Association of C4d deposition with clinical outcomes in IgA nephropathy

Monocytes and Macrophages in Kidney Disease and Homeostasis

The monocyte-macrophage lineage of inflammatory cells is characterized by significant morphologic and functional plasticity. Macrophages have broad M1 and M2 phenotype subgroups with distinctive functions and dual reno-toxic and reno-protective effects. Macrophages are a major contributor to injury in immune-complex-mediated, as well as pauci-immune, glomerulonephritis. Macrophages are also implicated in tubulointerstitial and vascular disease, though there have not been many human studies. Patrolling monocytes in the intravascular compartment have been reported in auto-immune injury in the renal parenchyma, manifesting as acute kidney injury. Insights into the pathogenetic roles of macrophages in renal disease suggest potentially novel therapeutic and prognostic biomarkers and targeted therapy. This review provides a concise overview of the macrophage-induced pathogenetic mechanism as a background for the latest findings about macrophages' roles in different renal compartments and common renal diseases.

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

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

Decrypting the Pathological Pathways in IgA Nephropathy

IgAN is the most common form of glomerulonephritis affecting 2000000 people annually. The disease ultimately progresses to chronic renal failure and ESRD. In this article, we focused on a comprehensive understanding of the pathogenesis of the disease and thus identifying different target proteins that could be essential in therapeutic approaches in the management of the disease. Aberrantly glycosylated IgA1 produced by the suppression of the enzyme β-1, 3 galactosyltransferase ultimately triggered the formation of IgG autoantibodies which form complexes with Gd-IgA1. The complex gets circulated through the blood vessels through monocytes and ultimately gets deposited in the glomerular mesangial cells via CD71 receptors present locally. This complex triggers the inflammatory pathways activating the alternate complement system, various types of T Cells, toll-like receptors, cytokines, and chemokines ultimately recruiting the phagocytic cells to eliminate the Gd-IgA complex. The inflammatory proteins cause severe mesangial and podocyte damage in the kidney which ultimately initiates the repair process following chronic inflammation by an important protein named TGFβ1. TGF β1 is an important protein produced during chronic inflammation mediating the repair process via various downstream transduction proteins and ultimately producing fibrotic proteins which help in the repair process but permanently damage the glomerular cells.

Treatment of IgA Nephropathy: A Rapidly Evolving Field

The pivotal event in the pathophysiology of IgA nephropathy is the binding of circulating IgA-containing immune complexes to mesangial cells, with secondary glomerular and tubulointerstitial inflammation and fibrosis. The paramount difficulty in the management of IgA nephropathy is the heterogeneity in its clinical presentation and prognosis, requiring an individualized treatment approach. Goal-directed supportive care remains the bedrock of therapy for all patients, regardless of risk of progression. Sodium-glucose transporter 2 inhibitors and sparsentan should be integral to contemporary supportive care, particularly in patients with chronic kidney damage. Pending the development of reliable biomarkers, it remains a challenge to identify patients prone to progression due to active disease and most likely to derive a net benefit from immunosuppression. The use of clinical parameters, including the degree of proteinuria, the presence of persistent microscopic hematuria, and the rate of eGFR loss, combined with the mesangial hypercellularity, endocapillary hypercellularity, segmental glomerulosclerosis, tubular atrophy/interstitial fibrosis, crescents score, is currently the best approach. Systemic glucocorticoids are indicated in high-risk patients, but the beneficial effects wane after withdrawal and come at the price of substantial treatment-associated toxicity. Therapies with direct effect on disease pathogenesis are increasingly becoming available. While targeted-release budesonide has garnered the most attention, anti-B-cell strategies and selective complement inhibition will most likely prove their added value. We propose a comprehensive approach that tackles the different targets in the pathophysiology of IgA nephropathy according to their relevance in the individual patient.

Validating the association of Oxford classification and renal function deterioration among Taiwanese individuals with Immunoglobulin A nephropathy

Validation of the Oxford classification (MEST and MEST-C) for Immunoglobulin A nephropathy (IgAN) in the Taiwanese population is lacking. Our study aimed to validate this classification and assess individual lesion impact. We conducted a retrospective cohort study at Taichung Veterans General Hospital, Taiwan (Jan 2011-Jul 2023). Composite renal outcomes were evaluated using clinical conditions and estimated glomerular filtration rate (eGFR). We used Kaplan-Meier, univariable/multivariable logistic regression and ROC curves. Subgroup analysis considered eGFR < or ≥ 30.0 ml/min/1.73 m2. In 366 renal biopsies, serum creatinine was 1.34 mg/dl, eGFR 53.8 ml/min/1.73 m2, urine protein-creatinine ratio 1159 mg/g. T1/T2 lesions had lowest baseline eGFR (39.6/11.5 ml/min/1.73 m2), correlating with poorest renal survival (median survival 54.7/34.4 months). Univariable analysis linked all individual variables to worse renal outcomes. Multivariable analysis (MEST/MEST-C) showed only T1/T2 linked to worse outcomes. T score had highest predictive power (AUC 0.728, sensitivity 60.2%, specificity 83.6%), with MEST having high AUC (0.758). No extra predictive power was seen transitioning MEST to MEST-C. Subgroup analysis (eGFR < 30.0 ml/min/1.73 m2) associated C1 with improved renal outcomes (odds ratio 0.14, 95% CI 0.03-0.65). T lesion correlated with worse outcomes across subgroups. The T lesion consistently correlated with worse renal outcomes across all groups and baseline statuses. Integrating the C lesion into the transition from MEST to MEST-C did not enhance predictive power. Importantly, the C1 lesion was linked to improved renal outcomes in the eGFR < 30.0 ml/min/1.73 m2 subgroup, likely due to treatment effects.

