IgA nephropathy: clearance kinetics of IgA-containing immune complexes

Downregulation of PPARα mediates FABP1 expression, contributing to IgA nephropathy by stimulating ferroptosis in human mesangial cells

Immunoglobulin A nephropathy (IgAN) is the commonest primary glomerulonephritis, and a major cause of end-stage renal disease; however, its pathogenesis requires elucidation. Here, a hub gene, FABP1, and signaling pathway, PPARα, were selected as key in IgAN pathogenesis by combined weighted gene correlation network analysis of clinical traits and identification of differentially expressed genes from three datasets. FABP1 and PPARα levels were lower in IgAN than control kidney, and linearly positively correlated with one another, while FABP1 levels were negatively correlated with urinary albumin-to-creatinine ratio, and GPX4 levels were significantly decreased in IgAN. In human mesangial cells (HMCs), PPARα and FABP1 levels were significantly decreased after Gd-IgA1 stimulation and mitochondria appeared structurally damaged, while reactive oxygen species (ROS) and malondialdehyde (MDA) were significantly increased, and glutathione and GPX4 decreased, relative to controls. GPX4 levels were decreased, and those of ACSL4 increased on siPPARα and siFABP1 siRNA treatment. In PPARα lentivirus-transfected HMCs stimulated by Gd-IgA1, ROS, MDA, and ACSL4 were decreased; glutathione and GPX4, and immunofluorescence colocalization of PPARα and GPX4, increased; and damaged mitochondria reduced. Hence, PPARα pathway downregulation can reduce FABP1 expression, affecting GPX4 and ACSL4 levels, causing HMC ferroptosis, and contributing to IgAN pathogenesis.

Serological Measurement of Poly-IgA Immune Complex Levels in IgA Nephropathy and IgA Vasculitis

Both IgA nephropathy and IgA vasculitis, formerly known as Henoch-Schӧnlein purpura, are immune deposition diseases. IgA nephropathy is caused by the deposition of aberrantly formed poly-IgA complexes from blood circulation to the kidney glomerulus; IgA vasculitis is characterized by IgA-dominant immune deposits to small vessels of the skin and other organs, including the kidney. Therefore, measuring the disease-causing poly-IgA contents in the plasma is needed to study these conditions. However, while clinical tests for the level of total plasma IgA are routinely performed, methods for specific detection of poly-IgA contents are unavailable in clinical medicine. In this protocol, we describe a practical solution for measuring poly-IgA in patient samples. The new method is based on the biological selectivity of IgA Fcα receptor I (FcαRI/CD89) toward poly-IgA species, in contrast to its relatively low affinity for normal monomeric IgA. By devising recombinant CD89 ectodomain as the "capturing" probe, we validated the feasibility of the assay for measuring plasma poly-IgA levels in a 96-well format. The methodology was able to differentiate plasma samples of IgA nephropathy, or related IgA vasculitis, from those of other autoimmune kidney disease types or from healthy controls. Moreover, the measured poly-IgA indices not only correlated with the severity of IgA nephropathy, but the levels also trended lower following corticosteroid or immunosuppressant treatments of patients. Therefore, we anticipate the new assay will provide useful measurements of the IgA nephropathy disease activity index for stratifying disease severity or for evaluating treatment response. Graphical abstract.

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 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.

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.

IgA and FcαRI: Versatile Players in Homeostasis, Infection, and Autoimmunity

Mucosal surfaces constitute the frontiers of the body and are the biggest barriers of our body for the outside world. Immunoglobulin A (IgA) is the most abundant antibody class present at these sites. It passively contributes to mucosal homeostasis via immune exclusion maintaining a tight balance between tolerating commensals and providing protection against pathogens. Once pathogens have succeeded in invading the epithelial barriers, IgA has an active role in host-pathogen defense by activating myeloid cells through divers receptors, including its Fc receptor, FcαRI (CD89). To evade elimination, several pathogens secrete proteins that interfere with either IgA neutralization or FcαRI-mediated immune responses, emphasizing the importance of IgA-FcαRI interactions in preventing infection. Depending on the IgA form, either anti- or pro-inflammatory responses can be induced. Moreover, the presence of excessive IgA immune complexes can result in continuous FcαRI-mediated activation of myeloid cells, potentially leading to severe tissue damage. On the one hand, enhancing pathogen-specific mucosal and systemic IgA by vaccination may increase protective immunity against infectious diseases. On the other hand, interfering with the IgA-FcαRI axis by monovalent targeting or blocking FcαRI may resolve IgA-induced inflammation and tissue damage. This review describes the multifaceted role of FcαRI as immune regulator between anti- and pro-inflammatory responses of IgA, and addresses potential novel therapeutic strategies that target FcαRI in disease.

