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

Sinomenine Hydrochloride Protects IgA Nephropathy Through Regulating Cell Growth and Apoptosis of T and B Lymphocytes

Purpose

Sinomenine hydrochloride (SH) is used to treat chronic inflammatory diseases such as rheumatoid arthritis and may also be efficacious against Immunoglobulin A nephropathy (IgAN). However, no trial has investigated the molecular mechanism of SH on IgAN. Therefore, this study aims to investigate the effect and mechanism of SH on IgAN.

Methods

The pathological changes and IgA and C3 depositions in the kidney of an IgAN rat model were detected by periodic acid-Schiff (PAS) and direct immunofluorescence staining. After extracting T and B cells using immunomagnetic beads, we assessed their purity, cell cycle phase, and apoptosis stage through flow cytometry. Furthermore, we quantified cell cycle-related and apoptosis-associated proteins by Western blotting.

Results

SH reduced IgA and C3 depositions in stage 4 IgAN, thereby decreasing inflammatory cellular infiltration and mesangial injury in an IgAN model induced using heteroproteins. Furthermore, SH arrested the cell cycle of lymphocytes T and B from the spleen of IgAN rats. Regarding the mechanism, our results demonstrated that SH regulated the Cyclin D1 and Cyclin E1 protein levels for arresting the cell cycle and it also regulated Bax and Bcl-2 protein levels, thus increasing Cleaved caspase-3 protein levels in Jurkat T and Ramos B cells.

Conclusion

SH exerts a dual regulation on the cell cycle and apoptosis of T and B cells by controlling cell cycle-related and apoptosis-associated proteins; it also reduces inflammatory cellular infiltration and mesangial proliferation. These are the major mechanisms of SH in IgAN.

Advances in Mass Spectrometry-Based Blood Metabolomics Profiling for Non-Cancer Diseases: A Comprehensive Review

Blood metabolomics profiling using mass spectrometry has emerged as a powerful approach for investigating non-cancer diseases and understanding their underlying metabolic alterations. Blood, as a readily accessible physiological fluid, contains a diverse repertoire of metabolites derived from various physiological systems. Mass spectrometry offers a universal and precise analytical platform for the comprehensive analysis of blood metabolites, encompassing proteins, lipids, peptides, glycans, and immunoglobulins. In this comprehensive review, we present an overview of the research landscape in mass spectrometry-based blood metabolomics profiling. While the field of metabolomics research is primarily focused on cancer, this review specifically highlights studies related to non-cancer diseases, aiming to bring attention to valuable research that often remains overshadowed. Employing natural language processing methods, we processed 507 articles to provide insights into the application of metabolomic studies for specific diseases and physiological systems. The review encompasses a wide range of non-cancer diseases, with emphasis on cardiovascular disease, reproductive disease, diabetes, inflammation, and immunodeficiency states. By analyzing blood samples, researchers gain valuable insights into the metabolic perturbations associated with these diseases, potentially leading to the identification of novel biomarkers and the development of personalized therapeutic approaches. Furthermore, we provide a comprehensive overview of various mass spectrometry approaches utilized in blood metabolomics research, including GC-MS, LC-MS, and others discussing their advantages and limitations. To enhance the scope, we propose including recent review articles supporting the applicability of GC×GC-MS for metabolomics-based studies. This addition will contribute to a more exhaustive understanding of the available analytical techniques. The Integration of mass spectrometry-based blood profiling into clinical practice holds promise for improving disease diagnosis, treatment monitoring, and patient outcomes. By unraveling the complex metabolic alterations associated with non-cancer diseases, researchers and healthcare professionals can pave the way for precision medicine and personalized therapeutic interventions. Continuous advancements in mass spectrometry technology and data analysis methods will further enhance the potential of blood metabolomics profiling in non-cancer diseases, facilitating its translation from the laboratory to routine clinical application.

