Autoantibodies against Linear Epitopes of Myeloperoxidase in Anti-Glomerular Basement Membrane Disease

Recommendations for the diagnosis and treatment of anti-neutrophil cytoplasmic autoantibody associated vasculitis

Anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis is characterised by small vessel necrotising inflammatory vasculitis. Prior to immunosupressant therapy availability it usually led to a fatal outcome. Current treatment has changed ANCA-associated vasculitis into a condition with a significant response rate, although with a not negligible relapse occurrence and cumulative organ lesions, mostly due to drug-related toxicities. The use of glucocorticoids, cyclophosphamide and other immunosupressants (such as azathioprine, mychophenolate and methotrexate) was optimised in a series of clinical trials that established the treatment of reference. In recent years, a better knowledge of B lymphocyte function and the role of complement inhibition has transformed the course of this disease while minimising treatment-related adverse effects. This multidisciplinary document of recommendations is based on the consensus of three scientific societies (Internal Medicine, Nephrology and Rheumatology) and on the best available evidence on diagnosis, treatment and follow-up of patients with ANCA-associated vasculitis, including some special situations. The aim of this document is to provide updated information and well-grounded clinical recommendations to practising physicians as to how to improve the diagnosis and treatment outcome of our patients.

Epitope Spreading in Immune-Mediated Glomerulonephritis: The Expanding Target

Epitope spreading is a critical mechanism driving the progression of autoimmune glomerulonephritis. This phenomenon, where immune responses broaden from a single epitope to encompass additional targets, contributes to the complexity and severity of diseases such as membranous nephropathy (MN), lupus nephritis (LN), and ANCA-associated vasculitis (AAV). In MN, intramolecular spreading within the phospholipase A2 receptor correlates with a worse prognosis, while LN exemplifies both intra- and intermolecular spreading, exacerbating renal involvement. Similarly, ANCA reactivity in AAV highlights the destructive potential of epitope diversification. Understanding these immunological cascades reveals therapeutic opportunities-targeting early epitope spreading could curb disease progression. Despite promising insights, the clinical utility of epitope spreading as a prognostic tool remains debated. This review provides a complete overview of the current evidence, exploring the dual-edged nature of epitope spreading, the intricate immune mechanisms behind it, and its therapeutic implications. By elucidating these dynamics, we aim to pave the way for more precise, targeted interventions in autoimmune glomerular diseases.

PUPAID: A R + ImageJ pipeline for thorough and semi-automated processing and analysis of multi-channel immunofluorescence data

PUPAID is a workflow written in R + ImageJ languages which is dedicated to the semi-automated processing and analysis of multi-channel immunofluorescence data. The workflow is designed to extract fluorescence signals within automatically-segmented cells, defined here as Areas of Interest (AOI), on whole multi-layer slides (or eventually cropped sections of them), defined here as Regions of Interest (ROI), in a simple and understandable yet thorough manner. The included (but facultative) R Shiny-based interactive application makes PUPAID also suitable for scientists who are not fluent with R programming. Furthermore, we show that PUPAID identifies significantly more cells, especially in high-density regions, as compared to already published state-of-the-art methods such as StarDist or Cellpose. For extended possibilities and downstream compatibility, single cell information is exported as FCS files (the standardized file format for single cell-based cytometry data) in order to be openable using any third-party cytometry analysis software or any analysis workflow which takes FCS files as input.

Clinical characteristics and outcome of double-seropositive patients with anti-glomerular basement membrane antibodies and anti-neutrophil cytoplasmic antibodies

OBJECTIVE

To explore the clinical characteristics of double-seropositive patients (DPPs) with anti-glomerular basement membrane (Anti-GBM) antibodies and anti-neutrophil cytoplasmic antibodies (ANCA).

METHODS

We collected patients with both ANCA and anti-GBM positive glomerulonephritis who were hospitalized in the Department of Nephrology at the First Affiliated Hospital of Nanjing Medical University from January 2010 to August 2022. Retrospective analysis of the baseline clinical characteristics of patients and follow-up to explore relevant factors affecting renal and patient survival.

