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

O-glycosylation of IgA1 and the pathogenesis of an autoimmune disease IgA nephropathy

IgA nephropathy is a kidney disease characterized by deposition of immune complexes containing abnormally O-glycosylated IgA1 in the glomeruli. Specifically, some O-glycans are missing galactose that is normally β1,3-linked to N-acetylgalactosamine of the core 1 glycans. These galactose-deficient IgA1 glycoforms are produced by IgA1-secreting cells due to a dysregulated expression and activity of several glycosyltransferases. Galactose-deficient IgA1 in the circulation of patients with IgA nephropathy is bound by IgG autoantibodies and the resultant immune complexes can contain additional proteins, such as complement C3. These complexes, if not removed from the circulation, can enter the glomerular mesangium, activate the resident mesangial cells, and induce glomerular injury. In this review, we briefly summarize clinical and pathological features of IgA nephropathy, review normal and aberrant IgA1 O-glycosylation pathways, and discuss the origins and potential significance of natural anti-glycan antibodies, namely those recognizing N-acetylgalactosamine. We also discuss the features of autoantibodies specific for galactose-deficient IgA1 and the characteristics of pathogenic immune complexes containing IgA1 and IgG. In IgA nephropathy, kidneys are injured by IgA1-containing immune complexes as innocent bystanders. Most patients with IgA nephropathy progress to kidney failure and require dialysis or transplantation. Moreover, most patients after transplantation experience a recurrent disease. Thus, a better understanding of the pathogenetic mechanisms is needed to develop new disease-specific treatments.

Pathogenic Immunoglobulin A-Producing Cells in Immunoglobulin A Nephropathy

Immunoglobulin A nephropathy (IgAN) is the most prevalent primary glomerular disease worldwide and it remains a leading cause of kidney failure. Clinical manifestations of IgA are exacerbated by infections, and emerging data suggest that aberrant mucosal immune responses are important contributors to the immunopathogenesis of this disease. However, the exact stimuli, location and mechanism of nephritis-inducing IgA production remains unclear. In this focused review we explore recent developments in our understanding of the contribution of the mucosal immune system and mucosal-derived IgA-producing cells to the development of IgAN.

The pathogenesis of IgA nephropathy and implications for treatment

IgA nephropathy (IgAN) is a common form of primary glomerulonephritis and represents an important cause of chronic kidney disease globally, with observational studies indicating that most patients are at risk of developing kidney failure within their lifetime. Several research advances have provided insights into the underlying disease pathogenesis, framed by a multi-hit model whereby an increase in circulating IgA1 that lacks galactose from its hinge region - probably derived from the mucosal immune system - is followed by binding of specific IgG and IgA antibodies, generating immune complexes that deposit within the glomeruli, which triggers inflammation, complement activation and kidney damage. Although treatment options are currently limited, new therapies are rapidly emerging that target different pathways, cells and mediators involved in the disease pathogenesis, including B cell priming in the gut mucosa, the cytokines APRIL and BAFF, plasma cells, complement activation and endothelin pathway activation. As more treatments become available, there is a realistic possibility of transforming the long-term outlook for many individuals with IgAN.

Gross Hematuria after the COVID-19 mRNA Vaccination: Nationwide Multicenter Prospective Cohort Study in Japan

Key Points

Little is known about the clinicopathological characteristics and renal outcomes in the patients with gross hematuria (GH) after the vaccination. To fill a clinicopathological knowledge gap regarding vaccination and GH, we conducted a nationwide multicenter prospective cohort study. GH is more likely to occur in patients with IgA nephropathy, with a female bias, but without progressive exacerbation of renal function.

Background

In the past 3 years, cases of gross hematuria (GH) after the vaccination for coronavirus disease 2019 in patients with IgA nephropathy (IgAN) have been frequently reported worldwide. However, the postevent renal prognosis of these patients, their clinical backgrounds, and underlying mechanisms remain unknown. Therefore, we conducted a nationwide multicenter prospective cohort study in Japan.

Methods

We analyzed laboratory findings at the time of the first presentation to the hospital and 3 and 6 months after in patients with GH after the vaccination and histopathological findings in their kidney biopsy specimens. Moreover, changes in pathological biomarkers of IgAN such as galactose-deficient IgA1 (Gd-IgA1) and its immune complexes were also evaluated.

Results

During the study period, 127 newly presenting patients with GH after the vaccination were enrolled, with a clear female bias (73.2%). GH was observed after the second or subsequent vaccinations in most patients (92.9%). Of the 37 patients undergoing kidney biopsy before the vaccination, 36 patients had been diagnosed with IgAN/IgA vasculitis (IgAV). In the remaining 90 patients, 69 of the 70 who newly underwent kidney biopsy were diagnosed with IgAN (n =67)/IgAV (n =2). Their histopathology did not show a high incidence of acute lesions such as endocapillary hypercellularity and crescentic lesions. Most cases showed a temporary increase in proteinuria, but no sustained worsening in renal function. Among the biomarkers measured, serum Gd-IgA1 and immune complexes were comparable throughout the observation period; however, only urinary Gd-IgA1 was increased at the time of GH.

Conclusions

We found that GH after the vaccination is more likely to occur in patients with IgAN/IgAV, with a female bias, but without progressive exacerbation of renal function. Although further investigation is needed regarding causal relationship between vaccination and GH, this study provides many insights into the molecular mechanisms of GH.

Causal association between peripheral immune cells and IgA nephropathy: a Mendelian randomization study

Background

The relationship between peripheral immune cells and immunoglobulin A nephropathy (IgAN) is widely known; however, causal evidence of this link is lacking. Here, we aimed to determine the causal effect of peripheral immune cells, specifically total white blood cells, lymphocytes, monocytes, basophils, eosinophils, and neutrophils, as well as lymphocyte subset traits, on the IgAN risk using a Mendelian randomization (MR) analysis.

Methods

The inverse-variance weighted (IVW) method was used for the primary analysis. We applied three complementary methods, including the weighted median, MR-Egger regression, and MR-PRESSO, to detect and correct for the effect of horizontal pleiotropy. Additionally, we performed a multivariable MR (MVMR) analysis, adjusting for the effects of C-reactive protein (CRP) levels. The roles of specific lymphocyte subtypes and their significance have garnered interest. Bidirectional two-sample MR analysis was performed to test the potential causal relationships between immune traits, including median fluorescence intensities (MFIs) and the relative cell count (AC), and IgAN.

Results

The IVW-MR analysis suggested a potential causal relationship between lymphocyte counts and IgAN in Europe (OR per 1-SD increase: 1.43, 95% CI: 1.08-1.88, P = 0.0123). The risk effect of lymphocytes remained even after adjusting for CRP levels using the MVMR method (OR per 1-SD increase: 1.44, 95% CI: 1.05-1.96, P = 0.0210). The other sensitivity analyses showed a consistent trend. The largest GWAS published to date was used for peripheral blood immunophenotyping to explore the potential causal relationship between peripheral immune cell subsets and IgAN. Six AC-IgAN and 14 MFI-IgAN pairs that reached statistical significance (P < 0.05) were detected. Notably, CD3, expressed in eight subsets of T cells, consistently showed a positive correlation with IgAN. The bidirectional MR analysis did not reveal any evidence of reverse causality. According to the sensitivity analysis, horizontal pleiotropy was unlikely to distort the causal estimates.

