Complement in ANCA-associated glomerulonephritis

Urinary complement biomarkers in immune-mediated kidney diseases

The complement system, an important part of the innate system, is known to play a central role in many immune mediated kidney diseases. All parts of the complement system including the classical, alternative, and mannose-binding lectin pathways have been implicated in complement-mediated kidney injury. Although complement components are thought to be mainly synthesized in the liver and activated in the circulation, emerging data suggest that complement is synthesized and activated inside the kidney leading to direct injury. Urinary complement biomarkers are likely a better reflection of inflammation within the kidneys as compared to traditional serum complement biomarkers which may be influenced by systemic inflammation. In addition, urinary complement biomarkers have the advantage of being non-invasive and easily accessible. With the rise of therapies targeting the complement pathways, there is a critical need to better understand the role of complement in kidney diseases and to develop reliable and non-invasive biomarkers to assess disease activity, predict treatment response and guide therapeutic interventions. In this review, we summarized the current knowledge on urinary complement biomarkers of kidney diseases due to immune complex deposition (lupus nephritis, primary membranous nephropathy, IgA nephropathy) and due to activation of the alternative pathway (C3 glomerulopathy, thrombotic microangiography, ANCA-associated vasculitis). We also address the limitations of current research and propose future directions for the discovery of urinary complement biomarkers.

Presentation and progression of MPO-ANCA interstitial lung disease

The association between MPO-ANCA-associated vasculitis (AAV) and interstitial lung disease (ILD) has been well established. Pulmonary fibrosis may coexist with, follow, or even precede the diagnosis of AAV, and its presence adversely affects the prognosis. The optimal approach to investigating ANCA in patients with ILD remains a subject of ongoing debate. Here we aim to describe presentation and progression of MPO-ANCA ILD. We conducted a retrospective evaluation of a cohort of individuals diagnosed with MPO-ANCA ILD, with or without accompanying renal impairment, at the Immunology and Cell Therapy Unit, Careggi University Hospital, Florence, Italy, between June 2016 and June 2022. Clinical records, imaging studies, pathologic examinations, and laboratory test results were collected. Among the 14 patients identified with MPO-ANCA ILD, we observed a significant association between MPO-ANCA titers assessed at the time of ILD diagnosis and renal involvement. Renal impairment in these cases often manifested as subclinical or slowly progressive kidney damage. Interestingly, complement C3 deposits were consistently found in all renal biopsy specimens, thereby suggesting the potential for novel therapeutic targets in managing renal complications associated with MPO-ANCA ILD. The presentation of MPO-ANCA vasculitis as ILD can be the first and only clinical manifestation. MPO-ANCA levels at ILD diagnosis could warn on the progression to renal involvement in patients with MPO-ANCA ILD, hence caution is needed because renal disease can be subclinical or smoldering.

Non-invasive biomarkers of disease activity and organ damage in ANCA-associated vasculitis: a systematic review

BACKGROUND

In anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV), histopathological assessment of affected tissue is often necessary for diagnosis and assessment of disease extent. There is a requirement for validated non-invasive biomarkers to avoid the need for serial tissue biopsies.

METHODS

A systematic review of scientific databases from 2012 until present was performed to identify studies fulfilling the inclusion criteria. Studies were assessed for quality using the Strengthening the Reporting of Observational Studies in Epidemiology checklist for cohort, case-control and cross-sectional studies and the Risk of Bias Assessment tool for Non-randomised Studies, or the Cochrane Risk of Bias tool 2.0 for randomised controlled trials. A descriptive synthesis of the data for non-invasive (blood-based or urinary) biomarkers of AAV-related disease activity and organ damage was performed.

RESULTS

Twenty-two high quality studies were included. These articles reported the value of blood-based and urinary biomarkers including anti-neutrophil cytoplasmic antibodies, immune cells, complement factors, gene expression profiles, cytokines, chemokines and other proteins in the assessment of disease activity and/or organ damage in patients with AAV. Many of these biomarkers involve the alternative complement pathway, neutrophil activation and macrophage activation.

CONCLUSION

This is the first contemporary systematic review synthesising the value of non-invasive biomarkers of AAV-related disease activity and organ damage. The incorporation of individual markers in combined biomarker profiles might enhance clinical decision-making. Many unmet needs were identified; few studies involve oeosinophilic granulomatosis with polyangiitis and patients with childhood-onset AAV. Further validation of the candidate biomarkers is warranted in large prospective studies to bridge the existing knowledge gaps and apply precision health to systemic vasculitis.

Are serum C3 levels or kidney C3 deposits useful markers for predicting outcomes in patients with ANCA-associated vasculitis?

Introduction

Complement activation emerged as a key actor of anti-neutrophil cytoplasmic antibodies-associated vasculitis (AAV). Whether serum levels of C3 (sC3) or C3 kidney deposition may help to refine the prognosis of AAV remains elusive.

Methods

Retrospective multicentric study that included 154 patients with a first flare of AAV and sC3 (n = 143) or C3 kidney staining (n = 95) available at diagnosis. Clinical presentations, kidney pathology, and survival of patients with normal or low sC3 were compared using univariate analyses, Kaplan-Maier curves with log-rank comparison, or multivariate Cox' model, as appropriate.

Results

20 patients (14 %) had low sC3. sC3 (as bivariate low/normal or as a continuous variable) was associated with 5-year mortality but not with kidney survival. C3 kidney deposition (C3+) was identified in 23 patients who were characterized by more frequent chronic hypertension and lower eGFR at presentation (p = 0.04). C3+ correlated with IgG, IgM, C1q deposition (p = 0.07, p < 0.0001 and p = 0.003, respectively). Chronicity and activity scores were similar in C3+ and C3- patients. Among C3+ patients, those with C3 deposition ≥2+ had lower eGFR at presentation (p = 0.006) and were more frequently classified as sclerotic using the Berden classification (p = 0.04) and as 'high risk' using the Brix score (p = 0.03). However, eGFR improvement following induction regimen was similar between C3+ and C3- patients, and kidney survival at 5 years was similar.

Conclusions

Correlation of sC3 with mortality confirms mechanistic links between complement pathways and AAV, but the lack of clear predictive sC3 cut-off and the similar kidney outcome irrespective of C3 deposition precludes their use as biomarkers of AAV outcomes and response to treatment.

Complement System and the Kidney: Its Role in Renal Diseases, Kidney Transplantation and Renal Cell Carcinoma

The crosstalk among the complement system, immune cells, and mediators of inflammation provides an efficient mechanism to protect the organism against infections and support the repair of damaged tissues. Alterations in this complex machinery play a role in the pathogenesis of different diseases. Core complement proteins C3 and C5, their activation fragments, their receptors, and their regulators have been shown to be active intracellularly as the complosome. The kidney is particularly vulnerable to complement-induced damage, and emerging findings have revealed the role of complement system dysregulation in a wide range of kidney disorders, including glomerulopathies and ischemia-reperfusion injury during kidney transplantation. Different studies have shown that activation of the complement system is an important component of tumorigenesis and its elements have been proved to be present in the TME of various human malignancies. The role of the complement system in renal cell carcinoma (RCC) has been recently explored. Clear cell and papillary RCC upregulate most of the complement genes relative to normal kidney tissue. The aim of this narrative review is to provide novel insights into the role of complement in kidney disorders.

Exosome Liberation by Human Neutrophils under L-Amino Acid Oxidase of Calloselasma rhodostoma Venom Action

L-Amino acid oxidase (LAAO) is an enzyme found in snake venom that has multifaceted effects, including the generation of hydrogen peroxide (H2O2) during oxidative reactions, leading to various biological and pharmacological outcomes such as apoptosis, cytotoxicity, modulation of platelet aggregation, hemorrhage, and neutrophil activation. Human neutrophils respond to LAAO by enhancing chemotaxis, and phagocytosis, and releasing reactive oxygen species (ROS) and pro-inflammatory mediators. Exosomes cellular nanovesicles play vital roles in intercellular communication, including immune responses. This study investigates the impact of Calloselasma rhodostoma snake venom-derived LAAO (Cr-LAAO) on human neutrophil exosome release, including activation patterns, exosome formation, and content. Neutrophils isolated from healthy donors were stimulated with Cr-LAAO (100 μg/mL) for 3 h, followed by exosome isolation and analysis. Results show that Cr-LAAO induces the release of exosomes with distinct protein content compared to the negative control. Proteomic analysis reveals proteins related to the regulation of immune responses and blood coagulation. This study uncovers Cr-LAAO's ability to activate human neutrophils, leading to exosome release and facilitating intercellular communication, offering insights into potential therapeutic approaches for inflammatory and immunological disorders.

