabetic nephropathy. In animals, a direct effect of MBL on diabetic kidney changes is observed. We hypothesized that MBL levels and detrimental complement activation increase as a consequence of diabetes. We measured plasma MBL before and 7 weeks after inducing diabetes by streptozotocin. Mice have two MBLs, MBL-A and MBL-C. Diabetes induction led to an increase in MBL-C concentration, whereas no change during the study was found in the control group. The increase in MBL-C was associated with the increasing plasma glucose levels. In accordance with the observed changes in circulating MBL levels, liver expression of Mbl2mRNA (encoding MBL-C) was increased in diabetes. Mbl1expression (encoding MBL-A) did not differ between diabetic and control animals. The estimated half-life of recombinant human MBL was significantly prolonged in mice with diabetes compared with control mice. Complement activation in plasma and glomeruli did not differ between groups. We demonstrate for the first time that MBL levels increase after induction of diabetes and in parallel with increasing plasma glucose. Our findings support the previous clinical observations of increased MBL in type 1 diabetes. This change may be explained by alternations in both MBL production and turnover.

Complement in glomerular disease

The role of the complement system in renal disease has long been recognized, but there have been major advances in our understanding of its role over the past decade. Complement plays a critical role not only in host's defense against infection and preventing damage to "self" tissues but also mediates tissue injury, both in the glomerulus and tubulointerstitium. Although injury may originate in the glomerulus, resulting proteinuria and complement activation within the tubular lumen may lead to tubulointerstitial damage and progressive renal disease. Recent advances in our understanding of the mechanisms by which complement mediates renal injury have led to the development of promising strategies with which complement may be targeted to prevent renal injury and its associated complications.

Basic and translational concepts of immune-mediated glomerular diseases

Genetically modified immune responses to infections and self-antigens initiate most forms of GN by generating pathogen- and danger-associated molecular patterns that stimulate Toll-like receptors and complement. These innate immune responses activate circulating monocytes and resident glomerular cells to release inflammatory mediators and initiate adaptive, antigen-specific immune responses that collectively damage glomerular structures. CD4 T cells are needed for B cell-driven antibody production that leads to immune complex formation in glomeruli, complement activation, and injury induced by both circulating inflammatory and resident glomerular effector cells. Th17 cells can also induce glomerular injury directly. In this review, information derived from studies in vitro, well characterized experimental models, and humans summarize and update likely pathogenic mechanisms involved in human diseases presenting as nephritis (postinfectious GN, IgA nephropathy, antiglomerular basement membrane and antineutrophil cytoplasmic antibody-mediated crescentic GN, lupus nephritis, type I membranoproliferative GN), and nephrotic syndrome (minimal change/FSGS, membranous nephropathy, and C3 glomerulopathies). Advances in understanding the immunopathogenesis of each of these entities offer many opportunities for future therapeutic interventions.

Diabetic angiopathy, the complement system and the tumor necrosis factor superfamily

Among the most serious consequences of diabetes mellitus is the development of diabetic angiopathy, of which the clinical features are cardiovascular disease, retinopathy, nephropathy and neuropathy. Diabetic kidney problems affect up to one third of all patients with diabetes mellitus and are a major cause of end-stage renal failure. Although a huge number of pharmaceutical interventions are available today, diabetic angiopathy remains a leading cause of mortality and morbidity in diabetes mellitus, therefore, an urgent need exists to develop new therapeutic strategies. Recent data support the hypothesis that dysregulation of the complement system and of members of the tumor necrosis factor (TNF) superfamily may be involved in the development of diabetic vascular complications. The mannose-binding lectin pathway-an overall regulatory component of the complement system-is a particularly promising biomarker as it is directly involved in the development of diabetic angiopathy. In addition, two components of the TNF superfamily, namely TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and osteoprotegerin, may be involved in the pathogenesis of diabetic angiopathy. Several ways of specifically manipulating the complement and TNF superfamily systems already exist, but whether or not these drugs provide new targets for intervention for late diabetic complications is still to be revealed.

