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

Introduction: C3 glomerulopathies (C3G) are ultra-rare complement-mediated diseases that lead to end-stage renal disease (ESRD) within 10 years of diagnosis in ~50% of patients. Overactivation of the alternative pathway (AP) of complement in the fluid phase and on the surface of the glomerular endothelial glycomatrix is the underlying cause of C3G. Although there are animal models for C3G that focus on genetic drivers of disease, in vivo studies of the impact of acquired drivers are not yet possible. Methods: Here we present an in vitro model of AP activation and regulation on a glycomatrix surface. We use an extracellular matrix substitute (MaxGel) as a base upon which we reconstitute AP C3 convertase. We validated this method using properdin and Factor H (FH) and then assessed the effects of genetic and acquired drivers of C3G on C3 convertase. Results: We show that C3 convertase readily forms on MaxGel and that this formation was positively regulated by properdin and negatively regulated by FH. Additionally, Factor B (FB) and FH mutants impaired complement regulation when compared to wild type counterparts. We also show the effects of C3 nephritic factors (C3Nefs) on convertase stability over time and provide evidence for a novel mechanism of C3Nef-mediated C3G pathogenesis. Discussion: We conclude that this ECM-based model of C3G offers a replicable method by which to evaluate the variable activity of the complement system in C3G, thereby offering an improved understanding of the different factors driving this disease process.

Genetic and pathological findings in a boy with psoriasis and C3 glomerulonephritis: A case report and literature review.

BACKGROUND: Psoriasis is a chronic inflammatory dermatosis with complex genetic basis supported by family investigation. Renal involvement in psoriasis is sparsely studied and its pathogenesis is still unclear. METHODS AND RESULTS: We describe the case of a 7-year-old boy presented new onset of nephropathy two weeks after a flare-up of psoriasis. His mother had a long history of psoriasis without abnormal urinalysis records. The case showed non-nephrotic range proteinuria, microscopic hematuria without any other abnormal results including renal function, complement cascade, and ultrasound. Renal pathological demonstrated the diagnosis of C3 glomerulonephritis (C3GN) showing mesangial proliferative glomerulonephritis with C3 staining only, effacement of podocyte process and intramembranous electron dense deposit by electric microscopy. Parent-child trio WES performed to screening the common variants of psoriasis susceptibility locus and also the rare variants associated with C3GN. We identified a missense single nucleotide polymorphism of CARD14 (*607211, rs34367357, p.Val585Ile) carried by the proband and his mother. Meta-analysis proved the association of rs34367357 and psoriasis (p = 0.006, OR = 1.23). A hemizygouse mutation of CLCN5 (*300008, c.1904A＞G,p.Asn635Ser) was identified for diagnosis of Dent disease (*300009). CONCLUSION: The case highlights the genetic study is necessary to facilitate disease differentiation in new onset of nephropathy with psoriasis in children.

Anti-Factor B Antibodies and Acute Postinfectious GN in Children.

BACKGROUND: The pathophysiology of the leading cause of pediatric acute nephritis, acute postinfectious GN, including mechanisms of the pathognomonic transient complement activation, remains uncertain. It shares clinicopathologic features with C3 glomerulopathy, a complement-mediated glomerulopathy that, unlike acute postinfectious GN, has a poor prognosis. METHODS: This retrospective study investigated mechanisms of complement activation in 34 children with acute postinfectious GN and low C3 level at onset. We screened a panel of anticomplement protein autoantibodies, carried out related functional characterization, and compared results with those of 60 children from the National French Registry who had C3 glomerulopathy and persistent hypocomplementemia. RESULTS: All children with acute postinfectious GN had activation of the alternative pathway of the complement system. At onset, autoantibodies targeting factor B (a component of the alternative pathway C3 convertase) were found in a significantly higher proportion of children with the disorder versus children with hypocomplementemic C3 glomerulopathy (31 of 34 [91%] versus 4 of 28 [14%], respectively). In acute postinfectious GN, anti-factor B autoantibodies were transient and correlated with plasma C3 and soluble C5b-9 levels. We demonstrated that anti-factor B antibodies enhance alternative pathway convertase activity in vitro, confirming their pathogenic effect. We also identified crucial antibody binding sites on factor B, including one correlated to disease severity. CONCLUSIONS: These findings elucidate the pathophysiologic mechanisms underlying acute postinfectious GN by identifying anti-factor B autoantibodies as contributing factors in alternative complement pathway activation. At onset of a nephritic syndrome with low C3 level, screening for anti-factor B antibodies might help guide indications for kidney biopsy to avoid misdiagnosed chronic glomerulopathy, such as C3 glomerulopathy, and to help determine therapy.