Animal models for anti-neutrophil cytoplasmic antibody-associated vasculitis: Are current models good enough?

Antineutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV) is a rare and severe systemic autoimmune disease characterized by pauci-immune necrotizing inflammation of small blood vessels. AAV involves multiple organ systems throughout the body. Our knowledge of the pathogenesis of AAV has increased considerably in recent years, involving cellular, molecular and genetic factors. Because of the controlled environment with no other confounding factors, animal models are beneficial for studying the mechanistic details of disease development and for providing novel therapeutic targets with fewer toxic side effects. However, the complexity and heterogeneity of AAV make it very difficult to establish a single animal model that can fully represent the entire clinical spectrum found in patients. The aim of this review is to overview the current status of animal models for AAV, outline the pros and cons of methods, and propose potential directions for future research.

Avacopan for the treatment of ANCA-associated vasculitis: an update

INTRODUCTION

Glucocorticoids (GC) have been part of the standard treatment of anti-neutrophil cytoplasm autoantibodies (ANCA)-associated vasculitides (AAV) for more than 60 years. Various therapeutic advances have occurred over the past 2 decades and led to a significant reduction of GC exposure, but most patients still have to suffer from complications of GC, including infections, metabolic abnormalities, and cardiovascular morbidity. In 2007, activation of the complement pathway was demonstrated to play a role in the pathogenesis of AAV. Avacopan, an oral competitive inhibitor of the C5a receptor (C5aR1, CD88), was then developed, with an additional aim to decrease the use of GC.

AREAS COVERED

In this article, we briefly summarize the rationale for targeting the complement pathway in AAV, and review relevant findings from pre-clinical, phase I, II, and III studies, subsequent and more recent case reports and series on the efficacy and safety of avacopan.

EXPERT OPINION

Based on the results of these studies, avacopan was approved in most countries since late 2021, as an adjunctive induction treatment for patients with AAV. Several newer questions now are pending answers, including as to how avacopan should be used in real-world practice, beyond how it was given in the original clinical trials.

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.

The complement system in antineutrophil cytoplasmic antibody-associated vasculitis: pathogenic player and therapeutic target

PURPOSE OF REVIEW

The purpose of this review is to discuss the role of the complement system in the pathogenesis of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) as well as the employment of complement inhibitors in AAV treatment.

RECENT FINDINGS

AAV has traditionally been considered a pauci-immune disease until recent findings demonstrated the pathogenic role of the complement system. The complement alternative pathway is crucial in AAV, and C5a seems to be a key molecule for AAV to develop. Avacopan, a C5a-receptor (C5aR) antagonist, proved effective in achieving AAV remission and ameliorating kidney function.

SUMMARY

The increased circulating levels of some complement components - as well as the consumption of others - in patients with AAV suggested a systemic activation of the complement system. Low C3 levels correlate with a more aggressive disease and a worse renal prognosis. In ANCA-associated glomerulonephritis, renal deposits of C3d and properdin, suggestive of local alternative pathway activation, correlate with glomerular crescents and proteinuria. The interaction between C5a and neutrophil triggers alternative pathway activation, suggesting the central role of C5a in AAV pathogenesis. Avacopan, a C5aR inhibitor, showed beneficial effects in AAV and represents a promising therapy to achieve sustained remission and to spare glucocorticoids.

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.

Novel aspects in the pathophysiology and diagnosis of glomerular diseases

Immune deposits/complexes are detected in a multitude of tissues in autoimmune disorders, but no organ has attracted as much attention as the kidney. Several kidney diseases are characterised by the presence of specific configurations of such deposits, and many of them are under a 'shared care' between rheumatologists and nephrologists. This review focuses on five different diseases commonly encountered in rheumatological and nephrological practice, namely IgA vasculitis, lupus nephritis, cryoglobulinaemia, anti-glomerular basement membrane disease and anti-neutrophil cytoplasm-antibody glomerulonephritis. They differ in disease aetiopathogenesis, but also the potential speed of kidney function decline, the responsiveness to immunosuppression/immunomodulation and the deposition of immune deposits/complexes. To date, it remains unclear if deposits are causing a specific disease or aim to abrogate inflammatory cascades responsible for tissue damage, such as neutrophil extracellular traps or the complement system. In principle, immunosuppressive therapies have not been developed to tackle immune deposits/complexes, and repeated kidney biopsy studies found persistence of deposits despite reduction of active inflammation, again highlighting the uncertainty about their involvement in tissue damage. In these studies, a progression of active lesions to chronic changes such as glomerulosclerosis was frequently reported. Novel therapeutic approaches aim to mitigate these changes more efficiently and rapidly. Several new agents, such as avacopan, an oral C5aR1 inhibitor, or imlifidase, that dissolves IgG within minutes, are more specifically reducing inflammatory cascades in the kidney and repeat tissue sampling might help to understand their impact on immune cell deposition and finally kidney function recovery and potential impact of immune complexes/deposits.

Role of complement activation in anti-neutrophil cytoplasmic antibody-associated glomerulonephritis

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is an autoimmune disease characterized by necrotizing inflammation of small or medium vessels, causing ANCA associated glomerulonephritis (AAGN). AAGN is defined as pauci-immune glomerulonephritis with no or little immune deposition; hence, activation of the complement system in AAV was overlooked until recently. However, many studies in mice and humans have revealed a crucial role for complement system activation in the development of AAGN. Circulating and urinary detection of various complement components associated with AP activation, which have been broadly correlated with the clinical activity of AAGN, has been reported and may be useful for predicting renal outcome at the time of diagnosis and setting up personalized treatments. Moreover, recent investigations have suggested the possible contribution of the complement classical or lectin pathway activation in the development of AAGN. Thus, as therapeutic options targeting complement components are making rapid strides, the primary complement pathway involved in AAGN disease progression remains to be elucidated: this will directly impact the development of novel therapeutic strategies with high specificity and reduced side effects. This review summarizes and discusses the most recent evidence on the crucial roles of the complement system in the development of AAGN and possible therapeutic strategies that target complement components for disease management.

Old known and possible new biomarkers of ANCA-associated vasculitis

Antineutrophil cytoplasm antibody (ANCA)-associated vasculitis (AAV) comprises a group of multisystem disorders involving severe, systemic, small-vessel vasculitis with short- and long term serious and life-threating complications. Despite the simplification of treatment, fundamental aspects concerning assessment of its efficacy and its adaptation to encountered complications or to the relapsing/remitting/subclinical disease course remain still unknown. The pathogenesis of AAV is complex and unique, and despite the progress achieved in the last years, much has not to be learnt. Foremost, there is still no accurate marker enabling us to monitoring disease and guide therapy. Therefore, the disease management relays often on clinical judgment and follows a" trial and error approach". In the recent years, an increasing number of new molecules s have been explored and used for this purpose including genomics, B- and T-cell subpopulations, complement system factors, cytokines, metabolomics, biospectroscopy and components of our microbiome. The aim of this review is to discuss both the role of known historical and clinically established biomarkers of AAV, as well as to highlight potential new ones, which could be used for timely diagnosis and monitoring of this devastating disease, with the goal to improve the effectiveness and ameliorate the complications of its demanding therapy.

