Circulating immune complexes in IgA nephropathy consist of IgA1 with galactose-deficient hinge region and antiglycan antibodies

Immune Characteristics of IgA Nephropathy With Minimal Change Disease

Background: IgA nephropathy (IgAN) has a high degree of heterogeneity in clinical and pathological features. Among all subsets of IgAN, the pathogenesis of IgAN with minimal change disease (MCD-IgAN) remained controversial.

Methods: We analyzed the clinical and pathological characteristics of MCD-IgAN patients in a retrospective cohort. Patients diagnosed with IgAN, excluding MCD-IgAN, were randomly selected as controls. Levels of plasma galactose-deficient IgA1 (GdIgA1), IgG autoantibodies against GdIgA1, GdIgA1 deposition in the glomerulus, and inflammatory reactivity of circulating poly-IgA1 complexes to cultured mesangial cells were evaluated.

Results: Patients with MCD-IgAN had significantly higher levels of proteinuria and estimated glomerular filtration rate (eGFR), lower levels of albumin and urine blood cells, and milder histological lesions by a light microscope compared to IgAN patients, which bears a resemblance to MCD. Lower levels of GdIgA1 (3.41 ± 1.68 vs. 4.92 ± 2.30 μg/ml, p = 0.009) and IgG antiglycan autoantibodies (23.25 ± 22.59 vs. 76.58 ± 71.22 IU/ml, p < 0.001) were found in MCD-IgAN patients than those in IgAN controls. Meanwhile, weaker fluorescence intensities of both IgA and GdIgA1 were observed in the glomerulus of MCD-IgAN patients compared to those in IgAN patients. Furthermore, poly-IgA1 complexes from MCD-IgAN patients induced weaker inflammatory effects on cultured mesangial cells than those from IgAN patients in vitro.

Conclusion: The results demonstrated that MCD-IgAN cases represent a dual glomerulopathy, namely, mild IgAN with superimposed MCD, which furthermore provides substantial evidence for the corticosteroids therapy in MCD-IgAN patients as the guidelines recommended.

Single-cell RNA-sequencing reveals distinct immune cell subsets and signaling pathways in IgA nephropathy

BACKGROUND

IgA nephropathy (IgAN) is the most common primary glomerulonephritis globally. Increasing evidence suggests the importance of host immunity in the development of IgAN, but its dynamics during the early stage of IgAN are still largely unclear.

RESULTS

Here we successfully resolved the early transcriptomic changes in immune cells of IgAN by conducting single-cell RNA-sequencing (scRNA-seq) with peripheral blood mononuclear cells. The differentially expressed genes (DEGs) between control and IgAN were predominantly enriched in NK cell-mediated cytotoxicity and cell killing pathways. Interestingly, we discovered that the number and cytotoxicity of NK cells are significantly reduced in IgAN patients, where both the number and marker genes of NK cells were negatively associated with the clinical parameters, including the levels of urine protein creatinine ratio (UPCR), serum galactose-deficient IgA1 and IgA. A distinctive B cell subset, which had suppressed NFκB signaling was predominantly in IgAN and positively associated with disease progression. Moreover, the DEGs of B cells were enriched in different viral infection pathways. Classical monocytes also significantly changed in IgAN and a monocyte subset expressing interferon-induced genes was positively associated with the clinical severity of IgAN. Finally, we identified vast dynamics in intercellular communications in IgAN.

CONCLUSIONS

We dissected the immune landscape of IgAN at the single-cell resolution, which provides new insights in developing novel biomarkers and immunotherapy against glomerulonephritis.

Pathogenesis of IgA Vasculitis: An Up-To-Date Review

Immunoglobin A (IgA) vasculitis (IgAV), formerly called the Henoch-Schönlein purpura (HSP), is a small vessel vasculitis, characterized by IgA1-dominant immune deposition at diseased vessel walls. IgAV is the most common form of vasculitis in children; typical symptoms include palpable purpura, arthritis or arthralgia, abdominal pain, and hematuria or proteinuria. Galactose-deficient IgA1 is detected in the tissues of the kidney and skin in patients with IgAV; it forms immune complexes leading to subsequent immune reactions and injuries. This report provides the recent advances in the understanding of environmental factors, genetics, abnormal innate and acquired immunity, and the role of galactose-deficient IgA1 immunocomplexes in the pathogenesis of IgAV.

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

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

Renal diseases and the role of complement: Linking complement to immune effector pathways and therapeutics

The complement system is an ancient and phylogenetically conserved key danger sensing system that is critical for host defense against pathogens. Activation of the complement system is a vital component of innate immunity required for the detection and removal of pathogens. It is also a central orchestrator of adaptive immune responses and a constituent of normal tissue homeostasis. Once complement activation occurs, this system deposits indiscriminately on any cell surface in the vicinity and has the potential to cause unwanted and excessive tissue injury. Deposition of complement components is recognized as a hallmark of a variety of kidney diseases, where it is indeed associated with damage to the self. The provenance and the pathophysiological role(s) played by complement in each kidney disease is not fully understood. However, in recent years there has been a renaissance in the study of complement, with greater appreciation of its intracellular roles as a cell-intrinsic system and its interplay with immune effector pathways. This has been paired with a profusion of novel therapeutic agents antagonizing complement components, including approved inhibitors against complement components (C)1, C3, C5 and C5aR1. A number of clinical trials have investigated the use of these more targeted approaches for the management of kidney diseases. In this review we present and summarize the evidence for the roles of complement in kidney diseases and discuss the available clinical evidence for complement inhibition.

Poly-IgA Complexes and Disease Severity in IgA Nephropathy

BACKGROUND AND OBJECTIVES

Poly-IgA immune complex formation and glomerular deposition play a key role in IgA nephropathy. Our study sought to develop a new methodology for one-step serologic detection of poly-IgA levels.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A novel ELISA method using recombinant CD89 as a "capturing" probe was established for detecting poly-IgA immune complex in plasma. We applied semiquantitative measurements of these poly-IgA indices in patients recruited at Peking University First Hospital who had IgA nephropathy or other kidney disease types, as compared with healthy controls. The longitudinal trend of the poly-IgA index and the association with pathologic parameters and treatment responses were evaluated. Finally, we analyzed the molecular composition of poly-IgA complexes in patients by mass spectrometry.

RESULTS

Recombinant CD89-mounted ELISA plates specifically captured plasma poly-IgA. The levels of poly-IgA immune complex (26.7 [interquartile range (IQR) 17.1-42.6] U/ml) in IgA nephropathy were significantly higher than those in healthy controls (15.5 [IQR 10.7-20.0] U/ml; P<0.001) or in controls with non-IgA nephropathy disease (14.8 [IQR 10.5-21.9] U/ml; P<0.001). Higher levels of poly-IgA immune complex were associated with lower eGFR and worse kidney outcome. Accuracy parameters and concordant statistics showed good discrimination between IgA nephropathy and healthy controls based on poly-IgA index levels (area under the curve [AUC], 0.78; 95% confidence interval [95% CI], 0.72 to 0.83; P<0.001), significantly outperforming galactose-deficient IgA1 levels (AUC, 0.70; P=0.05). Corticosteroid and immunosuppressant treatments lowered poly-IgA indices. After a recombinant CD89-directed workflow in conjunction with mass spectrometry, we also analyzed the molecular composition of IgA immune complex in patients with IgA nephropathy.

CONCLUSIONS

Higher level of recombinant CD89-bound poly-IgA immune complex was associated with the severity of the disease and with treatment response to steroids and immunosuppressants.