Targeting complement in IgA nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. Recent years have witnessed significant improvements in the understanding of the pathogenesis of IgAN and particularly, the pathogenic role of complement activation. The alternative complement pathway is the major complement cascade activator in IgAN, and glomerular C3 deposition has been shown to correlate with disease progression. In addition, several studies have provided insight into the pathogenic role of factor H-related proteins -1 and -5 in IgAN, as independent players in complement dysregulation. The lectin pathway has also been shown to be associated with the severity of IgAN. Glomerular deposition of C4d has been associated with increased histologic disease activity, faster decline in estimated glomerular filtration rate and higher risk of kidney failure. On the other hand, although overlooked in the Oxford classification, numerous studies have shown that the coexistence of thrombotic microangiopathy in IgAN is a significant indicator of a poorer prognosis. All the breakthroughs in the understanding of the contributing role of complement in IgAN have paved the way for the development of new complement-targeted therapies in this disease. Several ongoing trials are evaluating the efficacy of new agents against factor B (iptacopan, Ionis-FB-LRX), C3 (pegcetacoplan), factor D (vemircopan, pelecopan), C5 (ravulizumab, cemdisiran) and C5a receptor 1 (avacopan). In this study, we provide a comprehensive review of the role of complement in IgAN, including the emerging mechanisms of complement activation and the promising potential of complement inhibitors as a viable treatment option for IgAN.

IgA nephropathy

IgA nephropathy (IgAN), the most prevalent primary glomerulonephritis worldwide, carries a considerable lifetime risk of kidney failure. Clinical manifestations of IgAN vary from asymptomatic with microscopic or intermittent macroscopic haematuria and stable kidney function to rapidly progressive glomerulonephritis. IgAN has been proposed to develop through a 'four-hit' process, commencing with overproduction and increased systemic presence of poorly O-glycosylated galactose-deficient IgA1 (Gd-IgA1), followed by recognition of Gd-IgA1 by antiglycan autoantibodies, aggregation of Gd-IgA1 and formation of polymeric IgA1 immune complexes and, lastly, deposition of these immune complexes in the glomerular mesangium, leading to kidney inflammation and scarring. IgAN can only be diagnosed by kidney biopsy. Extensive, optimized supportive care is the mainstay of therapy for patients with IgAN. For those at high risk of disease progression, the 2021 KDIGO Clinical Practice Guideline suggests considering a 6-month course of systemic corticosteroid therapy; however, the efficacy of systemic steroid treatment is under debate and serious adverse effects are common. Advances in understanding the pathophysiology of IgAN have led to clinical trials of novel targeted therapies with acceptable safety profiles, including SGLT2 inhibitors, endothelin receptor blockers, targeted-release budesonide, B cell proliferation and differentiation inhibitors, as well as blockade of complement components.

Immune abnormalities in IgA nephropathy

Immunoglobulin A (IgA) nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and it is characterized by mesangial IgA deposition. Asymptomatic hematuria with various degrees of proteinuria is the most common clinical presentation and up to 20%-40% of patients develop end-stage kidney disease within 20 years after disease onset. The pathogenesis of IgAN involves four sequential processes known as the "four-hit hypothesis" which starts with the production of a galactose-deficient IgA1 (gd-IgA1), followed by the formation of anti-gd-IgA1 IgG or IgA1 autoantibodies and immune complexes that ultimately deposit in the glomerular mesangium, leading to inflammation and injury. Although several key questions about the production of gd-IgA1 and the formation of anti-gd-IgA1 antibodies remain unanswered, a growing body of evidence is shedding light on the innate and adaptive immune mechanisms involved in this complex pathogenic process. Herein, we will focus on these mechanisms that, along with genetic and environmental factors, are thought to play a key role in disease pathogenesis.

Tetrandrine inhibits the proliferation of mesangial cells induced by enzymatically deglycosylated human IgA1 via IgA receptor/MAPK/NF-κB signaling pathway