Intrarenal synthesis of complement C3 localized to distinct vascular compartments in ANCA-associated renal vasculitis

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a small vessel vasculitis affecting multiple organ systems, including the kidney. The activation of the complement system contributes essentially to its pathogenesis by autoantibody-antigen recognition directed against host cells in ANCA-associated renal vasculitis. We herein provide evidence for intrarenal synthesis of complement C3 localized to distinct vascular compartments in ANCA-associated renal vasculitis that associated with distinct inflammatory signaling pathways. Therefore, a total number of 43 kidney biopsies with ANCA-associated renal vasculitis were retrospectively included and evaluated for presence/absence of C3 deposits localized to distinct vascular compartments in association with clinicopathological biopsy findings. In addition, intrarenal C3 mRNA expression levels specifically from microdissected tubulointerstitial and glomerular compartments were extracted from transcriptome datasets. C3 deposits were present in the glomerular tuft, interlobular arteries, peritubular capillaries, and venules in ANCA-associated renal vasculitis. Most C3 deposits are localized to the glomerular tuft overlapping with peritubular capillaries. The presence of C3 deposits in the glomerular tuft correlated with impaired kidney function and overall short-term survival. Intrarenal complement C3 deposits were not associated with consumption of respective serum levels, supporting the concept of intrarenal C3 synthesis. Finally, intrarenal synthesis of complement C3 was linked to distinct inflammatory signaling pathways in the kidney that is especially relevant in ANCA-associated renal vasculitis. Considering recent advances in AAV therapy with the emergence of new therapeutics that inhibit complement activation, we here provide novel insights into intrarenal complement synthesis and associated inflammatory signaling pathways in ANCA-associated renal vasculitis.

What is the prognosis of ANCA-associated glomerulonephritis with immune deposition?

Objectives

This study aimed to analyze histological and clinical characteristics of patients with antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) showing renal involvement to investigate the associations between immune complexes (IC) and clinicopathological indicators, and explore the renal outcomes of AAV.

Methods

We retrospectively evaluated the histopathological features and clinical characteristics of 80 renal biopsies of patients with AAV with renal involvement. Renal morphology was classified into two (with and without the presence of IC and complement deposition). Endpoints included end-stage kidney disease (ESKD) and death.

Results

Compared with patients without IC, patients with immune deposition had lower complement C3 (0.80 ± 0.27 vs. 0.93 ± 0.20, p = 0.024), more severe hematuria [133 (46–299) vs. 33 (15–115), p = 0.001] but had milder chronic pathology, including chronic tubular atrophy (p = 0.03), chronic interstitial fibrosis (p = 0.049). Patients in the immune deposition group showed a tendency to have more severe crescent formation and less glomerulosclerosis, but the difference was not statistically significant. Endpoints such as death and ESKD were not significantly different between the two groups.

Conclusions

Immune deposition may indicate lower complement C3, more severe hematuria and glomerular lesions, milder tubular atrophy, and interstitial fibrosis, but it cannot predict the renal outcome.

Relevance of Complement C4 Deposits Localized to Distinct Vascular Compartments in ANCA-Associated Renal Vasculitis

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a small-vessel vasculitis affecting multiple organ systems, including the kidney. Small vessels in the kidney include small-sized arteries, capillaries, and venules. Intrarenal C4 deposits are now increasingly recognized as a potential marker and pathogenic mechanism of autoantibody-mediated tissue damage in ANCA-associated renal vasculitis. We here describe the relevance of complement C4 deposits localized to distinct vascular compartments in a cohort of biopsy-proven ANCA-associated renal vasculitis. A cohort of 43 biopsy-proven cases of ANCA-associated renal vasculitis with myeloperoxidase (MPO) or proteinase 3 (PR3) seropositivity were retrospectively enrolled in a single-center observational study. Univariate and multivariate regression analysis was performed to identify parameters associated with intrarenal C4 deposits in ANCA-associated renal vasculitis. We here show that C4 deposits localize to distinct vascular compartments in ANCA-associated renal vasculitis, and provide evidence for an association with better short-term survival (p = 0.008), implicating that this subgroup had a superior response to remission induction therapy. Second, C4 deposits in interlobular arteries were associated with eosinophilic infiltrates in renal vasculitis with MPO-ANCA seropositivity (p = 0.021). In renal vasculitis positive for MPO-ANCA, the absence of C4 deposits in the glomerular tuft was associated with sclerotic class ANCA-associated renal vasculitis (p < 0.001), and tubular RBC casts (p = 0.024). Fourth, complement C4 in interlobular arteries is associated with tubular atrophy specifically in renal vasculitis with PR3-ANCA seropositivity (p = 0.006). Finally, complement C4 deposits in peritubular capillaries associated specifically with hyaline casts in cases positive for PR3-ANCA (p = 0.025), implicating a role in tubular injury. Interestingly, C4 deposits were localized to distinct vascular compartments independent of the systemic activation of the complement system, reflected by the consumption of respective serum complement molecules in ANCA-associated renal vasculitis. In summary, we here show that C4 deposits localize to distinct vascular compartments in ANCA-associated renal vasculitis, and provide evidence for an association with survival and distinct histopathological lesions. Considering recent advances in AAV therapy with the emergence of new therapeutics that inhibit complement activation, we here provide novel insights into complement C4 as a potential marker to identify patients who may benefit most from these drugs. Thus, our results may contribute to a more personalized treatment approach of AAV depending on the relevance of distinct intrarenal complement deposits.

Clinical features and prognosis of MPO-ANCA and anti-GBM double-seropositive patients

BACKGROUND

Several lines of evidence implicate that there are distinct differences between patients with myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA) and anti-glomerular basement membrane (GBM) antibody double-seropositive patients (DPPs) and single-positive patients. Hence, we conducted a retrospective study from a single center in China to analyze the clinical and pathological features, and prognosis of DPPs.

METHODS

109 patients with MPO-ANCA-associated vasculitis (MPO-AAV), 20 DPPs and 23 patients diagnosed with anti-GBM disease from a large center in China were included in this study. The ratio of patients with renal biopsy in three groups were 100%, 50% and 100%, respectively. Their clinical and pathological characteristics, and outcomes were analyzed. The intensity of immune deposits in the kidney at diagnosis was detected by immunofluorescence (IF). Furthermore, multivariate Cox hazard model analysis was used to assess the clinical and histological predictors of end-stage renal disease (ESRD) and death for DPPs.

RESULTS

In our study, we found that patients in the DPPs group were older than the other two groups (p = 0.007, MPO-AAV vs. DPPs; p < 0.001, DPPs vs. anti-GBM). The DPPs group had a higher value of serum creatinine (p = 0.041) and lower estimated glomerular filtration rate (eGFR) (p = 0.032) compared with MPO-AAV patients. On the contrary, the DPPs group had a lower serum creatinine (p = 0.003) compared with patients with anti-GBM group. The proportion of patients with cardiac system involvement in the DPPs group was higher than anti-GBM patients (p = 0.014). Cellular crescents could be generally observed in renal biopsy of DPPs and patients with anti-GBM glomerulonephritis. In addition, Bowman's capsule rupture was more common in DPPs than MPO-AAV patients (p = 0.001). MPO-AAV had a better renal and overall survival outcome than DPPs (p < 0.001). There was no significant difference of renal and overall survival outcome between DPPs and patients with anti-GBM disease. The incidence of ESRD in DPPs was negatively associated with lymphocyte count (HR 0.153, 95% CI 0.027 to 0.872, p = 0.034) and eGFR (HR 0.847, 95% CI 0.726 to 0.989, p = 0.036). Elevated serum creatinine was confirmed as a risk factor of both renal (HR 1.003, 95% CI 1.000 to 1.005, p = 0.019) and patient survival in DPPs (HR1.461, 95% CI 1.050 to 2.033, p = 0.024).

CONCLUSION

In summary, compared with anti-GBM disease, DPPs tended to involve multi-organ damage rather than limited to the kidney. It is highlighted that serologic DPPs have a worse renal and patient prognosis than MPO-AAV. Moreover, we found that the risk factors of renal survival of DPPs include low lymphocyte count, elevated serum creatinine and reduced eGFR, and serum creatinine can predict patient survival.

The Potential Applications of Raman Spectroscopy in Kidney Diseases

Raman spectroscopy (RS) is a spectroscopic technique based on the inelastic interaction of incident electromagnetic radiation (from a laser beam) with a polarizable molecule, which, when scattered, carries information from molecular vibrational energy (the Raman effect). RS detects biochemical changes in biological samples at the molecular level, making it an effective analytical technique for disease diagnosis and prognosis. It outperforms conventional sample preservation techniques by requiring no chemical reagents, reducing analysis time even at low concentrations, and working in the presence of interfering agents or solvents. Because routinely utilized biomarkers for kidney disease have limitations, there is considerable interest in the potential use of RS. RS may identify and quantify urinary and blood biochemical components, with results comparable to reference methods in nephrology.

