Complement gene variants determine the risk of immunoglobulin-associated MPGN and C3 glomerulopathy and predict long-term renal outcome

Acquired and genetic determinants of disease phenotype and therapeutic strategies in C3 glomerulopathy and immunoglobulin-associated MPGN

C3 glomerulopathy (C3G), a prototype of complement-mediated disease, is characterized by significant heterogeneity, in terms of not only clinical, histological and biological presentation but also prognosis, and response to existing therapies. Recent advancements in understanding the factors responsible for alternative pathway dysregulation in the disease have highlighted its even more complex nature. Here, we propose a reexamination of the diversity of C3G presentations in light of the drivers of complement activation. Autoantibodies targeting complement proteins, genetic abnormalities in complement genes and monoclonal immunoglobulins are now well-known to drive disease occurrence. This review discusses how these drivers contribute to the heterogeneity in disease phenotype and outcomes, providing insights into tailored diagnostic and therapeutic approaches. In recent years, a broad spectrum of complement inhibitory therapies has emerged, soon to be available in clinical practice. The recognition of specific clinical, biological and histological patterns associated with different forms of C3G is crucial for personalized management, particularly treatment strategies.

Comparative Analysis of Proteinuria and Longitudinal Outcomes in Immune Complex Membranoproliferative Glomerulonephritis and C3 Glomerulopathy

Introduction

C3 glomerulopathy (C3G) and primary immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) are rare diseases that share a similar pathogenesis; however, the prognostic significance of proteinuria reduction remains poorly characterized. This study compared the outcomes in C3G and IC-MPGN and assessed the impact of changes in proteinuria on kidney prognosis.

Methods

This retrospective, longitudinal, multicenter study used joint linear mixed-effects models to assess proteinuria trajectories, and Cox regression to evaluate their association with kidney failure. In addition, time-averaged proteinuria (TA-P) was calculated to determine its impact on kidney prognosis.

Results

The study included 149 patients: 98 with C3G (66%) and 51 with IC-MPGN (34%) with a median age of 35 (interquartile range [IQR]: 22-53) years. During a median follow-up of 65 (IQR: 32-114) months, 44 patients (30%) progressed to kidney failure without differences across C3G or IC-MPGN. A strong association was observed between longitudinal increase in proteinuria and the risk of kidney failure. In addition, a ≥ 50% proteinuria reduction over time was associated with a lower risk of kidney failure (hazard ratio [HR]: 0.61; 95% confidence interval [CI]: 0.46-0.75, P < 0.001). Results were consistent in both C3G and IC-MPGN, and in those with baseline estimated glomerular filtration rate (eGFR) ≥ 30 ml/min per 1.73 m2 and proteinuria ≥ 1 g/d. A ≥30% proteinuria reduction at 6 months or a ≥50% proteinuria reduction at 12 months were associated with a slower eGFR decline. Patients were categorized into 4 subgroups based on TA-P levels, with TA-P values < 1 g/d indicating better kidney outcomes.

Conclusion

Proteinuria reduction was associated with improved kidney outcomes and slower eGFR decline in both C3G and IC-MPGN.

Autoantibodies Against Factor B and Factor H Without Pathogenic Effects in a Patient with Immune Complex-Mediated Membranoproliferative Glomerulonephritis

Background: Membranoproliferative glomerulonephritis (MPGN) is an umbrella term for chronic disorders affecting the glomeruli. MPGN is often accompanied by the presence of autoantibodies against complement components. However, the actual pathogenic effects of such autoantibodies, if any, are rarely studied. In this work, we investigated the role of anti-complement autoantibodies in an IC-MPGN patient. Methods: The presence of autoantibodies, their binding site, isotype, and titer were analyzed in ELISA. Antibody-antigen complexes were detected in the patient's serum using Western blot. Autoantibodies were studied in functional assays to analyze their effects on C3 convertase, complement deposition, cofactor activity, C3b binding, and hemolysis. Results: We identified autoantibodies against factor B (FB) and factor H (FH) in the patient's serum. Both FB-, and FH-autoantibodies were of IgG2, IgG3, IgG4, and IgGκ, IgGλ isotypes. FB-autoantibodies bound to the Ba and the enzymatically active Bb part of FB. FH-autoantibodies bound to the N- and C-termini of FH and cross-reacted with FHL-1 and FHR-1 proteins. In vivo formed complexes of the autoantibodies with both FB and FH were detected in the IgG fraction isolated from the serum. The autoantibodies did not influence solid-phase C3 convertase assembly and its FH-mediated decay. The free autoantibodies had no effect on complement deposition and on FH cofactor activity but slightly reduced C3b binding to FH. The IgG fraction of the patient dose-dependently inhibited complement-mediated rabbit red blood cell lysis, and the free autoantibodies decreased the solid phase C3 convertase activity. Conclusions: This case highlights that FB- and FH-autoantibodies are not necessarily pathogenic in IC-MPGN.

Acquired drivers of C3 glomerulopathy

C3 glomerulopathy (C3G) is a group of heterogeneous ultrarare kidney diseases characterized by dysregulated activation of the complement alternative pathway (AP) leading to excessive C3 cleavage. Diagnosis relies on kidney biopsy showing predominant C3 deposition in the glomerular basement membrane, with electron microscopy differentiating between dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). The main drivers of AP dysregulation in C3G are acquired rather than genetic and consist primarily of autoantibodies called nephritic factors (C3Nefs, C4Nefs and C5Nefs) that bind to and stabilize complement convertases, causing complement overactivation. Current therapies are largely supportive, and existing complement-targeting treatments, such as eculizumab, demonstrate limited efficacy. Challenges in studying C3G include variability in autoantibody detection and a lack of standardized assays, which complicates clinical interpretation. Comprehensive assessment involving autoantibody panels, complement biomarkers, functional assays and genetic testing provides a more complete understanding of disease dynamics; however, key knowledge gaps remain regarding Nef origins, mechanisms and their pathogenic role. In this review we discuss acquired drivers of C3G with an emphasis on C3Nefs and C5Nefs and suggest areas of interest that might benefit from future research.

Optimizing gene selection and module identification via ontology-based scoring and deep learning

Motivation

Understanding gene interactions and their biological significance is a key challenge in computational biology. The complexity of biological systems, coupled with high-dimensional omics data, necessitates robust methods for gene selection and interaction analysis. Traditional statistical techniques often struggle with the hierarchical nature of gene ontology (GO) terms, leading to redundancy and limited interpretability. Meanwhile, deep learning models require biologically meaningful input to enhance their predictive power.

Results

We present an integrated framework that enhances gene selection and uncovers gene interactions by combining a novel statistical algorithm with a deep neural network model. The statistical algorithm ranks differentially expressed genes by correlating their expression scores with the semantic similarity of their biological context, utilizing GO information to align genes with known pathways. The deep neural network then identifies interaction modules by integrating genes from different clusters based on regulatory pathway data. This model effectively navigates the hierarchical complexity of GO terms structured as directed acyclic graphs, employing a feed-forward architecture optimized via back-propagation. Our results demonstrate improved gene selection accuracy and enhanced discovery of biologically relevant interactions, providing valuable insights into complex disease mechanisms.

Factor B as a therapeutic target for the treatment of complement-mediated diseases

The complement system, consisting of three initiating pathways-classical, lectin and alternative, is an important part of innate immunity. Dysregulation of the complement system is implicated in the pathogenesis of several autoimmune and inflammatory diseases. Therapeutic inhibition of the complement system has been recognized as a viable approach to drug development and has been successful with the approval of a small number of complement inhibitors for diseases such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, neuromyelitis optica, myasthenia gravis and geographic atrophy. More recently, therapies selectively targeting the alternative pathway (AP), which drives the amplification of the complement responses, are being evaluated for these complement-mediated diseases. Complement Factor B, a serine protease, is a unique component of the AP that is essential for the catalytic activity of AP C3 convertase and AP C5 convertase. Inhibition of Factor B blocks the activity of the alternative pathway and the amplification loop, and subsequent generation of the membrane attack complex downstream; however, it has no effect on the initial activation mediated by the classical and lectin complement pathways. Therefore, Factor B is an attractive target for diseases in which the AP is overactivated. In this review, we provide an overview of Factor B and its critical role in the AP, discuss the benefit-risk of Factor B inhibition as a targeted therapeutic strategy, and describe the various Factor B inhibitors that are approved and/or in clinical development.