Targeting complement in IgA nephropathy

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

The role of complement in kidney disease

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

Evaluating Progression Risk in Patients With Immunoglobulin A Nephropathy

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

Complement inhibitors for kidney disease

A refined understanding of the role of complement in the pathogenesis of glomerular and other kidney diseases has, over the past two decades, been matched by the development of novel, complement-targeting therapies. As we increasingly recognize the important role that complement activation across all three pathways-classical, lectin and alternative-plays in glomerular lesions both rare (e.g. C3 glomerulopathy) and common (e.g. immunoglobulin A nephropathy), we can identify avenues for precise, targeted approaches to modifying the natural history of these kidney diseases. In this review, we survey the evidence on using complement inhibition from the earliest, small-scale studies focusing on C5-targeting agents to more recent, large, multicenter, randomized trials utilizing complement blockade higher up in the complement pathway at the level of C3. We conclude by examining where the field of complement targeting therapy may be headed in light of these studies.

A novel prognostic nomogram predicts premature failure of kidney allografts with IgA nephropathy recurrence

BACKGROUND

Recurrence of immunoglobulin A nephropathy (IgAN) limits graft survival in kidney transplantation. However, predictors of a worse outcome are poorly understood.

METHODS

Among 442 kidney transplant recipients (KTRs) with IgAN, 83 (18.8%) KTRs exhibited biopsy-proven IgAN recurrence between 1994 and 2020 and were enrolled in the derivation cohort. A multivariable Cox model predicting allograft loss based on clinical data at the biopsy and a web-based nomogram were developed. The nomogram was externally validated using an independent cohort (n = 67).

RESULTS

Patient age <43 years {hazard ratio [HR] 2.20 [95% confidence interval (CI) 1.41-3.43], P < .001}, female gender [HR 1.72 (95% CI 1.07-2.76), P = .026] and retransplantation status [HR 1.98 (95% CI 1.13-3.36), P = .016] were identified as independent risk factors for IgAN recurrence. Patient age <43 years [HR 2.77 (95% CI 1.17-6.56), P = .02], proteinuria >1 g/24 hours [HR 3.12 (95% CI 1.40-6.91), P = .005] and C4d positivity [HR 2.93 (95% CI 1.26-6.83), P = .013] were found to be associated with graft loss in patients with IgAN recurrence. A nomogram predicting graft loss was constructed based on clinical and histological variables, with a C statistic of 0.736 for the derivation cohort and 0.807 for the external validation cohort.

CONCLUSIONS

The established nomogram identified patients with recurrent IgAN at risk for premature graft loss with good predictive performance.

Challenges in IgA Nephropathy Management: An Era of Complement Inhibition

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

Present and Future of IgA Nephropathy and Membranous Nephropathy Immune Monitoring: Insights from Molecular Studies

IgA Nephropathy (IgAN) and Membranous Nephropathy (MN) are primary immune-mediated glomerular diseases with highly variable prognosis. Current guidelines recommend that greater immunologic activity and worse prognosis should guide towards the best treatment in an individualized approach. Nevertheless, proteinuria and glomerular filtration rate, the current gold standards for prognosis assessment and treatment guidance in primary glomerular diseases, may be altered with chronic damage and nephron scarring, conditions that are not related to immune activity. In recent years, thanks to the development of new molecular technologies, among them genome-wide genotyping, RNA sequencing techniques, and mass spectrometry, we have witnessed an outstanding improvement in understanding the pathogenesis of IgAN and MN. In addition, recent genome-wide association studies have suggested potential targets for immunomodulating agents, stressing the need for the identification of specific biomarkers of immune activity. In this work, we aim to review current evidence and recent progress, including the more recent use of omics techniques, in the identification of potential biomarkers for immune monitoring in IgAN and MN.

Clinicopathological Characteristics and Impacts on Clinical Outcomes of Thrombotic Microangiopathy Lesions in Patients with Immunoglobulin A Nephropathy in Thailand

INTRODUCTION

More reports of thrombotic microangiopathy (TMA) in immunoglobulin A (IgA) nephropathy suggest its association with poor clinical outcomes. However, the prevalence and clinical significance of TMA in IgA nephropathy have not been widely studied in different populations.

METHODS

Kidney biopsies of all patients with primary IgA nephropathy from 1995 to 2015 at the King Chulalongkorn Memorial Hospital, Thailand, were retrospectively reviewed and reclassified by two pathologists following the Oxford MEST-C classification. TMA lesions were detected based solely on light microscopic findings. Associations between the presence of TMA and clinical data, other pathologic findings, and clinical outcomes were studied.

RESULTS

Among 267 patients with primary IgA nephropathy, 166 had adequate clinical data and kidney tissues for the analysis. TMA was observed in 21 patients (13%) and was associated with higher mean arterial pressure (MAP), history of malignant hypertension, higher proteinuria, and lower estimated glomerular filtration rate (eGFR) at diagnosis compared to those without TMA. According to the Oxford MEST-C classification, TMA showed a significant association with severe tubular atrophy/interstitial fibrosis (T2) but not with mesangial hypercellularity (M1), endocapillary hypercellularity (E1), segmental glomerulosclerosis (S1), or crescents (C1-2). After a median follow-up of 50 months, patients with TMA had a significantly higher risk of progression to end-stage kidney disease (ESKD) (hazard ratio [HR] 5.8, 95% confidence interval [CI]: 3.1-10.9) and all-cause mortality (HR 3.4, 95% CI: 1.3-8.8). After adjusting for baseline eGFR, MAP, proteinuria, and other pathological lesions, TMA remained an independent predictor of ESKD (adjusted HR 2.4, 95% CI: 1.1-5.4).

CONCLUSIONS

Kidney TMA in IgA nephropathy is associated with advanced disease stages, carries a poor prognosis, and thus should be considered in the pathological classification of IgA nephropathy.

IgA Nephropathy: the Lectin Pathway and Implications for Targeted Therapy

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

Complement detection in kidney biopsies - utility and challenges

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Association between glomerular C4d deposition, proteinuria, and disease severity in children with IgA nephropathy

BACKGROUND

C4d may be used as a marker to evaluate the condition and prognosis of adults with IgA nephropathy, but there have been few studies of children with IgA nephropathy.

METHODS

C4d immunohistochemical staining was performed on samples from children with IgA nephropathy with C1q-negative immunofluorescence. The clinical and pathological treatment and prognostic characteristics of children in the C4d-positive and -negative groups were compared.