Association between kinetic of Anti-Neutrophil Cytoplasmic Antibody (ANCA), renal survival and relapse risk in ANCA-glomerulonephritis

BACKGROUND

Anti-neutrophil cytoplasmic antibody (ANCA) kinetic in ANCA-associated vasculitis with glomerulonephritis (AAV-GN) has been suggested to be associated with AAV relapse. Few studies focused on its association with renal prognosis. Thus, we aimed to investigate the relationship between i) ANCA specificity and evolutive profile, and ii) renal outcomes.

METHODS

This multicentric retrospective study included patients diagnosed with ANCA-GN since 01/01/2000. Patients without ANCA at diagnosis and with fewer than 3 ANCA determinations during follow-up were excluded. We analyzed eGFR variation, renal-free and relapse-free survival according to three ANCA profiles (negative, recurrent, persistent) and to ANCA specificity (MPO or PR3).

RESULTS

Over a follow-up of 56 [34-101] months, a median of 19 [13-25] ANCA determinations were performed for the 134 included patients. Patients with a recurrent/persistent ANCA profile had a lower relapse-free (p = 0.019) survival and tended to have a lower renal survival (p = 0.053) compared to those with negative ANCA profile. Patients with a recurrent/persistent MPO-ANCA profile had the shortest renal survival (p = 0.015) and those with recurrent/persistent PR3-ANCA profile had the worst relapse-free survival (p = 0.013) compared to other profiles. The negative ANCA profile was associated with a greater eGFR recovery. In multivariate regression analysis, it was an independent predictor of a two-fold increase of eGFR at 2-year (OR = 6.79, 95% CI [1.78, 31.4], p = 0.008).

CONCLUSION

ANCA kinetic after ANCA-GN diagnosis is associated with outcomes. MPO-ANCA recurrence/persistence identifies patients with a lower potential of renal recovery and a higher risk of kidney failure, while PR3-ANCA recurrence/persistence identifies patients with a greater relapse risk. Thus, ANCA kinetic may help identify patients with a smoldering disease.

Complement Inhibition in ANCA-Associated Vasculitis

Efficacy of immunosuppressive treatment of Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is complicated by its toxicity. With the replacement of cyclophosphamide with rituximab, serious adverse events seem to be associated especially with high-dose corticosteroids. Activation of alternative complement pathway plays an important role in the pathogenesis of AAV. Avacopan (C5a receptor inhibitor) was demonstrated to have at least similar efficacy and better safety (in terms of corticosteroid-related adverse events) compared with high-dose corticosteroids in the induction treatment of AAV. Other modes of the inhibition of alternative complement pathway are currently tested in AAV or could be considered on the basis of the experience in other glomerular diseases.

The Complement System and ANCA Associated Vasculitis in the Era of Anti-Complement Drugs

ANCA (anti-neutrophil cytoplasmic antibody)-associated vasculitis (AAV) is the condition in which ANCA, as an autoantibody, is associated with the pathogenesis of vasculitis in small blood vessels, mainly in the ear, nose, throat, kidney, lung, and nerves. These diseases are important because they can be fatal due to renal failure and pulmonary hemorrhage if not promptly and appropriately treated. Recently accumulated evidence has shown that C5a produced by the complement alternative pathway primes neutrophils, which in turn activate the complement alternative pathway, leading to the pathogenesis of AAV. Avacopan (CCX168), a C5aR antagonist was shown to be effective against AAV, and it has been a novel therapeutic option, becoming a novel anti-complement drug to modulate inflammatory diseases.

Early ultrastructural lesions of anti-neutrophil cytoplasmic antibody- versus complement-associated vasculitis

This study aims to describe electron microscopic findings of vasculitis associated with anti-neutrophil cytoplasmic antibody (ANCA) and complement. Sural nerve biopsy specimens were obtained from 10 patients with microscopic polyangiitis (MPA), a representative ANCA-associated vasculitis, and six patients with nonsystemic vasculitic neuropathy (NSVN), who were negative for ANCA but positive for complement deposition. In patients with MPA, attachment of neutrophils to epineurial vascular endothelial cells, migration of neutrophils to the extravascular space via the penetration of the endothelial layer, and release of neutrophil components to the extracellular space were observed. Such neutrophil-associated lesions were not observed in patients with NSVN. Nonetheless, morphological changes in epineurial vascular endothelial cells, such as increases in cytoplasmic organelles and cytoplasmic protrusions into the vascular lumen, were observed in patients with NSVN. Since these findings were observed where light microscopy-based findings suggestive of vasculitis (e.g., the disruption of vascular structures and fibrinoid necrosis) were absent, they were considered early lesions that preceded the formation of the so-called necrotizing vasculitis. In conclusion, this study enabled the visualization of distinctive early ultrastructural lesions associated with ANCA and complement. Further studies are needed to elucidate the molecular basis of the induction of these fine structural changes, which will contribute to the development of targeted therapies based on specific mechanisms of vasculitis.

Pathogenic T-Cell Responses in Immune-Mediated Glomerulonephritis

Glomerulonephritis (GN) comprises a group of immune-mediated kidney diseases affecting glomeruli and the tubulointerstitium. Glomerular crescent formation is a histopathological characteristic of severe forms of GN, also referred to as crescentic GN (cGN). Based on histological findings, cGN includes anti-neutrophil cytoplasmic antibody (ANCA)-associated GN, a severe form of ANCA-associated vasculitis, lupus nephritis associated with systemic lupus erythematosus, Goodpasture’s disease, and IgA nephropathy. The immunopathogenesis of cGN is associated with activation of CD4+ and CD8+ T cells, which particularly accumulate in the periglomerular and tubulointerstitial space but also infiltrate glomeruli. Clinical observations and functional studies in pre-clinical animal models provide evidence for a pathogenic role of Th1 and Th17 cell-mediated immune responses in cGN. Emerging evidence further argues that CD8+ T cells have a role in disease pathology and the mechanisms of activation and function of recently identified tissue-resident CD4+ and CD8+ T cells in cGN are currently under investigation. This review summarizes the mechanisms of pathogenic T-cell responses leading to glomerular damage and renal inflammation in cGN. Advanced knowledge of the underlying immune mechanisms involved with cGN will enable the identification of novel therapeutic targets for the replacement or reduction in standard immunosuppressive therapy or the treatment of refractory disease.

Digital Spatial Profiling of Individual Glomeruli From Patients With Anti-Neutrophil Cytoplasmic Autoantibody-Associated Glomerulonephritis

We previously showed that the rupture of Bowman’s capsule (BC) promotes the progression of crescentic glomerulonephritis by enhancing the entry of CD8⁺ T cells into the glomeruli. In the present study, we utilized digital spatial profiling to simultaneously profile the altered abundances of the messenger RNA (mRNA) transcripts and proteins in the glomerular and periglomerular areas of four biopsy samples of anti-neutrophil cytoplasmic autoantibody-associated glomerulonephritis (ANCA-GN) and two biopsy specimens of minimal change disease (MCD) controls. The paraffin-embedded biopsy samples were stained with collagen IV, CD45, and SYTO 13 to distinguish the glomeruli with periglomerular infiltration but intact BC, with focal BC rupture, and with extensive rupture of BC and glomeruli without crescent formation and leukocytic infiltration in ANCA-GN. By assessing multiple discrete glomerular areas, we found that the transcript expression levels of the secreted phosphoprotein-1 and its receptor CD44 were upregulated significantly in the glomeruli with more severe ruptures of BC, and their expression levels correlated positively with the fibrotic markers. We also found that both alternative and classic complement pathways were activated in the glomeruli from patients with ANCA-GN. Furthermore, M1 macrophages were involved mostly in the early stage of BC rupture, while M2 macrophages were involved in the late stage and may contribute to the fibrosis process of the crescents. Finally, loss of glomerular cells in ANCA-GN was likely mediated by apoptosis. Our results show that digital spatial profiling allows the comparative analysis of the mRNA and protein profiles in individual glomeruli affected differently by the disease process and the identification of potential novel mechanisms in ANCA-GN.