Challenges in the Diagnosis and Management of Immune Complex-Mediated Membranoproliferative Glomerulonephritis and Complement 3 Glomerulopathy

Immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) and complement 3 glomerulopathy (C3G) are rare, complement-mediated kidney diseases, previously classified under the group of kidney disorders termed membranoproliferative glomerulonephritis (MPGN) type 1, type 2, and type 3. Despite new advances in our understanding of IC-MPGN and C3G, several unmet needs persist in the diagnosis and management of patients with these nephropathies, due in part to their rarity and their overlapping clinical presentations, histologic features, and underlying pathophysiologies. This review summarizes our current understanding of the role of complement in IC-MPGN and C3G, and underlines the key histopathologic differences between the diseases. Using seven illustrative patient cases, we discuss consensus guideline treatment recommendations and the uncertainties, challenges, and considerations regarding the diagnosis and management of patients with IC-MPGN and C3G in clinical practice. The presented cases emphasize the need for a multidisciplinary approach encompassing primary care providers (PCPs), nephrologists, nephropathologists, and laboratory scientists. Key knowledge gaps are evaluated, including differential diagnoses, underlying pathologic mechanisms, and the lack of effective treatments targeting drivers of disease. As the therapeutic landscape evolves, an improved understanding of IC-MPGN and C3G is crucial to identifying optimal targeted-treatment strategies and facilitating a personalized approach to the management of these complex glomerular diseases.

Gene variant C3 glomerulonephritis with chronic urinary tract infection: A case report and literature review

RATIONALE

C3 glomerulonephritis (C3GN) is 1 type of C3 nephropathy which is a rare glomerular disease associated with abnormal regulation of the alternative complement pathway. This review reports a rare case of C3GN with repeated urinary tract infection and summarizes the clinical features, differential diagnosis, treatment, and outcome of patients with C3GN.

PATIENT CONCERNS

A 44-year-old woman was admitted to our hospital because of proteinuria increased for more than 8 years.

DIAGNOSES

The patient was finally diagnosed with C3GN by renal biopsy and gene testing.

INTERVENTIONS

The patient was worried about the side effects of drugs and strongly refused to use glucocorticoids and immunosuppressants, so she was given angiotensin II receptor blocker monotherapy for glomerulonephritis and quinolones for urinary tract infection.

OUTCOMES

Serum creatinine, proteinuria, and serum complement c3 levels remained stable. Our case is still under continuous follow-up.

LESSONS

Clinical diagnosis of C3GN is difficult to make as there are many differential diagnoses, especially post infection nephritis. This case emphasizes the importance of renal biopsy in the diagnosis of C3GN, but the relationship between gram-negative bacilli and C3GN is still unclear. In addition, gene mutation is also involved in the pathogenesis of C3GN, and the treatment of C3GN still needs to be explored.

Short-term renal and patient outcomes of primary immunoglobulin-associated mesangiocapillary glomerulonephritis: Insights from a developing country

BACKGROUND

Primary immunoglobulin (Ig)-associated mesangiocapillary glomerulonephritis (Ig-MCGN) is an immune complex glomerulonephritis of unknown etiology. It is a common cause of chronic kidney disease in developing countries. There is limited data available on renal and patient outcomes of this disease from developing countries.

AIM

To determine the short-term renal and patient outcomes of adults with a tissue-confirmed diagnosis of primary Ig-MCGN at a single center in Pakistan.

METHODS

A retrospective cohort study of adult patients was conducted on biopsy-proven Ig-MCGN cases diagnosed between 1998 and 2019 at the Sindh Institute of Urology and Transplantation, Karachi, Pakistan. Secondary causes were excluded. The primary endpoint was renal survival without end-stage kidney disease (ESKD) or mortality. The secondary endpoint was the rate of remission during the 2-year follow-up period. Survival curves were made with the use of Kaplan-Meier estimates.

RESULTS

A total of 163 patients were included in the study and their mean follow-up duration was 29.45 months ± 21.28 months. Among baseline characteristics, young age, lower estimated glomerular filtration rate, requirement of kidney replacement therapy, presence of crescents, and severity of interstitial fibrosis and tubular atrophy were found to have a significant association with renal outcomes. The renal outcomes were negatively correlated with the presence of hypertension, level of complements, and degree of proteinuria. In all, 63 (37.4%) patients were treated with steroids and 21 (13%) received combination therapy (cyclophosphamide with steroids). At 2 years, 124 (76.07%) patients were in complete remission or partial remission [56 (34.3%) and 68 (41.71%), respectively], while 32 (19.63%) patients progressed to ESKD and 7 (4.29%) patients died.

CONCLUSION

The outcomes of primary Ig-MCGN are guarded in Pakistan and require further prospective studies to improve our understanding of this relatively common disease so that more personalized treatment approaches can be developed.

Kidney transplantation in children and adolescents with C3 glomerulopathy or immune complex membranoproliferative glomerulonephritis: a real-world study within the CERTAIN research network

BACKGROUND

Complement 3 glomerulopathy (C3G) and immune complex membranoproliferative glomerulonephritis (IC-MPGN) are ultra-rare chronic kidney diseases with an overall poor prognosis, with approximately 40-50% of patients progressing to kidney failure within 10 years of diagnosis. C3G is characterized by a high rate of disease recurrence in the transplanted kidney. However, there is a lack of published data on clinical outcomes in the pediatric population following transplantation.

METHODS

In this multicenter longitudinal cohort study of the Cooperative European Paediatric Renal Transplant Initiative (CERTAIN) registry, we compared the post-transplant outcomes of pediatric patients with C3G (n = 17) or IC-MPGN (n = 3) with a matched case-control group (n = 20).

RESULTS

Eleven of 20 children (55%) with C3G or IC-MPGN experienced a recurrence within 5 years post-transplant. Patients with C3G or IC-MPGN had a 5-year graft survival of 61.4%, which was significantly (P = 0.029) lower than the 5-year graft survival of 90% in controls; five patients with C3G or IC-MPGN lost their graft due to recurrence during this observation period. Both the 1-year (20%) and the 5-year (42%) rates of biopsy-proven acute rejection episodes were comparable between patients and controls. Complement-targeted therapy with eculizumab, either as prophylaxis or treatment, did not appear to be effective.

CONCLUSIONS

These data in pediatric patients with C3G or IC-MPGN show a high risk of post-transplant disease recurrence (55%) and a significantly lower 5-year graft survival compared to matched controls with other primary kidney diseases. These data underscore the need for post-transplant patients for effective and specific therapies that target the underlying disease mechanism.

Clinicopathological characteristics and outcomes of adult patients with idiopathic membranoproliferative glomerulonephritis according to an immunofluorescence-based classification

BACKGROUND

The classification of membranoproliferative glomerulonephritis (MPGN) into immune complex-mediated MPGN and complement-mediated MPGN on immunofluorescence has provided insights into two distinct disease processes. There are limited data available on renal outcomes of MPGN from developing countries.

METHODS

A retrospective analysis was conducted on biopsy-proven MPGN cases diagnosed between 1998 and 2018 at the Sindh Institute of Urology and Transplantation (SIUT). Secondary causes were excluded. Patients were reclassified as immune complex-mediated-MPGN and complement-mediated-MPGN based on immunofluorescence results. The clinicopathological findings and outcomes of the two groups were compared.