RESULTS

A total of sixty-five children with IgA nephropathy were included in the study and were followed up for an average of 37 months. C4d was mainly deposited along the capillary loops. The urinary protein-to-creatinine ratio (UPCR) in the C4d-positive group was significantly higher than that in the C4d-negative group (3.97 vs. 0.81, P < 0.001), and the average integrated optical density value of each child was positively correlated with the UPCR (r = 0.441, P < 0.001). There was a significant difference in the proportions of children with mesangial hypercellularity (M1) (68.97% vs. 44.44%, P = 0.048) and segmental glomerulosclerosis (S1) (65.52% vs. 33.33%, P = 0.010) between the C4d-positive group and the C4d-negative group. The proportion of children who received immunosuppressants in the C4d-positive group was higher than that in the C4d-negative group (86.21% vs. 36.11%, P < 0.001). There was no significant difference in the proportion of children developing kidney failure between the two groups.

CONCLUSION

C4d was found to be associated with proteinuria, segmental lesions, and immunosuppressant treatment. Activation of the lectin pathway may reflect the severity of clinical and pathological manifestations of IgA nephropathy in children. A higher resolution version of the Graphical abstract is available as Supplementary information.

Cytoplasmic WT1 in IgA nephropathy, an indicator of poor prognosis associated with mesangial/peri-mesangial C4d

BACKGROUND

We aimed to investigate the immuno-histochemical expression of C4d, ADAM10 and WT1 in kidney biopsies of immunoglobulin A nephropathy (IgAN) patients and correlate the findings with clinical, laboratory and histopathologic features in the hope of defining new parameters to better understand the pathogenesis of the disease, and predict prognosis.

MATERIALS AND METHODS

Paraffin-embedded kidney biopsy samples of 128 IgAN patients were immuno-histochemically treated with C4d and ADAM10/WT1 dual stain. Results were evaluated according to Oxford classification parameters, epidemiologic features, laboratory findings at presentation and clinical follow-up.

RESULTS

We observed C4d positivity in 40.6% of our patients, 25% of which was mesangial/peri-mesangial (m/pm) staining. Only m/pmC4d positivity statistically correlated with progression to end-stage renal disease (ESRD). M/pmC4d positive patients had statistically significantly higher baseline proteinuria levels, presence of crescents and > 25% segmental sclerosis of glomeruli. There was cytoplasmic staining of WT1 in 11.2% of cases. Presence of cWT1 correlated with m/pmC4d positivity and progression to ESRD. There was no glomerular ADAM10 detected and tubular expression of this protein did not relate to amount of tubular damage or other parameters.

CONCLUSION

This study is the first to show that cWT1is involved in IgAN and appears as an independent variable for worse prognosis.

Clinical relevance of glomerular C4d deposition in children with early IgA nephropathy or Henoch-Schönlein purpura nephropathy

BACKGROUND

Both IgA nephropathy (IgAN) and Henoch-Schönlein purpura nephropathy (HSPN) are characterized by glomerular mesangial IgA deposition. Several large studies on adults have suggested that glomerular C4d deposition has prognostic value in IgAN. However, there are few relevant studies on the clinical value of C4d deposition in children with IgAN or HSPN.

METHODS

We performed a retrospective cohort study in pediatric patients with IgAN or HSPN. Clinicopathological data were collected at the time of kidney biopsy. Kidney C4d deposition was analyzed by immunohistochemistry. The end point was defined as a ≥ 20% decrease in estimated glomerular filtration from baseline.

RESULTS

We enrolled 75 children, including 36 children with IgAN and 39 with HSPN. The prevalence of C4d deposition was 36% (27/75). C4d deposition was more abundant in children with proteinuria ≥ 50 mg/kg/day (51.9% versus 20.8%, P = 0.006) or nephrotic syndrome (37.0% versus 10.4%, P = 0.006). Mesangial hypercellularity (hazard ratio [HR], 5.745, 95% confidence interval [CI], 1.670-19.761, P = 0.006) and IgM deposition (HR, 4.522, 95% CI, 1.321-15.478, P = 0.016) were associated with C4d deposition. After a median follow-up of 22 months, seven (19.4%) IgAN patients and one (2.6%) HSPN patient had decreased kidney function. In children with IgAN, positive C4d was associated with decreased kidney function (P = 0.047).

CONCLUSION

Glomerular C4d deposition was associated with mesangial hypercellularity and glomerular IgM deposition in IgAN and HSPN. Glomerular C4d deposition may be a risk factor for eGFR decline in children with IgAN. A higher resolution version of the Graphical abstract is available as Supplementary information.

IgA Nephropathy: Current Understanding and Perspectives on Pathogenesis and Targeted Treatment

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide, with varied clinical and histopathological features between individuals, particularly across races. As an autoimmune disease, IgAN arises from consequences of increased circulating levels of galactose-deficient IgA1 and mesangial deposition of IgA-containing immune complexes, which are recognized as key events in the widely accepted "multi-hit" pathogenesis of IgAN. The emerging evidence further provides insights into the role of genes, environment, mucosal immunity and complement system. These developments are paralleled by the increasing availability of diagnostic tools, potential biomarkers and therapeutic agents. In this review, we summarize current evidence and outline novel findings in the prognosis, clinical trials and translational research from the updated perspectives of IgAN pathogenesis.

