C3 nephritic factor associated with C3 glomerulopathy in children

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.

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.

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.

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.

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.

Long-term follow-up including extensive complement analysis of a pediatric C3 glomerulopathy cohort

BACKGROUND

C3 glomerulopathy (C3G) is a rare kidney disorder characterized by predominant glomerular depositions of complement C3. C3G can be subdivided into dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). This study describes the long-term follow-up with extensive complement analysis of 29 Dutch children with C3G.

METHODS

Twenty-nine C3G patients (19 DDD, 10 C3GN) diagnosed between 1992 and 2014 were included. Clinical and laboratory findings were collected at presentation and during follow-up. Specialized assays were used to detect rare variants in complement genes and measure complement-directed autoantibodies and biomarkers in blood.

RESULTS

DDD patients presented with lower estimated glomerular filtration rate (eGFR). C3 nephritic factors (C3NeFs) were detected in 20 patients and remained detectable over time despite immunosuppressive treatment. At presentation, low serum C3 levels were detected in 84% of all patients. During follow-up, in about 50% of patients, all of them C3NeF-positive, C3 levels remained low. Linear mixed model analysis showed that C3GN patients had higher soluble C5b-9 (sC5b-9) and lower properdin levels compared to DDD patients. With a median follow-up of 52 months, an overall benign outcome was observed with only six patients with eGFR below 90 ml/min/1.73 m² at last follow-up.

CONCLUSIONS

We extensively described clinical and laboratory findings including complement features of an exclusively pediatric C3G cohort. Outcome was relatively benign, persistent low C3 correlated with C3NeF presence, and C3GN was associated with higher sC5b-9 and lower properdin levels. Prospective studies are needed to further elucidate the pathogenic mechanisms underlying C3G and guide personalized medicine with complement therapeutics.

C3 glomerulonephritis associated with ANCA positivity: a case report

BACKGROUND

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

CASE PRESENTATION

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

CONCLUSIONS

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

Therapy and outcomes of C3 glomerulopathy and immune-complex membranoproliferative glomerulonephritis

BACKGROUND

Data on therapy and outcome of dense deposit disease (DDD), C3 glomerulonephritis (C3GN), and immune-complex MPGN (IC-MPGN) in children are limited.

METHODS

In this retrospective single-center study from 2007 to 2019, kidney biopsies were reviewed to include patients aged <18-years with C3 glomerulopathy and IC-MPGN. Initial immunosuppression comprised prednisolone, mycophenolate mofetil (n = 51), tacrolimus (n = 11), and/or IV cyclophosphamide (n = 20). Clinicopathological features, response to therapy, and adverse outcome (eGFR_cr < 15 mL/min/1.73 m² or death) were evaluated.

RESULTS

A total of 92 patients were classified as DDD (n = 48, 52.2%), C3GN (n = 26, 28.3%), and IC-MPGN (n = 18, 19.6%) by immunohistochemistry and electron microscopy; 8 patients with DDD were misclassified as IC-MPGN on immunofluorescence. At last follow-up (median 4.3 years), complete or partial remission occurred in 28.5, 36.1, and 16.7% patients with DDD, C3GN, and IC-MPGN, respectively. Serum albumin at onset < 2.5 g/dL (HR = 0.29, P = 0.005) and persistently low serum C3 (HR = 0.34, P = 0.02) were associated with lack of remission. The 5-year kidney survival was 62.6, 85.5, and 88.5% in patients with DDD, C3GN, and IC-MPGN, respectively (log-rank, P = 0.006). Presentation as rapidly progressive GN (HR = 11.2, P < 0.001), age > 10 years at onset (HR = 4.0, P = 0.004), and DDD (HR = 4.2, P = 0.02) were independently associated with adverse outcome; achieving remission was protective (HR = 0.04; P < 0.001).

CONCLUSION

Outcome in patients with C3 glomerulopathy and IC-MPGN was unsatisfactory, and only a small proportion of patients achieved complete or partial remission. Patients with DDD were more likely to present with rapidly progressive GN and were at higher risk of adverse outcomes, including kidney failure.