RESULTS

In total, 213 patients with idiopathic MPGN were identified. Among these, 163 (76.5%) were reclassified as immune complex-mediated-MPGN and 50 (23.4%) as complement-mediated-MPGN. No significant differences were found between the two groups regarding age, gender, clinical characteristics, biopsy indications, biopsy findings, and renal function at presentation. Overall, 63 subjects (38.7%) with immune complex-mediated-MPGN and 27 (54%) with complement-mediated-MPGN received immunosuppressive agents (p = 0.08). Complete and partial remission rates were higher in immune complex-mediated-MPGN than in complement-mediated-MPGN (76% vs 58%, p < 0.05). At two years, median estimated glomerualr filtration rate (eGFR) tended to be higher in patients with immune complex-mediated-MPGN 91.2 (45.4-113.7) vs 83.45(34.6-102.50) ml/min/1.73 m2, p = 0.22) with significantly better renal survival (76% vs 58%, p = 0.03). Comparatively, more patients progressed to end-stage kidney disease (ESKD) in the complement-mediated-MPGN group (32% vs 19.6%, p = 0.06), with increased overall mortality (5 (10%) vs 7 (4.3%), p = 0.12).

CONCLUSION

The clinicopathological features at presentation of complement-mediated-MPGN are similar to those of immune complex-mediated-MPGN. However, it is less frequent and overall prognosis is less favorable.

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.

Diagnostic Challenges and Emerging Pathogeneses of Selected Glomerulopathies

Recent progress in glomerular immune complex and complement-mediated diseases have refined diagnostic categories and informed mechanistic understanding of disease development in pediatric patients. Herein, we discuss selected advances in 3 categories. First, membranous nephropathy antigens are increasingly utilized to characterize disease in pediatric patients and include phospholipase A2 receptor (PLA2R), Semaphorin 3B (Sema3B), neural epidermal growth factor-like 1 (NELL1), and protocadherin FAT1, as well as the lupus membranous-associated antigens exostosin 1/2 (EXT1/2), neural cell adhesion molecule 1 (NCAM1), and transforming growth factor beta receptor 3 (TGFBR3). Second, we examine advances in techniques for paraffin and light chain immunofluorescence (IF), including the former's function as a salvage technique and their necessity for diagnosis in adolescent cases of membranous-like glomerulopathy with masked IgG kappa deposits (MGMID) and proliferative glomerulonephritis with monotypic Ig deposits (PGNMID), respectively. Finally, progress in understanding the roles of complement in pediatric glomerular disease is reviewed, with specific attention to overlapping clinical, histologic, and genetic or functional alternative complement pathway (AP) abnormalities among C3 glomerulopathy (C3G), infection-related and post-infectious GN, "atypical" post-infectious GN, immune complex mediated membranoproliferative glomerulonephritis (IC-MPGN), and atypical hemolytic uremic syndrome (aHUS).

Clinical characteristics and outcomes of immune-complex membranoproliferative glomerulonephritis and C3 glomerulopathy in Japanese children

BACKGROUND

Membranoproliferative glomerulonephritis (MPGN) can be divided into immune-complex MPGN (IC-MPGN) and C3 glomerulopathy (C3G), which includes dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). These conditions result from abnormalities in different complement pathways and may lead to different prognoses. However, there are limited studies describing the respective clinical courses.

METHODS

In this study, Japanese pediatric patients diagnosed with MPGN based on kidney biopsies conducted between February 2002 and December 2022 were reclassified as having IC-MPGN or C3G (DDD or C3GN). We retrospectively analyzed the clinical characteristics and outcomes of these patients.

RESULTS

Out of 25 patients with MPGN, three (12.0%) were diagnosed with DDD, 20 (80.0%) with C3GN, and two (8.0%) with IC-MPGN. There were 13 (65.0%) patients and one (33.3%) patient in remission after treatment for C3GN and DDD, respectively, and no patients with IC-MPGN achieved remission. The median follow-up period was 5.3 (2.5-8.9) years, and none of the patients in either group progressed to an estimated glomerular filtration rate < 15 ml/min/1.73 m2. Patients with C3GN presenting mild to moderate proteinuria (n = 8) received a renin-angiotensin system inhibitor (RAS-I) alone, and these patients exhibited a significant decrease in the urinary protein creatinine ratio and a notable increase in serum C3 levels at the last follow-up.

CONCLUSIONS

Most patients with MPGN were diagnosed with C3GN. The remission rate for C3GN was high, and no patients developed kidney failure during the approximately 5-year follow-up. Additionally, patients with C3GN with mild to moderate proteinuria had good outcomes with RAS-I alone, but continued vigilance is necessary to determine long-term prognosis.

Prognostic factors and validation of the histologic chronicity score for C3 glomerulopathy: a registry analysis

Background

Data on the prognostic factors for C3 glomerulopathy (C3G) are limited, and validation of the new C3G histologic index (C3G-HI) in different settings is still needed. We aimed to evaluate the chronicity score of C3G-HI and probable prognostic factors in our population.

Methods

In this registry study, 74 patients from 20 centers with adequate follow-up data were included. Total chronicity score (TCS) was calculated according to percentages of glomerulosclerosis, interstitial fibrosis, tubular atrophy, and presence of arterio- and arteriolosclerosis. Primary composite outcome was defined as doubling of serum creatinine from baseline, undergoing dialysis or transplantation, development of stage 5 chronic kidney disease, or death.

Results

Median age was 34 [interquartile range (IQR) 24-46] years, and 39 patients (52.7%) were male. Median follow-up duration was 36 (IQR 12-60) months, and median TCS was 3 (IQR 1-5). Overall, 19 patients (25.7%) experienced primary composite outcome. Multivariate Cox regression model showed that only hemoglobin [adjusted HR (aHR) 0.67, 95% confidence interval 0.46-0.97, P = .035] predicted primary composite outcome, and TCS fell short of the statistical significance (aHR 1.26, 0.97-1.64, P = .08). Receiver operating characteristic analysis demonstrated that TCS showed an area under the curve value of 0.68 (0.56-0.78, P = .028) in discriminating primary composite outcome at 3 years, and 3-year kidney survival was lower in patients with TCS ≥4 (72.4%) compared with TCS <4 (91.1%) in Kaplan-Meier analysis (P = .036).

Conclusions

Low hemoglobin levels predicted dismal outcomes in patients with C3G. TCS ≥4 was associated with a worse 3-year kidney survival, which validated the 3-year prognostic value of the TCS of C3G-HI in our population.

Unsupervised Clustering of Membranoproliferative Glomerulonephritis and C3 Glomerulopathy Patients Discovers Distinct Patient Groups unlike the Current Classification

INTRODUCTION

Membranoproliferative glomerulonephritis is currently divided into immunoglobulin-mediated glomerulonephritis (IC-MPGN) and C3 glomerulopathy (C3G); however, the patients often overlap with histology, complement, clinical and prognostic factors. Our aim was to investigate if an unsupervised clustering method finds different patient groups in 44 IC-MPGN/C3G patients using only histological and clinical data available in everyday clinical work.

METHODS

Primary IC-MPGN/C3G adult patients were included whose diagnostic (baseline) native biopsy was obtained in 2006-2017. The biopsies were reassessed and the clinical data at baseline and during follow-up were obtained from the medical records. There were 39 baseline histological and clinical variables included in the unsupervised clustering. Follow-up information was combined with the clustering results.

RESULTS

The clustering resulted in two clusters (n = 24 and n = 20 patients for clusters 1-2, respectively), where cluster 1 had a significantly higher baseline plasma creatinine (mean 213 vs. 104, respectively, p value <0.001) and a lower baseline eGFR than cluster 2 (mean 37 vs. 70, respectively, p value <0.001). Regarding histology, chronic changes such as lobulated glomeruli, mesangial matrix expansion, and glomeruli double contours were more prevalent in cluster 1 (p value <0.001). Biopsy morphology was more often crescentic and membranoproliferative in cluster 1 (p value <0.001). Although the differences were insignificant, cluster 1 patients were in dialysis in the last follow-up or had a progressive disease more often than cluster 2 patients (21% vs. 5%, 38% vs. 10%).