Circulating immune-complexes and complement activation through the classical pathway in myeloperoxidase-ANCA-associated glomerulonephritis

Background

Anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (AAGN) is the fulminant glomerular diseases with poor renal prognosis. Activation of the complement system has recently been reported in the pathogenesis of AAGN, but it remains to be clarified as to which complement pathway is mainly involved.

Methods

20 patients with myeloperoxidase (MPO)-AAGN were retrospectively evaluated. Using serum samples, circulating immune-complexes (CICs) were assessed by the monoclonal rheumatoid factor assay, and C5a and C5b-9 were assessed by ELISA. Complement activation through the classical pathway was further evaluated by the WIESLAB® Complement System Classical Pathway kit. The affinities of ANCAs were evaluated by a competitive inhibition method using ELISA, and were classified into the high, and low-affinity group. Deposition of complement components, such as C3, C5, C4d, C5b-9, factor Bb, mannan-binding lectin serine peptidase (MASP)-1, MASP-2, and mannose/mannan-binding lectin (MBL), in frozen renal sections were analyzed by immunofluorescence staining.

Results

CICs were found to be positive in 65% of the patients. All CIC-positive patients belonged to the high-affinity group. Furthermore, serum C5a and C5b-9 were significantly increased in MPO-AAGN patients, and these levels positively correlated with CIC levels. A significant negative correlation was also found between levels of WIESLAB® classical pathway kit and CICs. By immunofluorescence staining, glomerular deposition of C4d, C5, and C5b-9 were observed in similar distributions in MPO-AAGN patients, whereas the deposition of MASP-1, MASP-2, MBL, and factor Bb were minimal.

Conclusions

These results suggest the involvement of immune-complex induced complement activation through the classical pathway in the pathogenesis of MPO-AAGN.

A case of smoldering antineutrophil cytoplasmic antibody-associated vasculitis development during the course of primary Sjögren's syndrome

Various forms of glomerular lesions have been described in primary Sjögren's syndrome (pSjS); however, myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis is rarely reported, and the disease onset and clinical course of ANCA-associated vasculitis (AAV) complicated by pSjS are not well understood. A 51-year-old woman was referred to our hospital because of mild proteinuria and microscopic hematuria. She fulfilled the classification criteria for pSjS. We performed a kidney biopsy; however, it revealed no characteristic findings for pSjS, vasculitis, or other autoimmune diseases, including systemic lupus erythematosus. After 9 months, urinalysis abnormalities worsened and renal function was slowly declining, and ANCA was found to be positive. A second kidney biopsy was performed, revealing MPO-ANCA-associated pauci-immune segmental necrotizing glomerulonephritis with crescent formation. Even though immunofluorescence microscopy did not reveal any positive findings, additional electron microscopy demonstrated the presence of mesangial electron-dense deposits in both kidney biopsies. Based on kidney biopsy results and sequential serum ANCA measurements, we considered that smoldering ANCA-associated vasculitis had developed in this patient as this can develop during the clinical course of pSjS. She responded well to steroid therapy. Serum measurement, especially perinuclear, ANCA levels can be useful in patients with pSjS to detect the onset of ANCA-associated vasculitis, even in the absence of acute renal deterioration or severe urinary abnormalities.

Automated Computational Detection of Disease Activity in ANCA-Associated Glomerulonephritis Using Raman Spectroscopy: A Pilot Study

Biospectroscopy offers the ability to simultaneously identify key biochemical changes in tissue associated with a given pathological state to facilitate biomarker extraction and automated detection of key lesions. Herein, we evaluated the application of machine learning in conjunction with Raman spectroscopy as an innovative low-cost technique for the automated computational detection of disease activity in anti-neutrophil cytoplasmic autoantibody (ANCA)-associated glomerulonephritis (AAGN). Consecutive patients with active AAGN and those in disease remission were recruited from a single UK centre. In those with active disease, renal biopsy samples were collected together with a paired urine sample. Urine samples were collected immediately prior to biopsy. Amongst those in remission at the time of recruitment, archived renal tissue samples representative of biopsies taken during an active disease period were obtained. In total, twenty-eight tissue samples were included in the analysis. Following supervised classification according to recorded histological data, spectral data from unstained tissue samples were able to discriminate disease activity with a high degree of accuracy on blind predictive modelling: F-score 95% for >25% interstitial fibrosis and tubular atrophy (sensitivity 100%, specificity 90%, area under ROC 0.98), 100% for necrotising glomerular lesions (sensitivity 100%, specificity 100%, area under ROC 1) and 100% for interstitial infiltrate (sensitivity 100%, specificity 100%, area under ROC 0.97). Corresponding spectrochemical changes in paired urine samples were limited. Future larger study is required, inclusive of assigned variables according to novel non-invasive biomarkers as well as the application of forward feature extraction algorithms to predict clinical outcomes based on spectral features.

Beyond Systemic Lupus Erythematosus and Anti-Phospholipid Syndrome: The Relevance of Complement From Pathogenesis to Pregnancy Outcome in Other Systemic Rheumatologic Diseases

Evidence about the relevance of the complement system, a highly conserved constituent of the innate immunity response that orchestrates the elimination of pathogens and the inflammatory processes, has been recently accumulated in many different rheumatologic conditions. In rheumatoid arthritis, complement, mainly the classical pathway, contributes to tissue damage especially in seropositive subjects, with complement activation occurring in the joint. Data about complement pathways in psoriatic arthritis are dated and poorly consistent; among patients with Sjögren syndrome, hypocomplementemia exerts a prognostic role, identifying patients at risk of extra-glandular manifestations. Hints about complement involvement in systemic sclerosis have been recently raised, following the evidence of complement deposition in affected skin and in renal samples from patients with scleroderma renal crisis. In vasculitides, complement plays a dual role: on one hand, stimulation of neutrophils with anti-neutrophil cytoplasmic antibodies (ANCA) results in the activation of the alternative pathway, on the other, C5a induces translocation of ANCA antigens, favouring the detrimental role of antibodies. Complement deposition in the kidneys identifies patients with more aggressive renal disease; patients with active disease display low serum levels of C3 and C4. Even though in dermatomyositis sC5b-9 deposits are invariably present in affected muscles, data on C3 and C4 fluctuation during disease course are scarce. C3 and C1q serum levels have been explored as potential markers of disease activity in Takayasu arteritis, whereas data in Behçet disease are limited to in vitro observations. Pregnancies in women with rheumatologic conditions are still burdened by a higher rate of pregnancy complications, thus the early identification of women at risk would be invaluable. A fine-tuning of complement activation is required from a physiological progression of pregnancy, from pre-implantation stages, through placentation to labour. Complement deregulation has been implicated in several pregnancy complications, such as recurrent abortion, eclampsia and premature birth; low complement levels have been shown to reliably identify women at risk of complications. Given its physiologic role in orchestrating pregnancy progression and its involvement as pathogenic effector in several rheumatologic conditions, complement system is an attractive candidate biomarker to stratify the obstetric risk among women with rheumatologic conditions.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

Comparison of Complement Pathway Activation in Autoimmune Glomerulonephritis

Introduction

Methods

Results

Conclusion

ANCA-Associated Vasculitic Neuropathies: A Review

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis is a systemic disorder that frequently affects the peripheral nervous system and consists of three distinct conditions: microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA, previously Wegener’s granulomatosis), and eosinophilic granulomatosis with polyangiitis (EGPA, previously Churg-Strauss syndrome). The neuropathic features associated with this condition usually include mononeuritis multiplex, which reflects the locality of lesions. Findings suggestive of vasculitis are usually found in the epineurium and occur diffusely throughout the nerve trunk. Nerve fiber degeneration resulting from ischemia is sometimes focal or asymmetric and tends to become conspicuous at the middle portion of the nerve trunk. The attachment of neutrophils to endothelial cells in the epineurial vessels is frequently observed in patients with ANCA-associated vasculitis; neutrophils play an important role in vascular inflammation by binding of ANCA. The positivity rate of ANCA in EGPA is lower than that in MPA and GPA, and intravascular and tissue eosinophils appear to participate in neuropathy. Immunotherapy for ANCA-associated vasculitis involves the induction and maintenance of remission to prevent the relapse of the disease. A combination of glucocorticoids along with cyclophosphamide, rituximab, methotrexate, or mycophenolate mofetil is considered depending on the severity of the condition of the organ to induce remission. A combination of low-dose glucocorticoids and azathioprine, rituximab, methotrexate, or mycophenolate mofetil is recommended to maintain remission. The efficacy of anti-interleukin-5 therapy (i.e., mepolizumab) was demonstrated in the case of refractory or relapsing EGPA. Several other new agents, including avacopan, vilobelimab, and abatacept, are under development for the treatment of ANCA-associated vasculitis. Multidisciplinary approaches are required for the diagnosis and management of the disorder because of its systemic nature. Furthermore, active participation of neurologists is required because the associated neuropathic symptoms can significantly disrupt the day-to-day functioning and quality of life of patients with ANCA-associated vasculitis.