Conclusions

Genetically determined high lymphocyte counts were associated with IgAN, supporting that high lymphocyte counts is causal risk factor for IgAN.

Contemporary review of IgA nephropathy

IgA nephropathy (IgAN) is considered the most common primary glomerulonephritis worldwide with a predilection for Asian-Pacific populations and relative rarity in those of African descent. Perhaps 20%-50% of patients progress to kidney failure. The pathogenesis is incompletely understood. Mesangial deposition of immune complexes containing galactose-deficient IgA1 complexed with anti-glycan IgG or IgA antibodies results in mesangial cell activation and proliferation, inflammatory cell recruitment, complement activation, and podocyte damage. Diagnosis requires a biopsy interpreted by the Oxford criteria. Additional pathologic features include podocytopathy, thrombotic microangiopathy, and C4d staining. Biomarkers predicting adverse outcomes include proteinuria, reduced GFR, hypertension, and pathology. Acceptable surrogate endpoints for therapeutic trials include ongoing proteinuria and rate of eGFR decline. The significance of persisting hematuria remains uncertain. The mainstay of therapy is supportive, consisting of lifestyle modifications, renin-angiotensin inhibition (if hypertensive or proteinuric), sodium-glucose-transporter 2 inhibition (if GFR reduced or proteinuric), and endothelin-receptor antagonism (if proteinuric). Immunosuppression should be considered for those at high risk after maximal supportive care. Corticosteroids are controversial with the most positive results observed in Chinese. They carry a high risk of serious side effects. Similarly, mycophenolate may be most effective in Chinese. Other immunosuppressants are of uncertain benefit. Tonsillectomy appears efficacious in Japanese. Active areas of investigation include B-cell inhibition with agents targeting the survival factors BAFF and APRIL and complement inhibition with agents targeting the alternate pathway (Factors B and D), the lectin pathway (MASP-2), and the common pathway (C3 and C5). Hopefully soon, the who and the how of immunosuppression will be clarified, and kidney failure can be forestalled.

IgA Nephropathy: Significance of IgA1-Containing Immune Complexes in Clinical Settings

IgA nephropathy (IgAN) is considered to be an autoimmune disease characterized by the formation of IgA1-containing immune complexes in the circulation and glomerular immunodeposits. Extensive research has identified multiple genetic, immunological, and environmental factors contributing to disease development and progression. The pathogenesis of IgAN is considered a multifactorial process involving the formation of immune complexes wherein aberrantly O-glycosylated IgA1 is recognized as an autoantigen. Consequently, the clinical presentation of IgAN is highly variable, with a wide spectrum of manifestations ranging from isolated microscopic hematuria or episodic macroscopic hematuria to nephrotic-range proteinuria. Whereas some patients may exhibit a slowly progressive form of IgAN, others may present with a rapidly progressive glomerulonephritis leading to kidney failure. Development of the treatment for IgAN requires an understanding of the characteristics of the pathogenic IgA1-containing immune complexes that enter the glomerular mesangium and induce kidney injury. However, not all details of the mechanisms involved in the production of galactose-deficient IgA1 and immune-complex formation are fully understood. Here, we review what we have learned about the characteristics of nephritogenic IgA1 in the half-century since the first description of IgAN in 1968.

Intestinal epithelium dysfunctions cause IgA deposition in the kidney glomeruli of intestine-specific Ap1m2-deficient mice

BACKGROUND

Intestinal epithelial cells (IECs) serve as robust barriers against potentially hostile luminal antigens and commensal microbiota. Epithelial barrier dysfunction enhances intestinal permeability, leading to leaky gut syndrome (LGS) associated with autoimmune and chronic inflammatory disorders. However, a causal relationship between LGS and systemic disorders remains unclear. Ap1m2 encodes clathrin adaptor protein complex 1 subunit mu 2, which facilitates polarized protein trafficking toward the basolateral membrane and contributes to the establishment of epithelial barrier functions.

METHODS

We generated IEC-specific Ap1m2-deficient (Ap1m2ΔIEC) mice with low intestinal barrier integrity as an LSG model and examined the systemic impact.

FINDINGS

Ap1m2ΔIEC mice spontaneously developed IgA nephropathy (IgAN)-like features characterized by the deposition of IgA-IgG immune complexes and complement factors in the kidney glomeruli. Ap1m2 deficiency markedly enhanced aberrantly glycosylated IgA in the serum owing to downregulation and mis-sorting of polymeric immunoglobulin receptors in IECs. Furthermore, Ap1m2 deficiency caused intestinal dysbiosis by attenuating IL-22-STAT3 signaling. Intestinal dysbiosis contributed to the pathogenesis of IgAN because antibiotic treatment reduced aberrantly glycosylated IgA production and renal IgA deposition in Ap1m2ΔIEC mice.

INTERPRETATION

IEC barrier dysfunction and subsequent dysbiosis by AP-1B deficiency provoke IgA deposition in the mouse kidney. Our findings provide experimental evidence of a pathological link between LGS and IgAN.

FUNDING

AMED, AMED-CREST, JSPS Grants-in-Aid for Scientific Research, JST CREST, Fuji Foundation for Protein Research, and Keio University Program for the Advancement of Next Generation Research Projects.

Insights on Nefecon®, a Targeted-Release Formulation of Budesonide and Its Selective Immunomodulatory Effects in Patients with IgA Nephropathy

Immunoglobulin A nephropathy (IgAN) is a chronic, immune-mediated kidney disease characterized by the deposition of galactose-deficient immunoglobulin A1 (Gd-IgA1) in the kidneys. Excess Gd-IgA1 production in patients with IgAN is located within the mucosa-associated lymphoid tissue, particularly within the lamina propria in the distal ileum. Nefecon® is a targeted-release formulation of the corticosteroid budesonide, which became the first treatment approved by the US Food and Drug Administration (FDA; brand name, TARPEYO®) and European Medicines Agency (EMA; KINPEYGO®) for patients with primary IgAN at risk of rapid disease progression, after demonstrating clinically significant reduction of proteinuria in an interim analysis of the Phase III NefIgArd trial. After showing a significant reduction in estimated glomerular filtration rate decline in the full 2-year analysis of the trial, Nefecon was granted full approval by the FDA to reduce the loss of kidney function. Nefecon was specifically designed to deliver budesonide to the distal ileum, selectively targeting excess Gd-IgA1 production in the gut-associated lymphoid tissue. In this review, we describe the properties of Nefecon and the evidence to date that confirms its localized treatment effect. We also present unpublished evidence from Phase I trials investigating the pharmacokinetics and cortisol suppression effects of Nefecon in healthy participants. These studies demonstrated that Nefecon has a distinct pharmacokinetic profile from other budesonide products, allowing for targeted, localized action in the distal ileum. When considered alongside existing clinical trial data showing the effect of Nefecon on gut-associated biomarkers, available evidence indicates that Nefecon has a selective immunomodulatory mechanism of action and a direct disease-modifying effect in patients with IgAN, while having low systemic exposure and adverse effects.

New therapies for immunoglobulin A nephropathy: what's the standard of care in 2023?

PURPOSE OF REVIEW

As the most common primary glomerulonephritis, immunoglobulin A (IgA) nephropathy (IgAN) is an important cause of kidney failure and mortality. Until recently, therapeutic options were limited. Fortunately, there have been numerous recent clinical trials demonstrating efficacy of new therapies in slowing chronic kidney disease (CKD) progression at varying stages of disease.

