Epitope specificity of myeloperoxidase antibodies: identification of candidate human immunodominant epitopes

Presentation and progression of MPO-ANCA interstitial lung disease

The association between MPO-ANCA-associated vasculitis (AAV) and interstitial lung disease (ILD) has been well established. Pulmonary fibrosis may coexist with, follow, or even precede the diagnosis of AAV, and its presence adversely affects the prognosis. The optimal approach to investigating ANCA in patients with ILD remains a subject of ongoing debate. Here we aim to describe presentation and progression of MPO-ANCA ILD. We conducted a retrospective evaluation of a cohort of individuals diagnosed with MPO-ANCA ILD, with or without accompanying renal impairment, at the Immunology and Cell Therapy Unit, Careggi University Hospital, Florence, Italy, between June 2016 and June 2022. Clinical records, imaging studies, pathologic examinations, and laboratory test results were collected. Among the 14 patients identified with MPO-ANCA ILD, we observed a significant association between MPO-ANCA titers assessed at the time of ILD diagnosis and renal involvement. Renal impairment in these cases often manifested as subclinical or slowly progressive kidney damage. Interestingly, complement C3 deposits were consistently found in all renal biopsy specimens, thereby suggesting the potential for novel therapeutic targets in managing renal complications associated with MPO-ANCA ILD. The presentation of MPO-ANCA vasculitis as ILD can be the first and only clinical manifestation. MPO-ANCA levels at ILD diagnosis could warn on the progression to renal involvement in patients with MPO-ANCA ILD, hence caution is needed because renal disease can be subclinical or smoldering.

High-resolution epitope mapping of commercial antibodies to ANCA antigens by yeast surface display

Epitope mapping provides critical insight into antibody-antigen interactions. Epitope mapping of autoantibodies from patients with autoimmune diseases can help elucidate disease immunogenesis and guide the development of antigen-specific therapies. Similarly, epitope mapping of commercial antibodies targeting known autoantigens enables the use of those antibodies to test specific hypotheses. Anti-Neutrophil Cytoplasmic Autoantibody (ANCA) vasculitis results from the formation of autoantibodies to multiple autoantigens, including myeloperoxidase (MPO), proteinase-3 (PR3), plasminogen (PLG), and peroxidasin (PXDN). To perform high-resolution epitope mapping of commercial antibodies to these autoantigens, we developed a novel yeast surface display library based on a series of >5000 overlapping peptides derived from their protein sequences. Using both FACS and magnetic bead isolation of reactive yeast, we screened 19 commercially available antibodies to the ANCA autoantigens. This approach to epitope mapping resulted in highly specific, fine epitope mapping, down to single amino acid resolution in many cases. Our study also identified cross-reactivity between some commercial antibodies to MPO and PXDN, which suggests that patients with apparent autoantibodies to both proteins may be the result of cross-reactivity. Together, our data validate yeast surface display using maximally overlapping peptides as an excellent approach to linear epitope mapping.

Mechanisms of vascular damage in ANCA vasculitis

The discovery of anti-neutrophil cytoplasmic antibodies (ANCA) and their antigenic targets, myeloperoxidase (MPO) and proteinase 3 (PR3), has led to further understanding as to the pathophysiologic processes that underlie vascular and tissue damage in ANCA vasculitis. ANCA trigger neutrophil activation leading to vascular damage in ANCA vasculitis. However, decades of study have determined that neutrophil activation alone is not sufficient to cause disease. Inflammatory stimuli are drivers of ANCA autoantigen expression and ANCA production. Certain infections or bacterial peptides may be crucial players in the initial steps of ANCA immunopathogenesis. Genetic and epigenetic alterations of gene encoding for MPO and PR3 provide additional disturbances to the immune homeostasis which provide a substrate for pathogenic ANCA formation from an adaptive immune system predisposed to autoreactivity. Promoted by inflammatory cytokines, ANCA binding leads to neutrophil activation, a process characterized by conformational changes, production and release of cytotoxic substances, and alternative complement pathway activation, thus creating an intense inflammatory milieu. This cascade of events perpetuates a vicious cycle of further inflammatory cell recruitment and activation, culminating in tissue necrosis. Our understanding of the pathogenic process in ANCA vasculitis paves the way for the development of therapies targeting crucial steps in this process. The greater appreciation of the role for complement, monocytes, and the adaptive immune system has already led to novel complement blockers and is poised to lead to further innovations which will allow for tailored antigen- or cell-specific immunotherapy targeting the autoimmune process without exposure to undue risks or toxicities.