An update to the pathogenesis for monoclonal gammopathy of renal significance.

Monoclonal gammopathy of renal significance (MGRS) is characterized by the nephrotoxic monoclonal immunoglobulin secreted by an otherwise asymptomatic or indolent B cell or plasma cell clone, without hematologic criteria for treatment. These MGRS-associated diseases can involve one or more renal compartments, including glomeruli, tubules, and vessels. Hydrophobic residue replacement, N-glycosylated, increase in isoelectric point in monoclonal immunoglobulin (MIg) causes it to transform from soluble form to tissue deposition, and consequently resulting in glomerular damage. In addition to MIg deposition, complement deposition is also found in C3 glomerulopathy with monoclonal glomerulopathy, which is caused by an abnormality of the alternative pathway and may involve multiple factors including complement component 3 nephritic factor, anti-complement factor auto-antibodies, or MIg which directly cleaves C3. Furthermore, inflammatory factors, growth factors, and virus infection may also participate in the development of the diseases. In this review, for the first time, we discussed current highlights in the mechanism of MGRS-related lesions.

Immunological features of patients affected by Barraquer-Simons syndrome.

BACKGROUND: C3 hypocomplementemia and the presence of C3 nephritic factor (C3NeF), an autoantibody causing complement system over-activation, are common features among most patients affected by Barraquer-Simons syndrome (BSS), an acquired form of partial lipodystrophy. Moreover, BSS is frequently associated with autoimmune diseases. However, the relationship between complement system dysregulation and BSS remains to be fully elucidated. The aim of this study was to provide a comprehensive immunological analysis of the complement system status, autoantibody signatures and HLA profile in BSS. Thirteen subjects with BSS were recruited for the study. The circulating levels of complement components, C3, C4, Factor B (FB) and Properdin (P), as well as an extended autoantibody profile including autoantibodies targeting complement components and regulators were assessed in serum. Additionally, HLA genotyping was carried out using DNA extracted from peripheral blood mononuclear cells. RESULTS: C3, C4 and FB levels were significantly reduced in patients with BSS as compared with healthy subjects. C3NeF was the most frequently found autoantibody (69.2% of cases), followed by anti-C3 (38.5%), and anti-P and anti-FB (30.8% each). Clinical data showed high prevalence of autoimmune diseases (38.5%), the majority of patients (61.5%) being positive for at least one of the autoantibodies tested. The HLA allele DRB1*11 was present in 54% of BSS patients, and the majority of them (31%) were positive for *11:03 (vs 1.3% in the general population). CONCLUSIONS: Our results confirmed the association between BSS, autoimmunity and C3 hypocomplementemia. Moreover, the finding of autoantibodies targeting complement system proteins points to complement dysregulation as a central pathological event in the development of BSS.

C4 nephritic factor in patients with immune-complex-mediated membranoproliferative glomerulonephritis and C3-glomerulopathy.

