Glomeruli of Dense Deposit Disease contain components of the alternative and terminal complement pathway

Complement Terminal Pathway Activation and Intrarenal Immune Response in C3 Glomerulopathy

Key Points

Background

C3 glomerulopathy is a rare disease resulting from an overactivation of the complement alternative pathway. Although there is also evidence of terminal pathway activation, its occurrence and consequences on the disease have been poorly studied.

Methods

We retrospectively studied a cohort of 42 patients diagnosed with C3 glomerulopathy. We performed centralized extensive characterization of histological parameters. Kidney C5b-9 staining was performed as a marker of terminal pathway activation; intrarenal immune response was characterized through transcriptomic analysis.

Results

Eighty-eight percent of biopsies showed C5b-9 deposits in glomeruli. Biopsies were grouped according to the amount of C5b-9 deposits (no or low n=15/42, 36%; intermediate n=15/42, 36%; and high n=12/42, 28%). Patients with high C5b-9 deposits significantly differed from the two other groups of patients and were characterized by a significant higher histological chronicity score (P = 0.005) and lower outcome-free survival (P = 0.001). In multivariable analysis, higher glomerular C5b-9 remained associated with poor kidney prognosis after adjustment. One third of the 847 studied immune genes were upregulated in C3 glomerulopathy biopsies compared with controls. Unsupervised clustering on differentially expressed genes identified a group of kidney biopsies enriched in high glomerular C5b-9 with high immune and fibroblastic signature and showed high chronicity scores on histological examination.

Conclusions

In a cohort of patients with C3 glomerulopathy, intrarenal terminal pathway activation was associated with specific histological phenotype and disease prognosis.

Apolipoprotein E is enriched in dense deposits and is a marker for dense deposit disease in C3 glomerulopathy

C3 glomerulopathy (C3G) is a rare disease resulting from dysregulation of the alternative pathway of complement. C3G includes C3 glomerulonephritis (C3GN) and dense deposit disease (DDD), both of which are characterized by bright glomerular C3 staining on immunofluorescence studies. However, on electron microscopy (EM), DDD is characterized by dense osmiophilic mesangial and intramembranous deposits along the glomerular basement membranes (GBM), while the deposits of C3GN are not dense. Why the deposits appear dense in DDD and not in C3GN is not known. We performed laser microdissection (LCM) of glomeruli followed by mass spectrometry (MS) in 12 cases each of DDD, C3GN, and pretransplant kidney control biopsies. LCM/MS showed marked accumulation of complement proteins C3, C5, C6, C7, C8, C9 and complement regulating proteins CFHR5, CFHR1, and CFH in C3GN and DDD compared to controls. C3, CFH and CFHR proteins were comparable in C3GN and DDD. Yet, there were significant differences. First, there was a six-to-nine-fold increase of C5-9 in DDD compared to C3GN. Secondly, an unexpected finding was a nine-fold increase in apolipoprotein E (ApoE) in DDD compared to C3GN. Most importantly, immunohistochemical and confocal staining for ApoE mirrored the dense deposit staining in the GBM in DDD but not in C3GN or control cases. Validation studies using 31 C3G cases confirmed the diagnosis of C3GN and DDD in 80.6 % based on ApoE staining. Overall, there is a higher burden of terminal complement pathway proteins in DDD compared to C3GN. Thus, our study shows that dense deposits in DDD are enriched with ApoE compared to C3GN and control cases. Hence, ApoE staining may be used as an adjunct to EM for the diagnosis of DDD and might be valuable when EM is not available.

From Patterns to Proteins: Mass Spectrometry Comes of Age in Glomerular Disease

Laser capture microdissection and mass spectrometry (LCM/MS) is a technique that involves dissection of glomeruli from paraffin-embedded biopsy tissue, followed by digestion of the dissected glomerular proteins by trypsin, and subsequently mass spectrometry to identify and semiquantitate the glomerular proteins. LCM/MS has played a crucial role in the identification of novel types of amyloidosis, biomarker discovery in fibrillary GN, and more recently discovery of novel target antigens in membranous nephropathy (MN). In addition, LCM/MS has also confirmed the role for complement proteins in glomerular diseases, including C3 glomerulopathy. LCM/MS is now widely used as a clinical test and considered the gold standard for diagnosis and typing amyloidosis. For the remaining glomerular diseases, LCM/MS has remained a research tool. In this review, we discuss the usefulness of LCM/MS in other glomerular diseases, particularly MN, deposition diseases, and diseases of complement pathways, and advocate more routine use of LCM/MS at the present time in at least certain diseases, such as MN, for target antigen detection. We also discuss the limitations of LCM/MS, particularly the difficulties faced from moving from a research-based technique to a clinical test. Nonetheless, the role of LCM/MS in glomerular diseases is expanding. Currently, LCM/MS may be used to identify the etiology in certain glomerular diseases, but in the future, LCM/MS can play a valuable role in determining pathways of complement activation, inflammation, and fibrosis.

Complement and platelets: prothrombotic cell activation requires membrane attack complex-induced release of danger signals

Complement activation in the diseases paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS) results in cytolysis and fatal thrombotic events, which are largely refractory to anticoagulation and/or antiplatelet therapy. Anticomplement therapy, however, efficiently prevents thrombotic events in PNH and aHUS, but the underlying mechanisms remain unresolved. We show that complement-mediated hemolysis in whole blood induces platelet activation similarly to activation by adenosine 5'-diphosphate (ADP). Blockage of C3 or C5 abolished platelet activation. We found that human platelets failed to respond functionally to the anaphylatoxins C3a and C5a. Instead, complement activation did lead to prothrombotic cell activation in the whole blood when membrane attack complex (MAC)-mediated cytolysis occurred. Consequently, we demonstrate that ADP receptor antagonists efficiently inhibited platelet activation, although full complement activation, which causes hemolysis, occurred. By using an established model of mismatched erythrocyte transfusions in rats, we crossvalidated these findings in vivo using the complement inhibitor OmCI and cobra venom factor. Consumptive complement activation in this animal model only led to a thrombotic phenotype when MAC-mediated cytolysis occurred. In conclusion, complement activation only induces substantial prothrombotic cell activation if terminal pathway activation culminates in MAC-mediated release of intracellular ADP. These results explain why anticomplement therapy efficiently prevents thromboembolisms without interfering negatively with hemostasis.