Current understanding of IgA antibodies in the pathogenesis of IgA nephropathy

Immunoglobulin A (IgA) is the most abundant isotype of antibodies, provides a first line of defense at mucosal surfaces against pathogens, and thereby contributes to mucosal homeostasis. IgA is generally considered as a non-inflammatory antibody because of its main function, neutralizing pathogenic virus or bacteria. Meanwhile, IgA can induce IgA-mediated diseases, such as IgA nephropathy (IgAN) and IgA vasculitis. IgAN is characterized by the deposition of IgA and complement C3, often with IgG and/or IgM, in the glomerular mesangial region, followed by mesangial cell proliferation and excessive synthesis of extracellular matrix in glomeruli. Almost half a century has passed since the first report of patients with IgAN; it remains debatable about the mechanism how IgA antibodies selectively bind to mesangial region-a hallmark of IgAN-and cause glomerular injuries in IgAN. Previous lectin- and mass-spectrometry-based analysis have revealed that IgAN patients showed elevated serum level of undergalactosylated IgA1 in O-linked glycans of its hinge region, called galactose-deficient IgA1 (Gd-IgA1). Thereafter, numerous studies have confirmed that the glomerular IgA from IgAN patients are enriched with Gd-IgA1; thus, the first hit of the current pathogenesis of IgAN has been considered to increase circulating levels of Gd-IgA1. Recent studies, however, demonstrated that this aberrant glycosylation alone is not sufficient to disease onset and progression, suggesting that several additional factors are required for the selective deposition of IgA in the mesangial region and induce nephritis. Herein, we discuss the current understanding of the characteristics of pathogenic IgA and its mechanism of inducing inflammation in IgAN.

IgA nephropathy: an overview of drug treatments in clinical trials

INTRODUCTION

IgA nephropathy (IgAN) is the commonest primary glomerulonephritis worldwide and may progress to end-stage kidney disease (ESKD) within a 10-20 year period. Its slowly progressive course has made clinical trials challenging to perform, however the acceptance of proteinuria reduction as a surrogate end point has significantly improved the feasibility of conducting clinical trials in IgAN, with several novel and repurposed therapies currently undergoing assessment. Already, interim results are demonstrating value to some of these, offering great hope to those with IgAN.

AREAS COVERED

This review explores the rationale, candidates, clinical precedents, and trial status of therapies that are currently or have recently been evaluated for efficacy in IgAN. All IgAN trials registered with the U.S. National Library of Medicine; ClinicalTrials.gov were reviewed.

EXPERT OPINION

For the first time, effective treatment options beyond supportive care are becoming available for those with IgAN. This is the culmination of commendable international efforts and signifies a new era for those with IgAN. As more therapies become available, future challenges will revolve around deciding which treatments are most appropriate for individual patients, which is likely to push IgAN into the realm of precision medicine.

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

Introduction

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

Methods

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

Results

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

Conclusion

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

How to take advantage of easily available biomarkers in patients with IgA nephropathy: IgA and C3 in serum and kidney biopsies

IgA nephropathy (IgAN) is the most common glomerulonephritis worldwide. It is diagnosed based on clinical and histological features including predominant IgA deposits in kidney biopsy. The multi-hit theory, based on the production of GDIgA1 and anti-GDIgA1 antibodies, and complement activation via alternative and lectin pathways and also a genetic tendency are crucial in the pathogenesis of IgAN. The aim of the present review is to summarize the utility of routine diagnostic tests in IgA nephropathy, such as IgA and C3 in serum and kidney biopsy specimens, for predicting the disease progression. The paper also contains data on new markers used in the diagnosis and prognosis of IgA nephropathy.

The Role of Complement in Microangiopathic Lesions of IgA Nephropathy

Introduction

Methods

Results

Conclusion

The rational use of complement inhibitors in kidney diseases

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

Comparison of Complement Pathway Activation in Autoimmune Glomerulonephritis

Introduction

Methods

Results

Conclusion

Activation of Complement System in Henoch-Schönlein Purpura Nephritis

Introduction: We studied the association between Henoch-Schönlein purpura nephritis (HSPN) and complement system activation. Methods: We retrospectively reviewed the pathologic findings and medical records of 35 children and 12 adults with HSPN and compared the differences according to C4d positivity in three groups consisting of total 47 patients, 35 pediatric and 12 adult patients, respectively. C4d staining of renal biopsy was additionally performed at the time of diagnosis or retrospectively using archival biopsy material. Results: The overall rate of C4d positivity was 53.2%: 20 (57.1%) of the 35 children and five (41.7%) of the 12 adults. Among the groups there was no significant difference in the severity of proteinuria, renal function, presence of crescents or mesangial proliferation stratified by C4d positivity, unlike IgA nephropathy. Conclusions: We suggest that the activation of complement system is not correlated with the clinical or pathological severity of HSPN.

C4d as a Screening Tool and an Independent Predictor of Clinical Outcomes in Lupus Nephritis and IgA Nephropathy

Background

As an indispensable marker of complement cascades activation, C4d was confirmed of its crucial role in the pathogenesis of both lupus nephritis (LN) and IgA nephropathy (IgAN). While the studies directly comparing the diagnostic value, and outcomes predicting function of C4d between LN and IgAN are still absent.

Methods

A cohort of 120 LN patients, 120 IgAN patients who were diagnosed by renal biopsy between January 2015 and December 2017 and 24 healthy age matched controls were prospectively analyzed. The patients were followed till December 2020. The outcomes were adverse disease treatment response (disease relapse) and kidney disease progression event (decline of estimated glomerular filtration rate by more than 20% or end-stage kidney disease). The renal C4d deposition proportion and pattern were compared between IgAN and LN patients. In addition, the relationship between renal C4d deposition and disease subtypes, disease relapse as well as disease progression for LN and IgAN patients were also analyzed.

Results

The LN, IgAN patients and healthy controls were well matched in ages. The follow-up period was 38.5 (30.3–60.8) months for LN patients and 45.0 (30.5–57.0) months for IgAN patients. 78 patients (65.0%) with LN had renal C4d deposition, compared with only 39 IgAN patients (32.5%) with C4d deposition in renal tissues (P < 0.001). The LN patients shared different renal C4d distribution patterns with IgAN patients. Compared with IgAN patients, the C4d deposition in LN patients was significantly more in renal glomerulus (P < 0.001) and less in renal tubules (P = 0.003). For disease subtypes, renal C4d deposition was especially strong in class V membranous LN and IgAN with tubulointerstitial fibrosis (T1/T2) lesions. Renal C4d deposition was independently correlated with the disease relapse of LN patients (HR = 1.007, P = 0.040), and acted as an independent predictor of disease progression during the follow-up period for IgAN patients (HR = 1.821, P = 0.040).