Objective: Despite the use of renin-angiotensin system blockade and immunosuppressive drugs, including corticosteroids, the current treatment regimens for Immunoglobulins A nephropathy (IgAN) are severely limited. The proliferation of mesangial cell and deposition of deglycosylated human IgA1 immune complex are the most common pathologic features of IgAN. We examined the tetrandrine potential of suppressing the proliferation of mesangial cells and explored its underlying mechanisms with a focus on IgA receptor/MAPK/NF-κB signaling pathway. Methods: Standard human IgA (native IgA) were enzymatically desialylated (deS IgA) or further degalactosylated (deS/deGal IgA) using neuraminidase and β-galactosidase. Rat glomerular mesangial cells (HBZY-1) and human renal mesangial cells (HRMC) stimulated by IgA were used to observe the suppressive effect of tetrandrine. The MTT assay was used to detect the cell viability. The protein expression of IgA receptor/MAPK/NF-κB signaling pathway was examined by Western blot. Cell cycle analysis was measured by flow cytometer. Results: Native IgA and deS IgA showed limited stimulation effect on both HBZY-1 cells and HRMCs, whereas deS/deGal IgA significantly stimulated the proliferation of both HBZY-1 cells and HRMCs (p < 0.05). Compared with non-stimulation of deS/deGal IgA, 1-3 μM of tetrandrine had stronger inhibitory effect on the proliferation of HBZY-1 cells and HRMCs with the stimulation of deS/deGal IgA (p < 0.05), suggesting that tetrandrine possibly inhibited the proliferation of mesangial cells induced by deglycosylated human IgA1 specifically. Molecular mechanism study revealed that tetrandrine decreased the expression of IgA1 receptor, CD71 and β4GALT1, and inhibited the activation of MAPK/NF-κB significantly (p < 0.05). Moreover, these inhibitory effect of tetrandrine caused cell cycle arrest and stopped the cell growth in the S phase companied with the upregulating of cyclin A2 and downregulating of cyclin D1. Conclusion: Taken together, tetrandrine inhibited the proliferation of mesangial cells induced by enzymatically deglycosylated human IgA1 via IgA receptor/MAPK/NF-κB signaling pathway. Based on these potential molecular mechanisms, tetrandrine would be an appealing therapeutic option for IgAN.

Role of abnormal glycosylated IgA1 and interstitial transformation of glomerular endothelial cells in the development and progression of IgA nephropathy

BACKGROUND

IgA nephropathy (IgAN) is a common primary renal disease in childhood.

METHODS

Twenty blood samples and renal tissue from patients with IgAN, 20 blood samples from healthy children and 10 normal renal tissue were collected. Serum Gd-IgA1 and renal Gd-IgA1, CD31, α-SMA and vimentin were measured.

RESULTS

The serum Gd-IgA1 concentration in the IgAN group was significantly higher. Gd-IgA1 was not expressed in normal kidneys, which was positive in the IgAN group. Gd-IgA1 levels in serum and renal tissue were not related. The expression of CD31 decreased significantly in IgAN group, while the expression of α-SMA and vimentin increased significantly. There was no significant correlation between the renal concentration of Gd-IgA1 and CD31, α-SMA and vimentin.

CONCLUSION

The increased Gd-IgA1 in the serum and kidney may promote the pathogenesis of IgAN. The serum Gd-IgA1 cannot predict the extent of its deposition in the kidney. Endothelial mesenchymal transition (EndMT) may be involved in the pathogenesis of renal fibrosis in IgAN.

Mechanism of Yiqi Yangying Heluo Formula in the Treatment of IgA Nephropathy by Affecting Gd-IgA1 Based on BAFF Molecular Level and T Lymphocyte Immunity

Background

Galactose-deficient IgA1 (Gd-IgA1) is a critical initiating factor in the pathogenesis of IgA nephropathy (IgAN), which plays an important role in the diagnosis and evaluation of this disease. Moreover, the whole pathogenesis process has an intimate association with the immune response of T and B lymphocytes and their inflammatory factors. There is no specific therapy for IgAN at present. Yiqi Yangyin Formula can significantly reduce urinary protein and hematuria in patients with IgAN. Yiqi Yangying Heluo Formula (YYHF) is optimized on the basis of the above prescription, but its specific mechanism remains to be further studied.

Methods

The effect of YYHF on urinary protein and urinary red blood cell count in patients with IgAN was observed by a self-controlled clinical study before and after treatment. On this basis, flow cytometry was used to detect the proportion of T lymphocyte subsets in peripheral blood of patients with IgAN before and after treatment and healthy controls. Meanwhile, the levels of Gd-IgA1, B cell activation factor (BAFF), and their cytokines (IL-4, IL-6, and IL-17) in peripheral blood were detected by enzyme-linked immunosorbent assay. The changes in mechanism-related indicators of the two groups were observed and subject to correlation analysis.