IgA Vasculitis with Nephritis in Adults: Histological and Clinical Assessment

Patients with IgA vasculitis (IgAV), an immune complex-mediated disease, may exhibit kidney involvement-IgAV with nephritis (IgAVN). The kidney-biopsy histopathologic features of IgAVN are similar to those of IgA nephropathy, but little is known about histopathologic disease severity based on the interval between purpura onset and diagnostic kidney biopsy. We assessed kidney histopathology and clinical and laboratory data in a cohort of adult patients with IgAVN (n = 110). The cases were grouped based on the interval between the onset of purpura and kidney biopsy: Group 1 (G1, <1 month, n = 14), Group 2 (G2, 1-6 months, n = 58), and Group 3 (G3, >6 months, n = 38). Glomerular leukocytes were more common in G1 than in the other groups (p = 0.0008). The proportion of neutrophils among peripheral-blood leukocytes was the highest in the patients biopsied within a month after onset of purpura (G1: 71 ± 8%). In the patients with an interval >6 months, the neutrophil proportion was lower, 60%. Moreover, the glomerular mesangial proliferation score correlated with the serum total IgA concentration (p = 0.0056). In conclusion, IgAVN patients biopsied <1 month from purpura onset showed an elevated percentage of blood neutrophils and glomerular leukocytes, consistent with an acute-onset inflammatory reaction. In all IgAVN patients, the mesangial proliferation score correlated with the serum IgA level.

The microbiome and IgA nephropathy

The immunopathogenic mechanisms underlying immunoglobulin A nephropathy (IgAN) are poorly understood, yet it is one of the most common causes of kidney failure globally. The commonly referenced syndrome of synpharyngitic gross hematuria as a presenting feature of IgAN has led to a logical association between infections and development of IgAN, however no pathogenic organism has been clearly linked to IgAN. Advances in sequencing technology have enabled more detailed characterization of host microbial communities, and highlighted the interrelationship between microbiota and immune responses in health and disease. This review will summarize current thinking on the relationship between microbiota and development of IgAN with a focus on recent studies relating aberrant mucosal IgA-biased immune responses to microbiota and how this may be related to the immunopathogenesis of IgAN.

Propensity of IgA to self-aggregate via tailpiece cysteine-471 and treatment of IgA nephropathy using cysteamine

IgA nephropathy is caused by deposition of circulatory IgA1 in the kidney. Hypogalactosylated IgA1 has the propensity to form poly-IgA aggregates that are prone to deposition. Herein, we purified poly-IgA from the plasma of patients with IgA nephropathy and showed that the complex is susceptible to reducing conditions, suggesting intermolecular disulfide connections between IgA units. We sought to find the cysteine residue(s) that form intermolecular disulfide. Naturally assembled dimeric IgA, also known as secretory IgA, involves a J chain subunit connected with 2 IgA1 molecules via their penultimate cysteine-471 residue on a “tailpiece” segment of IgA heavy chain. It is plausible that, with the absence of J chain, the cysteine residue of mono-IgA1 might aberrantly form a disulfide bond in poly-IgA formation. Mutagenesis confirmed that cysteine-471 is capable of promoting IgA aggregation. These discoveries prompted us to test thiol-based drugs for stabilizing cysteine. Specifically, the cystine-reducing drug cysteamine used for treatment of cystinosis showed a remarkable potency in preventing self-aggregation of IgA. When administrated to rat and mouse models of IgA nephropathy, cysteamine significantly reduced glomerular IgA deposition. Collectively, our results reveal a potentially novel molecular mechanism for aberrant formation of IgA aggregates, to which the repurposed cystinosis drug cysteamine was efficacious in preventing renal IgA deposition.

Cosmc deficiency causes spontaneous autoimmunity by breaking B cell tolerance

Factors regulating the induction and development of B cell–mediated autoimmunity are not well understood. Here, we report that targeted deletion in murine B cells of X-linked Cosmc, encoding the chaperone required for expression of core 1 O-glycans, causes the spontaneous development of autoimmune pathologies due to a breakdown of B cell tolerance. BC-CosmcKO mice display multiple phenotypic abnormalities, including severe weight loss, ocular manifestations, lymphadenopathy, and increased female-associated mortality. Disruption of B cell tolerance in BC-CosmcKO mice is manifested as elevated self-reactive IgM and IgG autoantibodies. Cosmc-deficient B cells exhibit enhanced basal activation and responsiveness to stimuli. There is also an elevated frequency of spontaneous germinal center B cells in BC-CosmcKO mice. Mechanistically, loss of Cosmc confers enhanced B cell receptor (BCR) signaling through diminished BCR internalization. The results demonstrate that Cosmc, through control of core 1 O-glycans, is a previously unidentified immune checkpoint gene in maintaining B cell tolerance.

Immunoglobulin A, an Active Liaison for Host-Microbiota Homeostasis

Mucosal surfaces in the gastrointestinal tract are continually exposed to native, commensal antigens and susceptible to foreign, infectious antigens. Immunoglobulin A (IgA) provides dual humoral responses that create a symbiotic environment for the resident gut microbiota and prevent the invasion of enteric pathogens. This review features recent immunological and microbial studies that elucidate the underlying IgA and microbiota-dependent mechanisms for mutualism at physiological conditions. IgA derailment and concurrent microbiota instability in pathological diseases are also discussed in detail. Highlights of this review underscore that the source of IgA and its structural form can dictate microbiota reactivity to sustain a diverse niche where both host and bacteria benefit. Other important studies emphasize IgA insufficiency can result in the bloom of opportunistic pathogens that encroach the intestinal epithelia and disseminate into circulation. The continual growth of knowledge in these subjects can lead to the development of therapeutics targeting IgA and/or the microbiota to treat life threatening diseases.

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

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

Pathogenesis of IgA Nephropathy: Current Understanding and Implications for Development of Disease-Specific Treatment

IgA nephropathy, initially described in 1968 as a kidney disease with glomerular “intercapillary deposits of IgA-IgG”, has no disease-specific treatment and is a common cause of kidney failure. Clinical observations and laboratory analyses suggest that IgA nephropathy is an autoimmune disease wherein the kidneys are damaged as innocent bystanders due to deposition of IgA1-IgG immune complexes from the circulation. A multi-hit hypothesis for the pathogenesis of IgA nephropathy describes four sequential steps in disease development. Specifically, patients with IgA nephropathy have elevated circulating levels of IgA1 with some O-glycans deficient in galactose (galactose-deficient IgA1) and these IgA1 glycoforms are recognized as autoantigens by unique IgG autoantibodies, resulting in formation of circulating immune complexes, some of which deposit in glomeruli and activate mesangial cells to induce kidney injury. This proposed mechanism is supported by observations that (i) glomerular immunodeposits in patients with IgA nephropathy are enriched for galactose-deficient IgA1 glycoforms and the corresponding IgG autoantibodies; (ii) circulatory levels of galactose-deficient IgA1 and IgG autoantibodies predict disease progression; and (iii) pathogenic potential of galactose-deficient IgA1 and IgG autoantibodies was demonstrated in vivo. Thus, a better understanding of the structure–function of these immunoglobulins as autoantibodies and autoantigens will enable development of disease-specific treatments.

IgA glycosylation and immune complex formation in IgAN

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. This disease, discovered in 1968, is characterized by IgA-IgG glomerular immunodeposits with a mesangial pattern. It is thought that these immunodeposits originate from the immune complexes formed in the circulation. It is hypothesized that the pathogenesis of IgAN is driven by aberrant glycoforms of IgA1 (galactose-deficient IgA1, Gd-IgA1). Gd-IgA1, in genetically susceptible individuals, represents the initiating factor for the formation of circulating immune complexes due to its recognition by IgG autoantibodies and the subsequent formation of pathogenic IgA1-IgG immune complexes. Complement activation through alternative and/or lectin pathways is likely playing an important role in the pathogenic properties of these complexes and may further upregulate local inflammatory responses and glomerular injury.