CONCLUSIONS

Our results indicate that these patients share greater similarity than the current classification IC-MPGN versus C3G indicates.

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.

Complement gene mutations in children with C3 glomerulopathy: do they affect the response to mycophenolate mofetil?

BACKGROUND

C3 glomerulopathy (C3G) is a complement-mediated disease. Although genetic studies are not required for diagnosis, they are valuable for treatment planning and prognosis prediction. The aim of this study is to investigate the clinical phenotypes, kidney survival, and response to mycophenolate mofetil (MMF) treatment in pediatric C3G patients with and without mutations in complement-related genes.

METHODS

Sixty pediatric C3G patients were included, divided into two groups based on complement-related gene mutations. Demographic and clinical-pathological findings, treatment modalities, and outcome data were compared, and Kaplan-Meier analysis was performed for kidney survival.

RESULTS

Out of the 60 patients, 17 had mutations. The most common mutation was in the CFH gene (47%). The mean age at diagnosis was higher in the group with mutation (12.9 ± 3.6 vs. 11.2 ± 4.1 years, p = 0.039). While the patients without mutation most frequently presented with nephritic syndrome (44.2%), the mutation group was most likely to have asymptomatic urinary abnormalities (47.1%, p = 0.043). Serum parameters and histopathological characteristics were similar, but hypoalbuminemia was more common in patients without mutation. During 45-month follow-up,10 patients progressed to chronic kidney disease stage 5 (CKD5), with 4 having genetic mutation. The time to develop CKD5 was longer in the mutation group but not significant. MMF treatment had no effect on progression in either group.

CONCLUSIONS

This study is the largest pediatric C3G study examining the relationship between genotype and phenotype. We showed that the mutation group often presented with asymptomatic urinary abnormalities, was diagnosed relatively late but was not different from the without mutation group in terms of MMF treatment response and kidney survival.

The 14th International Podocyte Conference 2023: from podocyte biology to glomerular medicine

The 14th International Podocyte Conference took place in Philadelphia, Pennsylvania, USA from May 23 to 26, 2023. It commenced with an early-career researchers' meeting on May 23, providing young scientists with a platform to present and discuss their research findings. Throughout the main conference, 29 speakers across 9 sessions shared their insights on podocyte biology, glomerular medicine, novel technologic advancements, and translational approaches. Additionally, the event featured 3 keynote lectures addressing engineered chimeric antigen receptor T cell- and mRNA-based therapies and the use of biobanks for enhanced disease comprehension. Furthermore, 4 brief oral abstract sessions allowed scientists to present their findings to a broad audience. The program also included a panel discussion addressing the challenges of conducting human research within the American Black community. Remarkably, after a 5-year hiatus from in-person conferences, the 14th International Podocyte Conference successfully convened scientists from around the globe, fostering the presentation and discussion of crucial research findings, as summarized in this review. Furthermore, to ensure continuous and sustainable education, research, translation, and trial medicine related to podocyte and glomerular diseases for the benefit of patients, the International Society of Glomerular Disease was officially launched during the conference.

Recurrent C3 glomerulopathy after kidney transplantation

The complement system is part of innate immunity and is pivotal in protecting the body against pathogens and maintaining host homeostasis. Activation of the complement system is triggered through multiple pathways, including antibody deposition, a mannan-binding lectin, or activated complement deposition. C3 glomerulopathy (C3G) is a rare glomerular disease driven by complement dysregulation with high post-transplantation recurrence rates. Its treatment is mainly based on immunosuppressive therapies, specifically mycophenolate mofetil and glucocorticoids. Recent years have seen significant progress in understanding complement biology and its role in C3G pathophysiology. New complement-tergeting treatments have been developed and initial trials have shown promising results. However, challenges persist in C3G, with recurrent post-transplantation cases leading to suboptimal outcomes. This review discusses the pathophysiology and management of C3G, with a focus on its recurrence after kidney transplantation.

Exploring the impact and utility of genomic sequencing in established CKD

Clinical genetics is increasingly recognized as an important area within nephrology care. Clinicians require awareness of genetic kidney disease to recognize clinical phenotypes, consider use of genomics to aid diagnosis, and inform treatment decisions. Understanding the broad spectrum of clinical phenotypes and principles of genomic sequencing is becoming increasingly required in clinical nephrology, with nephrologists requiring education and support to achieve meaningful patient outcomes. Establishment of effective clinical resources, multi-disciplinary teams and education is important to increase application of genomics in clinical care, for the benefit of patients and their families. Novel applications of genomics in chronic kidney disease include pharmacogenomics and clinical translation of polygenic risk scores. This review explores established and emerging impacts and utility of genomics in kidney disease.

Thrombomodulin Gene Mutation and Associated Predisposing Factors in Familial Collapsing Glomerulopathy

Collapsing glomerulopathy (CG) is a rare glomerular disease and its familial form is even rarer. CG and non-collapsing forms of focal segmental glomerulosclerosis may both be caused by pathogenic variants in the same genes, but there is less information on genetics of the former disease. We hypothesized that different hits (viral infection and genetic variants) may be involved in the development of a familial CG here described. We performed renal and etiological routine evaluation, PVB19 serology, genetic tests including whole-exome analysis and dosage of serum thrombomodulin (THBD) in two siblings with CG, one healthy sister, and their mother. The THBD gene variant p.A43T in homozygosity was identified in the proband and her affected brother, both with CG. The same mutation was identified in their mother in heterozygosity. THBD levels were elevated in the serum of both affected siblings. They also had PVB19 positive serology and the G1 high-risk apolipoprotein L1 (APOL1) alleles in homozygosity. Their healthy sister had no PVB19-positive serology and no THBD nor APOL1 gene variants. In this case of familial CG, THBD, and APOL1 gene variants, and a previous PVB19 infection may be associated with the development of CG in a multihit process. In addition, the p.A43T THBD variant, identified in the affected siblings, has never been previously described in homozygosis, pointing to a likely autosomal recessive CG trait caused by this gene mutation.

Complement Activation in Nephrotic Glomerular Diseases

The nephrotic syndrome holds significant clinical importance and is characterized by a substantial protein loss in the urine. Damage to the glomerular basement membrane or podocytes frequently underlies renal protein loss. There is an increasing belief in the involvement of the complement system, a part of the innate immune system, in these conditions. Understanding the interactions between the complement system and glomerular structures continually evolves, challenging the traditional view of the blood-urine barrier as a passive filter. Clinical studies suggest that a precise inhibition of the complement system at various points may soon become feasible. However, a thorough understanding of current knowledge is imperative for planning future therapies in nephrotic glomerular diseases such as membranous glomerulopathy, membranoproliferative glomerulonephritis, lupus nephritis, focal segmental glomerulosclerosis, and minimal change disease. This review provides an overview of the complement system, its interactions with glomerular structures, and insights into specific glomerular diseases exhibiting a nephrotic course. Additionally, we explore new diagnostic tools and future therapeutic approaches.