RECENT FINDINGS

The TESTING trial has provided high-quality evidence for slowing estimated glomerular filtration rate (eGFR) decline with a reduced-dose glucocorticoid regimen, while demonstrating an improved safety profile. Targeted-release budesonide represents a well tolerated therapy for reducing eGFR decline. Mycophenolate mofetil may reduce CKD progression in some populations, while hydroxychloroquine is efficacious in reducing proteinuria. Sodium-glucose cotransporter (SGLT2) inhibitors and sparsentan are effective therapies for CKD due to IgAN, but should not be used in lieu of disease-modifying immunosuppressive therapy. Many new therapies are approaching readiness for clinical use.

SUMMARY

Numerous therapeutic options now exist and include disease-modifying and nephroprotective drugs. Identifying the right treatment for the right patient is now the clinical challenge and, with new drugs on the horizon, represents the primary unmet research need in this rapidly-developing field.

Differentiating primary and secondary FSGS using non-invasive urine biomarkers

Background

Focal segmental glomerulosclerosis (FSGS) is divided into genetic, primary (p), uncertain cause, and secondary (s) forms. The subclasses differ in management and prognosis with differentiation often being challenging. We aimed to identify specific urine proteins/peptides discriminating between clinical and biopsy-proven pFSGS and sFSGS.

Methods

Sixty-three urine samples were collected in two different centers (19 pFSGS and 44 sFSGS) prior to biopsy. Samples were analysed using capillary electrophoresis-coupled mass spectrometry. For biomarker definition, datasets of age-/sex-matched normal controls (NC, n = 98) and patients with other chronic kidney diseases (CKDs, n = 100) were extracted from the urinary proteome database. Independent specificity assessment was performed in additional data of NC (n = 110) and CKD (n = 170).

Results

Proteomics data from patients with pFSGS were first compared to NC (n = 98). This resulted in 1179 biomarker (P < 0.05) candidates. Then, the pFSGS group was compared to sFSGS, and in a third step, pFSGS data were compared to data from different CKD etiologies (n = 100). Finally, 93 biomarkers were identified and combined in a classifier, pFSGS93. Total cross-validation of this classifier resulted in an area under the receiving operating curve of 0.95. The specificity investigated in an independent set of NC and CKD of other etiologies was 99.1% for NC and 94.7% for CKD, respectively. The defined biomarkers are largely fragments of different collagens (49%).

Conclusion

A urine peptide-based classifier that selectively detects pFSGS could be developed. Specificity of 95%-99% could be assessed in independent samples. Sensitivity must be confirmed in independent cohorts before routine clinical application.

The role of BAFF and APRIL in IgA nephropathy: pathogenic mechanisms and targeted therapies

Immunoglobulin A nephropathy (IgAN), characterized by mesangial deposition of galactose-deficient-IgA1 (Gd-IgA1), is the most common biopsy-proven primary glomerulonephritis worldwide. Recently, an improved understanding of its underlying pathogenesis and the substantial risk of progression to kidney failure has emerged. The "four-hit hypothesis" of IgAN pathogenesis outlines a process that begins with elevated circulating levels of Gd-IgA1 that trigger autoantibody production. This results in the formation and deposition of immune complexes in the mesangium, leading to inflammation and kidney injury. Key mediators of the production of Gd-IgA1 and its corresponding autoantibodies are B-cell activating factor (BAFF), and A proliferation-inducing ligand (APRIL), each playing essential roles in the survival and maintenance of B cells and humoral immunity. Elevated serum levels of both BAFF and APRIL are observed in patients with IgAN and correlate with disease severity. This review explores the complex pathogenesis of IgAN, highlighting the pivotal roles of BAFF and APRIL in the interplay between mucosal hyper-responsiveness, B-cell activation, and the consequent overproduction of Gd-IgA1 and its autoantibodies that are key features in this disease. Finally, the potential therapeutic benefits of inhibiting BAFF and APRIL in IgAN, and a summary of recent clinical trial data, will be discussed.

A Phase 2 Trial of Sibeprenlimab in Patients with IgA Nephropathy

BACKGROUND

A proliferation-inducing ligand (APRIL) is implicated in the pathogenesis of IgA nephropathy. Sibeprenlimab is a humanized IgG2 monoclonal antibody that binds to and neutralizes APRIL.

METHODS

In this phase 2, multicenter, double-blind, randomized, placebo-controlled, parallel-group trial, we randomly assigned adults with biopsy-confirmed IgA nephropathy who were at high risk for disease progression, despite having received standard-care treatment, in a 1:1:1:1 ratio to receive intravenous sibeprenlimab at a dose of 2, 4, or 8 mg per kilogram of body weight or placebo once monthly for 12 months. The primary end point was the change from baseline in the log-transformed 24-hour urinary protein-to-creatinine ratio at month 12. Secondary end points included the change from baseline in the estimated glomerular filtration rate (eGFR) at month 12. Safety was also assessed.

RESULTS

Among 155 patients who underwent randomization, 38 received sibeprenlimab at a dose of 2 mg per kilogram, 41 received sibeprenlimab at a dose of 4 mg per kilogram, 38 received sibeprenlimab at a dose of 8 mg per kilogram, and 38 received placebo. At 12 months, the geometric mean ratio reduction (±SE) from baseline in the 24-hour urinary protein-to-creatinine ratio was 47.2±8.2%, 58.8±6.1%, 62.0±5.7%, and 20.0±12.6% in the sibeprenlimab 2-mg, 4-mg, and 8-mg groups and the placebo group, respectively. At 12 months, the least-squares mean (±SE) change from baseline in eGFR was -2.7±1.8, 0.2±1.7, -1.5±1.8, and -7.4±1.8 ml per minute per 1.73 m2 in the sibeprenlimab 2-mg, 4-mg, and 8-mg groups and the placebo group, respectively. The incidence of adverse events that occurred after the start of administration of sibeprenlimab or placebo was 78.6% in the pooled sibeprenlimab groups and 71.1% in the placebo group.

CONCLUSIONS

In patients with IgA nephropathy, 12 months of treatment with sibeprenlimab resulted in a significantly greater decrease in proteinuria than placebo. (Funded by Visterra; ENVISION ClinicalTrials.gov number, NCT04287985; EudraCT number, 2019-002531-29.).

Decrypting the Pathological Pathways in IgA Nephropathy

IgAN is the most common form of glomerulonephritis affecting 2000000 people annually. The disease ultimately progresses to chronic renal failure and ESRD. In this article, we focused on a comprehensive understanding of the pathogenesis of the disease and thus identifying different target proteins that could be essential in therapeutic approaches in the management of the disease. Aberrantly glycosylated IgA1 produced by the suppression of the enzyme β-1, 3 galactosyltransferase ultimately triggered the formation of IgG autoantibodies which form complexes with Gd-IgA1. The complex gets circulated through the blood vessels through monocytes and ultimately gets deposited in the glomerular mesangial cells via CD71 receptors present locally. This complex triggers the inflammatory pathways activating the alternate complement system, various types of T Cells, toll-like receptors, cytokines, and chemokines ultimately recruiting the phagocytic cells to eliminate the Gd-IgA complex. The inflammatory proteins cause severe mesangial and podocyte damage in the kidney which ultimately initiates the repair process following chronic inflammation by an important protein named TGFβ1. TGF β1 is an important protein produced during chronic inflammation mediating the repair process via various downstream transduction proteins and ultimately producing fibrotic proteins which help in the repair process but permanently damage the glomerular cells.