Comparative Long-Term Renal Allograft Outcomes of Recurrent Immunoglobulin A with Severe Activity in Kidney Transplant Recipients with and without Rituximab: An Observational Cohort Study

Recurrent IgA nephropathy (IgAN) remains an important cause of allograft loss in renal transplantation. Due to the limited efficacy of corticosteroid in the treatment of recurrent glomerulonephritis, rituximab was used in kidney transplant (KT) recipients with severe recurrent IgAN. A retrospective cohort study was conducted between January 2015 and December 2020. Accordingly, there were 64 KT recipients with biopsy-proven recurrent IgAN with similar baseline characteristics that were treated with the conventional standard therapy alone (controls, n = 43) or together with rituximab (cases, n = 21). All of the recipients had glomerular endocapillary hypercellularity and proteinuria (>1 g/d) with creatinine clearance (CrCl) > 30 mL/min/1.73 m² and well-controlled blood pressure using renin–angiotensin–aldosterone blockers. The treatment outcomes were renal allograft survival rate, proteinuria, and post-treatment allograft pathology. During 3.8 years of follow-up, the rituximab-based regimen rapidly decreased proteinuria within 12 months after rituximab administration and maintained renal allograft function—the primary endpoint—for approximately 3 years. There were eight recipients in the case group (38%), and none in the control group reached a complete remission (proteinuria < 250 mg/d) at 12 months after treatment. Notably, renal allograft histopathology from patients with rituximab-based regimen showed the less severe endocapillary hypercellularity despite the remaining strong IgA deposition. In conclusion, adjunctive treatment with rituximab potentially demonstrated favorable outcomes for treatment of recurrent severe IgAN post-KT as demonstrated by proteinuria reduction and renal allograft function in our cohort. Further in-depth mechanistic studies with the longer follow-up periods are recommended.

The immune landscape of IgA induction in the gut

Antibodies are key elements of protective immunity. In the mucosal immune system in particular, secretory immunoglobulin A (SIgA), the most abundantly produced antibody isotype, protects against infections, shields the mucosal surface from toxins and environmental factors, and regulates immune homeostasis and a peaceful coexistence with our microbiota. However, the dark side of IgA biology promotes the formation of immune complexes and provokes pathologies, e.g., IgA nephropathy (IgAN). The precise mechanisms of how IgA responses become deregulated and pathogenic in IgAN remain unresolved. Yet, as the field of microbiota research moved into the limelight, our basic understanding of IgA biology has been taking a leap forward. Here, we discuss the structure of IgA, the anatomical and cellular foundation of mucosal antibody responses, and current concepts of how we envision the interaction of SIgA and the microbiota. We center on key concepts in the field while taking account of both historic findings and exciting new observations to provide a comprehensive groundwork for the understanding of IgA biology from the perspective of a mucosal immunologist.

Aberrantly Glycosylated IgA1 in IgA Nephropathy: What We Know and What We Don't Know

IgA nephropathy (IgAN), the most common primary glomerular disease worldwide, is characterized by glomerular deposition of IgA1-containing immune complexes. The IgA1 hinge region (HR) has up to six clustered O-glycans consisting of Ser/Thr-linked N-acetylgalactosamine usually with β1,3-linked galactose and variable sialylation. Circulating levels of IgA1 with abnormally O-glycosylated HR, termed galactose-deficient IgA1 (Gd-IgA1), are increased in patients with IgAN. Current evidence suggests that IgAN is induced by multiple sequential pathogenic steps, and production of aberrantly glycosylated IgA1 is considered the initial step. Thus, the mechanisms of biosynthesis of aberrantly glycosylated IgA1 and the involvement of aberrant glycoforms of IgA1 in disease development have been studied. Furthermore, Gd-IgA1 represents an attractive biomarker for IgAN, and its clinical significance is still being evaluated. To elucidate the pathogenesis of IgAN, it is important to deconvolute the biosynthetic origins of Gd-IgA1 and characterize the pathogenic IgA1 HR O-glycoform(s), including the glycan structures and their sites of attachment. These efforts will likely lead to development of new biomarkers. Here, we review the IgA1 HR O-glycosylation in general and the role of aberrantly glycosylated IgA1 in the pathogenesis of IgAN in particular.

IgG deposits in the mesangium and capillary loops predict poor renal outcome in patients with IgA nephropathy: a single-center retrospective study

Background

Glomerular IgG deposition in patients with IgA nephropathy (IgAN) has been shown to be associated with poor renal survival; however, most published studies to date are too small-scale and inconsistent to provide guidance for clinical practice.

Methods

Based on renal biopsy findings, 742 patients were divided into the following groups: (i) IgA deposition alone (IgA) vs IgA + IgG deposition (IgA + IgG) and (ii) IgG co-deposition confined to the mesangium vs mesangium + capillary loops (CLs). The clinicopathological variables at biopsy and renal outcome were assessed.

Results

Of the 742 patients, 182 had IgG co-deposition and 51 had IgG deposits in the mesangium + CLs. Patients with IgG co-deposition were associated with severe clinical and pathological lesions, especially those with a location of IgG deposits in the mesangium +CLs. Kaplan–Meier analysis revealed that a lower renal cumulative survival rate was present in both patients with IgG co-deposition and those with a location of IgG deposits in the mesangium + CLs (all p < 0.05). Moreover, patients with a higher intensity of glomerular IgG deposits or C3 deposits or C1q deposits were also associated with a lower survival rate. A multivariate Cox regression model identified the location of IgG deposits in the mesangium + CLs as an independent risk factor for poor prognosis (HR, 2.11; 95% CI: 1.06–4.18; p = 0.005).

Conclusions

Glomerular IgG co-deposition and the location of glomerular IgG deposits in the mesangium + CLs were both associated with adverse renal outcomes, but only the location of glomerular IgG deposits in the CLs was an independent risk factor for poor prognosis in IgAN.

Fantastic IgA plasma cells and where to find them

IgA is produced in large quantities at mucosal surfaces by IgA⁺ plasma cells (PC), protecting the host from pathogens, and restricting commensal access to the subepithelium. It is becoming increasingly appreciated that IgA⁺ PC are not constrained to mucosal barrier sites. Rather, IgA⁺ PC may leave these sites where they provide both host defense and immunoregulatory function. In this review, we will outline how IgA⁺ PC are generated within the mucosae and how they subsequently migrate to their "classical" effector site, the gut lamina propria. From there we provide examples of IgA⁺ PC displacement from the gut to other parts of the body, referencing examples during homeostasis and inflammation. Lastly, we will speculate on mechanisms of IgA⁺ PC displacement to other tissues. Our aim is to provide a new perspective on how IgA⁺ PC are truly fantastic beasts of the immune system and identify new places to find them.

A Fresh Perspective on Monoclonal Gammopathies of Renal Significance

Monoclonal gammopathies of renal significance (MGRS) encompass a remarkable variety of kidney diseases that result from intrinsic nephrotoxic properties of certain monoclonal Igs or their subunits. Effective disease-modifying treatments rely on the targeting of a malignant B-cell clone that may be demonstrable but often is quite hypothetical. Hence, convincing arguments for the genuine monoclonal character of the causative mono-isotypic Ig tissue deposits is needed for design of appropriate treatment strategies. The purpose of this article was to critically analyze distinct situations of suspected MGRS that occur in the practice of pathologists, nephrologists, hematologists, and immunologists. A particular focus of interest is the group of conditions known as proliferative glomerulonephritis with mono-isotypic immunoglobulin deposits (PGNMIDs), which illustrates the difficulties and ambiguities surrounding a definitive assignment of MGRS status.

Renal deposition and clearance of recombinant poly-IgA complexes in a model of IgA nephropathy

IgA nephropathy (IgAN) is the most common type of glomerulonephritis worldwide, which follows a chronic but nonetheless highly variable course of progression. IgA immune complexes are the primary source of renal deposits in IgAN. Apart from the presence of granular IgA1 deposits in the glomerular mesangium and mesangial hypercellularity as common features, the detailed process of IgA1 deposition and clearance in the kidney remains unclear. We sought to examine the dynamics of IgA deposition and tissue plasticity in response to deposits including their intrarenal clearance. We followed a synthetic approach to produce a recombinant fusion between IgA Fc (rIgA) and a biotin tag, which was subsequently induced with streptavidin (SA) to form an oligomeric poly-IgA mimic. Both uninduced rIgA (mono-rIgA) and polymeric SA-rIgA (poly-rIgA) were injected intravenously into Wistar rats. Plasma IgA levels and renal and liver histology were examined in a time series. In contrast to mono-rIgA, this synthetic poly-rIgA analog formed renal deposits exclusively in the glomerulus and were mostly cleared in 3 h. However, repeated daily injections for 12 days caused long-lasting and stronger glomerular IgA deposition together with IgG and complement C3, in association with mesangial cell proliferation, matrix expansion, and variable degrees of albuminuria and hematuria that phenocopied IgAN. Ex vivo, poly-rIgA bound cultured mesangial cells and elicited cytokine production, in addition to activating plasma C3 that was consistent with the actions of IgA immune complexes in IgAN pathogenesis. Remarkably, the kidneys were able to reverse all pathologic manifestations and restore normal glomerular histology 2 weeks after injections were halted. The synthetic model showed the kinetics between the intricate balance of renal deposition and clearance, as well as glomerular plasticity towards healing. Together, the results revealed a priming effect of existing deposits in promoting stronger and longer-lasting IgA deposition to cause renal damage. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Hyposialylation Must Be Considered to Develop Future Therapies in Autoimmune Diseases