Complement: Functions, location and implications

The complement system, an arm of the innate immune system plays a critical role in both health and disease. The complement system is highly complex with dual possibilities, helping or hurting the host, depending on the location and local microenvironment. The traditionally known functions of complement include surveillance, pathogen recognition, immune complex trafficking, processing and pathogen elimination. The noncanonical functions of the complement system include their roles in development, differentiation, local homeostasis and other cellular functions. Complement proteins are present in both, the plasma and on the membranes. Complement activation occurs both extra- and intracellularly, which leads to considerable pleiotropy in their activity. In order to design more desirable and effective therapies, it is important to understand the different functions of complement, and its location-based and tissue-specific responses. This manuscript will provide a brief overview into the complex nature of the complement cascade, outlining some of their complement-independent functions, their effects at different locale, and their implication in disease settings.

Recurrent C3 Glomerulonephritis along with BK-Virus-Associated Nephropathy after Kidney Transplantation: A Case Report

C3 glomerulonephritis (C3GN) is a rare cause of end-stage kidney disease and frequently recurrent in allografts following kidney transplantation (KT). Herein, we describe the case of a kidney transplant recipient who developed recurrent C3GN along with BK-virus-associated nephropathy (BKVAN) following KT. A 33-year-old man diagnosed with membranoproliferative glomerulonephritis 17 years ago underwent preemptive KT with a donor kidney from his aunt. Proteinuria gradually increased after 3 months following KT, and graft biopsy was performed 30 months after KT. Histopathological examination revealed recurrent C3GN. The dosages of triple immunosuppressive maintenance therapy agents were increased. Subsequently, serum C3 levels recovered to normal levels. However, at 33 months following KT, the BK viral load increased and graft function gradually deteriorated; a second graft biopsy was performed at 46 months following KT, which revealed BKVAN and decreased C3GN activity. The dosages of immunosuppressive agents were decreased; subsequently, BKVAN improved and graft function was maintained with normal serum C3 levels at 49 months following KT. This case indicates that C3GN is highly prone to recurrence following KT and that immunosuppressive therapy for C3GN increases the risk of BKVAN.

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.

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

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

Proteomic Analysis of Complement Proteins in Glomerular Diseases

Introduction

Complement plays an important role in the pathogenesis of glomerulonephritis (GN). Even though the underlying etiology of GN might be different, complement activation with subsequent glomerular deposition of complement proteins result in glomerular injury and progression of the lesions. Routine immunofluorescence microscopy (IF) includes staining for only complement factors C3c and C1q. Therefore, with regard to evaluation of the complement pathways, routine kidney biopsy provides only limited information.

Methods

In this study, using laser microdissection of glomeruli followed by mass spectrometry, complement proteins and pathways involved in GN were analyzed.

Results

We found that C3 followed by C9 are the most abundant complement proteins in GN, indicating activation of classical or lectin or alternative, and terminal pathways, either exclusively or in a combination of pathways. Furthermore, depending on the type of GN, C4A and/or C4B were also present. Therefore, membranous nephropathy (MN), fibrillary GN, and infection-related GN showed C4A dominant pathways, whereas lupus nephritis (LN), proliferative GN with monoclonal Ig deposits, monoclonal Ig deposition disease (MIDD), and immunotactoid glomerulopathy showed C4B dominant pathways. Significant deposition of complement regulatory proteins, factor H-related protein-1 (FHR-1) and factor H-related protein-5 (FHR-5), were also detected in most GN.

Conclusions

This study shows accumulation of specific complement proteins in GN. The complement pathways, complement proteins, and the amount of complement protein deposition are variable in different types of GN. Selective targeting of complement pathways may be a novel option in the treatment of GN.

Alternative pathway diagnostics

Uncontrolled alternative pathway activation is the primary driver of several diseases, and it contributes to the pathogenesis of many others. Consequently, diagnostic tests to monitor this arm of the complement system are increasingly important. Defects in alternative pathway regulation are strong risk factors for disease, and drugs that specifically block the alternative pathway are entering clinical use. A range of diagnostic tests have been developed to evaluate and monitor the alternative pathway, including assays to measure its function, expression of alternative pathway constituents, and activation fragments. Genetic studies have also revealed many disease-associated variants in alternative pathway genes that predict the risk of disease and prognosis. Newer imaging modalities offer the promise of non-invasively detecting and localizing pathologic complement activation. Together, these various tests help in the diagnosis of disease, provide important prognostic information, and can help guide therapy with complement inhibitory drugs.

The susceptibility of the kidney to alternative pathway activation-A hypothesis

The glomerulus is often the prime target of dysregulated alternative pathway (AP) activation. In particular, AP activation is the key driver of two severe kidney diseases: atypical hemolytic uremic syndrome and C3 glomerulopathy. Both conditions are associated with a variety of predisposing molecular defects in AP regulation, such as genetic variants in complement regulators, autoantibodies targeting AP proteins, or autoantibodies that stabilize the AP convertases (C3- and C5-activating enzymes). It is noteworthy that these are systemic AP defects, yet in both diseases pathologic complement activation primarily affects the kidneys. In particular, AP activation is often limited to the glomerular capillaries. This tropism of AP-mediated inflammation for the glomerulus points to a unique interaction between AP proteins in plasma and this particular anatomic structure. In this review, we discuss the pre-clinical and clinical data linking the molecular causes of aberrant control of the AP with activation in the glomerulus, and the possible causes of this tropism. Based on these data, we propose a model for why the kidney is so uniquely and frequently targeted in patients with AP defects. Finally, we discuss possible strategies for preventing pathologic AP activation in the kidney.

Protein therapeutics and their lessons: Expect the unexpected when inhibiting the multi-protein cascade of the complement system

Over a century after the discovery of the complement system, the first complement therapeutic was approved for the treatment of paroxysmal nocturnal hemoglobinuria (PNH). It was a long-acting monoclonal antibody (aka 5G1-1, 5G1.1, h5G1.1, and now known as eculizumab) that targets C5, specifically preventing the generation of C5a, a potent anaphylatoxin, and C5b, the first step in the eventual formation of membrane attack complex. The enormous clinical and financial success of eculizumab across four diseases (PNH, atypical hemolytic uremic syndrome (aHUS), myasthenia gravis (MG), and anti-aquaporin-4 (AQP4) antibody-positive neuromyelitis optica spectrum disorder (NMOSD)) has fueled a surge in complement therapeutics, especially targeting diseases with an underlying complement pathophysiology for which anti-C5 therapy is ineffective. Intensive research has also uncovered challenges that arise from C5 blockade. For example, PNH patients can still face extravascular hemolysis or pharmacodynamic breakthrough of complement suppression during complement-amplifying conditions. These "side" effects of a stoichiometric inhibitor like eculizumab were unexpected and are incompatible with some of our accepted knowledge of the complement cascade. And they are not unique to C5 inhibition. Indeed, "exceptions" to the rules of complement biology abound and have led to unprecedented and surprising insights. In this review, we will describe initial, present and future aspects of protein inhibitors of the complement cascade, highlighting unexpected findings that are redefining some of the mechanistic foundations upon which the complement cascade is organized.

