Urinary excretion of terminal complement complexes in glomerular disease

Complement activation and effector pathways in membranous nephropathy

Complement activation has long been recognized as a central feature of membranous nephropathy (MN). Evidence for its role has been derived from the detection of complement products in biopsy tissue and urine from patients with MN and from mechanistic studies primarily based on the passive Heymann nephritis model. Only recently, more detailed insights into the exact mechanisms of complement activation and effector pathways have been gained from patient data, animal models, and in vitro models based on specific target antigens relevant to the human disease. These data are of clinical relevance, as they parallel the recent development of numerous specific complement therapeutics for clinical use. Despite efficient B-cell depletion, many patients with MN achieve only partial remission of proteinuria, which may be explained by the persistence of subepithelial immune complexes and ongoing complement-mediated podocyte injury. Targeting complement, therefore, represents an attractive adjunct treatment for MN, but it will need to be tailored to the specific complement pathways relevant to MN. This review summarizes the different lines of evidence for a central role of complement in MN and for the relevance of distinct complement activation and effector pathways, with a focus on recent developments.

The role of the complement system in primary membranous nephropathy: A narrative review in the era of new therapeutic targets

Primary membranous nephropathy (MN) is an important cause of nephrotic syndrome and chronic kidney disease (CKD) in the adult population. Although the discovery of different autoantibodies against glomerular/podocytic antigens have highlighted the role of B cells in the pathogenesis of MN, suboptimal response or even resistance to B cell-directed therapies occurs, suggesting that other pathophysiological mechanisms are involved in mediating podocyte injury. The complement system plays an important role in the innate immune response to infection, and dysregulation of the complement system has been observed in various kidney diseases. There is compelling evidence of complement cascade activation in primary MN, with the mannose-binding lectin (MBL) and alternative pathways particularly implicated. With appropriate validation, assays of complements and associated activation products could hold promise as adjunctive tools for non-invasive disease monitoring and prognostication. While there is growing interest to target the complement system in MN, there is concern regarding the risk of infection due to encapsulated organisms and high treatment costs, highlighting the need for clinical trials to identify patients most likely to benefit from complement-directed therapies.

Immune-Related Urine Biomarkers for the Diagnosis of Lupus Nephritis

The kidney is one of the main organs affected by the autoimmune disease systemic lupus erythematosus. Lupus nephritis (LN) concerns 30-60% of adult SLE patients and it is significantly associated with an increase in the morbidity and mortality. The definitive diagnosis of LN can only be achieved by histological analysis of renal biopsies, but the invasiveness of this technique is an obstacle for early diagnosis of renal involvement and a proper follow-up of LN patients under treatment. The use of urine for the discovery of non-invasive biomarkers for renal disease in SLE patients is an attractive alternative to repeated renal biopsies, as several studies have described surrogate urinary cells or analytes reflecting the inflammatory state of the kidney, and/or the severity of the disease. Herein, we review the main findings in the field of urine immune-related biomarkers for LN patients, and discuss their prognostic and diagnostic value. This manuscript is focused on the complement system, antibodies and autoantibodies, chemokines, cytokines, and leukocytes, as they are the main effectors of LN pathogenesis.

Expanding the Role of Complement Therapies: The Case for Lupus Nephritis

The complement system is an innate immune surveillance network that provides defense against microorganisms and clearance of immune complexes and cellular debris and bridges innate and adaptive immunity. In the context of autoimmune disease, activation and dysregulation of complement can lead to uncontrolled inflammation and organ damage, especially to the kidney. Systemic lupus erythematosus (SLE) is characterized by loss of tolerance, autoantibody production, and immune complex deposition in tissues including the kidney, with inflammatory consequences. Effective clearance of immune complexes and cellular waste by early complement components protects against the development of lupus nephritis, while uncontrolled activation of complement, especially the alternative pathway, promotes kidney damage in SLE. Therefore, complement plays a dual role in the pathogenesis of lupus nephritis. Improved understanding of the contribution of the various complement pathways to the development of kidney disease in SLE has created an opportunity to target the complement system with novel therapies to improve outcomes in lupus nephritis. In this review, we explore the interactions between complement and the kidney in SLE and their implications for the treatment of lupus nephritis.