Restricted myeloperoxidase epitopes drive the adaptive immune response in MPO-ANCA vasculitis

BACKGROUND

Treatment of autoimmune diseases has relied on broad immunosuppression. Knowledge of specific interactions between human leukocyte antigen (HLA), the autoantigen, and effector immune cells, provides the foundation for antigen-specific therapies. These studies investigated the role of HLA, specific myeloperoxidase (MPO) epitopes, CD4+ T cells, and ANCA specificity in shaping the immune response in patients with anti-neutrophil cytoplasmic autoantibody (ANCA) vasculitis.

METHODS

HLA sequence-based typing identified enriched alleles in our patient population (HLA-DPB1*04:01 and HLA-DRB4*01:01), while in silico and in vitro binding studies confirmed binding between HLA and specific MPO epitopes. Class II tetramers with MPO peptides were utilized to detect autoreactive CD4+ T cells. TCR sequencing was performed to determine the clonality of T cell populations. Longitudinal peptide ELISAs assessed the temporal nature of anti-MPO447-461 antibodies. Solvent accessibility combined with chemical modification determined the buried regions of MPO.

RESULTS

We identified a restricted region of MPO that was recognized by both CD4+ T cells and ANCA. The autoreactive T cell population contained CD4+CD25intermediateCD45RO+ memory T cells and secreted IL-17A. T cell receptor (TCR) sequencing demonstrated that autoreactive CD4+ T cells had significantly less TCR diversity when compared to naïve and memory T cells, indicating clonal expansion. The anti-MPO447-461 autoantibody response was detectable at onset of disease in some patients and correlated with disease activity in others. This region of MPO that is targeted by both T cells and antibodies is not accessible to solvent or chemical modification, indicating these epitopes are buried.

CONCLUSIONS

These observations reveal interactions between restricted MPO epitopes and the adaptive immune system within ANCA vasculitis that may inform new antigen-specific therapies in autoimmune disease while providing insight into immunopathogenesis.

Myeloperoxidase anti-neutrophil cytoplasmic antibody affinity is associated with the formation of neutrophil extracellular traps in the kidney and vasculitis activity in myeloperoxidase anti-neutrophil cytoplasmic antibody-associated microscopic polyangiitis

Anti‐neutrophil cytoplasmic antibody (ANCA) is associated with small‐vessel vasculitis particularly in the kidneys and can induce the formation of neutrophil extracellular traps (NETs) from primed neutrophils. Recently we have reported that the induction of NETs correlates with ANCA affinity for myeloperoxidase (MPO) and disease activity in patients with MPO‐ANCA‐associated microscopic polyangiitis. To investigate whether MPO‐ANCA affinity is associated with the formation of NETs in vivo, we examined the occurrence of NETs in the renal tissues of patients with MPO‐ANCA‐associated microscopic polyangiitis and ANCA affinity by double immunofluorescence staining for NET components of citrullinated histone, MPO and PAD4 and by ELISA competition with MPO, respectively. We divided 30 MPO‐ANCA‐associated microscopic polyangiitis patients into 2 groups based on their ANCA affinity levels (IC₅₀ for the high: 0.11 ± 0.04 µg/mL (Group1) and IC₅₀ for the low: 0.66 ± 0.24 µg/mL (Group2)). Group1 showed a higher Birmingham vasculitis activity score (15.6 ± 5.7) and 73% of the patients presented clinically with rapidly progressive glomerulonephritis and histologically with focal/crescentic glomerulonephritis (GN). Group 2 showed a lower Birmingham vasculitis activity score (9.2 ± 4.9) and 73% of the patients presented clinically with chronic renal failure and histologically with mixed/sclerotic GN. Group 1 showed a much higher occurrence of NETs than Group 2. Our findings indicate that ANCA affinity was associated with the in vivo formation of NETs, which might be involved in the pathophysiology of patients with MPO‐ANCA‐associated microscopic polyangiitis.

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.