Autoimmune disease development depends on multiple factors, including genetic and environmental. Abnormalities such as sialylation levels and/or quality have been recently highlighted. The adjunction of sialic acid at the terminal end of glycoproteins and glycolipids is essential for distinguishing between self and non-self-antigens and the control of pro- or anti-inflammatory immune reactions. In autoimmunity, hyposialylation is responsible for chronic inflammation, the anarchic activation of the immune system and organ lesions. A detailed characterization of this mechanism is a key element for improving the understanding of these diseases and the development of innovative therapies. This review focuses on the impact of sialylation in autoimmunity in order to determine future treatments based on the regulation of hyposialylation.

Streptococcus mutans induces IgA nephropathy-like glomerulonephritis in rats with severe dental caries

The mechanisms underlying immunoglobulin A nephropathy (IgAN), the most common chronic form of primary glomerulonephritis, remain poorly understood. Streptococcus mutans, a Gram-positive facultatively anaerobic oral bacterium, is a common cause of dental caries. In previous studies, S. mutans isolates that express Cnm protein on their cell surface were frequently detected in IgAN patients. In the present study, inoculation of Cnm-positive S. mutans in the oral cavities of 2-week-old specific-pathogen free Sprague-Dawley rats fed a high-sucrose diet for 32 weeks produced severe dental caries in all rats. Immunohistochemical analyses of the kidneys using IgA- and complement C3-specific antibodies revealed positive staining in the mesangial region. Scanning electron microscopy revealed a wide distribution of electron dense deposits in the mesangial region and periodic acid-Schiff staining demonstrated prominent proliferation of mesangial cells and mesangial matrix. These results suggest that IgAN-like glomerulonephritis was induced in rats with severe dental caries by Cnm-positive S. mutans.

Systolic blood pressure and chronic kidney disease progression in patients with primary glomerular disease

INTRODUCTION

Many current guidelines on optimal target blood pressure (BP) for chronic kidney disease (CKD) patients are largely based on studies in diabetic and hypertensive patients. However, there have been few studies in patients with glomerular diseases.

METHODS

We retrospectively studied the longitudinal association between BP and CKD progression in 1,066 biopsy-proven patients diagnosed with primary glomerular diseases, including IgA nephropathy, membranous nephropathy (MN), and focal segmental glomerulosclerosis (FSGS), between 2005 and 2017. The main predictor was time-updated systolic blood pressure (SBP) at every clinic visit. The primary outcome was a composite one including ≥ 50% decrease in estimated glomerular filtration rate (eGFR) from the baseline, and end-stage kidney disease (ESKD).

RESULTS

During 5009 person-years of follow-up, the primary outcome occurred in 157 (14.7%) patients. In time-varying Cox model, the adjusted hazard ratios (HRs) (95% confidence interval (CI)) for the primary outcome were 1.48 (0.96-2.29), 2.07 (1.22-3.52), and 2.53 (1.13-5.65) for SBP of 120-129, 130-139, and ≥ 140 mmHg, respectively, compared with SBP < 120 mmHg. This association was particularly evident in patients with elevated proteinuria. However, there was no association between baseline SBP and adverse kidney outcomes. Finally, prediction models failed to show the improvement of predictive performance of SBP compared with that of remission status. Moreover, patients with remission and less controlled SBP had better kidney outcomes than those with non-remission and well-controlled SBP.

CONCLUSION

Among patients with glomerular disease, higher time-updated SBP was significantly associated with higher risk of CKD progression. However, the clinical significance of blood pressure was less powerful than remission status.

Experimental evidence of pathogenic role of IgG autoantibodies in IgA nephropathy

BACKGROUND

IgA nephropathy is thought to be an autoimmune disease wherein galactose-deficient IgA1 (Gd-IgA1) is recognized by IgG autoantibodies, resulting in formation and renal accumulation of nephritogenic immune complexes. Although this hypothesis is supported by recent findings that, in renal immunodeposits of IgA nephropathy patients, IgG is enriched for Gd-IgA1-specific autoantibodies, experimental proof is still lacking.

METHODS

IgG isolated from sera of IgA nephropathy patients or produced as a recombinant IgG (rIgG) was mixed with human Gd-IgA1 to form immune complexes. IgG from healthy individuals served as a control. Nude and SCID mice were injected with human IgG and Gd-IgA1, in immune complexes or individually, and their presence in kidneys was ascertained by immunofluorescence. Pathologic changes in the glomeruli were evaluated by quantitative morphometry and exploratory transcriptomic profiling was performed by RNA-Seq.

RESULTS

Immunodeficient mice injected with Gd-IgA1 mixed with IgG autoantibodies from patients with IgA nephropathy, but not Gd-IgA1 mixed with IgG from healthy individuals, displayed IgA, IgG, and mouse complement C3 glomerular deposits and mesangioproliferative glomerular injury with hematuria and proteinuria. Un-complexed Gd-IgA1 or IgG did not induce pathological changes. Moreover, Gd-IgA1-rIgG immune complexes injected into immunodeficient mice induced histopathological changes characteristic of human disease. Exploratory transcriptome profiling of mouse kidney tissues indicated that these immune complexes altered gene expression of multiple pathways, in concordance with the changes observed in kidney biopsies of patients with IgA nephropathy.

CONCLUSIONS

This study provides the first in vivo evidence for a pathogenic role of IgG autoantibodies specific for Gd-IgA1 in the pathogenesis of IgA nephropathy.

Immunoglobulin A Glycosylation and Its Role in Disease

Human IgA is comprised of two subclasses, IgA1 and IgA2. Monomeric IgA (mIgA), polymeric IgA (pIgA), and secretory IgA (SIgA) are the main molecular forms of IgA. The production of IgA rivals all other immunoglobulin isotypes. The large quantities of IgA reflect the fundamental roles it plays in immune defense, protecting vulnerable mucosal surfaces against invading pathogens. SIgA dominates mucosal surfaces, whereas IgA in circulation is predominately monomeric. All forms of IgA are glycosylated, and the glycans significantly influence its various roles, including antigen binding and the antibody effector functions, mediated by the Fab and Fc portions, respectively. In contrast to its protective role, the aberrant glycosylation of IgA1 has been implicated in the pathogenesis of autoimmune diseases, such as IgA nephropathy (IgAN) and IgA vasculitis with nephritis (IgAVN). Furthermore, detailed characterization of IgA glycosylation, including its diverse range of heterogeneity, is of emerging interest. We provide an overview of the glycosylation observed for each subclass and molecular form of IgA as well as the range of heterogeneity for each site of glycosylation. In many ways, the role of IgA glycosylation is in its early stages of being elucidated. This chapter provides an overview of the current knowledge and research directions.

Glomerular C4d in Post-Transplant IgA Nephropathy is associated with decreased allograft survival

Background

Glomerulonephritis (GN), including post-transplant IgAN (post-Tx IgAN) is an important contributor to decreased long-term allograft survival. The immunopathological detection of the complement degradation product C4d in glomeruli (C4dG) has been recently described as a risk factor in native kidney IgAN, however little is known about C4dG deposition in post-Tx IgAN. We hypothesized that glomerular C4d may indicate a more aggressive disease course and worse allograft survival in patients with post-Tx IgAN.

