Difference in IgA1 O-glycosylation between IgA deposition donors and IgA nephropathy recipients

GalNAc-T14 may Contribute to Production of Galactose-Deficient Immunoglobulin A1, the Main Autoantigen in IgA Nephropathy

Introduction

Immunoglobulin A1 (IgA1) with galactose-deficient O-glycans (Gd-IgA1) play a key role in the pathogenesis of IgA nephropathy (IgAN). Mucosal-tissue infections increase IL-6 production and, in patients with IgAN, are often associated with macroscopic hematuria. IgA1-secreting cell lines derived from the circulation of patients with IgAN, compared to those of healthy controls (HCs), produce more IgA1 that has O-glycans with terminal or sialylated N-acetylgalactosamine (GalNAc). GalNAc residues are added to IgA1 hinge region by some of the 20 GalNAc transferases, the O-glycosylation-initiating enzymes. Expression of GALNT2, encoding GalNAc-T2, the main enzyme initiating IgA1 O-glycosylation, is similar in cells derived from patients with IgAN and HCs. In this report, we extend our observations of GALNT14 overexpression in IgA1-producing cell lines from patients with IgAN.

Methods

GALNT14 expression was analyzed in peripheral blood mononuclear cells (PBMCs) from patients with IgAN and from HCs. Moreover, the effect of GALNT14 overexpression or knock-down on Gd-IgA1 production in Dakiki cells was assessed.

Results

GALNT14 was overexpressed in PBMCs from patients with IgAN. IL-6 increased GALNT14 expression in PBMCs from patients with IgAN and HCs. We used IgA1-producing cell line Dakiki, a previously reported model of Gd-IgA1-producing cells, and showed that overexpression of GalNAc-T14 enhanced galactose deficiency of IgA1, whereas siRNA-mediated GalNAc-T14 knock-down reduced it. GalNAc-T14 was localized in trans-Golgi network, as expected.

Conclusions

Overexpression of GALNT14 due to inflammatory signals during mucosal infections may contribute to overproduction of Gd-IgA1 in patients with IgAN.

Serological and histopathological assessment of galactose-deficient immunoglobulin A1 deposition in kidney allografts: A multicenter prospective observational study

BACKGROUND

Recurrent immunoglobulin A (IgA) nephropathy is an important risk factor for kidney allograft loss. However, there is no classification system for IgA deposition in kidney allografts based on serological and histopathological evaluation of galactose-deficient IgA1 (Gd-IgA1). This study aimed to establish a classification system for IgA deposition in kidney allografts based on serological and histological evaluation of Gd-IgA1.

METHODS

This multicenter prospective study included 106 adult kidney transplant recipients in whom an allograft biopsy was performed. Serum and urinary Gd-IgA1 levels were investigated in 46 transplant recipients who were IgA-positive and classified into four subgroups according to the presence or absence of mesangial Gd-IgA1 (KM55 antibody) deposits and C3.

RESULTS

Minor histological changes without an acute lesion were observed in recipients with IgA deposition. Fourteen (30%) of the 46 IgA-positive recipients were KM55-positive and 18 (39%) were C3-positive. The C3 positivity rate was higher in the KM55-positive group. Serum and urinary Gd-IgA1 levels were significantly higher in KM55-positive/C3-positive recipients than in the other three groups with IgA deposition. Disappearance of IgA deposits was confirmed in 10 of 15 IgA-positive recipients in whom a further allograft biopsy was performed. The serum Gd-IgA1 level at the time of enrollment was significantly higher in recipients in whom IgA deposition continued than in those in whom it disappeared (p = 0.02).

CONCLUSIONS

The population with IgA deposition after kidney transplantation is serologically and pathologically heterogeneous. Serological and histological assessment of Gd-IgA1 is useful for identifying cases that should be carefully observed.

IgA Nephropathy: Current Understanding and Perspectives on Pathogenesis and Targeted Treatment

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide, with varied clinical and histopathological features between individuals, particularly across races. As an autoimmune disease, IgAN arises from consequences of increased circulating levels of galactose-deficient IgA1 and mesangial deposition of IgA-containing immune complexes, which are recognized as key events in the widely accepted "multi-hit" pathogenesis of IgAN. The emerging evidence further provides insights into the role of genes, environment, mucosal immunity and complement system. These developments are paralleled by the increasing availability of diagnostic tools, potential biomarkers and therapeutic agents. In this review, we summarize current evidence and outline novel findings in the prognosis, clinical trials and translational research from the updated perspectives of IgAN pathogenesis.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

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.

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.

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.

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.

O-glycoforms of polymeric IgA1 in the plasma of patients with IgA nephropathy are associated with pathological phenotypes

BACKGROUND

IgA1 O-glycosylation plays an important role in the pathogenesis of IgA nephropathy (IgAN). However, variations in IgA1 O-glycoforms have not been explored. We aimed to investigate the IgA1 O-glycoforms in the hinge region (HR) of polymeric IgA1 and then evaluate the association between IgA1 O-glycoforms and crescent formation in IgAN.