Results

(1) Compared with the levels before treatment, 24-hour urinary protein content decreased by 47.7% and urinary red blood cell number decreased by 67% in patients with IgAN intervened by YYHF after 48 weeks of follow-up. (2) Compared with the healthy control group, patients with IgAN showed a significantly increased proportion of Th1 cells, Th17 cells, Th1/Th2, Th1/Treg, Th2/Treg, and Th17/Treg, obviously reduced proportion of Th2 cells and Treg cells, and evidently elevated levels of Gd-IgA1, BAFF, and their cytokines (IL-4, IL-6, and IL-17) in the peripheral blood. (3) Following 48 weeks of follow-up after intervention treatment with YYHF, the levels of Gd-IgA1, BAFF, IL-6, and IL-17 were significantly lower, but the level of IL-4 was higher in peripheral blood of patients with IgAN than those before treatment and after 24 weeks of treatment; simultaneously, the proportion of Th1 cells, Th17 cells, Th1/Th2, Th1/Treg, Th2/Treg, and Th17/Treg decreased while that of Th2 cells and Treg cells increased after 48 weeks of follow-up compared with that before treatment in peripheral blood of patients with IgAN. (4) The results of correlation analysis revealed that the level of Gd-IgA1 in peripheral blood of patients with IgAN was positively correlated with the level of BAFF, as well as the proportion of Th1 cells, Th17 cells, Th1/Th2, IL-6, and IL-17 levels, and negatively correlated with the proportion of Treg cells. In addition, the level of Gd-IgA1 in peripheral blood was positively correlated with proteinuria, yet without correlation with hematuria.

Conclusion

YYHF can reduce the quantitative level of 24 h urinary protein and urinary red blood cell count in patients with IgAN. Patients with IgAN have obvious T cell immune imbalance. YYHF can significantly reduce the level of Gd-IgA1 in patients with IgAN, and its mechanism may be explained by the reduced level of BAFF in peripheral blood and improved immune balance of T cells.

IgA Nephropathy: Pleiotropic impact of Epstein-Barr virus infection on immunopathogenesis and racial incidence of the disease

IgA nephropathy (IgAN) is an autoimmune disease in which poorly galactosylated IgA1 is the antigen recognized by naturally occurring anti-glycan antibodies, leading to formation of nephritogenic circulating immune complexes. Incidence of IgAN displays geographical and racial disparity: common in Europe, North America, Australia, and east Asia, uncommon in African Americans, many Asian and South American countries, Australian Aborigines, and rare in central Africa. In analyses of sera and cells from White IgAN patients, healthy controls, and African Americans, IgAN patients exhibited substantial enrichment for IgA-expressing B cells infected with Epstein-Barr virus (EBV), leading to enhanced production of poorly galactosylated IgA1. Disparities in incidence of IgAN may reflect a previously disregarded difference in the maturation of the IgA system as related to the timing of EBV infection. Compared with populations with higher incidences of IgAN, African Americans, African Blacks, and Australian Aborigines are more frequently infected with EBV during the first 1-2 years of life at the time of naturally occurring IgA deficiency when IgA cells are less numerous than in late childhood or adolescence. Therefore, in very young children EBV enters "non-IgA" cells. Ensuing immune responses prevent infection of IgA B cells during later exposure to EBV at older ages. Our data implicate EBV-infected cells as the source of poorly galactosylated IgA1 in circulating immune complexes and glomerular deposits in patients with IgAN. Thus, temporal differences in EBV primo-infection as related to naturally delayed maturation of the IgA system may contribute to geographic and racial variations in incidence of IgAN.

