Phospholipase A2 Receptor-Related Membranous Nephropathy and Mannan-Binding Lectin Deficiency

The complement system in the pathogenesis and progression of kidney diseases: What doesn't kill you makes you older

The Complement System is an evolutionarily conserved component of immunity that plays a key role in host defense against infections and tissue homeostasis. However, the dysfunction of the Complement System can result in tissue damage and inflammation, thereby contributing to the development and progression of various renal diseases, ranging from atypical Hemolytic Uremic Syndrome to glomerulonephritis. Therapeutic interventions targeting the complement system have demonstrated promising results in both preclinical and clinical studies. Currently, several complement inhibitors are being developed for the treatment of complement-mediated renal diseases. This review aims to summarize the most recent insights into complement activation and therapeutic inhibition in renal diseases. Furthermore, it offers potential directions for the future rational use of complement inhibitor drugs in the context of renal diseases.

Complement activation and effector pathways in membranous nephropathy

Complement activation has long been recognized as a central feature of membranous nephropathy (MN). Evidence for its role has been derived from the detection of complement products in biopsy tissue and urine from patients with MN and from mechanistic studies primarily based on the passive Heymann nephritis model. Only recently, more detailed insights into the exact mechanisms of complement activation and effector pathways have been gained from patient data, animal models, and in vitro models based on specific target antigens relevant to the human disease. These data are of clinical relevance, as they parallel the recent development of numerous specific complement therapeutics for clinical use. Despite efficient B-cell depletion, many patients with MN achieve only partial remission of proteinuria, which may be explained by the persistence of subepithelial immune complexes and ongoing complement-mediated podocyte injury. Targeting complement, therefore, represents an attractive adjunct treatment for MN, but it will need to be tailored to the specific complement pathways relevant to MN. This review summarizes the different lines of evidence for a central role of complement in MN and for the relevance of distinct complement activation and effector pathways, with a focus on recent developments.

Complement Activation in Nephrotic Glomerular Diseases

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

Understanding the podocyte immune responses in proteinuric kidney diseases: from pathogenesis to therapy

The glomerular filtration barrier, comprising the inner layer of capillary fenestrated endothelial cells, outermost podocytes, and the glomerular basement membrane between them, plays a pivotal role in kidney function. Podocytes, terminally differentiated epithelial cells, are challenging to regenerate once injured. They are essential for maintaining the integrity of the glomerular filtration barrier. Damage to podocytes, resulting from intrinsic or extrinsic factors, leads to proteinuria in the early stages and eventually progresses to chronic kidney disease (CKD). Immune-mediated podocyte injury is a primary pathogenic mechanism in proteinuric glomerular diseases, including minimal change disease, focal segmental glomerulosclerosis, membranous nephropathy, and lupus nephritis with podocyte involvement. An extensive body of evidence indicates that podocytes not only contribute significantly to the maintenance of the glomerular filtration barrier and serve as targets of immune responses but also exhibit immune cell-like characteristics, participating in both innate and adaptive immunity. They play a pivotal role in mediating glomerular injury and represent potential therapeutic targets for CKD. This review aims to systematically elucidate the mechanisms of podocyte immune injury in various podocyte lesions and provide an overview of recent advances in podocyte immunotherapy. It offers valuable insights for a deeper understanding of the role of podocytes in proteinuric glomerular diseases, and the identification of new therapeutic targets, and has significant implications for the future clinical diagnosis and treatment of podocyte-related disorders.

Novel biomarkers and pathophysiology of membranous nephropathy: PLA2R and beyond

Research on membranous nephropathy truly exploded in the last 15 years. This happened because of the application of new techniques (laser capture microdissection, mass spectrometry, protein G immunoprecipitation, arrays) to the study of its pathogenesis. After the discovery of PLA2R as the major target antigen, many other antigens were identified and others are probably ongoing. Clinical and pathophysiology rebounds of new discoveries are relevant in terms of diagnosis and prognosis and it is time to make a first assessment of the innovative issues. In terms of classification, target antigens can be divided into: 'membrane antigens' and 'second wave' antigens. The first group consists of antigens constitutionally expressed on the podocyte membrane (as PLA2R) that may become a target of an autoimmune process because of perturbation of immune-tolerance. 'Second wave' antigens are antigens neo-expressed by the podocyte or by infiltrating cells after a stressing event: this allows the immune system to produce antibodies against them that intensify and maintain glomerular damage. With this abundance of target antigens it is not possible, at the moment, to test all antibodies at the bedside. In the absence of this possibility, the role of histological evaluation is still irreplaceable.

The role of complement in kidney disease

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

The role of complement in glomerulonephritis-are novel therapies ready for prime time?

The complement system plays a key pathogenic role in glomerular diseases with a diverse range of aetiologies, including C3 glomerulopathy, immunoglobulin A nephropathy, membranous nephropathy, ANCA-associated vasculitis and lupus nephritis. Several novel therapies targeting complement activity have recently been developed, which have now been approved or are in the late stages of clinical development. In this review, potential benefits and challenges of targeting the complement system in glomerular disease are discussed. We summarize current understanding of the role of complement, and the novel targeted therapies that are being developed for the treatment of glomerular disease.

Towards the Definition of the Molecular Hallmarks of Idiopathic Membranous Nephropathy in Serum Proteome: A DIA-PASEF Approach

Idiopathic membranous nephropathy (IMN) is a pathologically defined disorder of the glomerulus, primarily responsible for nephrotic syndromes (NS) in nondiabetic adults. The underlying molecular mechanisms are still not completely clarified. To explore possible molecular and functional signatures, an optimised mass spectrometry (MS) method based on next-generation data-independent acquisition combined with ion-mobility was applied to serum of patients affected by IMN (n = 15) or by other glomerulopathies (PN) (n = 15). The statistical comparison highlighted a panel of 57 de-regulated proteins with a significant increase in lipoprotein-related proteins (APOC1, APOB, APOA1, APOL1 and LCAT) and a substantial quantitative alteration of key serpins (including A4, D1, A7, A6, F2, F1 and 1) possibly associated with IMN or NS and podocyte stress. A critical dysregulation in metabolisms of lipids (e.g., VLDL assembly and clearance) likely to be related to known hyperlipidemia in IMN, along with involvement of non-classical complement pathways and a putative enrolment of ficolin-2 in sustaining the activation of the lectin-mediated complement system have been pinpointed. Moreover, mannose receptor CD206 (MRC1-down in IMN) and biotinidase (BTD-up in IMN) are able alone to accurately distinguish IMN vs. PN. To conclude, our work provides key proteomic insights into the IMN complexity, opening the way to an efficient stratification of MN patients.