Enhanced complement activation is part of the unfavourable cardiovascular risk profile in South Asians

South Asian immigrants in western societies exhibit a high burden of diabetes and subsequent vascular complications. Diabetic vascular complications are associated with vascular inflammation. We hypothesize that enhanced complement activation is involved. Therefore, levels of complement C3 and SC5b-9 - the soluble end product of complement activation - in a group of 200 South Asians were compared with an age- and sex-matched control group of native Caucasians. In addition, the association between complement levels and albuminuria, an indicator of renal damage and a cardiovascular risk marker, was assessed in the diabetic South Asian group. Compared with native Caucasians, South Asians had significantly higher levels of both serum C3 and plasma SC5b-9, even when only non-diabetic South Asians were considered. Diabetic South Asians had significantly higher C3 levels compared with non-diabetic South Asians. In diabetic South Asians, higher levels of SC5b-9 were associated with an increased prevalence of albuminuria (odds ratio 5.4, 95% confidence interval 1.8-15.8). These results suggest that enhanced complement activation is part of the unfavourable cardiovascular risk profile in South Asians.

Translational mini-review series on complement factor H: renal diseases associated with complement factor H: novel insights from humans and animals

Factor H is the major regulatory protein of the alternative pathway of complement activation. Abnormalities in factor H have been associated with renal disease, namely glomerulonephritis with C3 deposition including membranoproliferative glomerulonephritis (MPGN) and the atypical haemolytic uraemic syndrome (aHUS). Furthermore, a common factor H polymorphism has been identified as a risk factor for the development of age-related macular degeneration. These associations suggest that alternative pathway dysregulation is a common feature in the pathogenesis of these conditions. However, with respect to factor H-associated renal disease, it is now clear that distinct molecular defects in the protein underlie the pathogenesis of glomerulonephritis and HUS. In this paper we review the associations between human factor H dysfunction and renal disease and explore how observations in both spontaneous and engineered animal models of factor H dysfunction have contributed to our understanding of the pathogenesis of factor H-related renal disease.

Cutaneous vascular deposition of C5b-9 and its role as a diagnostic adjunct in the setting of diabetes mellitus and porphyria cutanea tarda

BACKGROUND

The cutaneous lesions of diabetes mellitus (DM) and porphyria cutanea tarda (PCT) exhibit distinctive microvascular changes including basement membrane zone thickening and lamellation, morphologically appearing as hyaline-like alterations of the vessel wall. Immunofluorescence demonstrates homogeneous mantles of immunoglobulin in the microvasculature. The staining intensity is variable and in some cases can closely approximate those immunofluorescent changes seen in photoaged skin.

OBJECTIVE

The purpose of this study was to establish an association between the microvascular changes seen in the skin from patients with DM and PCT and the presence of C5b-9 deposition, potentially defining the C5b-9 assay as an additional diagnostic adjunct.

METHODS

Routine light microscopy and immunofluorescence studies were conducted on skin biopsy specimens from 14 patients with cutaneous manifestations of DM and 17 patients with PCT. The immunofluorescence profile included IgG, IgM, IgA, C3, C3d, C4d, and C5b-9.

RESULTS

Fourteen of 14 DM and 17 of 17 PCT skin biopsy specimens revealed extensive granular and homogeneous vascular deposition of C5b-9; a similar pattern was observed for C3d and C4d. Control specimens from patients without DM and PCT, where C5b-9 was not an expected immunoreactant, were negative. Positive controls were cases of vasculitis, scleroderma, and dermatomyositis without DM and PCT where C5b-9 deposition was expected. C5b-9 deposition was observed and was of lesser magnitude than that encountered in patients with PCT or DM.

LIMITATIONS

We were unable to obtain detailed clinical information on some of the diabetic patients in regards to significant extracutaneous vascular complications. In addition, a correlation between hemoglobin 1 Ac levels and the extent of C5b-9 deposition could not be ascertained as the serum levels for hemoglobin 1 Ac were unknown.

CONCLUSION

Granular and homogeneous deposits of C5b-9 in vessels, along with homogeneous deposits of immunoglobulin within the blood vessels, are characteristic immunofluorescence findings in patients with DM and PCT. In regards to potential mechanisms of C5b-9 deposition, decreased metabolism of C5b-9 due to glycosylation of CD59 in the setting of DM and activation of complement by irradiated porphyrins in PCT are proposed. The extent of C5b-9 deposition suggests that this complex may play a pathogenetic role in the evolution of microvascular injury in patients with DM and PCT.