The Immunopathology of Complement Proteins and Innate Immunity in Autoimmune Disease

The complement is a powerful cascade of the innate immunity and also acts as a bridge between innate and acquired immune defence. Complement activation can occur via three distinct pathways, the classical, alternative and lectin pathways, each resulting in the common terminal pathway. Complement activation results in the release of a range of biologically active molecules that significantly contribute to immune surveillance and tissue homeostasis. Several soluble and membrane-bound regulatory proteins restrict complement activation in order to prevent complement-mediated autologous damage, consumption and exacerbated inflammation. The crucial role of complement in the host homeostasis is illustrated by association of both complement deficiency and overactivation with severe and life-threatening diseases. Autoantibodies targeting complement components have been described to alter expression and/or function of target protein resulting in a dysregulation of the delicate equilibrium between activation and inhibition of complement. The spectrum of diseases associated with complement autoantibodies depends on which complement protein and activation pathway are targeted, ranging from autoimmune disorders to kidney and vascular diseases. Nevertheless, these autoantibodies have been identified as differential biomarkers for diagnosis or follow-up of disease only in a small number of clinical conditions. For some autoantibodies, a clear relationship with clinical manifestations has been identified, such as anti-C1q, anti-Factor H, anti-C1 Inhibitor antibodies and C3 nephritic factor. For other autoantibodies, the origin and the functional consequences still remain to be elucidated, questioning about the pathophysiological significance of these autoantibodies, such as anti-mannose binding lectin, anti-Factor I, anti-Factor B and anti-C3b antibodies. The detection of autoantibodies targeting complement components is performed in specialized laboratories; however, there is no consensus on detection methods and standardization of the assays is a real challenge. This review summarizes the current panorama of autoantibodies targeting complement recognition proteins of the classical and lectin pathways, associated proteases, convertases, regulators and terminal components, with an emphasis on autoantibodies clearly involved in clinical conditions.

Practical management of C3 glomerulopathy and Ig-mediated MPGN: facts and uncertainties

In recent years, a substantial body of experimental and clinical work has been devoted to C3 glomerulopathy and Ig-mediated membranoproliferative glomerulonephritis. Despite the rapid accumulation of data, several uncertainties about these 2 rare forms of nephropathies persist. They concern their pathophysiology, classification, clinical course, relevance of biomarkers and of pathology findings, and assessment of the efficacy of the available therapies. The present review discusses the impact of these uncertainties on the clinical management of patients.

Clinical and Pathophysiological Insights Into Immunological Mediated Glomerular Diseases in Childhood

The kidney is often the target of immune system dysregulation in the context of primary or systemic disease. In particular, the glomerulus represents the anatomical entity most frequently involved, generally as the expression of inflammatory cell invasion or circulant or in situ immune-complex deposition. Glomerulonephritis is the most common clinical and pathological manifestation of this involvement. There are no universally accepted classifications for glomerulonephritis. However, recent advances in our understanding of the pathophysiological mechanisms suggest the assessment of immunological features, biomarkers, and genetic analysis. At the same time, more accurate and targeted therapies have been developed. Data on pediatric glomerulonephritis are scarce and often derived from adult studies. In this review, we update the current understanding of the etiologic events and genetic factors involved in the pathogenesis of pediatric immunologically mediated primitive forms of glomerulonephritis, together with the clinical spectrum and prognosis. Possible new therapeutic targets are also briefly discussed.

Nephritic Factors: An Overview of Classification, Diagnostic Tools and Clinical Associations

Nephritic factors comprise a heterogeneous group of autoantibodies against neoepitopes generated in the C3 and C5 convertases of the complement system, causing its dysregulation. Classification of these autoantibodies can be clustered according to their stabilization of different convertases either from the classical or alternative pathway. The first nephritic factor described with the capacity to stabilize C3 convertase of the alternative pathway was C3 nephritic factor (C3NeF). Another nephritic factor has been characterized by the ability to stabilize C5 convertase of the alternative pathway (C5NeF). In addition, there are autoantibodies against assembled C3/C5 convertase of the classical and lectin pathways (C4NeF). These autoantibodies have been mainly associated with kidney diseases, like C3 glomerulopathy and immune complex-associated-membranoproliferative glomerulonephritis. Other clinical situations where these autoantibodies have been observed include infections and autoimmune disorders such as systemic lupus erythematosus and acquired partial lipodystrophy. C3 hypocomplementemia is a common finding in all patients with nephritic factors. The methods to measure nephritic factors are not standardized, technically complex, and lack of an appropriate quality control. This review will be focused in the description of the mechanism of action of the three known nephritic factors (C3NeF, C4NeF, and C5NeF), and their association with human diseases. Moreover, we present an overview regarding the diagnostic tools for its detection, and the main therapeutic approach for the patients with nephritic factors.