BACKGROUND: Acquired or genetic abnormalities of the complement alternative pathway are the primary cause of C3glomerulopathy(C3G) but may occur in immune-complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) as well. Less is known about the presence and role of C4nephritic factor(C4NeF) which may stabilize the classical pathway C3-convertase. Our aim was to examine the presence of C4NeF and its connection with clinical features and with other pathogenic factors. RESULTS: One hunfe IC-MPGN/C3G patients were enrolled in the study. C4NeF activity was determined by hemolytic assay utilizing sensitized sheep erythrocytes. Seventeen patients were positive for C4NeF with lower prevalence of renal impairment and lower C4d level, and higher C3 nephritic factor (C3NeF) prevalence at time of diagnosis compared to C4NeF negative patients. Patients positive for both C3NeF and C4NeF had the lowest C3 levels and highest terminal pathway activation. End-stage renal disease did not develop in any of the C4NeF positive patients during follow-up period. Positivity to other complement autoantibodies (anti-C1q, anti-C3) was also linked to the presence of nephritic factors. Unsupervised, data-driven cluster analysis identified a group of patients with high prevalence of multiple complement autoantibodies, including C4NeF. CONCLUSIONS: In conclusion, C4NeF may be a possible cause of complement dysregulation in approximately 10-15% of IC-MPGN/C3G patients.

Novel Assays to Distinguish Between Properdin-Dependent and Properdin-Independent C3 Nephritic Factors Provide Insight Into Properdin-Inhibiting Therapy.

C3 glomerulopathy (C3G) is an umbrella classification for severe renal diseases characterized by predominant staining for complement component C3 in the glomeruli. The disease is caused by a dysregulation of the alternative pathway (AP) of the complement system. In more than half of C3G patients C3 nephritic factors (C3NeFs) are found. These autoantibodies bind to the AP C3 convertase, prolonging its activity. C3NeFs can be dependent or independent of the complement regulator properdin for their convertase-stabilizing function. However, studies to determine the properdin-dependency of C3NeFs are rare and not part of routine patient workup. Until recently, only supportive treatments for C3G were available. Complement-directed therapies are now being investigated. We hypothesized that patients with properdin-dependent C3NeFs may benefit from properdin-inhibiting therapy to normalize convertase activity. Therefore, in this study we validated two methods to distinguish between properdin-dependent and properdin-independent C3NeFs. These methods are hemolytic assays for measuring convertase activity and stability in absence of properdin. The first assay assesses convertase stabilization by patient immunoglobulins in properdin-depleted serum. The second assay measures convertase stabilization directly in patient serum supplemented with the properdin-blocking agent Salp20. Blood samples from 13 C3NeF-positive C3G patients were tested. Three patients were found to have properdin-dependent C3NeFs, whereas the C3NeF activity of the other ten patients was independent of properdin. The convertase-stabilizing activity in the samples of the patients with properdin-dependent C3NeFs disappeared in absence of properdin. These data indicate that inhibition of properdin in patients with properdin-dependent C3NeFs can normalize convertase activity and could represent a novel therapy for normalizing AP hyperactivity. Our assays provide a tool for identifying C3G patients who may benefit from properdin-inhibiting therapy and can be incorporated into standard C3G laboratory investigations.

Autoantibodies Against C3b-Functional Consequences and Disease Relevance.

The complement component C3 is at the heart of the complement cascade. It is a complex protein, which generates different functional activated fragments (C3a, C3b, iC3b, C3c, C3d). C3b is a constituent of the alternative pathway C3 convertase (C3bBb), binds multiple regulators, and receptors, affecting thus the functioning of the immune system. The activated forms of C3 are a target for autoantibodies. This review focuses on the discovery, disease relevance, and functional consequences of the anti-C3b autoantibodies. They were discovered about 70 years ago and named immunoconglutinins. They were found after infections and considered convalescent factors. At the end of the twentieth century IgG against C3b were found in systemic lupus erythematosus and recently in lupus nephritis, correlating with the disease severity and flare. Cases of C3 glomerulopathy and immune complex glomerulonephritis were also reported. These antibodies recognize epitopes, shared between C3(H2O)/C3b/iC3b/C3c and have overt functional activity. They correlate with low plasmatic C3 levels in patients. In vitro, they increase the activity of the alternative pathway C3 convertase, without being C3 nephritic factors. They perturb the binding of the negative regulators Complement Receptor 1 and Factor H. The clear functional consequences and association with disease severity warrant further studies to establish the link between the anti-C3b autoantibodies and tissue injury. Comparative studies with such antibodies, found in patients with infections, may help to uncover their origin and epitopes specificity. Patients with complement overactivation due to presence of anti-C3b antibodies may benefit from therapeutic targeting of C3.