C3G and Ig-MPGN-treatment standard

Among the broad spectrum of membranoproliferative glomerulonephritis (MPGN), immunofluorescence distinguishes C3 glomerulopathy (C3G), with predominant C3 deposits, and immunoglobulin-associated MPGN (Ig-MPGN), with combined C3 and Ig. However, there are several intersections between C3G and Ig-MPGN. Primary C3G and Ig-MPGN share the same prevalence of low serum C3 levels and of abnormalities of the alternative pathway of complement, and patients who present a bioptic pattern of Ig-MPGN at onset may show a C3G pattern in a subsequent biopsy. There is no specific therapy for primary C3G and Ig-MPGN and prognosis is unfavourable. The only recommended indications are inhibitors of the renin-angiotensin system, lipid-lowering agents and other renoprotective agents. The other drugs used currently, such as corticosteroids and mycophenolate mofetil, are often ineffective. The anti-C5 monoclonal antibody eculizumab has been tested in several patients, with mixed results. One reason for the uncertainty is the extremely variable clinical course, most likely reflecting a heterogeneous pathogenesis. An unsupervised clustering analysis that included histologic, biochemical, genetic and clinical data available at onset in patients with primary C3G and Ig-MPGN identified four clusters characterized by specific pathogenic mechanisms. This approach may facilitate accurate diagnosis and development of targeted therapies. Several trials are ongoing with drugs targeting different molecules of the complement cascade, however it is important to consider which component of the cascade may be the most appropriate for each patient. We review the current standards of treatment and discuss novel developments in the pathophysiology, diagnosis, outcome prediction and management of C3G and Ig-MPGN.

Iptacopan in Idiopathic Immune Complex-Mediated Membranoproliferative Glomerulonephritis: Protocol of the APPARENT Multicenter, Randomized Phase 3 Study

Introduction

Immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) is an ultra-rare, fast-progressing kidney disease that may be idiopathic (primary) or secondary to chronic infection, autoimmune disorders, or monoclonal gammopathies. Dysregulation of the alternative complement pathway is implicated in the pathophysiology of IC-MPGN; and currently, there are no approved targeted treatments. Iptacopan is an oral, highly potent proximal complement inhibitor that specifically binds to factor B and inhibits the alternative pathway (AP).

Methods

This randomized, double-blind, placebo-controlled phase 3 study (APPARENT; NCT05755386) will evaluate the efficacy and safety of iptacopan in patients with idiopathic (primary) IC-MPGN, enrolling up to 68 patients (minimum of 10 adolescents) aged 12 to 60 years with biopsy-confirmed IC-MPGN, proteinuria ≥1 g/g, and estimated glomerular filtration rate (eGFR) ≥30 ml/min per 1.73 m2. All patients will receive maximally tolerated angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and vaccination against encapsulated bacteria. Patients with any organ transplant, progressive crescentic glomerulonephritis, or kidney biopsy with >50% interstitial fibrosis/tubular atrophy, will be excluded. Patients will be randomized 1:1 to receive either iptacopan 200 mg twice daily (bid) or placebo for 6 months, followed by open-label treatment with iptacopan 200 mg bid for all patients for 6 months. The primary objective of the study is to evaluate the efficacy of iptacopan versus placebo in proteinuria reduction measured as urine protein-to-creatinine ratio (UPCR) (24-h urine) at 6 months. Key secondary end points will assess kidney function measured by eGFR, patients who achieve a proteinuria-eGFR composite end point, and patient-reported fatigue.

Conclusion

This study will provide evidence toward the efficacy and safety of iptacopan in idiopathic (primary) IC-MPGN.

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.

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.

Rare Variants in Complement Gene in C3 Glomerulopathy and Immunoglobulin-Mediated Membranoproliferative GN

BACKGROUND

C3 glomerulopathy and idiopathic immunoglobulin-mediated membranoproliferative GN (Ig-MPGN) are rare complement-mediated kidney diseases. Inherited forms of C3 glomerulopathy/Ig-MPGN are rarely described.

METHODS

Three hundred ninety-eight patients with C3 glomerulopathy ( n =296) or Ig-MPGN ( n =102) from a national registry were screened for three complement genes: factor H ( CFH ), factor I ( CFI ), and C3 . Patients with rare variant (minor allele frequency <0.1%) were included. Epidemiologic, clinical, and immunologic data at diagnosis and kidney outcomes of patients were retrospectively collected.

RESULTS

Fifty-three different rare variants, including 30 (57%), 13 (24%), and ten (19%) in CFH , CFI , and C3 variants, were identified in 66/398 (17%) patients. Thirty-eight (72%) variants were classified as pathogenic, including 20/30 (66%) and 11/13 (84%) variants in CFH and CFI , respectively, impairing synthesis of factor H or factor I regulators. Fifteen of 53 (27%) variants were of unknown significance. At diagnosis, 69% of patients were adult (median age of 31 years). With the exception of biologic stigma of thrombotic microangiopathy, which was more frequent in patients with CFI variants (5/14 [36%] versus 1/37 [3%] and 0% in the CFH group and C3 group, respectively, P < 0.001), the clinical and histologic features were similar among the three variants groups. The kidney outcome was poor regardless of the age at onset and treatment received. Sixty-five percent (43/66) of patients with rare variant reach kidney failure after a median delay of 41 (19-104) months, compared with 28% (55/195) after a median delay of 34 (12-143) months in the nonvariant group. Among 36 patients who received a kidney transplant, 2-year recurrence was frequent, occurring in 39% (12/31), without difference between variant groups, and led to graft failure in three cases.

CONCLUSIONS

In our cohort, 17% of C3 glomerulopathy/Ig-MPGN cases were associated with rare variants in the CFH , CFI , or C3 genes. In most cases, a quantitative deficiency in factor H or factor I was identified. The presence of a rare variant was associated with poor kidney survival.

PODCAST

This article contains a podcast at https://dts.podtrac.com/redirect.mp3/www.asn-online.org/media/podcast/CJASN/2023_11_08_CJN0000000000000252.mp3.

Adjustment of p-value expression to ontology using machine learning for genetic prediction, prioritization, interaction, and its validation in glomerular disease

The results of gene expression analysis based on p-value can be extracted and sorted by their absolute statistical significance and then applied to multiple similarity scores of their gene ontology (GO) terms to promote the combination and adjustment of these scores as essential predictive tasks for understanding biological/clinical pathways. The latter allows the possibility to assess whether certain aspects of gene function may be associated with other varieties of genes, to evaluate regulation, and to link them into networks that prioritize candidate genes for classification by applying machine learning techniques. We then detect significant genetic interactions based on our algorithm to validate the results. Finally, based on specifically selected tissues according to their normalized gene expression and frequencies of occurrence from their different biological and clinical inputs, a reported classification of genes under the subject category has validated the abstract (glomerular diseases) as a case study.

Genetic investigation of Nordic patients with complement-mediated kidney diseases

Background

Complement activation in atypical hemolytic uremic syndrome (aHUS), C3 glomerulonephropathy (C3G) and immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) may be associated with rare genetic variants. Here we describe gene variants in the Swedish and Norwegian populations.

Methods

Patients with these diagnoses (N=141) were referred for genetic screening. Sanger or next-generation sequencing were performed to identify genetic variants in 16 genes associated with these conditions. Nonsynonymous genetic variants are described when they have a minor allele frequency of <1% or were previously reported as being disease-associated.

Results

In patients with aHUS (n=94, one also had IC-MPGN) 68 different genetic variants or deletions were identified in 60 patients, of which 18 were novel. Thirty-two patients had more than one genetic variant. In patients with C3G (n=40) 29 genetic variants, deletions or duplications were identified in 15 patients, of which 9 were novel. Eight patients had more than one variant. In patients with IC-MPGN (n=7) five genetic variants were identified in five patients. Factor H variants were the most frequent in aHUS and C3 variants in C3G. Seventeen variants occurred in more than one condition.

Conclusion

Genetic screening of patients with aHUS, C3G and IC-MPGN is of paramount importance for diagnostics and treatment. In this study, we describe genetic assessment of Nordic patients in which 26 novel variants were found.