Soluble Membrane Attack Complex: Biochemistry and Immunobiology

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

Alternative complement pathway activation in thrombotic microangiopathy associated with lupus nephritis

INTRODUCTION/OBJECTIVE

Thrombotic microangiopathy (TMA) in systemic lupus erythematosus is a rare manifestation associated with activation of the complement system. This study aimed to compare plasma and urine complement activation products between patients with active lupus nephritis (aLN) and those with acute TMA plus concomitant active LN (aTMA+aLN).

METHODS

Plasma and urine samples were obtained from 20 patients with aTMA+aLN, 20 patients with aLN matched by the histological activity index, 5 patients with chronic TMA, 20 patients with inactive LN, and 10 kidney donors. Complement fragments C3a, C4a, C4d, Ba, C5a, C5bC9, and factor H were determined by ELISA; and kidney C4d deposition was detected by immunohistochemistry. Patients were followed for > 12 months and complement activation products re-measured after treatment in 10 aTMA+aLN patients.

RESULTS

Both aTMA+aLN and aLN groups had increased circulating C3a, Ba, and C5bC9; and decreased circulating C3, C4, C4a, C4d, and factor H. Urinary C3a, C5a, Ba, and C5bC9 were higher in patients with aTMA+aLN than in aLN. After treatment, levels of circulating C3, C4, and factor H increased; while levels of urinary C3a, C5a, Ba, and C5bC9 decreased in patients with aTMA+aLN. These changes were observed at each aTMA episode in two patients studied during repeated TMA episodes. There was no difference in C4d deposition in glomerular capillaries, tubular basement membrane, peritubular capillaries, and arterioles between patients with aLN and those aTMA+aLN.

CONCLUSIONS

Circulating and urine complement activation products suggest that thrombotic microangiopathy associated with LN is mediated through activation of the alternative complement pathway. Key Points • Immune-complex kidney disease in systemic lupus erythematosus (SLE) is associated with activation of the classical, lectin, and alternative complement pathways • Indirect evidence from measurement of circulating and urinary complement pathway activation products suggests that renal acute thrombotic microangiopathy in SLE is mediated by activation of the alternative complement pathway • C4d kidney immunohistochemistry may be positive in both immune complex nephritis and thrombotic microangiopathy. Therefore, it is not a specific marker of renal thrombotic microangiopathy in SLE.

Proteasuria in nephrotic syndrome-quantification and proteomic profiling

Nephrotic syndrome is characterized by urinary excretion of plasma proteases or proteasuria. There is a lack of data on the quantity, activity status and identity of these aberrantly filtered proteases. We established a fluorescence-based substrate assay to quantify protease activity in urine samples from healthy and nephrotic humans and mice. Protease class activity was determined after addition of specific inhibitors. Individual proteases were identified by tandem mass spectrometry (MS/MS). In spot urine samples from 10 patients with acute nephrotic syndrome of various etiology, urinary protease activity was significantly increased compared to that of healthy persons (753 ± 178 vs. 244 ± 65 relative units, p < 0.05). The corresponding proteases were highly sensitive to inhibition by the serine protease inhibitors AEBSF (reduction by 85 ± 6% and 72 ± 8%, respectively) and aprotinin (83 ± 9% vs. 25 ± 6%, p < 0.05). MS/MS of all urinary proteins or after AEBSF purification showed that most of them were active serine proteases from the coagulation and complement cascade. These findings were recapitulated in mice, pointing to a similar pathophysiology. In conclusion, nephrotic syndrome leads to increased urinary excretion of active plasma proteases which can be termed proteasuria. Serine proteases account for the vast majority of urinary protease activity in health and nephrotic syndrome. SIGNIFICANCE STATEMENT: In this study, we found that nephrotic urine samples of humans and mice have a significantly increased protease activity compared to healthy urine samples, using a universal pentapeptide substrate library. This was driven by increased excretion of aprotinin-sensitive serine proteases. With tandem mass spectrometry, we provide a comprehensive and systematic overview of all urinary proteases or the "urine proteasome". We identified renally expressed proteases in health and addition of proteases from the coagulation and complement cascade in the nephrotic state. These results set the basis to study the role of urinary proteases at both health and nephrotic syndrome to find diagnostic markers of renal disease as well as possible therapeutic targets.