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.

The properdin pathway: an "alternative activation pathway" or a "critical amplification loop" for C3 and C5 activation?

This review is not intended to cover in detail all aspects of the discovery and evolution of our understanding of the "alternative pathway" of complement activation, there are many excellent reviews that do this (see Fearon (CRC Crit Rev Immunol 1:1-32, 1979), Pangburn and Müller-Eberhard (Springer Semin Immunopathol 7:163-192, 1984)), but instead to give sufficient background for current concepts to be put in context. The prevailing textbook view, of components having a primary role as an alternative "pathway" for C3 activation, is challenged, with an argument developed for the primary role of the system being that of providing a surface-dependent amplification loop for both C3 and C5 activation. Whatever the mechanism by which the initial C3b molecule is generated, deposition onto a surface has the potential to target that surface for elimination. Elimination or escape from initial targeting is determined by a sophisticated and highly regulated amplification loop for C3 activation. This viewpoint of the system is then briefly developed to provide a context for therapeutic treatment of disease caused, at least in part, by dysregulated amplification of C3 activation, and to highlight some of the challenges that such therapies will face and need to address.

Cluster Analysis Identifies Distinct Pathogenetic Patterns in C3 Glomerulopathies/Immune Complex-Mediated Membranoproliferative GN.

Membranoproliferative GN (MPGN) was recently reclassified as alternative pathway complement-mediated C3 glomerulopathy (C3G) and immune complex-mediated membranoproliferative GN (IC-MPGN). However, genetic and acquired alternative pathway abnormalities are also observed in IC-MPGN. Here, we explored the presence of distinct disease entities characterized by specific pathophysiologic mechanisms. We performed unsupervised hierarchical clustering, a data-driven statistical approach, on histologic, genetic, and clinical data and data regarding serum/plasma complement parameters from 173 patients with C3G/IC-MPGN. This approach divided patients into four clusters, indicating the existence of four different pathogenetic patterns. Specifically, this analysis separated patients with fluid-phase complement activation (clusters 1-3) who had low serum C3 levels and a high prevalence of genetic and acquired alternative pathway abnormalities from patients with solid-phase complement activation (cluster 4) who had normal or mildly altered serum C3, late disease onset, and poor renal survival. In patients with fluid-phase complement activation, those in clusters 1 and 2 had massive activation of the alternative pathway, including activation of the terminal pathway, and the highest prevalence of subendothelial deposits, but those in cluster 2 had additional activation of the classic pathway and the highest prevalence of nephrotic syndrome at disease onset. Patients in cluster 3 had prevalent activation of C3 convertase and highly electron-dense intramembranous deposits. In addition, we provide a simple algorithm to assign patients with C3G/IC-MPGN to specific clusters. These distinct clusters may facilitate clarification of disease etiology, improve risk assessment for ESRD, and pave the way for personalized treatment.

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.

Defining the complement biomarker profile of C3 glomerulopathy.