The Expression of Non B Cell-Derived Immunoglobulins

Although V(D)J recombination and immunoglobulin (Ig) production are traditionally recognised to occur only in B lymphocytes and plasma cells, the expression of Igs in non-lymphoid cells, which we call non B cell-derived Igs (non B Igs), has been documented by growing studies. It has been demonstrated that non B-Igs can be widely expressed in most cell types, including, but not limited to, epithelial cells, cardiomyocytes, hematopoietic stem/progenitor cells, myeloid cells, and cells from immune-privileged sites, such as neurons and spermatogenic cells. In particular, malignant tumour cells express high level of IgG. Moreover, different from B-Igs that mainly localised on the B cell membrane and in the serum and perform immune defence function mainly, non B-Igs have been found to distribute more widely and play critical roles in immune defence, maintaining cell proliferation and survival, and promoting progression. The findings of non B-Igs may provide a wealthier breakthrough point for more therapeutic strategies for a wide range of immune-related diseases.

[State-of-the-art trends in the treatment of immune-mediated inflammatory kidney diseases: Translation of the fundamental science into clinical practice. A review]

Immune-mediated kidney diseases like glomerulonephritis and tubulointerstitial nephritis are not the most common cause of chronic kidney disease in the population, however the difficulties in their management, as well as a more rapid deterioration of kidney function, compared to diabetes mellitus and hypertension, justify the importance of this problem for internal medicine. Due to the fundamental discoveries in pathology and to the introduction of various methods of laboratory and instrumental investigation in the second half of the XX century substantial progress was made in the diagnostic approaches and treatment of these conditions. State-of-the-art diagnostic approach requires complex evaluation of the clinical, laboratory and morphological data to identify the nosological form of the disease. The accumulation of knowledge in the field of diseases' pathogenesis led to the revision of the current classification of glomerulonephritis that should be based on the immunopathogenesis of these conditions. The following phenotypes were suggested: autoimmunity-related, autoinflammation-related, alloimmunity-related, infections-related, and monoclonal gammopathy-related. The assessment of disease activity and chronicity in the kidney tissue should be mandatory. Personalized selection of the optimal treatment modality on the basis of the diagnosis, severity, and individual features of the patient is currently possible. The leading trends include rational prescription of glucocorticoids (steroid-sparing regimens) and cytotoxic agents, e.g. cyclophosphamide, as well as the introduction of multitarget regimens that include biologic agents or small molecules selectively suppressing B-cells or various complement pathways. Another mandatory component of treatment on par with immune suppression is nephroprotective therapy, which currently comprises not only traditional renin-angiotensin-aldosterone antagonists, but also endothelin receptor antagonists and sodium-glucose cotransporter-2 inhibitors. Current guidelines emphasize the importance of the non-pharmacological interventions for the implementation of the nephroprotective strategy. Rational combination of the aforementioned approaches allows for the optimization of the management of patients with immune-mediated kidney diseases, although it requires high competencies and strict adherence to the principles of the evidence-based medicine from the healthcare providers.

Tn antigen interactions of macrophage galactose-type lectin (MGL) in immune function and disease

Protein-carbohydrate interactions are essential in maintaining immune homeostasis and orchestrating inflammatory and regulatory immune processes. This review elucidates the immune interactions of macrophage galactose-type lectin (MGL, CD301) and Tn carbohydrate antigen. MGL is a C-type lectin receptor (CLR) primarily expressed by myeloid cells such as macrophages and immature dendritic cells. MGL recognizes terminal O-linked N-acetylgalactosamine (GalNAc) residue on the surface proteins, also known as Tn antigen (Tn). Tn is a truncated form of the elongated cell surface O-glycan. The hypoglycosylation leading to Tn may occur when the enzyme responsible for O-glycan elongation-T-synthase-or its associated chaperone-Cosmc-becomes functionally inhibited. As reviewed here, Tn expression is observed in many different neoplastic and non-neoplastic diseases, and the recognition of Tn by MGL plays an important role in regulating effector T cells, immune suppression, and the recognition of pathogens.

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.

Colocalization of IgG and IgA Heavy Chains with Kappa and Lambda Light Chains in Glomerular Deposits of IgA Nephropathy Patients Using High-Resolution Confocal Microscopy and Correlation with Oxford MEST-C Scores

Routine immunofluorescence microscopy of glomerular immunodeposits in IgA nephropathy shows IgA, C3, and lambda light chains, and sometimes IgG, IgM, and kappa light chains. However, a previous study using high-resolution confocal microscopy showed IgG in all IgA nephropathy cases, likely representing autoantibodies specific for galactose-deficient IgA1. Here, we used high-resolution confocal microscopy to examine the composition of glomerular immunodeposits and colocalization of kappa and lambda light chains with IgA or IgG heavy chains in kidney-biopsy samples from twenty patients with IgA nephropathy, seventeen without IgG, and nine with no or trace kappa light chains by routine immunofluorescence microscopy. IgG was detected in all biopsies by high-resolution confocal microscopy. Single-optical-plane images showed similar colocalization of IgG heavy chains with kappa and lambda light chains. Colocalization of IgA heavy chains was greater with lambda light chains than with kappa light chains. Colocalization of IgG heavy chain with kappa light chains was higher than with lambda light chains in biopsies with endocapillary hypercellularity and crescents, i.e., biopsies with active lesions. We confirmed the utility of high-resolution confocal microscopy to detect components of glomerular immunodeposits not apparent on routine immunofluorescence microscopy and for colocalization of different components, potentially clarifying the pathogenesis of IgA nephropathy.

Current Understanding of Complement Proteins as Therapeutic Targets for the Treatment of Immunoglobulin A Nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and a frequent cause of kidney failure. Currently, the diagnosis necessitates a kidney biopsy, with routine immunofluorescence microscopy revealing IgA as the dominant or co-dominant immunoglobulin in the glomerular immuno-deposits, often with IgG and sometimes IgM or both. Complement protein C3 is observed in most cases. IgAN leads to kidney failure in 20-40% of patients within 20 years of diagnosis and reduces average life expectancy by about 10 years. There is increasing clinical, biochemical, and genetic evidence that the complement system plays a paramount role in the pathogenesis of IgAN. The presence of C3 in the kidney immuno-deposits differentiates the diagnosis of IgAN from subclinical glomerular mesangial IgA deposition. Markers of complement activation via the lectin and alternative pathways in kidney-biopsy specimens are associated with disease activity and are predictive of poor outcome. Levels of select complement proteins in the circulation have also been assessed in patients with IgAN and found to be of prognostic value. Ongoing genetic studies have identified at least 30 loci associated with IgAN. Genes within some of these loci encode complement-system regulating proteins that can interact with immune complexes. The growing appreciation for the central role of complement components in IgAN pathogenesis highlighted these pathways as potential treatment targets and sparked great interest in pharmacological agents targeting the complement cascade for the treatment of IgAN, as evidenced by the plethora of ongoing clinical trials.