Conclusions

Renal C4d distribution proportion and pattern differed between LN and IgAN patients. The presence of C4d in renal tissue acted as an independent predictor of relapse for LN patients and disease progression for IgAN patients.

Utilizing the MEST score for prognostic staging in IgA nephropathy

Background

The Oxford classification/MEST score is an established histopathologic scoring system for patients with IgA nephropathy (IgAN). The objective of this study was to derive a prognostic model for IgAN based on the MEST score and histopathologic features.

Methods

A total of 306 patients with biopsy-proven primary IgAN were included. Histopathologic samples were retrieved from the Norwegian Kidney Biopsy Registry and reclassified according to the Oxford classification. The study endpoint was end-stage renal disease (ESRD). Patients were subclassified into three risk models based on histologic features (Model A), a composite score calculated from the adjusted hazard ratio values (Model B), and on quartiles (Model C).

Results

The mean follow-up time was 16.5 years (range 0.2–28.1). In total, 61 (20%) patients reached ESRD during the study period. Univariate analysis of M, E, S, T and C lesions demonstrated that all types were associated with an increased risk of ESRD; however, a multivariate analysis revealed that only S, T and C lesions were associated with poor outcomes. Statistical analysis of 15-year data demonstrated that Models A and B were as predictive as the MEST score, with an area-under-the-curve at 0.85. The Harrel c index values were 0.81 and 0.80 for the MEST score and Models A and B, respectively. In the present cohort, adding C lesions to the MEST score did not improve the models prognostic value.

Conclusions

Patients can be divided into risk classes based on their MEST scores. Histopathologic data provide valuable prognostic information at the time of diagnosis. Model B was the most suitable for clinical practice because it was the most user-friendly.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02653-y.

A mixed-method evaluation of the relationship between Oxford classification scores and longitudinal changes in proteinuria in patients with immunoglobulin A nephropathy

INTRODUCTION

This study aimed to investigate the relationship between Oxford Classification scores and longitudinal changes in proteinuria in patients with immunoglobulin A nephropathy (IgAN).

METHODS

The study was a single-center retrospective cohort study involving 358 patients with primary IgAN who were treated at the Shenzhen Second People's Hospital, China, between January 2011 and May 2021. Multivariate linear regression and generalized additive mixed models (GAMMs), adjusted for traditional risk confounders, were used to evaluate the correlation between scores for mesangial hypercellularity (M), endocapillary hypercellularity (E), segmental glomerulosclerosis (S), tubular atrophy/interstitial fibrosis (T), and crescents (C) (known as the Oxford Classification MEST-C score system), with proteinuria/creatinine ratio (PCR) at the time of renal biopsy and longitudinal changes in PCR, respectively.

RESULTS

The median PCR was 1061 mg/g, and it increased on average by 68.82 mg/g per year in these patients. Among patients with renal insufficiency, compared with patients without relative lesions, those with E present (E1) (1153.44; 95% confidence interval [CI], 188.99-2117.89 mg/g) and C > 0 (C1/2) (1063.58; 95% CI, 185.25-1941.90 mg/g) were associated with increased PCR levels at the time of renal biopsy. What's more, S present (S1) (194.96; 95% CI, 54.50-335.43 mg/g per year) was associated with the fastest PCR increase; C > 0 (C1/2) (147.59; 95% CI, 8.32-286.86 mg/g per year) and T >25% (T1/2) (77.04; 95% CI, 7.18-146.89 mg/g per year), were also correlated with a faster PCR increase. In patients with normal kidney function, associations between S1 (55.46; 95% CI, 8.93-101.99 mg/g per year) and E1 (94.02; 95% CI, 21.47-166.58 mg/g per year) and PCR change could be observed. Additionally, in patients with overweight/obesity, S1 (156.09; 95% CI, 52.41-259.77 mg/g per year), E1 (143.34; 95% CI, 35.30-251.38 mg/g per year), T1/2 (116.04; 95% CI, 22.58-209.51 mg/g per year), as well as C1/2 (134.03; 95% CI, 41.73-226.32 mg/g per year) were associated with noticeably quicker PCR increase.

CONCLUSIONS

Overall, E1 and C1/2 were independently associated with raised proteinuria levels at the time of renal biopsy, and S1, E1, T1/2, C1/2 were independently associated with a longitudinal increase in proteinuria in the patients with IgAN, especially in those with renal insufficiency or overweight/obesity, suggesting that currently available treatments might not be satisfactory, and weight control might be beneficial. Individual therapy development might benefit from the use of the Oxford Classification system.

The diagnostic value of immunohistochemical staining of the interstitial vascular C4d complement in membranous nephropathy

Membranous glomerulonephritis (MGN) is the most common cause of adulthood nephrotic syndrome. Diagnosis of membranous nephritis is based on light electron immunofluorescence microscopy and clinical signs. Immune complex deposition against podocyte antigens such as phospholipase A2 receptor (PLA2R) activates the complement system. Of this, complement Component C4d (C4d) is involved in the classical and lectin pathways. This marker may be used by immunohistochemistry to diagnose MGN when other methods are not available. In this work, C4d expression was monitored by immunohistochemical analysis in the glomerular capillaries of patients with primary MGN (study group, N=33) versus patients with minimal change disease (MCD, control group, N=20) in a cross-sectional evaluation performed based on the diagnosis confirmed by light microscopy and immunofluorescence. There was no significant demographic difference between the two groups except for age (P=0.002). C4d immune-expression was positive in glomerular capillary (2+ to 4+) in most of the MGN patients, while it was negative in the MCD group. The sensitivity and specificity of C4d immunostaining were 95% and 100%, respectively. The Pearson correlation coefficient was 0.74 between C4d (immunohistochemistry) and immunoglobulins (IgG; immunofluorescence) and 0.65 between C4d (immunohistochemistry) and the C3 complement product (immunofluorescence). Immunohistochemical evaluation of C4d is, therefore, a sensitive and specific method that has a high correlation with IgG immunofluorescence.