BACKGROUND AND OBJECTIVES: C3 glomerulopathy (C3G) applies to a group of renal diseases defined by a specific renal biopsy finding: a dominant pattern of C3 fragment deposition on immunofluorescence. The primary pathogenic mechanism involves abnormal control of the alternative complement pathway, although a full description of the disease spectrum remains to be determined. This study sought to validate and define the association of complement dysregulation with C3G and to determine whether specific complement pathway abnormalities could inform disease definition. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: This study included 34 patients with C3G (17 with C3 glomerulonephritis [C3GN] and 17 with dense deposit disease [DDD]) diagnosed between 2008 and 2013 selected from the C3G Registry. Control samples (n=100) were recruited from regional blood drives. Nineteen complement biomarkers were assayed on all samples. Results were compared between C3G disease categories and with normal controls. RESULTS: Assessment of the alternative complement pathway showed that compared with controls, patients with C3G had lower levels of serum C3 (P<0.001 for both DDD and C3GN) and factor B (P<0.001 for both DDD and C3GN) as well as higher levels of complement breakdown products including C3d (P<0.001 for both DDD and C3GN) and Bb (P<0.001 for both DDD and C3GN). A comparison of terminal complement pathway proteins showed that although C5 levels were significantly suppressed (P<0.001 for both DDD and C3GN) its breakdown product C5a was significantly higher only in patients with C3GN (P<0.05). Of the other terminal pathway components (C6-C9), the only significant difference was in C7 levels between patients with C3GN and controls (P<0.01). Soluble C5b-9 was elevated in both diseases but only the difference between patients with C3GN and controls reached statistical significance (P<0.001). Levels of C3 nephritic factor activity were qualitatively higher in patients with DDD compared with patients with C3GN. CONCLUSIONS: Complement biomarkers are significantly abnormal in patients with C3G compared with controls. These data substantiate the link between complement dysregulation and C3G and identify C3G interdisease differences.

Childhood-onset dense deposit disease: a rare cause of proteinuria.

INTRODUCTION: Dense deposit disease (DDD) is a rare renal disease related to the dysregulation of the alternative pathway of the complement cascade, caused by several factors including the presence of an autoantibody to C3 nephritic factor, mutations in factor H and autoantibodies to this protein. DDD is characterized by C3 accumulation with absent or scanty immunoglobulin deposition. CASE PRESENTATION: Herein we report the case of a child with benign course of DDD, who presented with moderate proteinuria and lack of clinical symptoms without immunosuppressive treatment. Laboratory testing revealed moderate proteinuria, normal serum creatinine, total protein, and albumin levels, but significantly decreased serum C3 level. The results of renal biopsy were consistent with DDD. Genetic analysis revealed that the patient carried one copy of the H402 risk allele of factor H. The level of proteinuria did not change during the follow-up period and no nephrotic syndrome signs occurred. Renal function was stable. CONCLUSION: In conclusion, a program of urine screening for asymptomatic proteinuria and hematuria to detect children with kidney disease before they experience loss of kidney functions should be considered. Children diagnosed with DDD should have the opportunity to get treatment early on and to be followed very closely.

C3 nephritic factor associated with C3 glomerulopathy in children.

BACKGROUND: C3 glomerulopathy (C3G) is characterized by predominant C3 deposits in glomeruli and dysregulation of the alternative pathway of complement. Half of C3G patients have a C3 nephritic factor (C3NeF). C3G incorporated entities with a range of features on microscopy including dense deposit diseases (DDD) and C3 glomerulonephritis (C3GN). The aim of this work was to study children cases of C3G associated with C3NeF. METHODS: We reviewed 18 cases of C3G with a childhood onset associated with C3NeF without identified mutations in CFH, CFI, and MCP genes. RESULTS: Clinical histories started with recurrent hematuria for seven patients, nephrotic syndrome for four, acute post-infectious glomerulonephritis for three and acute renal failure for four. Twelve patients had a low C3 at first investigation. Kidney biopsy showed ten C3GN and eight DDD. Twenty-three percent of the patients tested presented elevated sC5b9. Seven patients relapsed 3 to 6 years after the onset. At the end of follow-up, two patients were under dialysis, 11 had a persistent proteinuria, five had none; four patients did not follow any treatment. Steroids were first used in 80 % of cases. CONCLUSIONS: C3NeF associated C3G has a heterogeneous presentation and outcome. Anti-proteinuric agents may control the disease during follow-up, even after nephrotic syndrome at the onset. The efficiency of immunosuppressive therapy remains questionable.