Cross-talk between the complement system and endothelial cells in physiologic conditions and in vascular diseases

The endothelial layer represents a continuous physical barrier that controls coagulation and allows selective passage of soluble molecules and circulating cells across the vessel wall into the tissue. The functional activity of the endothelial cells may be influenced by their interaction with components of the complement system. In this review we shall discuss the complex interplay that can be established between the endothelium and complement proteins or activation products. Endothelial cells may also secrete several complement components which contribute to the circulating pool. This process can be regulated by cytokines and other pro-inflammatory stimuli. In addition, complement activation products stimulate endothelial cells to acquire a pro-inflammatory and pro-coagulant status. Expression of regulatory molecules on the cell surface provides protection against an undesired attack by complement activation products. Unrestricted complement activation under pathological conditions may lead to structural and functional changes of the endothelium resulting in vascular disease.

Complement activation and diabetic vascular complications

Diabetes mellitus is a major and increasing health problem worldwide. One of the most serious consequences of diabetes is the development of diabetic angiopathy, which includes cardiovascular disease, neuropathy, retinopathy and nephropathy. Diabetic nephropathy alone affects 15-25% of patients with type 1 diabetes and 30-40% of patients with type 2 diabetes and is the single-most important cause of end-stage renal failure in the Western World. Existing research has demonstrated the involvement of glycation factors, growth factors/cytokines, hemodynamic factors and intracellular changes in the pathogenesis of diabetic kidney disease. An emerging amount of recent data suggests that the complement system, especially the MBL pathway, plays an important role in the pathogenesis of diabetic vascular complications. Although the numerous therapeutic interventions available today may delay the development and progression of diabetes vascular complications, there is an ongoing need for new therapeutic strategies. In this article the evidence for a connection between the complement system and vascular dysfunction will be reviewed, with a special focus on the relation to diabetic kidney disease. Several ways of specifically manipulating the complement system already exist. However, whether or not these drugs provide new targets for intervention on diabetic vascular complications is still unknown.

Complement and glomerulonephritis: new insights

PURPOSE OF REVIEW

The last few years have seen a huge increase in our understanding of the role of the complement system and its regulation in glomerular disease. Our aim is to summarize the most important advances in this field.

RECENT FINDINGS

The role of complement in systemic lupus erythematosus continues to be elucidated. Classical pathway components protect from the development of autoimmunity, at least in part, through their role in the clearance of apoptotic cells. In contrast, the alternative pathway plays a direct role in exacerbating glomerular injury. Anti-C1q antibodies are related to activity in lupus nephritis and recent studies have shown that they are directly pathogenic in animal models. Proteinuria, whatever the cause, may lead to tubulointerstitial injury and complement activation adds to this process. In particular, deposition of terminal components of complement in the tubular lumen contributes to interstitial myofibroblast activation. There is increasing evidence for the role of complement regulatory proteins in glomerular injury. In particular, abnormalities of factor H or of CD46 may predispose to atypical haemolytic uraemic syndrome. The control proteins also protect against injury in immune complex glomerulonephritis.

SUMMARY

Advances in our understanding of the role of complement in glomerular injury point to the likely therapeutic benefits of targeting the complement system. Many new drugs are becoming available. Careful dissection of the pro and antiinflammatory effects of the complement system which the experimental models allow will assist in designing directed therapy that will avoid the detrimental effects of nonspecific systemic complement inhibition.

Glycation inactivation of the complement regulatory protein CD59: a possible role in the pathogenesis of the vascular complications of human diabetes

Micro- and macrovascular diseases are major causes of morbidity and mortality in the diabetic population, but the cellular and molecular mechanisms that link hyperglycemia to these complications remain incompletely understood. We proposed that in human diabetes, inhibition by glycation of the complement regulatory protein CD59 increases deposition of the membrane attack complex (MAC) of complement, contributing to the higher vascular risk. We report here 1) the generation and characterization of an anti-glycated human CD59 (hCD59) specific antibody, 2) the detection with this antibody of glycated hCD59 colocalized with MAC in kidneys and nerves from diabetic but not from nondiabetic subjects, and 3) a significantly reduced activity of hCD59 in erythrocytes from diabetic subjects, a finding consistent with glycation inactivation of hCD59 in vivo. Because hCD59 acts as a specific inhibitor of MAC formation, these findings provide a molecular explanation for the increased MAC deposition reportedly found in the target organs of diabetic complications. We conclude that glycation inactivation of hCD59 that leads to increased MAC deposition may contribute to the extensive vascular pathology that complicates human diabetes.

