Progressive IgA Nephropathy Is Associated With Low Circulating Mannan-Binding Lectin-Associated Serine Protease-3 (MASP-3) and Increased Glomerular Factor H-Related Protein-5 (FHR5) Deposition

The pathogenesis of IgA nephropathy and implications for treatment

IgA nephropathy (IgAN) is a common form of primary glomerulonephritis and represents an important cause of chronic kidney disease globally, with observational studies indicating that most patients are at risk of developing kidney failure within their lifetime. Several research advances have provided insights into the underlying disease pathogenesis, framed by a multi-hit model whereby an increase in circulating IgA1 that lacks galactose from its hinge region - probably derived from the mucosal immune system - is followed by binding of specific IgG and IgA antibodies, generating immune complexes that deposit within the glomeruli, which triggers inflammation, complement activation and kidney damage. Although treatment options are currently limited, new therapies are rapidly emerging that target different pathways, cells and mediators involved in the disease pathogenesis, including B cell priming in the gut mucosa, the cytokines APRIL and BAFF, plasma cells, complement activation and endothelin pathway activation. As more treatments become available, there is a realistic possibility of transforming the long-term outlook for many individuals with IgAN.

The role of complement in kidney disease: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference

Uncontrolled complement activation can cause or contribute to glomerular injury in multiple kidney diseases. Although complement activation plays a causal role in atypical hemolytic uremic syndrome and C3 glomerulopathy, over the past decade, a rapidly accumulating body of evidence has shown a role for complement activation in multiple other kidney diseases, including diabetic nephropathy and several glomerulonephritides. The number of available complement inhibitor therapies has also increased during the same period. In 2022, Kidney Diseases: Improving Global Outcomes (KDIGO) convened a Controversies Conference, "The Role of Complement in Kidney Disease," to address the expanding role of complement dysregulation in the pathophysiology, diagnosis, and management of various glomerular diseases, diabetic nephropathy, and other forms of hemolytic uremic syndrome. Conference participants reviewed the evidence for complement playing a primary causal or secondary role in progression for several disease states and considered how evidence of complement involvement might inform management. Participating patients with various complement-mediated diseases and caregivers described concerns related to life planning, implications surrounding genetic testing, and the need for inclusive implementation of effective novel therapies into clinical practice. The value of biomarkers in monitoring disease course and the role of the glomerular microenvironment in complement response were examined, and key gaps in knowledge and research priorities were identified.

Role of serum complement C3 and C4 on kidney outcomes in IgA nephropathy

IgA Nephropathy (IgAN) is the most prevalent glomerular disease worldwide. Complement system activation is crucial in its pathogenesis. Few studies correlated serum C3 and C4 with disease activity and prognosis. This retrospective study investigated the prognostic value of serum complement at the time of diagnosis in patients with IgAN. Specifically we evaluated whether adding serum C3 and C4 levels to established predictive models-one based on variables related to chronic kidney disease (CKD) progression and another incorporating variables from the International IgA Prediction Tool (IntIgAPT)-enhances the accuracy of outcome prediction. A composite renal outcome was defined as 50% decline in eGFR or onset of kidney failure. 101 patients were stratified according to baseline C3 levels in three groups (Low, Medium and High). During a median follow-up of 54 months, the Low group exhibited higher incidence of primary outcome (16.3 events vs 2.9 and 1.7 events × 100 pts/year, p = 0.0026). Model-1 (M1), consisting of CKD progression variables, and Model-3 (M3), comprising IntIgANPT variables, were implemented with baseline C3 and C4 to create Model-2 (M2) and Model-4 (M4), respectively. M2 demonstrated better predictive performance over M1, showing higher discrimination (lower AIC and BIC, higher C-index and NR2). Similarly, M4 outperformed M3, showing enhanced outcome prediction when C3 and C4 levels were added. Implementation of serum C3 and C4 can enhance prediction accuracy of already-validated prognostic models in IgAN. Lower C3 and higher C4 levels were associated with poorer prognosis, highlighting a more 'Complement-Pathic' subset of patients.

The complement system in the pathogenesis and progression of kidney diseases: What doesn't kill you makes you older

The Complement System is an evolutionarily conserved component of immunity that plays a key role in host defense against infections and tissue homeostasis. However, the dysfunction of the Complement System can result in tissue damage and inflammation, thereby contributing to the development and progression of various renal diseases, ranging from atypical Hemolytic Uremic Syndrome to glomerulonephritis. Therapeutic interventions targeting the complement system have demonstrated promising results in both preclinical and clinical studies. Currently, several complement inhibitors are being developed for the treatment of complement-mediated renal diseases. This review aims to summarize the most recent insights into complement activation and therapeutic inhibition in renal diseases. Furthermore, it offers potential directions for the future rational use of complement inhibitor drugs in the context of renal diseases.

Heterozygous mutations in factor H aggravate pathological damage in a stable IgA deposition model induced by Lactobacillus casei cell wall extract

Introduction

Activation of complement through the alternative pathway (AP) has a key role in the pathogenesis of IgA nephropathy (IgAN). We previously showed, by intraperitoneal injection of Lactobacillus casei cell wall extract (LCWE), C57BL/6 mice develop mild kidney damage in association with glomerular IgA deposition. To further address complement activity in causing glomerular histological alterations as suggested in the pathogenesis of IgAN, here we used mice with factor H mutation (FHW/R) to render AP overactivation in conjunction with LCWE injection to stimulate intestinal production of IgA.

Methods

Dose response to LCWE were examined between two groups of FHW/R mice. Wild type (FHW/W) mice stimulated with LCWE were used as model control.

Results

The FHW/R mice primed with high dose LCWE showed elevated IgA and IgA-IgG complex levels in serum. In addition to 100% positive rate of IgA and C3, they display elevated biomarkers of kidney dysfunction, coincided with severe pathological lesions, resembling those of IgAN. As compared to wild type controls stimulated by the same high dose LCWE, these FHW/R mice exhibited stronger complement activation in the kidney and in circulation.

Discussion

The new mouse model shares many disease features with IgAN. The severity of glomerular lesions and the decline of kidney functions are further aggravated through complement overactivation. The model may be a useful tool for preclinical evaluation of treatment response to complement-inhibitors.

The Lectin Pathway of the Complement System-Activation, Regulation, Disease Connections and Interplay with Other (Proteolytic) Systems

The complement system is the other major proteolytic cascade in the blood of vertebrates besides the coagulation-fibrinolytic system. Among the three main activation routes of complement, the lectin pathway (LP) has been discovered the latest, and it is still the subject of intense research. Mannose-binding lectin (MBL), other collectins, and ficolins are collectively termed as the pattern recognition molecules (PRMs) of the LP, and they are responsible for targeting LP activation to molecular patterns, e.g., on bacteria. MBL-associated serine proteases (MASPs) are the effectors, while MBL-associated proteins (MAps) have regulatory functions. Two serine protease components, MASP-1 and MASP-2, trigger the LP activation, while the third component, MASP-3, is involved in the function of the alternative pathway (AP) of complement. Besides their functions within the complement system, certain LP components have secondary ("moonlighting") functions, e.g., in embryonic development. They also contribute to blood coagulation, and some might have tumor suppressing roles. Uncontrolled complement activation can contribute to the progression of many diseases (e.g., stroke, kidney diseases, thrombotic complications, and COVID-19). In most cases, the lectin pathway has also been implicated. In this review, we summarize the history of the lectin pathway, introduce their components, describe its activation and regulation, its roles within the complement cascade, its connections to blood coagulation, and its direct cellular effects. Special emphasis is placed on disease connections and the non-canonical functions of LP components.