Pathogenesis-based new perspectives of management of ANCA-associated vasculitis

While the pathogenesis of anti-neutrophil cytoplasmic autoantibody associated vasculitis (AAV) is still not fully elucidated, there is a mounting evidence that it is initiated by inflammation and activation of innate immunity in the presence of predisposing factors, innate immunity, aberrant responses of the adaptive immune system, and complement system activation. Biologics targeting inflammation-related molecules in the immune system have been explored to treat AVV, and these treatments have provided revolutionary advances. When focusing on the pathogenic mechanisms of AVV, this review presents the new findings regarding novel therapeutic approaches for the management of these conditions.

Predictors of renal and patient outcomes in anti-neutrophil cytoplasmic antibody-associated vasculitis: Our single-center, tertiary care experience

Objectives

This study aims to assess the different predictors of renal and patient prognosis in anti-neutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) patients with and without renal involvement.

Patients and methods

A total of 79 patients (51 males, 28 females; mean age: 57.3±14.2 years; range, 18 to 71 years) with AAV between January 2006 and November 2019 were retrospectively analyzed. Demographic and laboratory data including the complement 3 (C3) serum levels and renal biopsy findings were extracted from the electronic and printed medical records of the hospital registry. Survival, renal survival, remission, and relapse outcomes were analyzed.

Results

A total of 35% of the patients with renal involvement progressed to end-stage renal disease (ESRD). The dialysis requirement at the time of admission (hazard ratio [HR]: 21.95 [2.93-164.22]; p=0.003), estimated glomerular filtration rate (eGFR) (HR: 0.97 [0.94-0.99]; p=0.024) and Five-Factor Score (FFS) ≥2 at the time of diagnosis (HR: 3.59 [1.08-11.94]; p=0.037) were the predictors of ESRD. The five-year patient survival rate was 87.1%. The only predictor of mortality was age (HR: 1.07 [1.01-1.14]; p=0.024). The patients with hypocomplementemia (22%) had a lower remission rate (p=0.049), FFS ≥2 at the time of diagnosis (p=0.026), and higher levels of hematuria (p=0.004) and proteinuria (p=0.037). The FFS ≥2 at the time of diagnosis was an independent predictor of relapse (HR: 8.9 [1.02-77.36]; p=0.047).

Conclusion

Our study suggests that the baseline renal function and FFS ≥2 at the time of diagnosis are the major prognostic factors for progression to ESRD in AAV patients. In addition, AAV patients with hypocomplementemia may have a lower remission rate.

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

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

Long-Term Renal Survival in Antineutrophil Cytoplasmic Antibody-Associated Glomerulonephritis With Complement C3 Deposition

Introduction

Recent studies have revealed the pivotal role of complement activation in the pathogenesis of antineutrophil cytoplasmic antibody–associated glomerulonephritis (ANCA-GN). This study investigated the clinicopathologic and prognostic significance of glomerular C3 deposition in the renal histopathology of patients with ANCA-GN.

Methods

We retrospectively identified 142 patients with ANCA-GN from 6 hospitals in Japan (2004–2020). C3 deposition was defined as C3 staining ≥1+ on a scale of 0 to 2+ using direct immunofluorescence (IF). The primary composite end points included a 30% reduction in estimated glomerular filtration rate (eGFR), end-stage kidney disease (ESKD), and death. We compared clinicopathologic features and long-term outcomes between patients with and without C3 deposition.

Results

C3 deposition was observed in 56 of 142 kidney biopsy samples (39.4%). Patients with C3 deposition had a lower serum C3 level (P = 0.002). During a median follow-up of 2.9 (interquartile range: 0.2–5.7) years, 69 events occurred and the cumulative event-free survival rate at 5 years was significantly lower in the C3-positive group than in the C3-negative group (log-rank: P = 0.002). In multivariable analysis, C3 deposition was significantly associated with the composite end points after adjusting for age, sex, baseline eGFR, serum C3 level, treatment, and the percentage of normal glomerulus, cellular crescents, global sclerosis, and interstitial damage (adjusted hazard ratio [HR] = 2.02, 95% confidence interval: 1.20–3.40, P = 0.008).

Conclusion

This study revealed that ANCA-GN patients with glomerular C3 deposition on IF had worse renal and overall survival rates.

Hypocomplementemia at Diagnosis of Pauci-immune Glomerulonephritis Is Associated With Advanced Histopathological Activity Index and High Probability of Treatment Resistance

Introduction

Recent evidence suggests that complement activation is important in the pathogenesis of pauci-immune (PI) vasculitis. This is a retrospective investigation of the frequency of hypocomplementemia at pauci-immune glomerulonephritis (PIGN) diagnosis, in relation to vasculitic manifestations, renal histopathology, and treatment outcomes.

Methods

A total of 115 patients with biopsy-proven PIGN were categorized based on their serum complement C3 (sC3). Histopathology evaluation included activity and chronicity indexes. The primary outcome of interest was treatment resistance, defined as a progressive decline in kidney function, with persistently active urine sediment, leading to dialysis dependency or vasculitis-related death.

Results

In all, 20.9% of patients had low sC3 levels associated with more advanced renal impairment (P < 0.01), requiring acute dialysis (P < 0.01) more frequently compared to patients with normal sC3. Within 1 year, 85.7% of patients with normal sC3 responded to therapy, versus 58.3% of those with low sC3 (P = 0.001). The probability of treatment resistance was strongly associated with low sC3 (P = 0.004), high serum creatinine (P < 0.001), acute dialysis requirement (P < 0.001), and high histopathological score of chronicity (P < 0.01). Advanced histopathological activity was related to more intense interstitial leukocyte infiltration (P = 0.005) and higher likelihood of fibrinoid necrosis documentation in a vessel wall (P = 0.02). The probability of treatment resistance was higher in patients with low sC3 (odds ratio [OR] = 6.47, 95% confidence interval [CI] 1.47-28.35, P = 0.013), oliguria (OR = 29.57, 95% CI = 4.74-184, P < 0.0001), and high chronicity score (OR = 1.77, 95% CI = 1.23-2.54, P = 0.002).

Conclusion

Low sC3 is emerging as an independent predictor of treatment resistance in patients with PIGN associated with higher index of histopathological activity at diagnosis compared to normal sC3.

Complement Components C3 and C4 Indicate Vasculitis Manifestations to Distinct Renal Compartments in ANCA-Associated Glomerulonephritis

Acute kidney injury (AKI) is a common and severe complication of antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) causing progressive chronic kidney disease (CKD), end-stage renal disease (ESRD) or death. Pathogenic ANCAs, in particular proteinase 3 (PR3) and myeloperoxidase (MPO), trigger a deleterious immune response resulting in pauci-immune necrotizing and crescentic glomerulonephritis (GN), a common manifestation of glomerular injury in AAV. However, there is growing evidence that activation of the complement pathway contributes to the pathogenesis and progression of AAV. We here aimed to compare glomerular and tubulointerstitial lesions in ANCA GN and extrarenal manifestation of AAV in association with levels of circulating complement components C3c and C4.

METHODS

Plasma levels of C3c and C4 in a total number of 53 kidney biopsies with ANCA GN were retrospectively included between 2015 and 2020. Glomerular and tubulointerstitial lesions were evaluated according to established scoring systems for ANCA GN and analogous to the Banff classification.