Early complement activation and decreased levels of glycosylphosphatidylinositol-anchored complement inhibitors in human and experimental diabetic retinopathy

Diabetic retinal microangiopathy is characterized by increased permeability, leukostasis, microthrombosis, and apoptosis of capillary cells, all of which could be caused or compounded by activation of complement. In this study, we observed deposition of C5b-9, the terminal product of complement activation, in the wall of retinal vessels of human eye donors with 9 +/- 3 years of type 2 diabetes, but not in the vessels of age-matched nondiabetic donors. C5b-9 often colocalized with von Willebrand factor in luminal endothelium. C1q and C4, the complement components unique to the classical pathway, were not detected in the diabetic retinas, suggesting that C5b-9 was generated via the alternative pathway, the spontaneous activation of which is regulated by complement inhibitors. The diabetic donors showed a prominent reduction in the retinal levels of CD55 and CD59, the two complement inhibitors linked to the plasma membrane by glycosylphosphatidylinositol anchors, but not in the levels of transmembrane CD46. Similar complement activation in retinal vessels and selective reduction in the levels of retinal CD55 and CD59 were observed in rats with a 10-week duration of streptozotocin-induced diabetes. Thus, diabetes causes defective regulation of complement inhibitors and complement activation that precede most other manifestations of diabetic retinal microangiopathy. These are novel clues for probing how diabetes affects and damages vascular cells.

New pharmacologic options for renal preservation

The understanding of the cause and pathophysiology of renal failure has guided the rational development of pharmacologic renoprotective strategies. Although traditionally anesthesiologists have focused on renal hemodynamic derangements, newer information suggests that cellular interactions amplify and perpetuate the insult. Consequently, the potential renoprotective armamentarium not only encompasses the more traditional vasoactive agents but also therapeutic approaches that may modify the cellular response to injury. Although few of these agents have reached the clinical arena, preliminary work suggests that this new approach to renal injury and protection may be promising.

Molecular basis for a link between complement and the vascular complications of diabetes

Activated terminal complement proteins C5b to C9 form the membrane attack complex (MAC) pore. Insertion of the MAC into endothelial cell membranes causes the release of growth factors that stimulate tissue growth and proliferation. The complement regulatory membrane protein CD59 restricts MAC formation. Because increased cell proliferation characterizes the major chronic vascular complications of human diabetes and because increased glucose levels in diabetes cause protein glycation and impairment of protein function, we investigated whether glycation could inhibit CD59. Glycation-inactivation of CD59 would cause increased MAC deposition and MAC-stimulated cell proliferation. Here, we report that (i) human CD59 is glycated in vivo, (ii) glycated human CD59 loses its MAC-inhibitory function, and (iii) inactivation of CD59 increases MAC-induced growth factor release from endothelial cells. We demonstrate by site-directed mutagenesis that residues K41 and H44 form a preferential glycation motif in human CD59. The presence of this glycation motif in human CD59, but not in CD59 of other species, may help explain the distinct propensity of humans to develop vascular proliferative complications of diabetes.

In situ complement activation in porcine membranoproliferative glomerulonephritis type II

Pigs genetically deficient in complement factor H all develop lethal membranoproliferative glomerulonephritis (MPGN) type II characterized by massive glomerular deposits of complement, intramembranous dense deposits, and mesangial hypercellularity. To elucidate the chronological relationship between these glomerular changes, and to precisely determine the localization of glomerular complement deposits, we studied kidney specimens from factor H-deficient piglets at different ages from fetal life until terminal kidney failure had developed. Deposits of C3 and the terminal complement complex localized within the glomerular basement membrane (GBM) were present already in factor H-deficient fetuses, without concurrent intramembranous dense deposits or mesangial hypercellularity. Incipient subendothelial dense deposits containing complement appeared no earlier than four days after birth, and intramembranous dense deposits in older piglets with established MPGN type II also contained large amounts of complement as detected by immune electron microscopy. Onset of kidney failure coincided with pronounced mesangial hypercellularity and expansion, compromising glomerular capillary patency. Formation of glomerular capillary wall double contours coincided with electron microscopic evidence of laminar disintegration of intramembranous dense deposits. Complement was also deposited in the mesangial matrix, but not on glomerular cells. We conclude that all components of the alternative and terminal pathways of complement have access into the GBM and the mesangial matrix. In the absence of factor H, complement is spontaneously activated and deposited in situ in these locations resulting in dense deposit formation. It is proposed that factor H dysfunction may play an essential role even in human MPGN type II.