Neutrophil extracellular trap components in fibrinoid necrosis of the kidney with myeloperoxidase-ANCA-associated vasculitis

In response to antineutrophil cytoplasmic antibody (ANCA), activated neutrophils release decondensed chromatin associated with cytoplasmic granular proteins composed of proteolytic enzymes and myeloperoxidase (MPO); these complexes are named neutrophil extracellular traps (NETs). NET formation requires peptidylarginine deiminase 4 (PAD4) to citrullinate chromatin histones and also MPO and neutrophil elastase to aid in decondensation and release of chromatin. Immunostaining of renal biopsy revealed that NET components of citrullinated histone, MPO and PAD4 were concurrently deposited both around fibrinoid necrosis in necrotizing glomerulonephritis in the early focal phase of ANCA-associated polyangiitis and along an interlobular arterial wall.

Autoimmune hepatitis in the Alaska Native population: autoantibody profile and HLA associations

BACKGROUND & AIMS

The Alaska Native population is one of few populations in the world with a high prevalence of autoimmune hepatitis. The objective of this study was to determine the frequency and HLA and clinical associations of autoantibodies in Alaska Native people with autoimmune hepatitis.

METHODS

Alaska Native individuals with autoimmune hepatitis were recruited in clinics conducted statewide. Sera were tested for the presence of autoantibodies described in either autoimmune hepatitis or rheumatic disease. Associations between autoantibodies and HLA alleles and clinical features were assessed.

RESULTS

Seventy-one patients were included. At the study visit, 34 patients (47.9%) had antibodies to double-stranded DNA by immunofluorescence; 27 (38.0%) had anti-neutrophil cytoplasmic antibodies; and 11 (15.5%) had anti-Ro antibodies. Only one person had antibodies against soluble liver antigen, and in that person, anti-Ro was absent. Associations were found between autoantibodies and HLA alleles, including positive associations between HLA DR3 and anti-double-stranded DNA antibodies and between HLA DR14 and antineutrophil cytoplasmic antibodies. There was no association between autoantibodies and clinical outcomes.

CONCLUSIONS

As in other populations, the prevalence of anti-double-stranded DNA antibodies and antineutrophil cytoplasmic antibodies is high in Alaska Native people with autoimmune hepatitis. In contrast to data from other populations, there is a lower prevalence of anti-soluble liver antigen and a lack of association between anti-Ro and anti-soluble liver antigen. In addition, the HLA profile and associations with autoantibodies are unique. No clear prognostic implications of autoantibodies have emerged in this population.

Anti-neutrophil cytoplasmic autoantibody pathogenicity revisited: pathogenic versus non-pathogenic anti-neutrophil cytoplasmic autoantibody

Anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV) is strongly associated with autoantibodies against myeloperoxidase (MPO) and proteinase 3 (PR3). No clear consensus has been reached on the pathogenicity of these autoantibodies. Animal models for MPO-ANCA, in vitro data suggesting pathogenicity of ANCA, and one case of a neonate showing symptoms of vasculitis after transplacental transfer of MPO, argue in favour of a pathogenic role for ANCA. On the other hand, the presence of natural MPO and PR3 autoantibodies in healthy individuals, lack of a strong correlation between ANCA titres and disease activity, and the occurrence of ANCA-negative AAV patients argue against pathogenicity of ANCA. Recent papers have drawn attention to the possibility of epitope specificity defining ANCA pathogenicity. Certain MPO epitopes were found to be specific for active disease, and others remained present during remission or were also present in healthy individuals. One linear epitope, aa447-459, was not only exclusive for active disease, but also detected in the total Ig fraction of ANCA-negative patients, reactivity being masked in serum by ceruloplasmin. So, not all ANCA seems to be equal, some could be pathogenic while others are not. For development of an autoimmune response, a specific ANCA repertoire is required, which may occur through intra-molecular epitope spreading in patients.

Epitope analysis of anti-myeloperoxidase antibodies in patients with ANCA-associated vasculitis

Objective

Increasing evidences have suggested the pathogenic role of anti-neutrophil cytoplasmic antibodies (ANCA) directing myeloperoxidase (MPO) in ANCA-associated vasculitis (AAV). The current study aimed to analyze the association between the linear epitopes of MPO-ANCA and clinicopathological features of patients with AAV.