Glomerulonephritis: immunopathogenesis and immunotherapy

'Glomerulonephritis' (GN) is a term used to describe a group of heterogeneous immune-mediated disorders characterized by inflammation of the filtration units of the kidney (the glomeruli). These disorders are currently classified largely on the basis of histopathological lesion patterns, but these patterns do not align well with their diverse pathological mechanisms and hence do not inform optimal therapy. Instead, we propose grouping GN disorders into five categories according to their immunopathogenesis: infection-related GN, autoimmune GN, alloimmune GN, autoinflammatory GN and monoclonal gammopathy-related GN. This categorization can inform the appropriate treatment; for example, infection control for infection-related GN, suppression of adaptive immunity for autoimmune GN and alloimmune GN, inhibition of single cytokines or complement factors for autoinflammatory GN arising from inborn errors in innate immunity, and plasma cell clone-directed or B cell clone-directed therapy for monoclonal gammopathies. Here we present the immunopathogenesis of GN and immunotherapies in use and in development and discuss how an immunopathogenesis-based GN classification can focus research, and improve patient management and teaching.

Membranous nephropathy: Mechanistic insights and therapeutic perspectives

Membranous nephropathy (MN) is one of the most common causes of non-diabetic nephrotic syndrome in adults. About 80% of cases are renal limited (primary MN) and 20% are associated with other systemic diseases or exposures (secondary MN). Autoimmune reaction is the main pathogenic factor of MN, and the discovery of autoantigens including the phospholipase A₂ receptor and thrombospondin type-1 domain-containing protein 7A has led to new insights into the pathogenesis, they can induce humoral immune responses led by IgG₄ makes them suitable for the diagnosis and monitoring of MN. In addition, complement activation, genetic susceptibility genes and environmental pollution are also involved in MN immune response. In clinical practice, due to the spontaneous remission of MN, the combination of supportive therapy and pharmacological treatment is widely used. Immunosuppressive drugs are the cornerstone of MN treatment, and the dangers and benefits of this approach vary from person to person. In summary, this review provides a more comprehensive review of the immune pathogenesis, interventions and unresolved issues of MN in the hope of providing some new ideas for clinical and scientific researchers in the treatment of MN.

Membranous nephropathy

Membranous nephropathy is a glomerulopathy, which main affected target is the podocyte, and has consequences on the glomerular basement membrane. It is more common in adults, especially over 50 years of age. The clinical presentation is nephrotic syndrome, but many cases can evolve with asymptomatic non-nephrotic proteinuria. The mechanism consists of the deposition of immune complexes in the subepithelial space of the glomerular capillary loop with subsequent activation of the complement system. Great advances in the identification of potential target antigens have occurred in the last twenty years, and the main one is the protein "M-type phospholipase-A2 receptor" (PLA2R) with the circulating anti-PLA2R antibody, which makes it possible to evaluate the activity and prognosis of this nephropathy. This route of injury corresponds to approximately 70% to 80% of cases of membranous nephropathy characterized as primary. In the last 10 years, several other potential target antigens have been identified. This review proposes to present clinical, etiopathogenic and therapeutic aspects of membranous nephropathy in a didactic manner, including cases that occur during kidney transplantation.

The classical pathway triggers pathogenic complement activation in membranous nephropathy

Membranous nephropathy (MN) is an antibody-mediated autoimmune disease characterized by glomerular immune complexes containing complement components. However, both the initiation pathways and the pathogenic significance of complement activation in MN are poorly understood. Here, we show that components from all three complement pathways (alternative, classical and lectin) are found in renal biopsies from patients with MN. Proximity ligation assays to directly visualize complement assembly in the tissue reveal dominant activation via the classical pathway, with a close correlation to the degree of glomerular C1q-binding IgG subclasses. In an antigen-specific autoimmune mouse model of MN, glomerular damage and proteinuria are reduced in complement-deficient mice compared with wild-type littermates. Severe disease with progressive ascites, accompanied by extensive loss of the integral podocyte slit diaphragm proteins, nephrin and neph1, only occur in wild-type animals. Finally, targeted silencing of C3 using RNA interference after the onset of proteinuria significantly attenuates disease. Our study shows that, in MN, complement is primarily activated via the classical pathway and targeting complement components such as C3 may represent a promising therapeutic strategy.

Lectin Complement Pathway Activation is Associated with Massive Proteinuria in PLA2R-Positive Membranous Nephropathy: A Retrospective Study

Introduction

Complement activation is highly involved in membranous nephropathy. Identifying the mechanism of the complement activation pathway carries crucial therapeutic implications yet remains controversial. This study explored lectin complement pathway activation in PLA2R-associated membranous nephropathy (MN).

Methods

One hundred and seventy-six patients with biopsy-proven PLA2R-associated MN were enrolled in the retrospective study and divided into the remission group (24-hour urine protein <0.75g and serum albumin >35 g/L) and the nephrotic syndrome group. The clinical manifestation and C3, C4d, C1q, MBL, and B factor in renal biopsy tissues and C3, C4, and immunoglobulins in serum were evaluated.

Results

Deposition of glomerular C3, C4d, and mannose-binding lectin (MBL) was significantly higher in the activated state than in the remission state in PLA2R-associated MN. MBL deposition was the risk factor for no remission. During follow-up, the persistent non-remission patients have significantly lower serum C3 levels.

Conclusion

Activation of the lectin complement pathway in PLA2R-associated MN may contribute to proteinuria progression and disease activity.

Factors affecting IgG4-mediated complement activation

Of the four human immunoglobulin G (IgG) subclasses, IgG4 is considered the least inflammatory, in part because it poorly activates the complement system. Regardless, in IgG4 related disease (IgG4-RD) and in autoimmune disorders with high levels of IgG4 autoantibodies, the presence of these antibodies has been linked to consumption and deposition of complement components. This apparent paradox suggests that conditions may exist, potentially reminiscent of in vivo deposits, that allow for complement activation by IgG4. Furthermore, it is currently unclear how variable glycosylation and Fab arm exchange may influence the ability of IgG4 to activate complement. Here, we used well-defined, glyco-engineered monoclonal preparations of IgG4 and determined their ability to activate complement in a controlled system. We show that IgG4 can activate complement only at high antigen and antibody concentrations, via the classical pathway. Moreover, elevated or reduced Fc galactosylation enhanced or diminished complement activation, respectively, with no apparent contribution from the lectin pathway. Fab glycans slightly reduced complement activation. Lastly, we show that bispecific, monovalent IgG4 resulting from Fab arm exchange is a less potent activator of complement than monospecific IgG4. Taken together, these results imply that involvement of IgG4-mediated complement activation in pathology is possible but unlikely.