Predictive factors for poor outcome in pediatric C3 glomerulonephritis

BACKGROUND

To clarify the predictive factors for poor outcome in pediatric C3 glomerulonephritis (C3GN), we retrospectively evaluated the relationship between the clinico-pathological findings and prognosis in cases of pediatric C3GN.

METHODS

We enrolled 18 patients diagnosed with C3GN. These patients were divided into two groups, four patients in the end-stage renal disease (ESRD) group and 14 patients in non-ESRD group, based on clinical status at the last examination. Patients in the non-ESRD group were further divided into Subgroup A, consisting of 6 treatment responders, and Subgroup B, consisting of 8 non- responders. The clinical and laboratory findings, as well as the histological findings were investigated for each group.

RESULTS

The frequency of nephrotic syndrome at onset in the ESRD group was higher than that in the non-ESRD group. Before treatment and at 2 years after treatment, urinary protein excretion levels and serum creatinine levels in the ESRD group were higher than those in the non-ESRD group. The mean serum C3 and CH50 levels at 2 years after treatment in the ESRD group were lower than those in the non-ESRD group. The degree of renal injury, level of mesangial deposits and degree of alpha SMA staining at the time of the first renal biopsy in the ESRD group were all higher than those in the non-ESRD group.

CONCLUSIONS

Our results suggest that the severity of C3GN at onset and persistent complements activity are associated with poor prognosis in C3GN.

Overactivity of Alternative Pathway Convertases in Patients With Complement-Mediated Renal Diseases

Overactivation of the alternative pathway of the complement system is associated with the renal diseases atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G). C3 nephritic factors (C3NeF) play an important role in C3G pathogenesis by stabilizing the key enzymatic complex of complement, the C3 convertase. However, the reliability of assays detecting these autoantibodies is limited. Therefore, in this study, we validated and optimized a prototype hemolytic method for robust detection and characterization of factors causing convertase overactivity in large patient cohorts. The assay assesses convertase activity directly in the physiological milieu of serum and therefore is not restricted to detection of stabilizing autoantibodies such as C3NeF but may also reveal genetic variants resulting in prolonged convertase activity. We first defined clear cutoff values based on convertase activity in healthy controls. Next, we evaluated 27 C3G patient samples and found 16 positive for prolonged convertase activity, indicating the presence of factors influencing convertase stability. In three patients, the overactive convertase profile was persistent over disease course while in another patient the increased stability normalized in remission. In all these four patients, the convertase-stabilizing activity resided in the purified immunoglobulin (Ig) fraction, demonstrating the autoantibody nature. By contrast, the Igs of a familial aHUS patient carrying the complement factor B mutation p.Lys323Glu did not reveal convertase stabilization. However, in serum prolonged convertase activity was observed and segregated with the mutation in both affected and unaffected family members. In conclusion, we present a robust and reliable method for the detection, characterization, and evaluation over time of factors prolonging convertase activity (C3NeF or certain mutations) in patient cohorts. This assay may provide new insights in disease pathogenesis and may contribute to the development of more personalized treatment strategies.

Membranoproliferative glomerulonephritis and C3 glomerulopathy in children: change in treatment modality? A report of a case series

Background

Membranoproliferative glomerulonephritis (MPGN) with immune complexes and C3 glomerulopathy (C3G) in children are rare and have a variable outcome, with some patients progressing to end-stage renal disease (ESRD). Mutations in genes encoding regulatory proteins of the alternative complement pathway and of complement C3 (C3) have been identified as concausative factors.

Methods

Three children with MPGN type I, four with C3G, i.e. three with C3 glomerulonephritis (C3GN) and one with dense deposit disease (DDD), were followed. Clinical, autoimmune data, histological characteristics, estimated glomerular filtration rate (eGFR), proteinuria, serum C3, genetic and biochemical analysis were assessed.

Results

The median age at onset was 7.3 years and the median eGFR was 72 mL/min/1.73 m². Six children had marked proteinuria. All were treated with renin-angiotensin-aldosterone system (RAAS) blockers. Three were given one or more immunosuppressive drugs and two eculizumab. At the last median follow-up of 9 years after diagnosis, three children had normal eGFR and no or mild proteinuria on RAAS blockers only. Among four patients without remission of proteinuria, genetic analysis revealed mutations in complement regulator proteins of the alternative pathway. None of the three patients with immunosuppressive treatment achieved partial or complete remission of proteinuria and two progressed to ESRD and renal transplantation. Two patients treated with eculizumab revealed relevant decreases in proteinuria.