Methods

Six recombinant linear fragments, covering the whole length amino acid sequence of a single chain of MPO, were produced from E.coli. Sera from 77 patients with AAV were collected at presentation. 13 out of the 77 patients had co-existence of serum anti-GBM antibodies. Ten patients also had sequential sera during follow up. The epitope specificities were detected by enzyme-linked immunosorbent assay using the recombinant fragments as solid phase ligands.

Results

Sera from 45 of the 77 (58.4%) patients with AAV showed a positive reaction to one or more linear fragments of the MPO chain. The Birmingham Vasculitis Activity Scores and the sera creatinine were significantly higher in patients with positive binding to the light chain fragment than that in patients without the binding. The epitopes recognized by MPO-ANCA from patients with co-existence of serum anti-GBM antibodies were mainly located in the N-terminus of the heavy chain. In 5 out of the 6 patients, whose sera in relapse recognize linear fragments, the reactivity to linear fragments in relapse was similar to that of initial onset.

Conclusion

The epitope specificities of MPO-ANCA were associated with disease activity and some clinicopathological features in patients with ANCA-associated vasculitis.

Epitope specificity determines pathogenicity and detectability in ANCA-associated vasculitis

Anti-neutrophil cytoplasmic antibody-associated (ANCA-associated) small vessel necrotizing vasculitis is caused by immune-mediated inflammation of the vessel wall and is diagnosed in some cases by the presence of myeloperoxidase-specific antibodies (MPO-ANCA). This multicenter study sought to determine whether differences in ANCA epitope specificity explain why, in some cases, conventional serologic assays do not correlate with disease activity, why naturally occurring anti-MPO autoantibodies can exist in disease-free individuals, and why ANCA are undetected in patients with ANCA-negative disease. Autoantibodies from human and murine samples were epitope mapped using a highly sensitive epitope excision/mass spectrometry approach. Data indicated that MPO autoantibodies from healthy individuals had epitope specificities different from those present in ANCA disease. Importantly, this methodology led to the discovery of MPO-ANCA in ANCA-negative disease that reacted against a sole linear sequence. Autoantibodies against this epitope had pathogenic properties, as demonstrated by their capacity to activate neutrophils in vitro and to induce nephritis in mice. The confounder for serological detection of these autoantibodies was the presence of a fragment of ceruloplasmin in serum, which was eliminated in purified IgG, allowing detection. These findings implicate immunodominant epitopes in the pathology of ANCA-associated vasculitis and suggest that autoantibody diversity may be common to other autoimmune diseases.

Anti-neutrophil cytoplasmic autoantibodies: methodological aspects and clinical significance in systemic vasculitis

Antineutrophil cytoplasmic antibodies (ANCA) are the serological hallmark of some idiopathic systemic vasculitides, such as granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA) and, to a lesser extent, Churg-Strauss syndrome (CCS), the so-called ANCA-associated vasculitides (AAV). ANCA were first detected by immunofluorescence (IIFT), subsequently the target antigens myeloperoxidase (MPO) and proteinase 3 (PR3) were identified, allowing the development of the quantitative, antigen-specific assays. According to the guidelines, combining IIFT and PR3-ANCA/MPO-ANCA assures the optimal diagnostic specificity. Antigen specificity does not effectively differentiate among the different AAV, however C-ANCA/PR3-ANCA are mainly found in GPA, while P-ANCA/MPO-ANCA are more prevalent in MPA and CSS. Despite their diagnostic value, the performance of the widespread immunometric assays for ANCA testing is disappointing, particularly for the low sensitivity. In recent years, more "sensitive" assays have been developed, using the microplate as well as fully the automated technologies, with promising preliminary results. ANCA, may be detected in a number of pathological conditions other than small vessel vasculitis. However, in most of these non-vasculitic patients ANCA do not recognize MPO or PR3 as target antigens, but other granulocyte components, often multiple or unknown specificities. A positive ANCA result by itself is not diagnostic for AAV, clinical evidence and possibly histological confirmation are always required. On the other hand, a negative test result cannot completely rule out a diagnosis of AAV, as AAV without detectable ANCA exist. The appropriate use of ANCA testing strongly improves the diagnostic accuracy and clinical usefulness of the results.