Complement factor I: Regulatory nexus, driver of immunopathology, and therapeutic

Complement factor I (FI) is the nexus for classical, lectin and alternative pathway complement regulation. FI is an 88 kDa plasma protein that circulates in an inactive configuration until it forms a trimolecular complex with its cofactor and substrate whereupon a structural reorganization allows the catalytic triad to cleave its substrates, C3b and C4b. In keeping with its role as the master complement regulatory enzyme, deficiency has been linked to immunopathology. In the setting of complete FI deficiency, a consumptive C3 deficiency results in recurrent infections with encapsulated microorganisms. Aseptic cerebral inflammation and vasculitic presentations are also less commonly observed. Heterozygous mutations in the factor I gene (CFI) have been demonstrated to be enriched in atypical haemolytic uraemic syndrome, albeit with a very low penetrance. Haploinsufficiency of CFI has also been associated with decreased retinal thickness and is a strong risk factor for the development of age-related macular degeneration. Supplementation of FI using plasma purified or recombinant protein has long been postulated, however, technical difficulties prevented progression into clinical trials. It is only using gene therapy that CFI supplementation has reached the clinic with GT005 in phase I/II clinical trials for geographic atrophy.

Clinico-Pathogenic Similarities and Differences between Infection-Related Glomerulonephritis and C3 Glomerulopathy

Recently, the comprehensive concept of "infection-related glomerulonephritis (IRGN)" has replaced that of postinfectious glomerulonephritis (PIGN) because of the diverse infection patterns, epidemiology, clinical features, and pathogenesis. In addition to evidence of infection, hypocomplementemia particularly depresses serum complement 3 (C3), with endocapillary proliferative and exudative GN developing into membranoproliferative glomerulonephritis (MPGN); also, C3-dominant or co-dominant glomerular immunofluorescence staining is central for diagnosing IRGN. Moreover, nephritis-associated plasmin receptor (NAPlr), originally isolated from the cytoplasmic fraction of group A Streptococci, is vital as an essential inducer of C3-dominant glomerular injury and is a key diagnostic biomarker for IRGN. Meanwhile, "C3 glomerulopathy (C3G)", also showing a histological pattern of MPGN due to acquired or genetic dysregulation of the complement alternative pathway (AP), mimics C3-dominant IRGN. Initially, C3G was characterized by intensive "isolated C3" deposition on glomeruli. However, updated definitions allow for glomerular deposition of other complement factors or immunoglobulins if C3 positivity is dominant and at least two orders of magnitude greater than any other immunoreactant, which makes it challenging to quickly distinguish pathomorphological findings between IRGN and C3G. As for NAPlr, it was demonstrated to induce complement AP activation directly in vitro, and it aggravates glomerular injury in the development of IRGN. A recent report identified anti-factor B autoantibodies as a contributing factor for complement AP activation in pediatric patients with PIGN. Moreover, C3G with glomerular NAPlr deposition without evidence of infection was reported. Taken together, the clinico-pathogenic features of IRGN overlap considerably with those of C3G. In this review, similarities and differences between the two diseases are highlighted.

Clinical Profiles and Patterns of Kidney Disease Progression in C3 Glomerulopathy

Key Points

Kidney survival in C3 glomerulopathy is significantly higher in patients with a disease chronicity score <4 and proteinuria <3.5 g/d, regardless of baseline eGFR. A faster eGFR decline in C3 glomerulopathy is associated with higher probability of kidney failure. Patients with glomerulopathy with a progressive reduction in proteinuria over time did not reach kidney failure.

Background

C3 glomerulopathy is a rare kidney disease, which makes it difficult to collect large cohorts of patients to better understand its variability. The aims of this study were to describe the clinical profiles and patterns of progression of kidney disease.

Methods

This was a retrospective, observational cohort study. Patients diagnosed with C3 glomerulopathy between 1995 and 2020 were enrolled. Study population was divided into clinical profiles by combining the following predictors: eGFR under/above 30 ml/min per 1.73 m2, proteinuria under/above 3.5 g/d, and histologic chronicity score under/above 4. The change in eGFR and proteinuria over time was evaluated in a subgroup with consecutive measurements of eGFR and proteinuria.

Results

One hundred and fifteen patients with a median age of 30 years (interquartile range 19–50) were included. Patients were divided into eight clinical profiles. Kidney survival was significantly higher in patients with a chronicity score <4 and proteinuria <3.5 g/d, both in those presenting with an eGFR under/above 30 ml/min per 1.73 m2. The median eGFR slope of patients who reached kidney failure was −6.5 ml/min per 1.73 m2 per year (interquartile range −1.6 to −17). Patients who showed a reduction in proteinuria over time did not reach kidney failure. On the basis of the rate of eGFR decline, patients were classified as faster eGFR decline (≥5 ml/min per 1.73 m2 per year), slower (<5 ml/min per 1.73 m2 per year), and those without decline. A faster eGFR decline was associated with higher probability of kidney failure.

Conclusions

Kidney survival is significantly higher in patients with a chronicity score <4 and proteinuria <3.5 g/d regardless of baseline eGFR, and a faster rate of decline in eGFR is associated with higher probability of kidney failure.

Diagnostic and Prognostic Comparison of Immune-Complex-Mediated Membranoproliferative Glomerulonephritis and C3 Glomerulopathy

Membranoproliferative glomerulonephritis (MPGN) is subdivided into immune-complex-mediated glomerulonephritis (IC-MPGN) and C3 glomerulopathy (C3G). Classically, MPGN has a membranoproliferative-type pattern, but other morphologies have also been described depending on the time course and phase of the disease. Our aim was to explore whether the two diseases are truly different, or merely represent the same disease process. All 60 eligible adult MPGN patients diagnosed between 2006 and 2017 in the Helsinki University Hospital district, Finland, were reviewed retrospectively and asked for a follow-up outpatient visit for extensive laboratory analyses. Thirty-seven (62%) had IC-MPGN and 23 (38%) C3G (including one patient with dense deposit disease, DDD). EGFR was below normal (≤60 mL/min/1.73 m²) in 67% of the entire study population, 58% had nephrotic range proteinuria, and a significant proportion had paraproteins in their serum or urine. A classical MPGN-type pattern was seen in only 34% of the whole study population and histological features were similarly distributed. Treatments at baseline or during follow-up did not differ between the groups, nor were there significant differences observed in complement activity or component levels at the follow-up visit. The risk of end-stage kidney disease and survival probability were similar in the groups. IC-MPGN and C3G have surprisingly similar characteristics, kidney and overall survival, which suggests that the current subdivision of MPGN does not add substantial clinical value to the assessment of renal prognosis. The high proportion of paraproteins in patient sera or in urine suggests their involvement in disease development.

Membranoproliferative glomerulonephritis: no longer the same disease and may need very different treatment

Membranoproliferative glomerulonephritis (MPGN) is a pattern of glomerular injury that may be primary or secondary to infections, autoimmune diseases and haematological disorders. Primary C3G and IC-MPGN are rare and the prognosis is unfavourable. Based on immunofluorescence findings, MPGN has been classified into complement-mediated C3 glomerulopathy (C3G) and immune complex-mediated MPGN (IC-MPGN). However, this classification leaves a number of issues unresolved. The finding of genetic and acquired complement abnormalities in both C3G and IC-MPGN indicates that they represent a heterogeneous spectrum rather than distinct diseases. An unsupervised hierarchical clustering in a cohort of patients with primary C3G and IC-MPGN identified four distinct pathogenetic patterns, characterized by specific histologic and clinical features, and genetic and acquired complement abnormalities. These results provide the groundwork for a more accurate diagnosis and the development of targeted therapies. The drugs that are currently used, such as corticosteroids and immunosuppressants, are frequently ineffective in primary C3G and IC-MPGN. Eculizumab, an anti-C5 monoclonal antibody, has been used occasionally in single cases or small series. However, only a few patients have achieved remission. This heterogeneous response could be related to the extent of terminal complement activation, which may vary substantially from patient to patient. Several drugs that target the complement system at different levels are under investigation for C3G and IC-MPGN. However, clinical trials to test new therapeutics will be challenging and heavily influenced by the heterogeneity of these diseases. This creates the need to characterize each patient to match the specific complement abnormality with the type of intervention.