Conclusions

In children with MPGN type I and C3G, the outcomes of renal function and response to treatment modality show great variability independent from histological diagnosis at disease onset. In case of severe clinical presentation at disease onset, early genetic and biochemical analysis of the alternative pathway dysregulation is recommended. Treatment with eculizumab appears to be an option to slow disease progression in single cases.

Complement as a diagnostic tool in immunopathology

The complement system is a complex and autoregulated multistep cascade at the interface of innate and adaptive immunity. It is activated by immune complexes or apoptotic cells (classical pathway), pathogen-associated glycoproteins (lectin pathway) or a variety of molecular and cellular surfaces (alternative pathway). Upon activation, complement triggers the generation of proteolytic fragments that allow the elimination of the activating surface by enhancing inflammation, opsonization, phagocytosis, and cellular lysis. Moreover, complement efficiently discriminates self from non-self surfaces by means of soluble and membrane-bound complement regulators which are critical for innate self-tolerance. Complement deficiency or dysfunction disturb complement homeostasis and give rise to diseases as diverse as bacterial infections, autoimmunity, or renal and neurological disorders. Research on complement-targeted therapies is an expanding field that has already improved the prognosis of severe diseases such as atypical Haemolytic Uremic syndrome or Paroxysmal Nocturnal Haemoglobinuria. Therefore, complement analysis and monitoring provides valuable information with deep implications for diagnosis and therapy. In addition to its important role as an extracellular defense system, it has now become evident that complement is also present intracellularly, and its activation has profound implications for leukocyte survival and function. In this review, we summarize the essential, up-to-date information on the use of complement as a diagnostic and therapeutic tool in the clinics.

C3 glomerulopathy and eculizumab: a report on four paediatric cases

BACKGROUND

Eculizumab may be used to treat C3-glomerulopathy (C3G), a rare but severe glomerular disease.

DIAGNOSIS AND TREATMENT

Patients 1, 2 and 3 were diagnosed with nephritic syndrome with alternative complement pathway activation (low C3, C3Nef-positive) and C3G at the age of 9, 13 and 12 years, respectively. Treatment with eculizumab normalized proteinuria within 1, 2 and 7 months, respectively. Proteinuria relapsed when eculizumab was withdrawn, but the re-introduction of eculizumab normalized proteinuria. Patient 4 was diagnosed with C3G at 9 years of age, with progression to end-stage renal disease within 2 years, followed by a first renal transplantation (R-Tx) with early disease recurrence and graft loss within 39 months. After a second R-Tx, she rapidly presented with biological and histological recurrence: therapy with eculizumab was started, with no effect on proteinuria after 5 months, in a complex clinical setting (i.e. association of C3G recurrence, humoral rejection and BK nephritis). Eculizumab was withdrawn due to multiple viral reactivations, but the re-introduction of the drug a few months later enabled a moderate decrease in proteinuria.

CONCLUSION

These cases illustrate the efficacy of eculizumab, at least on native kidneys, in paediatric C3G. However, larger international studies are warranted to confirm the benefit and safety of eculizumab therapy.

C3 Glomerulopathy

Recent advances in our understanding of the disease pathology of membranoproliferative glomerulonephritis has resulted in its re-classification as complement C3 glomerulopathy (C3G) and immune complex-mediated glomerulonephritis (IC-GN). The new consensus is based on its underlying pathomechanism, with a key pathogenetic role for the complement alternative pathway (AP), rather than on histomorphological characteristics. In C3G, loss of AP regulation leads to predominant glomerular C3 deposition, which distinguishes C3G from IC-GN with predominant immunoglobulin G staining. Electron microscopy further subdivides C3G into C3 glomerulonephritis and dense deposit disease depending on the presence and distribution pattern of electron-dense deposits within the glomerular filter. Mutations or autoantibodies affecting the function of AP activators or regulators, in particular the decay of the C3 convertase (C3 nephritic factor), have been detected in up to 80 % of C3G patients. The natural outcome of C3G is heterogeneous, but 50 % of patients progress slowly and reach end-stage renal disease within 10-15 years. The new classification not only marks significant advancement in the pathogenic understanding of this rare disease, but also opens doors towards more specific treatment with the potential for improved outcomes.