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

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

Nephritis-Associated Plasmin Receptor (NAPlr): An Essential Inducer of C3-Dominant Glomerular Injury and a Potential Key Diagnostic Biomarker of Infection-Related Glomerulonephritis (IRGN)

Nephritis-associated plasmin receptor (NAPlr) was originally isolated from the cytoplasmic fraction of group A Streptococci, and was found to be the same molecule as streptococcal glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and plasmin receptor (Plr) on the basis of nucleotide and amino acid sequence homology. Its main functions include GAPDH activity, plasmin-binding capacity, and direct activation of the complement alternative pathway (A-P). Plasmin trapped by deposited NAPlr triggers the degradation of extracellular matrix proteins, such as glomerular basement membranes and mesangial matrix, and the accumulation of macrophages and neutrophils, leading to the induction of plasmin-related endocapillary glomerular inflammation. Deposited NAPlr at glomerular endocapillary site directly activates the complement A-P, and the endocapillary release of complement-related anaphylatoxins, C3a and C5a, amplify the in situ endocapillary glomerular inflammation. Subsequently, circulating and in situ-formed immune complexes participate in the glomerular injury resulting in NAPlr-mediated glomerulonephritis. The disease framework of infection-related glomerulonephritis (IRGN) has been further expanded. GAPDH of various bacteria other than Streptococci have been found to react with anti-NAPlr antibodies and to possess plasmin-binding activities, allowing glomerular NAPlr and plasmin activity to be utilized as key biomarkers of IRGN.

Complement Factor I Variants in Complement-Mediated Renal Diseases

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

A Deep Insight Into Regulatory T Cell Metabolism in Renal Disease: Facts and Perspectives

Kidney disease encompasses a complex set of diseases that can aggravate or start systemic pathophysiological processes through their complex metabolic mechanisms and effects on body homoeostasis. The prevalence of kidney disease has increased dramatically over the last two decades. CD4⁺CD25⁺ regulatory T (Treg) cells that express the transcription factor forkhead box protein 3 (Foxp3) are critical for maintaining immune homeostasis and preventing autoimmune disease and tissue damage caused by excessive or unnecessary immune activation, including autoimmune kidney diseases. Recent studies have highlighted the critical role of metabolic reprogramming in controlling the plasticity, stability, and function of Treg cells. They are also likely to play a vital role in limiting kidney transplant rejection and potentially promoting transplant tolerance. Metabolic pathways, such as mitochondrial function, glycolysis, lipid synthesis, glutaminolysis, and mammalian target of rapamycin (mTOR) activation, are involved in the development of renal diseases by modulating the function and proliferation of Treg cells. Targeting metabolic pathways to alter Treg cells can offer a promising method for renal disease therapy. In this review, we provide a new perspective on the role of Treg cell metabolism in renal diseases by presenting the renal microenvironment、relevant metabolites of Treg cell metabolism, and the role of Treg cell metabolism in various kidney diseases.

A clinical approach to children with C3 glomerulopathy

C3 glomerulopathy is a relatively new clinical entity that represents a challenge both to diagnose and to treat. As new therapeutic agents that act as complement inhibitors become available, many with an oral formulation, a better understanding of this disease and of the underlying complement dysregulation driving it has become increasingly useful to optimize patient care. Moreover, recent advances in research have clarified the role of complement in other glomerular diseases in which its role was less established, namely in immune-complex membranoproliferative glomerulonephritis (IC-MPGN), ANCA-vasculitis, IgA nephropathy, and idiopathic membranous nephropathy. Complement inhibitors are being studied in adult and adolescent clinical trials for these indications. This review summarizes current knowledge and future perspectives on every aspect of the diagnosis and management of C3 glomerulopathy and elucidates current understanding of the role of complement in this condition and in other glomerular diseases in children. An overview of ongoing trials involving therapeutic agents targeting complement in glomerular diseases is also provided.

Complement C3-targeted therapy in C3 glomerulopathy, a prototype of complement-mediated kidney diseases

C3 glomerulopathy (C3G) is a rare and complex kidney disease that primarily affects young adults. Renal outcomes remain poor in the absence of specific treatment. C3G is driven by uncontrolled overactivation of the alternative complement pathway, which is mainly of acquired origin. Functional characterization of complement abnormalities (i.e., autoantibodies targeting complement components and variants in complement genes) identified in patients and experimental models of the disease improved the understanding of the disease, making C3G a prototype of complement-mediated diseases. The contribution of C3 convertase, as well as C5 convertase, in disease occurrence, phenotype, and severity is now well established, offering various potential therapeutic interventions. However, the lack of sufficient efficiency in anti-C5 therapy highlights the extreme complexity of the disease and the need for new therapeutic approaches based on C3 and C3 convertase axis inhibition. Here, we provide an overview of the complement activation mechanism involved in C3G and discuss therapeutic options based on complement inhibitors, with a specific focus on C3 inhibition.

Myeloperoxidase immunohistochemical staining can identify glomerular endothelial cell injury in dense deposit disease

BACKGROUND

Previous studies have demonstrated residual complement-mediated deposits in repeat kidney biopsies of C3 glomerulopathies (C3G) (dense deposit disease (DDD) and C3 glomerulonephritis) following eculizumab treatment, despite some clinical improvement. With residual complement deposition, it is difficult to determine whether there is a reduced complement-mediated endothelial cell injury. We validated that myeloperoxidase (MPO) immunohistochemical staining identified glomerular endothelial cell injury in crescentic glomerulonephritis and C3G.

CASE (DIAGNOSIS/TREATMENT)

We report that MPO staining in the glomerular endothelium of the post-treatment kidney biopsy was significantly reduced after 3 years of eculizumab treatment and clinical improvement in a 5-year-old boy with initial DDD and secondary crescent formation.

CONCLUSION

We find that immunostaining for MPO is a useful method to compare glomerular endothelial injury in C3G following eculizumab treatment. This finding also supports the notion that eculizumab, a C5 blocker, may not mainly block C3 deposits in the glomeruli but significantly blocks final activation of the complement cascade, thus reducing glomerular endothelial cell injury.

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.