Targeting complement in IgA nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. Recent years have witnessed significant improvements in the understanding of the pathogenesis of IgAN and particularly, the pathogenic role of complement activation. The alternative complement pathway is the major complement cascade activator in IgAN, and glomerular C3 deposition has been shown to correlate with disease progression. In addition, several studies have provided insight into the pathogenic role of factor H-related proteins -1 and -5 in IgAN, as independent players in complement dysregulation. The lectin pathway has also been shown to be associated with the severity of IgAN. Glomerular deposition of C4d has been associated with increased histologic disease activity, faster decline in estimated glomerular filtration rate and higher risk of kidney failure. On the other hand, although overlooked in the Oxford classification, numerous studies have shown that the coexistence of thrombotic microangiopathy in IgAN is a significant indicator of a poorer prognosis. All the breakthroughs in the understanding of the contributing role of complement in IgAN have paved the way for the development of new complement-targeted therapies in this disease. Several ongoing trials are evaluating the efficacy of new agents against factor B (iptacopan, Ionis-FB-LRX), C3 (pegcetacoplan), factor D (vemircopan, pelecopan), C5 (ravulizumab, cemdisiran) and C5a receptor 1 (avacopan). In this study, we provide a comprehensive review of the role of complement in IgAN, including the emerging mechanisms of complement activation and the promising potential of complement inhibitors as a viable treatment option for IgAN.

The role of complement in kidney disease

The complement cascade comprises soluble and cell surface proteins and is an important arm of the innate immune system. Once activated, the complement system rapidly generates large quantities of protein fragments that are potent mediators of inflammatory, vasoactive and metabolic responses. Although complement is crucial to host defence and homeostasis, its inappropriate or uncontrolled activation can also drive tissue injury. For example, the complement system has been known for more than 50 years to be activated by glomerular immune complexes and to contribute to autoimmune kidney disease. Notably, the latest research shows that complement is also activated in kidney diseases that are not traditionally thought of as immune-mediated, including haemolytic-uraemic syndrome, diabetic kidney disease and focal segmental glomerulosclerosis. Several complement-targeted drugs have been approved for the treatment of kidney disease, and additional anti-complement agents are being investigated in clinical trials. These drugs are categorically different from other immunosuppressive agents and target pathological processes that are not effectively inhibited by other classes of immunosuppressants. The development of these new drugs might therefore have considerable benefits in the treatment of kidney disease.

Evaluating Progression Risk in Patients With Immunoglobulin A Nephropathy

The highly variable rate of decline in kidney function in patients with immunoglobulin A nephropathy (IgAN) provides a major clinical challenge. Predicting which patients will progress to kidney failure, and how quickly, is difficult. Multiple novel therapies are likely to be approved in the short-term, but clinicians lack the tools to identify patients most likely to benefit from specific treatments at the right time. Noninvasive and validated markers for selecting at-risk patients and longitudinal monitoring are urgently needed. This review summarizes what is known about demographic, clinical, and histopathologic prognostic markers in the clinician's toolkit, including the International IgAN Prediction Tool. We also briefly review what is known on these topics in children and adolescents with IgAN. Although helpful, currently used markers leave clinicians heavily reliant on histologic features from the diagnostic kidney biopsy and standard clinical data to guide treatment choice, and very few noninvasive markers reflect treatment efficacy over time. Novel prognostic and predictive markers are under clinical investigation, with considerable progress being made in markers of complement activation. Other areas of research are the interplay between gut microbiota and galactose-deficient IgA1 expression; microRNAs; imaging; artificial intelligence; and markers of fibrosis. Given the rate of therapeutic advancement, the remaining gaps in biomarker research need to be addressed. We finish by describing our route to clinical utility of predictive and prognostic markers in IgAN. This route will provide us with the chance to improve IgAN prognosis by using robust, clinically practical markers to inform patient care.

The complement system in IgAN: mechanistic context for therapeutic opportunities

The complement system plays a crucial role in innate immunity, providing essential defense against pathogens. However, uncontrolled or prolonged activation of the complement cascade can significantly contribute to kidney damage, especially in cases of glomerulonephritis. Immunoglobulin A nephropathy (IgAN), the most prevalent form of primary glomerulonephritis, has growing evidence supporting the involvement of complement alternative and lectin pathways. In fact, patients with IgAN experience complement activation within their kidney tissue, which may be involved in the development of glomerular damage and the progression of IgAN. Complement activation has emerged as a significant area of interest in IgAN, with numerous complement-targeting agents currently being explored within this field. Nevertheless, the exact mechanisms of complement activation and their role in IgAN progression require comprehensive elucidation. This review seeks to contextualize the proposed mechanisms of complement activation within the various stages ("hits") of IgAN pathogenesis, while also addressing the clinical implications and anticipated outcomes of complement inhibition in IgAN.

IgA nephropathy

IgA nephropathy (IgAN), the most prevalent primary glomerulonephritis worldwide, carries a considerable lifetime risk of kidney failure. Clinical manifestations of IgAN vary from asymptomatic with microscopic or intermittent macroscopic haematuria and stable kidney function to rapidly progressive glomerulonephritis. IgAN has been proposed to develop through a 'four-hit' process, commencing with overproduction and increased systemic presence of poorly O-glycosylated galactose-deficient IgA1 (Gd-IgA1), followed by recognition of Gd-IgA1 by antiglycan autoantibodies, aggregation of Gd-IgA1 and formation of polymeric IgA1 immune complexes and, lastly, deposition of these immune complexes in the glomerular mesangium, leading to kidney inflammation and scarring. IgAN can only be diagnosed by kidney biopsy. Extensive, optimized supportive care is the mainstay of therapy for patients with IgAN. For those at high risk of disease progression, the 2021 KDIGO Clinical Practice Guideline suggests considering a 6-month course of systemic corticosteroid therapy; however, the efficacy of systemic steroid treatment is under debate and serious adverse effects are common. Advances in understanding the pathophysiology of IgAN have led to clinical trials of novel targeted therapies with acceptable safety profiles, including SGLT2 inhibitors, endothelin receptor blockers, targeted-release budesonide, B cell proliferation and differentiation inhibitors, as well as blockade of complement components.

Current Understanding of Complement Proteins as Therapeutic Targets for the Treatment of Immunoglobulin A Nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and a frequent cause of kidney failure. Currently, the diagnosis necessitates a kidney biopsy, with routine immunofluorescence microscopy revealing IgA as the dominant or co-dominant immunoglobulin in the glomerular immuno-deposits, often with IgG and sometimes IgM or both. Complement protein C3 is observed in most cases. IgAN leads to kidney failure in 20-40% of patients within 20 years of diagnosis and reduces average life expectancy by about 10 years. There is increasing clinical, biochemical, and genetic evidence that the complement system plays a paramount role in the pathogenesis of IgAN. The presence of C3 in the kidney immuno-deposits differentiates the diagnosis of IgAN from subclinical glomerular mesangial IgA deposition. Markers of complement activation via the lectin and alternative pathways in kidney-biopsy specimens are associated with disease activity and are predictive of poor outcome. Levels of select complement proteins in the circulation have also been assessed in patients with IgAN and found to be of prognostic value. Ongoing genetic studies have identified at least 30 loci associated with IgAN. Genes within some of these loci encode complement-system regulating proteins that can interact with immune complexes. The growing appreciation for the central role of complement components in IgAN pathogenesis highlighted these pathways as potential treatment targets and sparked great interest in pharmacological agents targeting the complement cascade for the treatment of IgAN, as evidenced by the plethora of ongoing clinical trials.