RESULTS

A total of 386 patients, including 69 ANCA negative anti-GBM glomerulonephritis patients, 296 anti-GBM negative ANCA associated vasculitis (AAV) patients, and 21 DPPs were enrolled in this study. Among the 21 DPPs aged 68.0 years (59.5, 74.0), there were 11 males and 10 females. The median serum creatinine at diagnosis was 629.0 (343.85, 788.75) μmol/L, and the median eGFR (CKD-EPI) was 7.58 (4.74, 13.77) mL/min. Fifteen cases (71.4 %) underwent initial RRT. After a follow-up of 40.0 (11.0, 73.0) months, 13 out of 21 DPPs (61.9 %) received maintenance RRT, while 49 out of 69 (71.0 %) ANCA negative anti-GBM-GN patients and 124 out of 296 (41.9 %) anti-GBM negative AAV patients received maintenance RRT (P < 0.001). Kaplan-Meier survival analysis showed that DPPs and ANCA negative anti-GBM-GN patients were more likely to progress to ESRD than anti-GBM negative AAV patients (P = 0.001). Among the 21 patients with DPPs, renal survival was significantly better in patients with better initial renal function, including those who did not receive initial RRT (P = 0.003), with lower serum creatinine levels (Cr < 629.0 μmol/L, P = 0.004) and higher eGFR levels (eGFR ≥ 7.60 ml/min, P = 0.005) than those with poor initial renal function. At the end of follow-up, 14 out of 21 DPPs (66.7 %) survived. Survival analysis showed no significant difference among patients in DPPs group, ANCA negative anti-GBM-GN group, and anti-GBM negative AAV group.

CONCLUSIONS

DPPs and ANCA negative anti-GBM-GN patients were more likely to progress to ESRD than anti-GBM negative AAV patients. In DPPs, the poor renal function at diagnosis might be a risk factor associated with poor renal survival.

Pathogenesis of Pulmonary Manifestations in ANCA-Associated Vasculitis and Goodpasture Syndrome

Pulmonary manifestations of vasculitis are associated with significant morbidity and mortality in affected individuals. They result from a complex interplay between immune dysregulation, which leads to vascular inflammation and tissue damage. This review explored the underlying pathogenesis of pulmonary involvement in vasculitis, encompassing various forms such as granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), eosinophilic granulomatosis with polyangiitis (EGPA), and anti-GBM disease. Mechanisms involving ANCA and anti-GBM autoantibodies, neutrophil activation, and neutrophil extracellular trap (NETs) formation are discussed, along with the role of the complement system in inducing pulmonary injury. Furthermore, the impact of genetic predisposition and environmental factors on disease susceptibility and severity was considered, and the current treatment options were presented. Understanding the mechanisms involved in the pathogenesis of pulmonary vasculitis is crucial for developing targeted therapies and improving clinical outcomes in affected individuals.

Anti-glomerular basement membrane disease developing 3 years after the development of Sweet syndrome and 1 year after the development of anti-neutrophil cytoplasmic antibody-associated vasculitis: a case report

A 73-year-old Japanese woman, with a history of Sweet syndrome diagnosed 3 years earlier and anti-myeloperoxidase (MPO) antibody anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis diagnosed 1 year earlier, presented with an episode of rapidly progressive glomerulonephritis (RPGN) with anti-glomerular basement membrane (GBM) disease. At the time of diagnosis of the ANCA-associated vasculitis 1 year earlier, serological testing yielded a negative result for anti-GBM antibody. However, at the present visit, serology for anti-MPO antibody was negative, while that for anti-GBM antibody was positive. This is the first report of anti-GBM disease developing sequentially after Sweet syndrome and ANCA-associated vasculitis. This case may provide clues to the potential immunological links among these three distinct conditions.

Vasculitis and Kidney Disease

Pediatric vasculitis is a complex group of disorders that commonly presents with multisystem involvement. Renal vasculitis can be isolated to the kidneys or can occur as part of a broader multiorgan vasculitis. Depending on severity, renal vasculitis may present as acute glomerulonephritis (AGN) often associated with hypertension and sometimes with a rapidly deteriorating clinical course. Prompt diagnosis and initiation of therapy are key to preserving kidney function and preventing long-term morbidity and mortality. This review focuses on the clinical presentation, diagnosis, and treatment objectives for common forms of renal vasculitis seen in pediatric patients.

Mass spectrometry-based N-glycosylation analysis in kidney disease

Kidney disease is a global health concern with an enormous expense. It is estimated that more than 10% of the population worldwide is affected by kidney disease and millions of patients would progress to death prematurely and unnecessarily. Although creatinine detection and renal biopsy are well-established tools for kidney disease diagnosis, they are limited by several inevitable defects. Therefore, diagnostic tools need to be upgraded, especially for the early stage of the disease and possible progression. As one of the most common post-translational modifications of proteins, N-glycosylation plays a vital role in renal structure and function. Deepening research on N-glycosylation in kidney disease provides new insights into the pathophysiology and paves the way for clinical application. In this study, we reviewed recent N-glycosylation studies on several kidney diseases. We also summarized the development of mass spectrometric methods in the field of N-glycoproteomics and N-glycomics.