Inhibition of terminal complement: a novel therapeutic approach for the treatment of systemic lupus erythematosus

The importance of the complement system in the pathophysiology of systemic lupus erythematosus (SLE) is clear although individual complement components play very different roles in the disease process. Early complement proteins are critical in the clearance of immune complexes and apoptotic bodies, and their absencepredisposesindividualsto SLE. Conversely, activationof terminalcomplement is associated with exacerbations of disease and damage to tissues and organs, particularly in lupus nephritis. Monoclonal antibodies that specifically inhibit terminal complement activation while preserving the critical functions of the early complement cascade have now been developed. These antibodies target the C5 complement protein, blocking its cleavage and the subsequent generation of potent proinflammatory molecules. Anti-C5 therapeutics have recently been investigated in an animal model of SLE and in a Phase I single dose study in humans. The results of these studiesand the multiple roles of complement in SLE are discussed.

C5b-9 does not mediate tubulointerstitial injury in experimental acute glomerular disease characterized by selective proteinuria

AIM: To determine whether complement membrane attack complex (C5b-9) has a pathogenic role in tubulointerstitial injury in a renal disease model characterized by acute highly selective proteinuria.

METHODS: Protein-overload nephropathy (PON) was induced in adult female Piebald-Viral-Glaxo rats with or without complement C6 deficiency (C6^- and C6⁺) by daily intraperitoneal injections of bovine serum albumin (BSA, 2 g/d), and examined on days 2, 4 and 8.

RESULTS: Groups with PON developed equivalent levels of heavy proteinuria within 24 h of BSA injection. In C6⁺ rats with PON, the tubulointerstitial expression of C5b-9 was increased and localized predominantly to the basolateral surface of tubular epithelial cells (TECs), whereas it was undetectable in C6^- animals. TEC proliferation (as assessed by the number of BrdU+ cells) increased by more than 50-fold in PON, peaking on day 2 and declining on days 4 to 8. There was a trend for a reduction in the number of BrdU+ TECs on day 4 in the C6^- PON group (P = 0.10 compared to C6⁺) but not at any other time-point. Kidney enlargement, TEC apoptosis (TUNEL⁺ cells) and markers of tubular injury (tubule dilatation, loss of TEC height, protein cast formation) were not altered by C6 deficiency in PON. Interstitial monocyte (ED-1+ cell) accumulation was partially reduced in C6^- animals with PON on day 4 (P = 0.01) but there was no change in myofibroblast accumulation.

CONCLUSION: These data suggest that C5b-9 does not mediate tubulointerstitial injury in acute glomerular diseases characterized by selective proteinuria.

Advances in our understanding of the pathogenesis of Henoch-Schönlein purpura and the implications for improving its diagnosis

Henoch-Schönlein purpura (HSP) is a leukocytoclastic vasculitis classically characterized by palpable purpura, arthritis, abdominal pain and renal disease. In this article, we summarize our current understanding of the pathogenesis of HSP and the implications for improving its diagnosis. Although the pathogenesis of HSP is not fully understood yet, exciting new information has emerged in recent years, leading to a better understanding of its pathogenesis. Here, we discuss genetic predisposition, immunoglobulins with a particular emphasis on IgA1, activated complements, cytokines and chemokines, abnormal coagulation and autoantibodies in the underlying pathogenic mechanisms. Finally, diagnostic criteria for HSP developed by institutions such as the American College of Rheumatology and the European League against Rheumatism/Paediatric Rheumatology European Society were proposed to improve early detection and diagnosis.

The complement cascade in kidney disease: from sideline to center stage

Activation of the complement pathway is implicated in the pathogenesis of many kidney diseases. The pathologic and clinical features of these diseases are determined in part by the mechanism and location of complement activation within the kidney parenchyma. This review describes the physiology, action, and control of the complement cascade and explains the role of complement overactivation and dysregulation in kidney disease. There have been recent advances in the understanding of the effects of upregulation of the complement cascade after kidney transplantation. Complement plays an important role in initiating and propagating damage to transplanted kidneys in ischemia-reperfusion injury, antibody-mediated rejection, and cell-mediated rejection. Complement-targeting therapies presently are in development, and the first direct complement medication for kidney disease was licensed in 2011. The potential therapeutic targets for anticomplement drugs in kidney disease are described. Clinical and experimental studies are ongoing to identify further roles for complement-targeting therapy.