Racial heterogeneity of IgA1 hinge-region O-glycoforms in patients with IgA nephropathy

Galactose (Gal)-deficient IgA1 (Gd-IgA1) is involved in IgA nephropathy (IgAN) pathogenesis. To reflect racial differences in clinical characteristics, we assessed disease- and race-specific heterogeneity in the O-glycosylation of the IgA1 hinge region (HR). We determined serum Gd-IgA1 levels in Caucasians (healthy controls [HCs], n = 31; IgAN patients, n = 63) and Asians (HCs, n = 20; IgAN patients, n = 60) and analyzed profiles of serum IgA1 HR O-glycoforms. Elevated serum Gd-IgA1 levels and reduced number of Gal residues per HR were observed in Caucasians. Reduced number of N-acetylgalactosamine (GalNAc) residues per HR and elevated relative abundance of IgA1 with three HR O-glycans were common features in IgAN patients; these features were associated with elevated blood pressure and reduced renal function. We speculate that the mechanisms underlying the reduced GalNAc content in IgA1 HR may be relevant to IgAN pathogenesis.

Identification and characterization of circulating immune complexes in IgA nephropathy

The underlying pathology of immunoglobulin A (IgA) nephropathy (IgAN), the most common glomerulonephritis worldwide, is driven by the deposition of immune complexes containing galactose-deficient IgA1 [Tn(+)IgA1] in the glomerular mesangium. Here, we report that novel anti-Tn circulating immune complexes (anti-Tn CICs) contain predominantly IgM, representing large macromolecular complexes of ~1.2 megadaltons to several megadalton sizes together with Tn(+)IgA1 and some IgG. These complexes are significantly elevated in sera of patients with IgAN, which contains higher levels of complement C3, compared to healthy individuals. Anti-Tn CICs are bioactive and induce specific proliferation of human renal mesangial cells. We found that these anti-Tn CICs can be dissociated with small glycomimetic compounds, which mimic the Tn antigen of Tn(+)IgA1, releasing IgA1 from anti-Tn CICs. This glycomimetic compound can also significantly inhibit the proliferative activity of anti-Tn CICs of patients with IgAN. These findings could enhance both the diagnosis of IgAN and its treatment, as specific drug treatments are now unavailable.

Metagenomics-based systematic analysis reveals that gut microbiota Gd-IgA1-associated enzymes may play a key role in IgA nephropathy

Background: IgA nephropathy (IgAN) is the most common type of glomerulonephritis in Asia. Its pathogenesis involves higher expression of galactose-deficient IgA1 (Gd-IgA1) and dysregulated intestinal mucosal immunity. The objective of this study was to explore whether specific gut microbiota and associated enzymes affect Gd-IgA1 in IgAN.

Methods: This study carried out shotgun metagenomic sequencing with Illumina on fecal samples collected from 20 IgAN patients (IgAN group) and 20 healthy controls (HCs group) who were recruited from January 2016 to December 2018 at the Second Clinical College of Guangzhou University of Chinese Medicine. Differences analysis in gut microbiota was performed to determine the overall microbiota composition, the representative enterotypes, and the microbiota abundance. Correlations between gut microbiota and clinical indicators were assessed by Spearman’s analysis. Moreover, the functional prediction of microbial communities and the quantitative calculation of enzymes encoded by microbiome were performed using the MetaCyc pathway and the bioBakery three platform, respectively.

Results:Bacteroides plebeius and Bacteroides vulgatus levels were higher, while Prevotella copri and Alistipes putredinis levels were lower in the IgAN group compared to HCs group. Enterotype I characterized by Bacteroides was closely related to the IgAN patients. Moreover, Bacteroides fragilis, Flavonifractor plautii and Ruminococcus gnavus were characteristic bacteria enriched in IgAN patients. Spearman’s correlation analysis found that Eggerthella lenta and Ruminococcus bromii were positively correlated with urine protein-creatinine ratio, while Ruminococcus gnavus showed a direct association with red blood cells in urine, and Bacteroides vulgatus and Ruminococcus gnavus were positively correlated with eGFR. These results indicated that intestinal dysbacteriosis occurred in IgAN patients and was associated with clinical and biochemical features. In addition, MetaCyc pathway analysis predicted microbiota-related metabolic pathways, including the biosynthesis of amino acids and glycans, were associated with the IgAN group. Microbial enzymes analysis highlighted that Gd-IgA1-associated α-galactosidase and α-N-acetyl-galactosaminidase secreted by Flavonifractor plautii were enriched in IgAN patients.

Conclusion: These findings suggested that α-galactosidase and α-N-acetyl-galactosaminidase secreted by Flavonifractor plautii might be related to the production of Gd-IgA1, indicating that enzymes originated from abnormal intestinal microbiota may contribute to the production of Gd-IgA1 and play an important role in the pathogenesis of IgAN.