Glomerular C4d Deposition and Kidney Disease Progression in IgA Nephropathy: A Systematic Review and Meta-analysis

Background

Glomerular deposition of C4d is a widely used biomarker for activation of the lectin pathway in the complement system and is reported to be associated with kidney progression in immunoglobulin A nephropathy (IgAN). The aim of this study was to evaluate whether glomerular C4d deposition, as a new biomarker, improves the prediction of kidney prognosis in IgAN.

Study Design

Systematic review and meta-analysis.

Setting & Population

Patients with biopsy-proven primary IgAN without age limitations.

Selection Criteria for Studies: Cross-sectional or cohort studies reporting the prevalence of glomerular C4d deposition or evaluating its association with IgAN progression.

Predictor

Glomerular C4d deposition.

Outcome

Composite progression event of a >30% decline in estimated glomerular filtration rate or end-stage kidney disease.

Results

12 studies with 1,251 patients were included. The prevalence of glomerular C4d deposition was 34% (95% CI, 27%-41%), with large heterogeneity (I² = 86%; P < 0.001). Patients with C4d deposition had lower estimated glomerular filtration rates (mean difference [MD], −11.48; 95% CI, −18.27 to −4.70; P < 0.001) as well as higher urinary protein-creatinine ratios (MD, 0.87; 95% CI, 0.53-1.21; P < 0.001) or 24-hour urinary protein excretion (MD, 0.99; 95% CI, 0.50-1.47; P < 0.001) and higher risk for hypertension (relative risk [RR], 1.45; 95% CI, 1.06-1.99; P = 0.02) than patients without C4d deposition. Glomerular C4d deposition was associated with a high Oxford classification score, including M1, E1, S1, and T1/2 lesions (all P ≤ 0.006). Patients with C4d deposition had higher rates of use of renin-angiotensin system blockers and immunosuppressants. Glomerular C4d was found to be a risk factor for the composite kidney event (RR, 3.17; 95% CI, 2.29-4.40; P < 0.001; adjusted HR, 2.05; 95% CI, 1.53-2.76; P < 0.001) and end-stage kidney disease (RR, 4.37; 95% CI, 3.15-6.07; P < 0.001) without evidence of heterogeneity.

Limitations

The definition of positive C4d was not uniform and not all studies provided data about kidney outcomes.

Conclusions

Glomerular C4d deposition is associated with an adverse prognosis and may be a useful biomarker of disease prediction in IgAN.

Prognostic Value of C4d Immunolabelling in Adult Patients With IgA Vasculitis

Background and Objectives: Glomerular C4d deposits are associated the severity and outcomes of IgA nephropathy. Whether this holds true in immunoglobulin A vasculitis (IgAV) is not known. The main objective of the study was to analyze the prognostic value of glomerular C4d immunolabelling on kidney impairment in adults with IgAV.

Design, Setting, Participants, Measurements: This retrospective cohort study included 120 adults with IgAV and a kidney biopsy performed between 1995 and 2018 in two French university hospital centers. All paraffin-embedded biopsies were reassessed according to Oxford classification. Immunofluorescence for C4d was performed in all cases. For analysis, patients were grouped according to positivity for C4d in the glomerular area. The main outcome was a composite endpoint of 50% increase in 24 h-proteinuria, or eGFR decrease by 50%, or kidney replacement therapy.

Results: The median follow-up was 28.3 months. Twenty-three patients met the composite endpoint, 12 for kidney replacement therapy, 6 for an eGFR decrease >50% and 5 for a >50% increase in proteinuria. At time of biopsy, the median proteinuria was 1.9 g/24 h and the median eGFR 73.5 mL/min/1.73 m². Among the 102 patients evaluable for C4d, 24 were positive on >30% glomeruli, mainly with a parieto-mesangial pattern. In this group, the initial proteinuria was more frequently nephrotic than in the C4d– group (60% vs. 33%, P = 0.039). Mesangial hypercellularity was more frequent in the C4d+ group (42% vs. 13%; P = 0.006) whereas macroscopic hematuria was more frequent in the C4d– group (18% vs. 0%; P = 0.03). After a median follow-up of 28 months, kidney survival did not differ according to C4d status.

Conclusion: In a population of adult IgAV patients, glomerular positivity for C4d was associated with the severity of the kidney disease at presentation, but not with subsequent renal function deterioration.

Collectin11 and Complement Activation in IgA Nephropathy

BACKGROUND AND OBJECTIVES

IgA nephropathy is the most common primary GN worldwide. Previous research demonstrated that collectin11, an initiator of the complement lectin pathway, was involved in both AKI and chronic tubulointerstitial fibrosis. Here, we investigated the potential role of collectin11 in the pathogenesis of IgA nephropathy.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The deposition of collectin11 and other complement proteins was detected in glomeruli of 60 participants with IgA nephropathy by immunofluorescence. In vitro, human mesangial cells were treated with IgA1-containing immune complexes derived from participants with IgA nephropathy. Then, the expression of collectin11 in mesangial cells was examined by quantitative RT-PCR and immunofluorescence. The codeposition of collectin11 with IgA1 or C3 on mesangial cells was detected by immunofluorescence and proximity ligation assays.

RESULTS

In total, 37% of participants with IgA nephropathy (22 of 60) showed codeposition of collectin11 with IgA in the glomerular mesangium. Using an injury model of mesangial cells, we demonstrated that IgA1-immune complexes derived from participants with IgA nephropathy increased the secretion of collectin11 in mesangial cells with the subsequent deposition of collectin11 on the cell surface via the interaction with deposited IgA1-immune complexes. In vitro, we found that collectin11 bound to IgA1-immune complexes in a dose-dependent but calcium-independent manner. Furthermore, deposited collectin11 initiated the activation of complement and accelerated the deposition of C3 on mesangial cells.

CONCLUSIONS

In situ-produced collectin11 by mesangial cells might play an essential role in kidney injury in a subset of patients with IgA nephropathy through the induction of complement activation.