The gut microbiome tango in the progression of chronic kidney disease and potential therapeutic strategies

Chronic kidney disease (CKD) affects more than 10% population worldwide and becomes a huge burden to the world. Recent studies have revealed multifold interactions between CKD and gut microbiome and their pathophysiological implications. The gut microbiome disturbed by CKD results in the imbalanced composition and quantity of gut microbiota and subsequent changes in its metabolites and functions. Studies have shown that both the dysbiotic gut microbiota and its metabolites have negative impacts on the immune system and aggravate diseases in different ways. Herein, we give an overview of the currently known mechanisms of CKD progression and the alterations of the immune system. Particularly, we summarize the effects of uremic toxins on the immune system and review the roles of gut microbiota in promoting the development of different kidney diseases. Finally, we discuss the current sequencing technologies and novel therapies targeting the gut microbiome.

[Diagnosis and Treatment of IgA Nephropathy-2023]

Immunoglobulin A nephropathy (IgAN) is the most common glomerulonephritis. It leads to end-stage kidney disease in about a third of the patients within 10 to 20 years. The pathogenesis of IgAN is incompletely understood. It is believed that a dysregulation of the mucosal immune system leads to undergalactosylation of IgA, followed by formation of IgG autoantibodies against undergalactosylated IgA, circulation of these IgG-IgA immune complexes, deposition of the immune complexes in the mesangium, ultimately resulting in glomerular inflammation. IgAN can occasionally be triggered by other diseases, these secondary causes of IgAN should be identified or ruled out (chronic inflammatory bowel disease, infections, tumors, rheumatic diseases). Characteristic findings of IgAN of variable extent are a nephritic urinary sediment (erythrocytes, acanthocytes, erythrocyte casts), proteinuria, impaired renal function, arterial hypertension, or intermittent painless macrohematuria, especially during infections of the upper respiratory tract. However, the diagnosis of IgAN can only be made by a kidney biopsy. A histological classification (MEST‑C score) should always be reported to be able to estimate the prognosis. The most important therapeutic measure is an optimization of the supportive therapy, which includes, among other things, a consistent control of the blood pressure, an inhibition of the RAS, and the administration of an SGLT2 inhibitor. A systemic immunosuppressive therapy with corticosteroids is discussed controversially, should be used restrictively and only administered after an individual benefit-risk assessment under certain conditions that speak for a progressive IgAN. New promising therapeutics are enteral Budesonide or the dual angiotensin-II-receptor- and endothelin-receptor-antagonist Sparsentan. Rapidly progressive IgAN should be treated with corticosteroids and cyclophosphamide like ANCA-associated vasculitis.

The Effects of Perilla frutescens Extracts on IgA Nephropathy: A Systematic Review and Meta-Analysis

(1) Background: Chronic renal disorders (CRD) are associated with significant comorbidities and necessitate complex therapeutic management. As time passed, Perilla frutescens (PF) became a promising therapeutic option for CRD. The aim of this systematic review and meta-analysis was to outline the therapeutic effects of PF extracts on various models of immunoglobulin a (IgA) nephropathy; (2) Methods: Medline, Embase, and Cochrane databases were used to find relevant studies. All prospective interventional studies that evaluated the effect of PF extract versus placebo on rat models of chronic renal disorders were assessed according to the international guidelines; (3) Results: Our search yielded 23 unique records, out of which only five were included in the analysis. Our results showed that administration of PF extracts led to a statistically significant reduction in proteinuria and PCNA levels in rats that received high doses of the extract as well as in the PCNA level and DNA synthesis in rats that received low doses of the extract. The evaluated outcomes benefited from a low degree of heterogeneity; (4) Conclusions: Some of the evaluated outcomes were significantly reduced by both high and low doses of extracts from Perilla frutescens. Further studies are needed to determine the exact effect over IgA nephropathy in human subjects.

IgA nephropathy associated with Crohn's disease

The relationship between IgA nephropathy (IgAN) and Crohn’s disease was reported. IgAN is the most common primary glomerulonephritis and one of the leading causes of chronic kidney disease and end-stage renal failure, and up to 50% of cases progressed to end-stage renal disease within 25 years after IgAN diagnosis. However, specific and effective therapeutic strategies are still lacking. In this review, we discuss the possibility of the mechanism involved in IgAN associated with Crohn’s disease based on the findings of basic and clinical studies. Although the etiology of IgAN associated with Crohn’s disease is not permanent and various factors are thought to be involved, the stabilization of the disease condition of Crohn’s disease is believed to help treat IgAN.

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.

IgA Nephropathy: the Lectin Pathway and Implications for Targeted Therapy

Many patients with IgA nephropathy (IgAN) progress to end-stage kidney disease even with optimal supportive care. An improved understanding of the pathophysiology of IgAN in recent years has led to the investigation of targeted therapies with acceptable tolerability that may address the underlying causes of IgAN or the pathogenesis of kidney injury. The complement system - particularly the lectin and alternative pathways of complement - have emerged as key mediators of kidney injury in IgAN and possible targets for investigational therapy. This review will focus on the lectin pathway. Examination of kidney biopsies has consistently shown glomerular deposition of mannan-binding lectin (one of six pattern-recognition molecules that activate the lectin pathway) together with IgA1 in up to 50% of patients with IgAN. Glomerular deposition of pattern-recognition molecules for the lectin pathway is associated with more severe glomerular damage and more severe proteinuria and hematuria. Emerging research suggests that the lectin pathway may also contribute to tubulointerstitial fibrosis in IgAN, and that collectin-11 is a key mediator of this association. This review summarizes the growing scientific and clinical evidence supporting the role of the lectin pathway in IgAN and examines the possible therapeutic role of lectin pathway inhibition for these patients.

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.

Omics are Getting Us Closer to Understanding IgA Nephropathy

During the last decade, thanks to omics technologies, new light has been shed on the pathogenesis of many diseases. Genomics, epigenomics, transcriptomics, and proteomics have helped to provide a better understanding of the origin and heterogeneity of several diseases. However, the risk factors for most autoimmune diseases remain unknown. The successes and pitfalls of omics have also been observed in nephrology, including immunoglobulin A nephropathy (IgAN), the most common form of glomerulonephritis and a principal cause of end-stage renal disease worldwide. Unfortunately, the immense progress in basic research has not yet been followed by the satisfactory development of a targeted treatment. Although, most omics studies describe changes in the immune system, there is still insufficient data to apply their results in the constantly evolving multi-hit pathogenesis model and thus do to provide a complete picture of the disease. Here, we describe recent findings regarding the pathophysiology of IgAN and link omics studies with immune system dysregulation. This review provides insights into specific IgAN markers, which may lead to the identification of potential targets for personalised treatment in the future.

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.

Results from part A of the multi-center, double-blind, randomized, placebo-controlled NefIgArd trial, which evaluated targeted-release formulation of budesonide for the treatment of primary immunoglobulin A nephropathy

The therapeutic potential of a novel, targeted-release formulation of oral budesonide (Nefecon) for the treatment of IgA nephropathy (IgAN) was first demonstrated by the phase 2b NEFIGAN trial. To verify these findings, the phase 3 NefigArd trial tested the efficacy and safety of nine months of treatment with Nefecon (16 mg/d) versus placebo in adult patients with primary IgAN at risk of progressing to kidney failure (ClinicalTrials.gov: NCT03643965). NefIgArd was a multicenter, randomized, double-blind, placebo-controlled two-part trial. In Part A, 199 patients with IgAN were treated with Nefecon or placebo for nine months and observed for an additional three months. The primary endpoint for Part A was 24-hour urine protein-to-creatinine ratio (UPCR) after nine months. Secondary efficacy outcomes evaluated included estimated glomerular filtration rate (eGFR) at nine and 12 months and the UPCR at 12 months. At nine months, UPCR was 27% lower in the Nefecon group compared with placebo, along with a benefit in eGFR preservation corresponding to a 3.87 ml/min/1.73 m2 difference versus placebo (both significant). Nefecon was well-tolerated, and treatment-emergent adverse events were mostly mild to moderate in severity and reversible. Part B is ongoing and will be reported on later. Thus, NefIgArd is the first phase 3 IgA nephropathy trial to show clinically important improvements in UPCR and eGFR and confirms the findings from the phase 2b NEFIGAN study.