Complement and the prothrombotic state

In 2007 and 2009 the regulatory approval of the first-in-class complement inhibitor Eculizumab has revolutionized the clinical management of two rare, life-threatening clinical conditions: paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). While being completely distinct diseases affecting blood cells and the glomerulus, PNH and aHUS remarkably share several features in their etiology and clinical presentation. An imbalance between complement activation and regulation at host surfaces underlies both diseases precipitating in severe thrombotic events that are largely resistant to anti-coagulant and/or anti-platelet therapies. Inhibition of the common terminal complement pathway by Eculizumab prevents the frequently occurring thrombotic events responsible for the high mortality and morbidity observed in patients not treated with anti-complement therapy. While many in vitro and ex vivo studies elaborate numerous different molecular interactions between complement activation products and hemostasis, this review focuses on the clinical evidence that links these two fields in humans. Several non-infectious conditions with known complement involvement are scrutinized for common patterns concerning a prothrombotic statues and the occurrence of certain complement activation levels. Next to PNH and aHUS, germline encoded CD59 or CD55 deficiency (the latter causing the disease Complement Hyperactivation, Angiopathic thrombosis, and Protein-Losing Enteropathy; CHAPLE), autoimmune hemolytic anemia (AIHA), (catastrophic) anti-phospholipid syndrome (APS, CAPS) and C3 glomerulopathy are considered. Parallels and distinct features among these conditions are discussed against the background of thrombosis, complement activation, and potential complement diagnostic and therapeutic avenues.

Inherited Kidney Complement Diseases

In the past 20 years, we have witnessed tremendous advances in our ability to diagnose and treat genetic diseases of the kidney caused by complement dysregulation. Staggering progress was realized toward a better understanding of the genetic underpinnings and pathophysiology of many forms of atypical hemolytic uremic syndrome (aHUS) and C3-dominant glomerulopathies that are driven by complement system abnormalities. Many of these seminal discoveries paved the way for the design and characterization of several innovative therapies, some of which have already radically improved patients' outcomes. This review offers a broad overview of the exciting developments that have occurred in the recent past, with a particular focus on single-gene (or Mendelian), complement-driven aHUS and C3-dominant glomerulopathies that should be of interest to both nephrologists and kidney researchers. The discussion is restricted to genes with robust associations with both aHUS and C3-dominant glomerulopathies (complement factor H, complement component 3, complement factor H-related proteins) or only aHUS (complement factor B, complement factor I, and membrane cofactor protein). Key questions and challenges are highlighted, along with potential avenues for future directions.

Inherited Kidney Complement Diseases

In the past 20 years, we have witnessed tremendous advances in our ability to diagnose and treat genetic diseases of the kidney caused by complement dysregulation. Staggering progress was realized toward a better understanding of the genetic underpinnings and pathophysiology of many forms of atypical hemolytic uremic syndrome (aHUS) and C3-dominant glomerulopathies that are driven by complement system abnormalities. Many of these seminal discoveries paved the way for the design and characterization of several innovative therapies, some of which have already radically improved patients' outcomes. This review offers a broad overview of the exciting developments that have occurred in the recent past, with a particular focus on single-gene (or Mendelian), complement-driven aHUS and C3-dominant glomerulopathies that should be of interest to both nephrologists and kidney researchers. The discussion is restricted to genes with robust associations with both aHUS and C3-dominant glomerulopathies (complement factor H, complement component 3, complement factor H-related proteins) or only aHUS (complement factor B, complement factor I, and membrane cofactor protein). Key questions and challenges are highlighted, along with potential avenues for future directions.

Deposition of the Membrane Attack Complex in Healthy and Diseased Human Kidneys

The membrane attack complex-also known as C5b-9-is the end-product of the classical, lectin, and alternative complement pathways. It is thought to play an important role in the pathogenesis of various kidney diseases by causing cellular injury and tissue inflammation, resulting in sclerosis and fibrosis. These deleterious effects are, consequently, targeted in the development of novel therapies that inhibit the formation of C5b-9, such as eculizumab. To clarify how C5b-9 contributes to kidney disease and to predict which patients benefit from such therapy, knowledge on deposition of C5b-9 in the kidney is essential. Because immunohistochemical staining of C5b-9 has not been routinely conducted and never been compared across studies, we provide a review of studies on deposition of C5b-9 in healthy and diseased human kidneys. We describe techniques to stain deposits and compare the occurrence of deposits in healthy kidneys and in a wide spectrum of kidney diseases, including hypertensive nephropathy, diabetic nephropathy, membranous nephropathy, IgA nephropathy, lupus nephritis, C3 glomerulopathy, and thrombotic microangiopathies such as the atypical hemolytic uremic syndrome, vasculitis, interstitial nephritis, acute tubular necrosis, kidney tumors, and rejection of kidney transplants. We summarize how these deposits are related with other histological lesions and clinical characteristics. We evaluate the prognostic relevance of these deposits in the light of possible treatment with complement inhibitors.

C3 glomerulopathy and atypical hemolytic uremic syndrome: an updated review of the literature on alternative complement pathway disorders

The complement system plays a significant role within the pathological process of C3 glomerulopathy (C3GP) and atypical hemolytic uremic syndrome (aHUS). In daily practice, clinicians should differentiate the subgroups of C3GP because of they should apply different treatment modalities. In the past, C3GP was considered as a part of membranoproliferative glomerulonephritis (MPGN). MPGN is defined as glomerular capillary thickening secondary to the synthesis of the new glomerular basement membrane and mesangial cellular hyperplasia with mesangial matrix expansion. Atypical hemolytic uremic syndrome is an ultra-rare disease that can be outlined by the triad of Coombs negative microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. Recent advances demonstrated that these diseases share common abnormalities of the control of the alternative complement system. Therefore, nowadays, most researchers advocate that there may be overlap in the pathogenesis of C3GP and aHUS. This review will provide recent novel mechanisms and treatment options in these diseases. For the purposes that we mentioned above and to help clinicians, we aimed to describe the etiology, pathophysiology, and treatment of C3GP and aHUS in this comprehensive review.

Function and Dysfunction of Complement Factor H During Formation of Lipid-Rich Deposits

Complement-mediated inflammation or dysregulation in lipid metabolism are associated with the pathogenesis of several diseases. These include age-related macular degeneration (AMD), C3 glomerulonephritis (C3GN), dense deposit disease (DDD), atherosclerosis, and Alzheimer’s disease (AD). In all these diseases, formation of characteristic lipid-rich deposits is evident. Here, we will discuss molecular mechanisms whereby dysfunction of complement, and especially of its key regulator factor H, could be involved in lipid accumulation and related inflammation. The genetic associations to factor H polymorphisms, the role of factor H in the resolution of inflammation in lipid-rich deposits, modification of macrophage functions, and complement-mediated clearance of apoptotic and damaged cells indicate that the function of factor H is crucial in limiting inflammation in these diseases.