An ex vivo test to investigate genetic factors conferring susceptibility to atypical haemolytic uremic syndrome

Introduction

Comprehensive genetic analysis is essential to clinical care of patients with atypical haemolytic uremic syndrome (aHUS) to reinforce diagnosis, and to guide treatment. However, the characterization of complement gene variants remains challenging owing to the complexity of functional studies with mutant proteins. This study was designed: 1) To identify a tool for rapid functional determination of complement gene variants; 2) To uncover inherited complement dysregulation in aHUS patients who do not carry identified gene variants.

Methods

To address the above goals, we employed an ex-vivo assay of serum-induced C5b-9 formation on ADP-activated endothelial cells in 223 subjects from 60 aHUS pedigrees (66 patients and 157 unaffected relatives).

Results

Sera taken from all aHUS patients in remission induced more C5b-9 deposition than control sera, independently from the presence of complement gene abnormalities. To avoid the possible confounding effects of chronic complement dysregulation related to aHUS status, and considering the incomplete penetrance for all aHUS-associated genes, we used serum from unaffected relatives. In control studies, 92.7% of unaffected relatives with known pathogenic variants exhibited positive serum-induced C5b-9 formation test, documenting a high sensitivity of the assay to identify functional variants. The test was also specific, indeed it was negative in all non-carrier relatives and in relatives with variants non-segregating with aHUS. All but one variants in aHUS-associated genes predicted in-silico as likely pathogenic or of uncertain significance (VUS) or likely benign resulted as pathogenic in the C5b-9 assay. At variance, variants in putative candidate genes did not exhibit a functional effect, with the exception of a CFHR5 variant. The C5b-9 assay in relatives was helpful in defining the relative functional effect of rare variants in 6 pedigrees in which the proband carried more than one genetic abnormality. Finally, for 12 patients without identified rare variants, the C5b-9 test in parents unmasked a genetic liability inherited from an unaffected parent.

Discussion

In conclusion, the serum-induced C5b-9 formation test in unaffected relatives of aHUS patients may be a tool for rapid functional evaluation of rare complement gene variants. When combined with exome sequencing the assay might be of help in variant selection, to identify new aHUS-associated genetic factors.

Modeling C3 glomerulopathies: C3 convertase regulation on an extracellular matrix surface

Introduction

C3 glomerulopathies (C3G) are ultra-rare complement-mediated diseases that lead to end-stage renal disease (ESRD) within 10 years of diagnosis in ~50% of patients. Overactivation of the alternative pathway (AP) of complement in the fluid phase and on the surface of the glomerular endothelial glycomatrix is the underlying cause of C3G. Although there are animal models for C3G that focus on genetic drivers of disease, in vivo studies of the impact of acquired drivers are not yet possible.

Methods

Here we present an in vitro model of AP activation and regulation on a glycomatrix surface. We use an extracellular matrix substitute (MaxGel) as a base upon which we reconstitute AP C3 convertase. We validated this method using properdin and Factor H (FH) and then assessed the effects of genetic and acquired drivers of C3G on C3 convertase.

Results

We show that C3 convertase readily forms on MaxGel and that this formation was positively regulated by properdin and negatively regulated by FH. Additionally, Factor B (FB) and FH mutants impaired complement regulation when compared to wild type counterparts. We also show the effects of C3 nephritic factors (C3Nefs) on convertase stability over time and provide evidence for a novel mechanism of C3Nef-mediated C3G pathogenesis.

Discussion

We conclude that this ECM-based model of C3G offers a replicable method by which to evaluate the variable activity of the complement system in C3G, thereby offering an improved understanding of the different factors driving this disease process.

Increased end-stage renal disease risk in age-related macular degeneration: a nationwide cohort study with 10-year follow-up

Common etiologies between age-related macular degeneration (AMD) and kidney disease advocate a close link between AMD and end-stage renal disease (ESRD). However, the risk of ESRD in people with AMD was not reported. Here, we investigated the association between AMD and the risk of ESRD by using a nationwide, population-based cohort data in Korea. 4,206,862 participants aged 50 years or older were categorized by presence of AMD and visual disability. Risk of ESRD was the primary outcome. Cox regression hazard model was used to examine the hazard ratios (HRs) with adjustment for potential confounders. Stratified analyses by age, sex, baseline kidney function, and cardiometabolic comorbidities were performed. During the mean 9.95 years of follow-up, there were 21,759 incident ESRD events (0.52%). AMD was associated with 33% increased risk of ESRD (adjusted HR [aHR] 1.33, 95% confidence interval [CI] 1.24-1.44), and the risk was even higher when accompanied by visual disability (aHR 2.05, 95% CI 1.68-2.50) than when not (aHR 1.26, 95% CI 1.17-1.37). Age, baseline kidney function, and cardiometabolic comorbidities significantly interact between AMD and the risk of ESRD. Our findings have clinical implications on disease prevention and risk factor management of ESRD in patients with AMD.

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.

Challenges in diagnostic testing of nephritic factors

Nephritic factors (NeFs) are autoantibodies promoting the activity of the central enzymes of the complement cascade, an important first line of defense of our innate immune system. NeFs stabilize the complement convertase complexes and prevent their natural and regulator-mediated decay. They are mostly associated with rare complement-mediated kidney disorders, in particular with C3 glomerulopathy and related diseases. Although these autoantibodies were already described more than 50 years ago, measuring NeFs for diagnostic purposes remains difficult, and this also complicates our understanding of their clinical associations. In this review, we address the multifactorial challenges of NeF diagnostics. We describe the diseases NeFs are associated with, the heterogenic mechanisms of action of different NeF types, the different methods available in laboratories used for their detection, and efforts for standardization. Finally, we discuss the importance of proper NeF diagnostics for understanding the clinical impact of these autoantibodies in disease pathophysiology and for considering future complement-directed therapy.

Results from a nationwide retrospective cohort measure the impact of C3 and soluble C5b-9 levels on kidney outcomes in C3 glomerulopathy

C3 glomerulopathy (C3G) is a rare complement-mediated disease. Specific treatments are not yet available and factors predictive of kidney survival such as age, kidney function and proteinuria are not specific to C3G. The prognostic value of biomarkers of complement activation, which are pathognomonic of the diseases, remains unknown. In a large cohort of 165 patients from the French National registry, we retrospectively assess the prognostic value of C3, soluble C5b-9 (sC5b-9), C3 nephritic factor, and rare disease-predicting variants in complement genes in predicting clinical outcome of patients. By multivariate analysis age (adult onset), reduced kidney function (defined by estimated glomerular filtration rate under 60ml/min) and presence of rare disease-predicting variants in complement genes predicted risk of progression to kidney failure. Moreover, by multivariate analysis, normal C3/high sC5b-9 levels or low C3/normal sC5b-9 levels remained independently associated with a worse kidney prognosis, with the relative risk 3.7- and 8-times higher, respectively. Subgroup analysis indicated that the complement biomarker profiles independently correlated to kidney prognosis in patients with adult but not pediatric onset. In this subgroup, we showed that profiles of biomarkers C3 and/or sC5b-9 correlated with intra glomerular inflammation and may explain kidney outcomes. In children, only the presence of rare disease-predicting variants correlated with kidney survival. Thus, in an adult population, we propose a three-point C3G prognostic score based on biomarker profiles at risk, estimated glomerular filtration rate at presentation and genetic findings, which may help stratify adult patients into subgroups that require close monitoring and more aggressive therapy.