Kidney diseases

Dysregulation and accelerated activation of the alternative pathway (AP) of complement is known to cause or accentuate several pathologic conditions in which kidney injury leads to the appearance of hematuria and proteinuria and ultimately to the development of chronic renal failure. Multiple genetic and acquired defects involving plasma- and membrane-associated proteins are probably necessary to impair the protection of host tissues and to confer a significant predisposition to AP-mediated kidney diseases. This review aims to explore how our current understanding will make it possible to identify the mechanisms that underlie AP-mediated kidney diseases and to discuss the available clinical evidence that supports complement-directed therapies. Although the value of limiting uncontrolled complement activation has long been recognized, incorporating complement-targeted treatments into clinical use has proved challenging. Availability of anti-complement therapy has dramatically transformed the outcome of atypical hemolytic uremic syndrome, one of the most severe kidney diseases. Innovative drugs that directly counteract AP dysregulation have also opened new perspectives for the management of other kidney diseases in which complement activation is involved. However, gained experience indicates that the choice of drug should be tailored to each patient's characteristics, including clinical, histologic, genetic, and biochemical parameters. Successfully treating patients requires further research in the field and close collaboration between clinicians and researchers who have special expertise in the complement system.

Unraveling complexity of antibody-mediated rejections, the mandatory way towards an accurate diagnosis and a personalized treatment

Antibody-mediated rejection (ABMR) remains one of the most challenging issues after organ transplantation and particularly after kidney transplantation. Despite many progresses during the last decade, ABMR is still the main cause of kidney graft loss and this all over the post- transplant period. In this review, we describe the recent knowledge about molecular and cellular mechanisms involved in ABMR. We focused our report on the role of the complement pathway in the process of ABMR and we give some insights into the role of inflammatory cells, NK lymphocytes and the role of endothelial cells. We further describe the potential role of non-HLA antibodies, of which the importance has been increasingly emphasized in recent years. Overall, this report could be of interest for all physicians who are working in the field of organ transplantation or who are working in the field of immunology. It gives essential information to understand new diagnosis advances and further therapeutic approaches. Antibody-mediated rejection (ABMR) is the leading cause of graft failure ([1,2]). In contrast to T-cell mediated rejection usually sensitive to steroids, active ABMR remains a therapeutic challenge. ABMR diagnosis relies on the presence of renal injuries and donor-specific antibodies (DSA) (HLA and non HLA antibodies) with sometimes the evidence of interaction between DSA and graft endothelium. Regularly revised during expert conferences, ABMR definition is currently categorized as active or chronic active. [3] The emergence of validated molecular assays targeting a better phenotyping of ABMR and the recent advances regarding the detrimental effect of DSA directed against minor antigens open the way to a better assessment of the heterogeneity of ABMR. In this review, we will address new aspects of ABMR regarding its mechanisms, diagnosis and treatments.

Membranous nephropathy: Clearer pathology and mechanisms identify potential strategies for treatment

Primary membranous nephropathy (PMN) is one of the common causes of adult-onset nephrotic syndrome and is characterized by autoantibodies against podocyte antigens causing in situ immune complex deposition. Much of our understanding of the disease mechanisms underpinning this kidney-limited autoimmune disease originally came from studies of Heymann nephritis, a rat model of PMN, where autoantibodies against megalin produced a similar disease phenotype though megalin is not implicated in human disease. In PMN, the major target antigen was identified to be M-type phospholipase A2 receptor 1 (PLA2R) in 2009. Further utilization of mass spectrometry on immunoprecipitated glomerular extracts and laser micro dissected glomeruli has allowed the rapid discovery of other antigens (thrombospondin type-1 domain-containing protein 7A, neural epidermal growth factor-like 1 protein, semaphorin 3B, protocadherin 7, high temperature requirement A serine peptidase 1, netrin G1) targeted by autoantibodies in PMN. Despite these major advances in our understanding of the pathophysiology of PMN, treatments remain non-specific, often ineffective, or toxic. In this review, we summarize our current understanding of the immune mechanisms driving PMN from animal models and clinical studies, and the implications on the development of future targeted therapeutic strategies.

The role of the complement system in primary membranous nephropathy: A narrative review in the era of new therapeutic targets

Primary membranous nephropathy (MN) is an important cause of nephrotic syndrome and chronic kidney disease (CKD) in the adult population. Although the discovery of different autoantibodies against glomerular/podocytic antigens have highlighted the role of B cells in the pathogenesis of MN, suboptimal response or even resistance to B cell-directed therapies occurs, suggesting that other pathophysiological mechanisms are involved in mediating podocyte injury. The complement system plays an important role in the innate immune response to infection, and dysregulation of the complement system has been observed in various kidney diseases. There is compelling evidence of complement cascade activation in primary MN, with the mannose-binding lectin (MBL) and alternative pathways particularly implicated. With appropriate validation, assays of complements and associated activation products could hold promise as adjunctive tools for non-invasive disease monitoring and prognostication. While there is growing interest to target the complement system in MN, there is concern regarding the risk of infection due to encapsulated organisms and high treatment costs, highlighting the need for clinical trials to identify patients most likely to benefit from complement-directed therapies.

Membranous nephropathy: Systems biology-based novel mechanism and traditional Chinese medicine therapy

Membranous nephropathy (MN) is a renal-limited non-inflammatory autoimmune disease in the glomerulus, which is the second or third main cause of end-stage kidney diseases in patients with primary glomerulonephritis. Substantial achievements have increased our understanding of the aetiology and pathogenesis of murine and human MN. The identification of nephritogenic autoantibodies against neutral endopeptidase, phospholipase A₂ receptor (PLA₂R) and thrombospondin type-1 domain-containing 7A (THSD7A) antigens provide more specific concept-driven intervention strategies for treatments by specific B cell-targeting monoclonal antibodies to inhibit antibody production and antibody-antigen immune complex deposition. Furthermore, additional antibody specificities for antigens have been discovered, but their pathogenic effects are uncertain. Although anti-PLA₂R and anti-THSD7A antibodies as a diagnostic marker is widely used in MN patients, many questions including autoimmune response development, antigenic epitopes, and podocyte damage signalling pathways remain unresolved. This review describes the current available evidence regarding both established and novel molecular mechanisms based on systems biology approaches (gut microbiota, long non-coding RNAs, metabolite biomarkers and DNA methylation) in MN, with an emphasis on clinical findings. This review further summarizes the applications of traditional Chinese medicines such as Tripterygium wilfordii and Astragalus membranaceus for MN treatment. Lastly, this review considers how the identification of novel antibodies/antigens and unresolved questions and future challenges reveal the pathogenesis of MN.