Challenges in IgA Nephropathy Management: An Era of Complement Inhibition

IgA nephropathy (IgAN) is the most common glomerular disease worldwide, with an estimated annual incidence of 25 per million adults. Despite optimized supportive care, some patients fail to achieve disease control and suffer progressive deterioration of kidney function. In this subpopulation of patients, the Kidney Disease: Improving Global Outcomes 2021 guidelines recommend consideration of corticosteroids; however, their use is associated with significant side effects. Ongoing clinical trials are expected to identify corticosteroid-sparing therapies to help improve treatment and prognosis for patients with IgAN. It has been well-documented that the complement system plays a significant role in IgAN pathogenesis, and several complement inhibitors are now entering late-stage clinical development. This review evaluates what we know about the role of complement in the pathophysiology of IgAN and considers how the availability of targeted complement inhibitors may impact future clinical practice. Key knowledge gaps are evaluated, and research opportunities are recommended to help guide clinical decision-making and optimize patient outcomes. Such gaps include evaluating the relative contribution of the alternative and lectin pathways to disease pathogenesis, and the importance of determining the dominant pathway driving IgAN progression. Continued research into the staining of complement proteins in kidney biopsies and identifying targeted biomarkers to assess disease progression and treatment responses will also be needed to support the implementation of newer therapies in clinical practice. Considering the future horizons for enhancing the care of patients with IgAN, tackling the outstanding challenges now will help prepare for the best possible future outcomes.

The role of complement in glomerulonephritis-are novel therapies ready for prime time?

The complement system plays a key pathogenic role in glomerular diseases with a diverse range of aetiologies, including C3 glomerulopathy, immunoglobulin A nephropathy, membranous nephropathy, ANCA-associated vasculitis and lupus nephritis. Several novel therapies targeting complement activity have recently been developed, which have now been approved or are in the late stages of clinical development. In this review, potential benefits and challenges of targeting the complement system in glomerular disease are discussed. We summarize current understanding of the role of complement, and the novel targeted therapies that are being developed for the treatment of glomerular disease.

Immune abnormalities in IgA nephropathy

Immunoglobulin A (IgA) nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and it is characterized by mesangial IgA deposition. Asymptomatic hematuria with various degrees of proteinuria is the most common clinical presentation and up to 20%-40% of patients develop end-stage kidney disease within 20 years after disease onset. The pathogenesis of IgAN involves four sequential processes known as the "four-hit hypothesis" which starts with the production of a galactose-deficient IgA1 (gd-IgA1), followed by the formation of anti-gd-IgA1 IgG or IgA1 autoantibodies and immune complexes that ultimately deposit in the glomerular mesangium, leading to inflammation and injury. Although several key questions about the production of gd-IgA1 and the formation of anti-gd-IgA1 antibodies remain unanswered, a growing body of evidence is shedding light on the innate and adaptive immune mechanisms involved in this complex pathogenic process. Herein, we will focus on these mechanisms that, along with genetic and environmental factors, are thought to play a key role in disease pathogenesis.

IgA Nephropathy: the Lectin Pathway and Implications for Targeted Therapy

Many patients with IgA nephropathy (IgAN) progress to end-stage kidney disease even with optimal supportive care. An improved understanding of the pathophysiology of IgAN in recent years has led to the investigation of targeted therapies with acceptable tolerability that may address the underlying causes of IgAN or the pathogenesis of kidney injury. The complement system - particularly the lectin and alternative pathways of complement - have emerged as key mediators of kidney injury in IgAN and possible targets for investigational therapy. This review will focus on the lectin pathway. Examination of kidney biopsies has consistently shown glomerular deposition of mannan-binding lectin (one of six pattern-recognition molecules that activate the lectin pathway) together with IgA1 in up to 50% of patients with IgAN. Glomerular deposition of pattern-recognition molecules for the lectin pathway is associated with more severe glomerular damage and more severe proteinuria and hematuria. Emerging research suggests that the lectin pathway may also contribute to tubulointerstitial fibrosis in IgAN, and that collectin-11 is a key mediator of this association. This review summarizes the growing scientific and clinical evidence supporting the role of the lectin pathway in IgAN and examines the possible therapeutic role of lectin pathway inhibition for these patients.

Complement detection in kidney biopsies - utility and challenges

PURPOSE OF REVIEW

This review discusses the important role of staining for components of the complement cascade in both native and transplant kidney biopsies. The use of complement staining as a marker of prognosis, disease activity, and as a potential future tool in identifying patients who may benefit from complement-targeted therapies is discussed.

RECENT FINDINGS

While staining for C3, C1q and C4d can yield valuable information about complement activation in kidney biopsies, to adequately assess complement activation and potential therapeutic targets, expanded staining panels looking at multiple split products and complement regulatory proteins are needed. Recent progress has been made in identifying markers of disease severity in C3 glomerulonephritis and IgA nephropathy, such as Factor H-related Protein-5, which may serve as future tissue biomarkers. In the transplant setting, the limitation of relying on C4d staining to identify antibody mediated rejection is giving way to molecular diagnostics, including The Banff Human Organ Transplant (B-HOT) panel, which includes numerous complement complement-related transcripts, with the classical, lectin, alternative, and common pathways.

SUMMARY

Staining for complement components in kidney biopsies to understand how complement is activated in individual cases may help to identify patients who may benefit from complement-targeted therapies.

Urinary complement profile in IgA nephropathy and its correlation with the clinical and pathological characteristics

Background and objectives

The activated complement profile in IgA nephropathy (IgAN) is still unclear. Our study investigated the profile of urinary complements in IgAN patients and its correlations with clinical and pathological characteristics.

Methods

Urinary protein abundance was detected by liquid chromatography-tandem mass spectrometry (LC–MS/MS) in 50 IgAN, 50 membranous nephropathy (MN), and 68 healthy controls (HC). Then, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed to identify differentially expressed proteins in IgAN patients. The differentially expressed complement proteins were screened in IgAN patients, and their correlations with laboratory or pathological parameters were analyzed. Thereafter, 7 complement components were validated by enzyme-linked immunosorbent assay (ELISA) in the urine samples of 45 IgAN patients.

Results

There were 786 differentially expressed proteins between IgAN and HC. KEGG analysis showed that differentially expressed urinary proteins in IgAN were enriched with complement. Of these, 67% of urinary complement protein abundance was associated with the estimated glomerular filtration rate. The urinary complement-related protein collectin12 (colec12), complement H factor (CFH), complement H factor-related protein 2 (CFHR2), and complement B factor (CFB) were positively correlated with serum creatinine; colec12, CFHR2, CFB, and C8g were positively correlated with glomerulosclerosis; CFH, CFHR2, C8g, and C9 were positively correlated with tubular atrophy/interstitial fibrosis.

Conclusion

Abnormally increased components of complement pathways significantly correlate with reduced renal function, proteinuria, and renal histological damage in IgAN. It could provide a potential biomarker panel for monitoring IgAN and provide clues for therapeutic choice targeting complement system of IgAN patients.

Identification of FCN1 as a novel macrophage infiltration-associated biomarker for diagnosis of pediatric inflammatory bowel diseases

BACKGROUND

The incidence of pediatric inflammatory bowel disease (PIBD) has been steadily increasing globally. Delayed diagnosis of PIBD increases the risk of complications and contributes to growth retardation. To improve long-term outcomes, there is a pressing need to identify novel markers for early diagnosis of PIBD.