Urinary C5b-9 excretion and clinical course in idiopathic human membranous nephropathy

Recent reports suggested that the presence of terminal complement complex (C5b-9) in urine from patients with idiopathic membranous nephropathy (IMN) may indicate on-going immunological damage. This report documents the relationship between C5b-9 excretion and clinical outcome in 35 adult patients with biopsy-proven IMN and progressively declining renal function. There were two groups of patients. Group I received one of three treatment regimens: prednisolone alone, prednisolone and chlorambucil, or prednisolone and cyclophosphamide (N = 22). Group II received no immunosuppressive therapy (N = 17). Three of the 18 patients receiving immunosuppressive drugs had more than one treatment regimen as they experienced a clinical relapse during the study period; hence 22 treatments were available for analysis. Urine samples were collected regularly and urinary C5b-9 (uC5b-9) was determined by ELISA. Both groups were similar with respect to age, sex distribution, and the duration of follow-up. An improvement in proteinuria and creatinine clearance was noted in the immunosuppressed group. Thirty-five patients were excreting C5b-9 initially (18 from group I and 17 from group II); 17 patients continued to excrete C5b-9 at the end of the observation period. These 17 patients had a significantly worse clinical outcome when compared to the 18 patients whose C5b-9 excretion became negative, either spontaneously or with treatment (P < 0.005). These results indicate that continuing C5b-9 excretion is correlated with a poor clinical outcome. They also suggest that uC5b-9 is a dynamic marker of ongoing immunological injury, and therefore may be useful in the initial assessment and monitoring of patients with IMN and in identifying patients who may derive benefit from immunosuppressive therapy.

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

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

Urinary excretion of protectin (CD59), complement SC5b-9 and cytokines in membranous glomerulonephritis

Protectin (CD59) is a low molecular weight glycophosphoinositol-anchored inhibitor of the membrane attack complex of complement (MAC) that is present, for example, on the membranes of endothelial cells and on epithelial cells of glomeruli and distal tubuli. To examine for the possibility that CD59 becomes detached from cell surfaces following cell injury, this study evaluated renal excretion of CD59 in patients with idiopathic membranous glomerulonephritis (MGN; N = 21), diabetic nephropathy (DNP; N = 15) and in healthy control subjects (N = 13). CD59 in human urine was quantitated by a competitive solid-phase radioimmunoassay having approximately 13 kDa soluble urinary CD59 as a standard. Immunofluorescence microscopy demonstrated a decreased expression of CD59 in the glomeruli of MGN patients. Using a Triton X-114 phase separation method 91 to 97% of urinary CD59 was found to be in a soluble form without anchor-associated phospholipid. The mean (+/- SEM) level of urinary CD59 was 5.6 +/- 0.2 micrograms/ml in MGN patients, 3.7 +/- 0.4 micrograms/ml in healthy controls (P < 0.001) and 2.6 +/- 0.1 in DNP patients (P < 0.001). When related to urinary creatinine (UCr) the corresponding values were 11.9 +/- 5.6, 4.8 +/- 0.3 (P = 0.021) and 4.4 +/- 0.2 (P < 0.002), respectively. The amount of CD59 in urine correlated with the urinary excretion of soluble terminal complement complexes, SC5b-9 (r = 0.594, P < 0.006) in MGN patients. The excretion of CD59 also correlated with the excretion of the inflammatory mediator IL-1 beta (r = 0.671, P = 0.001) but not with TNF-alpha (r = 0.314, P = 0.178). No correlation of CD59 excretion was observed with duration of the disease level of proteinuria, serum albumin concentration or serum creatinine level. Based on these findings we speculate that the increased excretion of CD59 into urine in MGN patients is due to complement activation and inflammation induced shedding of CD59 from glomerular cells.

Current topics in childhood lupus nephritis

Lupus nephritis is a major predictor of the prognosis of systemic lupus erythematosus (SLE). The present paper discusses lupus nephritis from clinical and immunopathological points of view. Although recent advances in diagnosis and treatment improve the prognosis of children with SLE, there remain many unsolved clinical problems. One of the current topics in the treatment for SLE is intermittent intravenous cyclophosphamide therapy which is effective even for the steroid-resistant patients with severe lupus nephritis, at least for short-term observation. Immunopathologically, the following issues are discussed: (i) The C5b-9 terminal complement complex plays an important role in the pathogenesis of lupus nephritis. The possible interaction of vitronectin and SP-40,40 is also mentioned; (ii) A semi-quantitative analysis of the charge barrier of the glomerular basement membrane reveals that the charge barrier dysfunction plays an important role in the pathogenesis of proteinuria in lupus nephritis. This study also demonstrates that the charge of immune deposits is important for the initiation of glomerular injury in lupus nephritis; (iii) It is demonstrated that the histopathological diversity of lupus nephritis is based on biological properties of nephritogenic auto-antibodies in murine lupus models.