Double Anti-neutrophil Cytoplasmic Antibody and Anti-glomerular Basement Membrane Antibody-positive Crescentic Glomerulonephritis, Following SARS-CoV-2 Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) affects kidneys mainly in the form of acute kidney injury but rarely can cause glomerular disease. On a very rare occasion, SARS-CoV-2 infection can be associated with anti-neutrophil cytoplasmic antigen (ANCA)-associated vasculitis and anti-glomerular basement membrane glomerulonephritis (anti-GBM GN). We report a case of a 59-year-old man who presented with progressive renal failure 8 weeks after contracting the viral infection, which progressed slowly to severe renal dysfunction. Renal biopsy showed crescentic glomerulonephritis (CrGN) accompanied by interrupted linear IgG deposits along the glomerular basement membrane (GBM) on immunofluorescence (IF) staining with associated mild acute tubular injury. The serology for anti-myeloperoxidase (MPO), as well as anti-GBM antibodies, was positive. He was treated with steroid and pulse intravenous cyclophosphamide, following which there was a significant improvement in the renal function and serological resolution of both the antibodies 6 months post-treatment. To the best of our knowledge, this is the first reported case of "double-antibody" positive CrGN following SARS-CoV-2 infection.

ANCA and anti-glomerular basement membrane double-positive patients: A systematic review of the literature

INTRODUCTION

Double-positive patients (DPP) exhibiting anti-glomerular basement membrane (GBM) and anti-neutrophil cytoplasmic antibodies (ANCAs) belong to an entity that is newly and poorly described, mainly in short series. We aimed to better characterize the epidemiological features, clinical presentation and therapeutic outcomes of these patients through a systematic review.

METHODS

We performed a systematic review of English-, German-, Spanish- and French-written publications from February 1987 to March 2020 reporting cases of DPP using the following databases: PubMed, Scielo, ScienceDirect, Google Scholar, The Cochrane Library, Open Grey, The Grey Literature Report, Clinicaltrials.gov and International Clinical Trial Registry Platform of the World Health Organization.

RESULTS

In total, 538 DPP were identified from 90 articles. Their clinical presentations were often severe, and the majority exhibited acute kidney failure (91.8%) with a median initial serum creatinine level of 873 μmol/L; 50.7% had alveolar haemorrhage. Other manifestations were present in 30.3% of DPP, mainly ear, nose, throat and articular manifestations. ANCAs were predominantly directed against MPO (n = 377/523; 72.1%) compared to PR3 (n = 107/523; 20.5%), with rare cases of triple positivity (n = 15/538; 2.9%). Although most patients received initial immunosuppressive therapy (n = 285/317; 89.9%), the one-year overall, renal and relapse-free survival rates were 64.8%, 38.7% and 71.1%, respectively.

CONCLUSION

DPP are associated with the characteristics of two eponymous vasculitis types, responsible for a poor overall and renal prognosis. Thus, simultaneous testing of both antibodies and systematic renal biopsy should be recommended in every patient with rapidly progressive glomerulonephritis to recognize this difficult-to-treat and rare disease.

The biology, pathogenetic role, clinical implications, and open issues of serum anti-neutrophil cytoplasmic antibodies

Anti-neutrophil cytoplasmic antibodies (ANCA) are a group of autoantibodies, predominantly IgG, involved in the pathogenesis of several autoimmune disorders, detected either through indirect immunofluorescence or enzyme-linked immunosorbent assay. By means of indirect immunofluorescence, the main patterns are C-ANCA (cytoplasmic) and P-ANCA (perinuclear), while proteinase 3 (PR3) and myeloperoxidase (MPO) represent the main autoantigens in granulomatosis with polyangiitis and microscopic polyangiitis, both belonging to the family of ANCA-associated vasculitis (AAV). While several experiments established the pathogenicity of MPO-ANCA, evidence remains elusive for PR3-ANCA and an additional target antigen, i.e. LAMP2, has been postulated with specific clinical relevance. The presence of a subset of AAV without ANCA may be explained by the presence of further target antigens or the presence of molecules in blood which make ANCA undetectable. A rise in ANCA titers is not necessarily predictive of a flare of disease in AAV if not accompanied by clinical manifestations. ANCA may develop through variable mechanisms, such as autoantigen complementarity, apoptosis impairment, neutrophil extracellular traps dysfunction and molecular mimicry. We will provide herein a comprehensive review of the available evidence on the biological mechanisms, pathogenetic role, and clinical implications of ANCA testing and disease management. Further, we will address the remaining open challenges in the field, including the role of ANCA in inflammatory bowel disease and in cocaine-induced vasculitis.