METHODS

The candidate biomarkers for PIBD were identified from the GSE117993 dataset by two machine learning algorithms, namely LASSO and mSVM-RFE, and externally validated in the GSE126124 dataset and our PIBD cohort. The role of ficolin-1 (FCN1) in PIBD and its association with macrophage infiltration was investigated using the CIBERSORT method and enrichment analysis of the single-cell dataset GSE121380, and further validated using immunoblotting, qRT-PCR, and immunostaining in colon biopsies from PIBD patients, a juvenile murine DSS-induced colitis model, and THP-1-derived macrophages.

RESULTS

FCN1 showed great diagnostic performance for PIBD in an independent clinical cohort with the AUC of 0.986. FCN1 expression was upregulated in both colorectal biopsies and blood samples from PIBD patients. Functionally, FCN1 was associated with immune-related processes in the colonic mucosa of PIBD patients, and correlated with increased proinflammatory M1 macrophage infiltration. Furthermore, single-cell transcriptome analysis and immunostaining revealed that FCN1 was almost exclusively expressed in macrophages infiltrating the colonic mucosa of PIBD patients, and these FCN1⁺ macrophages were related to hyper-inflammation. Notably, proinflammatory M1 macrophages derived from THP-1 expressed high levels of FCN1 and IL-1β, and FCN1 overexpression in THP-1-derived macrophages strongly promoted LPS-induced activation of the proinflammatory cytokine IL-1β via the NLRP3-caspase-1 axis.

CONCLUSIONS

FCN1 is a novel and promising diagnostic biomarker for PIBD. FCN1⁺ macrophages enriched in the colonic mucosa of PIBD exhibit proinflammatory phenotypes, and FCN1 promotes IL-1β maturation in macrophages via the NLRP3-caspase-1 axis.

Alternative pathway diagnostics

Uncontrolled alternative pathway activation is the primary driver of several diseases, and it contributes to the pathogenesis of many others. Consequently, diagnostic tests to monitor this arm of the complement system are increasingly important. Defects in alternative pathway regulation are strong risk factors for disease, and drugs that specifically block the alternative pathway are entering clinical use. A range of diagnostic tests have been developed to evaluate and monitor the alternative pathway, including assays to measure its function, expression of alternative pathway constituents, and activation fragments. Genetic studies have also revealed many disease-associated variants in alternative pathway genes that predict the risk of disease and prognosis. Newer imaging modalities offer the promise of non-invasively detecting and localizing pathologic complement activation. Together, these various tests help in the diagnosis of disease, provide important prognostic information, and can help guide therapy with complement inhibitory drugs.

Urinary complement proteins in IgA nephropathy progression from a relative quantitative proteomic analysis

Aim

IgA nephropathy (IgAN) is one of the leading causes of end-stage renal disease (ESRD). Urine testing is a non-invasive way to track the biomarkers used for measuring renal injury. This study aimed to analyse urinary complement proteins during IgAN progression using quantitative proteomics.

Methods

In the discovery phase, we analysed 22 IgAN patients who were divided into three groups (IgAN 1-3) according to their estimated glomerular filtration rate (eGFR). Eight patients with primary membranous nephropathy (pMN) were used as controls. Isobaric tags for relative and absolute quantitation (iTRAQ) labelling, coupled with liquid chromatography-tandem mass spectrometry, was used to analyse global urinary protein expression. In the validation phase, western blotting and parallel reaction monitoring (PRM) were used to verify the iTRAQ results in an independent cohort (N = 64).

Results

In the discovery phase, 747 proteins were identified in the urine of IgAN and pMN patients. There were different urine protein profiles in IgAN and pMN patients, and the bioinformatics analysis revealed that the complement and coagulation pathways were most activated. We identified a total of 27 urinary complement proteins related to IgAN. The relative abundance of C3, the membrane attack complex (MAC), the complement regulatory proteins of the alternative pathway (AP), and MBL (mannose-binding lectin) and MASP1 (MBL associated serine protease 2) in the lectin pathway (LP) increased during IgAN progression. This was especially true for MAC, which was found to be involved prominently in disease progression. Alpha-N-acetylglucosaminidase (NAGLU) and α-galactosidase A (GLA) were validated by western blot and the results were consistent with the iTRAQ results. Ten proteins were validated in a PRM analysis, and these results were also consistent with the iTRAQ results. Complement factor B (CFB) and complement component C8 alpha chain (C8A) both increased with the progression of IgAN. The combination of CFB and mucosal addressin cell adhesion molecule-1 (MAdCAM-1) also showed potential as a urinary biomarker for monitoring IgAN development.

Conclusion

There were abundant complement components in the urine of IgAN patients, indicating that the activation of AP and LP is involved in IgAN progression. Urinary complement proteins may be used as biomarkers for evaluating IgAN progression in the future.

Kidney diseases

Dysregulation and accelerated activation of the alternative pathway (AP) of complement is known to cause or accentuate several pathologic conditions in which kidney injury leads to the appearance of hematuria and proteinuria and ultimately to the development of chronic renal failure. Multiple genetic and acquired defects involving plasma- and membrane-associated proteins are probably necessary to impair the protection of host tissues and to confer a significant predisposition to AP-mediated kidney diseases. This review aims to explore how our current understanding will make it possible to identify the mechanisms that underlie AP-mediated kidney diseases and to discuss the available clinical evidence that supports complement-directed therapies. Although the value of limiting uncontrolled complement activation has long been recognized, incorporating complement-targeted treatments into clinical use has proved challenging. Availability of anti-complement therapy has dramatically transformed the outcome of atypical hemolytic uremic syndrome, one of the most severe kidney diseases. Innovative drugs that directly counteract AP dysregulation have also opened new perspectives for the management of other kidney diseases in which complement activation is involved. However, gained experience indicates that the choice of drug should be tailored to each patient's characteristics, including clinical, histologic, genetic, and biochemical parameters. Successfully treating patients requires further research in the field and close collaboration between clinicians and researchers who have special expertise in the complement system.

Factor H related proteins modulate complement activation on kidney cells

Complement activation at a particular location is determined by the balance of activating and inhibitory proteins. Factor H is a key regulator of the alternative pathway of complement, and genetic or acquired impairments in Factor H are associated with glomerular injury. The human Factor H-related proteins (FHRs) comprise a family of five proteins that are structurally related to Factor H. Variations in the genes or expression levels of the FHRs are also associated with glomerular disease, although the mechanisms of glomerular protection/injury are incompletely understood. To explore the role of the FHRs on complement regulation/dysregulation in the kidney, we expressed and purified recombinant murine FHRs (FHRs A, B, C and E). These four distinct FHRs contain binding regions with high amino acid sequence homology to binding regions within Factor H, but we observed different interactions of the FHRs with Factor H binding ligands, including heparin and C3d. There was differential binding of the FHRs to the resident kidney cell types (mesangial, glomerular endothelial, podocytes, and tubular epithelial). All four FHRs caused complement dysregulation on kidney cell surfaces in vitro, although the magnitude of the effect differed among the FHRs and also varied among the different kidney cells. However, only FHR E caused glomerular complement dysregulation when injected in vivo but did not exacerbate injury when injected into mice with ischemic acute kidney injury, an alternative pathway-mediated model. Thus, our experiments demonstrate that the FHRs have unique, and likely context-dependent, effects on the different cell types within the kidney.