Methods

In this retrospective study we assessed the presence and clinical relevance of C4dG in patients with post-transplant IgAN. We analyzed 885 renal allograft recipients, including 84 patients with post-transplant GN. All patients were transplanted between January 1999 and April 2006 and underwent at least one biopsy for differnt causes. The primary endpoint was death-censored graft survival, with a median follow-up of 9.6 (IQR 3.8–13.2) years.

Results

The prevalence of post-Tx GN was 9.5%. Twenty-seven patients with post-Tx IgAN were included. C4dG positive patients (N = 18, 66.7%) had significantly worse allograft survival compared to C4dG negative post-Tx IgAN patients and patients without post-Tx IgAN [C4dG positive: 27.8% vs. 55.6% and 66.0%; log-rank: p = 0.01]. C4dG remained a significant risk factor (HR 2.22, 95% CI 1.27–3.87) for allograft loss even after adjustment for T cell mediated rejection (TCMR) and antibody mediated rejection.

Conclusion

Glomerular C4d deposition is an independent risk factor for worse graft-survival in patients with post-Tx IgAN, even after adjusting for other risk factors such as antibody mediated rejection.  Assessment of glomerular C4d deposition may provide a valuable prognostic risk assessment tool to identify high risk patients in post-Tx IgAN.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s40620-020-00914-x) contains supplementary material, which is available to authorized users.

Quantitative assessment of successive carbohydrate additions to the clustered O-glycosylation sites of IgA1 by glycosyltransferases

Mucin-type O-glycosylation occurs on many proteins that transit the Golgi apparatus. These glycans impact structure and function of many proteins and have important roles in cellular biosynthetic processes, signaling and differentiation. Although recent technological advances have enhanced our ability to profile glycosylation of glycoproteins, limitations in the understanding of the biosynthesis of these glycan structures remain. Some of these limitations stem from the difficulty to track the biosynthetic process of mucin-type O-glycosylation, especially when glycans occur in dense clusters in repeat regions of proteins, such as the mucins or immunoglobulin A1 (IgA1). Here, we describe a series of nano-liquid chromatography (LC)-mass spectrometry (MS) analyses that demonstrate the range of glycosyltransferase enzymatic activities involved in the biosynthesis of clustered O-glycans on IgA1. By utilizing nano-LC-MS relative quantitation of in vitro reaction products, our results provide unique insights into the biosynthesis of clustered IgA1 O-glycans. We have developed a workflow to determine glycoform-specific apparent rates of a human UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltrasnfersase (GalNAc-T EC 2.4.1.41) and demonstrated how pre-existing glycans affect subsequent activity of glycosyltransferases, such as core 1 galactosyltransferase and α2,3- and α2,6-specific sialyltransferases, in successive additions in the biosynthesis of clustered O-glycans. In the context of IgA1, these results have potential to provide insight into the molecular mechanisms implicated in the pathogenesis of IgA nephropathy, an autoimmune renal disease involving aberrant IgA1 O-glycosylation. In a broader sense, these methods and workflows are applicable to the studies of the concerted and competing functions of other glycosyltransferases that initiate and extend mucin-type core 1 clustered O-glycosylation.

Emerging Modes of Treatment of IgA Nephropathy

IgA nephropathy is the most common primary glomerulonephritis with potentially serious outcome leading to end stage renal disease in 30 to 50% of patients within 20 to 30 years. Renal biopsy, which might be associated with risks of complications (bleeding and others), still remains the only reliable diagnostic tool for IgA nephropathy. Therefore, the search for non-invasive diagnostic and prognostic markers for detection of subclinical types of IgA nephropathy, evaluation of disease activity, and assessment of treatment effectiveness, is of utmost importance. In this review, we summarize treatment options for patients with IgA nephropathy including the drugs currently under evaluation in randomized control trials. An early initiation of immunosupressive regimens in patients with IgA nephropathy at risk of progression should result in the slowing down of the progression of renal function to end stage renal disease.

CDR3 sequences in IgA nephropathy are shorter and exhibit reduced diversity

Immunoglobulin (Ig) A nephropathy (IgAN) is the most common glomerulonephritis, which is characterized by the deposition of IgA antibody in the glomerulus. Systematic dissection of immune composition may contribute to a better understanding of the alternations in the immune system in IgAN. To this end, here we applied immune repertoire sequencing technology for parallel analysis of the complementary determining region 3 (CDR3) of all B cell receptors, including all five antibody subtypes (IgA, IgG, IgM, IgE and IgD), in 13 patients with IgAN and 7 healthy individuals. A significant decrease in CDR3 length was observed in the IgAN group. In particular, the JH6 family was significantly increased in IgAN. Amino acid usage was also altered in IgAN. Shannon, Simpson, Gini and Diversity 50 indices also revealed significant differences in the diversity of IgG, IgM and IgA antibodies as compared with controls. The proportions of IgA and IgG were increased, whereas IgM was decreased in IgAN. Moreover, a greater number of CDR3 sequences was shared between patients with IgAN. These findings suggest that the BCR immune repertoire is dramatically altered in IgAN, as characterized by shortened CDR3 length, as well as decreased overall diversity of CDR3.

Challenges in primary focal segmental glomerulosclerosis diagnosis: from the diagnostic algorithm to novel biomarkers

Primary or idiopathic focal segmental glomerulosclerosis (FSGS) is a kidney entity that involves the podocytes, leading to heavy proteinuria and in many cases progresses to end-stage renal disease. Idiopathic FSGS has a bad prognosis, as it involves young individuals who, in a considerably high proportion (∼15%), are resistant to corticosteroids and other immunosuppressive treatments as well. Moreover, the disease recurs in 30–50% of patients after kidney transplantation, leading to graft function impairment. It is suspected that this relapsing disease is caused by a circulating factor(s) that would permeabilize the glomerular filtration barrier. However, the exact pathologic mechanism is an unsettled issue. Besides its poor outcome, a major concern of primary FSGS is the complexity to confirm the diagnosis, as it can be confused with other variants or secondary forms of FSGS and also with other glomerular diseases, such as minimal change disease. New efforts to optimize the diagnostic approach are arising to improve knowledge in well-defined primary FSGS cohorts of patients. Follow-up of properly classified primary FSGS patients will allow risk stratification for predicting the response to different treatments. In this review we will focus on the diagnostic algorithm used in idiopathic FSGS both in native kidneys and in disease recurrence after kidney transplantation. We will emphasize those potential confusing factors as well as their detection and prevention. In addition, we will also provide an overview of ongoing studies that recruit large cohorts of glomerulopathy patients (Nephrotic Syndrome Study Network and Cure Glomerulonephropathy, among others) and the experimental studies performed to find novel reliable biomarkers to detect primary FSGS.

Immunoproteasome in IgA Nephropathy: State-of-Art and Future Perspectives

IgA nephropathy (IgAN) is a leading cause of chronic kidney disease and renal failure. The exact pathogenesis of IgAN is not well defined, but some genetic studies have led to a novel discovery that the immunoproteasome probably plays an important role in IgAN. The immunoproteasome is a proteasome variant that is expressed when cells are stressed or receive inflammatory signals. While immunoproteasome is suggested to be mainly involved in major histocompatibility complex-I (MHC-I) antigen presentation, recent studies indicate that it may assert broad functions in trafficking events that activate both innate and adaptive immunity. In this review, we first summarize new insights into its functions in immunity, and discuss how it underlies its associations with IgAN. We also highlight its potential as a therapeutic target for the future.