Revisiting post-infectious glomerulonephritis in the emerging era of C3 glomerulopathy

Background

Post-infectious glomerulonephritis (PIGN) is an immune complex-mediated glomerular injury that typically resolves. Dominant C3 deposition is characteristic of PIGN, but with the emergence of C3 glomerulonephritis (C3GN) as a distinct entity, it is unclear how the pathologic similarities between PIGN and C3GN should be reconciled. Therefore, nephrologists and nephropathologists need additional guidance at the time of biopsy.

Methods

We studied 23 pediatric and young adult patients diagnosed with PIGN. Patients were divided into two groups, one with co-dominance between C3 and immunoglobulins and the other meeting proposed diagnostic criteria for C3GN. Clinical and pathological features were compared.

Results

No clinical and/or pathological features could distinguish between those with C3-co-dominant deposits and those with C3 dominance. Nearly all patients in both groups regained their baseline renal function without clinical intervention.

Conclusions

Although the identification of abnormalities of the alternative pathway of complement is characteristic of C3GN, testing is not widely available and the turnaround time often exceeds 1 month. Our study found that PIGN with either co-dominant or dominant C3 deposition in a cohort of young patients has excellent short-term outcomes. Close clinical observation for persistent abnormalities, such as hypocomplementemia, prolonged hematuria or proteinuria, is recommended to single out patients that may harbor intrinsic complement abnormalities.

Overview of C3 Glomerulopathy

C3 glomerulopathy is an umbrella term, which includes several rare forms of glomerulonephritis (GN) with underlying defects in the alternate complement cascade. A common histological feature noted in all these GN is dominant C3 deposition in the glomerulus. In this review, we will provide an overview of the complement system as well as mediators, with an introduction to pharmaceutical agents that can alter the pathway.

Successful therapy of C3Nef-positive C3 glomerulopathy with plasma therapy and immunosuppression

BACKGROUND

C3 glomerulopathies (C3G) are characterized by uncontrolled activation of the alternative pathway of complement. In most patients these diseases progress towards end-stage renal disease, and the risk of recurrence after renal transplantation is high. In the majority of patients, only antibodies against the C3 convertase, termed C3Nef, can be found as a potential pathogenic factor. Although a large variety of therapeutic approaches have been used, no generally accepted therapy exists.

METHODS

In four consecutive patients with C3G in whom all known complement factor mutations were excluded and only C3Nef could be identified as a potential cause of disease, a multimodal therapeutic regimen with plasma therapy, corticosteroids and mycophenolate mofetil was used.

RESULTS

The multimodal regimen achieved normalization of renal function in all four patients, with complete remission in two patients and a distinct reduction of proteinuria in the other two patients. The single patient with C3 glomerulonephritis (C3GN) and marked terminal complement complex elevation only showed partial remission; further improvement was achieved following the addition of eculizumab to the therapeutic regimen. Repeatedly measured C3Nef levels did not correlate with disease course or therapeutic response in any of the patients.

CONCLUSIONS

As this multimodal therapeutic approach was effective in all four treated patients with suspected autoimmune etiology of C3G, it offers a treatment option for severely affected patients with this rare disease until more specific regimens are available.

A case of C3 glomerulonephritis successfully treated with eculizumab

BACKGROUND

C3 glomerulonephritis (C3GN) is a rare form of glomerulopathy that is characterized by predominant C3 deposits. Eculizumab, a humanized monoclonal C5 antibody, has recently emerged as a treatment option for C3GN. We report a C3GN patient successfully treated with eculizumab.

CASE DIAGNOSIS/TREATMENT

A 5-year-old boy who presented with proteinuria, hematuria, high ASO titers, and low C3 levels was initially diagnosed with post-streptococcal GN. His first kidney biopsy confirmed this diagnosis, but complement investigations identified three alternative pathway dysregulation factors: C3 nephritic factor, complement factor I heterozygous mutation (I398L), and anti-factor H autoantibodies (4,500 AU/ml). A second biopsy performed 11 months after initial presentation (nephrotic range proteinuria) showed a C3GN suggestive of isolated C3 deposits. Despite the use of intensive immunosuppressive therapy (rituximab, corticosteroids, mycophenolate), nephrotic-range proteinuria persisted and a third kidney biopsy showed the same C3GN pattern with more endocapillary proliferation. The serum C5b-9 level was elevated. Eculizumab was initiated and resulted in a significant decline of proteinuria (5.3 to 1.3 g/day) and an improvement in pathologic features. A transient interruption of eculizumab resulted in a rapid rise in proteinuria to 9.3 g/day, which decreased to 0.8 g/day after resumption of treatment.