Current understanding of IgA antibodies in the pathogenesis of IgA nephropathy

Immunoglobulin A (IgA) is the most abundant isotype of antibodies, provides a first line of defense at mucosal surfaces against pathogens, and thereby contributes to mucosal homeostasis. IgA is generally considered as a non-inflammatory antibody because of its main function, neutralizing pathogenic virus or bacteria. Meanwhile, IgA can induce IgA-mediated diseases, such as IgA nephropathy (IgAN) and IgA vasculitis. IgAN is characterized by the deposition of IgA and complement C3, often with IgG and/or IgM, in the glomerular mesangial region, followed by mesangial cell proliferation and excessive synthesis of extracellular matrix in glomeruli. Almost half a century has passed since the first report of patients with IgAN; it remains debatable about the mechanism how IgA antibodies selectively bind to mesangial region-a hallmark of IgAN-and cause glomerular injuries in IgAN. Previous lectin- and mass-spectrometry-based analysis have revealed that IgAN patients showed elevated serum level of undergalactosylated IgA1 in O-linked glycans of its hinge region, called galactose-deficient IgA1 (Gd-IgA1). Thereafter, numerous studies have confirmed that the glomerular IgA from IgAN patients are enriched with Gd-IgA1; thus, the first hit of the current pathogenesis of IgAN has been considered to increase circulating levels of Gd-IgA1. Recent studies, however, demonstrated that this aberrant glycosylation alone is not sufficient to disease onset and progression, suggesting that several additional factors are required for the selective deposition of IgA in the mesangial region and induce nephritis. Herein, we discuss the current understanding of the characteristics of pathogenic IgA and its mechanism of inducing inflammation in IgAN.

Low-molecular weight inhibitors of the alternative complement pathway

Dysregulation of the alternative complement pathway predisposes individuals to a number of diseases. It can either be evoked by genetic alterations in or by stabilizing antibodies to important pathway components and typically leads to severe diseases such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, C3 glomerulopathy, and age-related macular degeneration. In addition, the alternative pathway may also be involved in many other diseases where its amplifying function for all complement pathways might play a role. To identify specific alternative pathway inhibitors that qualify as therapeutics for these diseases, drug discovery efforts have focused on the two central proteases of the pathway, factor B and factor D. Although drug discovery has been challenging for a number of reasons, potent and selective low-molecular weight (LMW) oral inhibitors have now been discovered for both proteases and several molecules are in clinical development for multiple complement-mediated diseases. While the clinical development of these inhibitors initially focuses on diseases with systemic and/or peripheral tissue complement activation, the availability of LMW inhibitors may also open up the prospect of inhibiting complement in the central nervous system where its activation may also play an important role in several neurodegenerative diseases.

IgA nephropathy: an overview of drug treatments in clinical trials

INTRODUCTION

IgA nephropathy (IgAN) is the commonest primary glomerulonephritis worldwide and may progress to end-stage kidney disease (ESKD) within a 10-20 year period. Its slowly progressive course has made clinical trials challenging to perform, however the acceptance of proteinuria reduction as a surrogate end point has significantly improved the feasibility of conducting clinical trials in IgAN, with several novel and repurposed therapies currently undergoing assessment. Already, interim results are demonstrating value to some of these, offering great hope to those with IgAN.

AREAS COVERED

This review explores the rationale, candidates, clinical precedents, and trial status of therapies that are currently or have recently been evaluated for efficacy in IgAN. All IgAN trials registered with the U.S. National Library of Medicine; ClinicalTrials.gov were reviewed.

EXPERT OPINION

For the first time, effective treatment options beyond supportive care are becoming available for those with IgAN. This is the culmination of commendable international efforts and signifies a new era for those with IgAN. As more therapies become available, future challenges will revolve around deciding which treatments are most appropriate for individual patients, which is likely to push IgAN into the realm of precision medicine.

Mucosal Immune System Dysregulation in the Pathogenesis of IgA Nephropathy

The mucosal immune system, via a dynamic immune network, serves as the first line of defense against exogenous antigens. Mucosal immune system dysregulation is closely associated with the pathogenesis of immunoglobulin A nephropathy (IgAN), as illustrated by IgAN having the clinical feature of gross hematuria, often concurrent with mucosal infections. Notably, previous studies have demonstrated the efficacy of tonsillectomy and found that a targeted-release formulation of budesonide reduced proteinuria in patients with IgAN. However, it remains unclear how exogenous antigens interact with the mucosal immune system to induce or exacerbate IgAN. Thus, in this review, we focus on the dysregulation of mucosal immune response in the pathogenesis of IgAN.

Emerging Roles of the Unique Molecular Chaperone Cosmc in the Regulation of Health and Disease

The core-1 β1-3galactosyltransferase-specific chaperone 1 (Cosmc) is a unique molecular chaperone of core-1 β1-3galactosyltransferase(C1GALT1), which typically functions inside the endoplasmic reticulum (ER). Cosmc helps C1GALT1 to fold correctly and maintain activity. It also participates in the synthesis of the T antigen, O-glycan, together with C1GALT1. Cosmc is a multifaceted molecule with a wide range of roles and functions. It involves platelet production and the regulation of immune cell function. Besides that, the loss of function of Cosmc also facilitates the development of several diseases, such as inflammation diseases, immune-mediated diseases, and cancer. It suggests that Cosmc is a critical control point in diseases and that it should be regarded as a potential target for oncotherapy. It is essential to fully comprehend Cosmc's roles, as they may provide critical information about its involvement in disease development and pathogenesis. In this review, we summarize the recent progress in understanding the role of Cosmc in normal development and diseases.

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.

Gut-kidney axis in IgA nephropathy: Role on mesangial cell metabolism and inflammation

IgA Nephropathy (IgAN) is the commonest primary glomerular disease around the world and represents a significant cause of end-stage renal disease. IgAN is characterized by mesangial deposition of IgA-immune complexes and mesangial expansion. The pathophysiological process includes an abnormally glycosylated IgA1, which is an antigenic target. Autoantibodies specifically recognize galactose-deficient IgA1 forming immune complexes that are amplified in size by the soluble IgA Fc receptor CD89 leading to deposition in the mesangium through interaction with non-classical IgA receptors. The local production of cytokines promotes local inflammation and complement system activation, besides the stimulation of mesangial proliferation. The spectrum of clinical manifestations is quite variable from asymptomatic microscopic hematuria to rapidly progressive glomerulonephritis. Despite all the advances, the pathophysiology of the disease is still not fully elucidated. The mucosal immune system is quoted to be a factor in triggering IgAN and a "gut-kidney axis" is proposed in its development. Furthermore, many recent studies have demonstrated that food intake interferes directly with disease prognosis. In this review, we will discuss how mucosal immunity, microbiota, and nutritional status could be interfering directly with the activation of intrinsic pathways of the mesangial cells, directly resulting in changes in their function, inflammation and development of IgAN.