METHODS

The discovery cohort (cohort 1) comprised 11 crescentic IgAN patients, 10 noncrescentic IgAN patients and 10 healthy controls, and the validation cohort (cohort 2) comprised 11 crescentic IgAN patients, 9 noncrescentic IgAN patients, and 9 healthy controls. A total of 143 IgAN patients with different crescent percentages (cohort 3) were also included. Polymeric IgA1 was purified from the plasma of the participants. The variation in O-glycoforms was evaluated by estimating the molecular weights of IgA1 hinge glycopeptides using reversed-phase liquid chromatography (LC) and tandem mass spectrometry under electron-transfer/higher-energy collision dissociation (EThcD) fragmentation mode.

RESULTS

In discovery cohort (cohort 1), the numbers of GalNAc bound to one HR were lower in IgAN patients. The proportions of GalNAc3 (defined as O-glycans bound to one HR at 3 sites) and GalNAc4 were highest in crescentic IgAN patients followed by noncrescentic IgAN patients and were lowest in healthy controls (GalNAc 3: 9.92%±3.37% vs 6.65%±1.53% vs 4.05%±1.24%; P < 0.001; GalNAc4: 45.91%±4.75% vs 41.13%±2.95% vs 40.98%±2.95%; P = 0.004). The proportions of GalNAc5 and GalNAc6 were lowest in crescentic IgAN patients followed by noncrescentic IgAN patients and were highest in healthy controls (GalNAc5: 50.15%±4.27% vs 47.92%±4.09% vs 45.87%±3.79%, P = 0.028; GalNAc6: 6.58%±2.53% vs 6.04%±1.35% vs 4.65%±2.27%; P = 0.034). These results were consistent in the validation cohort (cohort 2); In another cohort with 143 patients with different crescents percentage (cohort 3), the numbers of GalNAc in polymeric IgA1 decreased with increasing percentage of crescents.

CONCLUSIONS

The numbers of GalNAc in IgA1 HRs were lower in IgAN patients, especially in crescentic IgAN patients, and may be associated with a severe IgAN phenotype.

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.

microRNA-630 Regulates Underglycosylated IgA1 Production in the Tonsils by Targeting TLR4 in IgA Nephropathy

IgA nephropathy (IgAN) is the most common primary glomerular disease. The characteristic pathology involves immune complexes formed by the deposition of IgA1 and underglycosylated IgA1 aggregates in the mesangial area, which may be accompanied by the deposition of IgG and/or IgM and complement components. However, the molecular mechanisms of IgAN remain unclear. In the present study, microarray analysis showed that the expression of microRNA-630 (miR-630) was significantly reduced in palatal tonsils from IgAN patients compared with chronic tonsillitis. Additionally, bioinformatic analysis showed that Toll-like receptor 4 (TLR4) was the predicted target gene of miR-630 and was regulated by miR-630. When miR-630 was overexpressed in palatal tonsil mononuclear cells from IgAN patients, the expression of TLR4 was reduced and the content of IgA1 in the cell culture supernatant was decreased, and the level of galactosylation in the IgA1 hinge region was increased. Moreover, immunohistochemical analysis showed that the expression of TLR4 in IgAN patients was significantly increased. After knocking down the expression of TLR4, both the concentration of IgA1 and the binding force of IgA1 with broad bean lectin were significantly reduced in IgAN. Furthermore, the mechanism study demonstrated that TLR4 might regulate the expression of IL-1β and IL-8 through NF-κB signaling pathway to modulate the concentration of IgA1 and the glycosylation level of IgA1. This interesting finding may offer new insight into the molecular mechanism of IgAN.

Mass spectrometry for the identification and analysis of highly complex glycosylation of therapeutic or pathogenic proteins

INTRODUCTION

Protein glycosylation influences characteristics such as folding, stability, protein interactions, and solubility. Therefore, glycan moieties of therapeutic proteins and proteins that are likely associated with disease pathogenesis should be analyzed in-depth, including glycan heterogeneity and modification sites. Recent advances in analytical methods and instrumentation have enabled comprehensive characterization of highly complex glycosylated proteins.

AREA COVERED

The following aspects should be considered when analyzing glycosylated proteins: sample preparation, chromatographic separation, mass spectrometry (MS) and fragmentation methods, and bioinformatics, such as software solutions for data analyses. Notably, analysis of glycoproteins with heavily sialylated glycans or multiple glycosylation sites requires special considerations. Here, we discuss recent methodological advances in MS that provide detailed characterization of heterogeneous glycoproteins.

EXPERT OPINION

As characterization of complex glycosylated proteins is still analytically challenging, the function or pathophysiological significance of these proteins is not fully understood. To reproducibly produce desired forms of therapeutic glycoproteins or to fully elucidate disease-specific patterns of protein glycosylation, a highly reproducible and robust analytical platform(s) should be established. In addition to advances in MS instrumentation, optimization of analytical and bioinformatics methods and utilization of glycoprotein/glycopeptide standards is desirable. Ultimately, we envision that an automated high-throughput MS analysis will provide additional power to clinical studies and precision medicine.