Overactivation of the complement system may be involved in intrarenal arteriolar lesions in IgA nephropathy

Introduction

IgA nephropathy (IgAN) encompasses a wide range of clinical and histology features. Some patients present without hematuria, with or without hypertension, still rapidly progress in renal function. Renal pathology of this part of patients were predominant intrarenal arteriolar lesions, rarely presented glomerular proliferative lesions. We aim to investigate the clinical and pathological characteristics and prognosis of these IgAN patients and initially explore whether the abnormal activation of complement is involved in the intrarenal arteriolar lesions of IgAN.

Methods

A total of 866 patients with renal biopsy-proven IgAN diagnosed at Beijing Anzhen Hospital were recruited. IgAN patients without intrarenal arteriolar lesions and proliferative lesions were excluded (n = 115), the rest were divided into arteriolar lesions group (n = 202) and proliferative lesions group (n = 549). Among them, 255 patients were regularly followed up for at least 1 year. Renal biopsy tissues of 104 IgAN patients were stained for complement components by immunohistochemistry and immunofluorescence.

Results

Compared with proliferative lesions group, the arteriolar lesions group experienced high percentage of hypertension (p = 0.004), low percentage of gross hematuria (p = 0.001), microscopic hematuria (p < 0.001) and less initial proteinuria (p = 0.033). Renal survival between the two groups was not significantly different (p = 0.133). MBL, C4d, FH and FHR5, C3c, and MAC deposited on intrarenal arteriole in arteriolar lesions group. Compare with the proliferative lesion group, the arteriolar lesions group exhibited a higher intensity of C3c deposition on the intrarenal arterioles (p = 0.048). C3c and CD31 co-deposited on intrarenal arterioles area in patients with intrarenal arteriolar lesions.

Conclusion

Renal survival of the IgAN patients in arteriolar lesions group was not better than those in proliferative lesions group. Abnormal activation of complement may be involved in the pathogenesis of arteriolar damage through the injury of endothelial cells in this clinical phenotype of IgAN.

The Rational Use of Complement Inhibitors in Kidney Diseases

The development of complement inhibitors represented one of the major breakthroughs in clinical nephrology in the last decade. Complement inhibition has dramatically transformed the outcome of one of the most severe kidney diseases, the atypical hemolytic uremic syndrome (aHUS), a prototypic complement-mediated disorder. The availability of complement inhibitors has also opened new promising perspectives for the management of several other kidney diseases in which complement activation is involved to a variable extent. With the rapidly growing number of complement inhibitors tested in a rapidly increasing number of indications, a rational use of this innovative and expensive new therapeutic class has become crucial. The present review aims to summarize what we know, and what we still ignore, regarding complement activation and therapeutic inhibition in kidney diseases. It also provides some clues and elements of thoughts for a rational approach of complement modulation in kidney diseases.

Circulating complement factor H levels are associated with disease severity and relapse in autoimmune hepatitis

Background & Aims

The complement system plays pivotal roles in innate immunity. Mannose-binding lectin-associated serine protease (MASP)-2 plays essential roles in the activation of the lectin complement pathway. Complement factor H acts as a critical negative regulator of the alternative complement pathway. The association of circulating MASP-2 and factor H with the clinical features of patients with autoimmune hepatitis (AIH) is unclear.

Methods

A total of 63 patients with AIH were recruited for this study. The serum levels of MASP-2, factor H, and C3a were measured, and their associations with the clinical features of AIH were analyzed.

Results

The circulating C3a levels were higher in patients with AIH than in the controls. The circulating MASP-2 and factor H levels were decreased depending on the severity of AIH. Multivariate logistic analysis showed that low circulating factor H levels were associated with features of severe AIH (odds ratio 0.36; 95% CI 0.15-0.84; p = 0.018). Multivariate Cox proportional hazards model analysis showed that low circulating factor H levels were associated with a high incidence of relapse (hazard ratio: 5.19; 95% CI 1.07–25.2; p = 0.041). Patients with low circulating factor H levels showed higher rates of relapse than the controls (log-rank, p = 0.006).

Conclusion

Circulating factor H levels were associated with severe disease and with the incidence of relapse, suggesting a role for the complement system in the pathophysiology of AIH.

Lay summary

Autoimmune hepatitis is an immune-mediated liver disease. Despite effective treatments, patients often relapse, which can lead to clinical deterioration and adverse outcomes. Herein, we studied the importance of the complement system (a form of innate immunity) in patients with autoimmune hepatitis. We found that the levels of a protein called factor H, which regulates the complement system, could be a potential biomarker of disease severity and relapse, and could even have therapeutic potential for patients with AIH.

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We measured serum MASP-2 and factor H in patients with AIH.

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Serum MASP-2 and factor H levels were lower in patients with severe AIH.

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Patients with AIH and low factor H before treatment showed a high rate of relapse.

New Insights into the Treatment of Glomerular Diseases: When Mechanisms Become Vivid

Treatment for glomerular diseases has been extrapolated from the experience of other autoimmune disorders while the underlying pathogenic mechanisms were still not well understood. As the classification of glomerular diseases was based on patterns of juries instead of mechanisms, treatments were typically the art of try and error. With the advancement of molecular biology, the role of the immune agent in glomerular diseases is becoming more evident. The four-hit theory based on the discovery of gd-IgA1 gives a more transparent outline of the pathogenesis of IgA nephropathy (IgAN), and dysregulation of Treg plays a crucial role in the pathogenesis of minimal change disease (MCD). An epoch-making breakthrough is the discovery of PLA2R antibodies in the primary membranous nephropathy (pMN). This is the first biomarker applied for precision medicine in kidney disease. Understanding the immune system's role in glomerular diseases allows the use of various immunosuppressants or other novel treatments, such as complement inhibitors, to treat glomerular diseases more reasonable. In this era of advocating personalized medicine, it is inevitable to develop precision medicine with mechanism-based novel biomarkers and novel therapies in kidney disease.

Comparison of Complement Pathway Activation in Autoimmune Glomerulonephritis

Introduction

Methods

Results

Conclusion

Complement multiplex testing: Concept, promises and pitfalls

Complement is a complex system. This complexity becomes more obvious when looking at complement analysis in health and disease, where one presentation can require a number of measurements to understand the full role of this cascade in the disease. The current state of clinical testing requires multiple tests to cover the whole of the complement cascade. There is a clear potential for multiplex testing to help address this need for comprehensive analysis of the state of complement deficiency, activation or inhibition. Fortunately, there are a number of potential methods for multiplex analysis, each with advantages and disadvantages that need to be considered in light of the intricacy of the complement cascade and its interconnection to other systems. Despite the complexities of such methods several groups have started utilizing multiplex analysis for research and even for diagnostic testing. The potential methods, current successes, and the type of testing that needs to be streamlined are reviewed in this text.

The Contribution of Complement to the Pathogenesis of IgA Nephropathy: Are Complement-Targeted Therapies Moving from Rare Disorders to More Common Diseases?

Primary IgA nephropathy (IgAN) is a leading cause of chronic kidney disease and kidney failure for which there is no disease-specific treatment. However, this could change, since novel therapeutic approaches are currently being assessed in clinical trials, including complement-targeting therapies. An improved understanding of the role of the lectin and the alternative pathway of complement in the pathophysiology of IgAN has led to the development of these treatment strategies. Recently, in a phase 2 trial, treatment with a blocking antibody against mannose-binding protein-associated serine protease 2 (MASP-2, a crucial enzyme of the lectin pathway) was suggested to have a potential benefit for IgAN. Now in a phase 3 study, this MASP-2 inhibitor for the treatment of IgAN could mark the start of a new era of complement therapeutics where common diseases can be treated with these drugs. The clinical development of complement inhibitors requires a better understanding by physicians of the biology of complement, the pathogenic role of complement in IgAN, and complement-targeted therapies. The purpose of this review is to provide an overview of the role of complement in IgAN, including the recent discovery of new mechanisms of complement activation and opportunities for complement inhibitors as the treatment of IgAN.