Identification of molecular mechanism and key biomarkers in membranous nephropathy by bioinformatics analysis

OBJECTIVES

Membranous nephropathy (MN) is an autoimmune nephropathy. The incidence of MN is increasing gradually in recent years. Previous studies focused on antibody production, complement activation and podocyte injury in MN. However, the etiology and underlying mechanism of MN remain to be further studied.

METHODS

GSE104948 and GSE108109 of glomerular expression profile were downloaded from Gene Expression Omnibus (GEO) database, GSE47184, GSE99325, GSE104954, GSE108112, GSE133288 of renal tubule expression profile, and GSE73953 of peripheral blood mononuclear cells (PBMCs) expression profile. After data integration by Networkanalyst, differentially expressed genes (DEGs) between MN and healthy samples were obtained. DEGs were enriched in gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG), and protein-protein interaction (PPI) networks of these genes were constructed through Metascape, etc. We further understood the function of hub genes through gene set enrichment analysis (GSEA). The diagnostic value of DEGs in MN was evaluated by receiver operating characteristic (ROC) analysis.

RESULTS

A total of 3 genes (TP53, HDAC5, and SLC2A3) were screened out. Among them, the up-regulated TP53 expression may be closely related to MN renal pathological changes. However, the expression of MN podocyte target antigen was not significantly different from that of healthy controls. In addition, the changes of Wnt signaling pathway in PBMCs and the effects of SLC2A3 on the differentiation of M2 monocyte need further study.

CONCLUSION

It is difficult to unify a specific mechanism for the changes of glomerulus, renal tubules and PBMCs in MN patients. This may be related to the pathogenesis, pathology and immune characteristics of MN. MN podocyte target antigen may not be the root cause of the disease, but a stage result in the pathogenesis process.

Modern view on the complement system role in membranous nephropathy

Membranous nephropathy (MN), an immune-mediated glomerular disease, is the most common cause of adult nephrotic syndrome. In MN, proteinuria is developed by podocyte damage due to the complement system activation in response to the subepithelial deposition of immune complexes containing various auto- and exogenous antigens. Membrane-attacking complex (MAC) is the terminal product of any complement pathways activation (classical, lectin or alternative) and plays the leading role in the complement-mediated podocytic damage. Thus far, the main pathway of complement activation leading to the formation of MAC in MN has not been established. The review highlights current evidence of various complement pathways activation in the development of MN, as well as recently established new molecular mechanisms of complement-mediated podocyte damage.

The Alternative Pathway Is Necessary and Sufficient for Complement Activation by Anti-THSD7A Autoantibodies, Which Are Predominantly IgG4 in Membranous Nephropathy

Membranous nephropathy (MN) is an immune kidney disease characterized by glomerular subepithelial immune complexes (ICs) containing antigen, IgG, and products of complement activation. Whereas proteinuria is caused by complement-mediated podocyte injury, the pathways of complement activation remain controversial due to the predominance of IgG4 in ICs, an IgG subclass considered unable to activate complement. THSD7A, a transmembrane protein expressed on podocytes, is the target autoantigen in ~3% of cases of primary MN. In this study, we analyzed sera from 16 patients with THSD7A-associated MN with regard to the anti-THSD7A IgG subclasses and their ability to fix complement in vitro. The serum concentration of anti-THSD7A IgG varied over two orders of magnitude (1.3-243 μg/mL). As a relative proportion of all IgG anti-THSD7A, IgG4 was by far the most abundant subclass (median 79%), followed by IgG1 (median 11%). IgG4 was the dominant subclass of anti-THSD7A antibodies in 14 sera, while IgG1 was dominant in one and co-dominant in another. One quarter of MN sera additionally contained low levels of anti-THSD7A IgA1. ICs formed by predominantly IgG4 anti-THSD7A autoantibodies with immobilized THSD7A were relatively weak activators of complement in vitro, compared to human IgG1 and IgG3 mAbs used as positive control. Complement deposition on THSD7A ICs was dose-dependent and occurred to a significant extent only at relatively high concentration of anti-THSD7A IgG. C3b fixation by THSD7A ICs was completely abolished in factor B-depleted sera, partially inhibited in C4-depleted sera, unchanged in C1q-depleted sera, and also occurred in Mg-EGTA buffer. These results imply that THSD7A ICs predominantly containing IgG4 activate complement at high IgG4 density, which strictly requires a functional alternative pathway, whereas the classical and lectin pathways are dispensable. These findings advance our understanding of how IgG4 antibodies activate complement.

Recent Advances in Clinical Diagnosis and Pharmacotherapy Options of Membranous Nephropathy

Membranous nephropathy (MN) is the most common cause of nephrotic syndrome among adults, which is the leading glomerular disease that recurs after kidney transplantation. Treatment for MN remained controversial and challenging, partly owing to absence of sensitive and specific biomarkers and effective therapy for prediction and diagnosis of disease activity. MN starts with the formation and deposition of circulating immune complexes on the outer area in the glomerular basement membrane, leading to complement activation. The identification of autoantibodies against the phospholipase A₂ receptor (PLA₂R) and thrombospondin type-1 domain-containing protein 7A (THSD7A) antigens illuminated a distinct pathophysiological rationale for MN treatments. Nowadays, detection of serum anti-PLA₂R antibodies and deposited glomerular PLA₂R antigen can be routinely applied to MN. Anti-PLA₂R antibodies exhibited much high specificity and sensitivity. Measurement of PLA₂R in immune complex deposition allows for the diagnosis of PLA₂R-associated MN in patients with renal biopsies. In the review, we critically summarized newer diagnosis biomarkers including PLA₂R and THSD7A tests and novel promising therapies by using traditional Chinese medicines such as Astragalus membranaceus, Tripterygium wilfordii, and Astragaloside IV for the treatment of MN patients. We also described unresolved questions and future challenges to reveal the diagnosis and treatments of MN. These unprecedented breakthroughs were quickly translated to clinical diagnosis and management. Considerable advances of detection methods played a critical role in diagnosis and monitoring of treatment.