The transient pore formed by homologous terminal complement complexes functions as a bidirectional route for the transport of autocrine and paracrine signals across human cell membranes

BACKGROUND

We have previously shown that the membrane attack complex (MAC) of complement stimulates cell proliferation and that insertion of homologous MAC into the membranes of endothelial cells results in the release of potent mitogens, including basic fibroblast growth factor (bFGF). The mechanism of secretion of bFGF and other polypeptides devoid of signal peptides, such as interleukin 1 (IL-1) is still an open problem in cell biology. We have hypothesized that the homologous MAC pore itself could constitute a transient route for the diffusion of biologically active macromolecules in and out of the target cells.

MATERIALS AND METHODS

Human red blood cell ghosts and artificial lipid vesicles were loaded with labeled growth factors, cytokines and IgG, and exposed to homologous MAC. The release of the 125I-macromolecules was followed as a function of time. The incorporation of labeled polypeptides and fluorescent dextran (MW: 10,000) was measured in MAC-impacted human red blood cells and human umbilical endothelial cells (HUVEC), respectively.

RESULTS

Homologous MAC insertion into HUVEC resulted in the massive uptake of 10-kD dextran and induced the release of bFGF, in the absence of any measurable lysis. Red blood cell ghosts preloaded with bFGF, IL-1 beta, and the alpha-chain of interferon-gamma (IFN-gamma) released the polypeptides upon MAC insertion, but they did not release preloaded IgG. MAC-impacted ghosts took up radioactive IFN-gamma from the extracellular medium. Vesicles loaded with IL-I released the polypeptide when exposed to MAC.

CONCLUSIONS

The homologous MAC pore in its nonlytic form allows for the export of cytosolic proteins devoid of signal peptides that are not secreted through the classical endoplasmic reticulum/Golgi exocytotic pathways. Our results suggest that the release, and perhaps the uptake, of biologically active macromolecules through the homologous MAC pore is a novel biological function of the complement system in mammals.

Role of complement in renal tubular damage

Deposition of immunoglobulins, complement proteins C1q, C3c, C3d, C4, C5, C6, C7, C8, C9, and terminal complement complex neoantigens in the renal tubulointerstitium was studied in serial sections by immunofluorescence microscopy. Renal tissue from 45 cases with various glomerular diseases, including 8 controls, was studied. The patients were divided into groups; one with tubulointerstitial lesions (24 cases) and the other without (13 cases). The immunoproteins were deposited mainly in the tubular basement membrane and blood vessels. Compared with controls there was a significantly increased staining score for C5 to C9 in the tubular basement membrane in both disease groups. However, the increase in terminal complement complex neoantigens score was significant only in the disease group with tubulo interstitial lesions. The changes in C3d score were not significant. Serial sections showed consistent and heavy ribbon-like deposits of complement proteins C3d, C5 to C9, and terminal complement complex neoantigens in corresponding locations of the segments of tubular basement membrane, mainly in the disease group with tubulointerstitial lesions and especially in the damaged tubules. These findings suggest that in situ activation of the complement cascade leads to the deposition of terminal complement complex neoantigens. Complement activation in the basal area of the tubules may, therefore, be an important pathogenetic mechanism in tubulointerstitial damage.

Distinct sites of production and deposition of the putative cell death marker clusterin in the human thymus

Clusterin is a multifunctional protein endowed with cell-aggregating, complement-inhibitory, and lipid-binding properties. Since several studies have demonstrated highly increased clusterin gene expression in epithelial and nervous tissues regressing as a consequence of tissue involution and apoptotic cell death, clusterin is also considered as a specific marker of dying cells. To determine whether clusterin expression is also upregulated during thymocyte death occurring during the negative selection process we analyzed the cellular distribution of clusterin mRNA and protein by in situ hybridization and immunocytochemistry in the human thymus. We observed that the expression of clusterin mRNA was confined to cells present in the thymic medulla, concentrated mainly around Hassal's bodies. Immunostaining of adjacent sections with antikeratin Ab revealed that cells containing clusterin mRNA were predominantly epithelial. By contrast no clusterin mRNA was found in thymocytes by in situ hybridization and Northern blot analysis of total RNA from purified thymocyte populations. Clusterin protein colocalized with the membrane attack complex of complement and vitronectin in the center of the largest Hassal's bodies, but was not detectable by immunocytochemistry in or at the surface of epithelial cells. Our results demonstrate that clusterin gene expression does not take place in apoptotic thymocytes, and therefore that clusterin synthesis by the dying cell is probably not a prerequisite to its death. However, synthesis of clusterin by medullary epithelial cells may be related to their terminal differentiation, and, furthermore, its presence in Hassal's bodies raises the possibility that the secreted protein is involved in the disposal of cell debris resulting from thymocyte apoptosis.