Immunological drivers of IgA nephropathy: Exploring the mucosa-kidney link

IgA nephropathy (IgAN) is the most common pattern of primary glomerular disease reported worldwide. Up to 40% of those with IgAN progress to end-stage kidney disease within 20 years of diagnosis, with no currently available disease-specific treatment. This is likely to change rapidly, with evolving insights into the mechanisms driving this disease. IgAN is an immune-complex-mediated disease, and its pathophysiology has been framed by the 'four-hit hypothesis', which necessitates four events to occur for clinically significant disease to develop. However, this hypothesis does not explain the wide variability observed in its presentation or clinical progression. Recently, there has been great interest in exploring the role of the mucosal immune system in IgAN, especially given the well-established link between mucosal infections and disease flares. Knowledge of antigen-mucosal interactions is now being successfully leveraged for therapeutic purposes; the gut-directed drug Nefecon (targeted release formulation-budesonide) is on track to become the first medication to be approved specifically for the treatment of IgAN. In this review, we examine established immunological paradigms in IgAN, explore how antigen-mucosal immune responses drive disease, and discuss how this knowledge is being used to develop new treatments.

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.

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

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

Complement Activation Is Associated With Crescents in IgA Nephropathy

Introduction

Crescents, especially those found at a percentage greater than 50%, are often associated with rapid progression of kidney disease in IgA nephropathy (IgAN). The mechanism of crescents forming in IgAN is still unclear. In this study, we aimed to evaluate whether excess complement activation participates in the formation of crescents in IgAN.

Methods

One hundred IgAN patients with various proportions of crescents—24 with 1%–24%, 27 with 25%–49%, 21 with 50%–74% 12 with more than 75%, and 16 without crescents—were included. Urinary concentrations of mannose-binding lectin (MBL), Bb, C4d, C3a, C5a, and soluble C5b-9 (sC5b-9) were measured at the time of biopsy. Receiver operating characteristic (ROC) curves were performed to evaluate predictive ability of renal survival for urine complement activation. In addition, historical C4d, C5b-9, and C3d were stained by immunohistochemistry.

Results

IgAN patients with more than 50% crescent formation showed higher complement activation levels than the other patients (urinary C3a/creatinine (C3a/Cr): 6.7295 ng/mg, interquartile range (IQR) 1.4652–62.1086 ng/mg vs. 0.1055 ng/mg, IQR 0–1.4089 ng/mg; urinary C5a/Cr: 15.6202 ng/mg, 4.3127–66.7347 ng/mg vs. 0.3280 ng/mg, IQR 0.0859–2.4439 ng/mg; urinary sC5b-9/Cr: 98.6357 ng/mg, 8.8058–1,087.4578 ng/mg vs. 1.4262 ng/mg, 0.0916–11.0858 ng/mg, all p-values <0.001). The levels of urinary MBL and C4d representing lectin complement pathway showed a linear association with the proportion of crescents (r = 0.457 and 0.562, respectively, both p-values <0.001). Combined urine complement products could increase the predictive ability compared with crescents alone from 0.904 to 0.944 (p = 0.062) with borderline significance. Moreover, the glomerular C4d deposition rate elevated with the increase of proportions of crescents.

Conclusion

Excess complement activation may be involved in the formation of crescents, especially diffuse crescent formation, in patients with IgAN. Urinary C4d correlated with the proportion of crescents and was a potential biomarker for disease monitoring in crescentic IgAN.

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

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

Autoimmune-mediated renal disease and hypertension

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

IgA vasculitis nephritis in children and adults: one or different entities?

The clinical features of the kidney involvement in immunoglobulin A (IgA) vasculitis (IgAVN) differ in children and adults for both clinical presentation and progression. IgAVN in children has mostly a self-limiting course and favorable resolution, while in adults the kidney involvement is frequently severe with unfavorable outcome. However, a subset of children is at risk of progression within the pediatric age or decades later in adulthood, particularly when the diagnosis and a prompt intervention are delayed. Factors predicting progression and outcome in the whole spectrum of age have been investigated in recent research, as well as the relationship between IgAVN and primary IgAN, which share the same pathology features, in the light of peculiar clinical differences and progression tendencies, and hence need for selective treatments. The search for a personalized treatment in children with IgAV and in different ages of life should rely on the identification of different risks for progression. This review will focus on recent studies which contribute to improve our knowledge in this still largely unclear area.

Complement activation in IgA nephropathy

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

The Association of the Oxford Classification Score with Longitudinal Estimated Glomerular Filtration Rate Decline in Patients with Immunoglobulin A Nephropathy: A Mixed-Method Study

Introduction

The Oxford Classification score, which predicts renal outcomes for immunoglobulin A nephropathy (IgAN), is widely used in clinical practice. Nevertheless, the relationship between these markers and longitudinal changes in renal function are poorly understood.

Methods

This was a population-based retrospective cohort study of 280 adults with biopsy-proven primary IgAN from 2011 to 2018. We used generalized additive mixed models to control for traditional kidney disease risk factors to analyze the associations between Oxford Classification MEST-C scores (mesangial hypercellularity, M; endocapillary hypercellularity, E; segmental glomerulosclerosis, S; tubular atrophy/interstitial fibrosis, T; crescents, C) and longitudinal changes in the estimated glomerular filtration rate (eGFR) after renal biopsy.

Results

The median eGFR was 78.2 mL/min/1.73 m² at baseline, and then it decreased on average by 1.3 mL/min/1.73 m² per year in the entire cohort. In adjusted models, compared with patients without relative lesions, the presence of T > 50% (T2) (−5.7; 95% confidence interval [CI], −9.5 to −2.0 mL/min/1.73m² per year) was associated with the fastest eGFR decline. S present (S1) (−2.9; 95% CI, −4.6 to −1.1 mL/min/1.73m² per year) and C > 25% glomeruli (C2) (−3.4; 95% CI, −6.4 to −0.5 mL/min/1.73m² per year) also demonstrated steeper eGFR declines. However, we found no association between M > 0.5 (M1), E present (E1), T 26%–50% (T1), and C present ≥ 1 glomerulus (C1), and progressive eGFR decline (p > 0.05).

Conclusion

The Oxford Classification scores, S1, T2, and C2, were independently associated with the longitudinal decreases in renal function in patients with IgAN. These findings suggested therapies targeted at improving early damage to these lesions might be essential to delay renal progression.