Definition of IgG Subclass-Specific Glycopatterns in Idiopathic Membranous Nephropathy: Aberrant IgG Glycoforms in Blood

The podocyte injury, and consequent proteinuria, that characterize the pathology of idiopathic membranous nephropathy (IMN) is mediated by an autoimmune reaction against podocyte antigens. In particular, the activation of pathways leading to abundant renal deposits of complement is likely to involve the binding of mannose-binding lectin (MBL) to aberrant glycans on immunoglobulins. To obtain a landscape of circulatory IgG Fc glycosylation characterizing this disease, we conducted a systematic N-glycan profiling study of IgG1, 2, and 4 by mass spectrometry. The cohort included 57 IMN patients, a pathological control group with nephrotic syndrome (PN) (n = 20), and 88 healthy control subjects. The effect of sex and age was assessed in all groups and controlled by rigorous matching. Several IgG Fc glycan traits were found to be associated with IMN. Interestingly, among them, only IgG4-related results were specific for IMN and not for PN. Hypo-galactosylation of IgG4, already shown for IMN, was observed to occur in the absence of core fucose, in line with a probable increase of pro-inflammatory IgG. In addition, elevated levels of fucosylated IgG4, along with low levels of hybrid-type glycans, were detected. Some of these IgG4 alterations are likely to be more pronounced in high PLA2R (phospholipase A2 receptor) patients. IgG Fc glycosylation patterns associated with IMN warrant further studies of their role in disease mechanisms and may eventually enrich the diagnostic spectrum regarding patient stratification.

Association of serum mannose-binding lectin, anti-phospholipase A2 receptor antibody and renal outcomes in idiopathic membranous nephropathy and atypical membranous nephropathy: a single center retrospective cohort study

Objectives

Idiopathic membranous nephropathy (iMN) is a major cause of nephrotic syndrome. Atypical membranous nephropathy (aMN) is a new type of nephropathy in China, characterized by a ‘full-house’ on immunofluorescent examination, that is IgG, IgA, IgM, C3, C1q positive, but without clinical evidence of a secondary cause. Phospholipase A2 receptor (PLA2R) was the major target antigens in iMN patients. Activation of the mannose-binding lectin (MBL) pathway plays a vital role in the development of MN. Our objective was to investigate the role of PLA2R and MBL in the pathogenesis of iMN and aMN.

Methods

We conducted a retrospective observational study using propensity score matching by age, gender, and eGFR. All clinical, laboratory data, and follow-up data of the patients were collected. Serum levels of anti-PLA2R antibodies and MBL were tested.

Results

Finally, 30 iMN patients and 30 aMN patients were included, and 20 healthy controls were retrospectively collected in this study. The 24 h proteinuria level was higher and serum albumin was lower in anti-PLA2R (+) patients than in anti-PLA2R (−) patients in both iMN and aMN groups. In aMN patients, MBL levels were significantly higher in anti-PLA2R (+) patients than in anti-PLA2R (−) patients (p = .045). The serum level of anti-PLA2R positively correlated with no-remission in both iMN and aMN groups.

Conclusions

The complement lectin pathway has an association with the development of MN, especially in patients with positive anti-PLA2R antibodies. Serum MBL cannot differentiate between the two diseases. Serum MBL levels are not associated with clinical manifestations, nor with prognosis.

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.

The rational use of complement inhibitors in kidney diseases

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

Exploring the Differences in Molecular Mechanisms and Key Biomarkers Between Membranous Nephropathy and Lupus Nephritis Using Integrated Bioinformatics Analysis

Background: Both membranous nephropathy (MN) and lupus nephritis (LN) are autoimmune kidney disease. In recent years, with the deepening of research, some similarities have been found in the pathogenesis of these two diseases. However, the mechanism of their interrelationship is not clear. The purpose of this study was to investigate the differences in molecular mechanisms and key biomarkers between MN and LN. Method: The expression profiles of GSE99325, GSE99339, GSE104948 and GSE104954 were downloaded from GEO database, and the differentially expressed genes (DEGs) of MN and LN samples were obtained. We used Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for enrichment analysis of DEGs. A protein-protein interaction (PPI) network of DEGs was constructed using Metascape. We filtered DEGs with NetworkAnalyst. Finally, we used receiver operating characteristic (ROC) analysis to identify the most significant DEGs for MN and LN. Result: Compared with LN in the glomerulus, 14 DEGs were up-regulated and 77 DEGs were down-regulated in MN. Compared with LN in renal tubules, 21 DEGs were down-regulated, but no up-regulated genes were found in MN. According to the result of GO and KEGG enrichment, PPI network and Networkanalyst, we screened out six genes (IFI6, MX1, XAF1, HERC6, IFI44L, IFI44). Interestingly, among PLA2R, THSD7A and NELL1, which are the target antigens of podocyte in MN, the expression level of NELL1 in MN glomerulus is significantly higher than that of LN, while there is no significant difference in the expression level of PLA2R and THSD7A. Conclusion: Our study provides new insights into the pathogenesis of MN and LN by analyzing the differences in gene expression levels between MN and LN kidney samples, and is expected to be used to prepare an animal model of MN that is more similar to human.

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

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

How Times Have Changed! A Cornucopia of Antigens for Membranous Nephropathy

The identification of the major target antigen phospholipase A2 receptor (PLA2R) in the majority of primary (idiopathic) cases of membranous nephropathy (MN) has been followed by the rapid identification of numerous minor antigens that appear to define phenotypically distinct forms of disease. This article serves to review all the known antigens that have been shown to localize to subepithelial deposits in MN, as well as the distinctive characteristics associated with each subtype of MN. We will also shed light on the novel proteomic approaches that have allowed identification of the most recent antigens. The paradigm of an antigen normally expressed on the podocyte cell surface leading to in-situ immune complex formation, complement activation, and subsequent podocyte injury will be discussed and challenged in light of the current repertoire of multiple MN antigens. Since disease phenotypes associated with each individual target antigens can often blur the distinction between primary and secondary disease, we encourage the use of antigen-based classification of membranous nephropathy.

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.

Membranous nephropathy

Membranous nephropathy (MN) is a glomerular disease that can occur at all ages. In adults, it is the most frequent cause of nephrotic syndrome. In ~80% of patients, there is no underlying cause of MN (primary MN) and the remaining cases are associated with medications or other diseases such as systemic lupus erythematosus, hepatitis virus infection or malignancies. MN is an autoimmune disease characterized by a thickening of the glomerular capillary walls due to immune complex deposition. Identification of the phospholipase A2 receptor (PLA2R) as the major antigen in adults in 2009 induced a paradigm shift in disease diagnosis and monitoring and several other antigens have since been characterized. Disease outcome is difficult to predict and around one-third of patients will undergo spontaneous remission. In those at high risk of progression, immunosuppressive therapy with cyclophosphamide plus corticosteroids has substantially reduced the need for kidney replacement therapy. Owing to carcinogenic risk, other treatments (calcineurin inhibitors and CD20-targeted B cell depletion therapy (rituximab)) have been developed. However, disease relapses are frequent when calcineurin inhibitors are stopped and the remission rate with rituximab is lower than with cyclophosphamide, particularly in patients with high PLA2R antibody titres. Other new drugs are already available and antigen-specific immunotherapies are being developed.