Treatment of C3 Glomerulopathy in Adult Kidney Transplant Recipients: A Systematic Review

BACKGROUND

C3 glomerulopathy (C3G), a rare glomerular disease mediated by alternative complement pathway dysregulation, is associated with a high rate of recurrence and graft loss after kidney transplantation (KTx). We aimed to assess the efficacy of different treatments for C3G recurrence after KTx.

METHODS

Databases (MEDLINE, EMBASE, and Cochrane Database) were searched from inception through 3 May, 2019. Studies were included that reported outcomes of adult KTx recipients with C3G. Effect estimates from individual studies were combined using the random-effects, generic inverse variance method of DerSimonian and Laird., The protocol for this meta-analysis is registered with PROSPERO (no. CRD42019125718).

RESULTS

Twelve studies (7 cohort studies and 5 case series) consisting of 122 KTx patients with C3G (73 C3 glomerulonephritis (C3GN) and 49 dense deposit disease (DDD)) were included. The pooled estimated rates of allograft loss among KTx patients with C3G were 33% (95% CI: 12-57%) after eculizumab, 42% (95% CI: 2-89%) after therapeutic plasma exchange (TPE), and 81% (95% CI: 50-100%) after rituximab. Subgroup analysis based on type of C3G was performed. Pooled estimated rates of allograft loss in C3GN KTx patients were 22% (95% CI: 5-46%) after eculizumab, 56% (95% CI: 6-100%) after TPE, and 70% (95% CI: 24-100%) after rituximab. Pooled estimated rates of allograft loss in DDD KTx patients were 53% (95% CI: 0-100%) after eculizumab. Data on allograft loss in DDD after TPE (1 case series, 0/2 (0%) allograft loss at 6 months) and rituximab (1 cohort, 3/3 (100%) allograft loss) were limited. Among 66 patients (38 C3GN, 28 DDD) who received no treatment (due to stable allograft function at presentation and/or clinical judgment of physicians), pooled estimated rates of allograft loss were 32% (95% CI: 7-64%) and 53% (95% CI: 28-77%) for C3GN and DDD, respectively. Among treated C3G patients, data on soluble membrane attack complex of complement (sMAC) were limited to patients treated with eculizumab (N = 7). 80% of patients with elevated sMAC before eculizumab responded to treatment. In addition, all patients who responded to eculizumab had normal sMAC levels after post-eculizumab.

CONCLUSIONS

Our study suggests that the lowest incidence of allograft loss (33%) among KTX patients with C3G are those treated with eculizumab. Among those who received no treatment for C3G due to stable allograft function, there is a high incidence of allograft loss of 32% in C3GN and 53% in DDD. sMAC level may help to select good responders to eculizumab.

Interaction of the Factor H Family Proteins FHR-1 and FHR-5 With DNA and Dead Cells: Implications for the Regulation of Complement Activation and Opsonization

Complement plays an essential role in the opsonophagocytic clearance of apoptotic/necrotic cells. Dysregulation of this process may lead to inflammatory and autoimmune diseases. Factor H (FH), a major soluble complement inhibitor, binds to dead cells and inhibits excessive complement activation on their surface, preventing lysis, and the release of intracellular material, including DNA. The FH-related (FHR) proteins share common ligands with FH, due to their homology with this complement regulator, but they lack the domains that mediate the complement inhibitory activity of FH. Because their roles in complement regulation is controversial and incompletely understood, we studied the interaction of FHR-1 and FHR-5 with DNA and dead cells and investigated whether they influence the regulatory role of FH and the complement activation on DNA and dead cells. FH, FHR-1, and FHR-5 bound to both plasmid DNA and human genomic DNA, where both FHR proteins inhibited FH-DNA interaction. The FH cofactor activity was inhibited by FHR-1 and FHR-5 due to the reduced binding of FH to DNA in the presence of the FHRs. Both FHRs caused increased complement activation on DNA. FHR-1 and FHR-5 bound to late apoptotic and necrotic cells and recruited monomeric C-reactive protein and pentraxin 3, and vice versa. Interactions of the FHRs with pentraxins resulted in enhanced activation of both the classical and the alternative complement pathways on dead cells when exposed to human serum. Altogether, our results demonstrate that FHR-1 and FHR-5 are competitive inhibitors of FH on DNA; moreover, FHR-pentraxin interactions promote opsonization of dead cells.

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.

Proteomic Analysis of Complement Proteins in Membranous Nephropathy

Introduction

Membranous nephropathy (MN) is the most common cause of nephrotic syndrome in Caucasian adults. Phospholipase A2 receptor (PLA2R)– and exostosin 1 (EXT1)/exostosin 2 (EXT2)–associated MN represent the most common primary and secondary forms of MN. The complement profile using a proteomics approach has not been studied in these 2 common forms of MN.

Methods

We used laser microdissection and mass spectrometry (MS/MS) to dissect glomeruli and identify glomerular complement proteins in PLA2R-associated (n = 7), EXT1/EXT2-associated MN (n = 21), and 11 control cases (time 0 transplant biopsies).

Results

MS/MS identified high total spectral counts for PLA2R and EXT1/EXT2 in corresponding cases of PLA2R- and EXT1/EXT2-positive MN. Both PLA2R- and EXT1/EXT2-associated MN had high spectral counts of complement proteins C3, C4, C5, C6, C7, C8, and C9. Complement protein C1 was present in low spectral counts in EXT1/EXT2-associated MN. Regulators of complement activation that were detected in MN included higher spectral counts of FH, FHR-1, FHR-5, clusterin, vitronectin and lower spectral counts of FHR-3, FHR-4, and CD59. Low spectral counts of FB and properdin, key components of the alternative pathway, also were detected. IgG4 and IgG1 were the most abundant IgG subclasses in PLA2R- and EXT1/EXT2-associated MN. Lower spectral counts for C3, C4, and C5 were detected in control cases when compared with MN.

Conclusion

Significant complement activation is present in MN as evidenced by large spectral counts of complement proteins from C3- and C4-based pathways, including regulatory proteins of complement pathways. These data suggest that anticomplement drugs may be effective in treatment for MN.

Outcome of C3 glomerulopathy patients: largest single-centre experience from South Asia

BACKGROUND

C3 glomerulopathy (C3G) is related to dysfunction of alternative complement pathway (ACP) because of its hyperactivation. Triggering factors and genetic profile are likely to be different in developing countries as compared to the Western world. Data regarding C3G from South Asian is scanty.