C3 glomerulopathy: Understanding an ultra-rare complement-mediated renal disease

C3 glomerulopathy (C3G) describes a pathologic pattern of injury diagnosed by renal biopsy. It is characterized by the dominant deposition of the third component of complement (C3) in the renal glomerulus as resolved by immunofluorescence microscopy. The underlying pathophysiology is driven by dysregulation of the alternative pathway of complement in the fluid-phase and in the glomerular microenvironment. Characterization of clinical features and a targeted evaluation for indices and drivers of complement dysregulation are necessary for optimal patient care. Autoantibodies to the C3 and C5 convertases of complement are the most commonly detected drivers of complement dysregulation, although genetic mutations in complement genes can also be found. Approximately half of patients progress to end-stage renal disease within 10 years of diagnosis, and, while transplantation is a viable option, there is high risk for disease recurrence and allograft failure. This poor outcome reflects the lack of disease-specific therapy for C3G, relegating patients to symptomatic treatment to minimize proteinuria and suppress renal inflammation. Fortunately, the future is bright as several anti-complement drugs are currently in clinical trials.

Association of Histologic Parameters with Outcome in C3 Glomerulopathy and Idiopathic Immunoglobulin-Associated Membranoproliferative Glomerulonephritis

BACKGROUND AND OBJECTIVES

C3 glomerulopathy and idiopathic Ig-associated membranoproliferative GN are kidney diseases characterized by abnormal glomerular complement C3 deposition. These conditions are heterogeneous in outcome, but approximately 50% of patients develop kidney failure within 10 years.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

To improve identification of patients with poor prognosis, we performed a detailed analysis of percutaneous kidney biopsies in a large cohort of patients. Using a validated histologic scoring system, we analyzed 156 native diagnostic kidney biopsies from a retrospective cohort of 123 patients with C3 glomerulopathy and 33 patients with Ig-associated membranoproliferative GN. We used linear regression, survival analysis, and Cox proportional hazards models to assess the relationship between histologic and clinical parameters with outcome.

RESULTS

Frequent biopsy features were mesangial expansion and hypercellularity, glomerular basement membrane double contours, and endocapillary hypercellularity. Multivariable analysis showed negative associations between eGFR and crescents, interstitial inflammation, and interstitial fibrosis/tubular atrophy. Proteinuria positively associated with endocapillary hypercellularity and glomerular basement membrane double contours. Analysis of second native biopsies did not demonstrate associations between immunosuppression treatment and improvement in histology. Using a composite outcome, risk of progression to kidney failure associated with eGFR and proteinuria at the time of biopsy, cellular/fibrocellular crescents, segmental sclerosis, and interstitial fibrosis/tubular atrophy scores.

CONCLUSIONS

Our detailed assessment of kidney biopsy data indicated that cellular/fibrocellular crescents and interstitial fibrosis/tubular atrophy scores were significant determinants of deterioration in kidney function.

An Interdisciplinary Diagnostic Approach to Guide Therapy in C3 Glomerulopathy

Since the re-classification of membranoproliferative glomerulonephritis the new disease entity C3 glomerulopathy is diagnosed if C3 deposition is clearly dominant over immunoglobulins in immunohistochemistry or immunofluorescence. Although this new definition is more orientated at the pathophysiology as mediated by activity of the alternative complement pathway C3 glomerulopathy remains a heterogenous group of disorders. Genetic or autoimmune causes are associated in several but not in all patients with this disease. However, prognosis is poorly predictable, and clinicians cannot directly identify patients that might benefit from therapy. Moreover, therapy may range from supportive care alone, unspecific immune suppression, plasma treatment, or plasma exchange to complement inhibition. The current biopsy based diagnostic approaches sometimes combined with complement profiling are not sufficient to guide clinicians neither (i) whether to treat an individual patient, nor (ii) to choose the best therapy. With this perspective, we propose an interdisciplinary diagnostic approach, including detailed analysis of the kidney biopsy for morphological alterations and immunohistochemical staining, for genetic analyses of complement genes, complement activation patterning in plasma, and furthermore for applying novel approaches for convertase typing and complement profiling directly in renal tissue. Such a combined diagnostic approach was used here for a 42-year-old female patient with a novel mutation in the Factor H gene, C3 glomerulopathy and signs of chronic endothelial damage. We present here an approach that might in future help to guide therapy of renal diseases with relevant complement activation, especially since diverse new anti-complement agents are under clinical investigation.

Complement Factor I Variants in Complement-Mediated Renal Diseases

C3 glomerulopathy (C3G) and atypical hemolytic uremic syndrome (aHUS) are two rare diseases caused by dysregulated activity of the alternative pathway of complement secondary to the presence of genetic and/or acquired factors. Complement factor I (FI) is a serine protease that downregulates complement activity in the fluid phase and/or on cell surfaces in conjunction with one of its cofactors, factor H (FH), complement receptor 1 (CR1/CD35), C4 binding protein (C4BP) or membrane cofactor protein (MCP/CD46). Because altered FI activity is causally related to the pathogenesis of C3G and aHUS, we sought to test functional activity of select CFI missense variants in these two patient cohorts. We identified 65 patients (16, C3G; 48, aHUS; 1 with both) with at least one rare variant in CFI (defined as a MAF < 0.1%). Eight C3G and eleven aHUS patients also carried rare variants in either another complement gene, ADAMTS13 or THBD. We performed comprehensive complement analyses including biomarker profiling, pathway activity and autoantibody testing, and developed a novel FI functional assay, which we completed on 40 patients. Seventy-eight percent of rare CFI variants (31/40) were associated with FI protein levels below the 25th percentile; in 22 cases, FI levels were below the lower limit of normal (type 1 variants). Of the remaining nine variants, which associated with normal FI levels, two variants reduced FI activity (type 2 variants). No patients carried currently known autoantibodies (including FH autoantibodies and nephritic factors). We noted that while rare variants in CFI predispose to complement-mediated diseases, phenotypes are strongly contingent on the associated genetic background. As a general rule, in isolation, a rare CFI variant most frequently leads to aHUS, with the co-inheritance of a CD46 loss-of-function variant driving the onset of aHUS to the younger age group. In comparison, co-inheritance of a gain-of-function variant in C3 alters the phenotype to C3G. Defects in CFH (variants or fusion genes) are seen with both C3G and aHUS. This variability underscores the complexity and multifactorial nature of these two complement-mediated renal diseases.

Ex Vivo Test for Measuring Complement Attack on Endothelial Cells: From Research to Bedside

As part of the innate immune system, the complement system plays a key role in defense against pathogens and in host cell homeostasis. This enzymatic cascade is rapidly triggered in the presence of activating surfaces. Physiologically, it is tightly regulated on host cells to avoid uncontrolled activation and self-damage. In cases of abnormal complement dysregulation/overactivation, the endothelium is one of the primary targets. Complement has gained momentum as a research interest in the last decade because its dysregulation has been implicated in the pathophysiology of many human diseases. Thus, it appears to be a promising candidate for therapeutic intervention. However, detecting abnormal complement activation is challenging. In many pathological conditions, complement activation occurs locally in tissues. Standard routine exploration of the plasma concentration of the complement components shows values in the normal range. The available tests to demonstrate such dysregulation with diagnostic, prognostic, and therapeutic implications are limited. There is a real need to develop tools to demonstrate the implications of complement in diseases and to explore the complex interplay between complement activation and regulation on human cells. The analysis of complement deposits on cultured endothelial cells incubated with pathologic human serum holds promise as a reference assay. This ex vivo assay most closely resembles the physiological context. It has been used to explore complement activation from sera of patients with atypical hemolytic uremic syndrome, malignant hypertension, elevated liver enzymes low platelet syndrome, sickle cell disease, pre-eclampsia, and others. In some cases, it is used to adjust the therapeutic regimen with a complement-blocking drug. Nevertheless, an international standard is lacking, and the mechanism by which complement is activated in this assay is not fully understood. Moreover, primary cell culture remains difficult to perform, which probably explains why no standardized or commercialized assay has been proposed. Here, we review the diseases for which endothelial assays have been applied. We also compare this test with others currently available to explore complement overactivation. Finally, we discuss the unanswered questions and challenges to overcome for validating the assays as a tool in routine clinical practice.