Acute renal failure in systemic sclerosis revealing Goodpasture syndrome: "All that glitters is not scleroderma renal crisis"

The most common cause of acute renal failure in systemic sclerosis patients is scleroderma renal crisis but other etiologies have to be considered such as another autoimmune disease. We report the case of a 60-year-old male admitted to our hospital with a renal failure. His medical history included a diagnosis of systemic sclerosis 6 months ago. Antinuclear antibodies were positive at a titer of 1:1280 with positive anti-Scl-70 and anti-myeloperoxidase (34 U/mL) antibodies. Scleroderma renal crisis was suspected. However, antineutrophil cytoplasmic antibody-associated vasculitis could not be excluded and a renal biopsy was performed. Histopathology revealed crescentic glomerulonephritis and rupture of Bowman's capsule. Anti-glomerular basement membrane antibodies were detected in serum and the diagnosis of Goodpasture syndrome was confirmed by kidney's immunofluorescence analysis showing typical deposits. Only three other cases of systemic sclerosis associated with Goodpasture syndrome have been reported in the literature. Also, rapidly progressive glomerulonephritis with positivity of both antineutrophil cytoplasmic antibody and anti-glomerular basement membrane antibodies has been described. Several studies have suggested that antineutrophil cytoplasmic antibody positivity occurs first leading to damages of the glomerular basement membrane, to the release of alpha-3 NC1 antigen, and ultimately to anti-glomerular basement membrane antibody production. Although rare, antineutrophil cytoplasmic antibody-associated vasculitis and Goodpasture syndrome should be searched for in systemic sclerosis patients with acute renal failure.

[ANCA and anti-MBG double-positive vasculitis: An update on the clinical and therapeutic specificities and comparison with the two eponymous vasculitis]

Double-positive vasculitis with anti-polynuclear cytoplasm (ANCA) and anti-glomerular basement membrane (GBM) antibodies is a rare entity of systemic vasculitis defined by the presence of ANCA and anti-GBM antibodies. The gradual accumulation of clinical and therapeutic data shows the usefulness of identifying and differentiating this entity from the two vasculitis respectively associated with the isolated presence of each of these two antibodies. Indeed, the double-positive ANCA and anti-GBM vasculitis appears to associate the characteristics of the demography and the extra-renal and pulmonary involvement of the ANCA-associated vasculitis on the one hand, and of the histological type and severe renal prognosis of the anti-MBG vasculitis on the other hand, with the renal involvement which is the only involvement consistently observed in double-positive vasculitis. The aim of this focus is to describe the epidemiological, clinico-biological, histological and prognostic characteristics of this entity, in light of recent literature and ongoing therapeutic changes in the two eponymous vasculitis.

[Review on anti-glomerular basement membrane disease or Goodpasture's syndrome]

Anti-glomerular basement membrane (anti-GBM) disease or Goodpasture's syndrome is a small vessel vasculitis affecting the capillary beds of kidneys and lungs. It is an autoimmune disease mediated by autoantibodies targeting the glomerular and alveolar basement membranes, leading to pneumorenal syndrome. It is a rare, monophasic and severe disease, associating rapidly progressive glomerulonephritis and alveolar hemorrhage. The presence of antineutrophil cytoplasmic antibodies (ANCA) is reported in 20 to 60% of cases. Management should be prompt and combine plasma exchange with systemic corticosteroids and immunosuppressive therapy by cyclophosphamide. The objective of this review is: 1) to describe the pathogenesis, clinical and histological features of the disease; 2) to characterize double-positive anti-GBM/ANCA patients; 3) to highlight the prognostic factors of renal and global survival, and 4) to focus on the treatment of anti-GBM disease.