STUDY DESIGN

In the present study, 115 patients of C3G from 2012 to 2017 were analyzed. Clinical details were reviewed; serological levels of C3, C4, complement factor H or B and autoantibody testing was done by nephelometry/ELISA. Limited genetics workup for CFH and CFHR5 genes was done.

RESULTS

The prevalence of C3G was 1.52%. There was no difference in demographic and histopathologic profiles of C3G patients. Majority of patients had low functional assay and C3 levels. C3 nephritic factor was present in 47.5% of DDD and 38.6% of C3GN. Autoantibodies to CFH were present more often in the patients of C3GN (29.5%) than DDD (12.5%). Autoantibodies to CFB were equally common in both groups. Past history of infections was present in one-third patients and monoclonal paraproteins were present only in two patients. No pathogenic variants were noted in CFH/CFHR5 gene. On follow-up (3.2 + 1.6 years), complete and partial remission was achieved in one-fourth patients and 26% had resistance disease. About 40% progressed to ESRD and 18 underwent renal transplantation of which nine had a post-transplant recurrence.

CONCLUSIONS

Indian cohort had some differences in the immunological and genetic profile when compared to the Western literature; most significant was the absence of monoclonal immunoglobulins as a trigger for C3G.

Emerging role of clinical mass spectrometry in pathology

Mass spectrometry-based assays have been increasingly implemented in various disciplines in clinical diagnostic laboratories for their combined advantages in multiplexing capacity and high analytical specificity and sensitivity. It is now routinely used in areas including reference methods development, therapeutic drug monitoring, toxicology, endocrinology, paediatrics, immunology and microbiology to identify and quantify biomolecules in a variety of biological specimens. As new ionisation methods, instrumentation and techniques are continuously being improved and developed, novel mass spectrometry-based clinical applications will emerge for areas such as proteomics, metabolomics, haematology and anatomical pathology. This review will summarise the general principles of mass spectrometry and specifically highlight current and future clinical applications in anatomical pathology.

Mechanisms of haemolysis-induced kidney injury

Intravascular haemolysis is a fundamental feature of chronic hereditary and acquired haemolytic anaemias, including those associated with haemoglobinopathies, complement disorders and infectious diseases such as malaria. Destabilization of red blood cells (RBCs) within the vasculature results in systemic inflammation, vasomotor dysfunction, thrombophilia and proliferative vasculopathy. The haemoprotein scavengers haptoglobin and haemopexin act to limit circulating levels of free haemoglobin, haem and iron - potentially toxic species that are released from injured RBCs. However, these adaptive defence systems can fail owing to ongoing intravascular disintegration of RBCs. Induction of the haem-degrading enzyme haem oxygenase 1 (HO1) - and potentially HO2 - represents a response to, and endogenous defence against, large amounts of cellular haem; however, this system can also become saturated. A frequent adverse consequence of massive and/or chronic haemolysis is kidney injury, which contributes to the morbidity and mortality of chronic haemolytic diseases. Intravascular destruction of RBCs and the resulting accumulation of haemoproteins can induce kidney injury via a number of mechanisms, including oxidative stress and cytotoxicity pathways, through the formation of intratubular casts and through direct as well as indirect proinflammatory effects, the latter via the activation of neutrophils and monocytes. Understanding of the detailed pathophysiology of haemolysis-induced kidney injury offers opportunities for the design and implementation of new therapeutic strategies to counteract the unfavourable and potentially fatal effects of haemolysis on the kidney.

Autoimmune abnormalities of the alternative complement pathway in membranoproliferative glomerulonephritis and C3 glomerulopathy

Membranoproliferative glomerulonephritis (MPGN) is a rare chronic kidney disease associated with complement activation. Recent immunofluorescence-based classification distinguishes between immune complex (IC)-mediated MPGN, with glomerular IgG and C3 deposits, and C3 glomerulopathies (C3G), with predominant C3 deposits. Genetic and autoimmune abnormalities causing hyperactivation of the complement alternative pathway have been found as frequently in patients with immune complex-associated MPGN (IC-MPGN) as in those with C3G. In the last decade, there have been great advances in research into the autoimmune causes of IC-MPGN and C3G. The complement-activating autoantibodies called C3-nephritic factors (C3NeFs), which are present in 40-80% of patients, form a heterogeneous group of autoantibodies that stabilise the C3 convertase or the C5 convertase of the alternative pathway or both. A few patients, mainly with IC-MPGN, carry autoantibodies directed against the two components of the alternative pathway C3 convertase, factors B and C3b. Finally, autoantibodies against factor H, the main regulator of the alternative pathway, have been reported in a small proportion of patients with IC-MPGN or C3G. The identification of distinct pathogenetic patterns leading to kidney injury and of targets in the complement cascade may pave the way for tailored therapies for IC-MPGN and C3G, with specific complement inhibitors in the development pipeline.

C3 glomerulopathy - understanding a rare complement-driven renal disease

The C3 glomerulopathies are a group of rare kidney diseases characterized by complement dysregulation occurring in the fluid phase and in the glomerular microenvironment, which results in prominent complement C3 deposition in kidney biopsy samples. The two major subgroups of C3 glomerulopathy - dense deposit disease (DDD) and C3 glomerulonephritis (C3GN) - have overlapping clinical and pathological features suggestive of a disease continuum. Dysregulation of the complement alternative pathway is fundamental to the manifestations of C3 glomerulopathy, although terminal pathway dysregulation is also common. Disease is driven by acquired factors in most patients - namely, autoantibodies that target the C3 or C5 convertases. These autoantibodies drive complement dysregulation by increasing the half-life of these vital but normally short-lived enzymes. Genetic variation in complement-related genes is a less frequent cause. No disease-specific treatments are available, although immunosuppressive agents and terminal complement pathway blockers are helpful in some patients. Unfortunately, no treatment is universally effective or curative. In aggregate, the limited data on renal transplantation point to a high risk of disease recurrence (both DDD and C3GN) in allograft recipients. Clinical trials are underway to test the efficacy of several first-generation drugs that target the alternative complement pathway.