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.

Localization of clusterin in the epimembranous deposits of passive Heymann nephritis

The membrane attack complex of complement (MAC) plays an important role in the mediation of proteinuria in experimental membranous nephropathy induced by Heymann antiserum. SP-40,40 is a recently described serum protein which appears to inhibit the formation of cytolytic MAC in a manner analogous to S protein/vitronectin. SP-40,40 is homologous to proteins originally isolated from rat and ram seminal fluid (sulfated glycoprotein 2 and clusterin, respectively). By current convention, these proteins are considered clusterin homologues. The objective of this study was to examine the participation of rat clusterin in passive Heymann nephritis. Using an antibody to rat clusterin as an immunofluorescent probe, clusterin deposits were demonstrated along the glomerular capillary wall in an identical pattern to rat C3 and C5b-9. Decomplementation using cobra venom factor prevented proteinuria and intraglomerular MAC formation. The epimembranous clusterin were not detected in the complement-depleted animals. The role of clusterin in the mediation of glomerular injury remains unknown, but it is probably related to in situ formation of the terminal complement cascade where it may play a regulatory role.

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.

Complement-mediated endothelial cell damage in immune complex vasculitis of the skin: ultrastructural localization of the membrane attack complex

Activation of the complement system is an important element in our concept of the pathomechanism of immune complex (IC) vasculitis. Both deposition of IC and attraction of polymorphonuclear leukocytes (PMN) are effected by products of complement activation. Actual tissue damage, however, is believed to be caused by PMN penetrating the vessel wall. Our former finding that deposits of membrane attack complex of complement (MAC) are found predominantly in skin lesions of patients with IC vasculitis and not in perilesional skin, has raised the question whether the complement system itself (by way of the MAC) contributes to tissue damage. Our present study shows the ultrastructural localization of MAC in lesional and clinically uninvolved skin in two patients with a cutaneous IC vasculitis. Lesional skin deposits of MAC were found on endothelial cells (EC) of upper dermal vessels and on infiltrating PMN. Uninvolved skin deposits of MAC were found on some EC, but clearly to a lesser extent than on EC of the lesional skin. In the skin of two healthy controls MAC was only found sporadically on EC. Deposits of MAC on EC in the lesional skin were often associated with a typical form of local cell swelling. This local form of endothelial cell swelling was incidentally seen in vessels of clinically uninvolved skin, but not in the skin of the two controls. The association of the endothelial cell swelling with deposits of MAC suggests that the complement system can have a direct damaging effect on EC in IC vasculitis by the assembly of MAC on the endothelial cell membrane.

Increased urinary excretion of C5b-9 distinguishes passive Heymann nephritis in the rat

Increased urinary excretion of C5b-9 distinguishes passive Heymann nephritis from other forms of experimental glomerulonephritis in the rat. In the passive Heymann nephritis (PHN) model of membranous nephropathy (MN) subepithelial deposits form from anti-Fx1A antibody reacting with antigen expressed on the glomerular epithelial cell membrane followed by membrane patching and shedding of immune complexes. Immune complex deposits are accompanied by deposits of C5b-9 which is required for the mediation of proteinuria. We tested the hypothesis that C5b-9 assembly on the epithelial cell membrane might result in C5b-9 excretion in the urine, which would distinguish this autoimmune mechanism of MN from other processes that result in subepithelial immune complex deposits. Using monoclonal antibodies developed to rat C6 and a rat C5b-9 neoantigen, in a sensitive ELISA assay, elevated urinary excretion of rat C5b-9 was documented in PHN associated with on-going glomerular immune deposit formation. No urinary C5b-9 was detectable in MN induced by an exogenous antigen (cationized IgG) despite equivalent glomerular C5b-9 deposits, or in models of nephrotoxic nephritis, subendothelial immune complex nephritis, anti-mesangial cell membrane antibody-induced nephritis or two non-immune nephropathies. Infusion of preformed C5b-9 in proteinuric animals excluded glomerular filtration of C5b-9 as a contributing mechanism to urinary C5b-9 excretion. We conclude that in the rat, increased urinary excretion of C5b-9 is a marker of MN induced by antibody to a glomerular epithelial cell antigen. Urine C5b-9 excretion reflects active glomerular immune deposit formation and distinguishes MN induced by this mechanism from other forms of MN as well as from other glomerular diseases with equivalent glomerular C5b-9 deposits.