Perspectives in membranous nephropathy

The identification of the phospholipase A2 receptor 1 (PLA2R) and thrombospondin type-1 domain-containing protein 7A (THSD7A) as podocyte antigens in adult patients with membranous nephropathy (MN) has strongly impacted both experimental and clinical research on this disease. Evidence has been furnished that podocyte-directed autoantibodies can cause MN, and novel PLA2R- and THSD7A-specific animal models have been developed. Today, measurement of serum autoantibody levels and staining of kidney biopsies for the target antigens guides MN diagnosis and treatment worldwide. Additionally, anti-PLA2R antibodies have been proven to be valuable prognostic biomarkers in MN. Despite these impressive advances, a variety of questions regarding the disease pathomechanisms, clinical use of antibody measurement, and future treatments remain unanswered. In this review, we will outline recent advances made in the field of MN and discuss open questions and perspectives with a focus on novel antigen identification, mechanisms of podocyte injury, clinical use of antibody measurement to guide diagnosis and treatment, and the potential of innovative, pathogenesis-based treatment strategies.

Primary membranous nephropathy in the era of autoantibodies and biological therapies

Primary membranous nephropathy is an autoimmune kidney disease and the most common cause of nephrotic syndrome in adults. About 70%-80% of cases are caused by anti-PLA2R antibodies. Its association with anti-THSD7A antibodies and other autoantibodies has also been described. Recent pilot studies and clinical trials have shown that several biological agents targeting autoantibody-producing cells are effective in controlling the disease with an acceptable safety profile. In this narrative review, we update key concepts about the pathogenesis, autoantibody-based diagnosis, and kidney biopsy findings in primary membranous nephropathy. In addition, we propose a diagnostic and therapeutic algorithm, including guidance on monitoring the response to therapy. We compare the efficacy and safety of currently available treatments, including rituximab and new biological agents, and identify unmet clinical needs.

Mechanisms of Primary Membranous Nephropathy

Membranous nephropathy (MN) is an autoimmune disease of the kidney glomerulus and one of the leading causes of nephrotic syndrome. The disease exhibits heterogenous outcomes with approximately 30% of cases progressing to end-stage renal disease. The clinical management of MN has steadily advanced owing to the identification of autoantibodies to the phospholipase A2 receptor (PLA2R) in 2009 and thrombospondin domain-containing 7A (THSD7A) in 2014 on the podocyte surface. Approximately 50–80% and 3–5% of primary MN (PMN) cases are associated with either anti-PLA2R or anti-THSD7A antibodies, respectively. The presence of these autoantibodies is used for MN diagnosis; antibody levels correlate with disease severity and possess significant biomarker values in monitoring disease progression and treatment response. Importantly, both autoantibodies are causative to MN. Additionally, evidence is emerging that NELL-1 is associated with 5–10% of PMN cases that are PLA2R- and THSD7A-negative, which moves us one step closer to mapping out the full spectrum of PMN antigens. Recent developments suggest exostosin 1 (EXT1), EXT2, NELL-1, and contactin 1 (CNTN1) are associated with MN. Genetic factors and other mechanisms are in place to regulate these factors and may contribute to MN pathogenesis. This review will discuss recent developments over the past 5 years.

Altered glycosylation of IgG4 promotes lectin complement pathway activation in anti-PLA2R1-associated membranous nephropathy

Primary membranous nephropathy (pMN) is a leading cause of nephrotic syndrome in adults. In most cases, this autoimmune kidney disease is associated with autoantibodies against the M-type phospholipase A2 receptor (PLA2R1) expressed on kidney podocytes, but the mechanisms leading to glomerular damage remain elusive. Here, we developed a cell culture model using human podocytes and found that anti-PLA2R1-positive pMN patient sera or isolated IgG4, but not IgG4-depleted sera, induced proteolysis of the 2 essential podocyte proteins synaptopodin and NEPH1 in the presence of complement, resulting in perturbations of the podocyte cytoskeleton. Specific blockade of the lectin pathway prevented degradation of synaptopodin and NEPH1. Anti-PLA2R1 IgG4 directly bound mannose-binding lectin in a glycosylation-dependent manner. In a cohort of pMN patients, we identified increased levels of galactose-deficient IgG4, which correlated with anti-PLA2R1 titers and podocyte damage induced by patient sera. Assembly of the terminal C5b-9 complement complex and activation of the complement receptors C3aR1 or C5aR1 were required to induce proteolysis of synaptopodin and NEPH1 by 2 distinct proteolytic pathways mediated by cysteine and aspartic proteinases, respectively. Together, these results demonstrated a mechanism by which aberrantly glycosylated IgG4 activated the lectin pathway and induced podocyte injury in primary membranous nephropathy.

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.

A Review of the Current Practice of Diagnosis and Treatment of Idiopathic Membranous Nephropathy in China

Idiopathic membranous nephropathy (IMN), a common pathological type of nephrotic syndrome, is one of the main causes of kidney failure. With an increasing prevalence, IMN has received considerable attention in China. Based on recent studies, we discuss advances in the diagnosis of IMN and the understanding of its genetic background. Although the pathogenesis of IMN remains unclear, our understanding has been substantially enhanced by the discovery of new antigens such as phospholipase A₂ receptor, thrombospondin type-1 domain-containing 7A, exostosin1/exostosin2, neural epidermal growth factor-like 1 protein, neural cell adhesion molecule 1, semaphorin 3B, and factor H autoantibody. However, due to ethnic, environmental, economic, and lifestyle differences and other factors, a consensus has not yet been reached regarding IMN treatment. In view of the differences between Eastern and Western populations, in-depth clinical evaluations of biomarkers for IMN diagnosis are necessary. This review details the current treatment strategies for IMN in China, including renin-angiotensin system inhibitors, corticosteroid monotherapy, cyclophosphamide, calcineurin inhibitors, mycophenolate mofetil, adrenocorticotropic hormone, and traditional Chinese medicine, as well as biological preparations such as rituximab. In terms of management, the 2012 Kidney Disease Improving Global Outcomes (KDIGO) clinical practice guidelines do not fully consider the characteristics of the Chinese population. Therefore, this review aims to present the current status of IMN diagnosis and treatment in Chinese patients, and includes a discussion of new approaches and remaining clinical challenges.

Common Denominators in the Immunobiology of IgG4 Autoimmune Diseases: What Do Glomerulonephritis, Pemphigus Vulgaris, Myasthenia Gravis, Thrombotic Thrombocytopenic Purpura and Autoimmune Encephalitis Have in Common?