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Identification of a novel interplay between intestinal bacteria and metabolites in Chinese patients with IgA nephropathy via integrated microbiome and metabolome approaches

Background

Immunoglobulin A nephropathy (IgAN) is the most common form of primary glomerulonephritis. The intestinal microbial ecosystem and metabolic network of IgAN have not been systematically analyzed. The present study aims to improve understanding of the gut microbiota and its metabolic capabilities to facilitate the development of diagnostic, therapeutic, and prognostic methods for IgAN.

Methods

We characterized the gut microbiota and metabolic patterns of fecal and serum samples of IgAN patients and healthy controls from the south of China using 16s ribosomal RNA gene sequencing and liquid chromatography-tandem mass spectrometry, respectively, and bioinformatics approaches.

Results

We found that the relative abundances of Streptococcus and Enterococcus were higher in IgAN patients, whereas Bacteroidetes and Bacteroides were lower. Changes in the gut microbiota of IgAN affected the metabolism and absorbance of microbiota-associated metabolites, in particular polyunsaturated fatty acids, free amino acid, and oligopeptides, and activated the phenylalanine metabolism pathway, thereby constructing a unique metabolic system of IgAN. We identified six pivotal metabolites, including bilirubin, trimethoprim, stearamide, phenylalanine, cis-9,10-epoxystearic acid, and phosphatidylethanolamine 17:0, that connected the metabolic networks of the gut and blood. Additionally, 5-hydroxyeicosatetraenoic acid and 5-hydroxy-6E,8Z,11Z-eicosatrienoic acid were shown to be associated with the classification of glomerular sclerosis.

Conclusions

We establish a relational network between microbiota, fecal metabolites, and serum metabolites in IgAN. The core microbiota and metabolites identified have promising value in therapeutic applications.

Perspectives on how mucosal immune responses, infections and gut microbiome shape IgA nephropathy and future therapies

Infections have been considered to play a critical role in the pathogenesis of IgA nephropathy (IgAN) because synpharyngitic hematuria is a common feature in IgAN. However, how infections participate in this process is still debated. More recent studies have also revealed that the alteration of the gut microbiome exerts a profound effect on host immune responses, contributing to the etiology or progression of autoimmunity. Considering IgA as the first line of defense against bacterial and viral antigens, this review evaluates the relationships among intestinal infections, gut microbiome, and IgA for a better understanding of the pathogenesis of IgAN. Moreover, as a prototype of IgA immunity, we provide detailed clarification of IgAN pathogenesis to shed light on other diseases in which IgA plays a role. Finally, we discuss potential therapies focusing on microbes and mucosal immune responses in IgAN.

IgA Nephropathy Concomitant With Karyomegalic Interstitial Nephritis

Immunoglobulin A (IgA) nephropathy is one of the most common glomerulonephritis characterized by the deposition of IgA in glomerular mesangium. Karyomegalic interstitial nephritis (KIN) is a rare interstitial nephritis with potential hereditary factors. IgA nephropathy concomitant with KIN has not yet been reported. Herein, we describe the clinical course, ultrasonic images and gastrointestinal endoscopy findings of a 28-year-old-male patient with IgA nephropathy with KIN. The pathologic examination of the renal biopsy specimen demonstrated mild mesangial proliferative IgA nephropathy with KIN. Molecular genetic testing detected an abnormality in FAN1 gene. The heterozygous mutation was present on chromosome 15q13.3. However, IgA nephropathy with KIN is a rare disorder, and its pathogenesis is yet to be clarified.

Serum Soluble CD89-IgA Complexes Are Elevated in IgA Nephropathy without Immunosuppressant History

Purpose

CD89 (FcαRI), the receptor of IgA, can shed from cells to form complexes with IgA in serum and is supposed to participate in the pathogenesis of IgA nephropathy (IgAN). There are contradictory results on their utility in clinical practice. This study is aimed at investigating whether sCD89-IgA complexes can help in the diagnosis or evaluation of the disease.