Successful treatment of steroid-refractory double-positive ANCA and anti-GBM disease with a combination of plasma exchange and immunosuppression: A case report and literature review

The concurrence of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) and anti-glomerular basement membrane (GBM) disease, known as double-positive disease, is rare, but it occurs at a much higher frequency than expected by chance. Double-positive disease has an aggressive clinical course, with no optimal treatment strategy. Here we describe a patient with steroid-refractory double-positive disease who was treated successfully with the addition of plasma exchange (PE) and cyclophosphamide (CPA). A 78-year-old Japanese woman who was diagnosed with diffuse alveolar hemorrhage and rapidly progressive glomerulonephritis received two cycles of pulse steroid therapy. However, her respiratory and renal condition deteriorated. She was found to be positive for both myeloperoxidase-ANCA and anti-GBM antibodies. The combination of PE and CPA improved her systemic condition. This is the first case report of a patient with steroid-refractory double-positive disease who was successfully treated with the addition of PE and CPA. The marked contrast in therapeutic response to corticosteroids alone and the addition of PE and CPA in this case strongly implies that earlier induction of combination therapy aimed at rapid removal of pathogenic autoantibodies and prevention of ongoing antibody production might improve the outcome of this life-threatening disease.

Stability of important antibodies for kidney disease: pre-analytic methodological considerations

Background

The importance of circulating antibodies as biomarkers of kidney disease has recently been recognized. However, no study has systematically described the methodology of sample preparation and storage regarding antibodies as biomarkers of kidney disease. It remains unknown whether repetitive freeze-thaw cycles, physical disturbances, storage at different temperatures or for different periods of time, or haemolytic or turbid serum samples affect antibody measurements. The aim of this study was to investigate the stabilities of antibodies associated with kidney disease in serum samples under various relevant clinical and research conditions.

Methods

We stored serum samples in the following different conditions: repetitive freeze-thaw cycles (1, 6 or 12 times), long-term storage (7 or 12 months at -80 °C), physical disturbance (1 or 8 h), and storage at 4 °C (1, 3 or 6 weeks) and room temperature (1 or 7 days). The stabilities of the anti-phospholipase A2 receptor (anti-PLA2R), anti-glomerular basement membrane, anti-myeloperoxidase and anti-proteinase 3 antibodies were evaluated with enzyme-linked immunosorbent assays (ELISA).

Results

We found that repetitive freeze-thaw cycles did not have a significant effect on the stabilities of the abovementioned antibodies in clear serum samples. The ELISA readings of haemolytic and turbid serum samples tended to increase and decrease, respectively. Neither long-term storage at -80 °C nor physical disturbance had a significant effect on anti-PLA2R antibody stability in sealed serum samples. The concentrations of most of these antibodies increased in unsealed serum samples that were stored at 4 °C for more than 6 weeks or at room temperature for more than 7 days.

Discussion

Our findings revealed that the abovementioned circulating antibodies that are used as biomarkers for kidney disease had stable physicochemical properties, structures and immunoreactivities such that they were not influenced by repetitive freeze-thaw cycles, physical disturbances or long-term storage at -80 °C. However, the ELISA readings tended to change for haemolytic, turbid and unsealed serum samples.

Anti-Glomerular Basement Membrane Disease

Anti-glomerular basement membrane (anti-GBM) disease is a rare small vessel vasculitis that affects the capillary beds of the kidneys and lungs. It is an archetypic autoimmune disease, caused by the development of directly pathogenic autoantibodies targeting a well characterized autoantigen expressed in the basement membranes of these organs, although the inciting events that induce the autoimmune response are not fully understood. The recent confirmation of spatial and temporal clustering of cases suggests that environmental factors, including infection, may trigger disease in genetically susceptible individuals. The majority of patients develop widespread glomerular crescent formation, presenting with features of rapidly progressive GN, and 40%-60% will have concurrent alveolar hemorrhage. Treatment aims to rapidly remove pathogenic autoantibody, typically with the use of plasma exchange, along with steroids and cytotoxic therapy to prevent ongoing autoantibody production and tissue inflammation. Retrospective cohort studies suggest that when this combination of treatment is started early, the majority of patients will have good renal outcome, although presentation with oligoanuria, a high proportion of glomerular crescents, or kidney failure requiring dialysis augur badly for renal prognosis. Relapse and recurrent disease after kidney transplantation are both uncommon, although de novo anti-GBM disease after transplantation for Alport syndrome is a recognized phenomenon. Copresentation with other kidney diseases such as ANCA-associated vasculitis and membranous nephropathy seems to occur at a higher frequency than would be expected by chance alone, and in addition atypical presentations of anti-GBM disease are increasingly reported. These observations highlight the need for future work to further delineate the immunopathogenic mechanisms of anti-GBM disease, and how to better refine and improve treatments, particularly for patients presenting with adverse prognostic factors.