Complement catalyzing glomerular diseases

Complement is an evolutionarily conserved system which is important in the defense against microorganisms and also in the elimination of modified or necrotic elements of the body. Complement is activated in a cascade type manner and activation and all steps of cascade progression are tightly controlled and regulatory interleaved with many processes of inflammatory machinery. Overshooting of the complement system due to dysregulation can result in the two prototypes of primary complement mediated renal diseases: C3 glomerulopathy and thrombotic microangiopathy. Apart from these, complement also is highly activated in many other inflammatory native kidney diseases, such as membranous nephropathy, ANCA-associated necrotizing glomerulonephritis, and IgA nephropathy. Moreover, it likely plays an important role also in the transplant setting, such as in antibody-mediated rejection or in hematopoietic stem cell transplant associated thrombotic microangiopathy. In this review, these glomerular disorders are discussed with regard to the role of complement in their pathogenesis. The consequential, respective clinical trials for complement inhibitory therapy strategies for these diseases are described.

Is complement the main accomplice in IgA nephropathy? From initial observations to potential complement-targeted therapies

IgA Nephropathy (IgAN) is the main cause of primary glomerulonephritis, globally. This disease is associated with a wide range of clinical presentations, variable prognosis and a spectrum of histological findings. More than fifty years after its first description, this heterogeneity continues to complicate efforts to understand the pathogenesis. Nevertheless, involvement of the complement system in IgAN was identified early on. Dysfunction of the immunoglobulin A (IgA) system, the principal offender in this disease, including modification of isoforms and glycoforms of IgA1, the nature of immune complexes and autoantibodies to galactose deficient IgA1 might all be responsible for complement activation in IgAN. However, the specific mechanisms engaging complement are still under examination. Research in this domain should allow for identification of patients that may benefit from complement-targeted therapy, in the foreseeable future.

Autoimmune-mediated renal disease and hypertension

Hypertension is a major risk factor for cardiovascular disease, chronic kidney disease (CKD), and mortality. Troublingly, hypertension is highly prevalent in patients with autoimmune renal disease and hastens renal functional decline. Although progress has been made over the past two decades in understanding the inflammatory contributions to essential hypertension more broadly, the mechanisms active in autoimmune-mediated renal diseases remain grossly understudied. This Review provides an overview of the pathogenesis of each of the major autoimmune diseases affecting the kidney that are associated with hypertension, and describes the current state of knowledge regarding hypertension in these diseases and their management. Specifically, discussion focuses on Systemic Lupus Erythematosus (SLE) and Lupus Nephritis (LN), Immunoglobulin A (IgA) Nephropathy, Idiopathic Membranous Nephropathy (IMN), Anti-Neutrophil Cytoplasmic Antibody (ANCA)-associated glomerulonephritis, and Thrombotic Thrombocytopenic Purpura (TTP). A summary of disease-specific animal models found to exhibit hypertension is also included to highlight opportunities for much needed further investigation of underlying mechanisms and novel therapeutic approaches.

Complement activation in IgA nephropathy

IgA nephropathy pathogenesis is incompletely understood, and this limits the development of disease-specific biomarkers and effective therapies. Evidence of complement activity in IgA nephropathy is well established. However, a growing body of research indicates complement activity is an important contributor to IgA nephropathy pathology. In particular, multiple associations have been identified between complement alternative, lectin and terminal pathway proteins and IgA nephropathy severity. Recently, we have also gained insight into possible mechanisms that could link glomerular IgA deposition, complement activity, glomerular inflammation and disease severity. Ongoing clinical trials of therapeutic complement inhibitors will provide insight into the importance of complement activity to IgA nephropathy pathogenesis. Further research into mechanisms of complement activity is essential to improving our understanding and management of patients with IgA nephropathy.

Defining the Glycosaminoglycan Interactions of Complement Factor H-Related Protein 5

Complement activation is an important mediator of kidney injury in glomerulonephritis. Complement factor H (FH) and FH-related protein 5 (FHR-5) influence complement activation in C3 glomerulopathy and IgA nephropathy by differentially regulating glomerular complement. FH is a negative regulator of complement C3 activation. Conversely, FHR-5 in vitro promotes C3 activation either directly or by competing with FH for binding to complement C3b. The FH-C3b interaction is enhanced by surface glycosaminoglycans (GAGs) and the FH-GAG interaction is well-characterized. In contrast, the contributions of carbohydrates to the interaction of FHR-5 and C3b are unknown. Using plate-based and microarray technologies we demonstrate that FHR-5 interacts with sulfated GAGs and that this interaction is influenced by the pattern and degree of GAG sulfation. The FHR-5-GAG interaction that we identified has functional relevance as we could show that the ability of FHR-5 to prevent binding of FH to surface C3b is enhanced by surface kidney heparan sulfate. Our findings are important in understanding the molecular basis of the binding of FHR-5 to glomerular complement and the role of FHR-5 in complement-mediated glomerular disease.

Alternative Complement Pathway Is Activated and Associated with Galactose-Deficient IgA1 Antibody in IgA Nephropathy Patients

Background

Galactose-deficient IgA₁ (Gd-IgA₁) and alternative complement pathway activation are considered to be involved in the pathogenesis of IgA nephropathy (IgAN). Nevertheless, the relationships between alternative pathway activation and disease activity or Gd-IgA₁ level remains unclear.

Methods

Ninety-eight biopsy-diagnosed IgAN, twenty-five primary focal segmental sclerosis (FSGS) patients and forty-two healthy individuals were recruited in this study. Among them, fifty IgAN patients received immunosuppression. Follow-up blood samples at 1 and 3~6 months after immunosuppression were collected. Plasma levels of complement C5a, factor Ba and Gd-IgA₁ were measured and analyzed. Immunostaining for complement was performed in twenty-five IgAN and FSGS patients.

Results

At baseline, IgAN patients had higher levels of plasma C5a, factor Ba and Gd-IgA₁ than control subjects. Gd-IgA₁ levels positively correlated with plasma C5a and factor Ba. In addition, levels of factor Ba and Gd-IgA₁ were positively associated with proteinuria and negatively associated with renal function. Immunostaining revealed positive staining for factor Bb and C3c in glomeruli in IgAN patients, but not in FSGS patients. At baseline, patients receiving immunosuppression had more severe proteinuria and higher factor Ba. After 6 months, eGFR declined and proteinuria persisted in patients without immunosuppression. In contrast, patients who received immunosuppression exhibited decreased plasma levels of C5a, factor Ba, and Gd-IgA₁ as early as 1 month after treatment. Proteinuria decreased and renal function also remained stable 6 months after immunosuppression.

Conclusions

Our results indicate a close relationship between alternative complement pathway activation, Gd-IgA₁ concentration and clinical severity of IgAN. Level of complement factor B may be a potential marker for disease activity and therapeutic target in IgAN patients.