Complement Factor H and Apolipoprotein E Participate in Regulation of Inflammation in THP-1 Macrophages

The alternative pathway (AP) of complement is constantly active in plasma and can easily be activated on self surfaces and trigger local inflammation. Host cells are protected from AP attack by Factor H (FH), the main AP regulator in plasma. Although complement is known to play a role in atherosclerosis, the mechanisms of its contribution are not fully understood. Since FH via its domains 5-7 binds apoliporotein E (apoE) and macrophages produce apoE we examined how FH could be involved in the antiatherogenic effects of apoE. We used blood peripheral monocytes and THP-1 monocyte/macrophage cells which were also loaded with acetylated low-density lipoprotein (LDL) to form foam cells. Binding of FH and apoE on these cells was analyzed by flow cytometry. High-density lipoprotein (HDL)-mediated cholesterol efflux of activated THP-1 cells was measured and transcriptomes of THP-1 cells using mRNA sequencing were determined. We found that binding of FH to human blood monocytes and cholesterol-loaded THP-1 macrophages increased apoE binding to these cells. Preincubation of fluorescent cholesterol labeled THP-1 macrophages in the presence of FH increased cholesterol efflux and cholesterol-loaded macrophages displayed reduced transcription of proinflammatory/proatherogenic factors and increased transcription of anti-inflammatory/anti-atherogenic factors. Further incubation of THP-1 cells with serum reduced C3b/iC3b deposition. Overall, our data indicate that apoE and FH interact with monocytic cells in a concerted action and this interaction reduces complement activation and inflammation in the atherosclerotic lesions. By this way FH may participate in mediating the beneficial effects of apoE in suppressing atherosclerotic lesion progression.

Thrombotic microangiopathy after renal transplantation: Current insights in de novo and recurrent disease

Thrombotic microangiopathy (TMA) is one of the most devastating sequalae of kidney transplantation. A number of published articles have covered either de novo or recurrent TMA in an isolated manner. We have, hereby, in this article endeavored to address both types of TMA in a comparative mode. We appreciate that de novo TMA is more common and its prognosis is poorer than recurrent TMA; the latter has a genetic background, with mutations that impact disease behavior and, consequently, allograft and patient survival. Post-transplant TMA can occur as a recurrence of the disease involving the native kidney or as de novo disease with no evidence of previous involvement before transplant. While atypical hemolytic uremic syndrome is a rare disease that results from complement dysregulation with alternative pathway overactivity, de novo TMA is a heterogenous set of various etiologies and constitutes the vast majority of post-transplant TMA cases. Management of both diseases varies from simple maneuvers, e.g., plasmapheresis, drug withdrawal or dose modification, to lifelong complement blockade, which is rather costly. Careful donor selection and proper recipient preparation, including complete genetic screening, would be a pragmatic approach. Novel therapies, e.g., purified products of the deficient genes, though promising in theory, are not yet of proven value.

Dense Deposit Disease Involving C3 and C4d Deposits

Dense deposit disease (DDD), earlier called Type II membranoproliferative glomerulonephritis is distinct disease having frequent relapses reaching end-stage kidney disease by 10-year in up to 50%-60% of cases and high recurrence rate in the allograft. The term DDD is derived from its distinctive ribbon-like osmiophilic deposits in the lamina densa of glomerular basement membrane by electron microscopy. Pathogenetically, alternate pathway dysfunction leads to this disease, which is diagnosed by ultrastructure. Herein, we describe our observation of C4d positivity in an adolescent boy with DDD.

FHR5 Binds to Laminins, Uses Separate C3b and Surface-Binding Sites, and Activates Complement on Malondialdehyde-Acetaldehyde Surfaces

Factor H related-protein 5 (CFHR5) is a surface-acting complement activator and variations in the CFHR5 gene are linked to CFHR glomerulonephritis. In this study, we show that FHR5 binds to laminin-521, the major constituent of the glomerular basement membrane, and to mesangial laminin-211. Furthermore, we identify malondialdehyde-acetaldehyde (MAA) epitopes, which are exposed on the surface of human necrotic cells (Homo sapiens), as new FHR5 ligands. Using a set of novel deletion fragments, we show that FHR5 binds to laminin-521, MAA epitopes, heparin, and human necrotic cells (HUVECs) via the middle region [short consensus repeats (SCRs) 5-7]. In contrast, surface-bound FHR5 contacts C3b via the C-terminal region (SCRs8-9). Thus, FHR5 uses separate domains for C3b binding and cell surface interaction. MAA epitopes serve as a complement-activating surface by recruiting FHR5. The complement activator FHR5 and the complement inhibitor factor H both bind to oxidation-specific MAA epitopes and FHR5 competes with factor H for binding. The C3 glomerulopathy-associated FHR2_1-2-FHR5 hybrid protein is more potent in MAA epitope binding and activation compared with wild-type FHR5. The implications of these results for pathology of CFHR glomerulonephritis are discussed. In conclusion, we identify laminins and oxidation-specific MAA epitopes as novel FHR5 ligands and show that the surface-binding site of FHR5 (SCRs5-7) is separated from the C3b binding site (SCRs8-9). Furthermore, FHR5 competes with factor H for binding to MAA epitopes and activates complement on these modified structures.

Understanding AMD by analogy: systematic review of lipid-related common pathogenic mechanisms in AMD, AD, AS and GN

RATIONALE

Age-related macular degeneration (AMD) is one of the leading causes of blindness among the elderly. Due to its complex etiology, current treatments have been insufficient. Previous studies reveal three systems closely involved in AMD pathogenesis: lipid metabolism, oxidation and inflammation. These systems are also involved in Alzheimer's disease, atherosclerosis and glomerulonephritis. Understanding commonalities of these four diseases may provide insight into AMD etiology.

OBJECTIVES

To understand AMD pathogenesis by analogy and suggest ideas for future research, this study summarizes main commonalities in disease pathogenesis of AMD, Alzheimer's disease, atherosclerosis and glomerulonephritis.

METHODS

Articles were identified through PubMed, Ovid Medline and Google Scholar. We summarized the common findings and synthesized critical differences.

RESULTS

Oxidation, lipid deposition, complement activation, and macrophage recruitment are involved in all four diseases shown by genetic, molecular, animal and human studies. Shared genetic variations further strengthen their connection. Potential areas for future research are suggested throughout the review.

CONCLUSIONS

The four diseases share many steps of an overall framework of pathogenesis. Various oxidative sources cause oxidative stress. Oxidized lipids and related molecules accumulate and lead to complement activation, macrophage recruitment and pathology. Investigations that arise under this structure may aid us to better understand AMD pathology.

Characterization of C3 in C3 glomerulopathy

Background

C3 glomerulopathy (C3G) is caused by overactivity of the alternative pathway of complement that results in bright glomerular C3 staining with minimal or no deposition of immunoglobulins on immunofluorescence microscopy. Laser microdissection and mass spectrometry of the two subtypes, C3 glomerulonephritis (C3GN) and dense deposit disease (DDD), have identified C3 as the predominant glomerular complement protein, although lesser amounts of C9, C5, C6, C7 and C8 are detectable. C3 plays a central role in complement activity, with its proteolytic cleavage first generating C3a and C3b, followed by inactivation of C3b generating iC3b (which includes C3α and C3β), which undergoes further breakdown yielding C3c and terminal breakdown fragment C3dg. The composition of C3 breakdown products in C3G is not known.