RESULTS

We here show that circulating levels of C3c and C4 in ANCA GN were comparable to the majority of other renal pathologies. Furthermore, hypocomplementemia was only detectable in a minor subset of ANCA GN and not correlated with renal or extrarenal AAV manifestations. However, low levels of circulating C3c correlated with AKI severity in ANCA GN independent of systemic disease activity or extrarenal AAV manifestation. By systematic scoring of glomerular and tubulointerstitial lesions, we provide evidence that low levels of circulating C3c and C4 correlated with vasculitis manifestations to distinct renal compartments in ANCA GN.

CONCLUSIONS

We here expand our current knowledge about distinct complement components in association with vasculitis manifestations to different renal compartments in ANCA GN. While low levels of C4 correlated with glomerulitis, our observation that low levels of circulating complement component C3c is associated with interstitial vasculitis manifestation reflected by intimal arteritis implicates that C3c contributes to tubulointerstitial injury in ANCA GN.

Granulomatous Inflammation in ANCA-Associated Vasculitis

ANCA-associated vasculitis (AAV) comprises granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA). While systemic vasculitis is a hallmark of all AAV, GPA is characterized by extravascular granulomatous inflammation, preferentially affecting the respiratory tract. The mechanisms underlying the emergence of neutrophilic microabscesses; the appearance of multinucleated giant cells; and subsequent granuloma formation, finally leading to scarred or destroyed tissue in GPA, are still incompletely understood. This review summarizes findings describing the presence and function of molecules and cells contributing to granulomatous inflammation in the respiratory tract and to renal inflammation observed in GPA. In addition, factors affecting or promoting the development of granulomatous inflammation such as microbial infections, the nasal microbiome, and the release of damage-associated molecular patterns (DAMP) are discussed. Further, on the basis of numerous results, we argue that, in situ, various ways of exposure linked with a high number of infiltrating proteinase 3 (PR3)- and myeloperoxidase (MPO)-expressing leukocytes lower the threshold for the presentation of an altered PR3 and possibly also of MPO, provoking the local development of ANCA autoimmune responses, aided by the formation of ectopic lymphoid structures. Although extravascular granulomatous inflammation is unique to GPA, similar molecular and cellular patterns can be found in both the respiratory tract and kidney tissue of GPA and MPA patients; for example, the antimicrobial peptide LL37, CD163⁺ macrophages, or regulatory T cells. Therefore, we postulate that granulomatous inflammation in GPA or PR3-AAV is intertwined with autoimmune and destructive mechanisms also seen at other sites.

Avacopan for the treatment of ANCA-associated vasculitis

Introduction: Anti-neutrophil cytoplasm autoantibodies (ANCA)-associated vasculitides (AAVs) are a group of rare heterogeneous diseases characterized by blood vessel inflammation resulting in organ destruction and death. Although various treatment strategies have resulted in marked improvement in vasculitis-specific outcomes, many patients with AAV continue to suffer from complications related to the prolonged use of glucocorticoids (GC) such as infections, metabolic abnormalities, and increased cardiovascular morbidity. Recently, activation of the alternative complement pathway has been implicated in the augmentation of the damage caused by AAV via the complement C5a receptor (C5aR1, CD88). Specifically targeting this pathway may lead to improved outcomes in patients with AAV.Areas covered: In this article, we have summarized the rationale for targeting the complement pathway in AAV. The relevant pre-clinical, phase I, II and III findings with emphasis on the efficacy, and safety of avacopan, a new oral competitive inhibitor that interferes with the binding of C5a to C5aR1 (CD88), are reviewed.Expert opinion: These results are encouraging, may led to major changes in the treatment approach for AAV, and give rise to future studies utilizing complement inhibitors in AAV patients, and potentially in other immune mediated diseases.

Characterization of a Rat Model of Myeloperoxidase-Anti-Neutrophil Cytoplasmic Antibody-Associated Crescentic Glomerulonephritis

BACKGROUND/AIM

Necrotizing crescentic glomerulonephritis (GN) associated with anti-neutrophil cytoplasmic antibodies (ANCA) against myeloperoxidase (MPO) is a devastating disease that quickly progresses to kidney failure. Current therapies are broadly immunosuppressive and associated with adverse effects. We wanted to set up a model that could be suitable for testing narrowly targeted therapies.

METHODS

The model was constructed in male Wistar Kyoto rats through injections of human MPO (hMPO) and pertussis toxin, followed by a sub-nephritogenic dose of sheep anti-rat glomerular basement membrane (GBM) serum to boost the disease. Rats were monitored for 35 days. Rats given hMPO alone, saline, or human serum albumin with or without anti-GBM serum were also studied.

RESULTS

Rats receiving hMPO developed circulating anti-hMPO and anti-rat MPO antibodies. Challenging hMPO-immunized rats with the anti-GBM serum led to more glomerular neutrophil infiltration and MPO release, and severe haematuria, heavy proteinuria, and higher blood urea nitrogen than hMPO alone. Pauci-immune GN developed with crescents, affecting 25% of glomeruli. The majority of crescents were fibrocellular. Necrotizing lesions and Bowman capsule ruptures were detected. Cells double positive for claudin-1 (a marker of parietal epithelial cells [PECs]) and neural cell adhesion molecule (NCAM; progenitor PECs) were present in crescents. Double staining for NCAM and Ki-67 established proliferative status of progenitor PECs. Podocyte damage was associated with endothelial and GBM changes by electron microscopy. Monocyte/macrophages and CD4+ and CD8+ T cells accumulated in glomeruli and the surrounding area and in the tubulointerstitium. Lung haemorrhage also manifested.

CONCLUSION

This model reflects histological lesions of human ANCA-associated rapidly progressive GN and may be useful for investigating new therapies.

C3 glomerulonephritis associated with ANCA positivity: a case report

BACKGROUND

C3 glomerulopathy (C3G) is a recent disease classification that is characterized by the presence of glomerular deposits (composed of C3) in the absence of significant amounts of immunoglobulin and comprises dense deposit disease and C3 glomerulonephritis (C3GN). Most C3GN manifests as membranoproliferative, mesangial proliferative glomerulonephritis patterns via light microscopy. Pure membranous nephropathy (MN)-like glomerular lesions are rare manifestations of C3GN. Anti-neutrophil cytoplasmic antibodies (ANCAs) are also seldomly reported to be positive in C3GN. Herein, we report the case of a C3GN patient presenting with an MN-like glomerular pattern with ANCA positivity.

CASE PRESENTATION

A 68-year-old woman was admitted to a local hospital with elevated serum creatinine for two weeks. Laboratory tests showed a hemoglobin level of 85 g/L. Urinalysis was positive for 2 + protein and 360 RBCs/HPF. Blood biochemistry analysis revealed the following concentrations: albumin, 30.3 g/L; globulin, 46.2 g/L; blood urea nitrogen, 19.9 mmol/L; and serum creatinine, 234 µmol/L. The serum C3 level was 0.4950 g/L, and the serum C4 level was 0.1050 g/L. The direct Coombs test was positive. Serologic testing for ANCA revealed the presence of p-ANCA (1:10) by indirect immunofluorescence microscopy assay, as well as the presence of PR3 1.2 (normal range < 1) and MPO 3.5 (normal range < 1) by enzyme immunoassay. Renal biopsy sample pathology showed 2/6 cellular crescents and thickened glomerular basement membranes. Immunofluorescence testing revealed only diffuse, finely granular depositions of C3 along the glomerular capillary walls in frozen and paraffin-embedded tissue sections. Electron microscopy demonstrated the presence of subepithelial electron-dense deposits, similar to those that are observed in membranous nephropathy. Corticosteroid and cyclophosphamide were administered, with a subsequent improvement in renal function.

CONCLUSIONS

We present the rare case of a patient with MN-like C3GN with ANCA positivity. C3GN with ANCA positivity may be represented by more crescents, severe renal dysfunction and more extrarenal manifestations. More cases are needed to elucidate the clinicopathologic features and optimal treatments of these patients.