The analysis of treatment of Immunoglobulin A-nephropathy

Aim. An evaluation of the effectiveness of immunosuppressive therapy (IST) and tonsillectomy (TE) in patients with IgA nephropathy (IgAN). Materials and methods. A retrospective cohort of the study included cases with biopsy proven primary IgAN (n=367, age 3412 years, men 55%). We used demographic and clinical and morphological parameters at the time of biopsy. Median followup period was 26 (10; 61) months. Outcomes were remission (complete or partial) and the progression of IgAN (defined as the start of dialysis or a decrease in glomerular filtration rate 50% from baseline). All patients received treatment with renin angiotensin system blockers. Evaluation of the effectiveness of therapy was carried out using propensity score (PS) methods matching, conventional double robust regression models with PS as independent covariate, and inverse probability weighting. Following patient subgroups were used for comparative analyses: with IST (n=176) and without IST (n=191); with TE (n=63) and without TE (n=304); without IST and without TE (IST-TE-; n=162); with TE and without IST (IST-TE+; n=29); with IST and without TE (IST+TE-; n=142); with IST and with TE (IST+ TE+; n=34). Results. All PS methods used gave close estimates of the comparative effectiveness of treatment in different subgroups: 1) patients on monotherapy with corticosteroids (CS) and combination of CS with other immunosuppressants did not have significant differences in probabilities of IgAN progression (hazard ratio 0.919; 95% CI 0.3332.950) and remission (odds ratio 0.919; 95% CI 0.3792.344) and were further combined into a group of IST; 2) IST was significantly associated with the lower risk of disease progression and increased odds ratio for remission; 3) the positive effects of IST were limited to cases with proteinuria 2 g/24 h; 4) the likelihood of IgAN remission and progression did not differ significantly between TE+ and TE-, IST-TE+ and IST-TE- groups. There were no cases of disease progression in the IST+TE+ group. The cumulative renal survival was higher in the IST+TE+ group compared to IST+ TE- group (p=0.010), while the probability of remission did not differ. Conclusion. IST was associated with a lower risk of IgAN progression and increased probability of remission, while these effects of IST were limited to patients with proteinuria 2 g/24 h. TE in combination with IST is associated with an additional reduction in the risk of disease progression.

Role of CD8+ T cells in crescentic glomerulonephritis

Crescentic glomerulonephritis (cGN) comprises three main types according to the pathogenesis and immunofluorescence patterns: anti-glomerular basement membrane antibody cGN, vasculitis-associated cGN and post-infectious immune complex cGN. In this brief review of the immune-pathogenesis of cGN, the focus is mainly on the role of CD8+ T cells in the progression of cGN. Under control conditions, Bowman's capsule (BC) provides a protected immunological niche by preventing access of cytotoxic CD8+ T cells to Bowman's space and thereby podocytes. Even in experimental nephrotoxic nephritis, leukocytes accumulate around the glomeruli, but remain outside of BC, as long as the latter remains intact. However, when and where breaches in BC occur, the inflammatory cells can gain access to and destroy podocytes, thus converting cGN into rapidly progressive glomerulonephritis (RPGN). These conclusions also apply to human cGN, where biopsies show that loss of BC integrity is associated with RPGN and progression to end-stage kidney disease. We propose a two-hit hypothesis for the role of cytotoxic CD8+ T cells in the progression of cGN. The initial insult occurs in response to the immune complex formation or deposition, resulting in local capillary and podocyte injury (first hit). The injured podocytes release neo-epitopes, eventually causing T-cell activation and migration to the glomerulus. Upon generation of breaches in BC, macrophages and CD8+ T cells can now gain access to the glomerular space and destroy neo-epitope expressing podocytes (second hit), resulting in RPGN. While further investigation will be required to test this hypothesis, future therapeutic trials should consider targeting of CD8+ T cells in the therapy of progressive cGN.

The Role of Immune Modulation in Pathogenesis of IgA Nephropathy

IgA nephropathy (IgAN) is the most prevalent primary glomerulonephritis worldwide, with diverse clinical manifestations characterized by recurrent gross hematuria or microscopic hematuria, and pathological changes featuring poorly O-galactosylated IgA1 deposition in the glomerular mesangium. Pathogenesis has always been the focus of IgAN studies. After 50 years of research, most scholars agree that IgAN is a group of clinicopathological syndromes with certain common immunopathological characteristics, and multiple mechanisms are involved in its pathogenesis, including immunology, genetics, and environmental or nutritional factors. However, the precise pathogenetic mechanisms have not been fully determined. One hypothesis about the pathogenesis of IgAN suggests that immunological factors are engaged in all aspects of IgAN development and play a critical role. A variety of immune cells (e.g., dendritic cells, NK cells, macrophages, T-lymphocyte subsets, and B-lymphocytes, etc.) and molecules (e.g., IgA receptors, Toll-like receptors, complements, etc.) in innate and adaptive immunity are involved in the pathogenesis of IgAN. Moreover, the abnormality of mucosal immune regulation is the core of IgAN immunopathogenesis. The roles of tonsil immunity or intestinal mucosal immunity, which have received more attention in recent years, are supported by mounting evidence. In this review, we will explore the latest research insights on the role of immune modulation in the pathogenesis of IgAN. With a better understanding of immunopathogenesis of IgAN, emerging therapies will soon become realized.

Simultaneous Immunoglobulin A and G Glycopeptide Profiling for High-Throughput Applications

Immunoglobulin (Ig) glycosylation is recognized for its influence on Ig turnover and effector functions. However, the large-scale profiling of Ig glycosylation in a biomedical setting is challenged by the existence of different Ig isotypes and subclasses, their varying serum concentrations, and the presence of multiple glycosylation sites per Ig. Here, a high-throughput nanoliquid chromatography (LC)- mass spectrometry (MS)-based method for simultaneous analysis of IgG and IgA glycopeptides was developed and applied on a serum sample set from 185 healthy donors. Sample preparation from minute amounts of serum was performed in 96-well plate format. Prior to trypsin digestion, IgG and IgA were enriched simultaneously, followed by a one-step denaturation, reduction, and alkylation. The obtained nanoLC-MS data were subjected to semiautomated, targeted feature integration and quality control. The combined and simplified protocol displayed high overall method repeatability, as assessed using pooled plasma and serum standards. Taking all samples together, 143 individual N- and O-glycopeptides were reliably quantified. These glycopeptides were attributable to 11 different peptide backbones, derived from IgG1, IgG2/3, IgG4, IgA1, IgA2, and the joining chain from dimeric IgA. Using this method, novel associations were found between IgA N- and O-glycosylation and age. Furthermore, previously reported associations of IgG Fc glycosylation with age in healthy individuals were confirmed. In conclusion, the new method paves the way for high-throughput multiprotein plasma glycoproteomics.

Role of Epstein-Barr Virus in Pathogenesis and Racial Distribution of IgA Nephropathy

IgA nephropathy (IgAN) is the dominant type of primary glomerulonephritis worldwide. However, IgAN rarely affects African Blacks and is uncommon in African Americans. Polymeric IgA1 with galactose-deficient hinge-region glycans is recognized as auto-antigen by glycan-specific antibodies, leading to formation of circulating immune complexes with nephritogenic consequences. Because human B cells infected in vitro with Epstein-Barr virus (EBV) secrete galactose-deficient IgA1, we examined peripheral blood B cells from adult IgAN patients, and relevant controls, for the presence of EBV and their phenotypic markers. We found that IgAN patients had more lymphoblasts/plasmablasts that were surface-positive for IgA, infected with EBV, and displayed increased expression of homing receptors for targeting the upper respiratory tract. Upon polyclonal stimulation, these cells produced more galactose-deficient IgA1 than did cells from healthy controls. Unexpectedly, in healthy African Americans, EBV was detected preferentially in surface IgM- and IgD-positive cells. Importantly, most African Blacks and African Americans acquire EBV within 2 years of birth. At that time, the IgA system is naturally deficient, manifested as low serum IgA levels and few IgA-producing cells. Consequently, EBV infects cells secreting immunoglobulins other than IgA. Our novel data implicate Epstein-Barr virus infected IgA⁺ cells as the source of galactose-deficient IgA1 and basis for expression of relevant homing receptors. Moreover, the temporal sequence of racial-specific differences in Epstein-Barr virus infection as related to the naturally delayed maturation of the IgA system explains the racial disparity in the prevalence of IgAN.