Methods

In this study, we chose six cases each of C3GN and DDD to analyze the composition of C3 deposits. We analyzed the amino acid sequence of C3 spectra detected by mass spectrometry to determine the relative abundance of C3 fragments in C3G. Thus we were able to determine the amino acid sequences mapping to the various C3 activation products including C3dg, C3α (C3α1 and α2), and C3β that are part of C3b/iC3b/C3c.

Results

C3dg is the predominant cleavage product detected with the highest amino acid coverage. The remaining amino acids map to C3α (C3α1 and α2) and C3β. Amino acids mapping to C3a and C3f are absent. Taken together, the C3α and C3β amino acids represent iC3b prior to or after C3c cleavage of C3dg. The C3 spectra for both C3GN and DDD are surprisingly similar.

Conclusion

The finding of large amounts of C3dg suggests that C3b deposition in the glomerulus is an active process triggered by thioester binding of C3b to the glycocalyx overlying the glomerular endothelial cells and glomerular basement membrane. Regulatory protein-mediated inactivation of C3b results in the generation of iC3b. After additional cleavages, mostly C3dg remains.

Complement activation in pauci-immune necrotizing and crescentic glomerulonephritis: results of a proteomic analysis

Background

Complement activation plays an important role in the pathophysiology of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV), although it remains unclear which pathway is activated. Whether pauci-immune necrotizing crescentic glomerulonephritis (pauci-immune GN) with negative ANCA serology is part of the spectrum of AAV or a different disease entity is essentially unknown.

Methods

We used proteomic analysis to delineate the complement profile in a series of 13 kidney biopsies of patients with pauci-immune GN, with either proteinase 3 (PR3) (five patients) or myeloperoxidase (MPO) antibodies (four patients) or with consistently negative ANCA serology (four patients). Immunofluorescence staining of glomeruli was essentially negative in the PR3-ANCA and MPO-ANCA groups, while a mild staining for C3 was seen in the ANCA-negative cases. No electron-dense deposits were found in the PR3-ANCA and MPO-ANCA groups, but mesangial and few subepithelial deposits were clearly present in the ANCA-negative specimens.

Results

Mass spectrometry revealed low spectra numbers for C3 and immunoglobulins in both PR3-positive and MPO-positive patients with minimal or no C4 and C9. In contrast, larger spectra numbers for C3, moderate spectra numbers for C9, complement factor H-related protein-1 and low spectra numbers for C4, C5 and immunoglobulins were found in the ANCA-negative cases.

Conclusion

While complement activation is noted in AAV, the complement activation appears to be more prominent in the ANCA-negative glomerulonephritis. The larger amount of C3 and moderate amount of C9 in the ANCA-negative glomerulonephritis implies activation of the alternate and terminal pathway of complement, suggesting that this entity may be caused or promoted by a genetic or acquired defect in the alternative pathway.

Serum C3 and Renal Outcome in Patients with Primary Focal Segmental Glomerulosclerosis

The role of complement (C) in the pathogenesis or progression of focal segmental glomerulosclerosis (FSGS) is uncertain. The present study assessed the relationship between serum C3, the baseline characteristics, and the progression of FSGS in the cohort and identified the clinical implications of serum C3 levels in patients with FSGS. Compared to the patients with C3 ≥ 85 mg/dL (N = 474), those with C3 < 85 mg/dL (N = 117) presented a higher level of serum creatinine, lower levels of eGFR, hemoglobin, proteinuria, triglyceride, cholesterol, IgA, as well as, severe tubulointerstitial injury (TI). Of the 221 patients with a mean follow-up of 53.3 months, the risk of reaching end-stage renal disease (ESRD) was significantly higher in patients with low serum C3 level (p < 0.001). An additional 40 patients with primary FSGS revealed a significant correlation between MAC and AP (p = 0.003), MAC and serum C3 (p = 0.018), and AP and serum C3 (p = 0.028). Compared to patients with none-to-mild TI, those with moderate-to-severe TI exhibited a lower level of serum C3 and AP, and a higher level of serum MAC. In conclusion, complement activation occurring in patients with FSGS is associated with clinical and histological severities. Low serum C3 was an independent risk factor for poor renal outcome in patients with FSGS.

Potential influences of complement factor H in autoimmune inflammatory and thrombotic disorders

Complement system homeostasis is important for host self-protection and anti-microbial immune surveillance, and recent research indicates roles in tissue development and remodelling. Complement also appears to have several points of interaction with the blood coagulation system. Deficiency and altered function due to gene mutations and polymorphisms in complement effectors and regulators, including Factor H, have been associated with familial and sporadic autoimmune inflammatory - thrombotic disorders, in which autoantibodies play a part. These include systemic lupus erythematosus, rheumatoid arthritis, atypical haemolytic uremic syndrome, anti-phospholipid syndrome and age-related macular degeneration. Such diseases are generally complex - multigenic and heterogeneous in their symptoms and predisposition/susceptibility. They usually need to be triggered by vascular trauma, drugs or infection and non-complement genetic factors also play a part. Underlying events seem to include decline in peripheral regulatory T cells, dendritic cell, and B cell tolerance, associated with alterations in lymphoid organ microenvironment. Factor H is an abundant protein, synthesised in many cell types, and its reported binding to many different ligands, even if not of high affinity, may influence a large number of molecular interactions, together with the accepted role of Factor H within the complement system. Factor H is involved in mesenchymal stem cell mediated tolerance and also contributes to self-tolerance by augmenting iC3b production and opsonisation of apoptotic cells for their silent dendritic cell engulfment via complement receptor CR3, which mediates anti-inflammatory-tolerogenic effects in the apoptotic cell context. There may be co-operation with other phagocytic receptors, such as complement C1q receptors, and the Tim glycoprotein family, which specifically bind phosphatidylserine expressed on the apoptotic cell surface. Factor H is able to discriminate between self and nonself surfaces for self-protection and anti-microbe defence. Factor H, particularly as an abundant platelet protein, may also modulate blood coagulation, having an anti-thrombotic role. Here, we review a number of interaction pathways in coagulation and in immunity, together with associated diseases, and indicate where Factor H may be expected to exert an influence, based on reports of the diversity of ligands for Factor H.