Patients double-seropositive for ANCA and anti-GBM antibodies have varied renal survival, frequency of relapse, and outcomes compared to single-seropositive patients

Co-presentation with both ANCA and anti-GBM antibodies is thought to be relatively rare. Current studies of such ‘double-positive’ cases report small numbers and variable outcomes. To study this further we retrospectively analyzed clinical features and long-term outcomes of a large cohort of 568 contemporary patients with ANCA-associated vasculitis, 41 patients with anti-GBM disease, and 37 double-positive patients with ANCA and anti-GBM disease from four European centers. Double-positive patients shared characteristics of ANCA-associated vasculitis (AAV), such as older age distribution and longer symptom duration before diagnosis, and features of anti-GBM disease, such as severe renal disease and high frequency of lung hemorrhage at presentation. Despite having more evidence of chronic injury on renal biopsy compared to patients with anti-GBM disease, double-positive patients had a greater tendency to recover from being dialysis-dependent after treatment and had intermediate long-term renal survival compared to the single-positive patients. However, overall patient survival was similar in all three groups. Predictors of poor patient survival included advanced age, severe renal failure, and lung hemorrhage at presentation. No single-positive anti-GBM patients experienced disease relapse, whereas approximately half of surviving patients with AAV and double-positive patients had recurrent disease during a median follow-up of 4.8 years. Thus, double-positive patients have a truly hybrid disease phenotype, requiring aggressive early treatment for anti-GBM disease, and careful long-term follow-up and consideration for maintenance immunosuppression for AAV. Since double-positivity appears common, further work is required to define the underlying mechanisms of this association and define optimum treatment strategies.

Plasma from patients with anti-glomerular basement membrane disease could recognize microbial peptides

Infection has long been suspected as a trigger of autoimmune diseases, and molecular mimicry mechanism was hypothesized in this study. Microbe originated peptides were searched from the Uniprot database based on a previous defined critical amino acid motif within α3_129−150, isoleucine137, tryptophan140, glycine142, phenylalanine 143 and phenylalanine 145. 23826 microbial peptides were identified using our searching strategy, among which seven were related with human infections. Circulating IgG and IgM antibodies against the seven microbial peptides were detected using ELISA in 76 patients with anti-GBM disease. Four peptides were recognized by both IgG and IgM antibodies, and one peptide was recognized by IgG antibodies only. Peptides from Bacteroides, Saccharomyces cerevisiae, and Bifidobacterium thermophilum possessed the highest recognition frequency with the prevalence of 73.7%, 61.8% and 67.1% for IgG, 56.6%, 44.7% and 67.1% for IgM in anti-GBM patients. Patients with antibodies against these microbial peptides showed more severe kidney injury, including higher serum creatinine and higher percentage of crescent formation. In conclusion, antibodies against microbial peptides were identified in the circulation of anti-GBM patients, implying its etiological role in eliciting autoimmune response against α3(IV)NC1 through molecular mimicry.

Deglycosylation of myeloperoxidase uncovers its novel antigenicity

Myeloperoxidase (MPO) is a common target antigen of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis and is recognized in one-third of patients with anti-glomerular basement membrane (GBM) disease. Our previous study identified over 60% of patients with anti-GBM disease recognizing linear peptides of MPO heavy chain. Here we tested whether aberrant glycosylation alters MPO antigenicity through exposure of neo-epitopes on MPO molecules. Atypical glycosylated MPO molecules, including all possible glycosylation types, were prepared by exoglycosidase and endoglycosidase treatments. Antibodies were detected from the sera of 40 patients with anti-GBM disease without the coexistence of MPO-ANCA. Circulating antibodies against aberrant glycosylated MPO existed in 21 of these patients. Non-glycan MPO and MPO with only N-acetylglucosamine had high frequencies of recognition (16 and 15 patients, respectively). Antibodies binding to aberrant glycosylated MPO could not be inhibited by intact MPO or GBM antigen. When applied to ethanol-fixed neutrophils from normal individuals, these antibodies yielded a typical cytoplasmic staining pattern (c-ANCA). Antigen specificity was detected in 90% of the antibodies using five peptides containing one of the five N-glycosylation sites each, mostly on N323, N355, and N391. The antibodies were restricted to IgG1 subclass, could activate complement, and induce neutrophil degranulation in vitro. Thus, aberrant glycosylated MPO exposed neo-epitopes and was recognized by half of the patients with anti-GBM disease. Their antibodies possessed pathogenic characteristics and may be associated with kidney injury.