Immunoelectron microscopic localization of membrane attack complex and hepatitis B e antigen in membranous nephropathy

Immunoelectron microscopy was used to localize membrane attack complex (MAC) and hepatitis B e (HBe) antigen in renal tissue specimens from a total of 9 patients with membranous nephropathy (MN); 6 with MN associated with a hepatitis B virus (HBV) infection, 2 with idiopathic MN, and 1 with lupus nephritis. All the patients were proteinuric, and 2 patients were classified as stage I-II, 6 as stage II, and 1 as stage IV. MAC, along with IgG and C3, was distributed within the subepithelial electron dense deposits in all the stages. MAC was also stained in the striated membranous structures within the glomerular basement membrane and mesangial matrix of some patients. In HBV-associated MN, HBe antigen was localized in the subepithelial electron dense deposits of 5 patients, while it was absent from the subepithelial deposits in a patient that was sero-positive for hepatitis B s antigen but negative for HBe antigen. This patient also lacked MAC deposition in these loci. These results suggest that MAC is associated with the formation of subepithelial deposits and proteinuria in MN. In HBV-associated MN, HBe antigen-antibody immune complex makes up the subepithelial deposits and is likely to activate the terminal components of complement in situ.

Linear C3 deposits on the tubular basement membrane in renal allograft biopsies

Linear and granular tubular basement membrane (TBM) deposits of C3 occur in renal allograft biopsies, but their significance is unknown. We retrospectively analyzed the predictive importance of C3 deposits in 88 biopsied transplant patients with allograft dysfunction. All patients were followed for greater than or equal to 2 years from biopsy. Patients were divided into three groups: group I: no C3 deposits, 47 patients; group II: granular deposits of C3, 28 patients; and group III: linear TBM deposits, 13 patients. The incidence of acute and chronic rejection was not different. In group III, 12 grafts were lost by 5 years (92%), and the remaining patient has chronic rejection. Group III survival was significantly less than groups I and II (Kaplan-Meier curves), P = 0.02, but graft survival in groups I and II were similar. There was no association of anti-TBM antibody deposits with C3, and the mechanism of deposition is unknown. We conclude that the presence of linear C3 deposits along the TBM in the setting of allograft dysfunction is associated with decreased allograft survival.

Immunohistochemical study of the membrane attack complex of complement in IgA nephropathy

The localization of the membrane attack complex of complement (MAC) was examined in the normal human kidneys and in biopsy specimens from patients with primary IgA nephropathy by immunofluorescent and immunoelectron microscopies. Immunofluorescent staining for MAC was significantly more intense than in the normal kidneys, and was observed in the mesangium and occasionally along the glomerular capillary walls of 22 of 30 patients with IgA nephropathy. By dual-staining, the MAC deposits were generally concordant with the deposits of IgA, C3, C5 and C9, or of IgG, when present. C1q or C4 was infrequently observed in the glomeruli. Immunoelectron microscopy revealed various staining patterns of glomerular MAC deposition; homogeneous fine-granular staining beneath the glomerular basement membrane (GBM) in the paramesangial zone, patchy staining within the mesangial electron dense deposits (EDD), and ring-shaped or ribbon-like staining, associated with the striated membrane structures (SMS), in the matrix of the mesangium, GBM and tubular basement membrane (TBM). This study suggests that the terminal complement system is activated, mainly by an alternative complement pathway mechanism, in the mesangium of IgA nephropathy, and is associated with the paramesangial lesion and EDD. MAC deposition in glomerular SMS may also result from in situ activation rather than trapping from the circulation. There was little correlation between glomerular MAC deposition and proteinuria or renal histology of patients with IgA nephropathy.