[An approach to research on nephritic factor and membranoproliferative glomerulonephritis].

Although it has been 45 years since membranoproliferative glomerulonephritis (MPGN) and C3 nephritic factor (3NeF) were first reported, the pathophysiology of MPGN is not fully understood at present. Careful analysis of previous case reports of MPGN has been the key to developing approaches to study the mechanisms of MPGN pathophysiology. In this review, previous studies on MPGN, mainly on its association with C3NeF, are discussed and important issues on MPGN as yet unknown are stated. Complements play important roles in MPGN. Studies on complements, particularly C3NeF, advanced as studies on pathophysiology of MPGN progressed and consequently, complement activation alternative pathways were clarified. Important, although not well-known research subjects on MPGN, include characteristics of several kinds of NeF and type I, III and dense deposit disease (DDD). Detailed reading of case reports often yields new research ideas and study directions of the disease and its treatment. The present review demonstrates that reviewing previous case reports is one approach to further advancing our understanding of the complicated disease of MPGN.

Sensitive and specific assays for C3 nephritic factors clarify mechanisms underlying complement dysregulation.

C3 nephritic factors are autoantibodies that prolong the half-life or prevent regulation of the alternative pathway C3 convertase, resulting in uncontrolled complement activation. They are strongly associated with renal disease but their role in pathogenesis remains controversial. Here we optimized and compared a panel of assays to identify and interrogate nephritic factor activities. Of 101 patients with histologic or clinically evident disease, 48 were positive in some or all assays. In the presence of properdin, binding of autoantibody was detected in 39 samples and convertase stabilization was detected in 36. Forty-two of 48 nephritic factors tested prevented convertase decay by factor H, and most of these by decay accelerating factor (28) and complement receptor 1 (34). Representative properdin-independent nephritic factors had no effect on C5 cleavage and terminal pathway activity, while properdin-dependent nephritic factors enhanced activity. Biacore analysis of four purified IgG samples confirmed resistance to decay and showed that properdin-independent nephritic factors increased convertase half-life over 50-fold, whereas properdin-dependent nephritic factors increased the half-life 10- to 20-fold and also increased activity of the C3 convertase up to 10-fold. Thus, our study provides a rational approach to detect and characterize nephritic factors in patients.

Acquired and genetic complement abnormalities play a critical role in dense deposit disease and other C3 glomerulopathies.

Dense deposit disease and glomerulonephritis with isolated C3 deposits are glomerulopathies characterized by deposits of C3 within or along the glomerular basement membrane. Previous studies found a link between dysregulation of the complement alternative pathway and the pathogenesis of these diseases. We analyzed the role of acquired and genetic complement abnormalities in a cohort of 134 patients, of whom 29 have dense deposit disease, 56 have glomerulonephritis with isolated C3 deposits, and 49 have primary membranoproliferative glomerulonephritis type I, with adult and pediatric onset. A total of 53 patients presented with a low C3 level, and 65 were positive for C3 nephritic factor that was significantly more frequently detected in patients with dense deposit disease than in other histological types. Mutations in CFH and CFI genes were identified in 24 patients associated with a C3 nephritic factor in half the cases. We found evidence for complement alternative pathway dysregulation in 26 patients with membranoproliferative glomerulonephritis type I. The complement factor H Y402H variant was significantly increased in dense deposit disease. We identified one at-risk membrane cofactor protein (MCP) haplotype for glomerulonephritis with isolated C3 deposits and membranoproliferative glomerulonephritis type I. Thus, our results suggest a critical role of fluid-phase alternative pathway dysregulation in the pathogenesis of C3 glomerulopathies as well as in immune complex-mediated glomerular diseases. The localization of the C3 deposits may be under the influence of MCP expression.