IgG4 autoimmune diseases (IgG4-AID) are an emerging group of autoimmune diseases that are caused by pathogenic autoantibodies of the IgG4 subclass. It has only recently been appreciated, that members of this group share relevant immunobiological and therapeutic aspects even though different antigens, tissues and organs are affected: glomerulonephritis (kidney), pemphigus vulgaris (skin), thrombotic thrombocytopenic purpura (hematologic system) muscle-specific kinase (MuSK) in myasthenia gravis (peripheral nervous system) and autoimmune encephalitis (central nervous system) to give some examples. In all these diseases, patients’ IgG4 subclass autoantibodies block protein-protein interactions instead of causing complement mediated tissue injury, patients respond favorably to rituximab and share a genetic predisposition: at least five HLA class II genes have been reported in individual studies to be associated with several different IgG4-AID. This suggests a role for the HLA class II region and specifically the DRβ1 chain for aberrant priming of autoreactive T-cells toward a chronic immune response skewed toward the production of IgG4 subclass autoantibodies. The aim of this review is to provide an update on findings arguing for a common pathogenic mechanism in IgG4-AID in general and to provide hypotheses about the role of distinct HLA haplotypes, T-cells and cytokines in IgG4-AID.

The Potential Role of Regulatory B Cells in Idiopathic Membranous Nephropathy

Regulatory B cells (Breg) are widely regarded as immunomodulatory cells which play an immunosuppressive role. Breg inhibits pathological autoimmune response by secreting interleukin-10 (IL-10), transforming growth factor-β (TGF-β), and adenosine and through other ways to prevent T cells and other immune cells from expanding. Recent studies have shown that different inflammatory environments induce different types of Breg cells, and these different Breg cells have different functions. For example, Br1 cells can secrete IgG4 to block autoantigens. Idiopathic membranous nephropathy (IMN) is an autoimmune disease in which the humoral immune response is dominant and the cellular immune response is impaired. However, only a handful of studies have been done on the role of Bregs in this regard. In this review, we provide a brief overview of the types and functions of Breg found in human body, as well as the abnormal pathological and immunological phenomena in IMN, and propose the hypothesis that Breg is activated in IMN patients and the proportion of Br1 can be increased. Our review aims at highlighting the correlation between Breg and IMN and proposes potential mechanisms, which can provide a new direction for the discovery of the pathogenesis of IMN, thus providing a new strategy for the prevention and early treatment of IMN.

Is primary membranous nephropathy a complement mediated disease?

Membranous nephropathy (MN) is an immune complex mediated disease. Although limited to the kidney, in up to 20% of patients MN is associated with other autoimmune, infectious or malignant diseases. The initial pathogenetic event in what is still considered "primary" MN is the binding of circulating autoantibodies to proteins (autoantigens) expressed in glomerular podocytes. This antibody binding leads to the formation of immune complexes in the glomerular basement membrane. There is clinical and experimental evidence that these immune deposits lead to the activation of the complement system. Experimental studies in the MN model of Heymann's nephritis show that the terminal membrane attack complex (MAC) of the complement system induces a disturbance of the glomerular filtration barrier and leads to proteinuria, the clinical hallmark of MN. After the discovery of the phospholipase A₂ receptor 1 and thrombospondin type 1 domain containing protein 7A as endogenous antigens, it is assumed that IgG4 antibodies directed against these proteins induce MN in over 85% of patients with primary MN. As a result, the role of complement in the pathogenesis of MN needs to be defined in light of these developments. In this review we describe the current knowledge on the function of the complement system in primary MN and discuss the open questions, which have to be solved for a better understanding of the potential role of complement in the pathophysiology of primary MN.

Update on IgG4-mediated autoimmune diseases: New insights and new family members

Antibodies of IgG4 subclass are exceptional players of the immune system, as they are considered to be immunologically inert and functionally monovalent, and as such may be part of classical tolerance mechanisms. IgG4 antibodies are found in a range of different diseases, including IgG4-related diseases, allergy, cancer, rheumatoid arthritis, helminth infection and IgG4 autoimmune diseases, where they may be pathogenic or protective. IgG4 autoimmune diseases are an emerging new group of diseases that are characterized by pathogenic, antigen-specific autoantibodies of IgG4 subclass, such as MuSK myasthenia gravis, pemphigus vulgaris and thrombotic thrombocytopenic purpura. The list of IgG4 autoantigens is rapidly growing and to date contains 29 candidate antigens. Interestingly, IgG4 autoimmune diseases are restricted to four distinct organs: 1) the central and peripheral nervous system, 2) the kidney, 3) the skin and mucous membranes and 4) the vascular system and soluble antigens in the blood circulation. The pathogenicity of IgG4 can be validated using our classification system, and is usually excerted by functional blocking of protein-protein interaction.

Long-term renal survival and related risk factors for primary membranous nephropathy in Chinese children: a retrospective analysis of 217 cases

BACKGROUND

Primary membranous nephropathy (PMN) is a rare pathological finding in paediatric patients. Data on PMN in children have been restricted to studies with small samples and fairly short follow-up periods. Therefore, we conducted this single-centre study to evaluate the long-term renal survival and related risk factors for PMN in children, and the clinical and histological characteristics were also described.

METHOD

Two hundred and seventeen children with PMN were enrolled from July 2008 to September 2017. Patients with follow-up durations < 12 months were excluded, except for patients who progressed to end-stage kidney disease (ESKD) or experienced a related death within 12 months. Long-term renal survival and related risk factors were analysed.

RESULT

The sex ratio was 1.33:1 (male vs female), and the median age was 15.0 (14.0-17.0) years old. The most prominent clinical manifestation was nephrotic syndrome (130 59.9%), which was accompanied by various degrees of oedema (142 65.4%), hyperlipidaemia (151 69.6%), hypoalbuminemia (130 59.9%), and nephrotic proteinuria (135 62.2%). Hypertension occurred in 36.4% of children with PMN. After a median follow-up of 45.0 (23.5-74.0) months, 11 patients (5.1%) developed ESKD, and the cumulative kidney survival rates of ESKD at 5 and 10 years after renal biopsy were 95.3% and 67.8%, respectively. The cumulative kidney survival rates of the combined event of ESKD and/or 30% decline in estimated glomerular filtration rate (eGFR) at 5 and 10 years after renal biopsy were 92.6% and 59.5%, respectively. Cox multivariate regression and Kaplan-Meier analysis demonstrated that hypertension and proteinuria ≥ 50 mg/kg/day were associated with renal outcome.

CONCLUSION

In this study, the 5-year and 10-year cumulative renal survival rates of ESKD in children with PMN were reported for the first time as 95.3% and 67.8%, respectively. In addition, this is the first report to find that hypertension and proteinuria  ≥ 50 mg/kg/day are associated with renal outcome in children with PMN.