CONCLUSIONS

The administration of anti-C5 antibodies may represent a valuable therapeutic option in patients with C3GN.

C4d as a Diagnostic Tool in Proliferative GN

Proliferative GN is classified as immune complex-mediated or complement-mediated (C3 glomerulopathy). Immune complex-mediated GN results from glomerular deposition of immune-complexes/Ig and C3; the C3 is derived from activation of the classical and/or lectin pathways of complement. C3 glomerulopathy results from deposition of C3 and other complement fragments with minimal or no deposition of immune complexes/Ig; the C3 is derived from activation of the alternative pathway of complement. C4d is a byproduct of activation of the classic and lectin pathways. Although widely used as a marker for antibody-mediated rejection, the significance of C4d in C3 glomerulopathy is undetermined. We studied glomerular C4d staining in 18 biopsy specimens of immune-complex GN, 30 biopsy specimens of C3 GN, and 13 biopsy specimens of postinfectious GN. All specimens of immune complex-mediated GN, except two specimens of IgA nephropathy and one specimen of sclerosing membranoproliferative GN, showed bright (2-3+) C4d staining. The staining pattern of C4d mirrored the staining patterns of Ig and C3. Conversely, C4d staining was completely negative in 24 (80%) of 30 specimens of C3 glomerulopathy, and only trace/1+ C4d staining was detected in six (20%) specimens. With regard to postinfectious GN, C4d staining was negative in six (46%) of 13 specimens, suggesting an abnormality in the alternative pathway, and it was positive in seven (54%) specimens. To summarize, C4d serves as a positive marker for immune complex-mediated GN but is absent or minimally detected in C3 glomerulopathy.

Eculizumab for treatment of rapidly progressive C3 glomerulopathy

C3 glomerulopathy (C3G) is a prototypic complement-mediated kidney disease. Rapidly progressive forms of C3G usually respond poorly to conventional treatments. We report on the efficacy of the terminal complement inhibitor eculizumab in 3 adult patients with rapidly progressive C3G. In all 3 patients, serum creatinine levels had increased by >50% in the 2 months preceding initiation of eculizumab treatment despite the use of conventional immunosuppressive drugs and/or plasma exchanges in 2 of these individuals. Of note, 2 patients had long-standing nephrotic syndrome. Kidney biopsy performed prior to eculizumab treatment disclosed marked glomerular inflammatory changes and increased C5b-9 deposition in all patients. Eculizumab use was associated with significant improvement in kidney function, with estimated glomerular filtration rates of patients increasing 22 to 38 mL/min/1.73 m(2). Eculizumab use also was associated with remission of nephrotic syndrome in the 2 affected patients, an effect observed as early as one week after treatment initiation. Repeat kidney biopsy disclosed regression of glomerular inflammatory changes and decreases in glomerular staining for C5b-9 in all patients. These results warrant further assessment of eculizumab for treatment of rapidly progressive forms of C3G with markedly increased glomerular C5b-9 deposits.

Successful management of a patient with a C3 Glomerulonephritis and crescentic pattern: a case report

BACKGROUND

Crescentic glomerulonephritis is a rare condition in children and is typically associated with renal insufficiency. Dysfunction of the alternative complement pathway is an unusual aetiology with an unknown mechanism.

CASE PRESENTATION

We report a case of a previously healthy 12-year-old Caucasian girl who was examined on emergency owing to an asymptomatic gross haematuria. An active urinary sediment and nephrotic-range proteinuria were identified, and serologic examination showed a decreased serum C3 concentration not associated with any immunologic or infectious cause. Oedema, hypertension, and renal insufficiency were not observed. A renal biopsy was performed, and crescentic glomerulonephritis associated with C3 glomerulonephritis was diagnosed. Prompt treatment with intravenous steroids resulted in complete resolution of the gross haematuria. Further examination did not detect any underlying acquired cause. A combination of oral steroids and cyclophosphamide, followed by mycophenolate mofetil, was maintained and resulted in clinical remission during an 8-month follow-up.

CONCLUSION

The presence of severe injury such as crescentic glomerulonephritis secondary to C3 glomerulonephritis is extremely unusual in children. This is the first known case of paediatric crescentic glomerulonephritis secondary to C3 glomerulonephritis that presented with gross haematuria and was treated early and effectively with immunosuppressive therapy based on its severe histologic features.