Gut Microbiome Characteristics in IgA Nephropathy: Qualitative and Quantitative Analysis from Observational Studies

Background

Recent data indicate the importance of gut-kidney axis in the pathogenesis of Immunoglobulin A nephropathy (IgAN). Growing evidence suggests the alterations of diversity and composition of gut microbiome among patients with IgAN, however, the details are not yet fully understood.

Methods

Eligible studies comparing the gut microbiome between patients with IgAN and non-IgAN individuals were systematically searched from PubMed, Embase, Web of Science, Cochrane Library, China National Knowledge Infrastructure, and ClinicalTrials.gov. The primary outcomes were alpha- and beta-diversity, and the differences in gut microbiota composition between patients with IgAN and non-IgAN persons. Qualitative analysis and meta-analysis were performed according to available data.

Results

Eleven cross-sectional studies, including 409 patients with IgAN and 243 healthy controls, were enrolled. No significant differences in the diversity and enrichment of gut bacteria were found between IgAN and healthy individuals, whereas the beta-diversity consistently showed significant microbial dissimilarities among the two groups. Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Fusobacteria, and Verrucomicrobia were the dominant phyla, however, no significant differences were found between IgAN patients and healthy controls at the phylum level. The genera, Streptococcus and Paraprevotella showed a higher proportion in patients with IgAN compared to healthy individuals, whereas Fusicatenibacter showed a lower abundance according to meta-analysis. Qualitative analyses suggested that Escherichia-Shigella might be increased in IgAN patients; the genera, Clostridium, Prevotella 9,and Roseburia, members of Ruminococcaceae and Lachnospiraceae families, were likely to have decreased abundances in patients with IgAN compared to healthy individuals.

Conclusion

Gut microbiota dysbiosis was demonstrated in IgAN, which might be involved in the pathogenesis of IgAN. Further studies are needed to confirm the findings of this study, due to the substantial heterogeneity.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/, identifier PROSPERO (CRD42022304034).

Sialylation as an Important Regulator of Antibody Function

Antibodies play a critical role in linking the adaptive immune response to the innate immune system. In humans, antibodies are categorized into five classes, IgG, IgM, IgA, IgE, and IgD, based on constant region sequence, structure, and tropism. In serum, IgG is the most abundant antibody, comprising 75% of antibodies in circulation, followed by IgA at 15%, IgM at 10%, and IgD and IgE are the least abundant. All human antibody classes are post-translationally modified by sugars. The resulting glycans take on many divergent structures and can be attached in an N-linked or O-linked manner, and are distinct by antibody class, and by position on each antibody. Many of these glycan structures on antibodies are capped by sialic acid. It is well established that the composition of the N-linked glycans on IgG exert a profound influence on its effector functions. However, recent studies have described the influence of glycans, particularly sialic acid for other antibody classes. Here, we discuss the role of glycosylation, with a focus on terminal sialylation, in the biology and function across all antibody classes. Sialylation has been shown to influence not only IgG, but IgE, IgM, and IgA biology, making it an important and unappreciated regulator of antibody function.

Diagnostic Potential of Plasma IgA1 O-Glycans in Discriminating IgA Nephropathy From Other Glomerular Diseases and Healthy Participants

Background: Aberrant O-glycosylation of IgA1 plays an important role in IgA nephropathy pathogenesis. Previous proteomic studies analyzed O-glycans of the circulating IgA1 hinge region and found that the N-acetylgalactosamine (GalNAc) and galactose numbers in the hinge region of IgA1 of patients with IgA nephropathy were lower than those in healthy participants. However, the diagnostic performance of the O-glycosylation traits in the hinge region of plasma IgA1 for IgA nephropathy remains unelucidated. The present study aimed to determine the difference in plasma IgA1 hinge region O-glycoforms among IgA nephropathy, non-IgA nephropathy disease controls, and healthy participants, and to further evaluate the diagnostic performance of plasma IgA1 glycosylation traits.