Increased retinal drusen in IgA glomerulonephritis are further evidence for complement activation in disease pathogenesis

Drusen are retinal deposits comprising cell debris, immune material and complement that are characteristic of macular degeneration but also found in glomerulonephritis. This was a pilot cross-sectional study to determine how often drusen occurred in IgA glomerulonephritis and their clinical significance. Study participants underwent non-mydriatic retinal photography, and their deidentified retinal images were examined for drusen by two trained graders, who compared central drusen counts, counts ≥ 10 and drusen size with those of matched controls. The cohort comprised 122 individuals with IgA glomerulonephritis including 89 males (73%), 49 individuals (40%) of East Asian or Southern European ancestry, with an overall median age of 54 years (34-64), and median disease duration of 9 years (4-17). Thirty-nine (33%) had an eGFR < 60 ml/min/1.73 m² and 72 had previously reached kidney failure (61%). Overall mean drusen counts were higher in IgA glomerulonephritis (9 ± 27) than controls (2 ± 7, p < 0.001). Central counts ≥ 10 were also more common (OR = 3.31 (1.42-7.73, p = 0.006), and were associated with longer disease duration (p = 0.03) but not kidney failure (p = 0.31). Larger drusen were associated with more mesangial IgA staining (p = 0.004). Increased drusen counts were also present in IgA glomerulonephritis secondary to Crohn's disease but not with Henoch-Schonlein purpura. The finding of retinal drusen in IgA glomerulonephritis is consistent with complement activation and represents a model for better understanding glomerular immune deposition and a supporting argument for treatment with anti-complement therapies.

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.

Metabolic signatures of immune cells in chronic kidney disease

Immune cells play a key role in maintaining renal dynamic balance and dealing with renal injury. The physiological and pathological functions of immune cells are intricately connected to their metabolic characteristics. However, immunometabolism in chronic kidney disease (CKD) is not fully understood. Pathophysiologically, disruption of kidney immune cells homeostasis causes inflammation and tissue damage via triggering metabolic reprogramming. The diverse metabolic characteristics of immune cells at different stages of CKD are strongly associated with their different pathological effect. In this work, we reviewed the metabolic characteristics of immune cells (macrophages, natural killer cells, T cells, natural killer T cells and B cells) and several non-immune cells, as well as potential treatments targeting immunometabolism in CKD. We attempt to elaborate on the metabolic signatures of immune cells and their intimate correlation with non-immune cells in CKD.

IgA vasculitis update: Epidemiology, pathogenesis, and biomarkers

Immunoglobulin A vasculitis (IgAV), formerly known as Henoch-Schönlein purpura, is the most common systemic vasculitis in children, characterized by diverse clinical manifestations with a wide spectrum ranging from isolated cutaneous vasculitis to systemic involvement. The incidence of IgAV is geographically and ethnically variable, with a prevalence in autumn and winter, suggesting a driving role that genetic and environmental factors play in the disease. Although IgAV has a certain degree of natural remission, it varies widely among individuals. Some patients can suffer from severe renal involvement and even progress to end-stage renal disease. Its pathogenesis is complex and has not been fully elucidated. The formation of galactose-deficient IgA1 (Gd-IgA1) and related immune complexes plays a vital role in promoting the occurrence and development of IgAV nephritis. In addition, neutrophil activation is stimulated through the binding of IgA to the Fc alpha receptor I expressed on its surface, resulting in systemic vascular inflammation and tissue damage. Starting from the epidemiological characteristics, this article will review the role of immunological factors such as Gd-IgA1, autoantibodies, circulating immune complexes, complement system, cellular immunization, and the contributions of environmental and genetic factors in the pathogenesis of IgAV, and conclude with the major biomarkers for IgAV.

The Gut and Kidney Crosstalk in Immunoglobulin A Nephropathy

Immunoglobulin A nephropathy(IgAN) is the most common primary glomerulonephritis worldwide. The working model for the pathogenesis of IgAN involves a multistep process starting from the production of galactose-deficient and polymeric immunoglobulin A-1 (gd-IgA1) that enters systemic circulation from gut-associated lymphoid tissue (GALT). Galactose-deficient IgA are targeted by endogenous IgG, leading to the formation of circulating immune complexes that deposit in the mesangium and resulting in glomerular inflammation. Disease onset and relapses are often associated with gut infections, supporting the hypothesis that the gut plays an important pathogenic role. In the presence of microbial pathogens or food antigens, activated dendritic cells in the gut mucosa induce T cell dependent and independent B cell differentiation into IgA secreting plasma cells. In IgAN patients, this promotes the systemic release of mucosal gd-IgA1. Not all bacterial strains have the same capacity to elicit IgA production, and little is known about the antigen specificity of the pathogenic gd-IgA1. However, efficacy of treatments targeting gut inflammation support a pathogenic link between the bowel immune system and IgAN. Herein, we review the evidence supporting the role of gut inflammation in IgAN pathogenesis.

Simultaneous Presence of Campylobacter rectus and Cnm-Positive Streptococcus mutans in the Oral Cavity Is Associated with Renal Dysfunction in IgA Nephropathy Patients: 5-Year Follow-Up Analysis

BACKGROUND

The simultaneous presence of Streptococcus mutans expressing the Cnm protein encoded by cnm (i.e., cnm-positive S. mutans) and Campylobacter rectus in the oral cavity has been associated with proteinuria in patients with IgA nephropathy (IgAN).

OBJECTIVES

The present study evaluated the relationship between renal function and oral bacteria in patients with IgAN over 5 years of follow-up.

METHODS

The presence of C. rectus and cnm-positive S. mutans in saliva samples of 117 patients with IgAN was initially evaluated by polymerase chain reaction. Patients were then divided into four groups according to the results of C. rectus and cnm-positive S. mutans detection: group A: C. rectus (-), cnm-positive S. mutans (-); group B: C. rectus (+), cnm-positive S. mutans (-); group C: C. rectus (-), cnm-positive S. mutans (+); and group D: C. rectus (+), cnm-positive S. mutans (+). Clinical characteristics were prospectively followed for 5 years.

RESULTS

Serum creatinine levels were significantly higher in group D than in group A over 5 years of follow-up. Additionally, the proportion of patients with an estimated glomerular filtration rate <45 mL/min increased over time; it was significantly greater in group D than in group A over 5 years of follow-up.

CONCLUSION

These results suggest that the simultaneous presence of C. rectus and cnm-positive S. mutans in the oral cavity is associated with renal dysfunction in IgAN patients.