Methods

A sandwich ELISA was established using anti-CD89 as a capture antibody and HRP-conjugated anti-IgA as a detection antibody. This method was used to measure serum levels of sCD89-IgA complexes in IgAN patients without immunosuppressant history and healthy subjects. Correlations between serum levels of sCD89-IgA complexes and disease severity were analyzed.

Results

Serum sCD89-IgA complexes increased with age (P < 0.001). IgAN patients had higher sCD89-IgA complex levels compared with age- and gender-matched normal healthy individuals (P < 0.001). IgAN patients had higher sCD89-IgA complex levels compared with age- and gender-matched normal healthy individuals (P < 0.001). IgAN patients had higher sCD89-IgA complex levels compared with age- and gender-matched normal healthy individuals (

Conclusions

Serum sCD89-IgA complexes can guide diagnosis of IgAN in patients without immunosuppressant history, but provide limited help in clinicopathologic prediction.

Triptolide inhibits tonsillar IgA production by upregulating FDC-SP in IgA nephropathy

IgA nephropathy (IgAN) is primarily resulted of qualitative abnormality of IgA. The occurrence of IgAN is associated with affected tonsils which enhances the IgA production via IgA class switching and immuno-activation. Follicular dendritic cell-secreted protein (FDC-SP) was found to be a negative effect for IgA production in tonsil. The previous studies suggested that Triptolide might reduce IgA production by its immunosuppression role. Given this background, this study investigated the mechanisms underlying the role of Triptolide and FDC-SP in the generation of IgA and IgA class switching in tonsil of IgAN patients. Immunohistochemistry and reverse transcription-polymerase chain reaction revealed that the expression of FDC-SP was increased in the tonsils of IgAN patients with Triptolide treatment compared with those without treatment. Meanwhile, the expression of FDC-SP was negatively correlated with IgA inducing cytokines in the tonsils of IgAN patients treated with Triptolide, due to the significant decreased IgA-bearing cells. The expression of FDC-SP in tonsillar tissue was confirmed by double immunofluorescence. Importantly, Triptolide promoted FDC-SP secretion, and correlated negatively with decreased IgA production in isolated FDC-associated clusters, which had been isolated from patients without TW treatment previously. Our study demonstrated that Triptolide might have an impact on FDC-SP production and downregulation of IgA synthesis in the tonsils of IgAN patients, which could be a promising strategy for therapeutic intervention in IgAN patients.

IgA and FcαRI: Pathological Roles and Therapeutic Opportunities

Immunoglobulin A (IgA) is the most abundant antibody class present at mucosal surfaces. The production of IgA exceeds the production of all other antibodies combined, supporting its prominent role in host-pathogen defense. IgA closely interacts with the intestinal microbiota to enhance its diversity, and IgA has a passive protective role via immune exclusion. Additionally, inhibitory ITAMi signaling via the IgA Fc receptor (FcαRI; CD89) by monomeric IgA may play a role in maintaining homeostatic conditions. By contrast, IgA immune complexes (e.g., opsonized pathogens) potently activate immune cells via cross-linking FcαRI, thereby inducing pro-inflammatory responses resulting in elimination of pathogens. The importance of IgA in removal of pathogens is emphasized by the fact that several pathogens developed mechanisms to break down IgA or evade FcαRI-mediated activation of immune cells. Augmented or aberrant presence of IgA immune complexes can result in excessive neutrophil activation, potentially leading to severe tissue damage in multiple inflammatory, or autoimmune diseases. Influencing IgA or FcαRI-mediated functions therefore provides several therapeutic possibilities. On the one hand (passive) IgA vaccination strategies can be developed for protection against infections. Furthermore, IgA monoclonal antibodies that are directed against tumor antigens may be effective as cancer treatment. On the other hand, induction of ITAMi signaling via FcαRI may reduce allergy or inflammation, whereas blocking FcαRI with monoclonal antibodies, or peptides may resolve IgA-induced tissue damage. In this review both (patho)physiological roles as well as therapeutic possibilities of the IgA-FcαRI axis are addressed.