Methods: Sixty-two patients with biopsy-proven primary IgA nephropathy, 30 age- and sex-matched non-IgA nephropathy disease controls (10 patients with membranous nephropathy, 10 with focal segmental glomerulosclerosis, and 10 with minimal change disease), and 30 healthy participants were prospectively recruited. Plasma galactose deficient-IgA1 levels were measured using a KM55 kit. Plasma IgA was extracted using IgA immunoaffinity beads. After de-N-glycosylation, reduction, alkylation, trypsin digestion, and O-glycopeptide enrichment via hydrophilic interaction liquid chromatography, liquid chromatography tandem mass spectrometry (LC-MS/MS) was applied to analyze the IgA1 O-glycosylation patterns and we derived the plasma IgA1 O-glycosylation traits.

Results: Plasma IgA1 O-glycosylation patterns were significantly changed in IgA nephropathy patients compared to those with non-IgA nephropathy disease controls and healthy participants. The GalNAc number was lowest in IgA nephropathy patients. In addition, a similar result was observed for the galactose number in the IgA1 hinge region. These values showed moderate potential for discriminating between IgA nephropathy and the controls. When these values were combined, the area under the curve increased compared to when they were considered individually. When further adding a clinical indicator, the area under the curve of the GalNAc-galactose-IgA panel exceed 0.9 in discriminating IgA nephropathy from the controls.

Conclusion: The amount of GalNAc and galactose in plasma IgA1 hinge region identified by glycoproteomics could be used as a diagnostic biomarker for IgA nephropathy. The panel containing GalNAc, galactose, and circulating IgA displayed excellent diagnostic performance and is promising for practical clinical applications.

IgA vasculitis nephritis

PURPOSE OF REVIEW

The purpose of this update is to summarize current knowledge on the pathophysiology of immunglobulin A (IgA) vasculitis nephritis (IgAVN) as well as to critically review evidence for established therapeutic regimes and available biomarkers. An additional purpose is to raise the discussion what could be done to further improve our understanding of IgAVN, identify patients at risk for adverse outcome and increase the evidence for therapy recommendations.

RECENT FINDINGS

Clinical and experimental studies have established the concept of a multilevel pathogenesis. Toll-like-receptor activation, B cell proliferation, micro-RNAs and complement activation have been identified or confirmed as potential therapeutic targets which can modify the course of the disease. Currently, kidney injury molecule-1, monocyte chemotactic protein-1, N-acetyl-β-glucosaminidase, and angiotensinogen are the most promising urinary biomarkers for early diagnosis of renal involvement in IgA vasculitis.

SUMMARY

Close surveillance of all IgAV patients for renal involvement is recommended. Given the multilevel pathogenesis, early treatment of even mild cases should be initiated. Further therapeutic options should be considered in case first-line therapy (mostly corticosteroids) has no effect. The evidence supporting current therapeutic regimes is predominantly based on expert opinion. Prospective studies are needed and should involve substances inhibiting B cell proliferation and complement activation.

New Insights into the Treatment of Glomerular Diseases: When Mechanisms Become Vivid

Treatment for glomerular diseases has been extrapolated from the experience of other autoimmune disorders while the underlying pathogenic mechanisms were still not well understood. As the classification of glomerular diseases was based on patterns of juries instead of mechanisms, treatments were typically the art of try and error. With the advancement of molecular biology, the role of the immune agent in glomerular diseases is becoming more evident. The four-hit theory based on the discovery of gd-IgA1 gives a more transparent outline of the pathogenesis of IgA nephropathy (IgAN), and dysregulation of Treg plays a crucial role in the pathogenesis of minimal change disease (MCD). An epoch-making breakthrough is the discovery of PLA2R antibodies in the primary membranous nephropathy (pMN). This is the first biomarker applied for precision medicine in kidney disease. Understanding the immune system’s role in glomerular diseases allows the use of various immunosuppressants or other novel treatments, such as complement inhibitors, to treat glomerular diseases more reasonable. In this era of advocating personalized medicine, it is inevitable to develop precision medicine with mechanism-based novel biomarkers and novel therapies in kidney disease.

Sialylation of IgG inhibits the formation of galactose-deficient IgA1-containing immune complexes and protects mesangial cells from injury in IgA nephropathy

Background

The addition of sialic acid alters IgG from a pro-inflammatory state to an anti-inflammatory state. However, there is a lack of research on the changes of IgG sialylation in IgA nephropathy (IgAN).