Precision medicine in immunoglobulin A nephropathy: still a journey ahead

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerular disease worldwide and since its first description extensive research has identified a number of key central pathogenetic contributors, including genetic, immunological and environmental factors. Along with its multifaceted pathophysiology, the clinical presentation of IgAN varies, ranging from mild forms with only minor urinary findings and preserved renal function to cases that rapidly progress to end-stage renal disease. Because of this, early identification of patients at risk for a progressive course is urgently needed. The search for valid and easily accessible biomarkers showed urinary Dickkopf-3 as a promising candidate to predict the course of kidney function. In addition, a recently established IgAN risk prediction tool derived from an international cohort of IgAN patients allows estimation of the risk of a 50% loss of kidney function over several years upon diagnosis. This might serve as a significant tool to individually predict the course of renal function by combining biometric, clinical, histological and treatment information at the time of diagnosis. Today there is no doubt that a comprehensive supportive treatment regimen is the main pillar for all IgAN patients. The value of an additional immunosuppressive treatment in IgAN patients at risk for disease progression is less clear. Early risk stratification and individualized therapies would be desirable for IgAN patients to facilitate the choice of treatment strategies, which is still a matter of ongoing discussion.

An Update on the Current State of Management and Clinical Trials for IgA Nephropathy

IgA nephropathy remains the most common primary glomerular disease worldwide. It affects children and adults of all ages, and is a leading cause of end-stage kidney disease, making it a considerable public health issue in many countries. Despite being initially described over 50 years ago, there are still no disease specific treatments, with current management for most patients being focused on lifestyle measures and renin-angiotensin-aldosterone system blockade. However, significant advances in the understanding of its pathogenesis have been made particularly over the past decade, leading to great interest in developing new therapeutic strategies, and a significant rise in the number of interventional clinical trials being performed. In this review, we will summarise the current state of management of IgAN, and then describe major areas of interest where new therapies are at their most advanced stages of development, that include the gut mucosal immune system, B cell signalling, the complement system and non-immune modulators. Finally, we describe clinical trials that are taking place in each area and explore future directions for translational research.

Glomerular C4 deposition and glomerulosclerosis predict worse renal outcomes in Chinese patients with IgA nephropathy

Background

Immunoglobulin A nephropathy (IgAN) is a clinical and pathological syndrome with heterogenous manifestation and progression. Complement activation is involved in the disease. However, the clinical significance of C4 deposition in IgAN is obscure.

Methods

A multicenter retrospective study was conducted in biopsy-proven IgAN patients. Based on mesangial C4 deposition, patients were divided into two groups. The baseline clinical data and immunopathological phenotypes were compared. The composite endpoint was defined as eGFR decline greater than 50%, doubling of baseline serum creatinine, the occurrence of end-stage renal disease (ESRD).

Results

A total of 642 IgAN patients were recruited, with 41 patients showing mesangial C4 deposition. The mesangial C4 positive group showed lower serum albumin, higher proteinuria, and a higher rate of IgG, IgM, and C1q mesangial deposition. After a median follow-up of 43.18 months, 81 (12.62%) patients achieved the composite endpoint. The multivariate Cox regression models identified glomerular C4 deposition (hazard ratios [HR] = 3.22, 95% confidence intervals [CI] = 1.51–6.87, p < 0.01), global sclerosis (G1 vs. G0, HR = 1.90, 95%CI = 1.02–3.52, p = 0.04; G2 vs. G0, HR = 3.72, 95%CI = 1.98–7.00, p < 0.01), male (HR = 1.80, 95%CI = 1.10–2.97, p = 0.02), serum creatinine (HR = 1.01, 95%CI = 1.00–1.01, p < 0.01), triglyceride (HR = 1.17, 95%CI = 1.01–1.35, p = 0.04), proteinuria (HR = 1.07, 95%CI = 1.01–1.13, p = 0.02), serum C3 level (HR = 0.05, 95%CI = 0.01–0.25, p < 0.01), and serum C4 level (HR = 99.59, 95%CI = 8.69–1140.89, p < 0.01) as independent risk factors for poor renal outcomes.

Conclusions

Glomerular mesangial C4 deposition and global sclerosis are independent predictors for poor prognosis in IgAN patients.

C4d at Crossroads Between Post-Infectious Glomerulonephritis and C3 Glomerulopathy

Background

Post-infectious glomerulonephritis (PIGN) (immune complex-mediated glomerulonephritis) and C3 glomerulopathy are sub-types of glomerulonephritis (GN) with hypercellularity. Both have overlapping clinical and morphologic features on a kidney biopsy, however, the treatment and prognosis of these diseases are quite different making their distinction of utmost importance. Immune complex-mediated glomerulonephritis arises from glomerular deposition of immune-complexes (Igs) and C3 as a result of activation of classical (CP) and lectin pathways (LP). C4d is produced as a result of activation of the CP/LP. On the other hand, C3 glomerulopathy results from activation of alternative pathway of complement.

Aim

To distinguish between PIGN and C3 glomerulopathy with the help of C4d IHC stain.

Materials and Methods

We studied 28 biopsies reported as GN with hypercellularity from January 2015 to January 2020. Clinical information, histological features and immunofluorescence patterns were analyzed. C4d IHC was performed on all the biopsies. Six known cases of immune complex-mediated GN were selected to act as a positive control for C4d staining.

Results

Amongst 28 cases originally reported as GN with hypercellularity, 18 were labeled as post-infectious GN and 10 as C3 glomerulopathy based on clinical information and serological findings. 13 of 18 (72.2%) cases of PIGN had mild to moderate (1-2+) C4d staining, 2 (11.1%) had strong (3+) staining and 3 (16.7%) cases were negative for C4d staining. In the 10 biopsies of C3 glomerulopathy, mild (1+) C4d staining was noted only in 3 (30%) biopsies. C4d had moderate to strong (2-3+) staining in the control group.

Conclusion

C4d IHC stain can be helpful in distinguishing PIGN from C3 glomerulopathy.