A New Vision of IgA Nephropathy: The Missing Link

IgA Nephropathy (IgAN) is a primary glomerulonephritis problem worldwide that develops mainly in the 2nd and 3rd decade of life and reaches end-stage kidney disease after 20 years from the biopsy-proven diagnosis, implying a great socio-economic burden. IgAN may occur in a sporadic or familial form. Studies on familial IgAN have shown that 66% of asymptomatic relatives carry immunological defects such as high IgA serum levels, abnormal spontaneous in vitro production of IgA from peripheral blood mononuclear cells (PBMCs), high serum levels of aberrantly glycosylated IgA1, and an altered PBMC cytokine production profile. Recent findings led us to focus our attention on a new perspective to study the pathogenesis of this disease, and new studies showed the involvement of factors driven by environment, lifestyle or diet that could affect the disease. In this review, we describe the results of studies carried out in IgAN patients derived from genomic and epigenomic studies. Moreover, we discuss the role of the microbiome in the disease. Finally, we suggest a new vision to consider IgA Nephropathy as a disease that is not disconnected from the environment in which we live but influenced, in addition to the genetic background, also by other environmental and behavioral factors that could be useful for developing precision nephrology and personalized therapy.

Location of glomerular immune deposits, not codeposition of immunoglobulin G, influences definitive renal outcomes in immunoglobulin A nephropathy

Background

It has been suggested that the prognosis of immunoglobulin (IgA) nephropathy (IgAN) is adversely affected if there is codeposition of IgG in the glomeruli or if immune deposits are present in the glomerular capillary walls. We sought to understand how these variables affect clinical outcome.

Methods

A total of 80 IgAN biopsies were retrospectively divided into groups: (i) IgA without IgG deposition versus IgA + IgG and (ii) immune deposits restricted to the mesangium versus mesangium and peripheral capillary walls (PCWs). The association of these groups with the composite primary outcome of renal replacement therapy, renal transplant, death or doubling of serum creatinine (SCr) concentration was determined. The change in estimated glomerular filtration rate (eGFR) was also assessed. Covariates examined were age, sex, race, SCr and proteinuria level at biopsy and at follow-up, duration of follow-up, treatment, Oxford score and presence of crescents.

Results

IgG codeposition showed a trend toward endocapillary hypercellularity (P = 0.082); there were no other baseline differences between the IgA (n = 55) and IgA + IgG (n = 25) groups. At a median follow-up time of 29 months, the combined primary outcome was reached in 24 patients, 16 with IgA and 8 with IgA + IgG (P = 0.82). Patients with immune deposits in the PCWs (n = 21) presented with higher baseline proteinuria than those with deposits limited to the mesangium (n = 59; P = 0.025), were more likely to have crescents/segmental glomerular necrosis on biopsy (P = 0.047) and were more likely to reach the combined primary outcome (P = 0.026). Biopsies with crescents/segmental glomerular necrosis were associated with endocapillary hypercellularity (P < 0.001).

Conclusions

In this multicenter IgAN cohort, IgG co-deposition and the location of glomerular immune deposits in the PCWs were both associated with greater histologic activity on renal biopsy, but only the location of glomerular immune deposits in the PCWs was associated with a significantly increased risk for end-stage renal disease, transplant, death and/or doubling of SCr.

Serum Levels of Joining Chain-Containing IgA1 Are Not Elevated in Patients with IgA Nephropathy

Background

It has been suggested that mesangial IgA deposits are dimeric or polymeric in IgA nephropathy (IgAN). However, evidence concerning the molecular form of serum IgA in IgAN is controversial. And there is no direct evidence that the serum levels of joining chain- (J chain-) containing IgA (J-IgA) are elevated in IgAN. In this study, we aimed to measure serum J-IgA and glomerular J chain deposition with anti-J chain monoclonal antibody in IgAN.

Methods

BALB/c mice were immunized with human J chain-GST recombinant peptide to obtain anti-J chain monoclonal antibody. The levels of serum total IgA and J-IgA were measured by sandwich enzyme-linked immunosorbent assay in 115 patients with IgAN and 117 healthy volunteers. J chain deposition in kidney specimens was analyzed by immunohistochemistry staining.

Results

Serum levels of total IgA1 were elevated in IgAN patients compared to healthy subjects. However, serum levels of IgA, J-IgA, and J chain-containing IgA1 (J-IgA1), the J-IgA to total IgA ratio, and the J-IgA1 to total IgA1 ratio were not significantly different between IgAN patients and healthy subjects. Western blot analysis and gel filtration analysis using purified IgA1 also showed that the proportion of J chain-containing polymeric IgA1 was lower in IgAN patients compared to healthy subjects. No correlation was found between serum J-IgA or J-IgA1 and clinical features in IgAN. Immunohistochemistry analysis showed that glomerular J chain was positive in 12 IgAN patients (57.1%). The values of the J-IgA to IgA ratio and J-IgA1 to IgA ratio were significantly higher in IgAN patients with glomerular J chain deposition than those without. However, the serum levels of J-IgA and J-IgA1 and the J-IgA1 to IgA1 ratio were not significantly higher in two subgroups.

Conclusions

Although serum levels of total IgA1 were elevated in IgAN, the serum levels of J-IgA1 were not elevated. And serum J-IgA, serum J-IgA1, and J chain deposition were not correlated with disease severity in IgAN.

The role of complement in IgA nephropathy

IgA nephropathy (IgAN) is common and often progresses to end stage renal disease. IgAN encompasses a wide range of histology and clinical features. IgAN pathogenesis is incompletely understood; the current multi-hit hypothesis of IgAN pathogenesis does not explain the range of glomerular inflammation and renal injury associated with mesangial IgA deposition. Although associations between IgAN and glomerular and circulating markers of complement activation are established, the mechanism of complement activation and contribution to glomerular inflammation and injury are not defined. Recent identification of specific complement pathways and proteins in severe IgAN cases had advanced our understanding of complement in IgAN pathogenesis. In particular, a growing body of evidence implicates the complement factor H related proteins 1 and 5 and lectin pathway as pathogenic in a subset of patients with severe disease. These data suggest complement deregulation and activity may be dominant drivers of renal injury in IgAN. Thereby, markers of complement activation may identify IgAN patients likely to progress to significant renal impairment and complement inhibition may emerge as an effective method of preventing and reducing glomerular injury in IgAN.

IgA1 hinge-region clustered glycan fidelity is established early during semi-ordered glycosylation by GalNAc-T2

GalNAc-type O-glycans are often added to proteins post-translationally in a clustered manner in repeat regions of proteins, such as mucins and IgA1. Observed IgA1 glycosylation patterns show that glycans occur at similar sites with similar structures. It is not clear how the sites and number of glycans added to IgA1, or other proteins, can follow a conservative process. GalNAc-transferases initiate GalNAc-type glycosylation. In IgA nephropathy, an autoimmune disease, the sites and O-glycan structures of IgA1 hinge-region are altered, giving rise to a glycan autoantigen. To better understand how GalNAc-transferases determine sites and densities of clustered O-glycans, we used IgA1 hinge-region (HR) segment as a probe. Using LC-MS, we demonstrated a semi-ordered process of glycosylation by GalNAc-T2 towards the IgA1 HR. The catalytic domain was responsible for selection of four initial sites based on amino-acid sequence recognition. Both catalytic and lectin domains were involved in multiple second site-selections, each dependent on initial site-selection. Our data demonstrated that multiple start-sites and follow-up pathways were key to increasing the number of glycans added. The lectin domain predominately enhanced IgA1 HR glycan density by increasing synthesis pathway exploration by GalNAc-T2. Our data indicated a link between site-specific glycan addition and clustered glycan density that defines a mechanism of how conserved clustered O-glycosylation patterns and glycoform populations of IgA1 can be controlled by GalNAc-T2. Together, these findings characterized a correlation between glycosylation pathway diversity and glycosylation density, revealing mechanisms by which a single GalNAc-T isozyme can limit and define glycan heterogeneity in a disease-relevant context.

Modulation of the microbiota by oral antibiotics treats immunoglobulin A nephropathy in humanized mice

BACKGROUND

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. IgA is mainly produced by the gut-associated lymphoid tissue (GALT). Both experimental and clinical data suggest a role of the gut microbiota in this disease. We aimed to determine if an intervention targeting the gut microbiota could impact the development of disease in a humanized mouse model of IgAN, the α1KI-CD89Tg mice.