Advances in Pathogenesis of Idiopathic Membranous Nephropathy

Background

Membranous nephropathy (MN), a major cause of nephrotic syndrome, has attracted people's attention in recent years for its growing prevalence. It is the second or third leading cause of ESRD in patients with primary glomerulonephritis and is the leading glomerulopathy that recurs after kidney transplantation.

Summary

MN can be classified as idiopathic membranous nephropathy (IMN) and secondary MN. The discovery of the M-type phospholipase A2 receptor (PLA2R) and thrombospondin type-1 domain-containing 7A (THSD7A) provides the new diagnostic methods and treatment strategies for IMN on the molecular level. The study on single nucleotide polymorphism of IMN genes, such as the single M-type phospholipase A2 receptor 1 (PLA2R1) gene and human leukocyte antigen (HLA) gene, explains the pathogenesis of the disease from the perspective of genetics and conforms to the trend of the era of precision medicine.

Key Messages

This review focuses on advances in the pathogenesis of IMN, including molecular and genetic pathogenesis, as well as discussing the diagnostic and treatment guiding value brought by these new discoveries.

Role of properdin in complement-mediated kidney diseases

As part of the innate immune system, the complement system is an important mechanism in our first line of defence, but it can also contribute to the onset of various diseases. In renal diseases, the dysregulation of the complement system is often caused by mutations in-and autoantibodies directed against-members of the complement system, and contributes to disease onset and severity. As the only known positive regulator of the complement system, the role of properdin in complement-mediated diseases is largely unknown. In this review, we provide an overview of the detection of properdin in kidney biopsies and urine, serum or plasma samples from patients with complement-mediated renal diseases, such as immune complex-mediated glomerulonephritis and C3 glomerulopathy. Advances towards a better understanding of the role of properdin in (local) complement activation will provide insight into its potential role and offer opportunities to improve diagnosis and therapeutic interventions.

Advances of the experimental models of idiopathic membranous nephropathy (Review)

Idiopathic membranous nephropathy (IMN) is one of the main types of chronic kidney disease in adults and one of the most common causes of end-stage renal disease. In recent years, the morbidity of IMN among primary glomerular diseases has markedly increased, while the pathogenesis of the disease remains unclear. To address this, a number of experimental models, including Heymann nephritis, anti-thrombospondin type-1 domain-containing 7A antibody-induced IMN, cationic bovine serum albumin, anti-human podocyte antibodies and zymosan-activated serum-induced C5b-9, have been established. This review comprehensively summarized the available animal and cell models for IMN. The limitations and advantages of the current models were discussed and two improved models were introduced to facilitate the selection of an appropriate model for further studies on IMN.

Anti-PLA2R1 Antibodies Containing Sera Induce In Vitro Cytotoxicity Mediated by Complement Activation

The phospholipase A2 receptor (PLA2R1) is the major autoantigen in idiopathic membranous nephropathy (MN). However, the pathogenic role of anti-PLA2R1 autoantibodies is unclear. Our aim was to evaluate the in vitro cytotoxicity of anti-PLA2R1 antibodies mediated by complement. Forty-eight patients with PLA2R1-related MN from the prospective cohort SOURIS were included. Anti-PLA2R1 titer, epitope profile, and anti-PLA2R1 IgG subclasses were characterized by ELISA. Cell cytotoxicity was evaluated by immunofluorescence in HEK293 cells overexpressing PLA2R1 incubated with patient or healthy donor sera in the presence or absence of rabbit complement or complement inhibitors. Mean cytotoxicity of anti-PLA2R1 sera for HEK293 cells overexpressing PLA2R1 was 2 ± 2%, which increased to 24 ± 6% after addition of rabbit complement (p < 0.001) (n = 48). GVB-EDTA, which inhibits all complement activation pathways, completely blocked cell cytotoxicity, whereas Mg-EGTA, which only inhibits the classical and lectin pathways, highly decreased suggesting a limited role of the alternative pathway. A higher diversity of IgG subclasses beyond IgG4 and high titer of total IgG anti-PLA2R1 were associated with increased cytotoxicity (p = 0.01 and p = 0.03 respectively). In a cohort of 37 patients treated with rituximab, high level of complement-mediated cytotoxicity was associated with less and delayed remission at month 6 after rituximab therapy (5/12 vs. 20/25 (p = 0.03) in 8.5 months ± 4.4 vs. 4.8 ± 4.0 (p = 0.02)). Kaplan-Meier analysis demonstrated that high level of cytotoxicity (≥40%) (p = 0.005), epitope spreading (defined by immunization beyond the immunodominant CysR domain) (p = 0.002), and high titer of anti-PLA2R1 total IgG (p = 0.01) were factors of poor renal prognosis. Anti-PLA2R1 antibodies containing sera can induce in vitro cytotoxicity mediated by complement activation, and the level of cytotoxicity increases with the diversity and the titer of anti-PLA2R1 IgG subclasses. These patients with high level of complement-mediated cytotoxicity could benefit from adjuvant therapy using complement inhibitor associated with rituximab to induce earlier remission and less podocyte injury.

Baseline proteinuria level is associated with prognosis in idiopathic membranous nephropathy

Objectives: This study aimed to investigate the unique prognostic, clinical, and renal histopathological characteristics of patients with idiopathic membranous nephropathy (IMN) with different levels of proteinuria.

Methods: This retrospective observational study included 190 IMN patients with low levels of proteinuria (low group), 193 IMN patients with medium levels of proteinuria (medium group), and 123 IMN patients with high levels of proteinuria (high group) treated between September 2006 and November 2015. Prognostic and baseline clinical and histopathological data were compared among the three groups. Poor prognostic events included the occurrence of a persistent 50% reduction in estimated glomerular filtration rate (eGFR), end-stage renal disease, or all-cause mortality.

Results: The severity of clinical symptoms and laboratory indices, such as blood pressure; extent of edema and hematuria; levels of fibrinogen, immunoglobulin (Ig)-G, complement (C)-4, total protein, albumin (ALB), and serum creatinine (SCr); and eGFR increased with increasing proteinuria (all p< .001). Based on renal histopathology, the extent of segmental sclerosis and balloon adhesion and renal interstitial lesion stage also increased in severity with increasing proteinuria (all p< .001). The Kaplan–Meier analysis showed that compared with patients with low and medium levels of proteinuria, patients with high levels of proteinuria had significantly lower cumulative poor event-free renal survival rates (p= .0039).

Conclusions: Baseline proteinuria level is indicative of prognosis in IMN patients; the greater the extent of proteinuria is, the worse the prognosis.