Phosphatase control of cytokine-mediated overproduction of galactose-deficient IgA1, the main autoantigen in IgA nephropathy

IgA nephropathy (IgAN) is an autoimmune disease characterized by the deposition of galactose-deficient IgA1 (Gd-IgA1)-containing immune complexes in the kidneys. Elevated serum levels of Gd-IgA1, the main autoantigen in IgAN, are associated with mucosal infections and poor renal outcome in IgAN patients, but little is known about the activation of IgA1-secreting cells overproducing this autoantigen. We found that in peripheral blood mononuclear cells (PBMCs), cytokine stimulation elevated Gd-IgA1 production in B cells from IgAN patients but not in those from healthy controls (p < 0.01). These results were replicated in immortalized B cells derived from PBMCs of IgAN patients and healthy controls. Using single-cell transcriptomics, we identified subsets of IgA1-secreting cells from IgAN patients, but not from healthy controls, with decreased expression of C1GALT1 in response to cytokine stimulation. The C1GALT1-encoded glycosyltransferase is responsible for addition of galactose to IgA1 O-glycans, and its reduced activity is associated with elevated serum levels of Gd-IgA1. These newly identified subsets of IgA1-secreting cells with reduced C1GALT1 expression exhibited reduced expression of several genes related to cytokine-mediated signaling, including those encoding phosphatases, such as SOCS1. siRNA knock-down of SOCS1, and the related SOCS3, increased Gd-IgA1 production in cells derived from PBMCs of healthy controls, indicating a role of these regulators in abnormal cytokine signaling and Gd-IgA1 overproduction. These results revealed that specific subsets of IgA1-secreting cells may be responsible for autoantigen production in IgAN due to abnormal regulation of cytokine-mediated signaling, a process that may occur in inflammatory responses in IgAN patients.

Liuwei Dihuang Pills Inhibit Podocyte Injury and Alleviate IgA Nephropathy by Directly Altering Mesangial Cell-Derived Exosome Function and Secretion

Background: Immunoglobulin A nephropathy (IgAN) is the most common glomerular disease worldwide. Its pathological features include IgA immune complex deposition, accompanied by mesangial cell proliferation and mesangial matrix expansion. This study was conducted to investigate the effects of Liuwei Dihuang pills (LWDHW) on IgAN in mice and human podocytes, as well as to determine their underlying mechanisms of action.

Methods: For in vitro experiments, podocytes were exposed to the human mesangial cell culture medium supernatant of glomerular cells treated with aggregated IgA1 (aIgA1) and LWDHW-containing serum. Cell viability and the proportion of positive cells were evaluated using CCK-8 and flow apoptosis kits, respectively. The cells were collected for western blot analysis. Twenty-four mice with IgAN induced by oral bovine serum albumin administration combined with tail vein injection of staphylococcal enterotoxin B were randomly divided into four groups of six mice each: untreated model group, model + LWDHW group, model + rapamycin group, and model + LWDHW + rapamycin group. The normal control group contained six mice. The red blood cell count in the urine, urine protein, blood urea nitrogen, serum creatinine, and IgA deposition were determined, and TUNEL and western blotting were performed in the mouse kidney tissues.

Results:In vitro experiments showed that LWDHW promoted autophagy by regulating the PI3K/Akt/mTOR signalling pathway and improved the damage to podocytes caused by the aIgA1-treated mesangial cell supernatant. This study demonstrates the effectiveness of LWDHW for treating IgAN. In the animal experiments, LWDHW significantly reduced the urine red blood cell count, serum creatinine and urea nitrogen contents, and 24 h urinary protein function and improved IgA deposition in the kidney tissues, glomerular volume, glomerular cell proliferation and polysaccharide deposition, and glomerular cell apoptosis. The pills also reversed the changes in the LC3II/I ratio and p62 content in the kidney tissues. The combination of LWDHW and rapamycin showed stronger inhibitory effects compared to those of LWDHW or rapamycin alone.

Conclusion: LWDHW may improve regulation of the PI3K-Akt-mTOR pathway and inhibit autophagy in podocytes, as well as alleviate IgA nephropathy by directly altering mesangial cell exosomes.

Crohn's disease may promote inflammation in IgA nephropathy: a case-control study of patients undergoing kidney biopsy

Intestinal immunity has been closely associated with the pathogenesis and progression of renal diseases, a relationship known as the "gut-kidney axis." To determine the association between immunoglobulin A nephropathy (IgAN) and Crohn's disease (CD), a clinico-pathological study was performed on patients who had IgAN with CD (CD-IgAN) and without CD (NOS-IgAN). We enrolled 29 patients diagnosed with IgAN via renal biopsy at the Tokyo Yamate Medical Center from 2009 to 2017. The patients were divided into CD-IgAN (n = 18) and NOS-IgAN (n = 11) and evaluated for clinical and pathological findings. IgA subclasses and galactose-deficient IgA1 (Gd-IgA1) were examined via immunohistochemistry using formalin-fixed paraffin-embedded sections from renal biopsy. Our results showed no significant difference in the extent of mesangial IgA subclasses or Gd-IgA1 deposition according to the presence or absence of CD. Pathologically, however, those with CD-IgAN had remarkably higher percentage of global glomerulosclerosis and extent of interstitial fibrosis and tubular atrophy (IF/TA) compared to those with NOS-IgAN. Moreover, the extent of macrophage infiltration in the glomerulus and interstitium was significantly higher in CD-IgAN than in NOS-IgAN. Clinically, the CD-IgAN group had significantly worse responsiveness to steroid treatment compared to the NOS-IgAN group. In conclusion, the similar immunological characteristics of deposited IgA molecules in the glomeruli between the CD-IgAN and NOS-IgAN groups might suggest their etiological similarity. However, a renal pathology showing advanced glomerular and tubulointerstitial sclerosis accompanying increased macrophage infiltration and highly resistant clinical features in patients with CD-IgAN suggests that some pathophysiological factors in CD, including abnormal intestinal immunity, may promote and activate the inflammatory process in IgAN via undetermined mechanisms.

IgA Nephropathy that Developed as an Immune-related Adverse Event of Pembrolizumab Complicated with Interstitial Nephritis

A 70-year-old man received pembrolizumab as a second-line treatment for squamous cell lung cancer of the lower right lobe. After three courses, proteinuria and hematuria were observed, which worsened after seven courses. He was diagnosed with a combination of IgA nephropathy and active interstitial nephritis. Steroid pulse therapy was started, and the dose of prednisolone was gradually reduced from 60 mg/day. Renal dysfunction as an immune-related adverse event of pembrolizumab monotherapy for non-small cell lung cancer has been reported previously. Therefore, establishing a system for the early detection and treatment that distinguishes immune-related glomerular diseases is essential.

Cytokines and Production of Aberrantly O-Glycosylated IgA1, the Main Autoantigen in IgA Nephropathy

Immunoglobulin A (IgA) nephropathy is the most common primary glomerulonephritis worldwide, with no disease-specific treatment and up to 40% of patients progressing to kidney failure. IgA nephropathy (IgAN), characterized by IgA1-containing immunodeposits in the glomeruli, is considered to be an autoimmune disease in which the kidneys are injured as innocent bystanders. Glomerular immunodeposits are thought to originate from the circulating immune complexes that contain aberrantly O-glycosylated IgA1, the main autoantigen in IgAN, bound by IgG autoantibodies. A common clinical manifestation associated with IgAN includes synpharyngitic hematuria at disease onset or during disease activity. This observation suggests a connection of disease pathogenesis with an activated mucosal immune system of the upper-respiratory and/or gastrointestinal tract and IgA1 glycosylation. In fact, some cytokines can enhance production of aberrantly O-glycosylated IgA1. This process involves abnormal cytokine signaling in IgA1-producing cells from patients with IgAN. In this article, we present our view of pathogenesis of IgAN and review how some cytokines can contribute to the disease process by enhancing production of aberrantly glycosylated IgA1. We also review current clinical trials of IgAN based on cytokine-targeting therapeutic approaches.