Case Report: A Rare Truncating Variant of the CFHR5 Gene in IgA Nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. Despite appropriate therapy, 20–40% of affected-patients evolve toward end-stage kidney disease (ESKD). Mesangial IgA deposits are the hallmark of IgAN, and complement deposition (C3) seems to differentiate latent IgA mesangial deposits from active IgAN. Atypical hemolytic uremic syndrome (aHUS), another disease in which complement plays an important role, is caused by inherited or acquired deregulation of the alternative pathway (AP) of complement. A subgroup of IgAN shows thrombotic microangiopathy (TMA) lesions in kidney biopsies, the histological characteristic of aHUS. Genetic variants of complement Factor H (CFH), known to be present in aHUS, have been associated with rapidly progressive forms of IgAN and a clinical pattern of aHUS. Genome-wide association studies (GWAS) have confirmed that the 1q32 region, encoding for CFH and its related proteins, is an IgAN susceptibility locus. A 30 year-old man was admitted for seizures and malignant hypertension. The kidney biopsy showed IgAN associated with features of TMA. Despite five plasma exchanges, the patient remained dialysis-dependent, and ESKD was diagnosed. Functional and genetic complement analysis were performed. A monoallelic protein-truncating, likely loss-of-function variant was identified in the CFHR5 gene. Eculizumab is the treatment of aHUS. As it has been successfully used in a few cases of rapidly progressive IgAN, it was decided to administer eculizumab over a period of 12 months in addition to the usual immunosuppression for renal transplantation. After a follow-up of 3 years, there was no clinical disease recurrence. Systematic biologic and genetic screening of complement in individuals with IgAN might be useful to better delineate the role of the AP of complement in renal disease progression, and this may have therapeutic implications.

Gain-of-function factor H-related 5 protein impairs glomerular complement regulation resulting in kidney damage

Genetic variation within the factor H-related (FHR) genes is associated with the complement-mediated kidney disease, C3 glomerulopathy (C3G). There is no definitive treatment for C3G, and a significant proportion of patients develop end-stage renal disease. The prototypical example is CFHR5 nephropathy, through which an internal duplication within a single CFHR5 gene generates a mutant FHR5 protein (FHR5mut) that leads to accumulation of complement C3 within glomeruli. To elucidate how abnormal FHR proteins cause C3G, we modeled CFHR5 nephropathy in mice. Animals lacking the murine factor H (FH) and FHR proteins, but coexpressing human FH and FHR5mut (hFH-FHR5mut), developed glomerular C3 deposition, whereas mice coexpressing human FH with the normal FHR5 protein (hFH-FHR5) did not. Like in patients, the FHR5mut had a dominant gain-of-function effect, and when administered in hFH-FHR5 mice, it triggered C3 deposition. Importantly, adeno-associated virus vector-delivered homodimeric mini-FH, a molecule with superior surface C3 binding compared to FH, reduced glomerular C3 deposition in the presence of the FHR5mut. Our data demonstrate that FHR5mut causes C3G by disrupting the homeostatic regulation of complement within the kidney and is directly pathogenic in C3G. These results support the use of FH-derived molecules with enhanced C3 binding for treating C3G associated with abnormal FHR proteins. They also suggest that targeting FHR5 represents a way to treat complement-mediated kidney injury.

A Family Affair: Addressing the Challenges of Factor H and the Related Proteins

Inflammation is a common denominator of diseases. The complement system, an intrinsic part of the innate immune system, is a key driver of inflammation in numerous disorders. Recently, a family of proteins has been suggested to be of vital importance in conditions characterized by complement dysregulation: the human Factor H (FH) family. This group of proteins consists of FH, Factor H-like protein 1 and five Factor H-related proteins. The FH family has been linked to infectious, vascular, eye, kidney and autoimmune diseases. In contrast to FH, the functions of the other highly homologous proteins are largely unknown and, hence, their role in the different disease-specific pathogenic mechanisms remains elusive. In this perspective review, we address the major challenges ahead in this emerging area, including 1) the controversies about the functional roles of the FH protein family, 2) the discrepancies in quantification of the FH protein family, 3) the unmet needs for validated tools and 4) limitations of animal models. Next, we also discuss the opportunities that exist for the immunology community. A strong multidisciplinary approach is required to solve these obstacles and is only possible through interdisciplinary collaboration between biologists, chemists, geneticists and physicians. We position this review in light of our own perspective, as principal investigators of the SciFiMed Consortium, a consortium aiming to create a comprehensive analytical system for the quantitative and functional assessment of the entire FH protein family.

Complement and kidney disease, new insights

PURPOSE OF REVIEW

In this review, we discuss recent studies showing the importance of the complement pathway in kidney disease.

RECENT FINDINGS

Recent findings in C3 glomerulopathy (C3G) include: acute postinfectious glomerulonephritis is characterised by the presence of antifactor B antibodies; human leukocyte antigen type, but not rare complement gene variation, is associated with primary immunoglobulin-associated membranoproliferative GN and C3G. Immunohistochemistry in C3G shows that factor H related protein 5 (FHR5) is the most prevalent complement protein and correlates with kidney function. A multicentre study supported the use of mycophenolate mofetil (MMF) in C3G even after a propensity matching analysis. In immunoglobulin A nephropathy (IgAN) several studies have emphasised the importance of complement. Imbalances of circulating FH and FHR1 and FHR5, which interfere with the regulatory functions of FH, associate with IgAN. Immunohistochemistry has shown associations between glomerular FHR5 deposition and C3 activation; glomerular FHR5 associated with clinical markers of IgAN severity. Data also suggest the lectin complement pathway contributes to IgAN severity. We also discuss complement activation in thrombotic microangiopathy and other kidney diseases.

SUMMARY

Complement activity can be detected in a wide range of kidney diseases and this provides pathogenic insight and potential for therapy with the ongoing development of several drugs directed at complement activation.

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.

Emerging role of ficolins in autoimmune diseases

Ficolins are pattern-recognition molecules (PRMs) that could form complexes with mannose-binding lectin-associated serine proteases (MASPs) to trigger complement activation via the lectin pathway, thereby mediating a series of immune responses including opsonization, phagocytosis and cytokine production. In the past few decades, accumulating evidence have suggested that ficolins play a major role in the onset and development of several autoimmune diseases (ADs), including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), Type 1 diabetes (T1D), inflammatory bowel disease (IBD), etc. In this review, we synthesized previous literatures and recent advances to elucidate the immunological regulations of ficolins and discuss the potential diagnostic ability of ficolins in ADs, as well as giving an insight into the future therapeutic options for ficolins in ADs.

Glomerular Complement Factor H-Related Protein 5 is Associated with Histologic Injury in Immunoglobulin A Nephropathy

Introduction

Immunoglobulin A nephrology (IgAN), characterized by co-deposition of IgA and complement components, is an activation of complement system involved disease. Factor H-related protein 5 (FHR-5) antagonized the ability of factor H to negatively regulate C3 activation, which leads to overactivation of the alternative pathway. Here we explore the relationship of intensity of glomerular FHR-5 deposition and severity of IgAN.

Methods

Renal staining of FHR-5 was detected by immunofluorescence, and plasma FHR-5 was detected by enzyme-linked immunosorbent assay in 56 patients with IgAN. The relationship of intensity of glomerular FHR-5 and clinical and pathologic features of these patients were further analyzed.