METHODS

Four- and 12-week old mice were divided into two groups to receive either antibiotics or vehicle control. Faecal bacterial load and proteinuria were quantified both at the beginning and at the end of the experiment, when blood, kidneys and intestinal tissue were collected. Serum mouse immunoglobulin G (mIgG) and human immunoglobulin A1 (hIgA1)-containing complexes were quantified. Renal and intestinal tissue were analysed by optical microscopy after haematoxylin and eosin colouration and immunohistochemistry with anti-hIgA and anti-mouse CD11b antibodies.

RESULTS

Antibiotic treatment efficiently depleted the faecal microbiota, impaired GALT architecture and impacted mouse IgA production. However, while hIgA1 and mIgG serum levels were unchanged, the antibiotic treatment markedly prevented hIgA1 mesangial deposition, glomerular inflammation and the development of proteinuria. This was associated with a significant decrease in circulating hIgA1-mIgG complexes. Notably, final faecal bacterial load strongly correlated with critical clinical and pathophysiological features of IgAN such as proteinuria and hIgA1-mIgG complexes. In addition, treatment with broad-spectrum antibiotics reverted established disease.

CONCLUSIONS

These data support an essential role of the gut microbiota in the generation of mucosa-derived nephrotoxic IgA1 and in IgAN development, opening new avenues for therapeutic approaches in this disease.

Mesangial Deposition Can Strongly Involve Innate-Like IgA Molecules Lacking Affinity Maturation

BACKGROUND

IgA nephropathy (IgAN) often follows infections and features IgA mesangial deposition. Polymeric IgA deposits in the mesangium seem to have varied pathogenic potential, but understanding their pathogenicity remains a challenge. Most mesangial IgA1 in human IgAN has a hypogalactosylated hinge region, but it is unclear whether this is required for IgA deposition. Another important question is the role of adaptive IgA responses and high-affinity mature IgA antibodies and whether low-affinity IgA produced by innate-like B cells might also yield mesangial deposits.

METHODS

To explore the effects of specific qualitative variations in IgA and whether altered affinity maturation can influence IgA mesangial deposition and activate complement, we used several transgenic human IgA1-producing models with IgA deposition, including one lacking the DNA-editing enzyme activation-induced cytidine deaminase (AID), which is required in affinity maturation. Also, to explore the potential role of the IgA receptor CD89 in glomerular inflammation, we used a model that expresses CD89 in a pattern observed in humans.

RESULTS

We found that human IgA induced glomerular damage independent of CD89. When comparing mice able to produce high-affinity IgA antibodies with mice lacking AID-enabled Ig affinity maturation, we found that IgA deposition and complement activation significantly increased and led to IgAN pathogenesis, although without significant proteinuria or hematuria. We also observed that hinge hypoglycosylation was not mandatory for IgA deposition.

CONCLUSIONS

In a mouse model of IgAN, compared with high-affinity IgA, low-affinity innate-like IgA, formed in the absence of normal antigen-driven maturation, was more readily involved in IgA glomerular deposition with pathogenic effects.

Association between anti-α-1,4-D-polygalacturonic acid antibodies and Henoch-Schönlein purpura in children

Objective

To investigate the relationship between anti-α-1,4-D-polygalacturonic acid (PGA) antibodies, particularly immunoglobulin (Ig)A, and Henoch-Schönlein purpura (HSP) in children.

Methods

This observational case–control study investigated PGA-IgA, PGA-IgG, and PGA/PGA-IgA circulating immune complex (PGA/PGA-IgA CIC) in paediatric patients with HSP versus controls. Children with HSP were also evaluated for food specific IgG and food intolerance. Between-group differences in anti-PGA antibodies were analysed.

Results

Serum PGA-IgA and PGA-IgG levels were significantly increased in patients with acute HSP (n = 251) versus those with urticaria (n = 48), acute respiratory infections (n = 95), surgical controls (n = 53) and neonates (n = 92). PGA/PGA-IgA CIC levels were also significantly higher in the acute HSP group versus surgical control and neonate groups. Levels of PGA/PGA-IgA CIC and PGA-IgA were significantly correlated (r = 0.997), and PGA-IgA showed high diagnostic specificity for HSP. No statistically significant differences were observed in PGA-IgA and PGA-IgG between various degrees of food intolerance in children with HSP.

Conclusion

Increased anti-PGA antibodies, particularly PGA-IgA and PGA/PGA-IgA CIC, were significantly associated with acute HSP in children. Food intolerance was not found to be associated with increased anti-PGA antibodies in children with HSP.

Murine Models of Human IgA Nephropathy

IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis in the world. IgAN is characterized by mesangial deposits of IgA1-containing immune complexes. IgA1 usually co-deposits with complement C3 and variable IgG and/or IgM. Exactly 50 years have passed since IgAN was described, however, the pathogenesis of disease onset and progression have not been fully clarified. Animal models can re-create the complex immunologic microenvironments that foster human autoimmunity and nephritis and provide access to tissue compartments not readily examined in patients. Thus, multiple models that may be helpful in studies of specific aspects of IgAN have been developed. A unique spontaneous animal model of IgAN, the ddY mouse, was reported in 1985. These mice show mild proteinuria and glomerular IgA deposits, with a highly variable incidence and degree of glomerular injury owing to a heterogeneous genetic background. Thus, we intercrossed an early onset group of ddY mice in which the development of IgAN resulted in the establishment of a novel 100% early onset-grouped ddY mouse model with increased levels of aberrantly glycosylated IgA and immune complexes. Although the molecular features of human IgA1 are different from rodent IgA, human IgA1 knock-in (α1KI)-CD89 transgenic mice, which express both human IgA1 and CD89, show circulating and mesangial deposits of IgA1-soluble CD89 complexes that result in kidney inflammation, hematuria, and proteinuria. In this review, we introduce several murine models of IgAN that can be useful tools for the analysis of multiple aspects of the pathogenesis of IgAN, which may aid in the assessment of approaches for the treatment of IgAN.

Biomarkers and Precision Medicine in IgA Nephropathy

The field of biomarker research in IgA nephropathy has experienced a major boost in recent years with the publication of a large number of scientific reports. Candidate biomarkers from blood, urine, and renal tissue obtained through the use of clinical chemistry, molecular biology, and omics have been proposed for translation in clinical practice. Nevertheless, individual biomarkers often lack sensitivity and specificity with the consequent impairment of disease specificity. This review, moving on from the analysis of the four-hit hypothesis, illustrates the biomarkers linked to the abnormal glycosylation process of IgA1 and the immune complex formation. It also describes other serum and urinary biomarkers. Given the profound insights into the pleiotropic function of a single biomarker that is specific for a pathophysiological mechanism, this review suggests a novel approach based on a panel of biomarkers that covers the entire pathogenic process of the disease. Clinical bioinformatics that integrate genetic, clinical, and bioinformatics data sets could optimize the specific value of each biomarker in a multimarker panel. This is a promising approach for precision medicine and personalized therapy in IgA nephropathy.

Genetic Determinants of IgA Nephropathy: Western Perspective

IgA nephropathy (IgAN) represents a genetically complex multifactorial trait. Its prevalence and clinical features vary geographically, and the disease has a range of clinical presentations that suggest multiple subtypes. Although familial aggregation of IgAN has been reported and prior linkage studies have highlighted significant locus heterogeneity, specific genetic variants underlying familial IgAN have not yet been defined. Population-based genome-wide association studies (GWAS) have discovered nearly 20 IgAN risk loci, providing novel insights into disease epidemiology and molecular mechanisms, shifting old paradigms of the disease pathogenesis. Follow-up fine-mapping studies have identified specific causal variants, and genotype-phenotype correlation studies have begun to delineate clinical consequences of GWAS risk alleles. The association between IgAN and galactose-deficient IgA1 (Gd-IgA1), a validated serum biomarker of IgAN, presented another avenue for genetic discovery because elevated serum levels of Gd-IgA1 are highly heritable. Recent GWAS for serum Gd-IgA1 levels provided novel insights into genetic regulation of this trait, but the genetic link between Gd-IgA1 and IgAN has not yet been established. In this review, we discuss these developments in the broader context of modern genetic approaches to complex traits, and provide our perspective on the critical challenges that need to be addressed to advance the field.