Methods

This study included a total of 184 IgAN patients. The sialylated IgG (SA-IgG), IgG-galactose-deficient IgA1 complex (IgG-Gd-IgA1-IC), IL-6, TNF-α, and TGF-β were detected using commercial ELISA kits. SA-IgG, non-sialylated IgG (NSA-IgG), sialylated IgG-IgA1 complex (SA-IgG-IgA1), and non-sialylated IgG-IgA1 complex (NSA-IgG-IgA1) were purified from IgAN patients and healthy controls (HCs).

Results

The mean SA-IgG levels in plasma and B lymphocytes in IgAN patients were significantly higher than those of healthy controls. A positive correlation was found between SA-IgG levels in plasma and B lymphocytes. In vitro, the results showed that the release of IgG-Gd-IgA1-IC was significantly decreased in peripheral blood mononuclear cells (PBMCs) cultured with SA-IgG from both IgAN patients and healthy controls. The proliferation ability and the release of IL-6, TNF-α, and TGF-β in human mesangial cells (HMCs) were measured after stimulating with SA-IgG-IgA1-IC and NSA-IgG-IgA1-IC. The mesangial cell proliferation levels induced by NSA-IgG-IgA1-IC derived from IgAN patients were significantly higher than those caused by SA-IgG-IgA1-IC derived from IgAN patients and healthy controls. Compared with NSA-IgG-IgA1 from healthy controls, IgAN-NSA-IgG-IgA1 could significantly upregulate the expression of IL-6 and TNF-α in mesangial cells. The data showed that there weren’t any significant differences in the levels of IL-6, TNF-α, and TGF-β when treated with IgAN-SA-IgG-IgA1 and HC-NSA-IgG-IgA1.

Conclusions

The present study demonstrated that the sialylation of IgG increased in patients with IgA nephropathy. It exerted an inhibitory effect on the formation of Gd-IgA1-containing immune complexes in PBMCs and the proliferation and inflammation activation in mesangial cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02657-8.

Development of Urinary Diagnostic Biomarker for IgA Nephropathy by Lectin Microarray

INTRODUCTION

The pathogenic roles of aberrantly glycosylated IgA1 have been reported. However, it is unexplored whether the profiling of urinary glycans contributes to the diagnosis of IgAN.

METHODS

We conducted a retrospective study enrolling 493 patients who underwent renal biopsy at Okayama University Hospital between December 2010 and September 2017. We performed lectin microarray in urine samples and investigated whether c-statistics of the reference standard diagnosis model employing hematuria, proteinuria, and serum IgA were improved by adding the urinary glycan intensity.

RESULTS

Among 45 lectins, 3 lectins showed a significant improvement of the models: Amaranthus caudatus lectin (ACA) with the difference of c-statistics 0.038 (95% CI: 0.019-0.058, p < 0.001), Agaricus bisporus lectin (ABA) 0.035 (95% CI: 0.015-0.055, p < 0.001), and Maackia amurensis lectin (MAH) 0.035 (95% CI: 0.015-0.054, p < 0.001). In 3 lectins, each signal plus reference standard showed good reclassification (category-free NRI and relative IDI) and good model fitting associated with the improvement of AIC and BIC. Stratified by eGFR, the discriminatory ability of ACA plus reference standard was maintained, suggesting the robust renal function-independent diagnostic performance of ACA. By decision curve analysis, there was a 3.45% net benefit by adding urinary glycan intensity of ACA to the reference standard at the predefined threshold probability of 40%.

CONCLUSIONS

The reduction of Gal(β1-3)GalNAc (T-antigen), Sia(α2-3)Gal(β1-3)GalNAc (Sialyl T), and Sia(α2-3)Gal(β1-3)Sia(α2-6)GalNAc (disialyl-T) was suggested by binding specificities of 3 lectins. C1GALT1 and COSMC were responsible for the biosynthesis of these glycans, and they were known to be downregulated in IgAN. The urinary glycan analysis by ACA is a useful and robust noninvasive strategy for the diagnosis of IgAN.

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.