Glomerular Immune Deposition in MPO-ANCA Associated Glomerulonephritis Is Associated With Poor Renal Survival

Background

Rapidly progressive glomerulonephritis caused by antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is typically characterized as pauci-immune glomerulonephritis. However, immune complex (IC) deposition in the glomerulus has been reported in a growing number of studies. Here, we assess the presence of glomerular immune deposits alongside renal outcome in myeloperoxidase (MPO)-ANCA associated glomerulonephritis (MPO-ANCA GN).

Methods

Clinical and histopathologic characteristics of 97 patients with MPO-ANCA GN classified by renal biopsy from January 2008 to December 2019 were extracted retrospectively from electronic medical records. The extent of immune deposits in the kidney (C3, C4, C1q, IgA, IgG, IgM) at diagnosis were analyzed by immunofluorescence (IF). Patients were followed up for a median period of 15 months. The response to treatment and outcomes of renal and histological lesion changes were also assessed.

Results

In our study, 41% (40/97) of patients showed positive IF (≥2+) for at least one of the six immunoglobulin or complement components tested. Patients with IC deposits showed higher levels of serum creatinine (p=0.025), lower platelet counts (p=0.009), lower serum complement C3 (sC3) (≤790 ml/L) (p=0.013) and serum IgG (p=0.018) than patients with pauci-immune (PI) deposition at diagnosis. End-stage renal disease was negatively associated with eGFR (HR 0.885, 95% CI 0.837 to 0.935, p<0.0001), platelet count (HR 0.996, 95% CI 0.992 to 1.000, p=0.046) and serum globulin (HR 0.905, 95% CI 0.854 to 0.959, p=0.001). Patients with lower sC3 levels showed a worse renal outcome than the patients with normal sC3 at diagnosis (p=0.003). Analysis of the components of the renal deposits found that patients with IgG deposits exhibited a poorer renal outcome compared to patients that were IgG negative (p=0.028). Moreover, Bowman’s capsule rupture occurred less frequently in patients with IgM deposition compared with IgM negative counterparts (p=0.028). Vascular lesions and granuloma-like lesions had been seen more frequently in cases with IgA deposition than those without IgA deposition (p=0.03 and 0.015, respectively).

Conclusion

In conclusion, patients with immune complex deposits in the kidney showed less platelet count, lower sC3 and sIgG levels, and higher serum creatinine levels. Patients with low sC3 at initial and with continued low sC3 during the treatment displayed a trend toward poorer kidney survival. Moreover, the IC group showed a worse renal outcome than the PI group, further enforcing the present strategy of introducing complement targeted therapies in AAV.

Severe glomerular C3 deposition indicates severe renal lesions and a poor prognosis in patients with immunoglobulin A nephropathy

AIMS

Glomerular complement 3 (C3) deposition is often observed in renal biopsies of patients with IgA nephropathy (IgAN); however, the relationship between the intensity of C3 deposition and the long-term prognosis of IgAN has rarely been reported. In this retrospective study, we aimed to evaluate the prognostic value of glomerular C3 deposition for IgAN progression.

METHODS AND RESULTS

From June 2009 to June 2010, a total of 136 adult patients with IgAN were enrolled in the study. According to the intensity of glomerular C3 deposition, patients were divided into a glomerular C3^high group (34 patients) and a glomerular C3^low group (102 patients). The levels of clinical parameters, glomerular immune complexes, histopathological features, and serum cytokines of the two groups were compared. On the basis of an average of 105 months of follow-up, the predictive value of glomerular C3 deposition for IgAN progression was also investigated. Patients in the C3^high group had more severe glomerular IgA, IgG, IgM, and complement factor H deposition, a higher percentage of mesangial hypercellularity (M1), and higher levels of segmental glomerulosclerosis (S1), tubular atrophy/interstitial fibrosis (T2), and crescents (C2) than those in the C3^low group. Renal biopsies in the C3^high group showed higher densities of interstitial inflammatory cells and higher levels of serum interferon-γ than those in the C3^low group. Multivariate Cox regression analysis revealed that a higher intensity of glomerular C3 deposition remained as an independent predictor of serum creatinine doubling and end-stage renal disease.

CONCLUSIONS

A high intensity of glomerular C3 deposition is associated with the severity of renal lesions, and predicts long-term poor renal survival for IgAN patients.

Pathogenicity of Proteinase 3-Anti-Neutrophil Cytoplasmic Antibody in Granulomatosis With Polyangiitis: Implications as Biomarker and Future Therapies

Granulomatosis with polyangiitis (GPA) is a rare but serious necrotizing auto-immune vasculitis. GPA is mostly associated with the presence of Anti-Neutrophil Cytoplasmic Antibody (ANCA) targeting proteinase 3 (PR3-ANCA), a serine protease contained in neutrophil granules but also exposed at the membrane. PR3-ANCAs have a proven fundamental role in GPA: they bind neutrophils allowing their auto-immune activation responsible for vasculitis lesions. PR3-ANCAs bind neutrophil surface on the one hand by their Fab binding PR3 and on the other by their Fc binding Fc gamma receptors. Despite current therapies, GPA is still a serious disease with an important mortality and a high risk of relapse. Furthermore, although PR3-ANCAs are a consistent biomarker for GPA diagnosis, relapse management currently based on their level is inconsistent. Indeed, PR3-ANCA level is not correlated with disease activity in 25% of patients suggesting that not all PR3-ANCAs are pathogenic. Therefore, the development of new biomarkers to evaluate disease activity and predict relapse and new therapies is necessary. Understanding factors influencing PR3-ANCA pathogenicity, i.e. their potential to induce auto-immune activation of neutrophils, offers interesting perspectives in order to improve GPA management. Most relevant factors influencing PR3-ANCA pathogenicity are involved in their interaction with neutrophils: level of PR3 autoantigen at neutrophil surface, epitope of PR3 recognized by PR3-ANCA, isotype and glycosylation of PR3-ANCA. We detailed in this review the advances in understanding these factors influencing PR3-ANCA pathogenicity in order to use them as biomarkers and develop new therapies in GPA as part of a personalized approach.

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.

The biology, pathogenetic role, clinical implications, and open issues of serum anti-neutrophil cytoplasmic antibodies

Anti-neutrophil cytoplasmic antibodies (ANCA) are a group of autoantibodies, predominantly IgG, involved in the pathogenesis of several autoimmune disorders, detected either through indirect immunofluorescence or enzyme-linked immunosorbent assay. By means of indirect immunofluorescence, the main patterns are C-ANCA (cytoplasmic) and P-ANCA (perinuclear), while proteinase 3 (PR3) and myeloperoxidase (MPO) represent the main autoantigens in granulomatosis with polyangiitis and microscopic polyangiitis, both belonging to the family of ANCA-associated vasculitis (AAV). While several experiments established the pathogenicity of MPO-ANCA, evidence remains elusive for PR3-ANCA and an additional target antigen, i.e. LAMP2, has been postulated with specific clinical relevance. The presence of a subset of AAV without ANCA may be explained by the presence of further target antigens or the presence of molecules in blood which make ANCA undetectable. A rise in ANCA titers is not necessarily predictive of a flare of disease in AAV if not accompanied by clinical manifestations. ANCA may develop through variable mechanisms, such as autoantigen complementarity, apoptosis impairment, neutrophil extracellular traps dysfunction and molecular mimicry. We will provide herein a comprehensive review of the available evidence on the biological mechanisms, pathogenetic role, and clinical implications of ANCA testing and disease management. Further, we will address the remaining open challenges in the field, including the role of ANCA in inflammatory bowel disease and in cocaine-induced vasculitis.

Complement as a Therapeutic Target in Systemic Autoimmune Diseases

The complement system (CS) includes more than 50 proteins and its main function is to recognize and protect against foreign or damaged molecular components. Other homeostatic functions of CS are the elimination of apoptotic debris, neurological development, and the control of adaptive immune responses. Pathological activation plays prominent roles in the pathogenesis of most autoimmune diseases such as systemic lupus erythematosus, antiphospholipid syndrome, rheumatoid arthritis, dermatomyositis, and ANCA-associated vasculitis. In this review, we will review the main rheumatologic autoimmune processes in which complement plays a pathogenic role and its potential relevance as a therapeutic target.