Results

Glomerular staining for FHR-5 was observed in a predominantly mesangial pattern in 32 biopsy specimens (57.1%). FHR-5 co-deposited with IgA and C3c in glomerular mesangial and capillary area in patients with IgAN. Patients with IgAN with Oxford endocapillary hypercellularity (P = 0.007) and segmental glomerulosclerosis (P = 0.049) presented with greater intensity of FHR-5 deposition. There were more cases with 2+ and 3+ FHR-5 staining in cohorts of 2+ and 3-4+ mesangial C3 deposition (P = 0.034) and IgA deposition (P = 0.019). Interestingly, the glomerular FHR-5 depositions were more abundant in male versus female in patients with IgAN (P = 0.002). Besides, circulating FHR-5 levels were elevated in patients with IgAN compared with healthy control subjects. Plasma FHR-5 levels were significantly higher in patients with mesangial hypercellularity at diagnosis than those with nonmesangial hypercellularity.

Conclusions

We found that glomerular intensity of FHR-5 deposition could indicate the severity of histologic lesions of IgAN.

Monitoring Immune Responses in IgA Nephropathy: Biomarkers to Guide Management

IgA nephropathy (IgAN) is the commonest biopsy-reported primary glomerulonephritis worldwide. It has an incidence which peaks among young adults, and 30 to 40% of patients' progress to end stage kidney disease within twenty years of diagnosis. Ten-year kidney survival rates have been reported to be as low as 35% in some parts of the world. The successful management of IgAN is limited by an incomplete understanding of the pathophysiology of IgAN and a poor understanding of how pathophysiology may vary both from patient to patient and between patient groups, particularly across races. This is compounded by a lack of rigorously designed and delivered clinical trials in IgAN. This is slowly changing, with a number of Phase 2 and 3 clinical trials of novel therapies targeting a number of different putative pathogenic pathways in IgAN due to report in the next 5 years. From our current, albeit limited, understanding of the pathophysiology of IgAN it is unlikely a single therapy will be effective in all patients with IgAN. The successful management of IgAN in the future is, therefore, likely to be reliant on targeted therapies, carefully selected based on an individualized understanding of a patient's risk of progression and underlying pathophysiology. The potential role of biomarkers to facilitate personalization of prognostication and treatment of IgAN is immense. Here we review the progress made over the past decade in identifying and validating new biomarkers, with a particular focus on those that reflect immunological responses in IgAN.

Complement activation is associated with crescent formation in IgA nephropathy

IgA nephropathy (IgAN) is common chronic glomerulonephritis with variable prognosis, ranging from minor urinary abnormalities to end-stage renal disease. The revised Oxford classification of IgAN explains that cellular/fibrocellular crescents are associated with poor renal prognosis, proposing an extension to the MEST-C score. C3 immunofluorescent staining follows a distribution similar to IgA staining. Therefore, complement activation was reported to play a pivotal role in IgAN pathogenesis. This study included 132 IgAN patients diagnosed by renal biopsies. The clinical parameters at the time of the biopsies were obtained from patient data records. We classified the patients into C ≥ 1 and C0 groups, and compared clinical, light microscopic, and immunofluorescent features. In the C ≥ 1 group, 2 (1.5%) and 31 (23.5%) patients were assigned to C2 and C1, respectively. The remaining 99 patients (75%) were classified as C0. The C ≥ 1 group had lower average age and rate of hypertension, and higher score of urinary occult blood and E score. The C ≥ 1 group had significantly higher average immunofluorescence scores for IgA, C5b-9, mannose-associated serine protease (MASP) 1/3, MASP2, properdin, factor B, and kappa. The steroid use rate was significantly higher in the C ≥ 1 group. During the follow-up period of 2.90 years on average, the rate of renal dysfunction was not significantly different between groups. Crescent formation in IgAN was associated with activation of the lectin and alternative pathways. The C ≥ 1 group had significantly increased use of steroids, which probably caused comparable renal function during the follow-up period.

The Role of Immune Modulation in Pathogenesis of IgA Nephropathy

IgA nephropathy (IgAN) is the most prevalent primary glomerulonephritis worldwide, with diverse clinical manifestations characterized by recurrent gross hematuria or microscopic hematuria, and pathological changes featuring poorly O-galactosylated IgA1 deposition in the glomerular mesangium. Pathogenesis has always been the focus of IgAN studies. After 50 years of research, most scholars agree that IgAN is a group of clinicopathological syndromes with certain common immunopathological characteristics, and multiple mechanisms are involved in its pathogenesis, including immunology, genetics, and environmental or nutritional factors. However, the precise pathogenetic mechanisms have not been fully determined. One hypothesis about the pathogenesis of IgAN suggests that immunological factors are engaged in all aspects of IgAN development and play a critical role. A variety of immune cells (e.g., dendritic cells, NK cells, macrophages, T-lymphocyte subsets, and B-lymphocytes, etc.) and molecules (e.g., IgA receptors, Toll-like receptors, complements, etc.) in innate and adaptive immunity are involved in the pathogenesis of IgAN. Moreover, the abnormality of mucosal immune regulation is the core of IgAN immunopathogenesis. The roles of tonsil immunity or intestinal mucosal immunity, which have received more attention in recent years, are supported by mounting evidence. In this review, we will explore the latest research insights on the role of immune modulation in the pathogenesis of IgAN. With a better understanding of immunopathogenesis of IgAN, emerging therapies will soon become realized.

Complement in IgA Nephropathy: The Role of Complement in the Pathogenesis, Diagnosis, and Future Management of IgA Nephropathy

Immunoglobulin A (IgA) nephropathy (IgAN) is an important cause of chronic and end-stage kidney disease. IgAN pathogenesis is incompletely understood. In particular, we cannot adequately explain the heterogeneity in clinical and histologic features and severities that characterizes IgAN. This limits patient stratification to appropriate and effective treatments and the development of disease-targeted therapies. Studies of the role of the alternative, lectin, and terminal complement pathways in IgAN have enhanced our understanding of disease pathogenesis and inform the development of novel diagnostic and therapeutic strategies. For example, recent genetic, serologic, and immunohistologic evidence suggests that imbalances between the main alternative complement pathway regulator protein (factor H) and competitor proteins that deregulate complement activity (factor H-related proteins 1 and 5, FHR1, and FHR5) associate with IgAN severity: a relative abundance of FHR1 and FHR5 amplifies complement-dependent inflammation and exacerbates kidney injury. Ongoing characterization of the mechanisms by which complement activity contributes to IgAN pathogenesis will facilitate the development of complement-based diagnostic techniques, biomarkers of disease activity and severity, and novel targeted therapies.

A Narrative Review on C3 Glomerulopathy: A Rare Renal Disease

In April 2012, a group of nephrologists organized a consensus conference in Cambridge (UK) on type II membranoproliferative glomerulonephritis and decided to use a new terminology, "C3 glomerulopathy" (C3 GP). Further knowledge on the complement system and on kidney biopsy contributed toward distinguishing this disease into three subgroups: dense deposit disease (DDD), C3 glomerulonephritis (C3 GN), and the CFHR5 nephropathy. The persistent presence of microhematuria with or without light or heavy proteinuria after an infection episode suggests the potential onset of C3 GP. These nephritides are characterized by abnormal activation of the complement alternative pathway, abnormal deposition of C3 in the glomeruli, and progression of renal damage to end-stage kidney disease. The diagnosis is based on studying the complement system, relative genetics, and kidney biopsies. The treatment gap derives from the absence of a robust understanding of their natural outcome. Therefore, a specific treatment for the different types of C3 GP has not been established. Recommendations have been obtained from case series and observational studies because no randomized clinical trials have been conducted. Current treatment is based on corticosteroids and antiproliferative drugs (cyclophosphamide, mycophenolate mofetil), monoclonal antibodies (rituximab) or complement inhibitors (eculizumab). In some cases, it is suggested to include sessions of plasma exchange.