Anti-myeloperoxidase autoantibodies react with native but not denatured myeloperoxidase

ANCA-associated vasculitis

The anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAVs) are a group of disorders involving severe, systemic, small-vessel vasculitis and are characterized by the development of autoantibodies to the neutrophil proteins leukocyte proteinase 3 (PR3-ANCA) or myeloperoxidase (MPO-ANCA). The three AAV subgroups, namely granulomatosis with polyangiitis (GPA), microscopic polyangiitis and eosinophilic GPA (EGPA), are defined according to clinical features. However, genetic and other clinical findings suggest that these clinical syndromes may be better classified as PR3-positive AAV (PR3-AAV), MPO-positive AAV (MPO-AAV) and, for EGPA, by the presence or absence of ANCA (ANCA⁺ or ANCA^-, respectively). Although any tissue can be involved in AAV, the upper and lower respiratory tract and kidneys are most commonly and severely affected. AAVs have a complex and unique pathogenesis, with evidence for a loss of tolerance to neutrophil proteins, which leads to ANCA-mediated neutrophil activation, recruitment and injury, with effector T cells also involved. Without therapy, prognosis is poor but treatments, typically immunosuppressants, have improved survival, albeit with considerable morbidity from glucocorticoids and other immunosuppressive medications. Current challenges include improving the measures of disease activity and risk of relapse, uncertainty about optimal therapy duration and a need for targeted therapies with fewer adverse effects. Meeting these challenges requires a more detailed knowledge of the fundamental biology of AAV as well as cooperative international research and clinical trials with meaningful input from patients.

Development of Monoclonal Antibodies Recognizing Linear Epitope: Illustration by Three Bacillus thuringiensis Crystal Proteins of Genetically Modified Cotton, Maize, and Tobacco

Bacillus thuringiensis Cry1Ac, Cry1Ia1, and Cry1Ie are δ-endotoxin insecticidal proteins widely implemented in genetically modified organisms (GMO), such as cotton, maize, and potato. Western blot assay integrates electrophoresis separation power and antibody high specificity for monitoring specific exogenous proteins expressed in GMO. Procedures for evoking monoclonal antibody (mAb) for Western blot were poorly documented. In the present study, Cry1Ac partially denatured at 100 °C for 5 min was used as an immunogen to develop mAbs selectively recognizing a linear epitope of Cry1Ac for Western blot. mAb 5E9C6 and 3E6E2 selected with sandwich ELISA strongly recognized the heat semidenatured Cry1Ac. Particularly, 3E6E2 recognized both E. coli and cotton seed expressed Cry1Ac in Western blot. Such strategy of using partially denatured proteins as immunogens and using sandwich ELISA for mAb screening was also successfully demonstrated with production of mAbs against Cry1Ie for Western blot assay in maize.

Epitope analysis of anti-myeloperoxidase antibodies in propylthiouracil-induced antineutrophil cytoplasmic antibody-associated vasculitis

Introduction

Increasing evidence has suggested that linear epitopes of antineutrophil cytoplasmic antibody (ANCA) directed to myeloperoxidase (MPO) might provide clues to the pathogenesis of propylthiouracil (PTU)-induced ANCA-associated vasculitis (AAV). This study mapped epitopes of MPO-ANCA in sera from patients with PTU-induced MPO-ANCA (with or without vasculitis) and primary AAV, aiming to analyze certain epitopes associated with the development of PTU-induced AAV.

Methods

Six recombinant linear fragments, covering the whole amino acid sequence of a single chain of MPO, were produced from Escherichia coli. Sera from 17 patients with PTU-induced AAV, 17 patients with PTU-induced MPO-ANCA but without clinical evidence of vasculitis, and 64 patients with primary AAV were collected at presentation. Of the 17 patients with PTU-induced AAV, 12 also had sera at remission. The epitope specificities were detected by enzyme-linked immunosorbent assay by using the recombinant fragments as solid-phase ligands.

Results

Compared with patients with PTU-induced MPO-ANCA but without clinical vasculitis, sera from PTU-induced AAV patients showed significantly higher reactivity against the H1 fragment of MPO (optical density values: 0.17 (0.10 to 0.35) versus 0.10 (0.04 to 0.21), P = 0.038) and could recognize a significantly higher number of fragments (two (none to four) versus one (none to two), P = 0.026). Compared with sera from primary AAV patients, sera from PTU-induced AAV patients had significantly higher reactivity to the P fragment and the H4 fragment (47.1% versus 14.1% P < 0.001; 41.2% versus 14.1%, P = 0.034, respectively), and could recognize a significantly higher number of fragments (two (none to four) versus one (none to two), P = 0.013]. Among the 12 PTU-induced AAV patients with sequential samples, the number of fragments recognized in remission was significantly less than that in initial onset (two (none to four) versus none (none to 0.75), P < 0.001].

Conclusions

Linear epitopes of MPO molecules might be associated closely with PTU-induced AAV. In particular, the P and H4 fragments may be important epitopes in PTU-induced AAV.

Neutrophil extracellular traps mediate transfer of cytoplasmic neutrophil antigens to myeloid dendritic cells toward ANCA induction and associated autoimmunity

Antineutrophil cytoplasmic antibodies (ANCAs) target proteins normally retained within neutrophils, indicating that cell death is involved in the autoimmunity process. Still, ANCA pathogenesis remains obscure. ANCAs activate neutrophils inducing their respiratory burst and a peculiar form of cell death, named NETosis, characterized by formation of neutrophil extracellular traps (NETs), decondensed chromatin threads decorated with cytoplasmic proteins endorsed with antimicrobial activity. NETs have been consistently detected in ANCA-associated small-vessel vasculitis, and this association prompted us to test whether the peculiar structure of NET favors neutrophil proteins uploading into myeloid dendritic cells and the induction of ANCAs and associated autoimmunity. Here we show that myeloid DCs uploaded with and activated by NET components induce ANCA and autoimmunity when injected into naive mice. DC uploading and autoimmunity induction are prevented by NET treatment with DNAse, indicating that NET structural integrity is needed to maintain the antigenicity of cytoplasmic proteins. We found NET intermingling with myeloid dendritic cells also positive for neutrophil myeloperoxidase in myeloperoxidase-ANCA-associated microscopic poliangiitis providing a potential correlative picture in human pathology. These data provide the first demonstration that NET structures are highly immunogenic such to trigger adaptive immune response relevant for autoimmunity.

Influence of myeloperoxidase-catalyzing reaction on the binding between myeloperoxidase and anti-myeloperoxidase antibodies

In the current study, whether myeloperoxidase (MPO)-catalyzing reaction could influence the antigenicity of MPO was investigated. Hypochlorite acid, the main product of the catalytic reaction, could lower the binding between MPO-antineutrophil cytoplasmic antibodies (ANCA) and MPO when the available chlorine was higher than 0.031×10(-3) g/l. After MPO-catalyzing reaction with H(2)O(2) lower than 0.469 g/l, the binding level between MPO-ANCAcontaining plasma and MPO increased slightly. The peak binding level was 1.135 ± 0.205 (expressed by the absorbance value at 405 nm). However, with the existence of hydrogen donor (o-phenylenediamine) in the reaction system, the peak binding level between MPO-ANCA-containing plasma and post-catalyzing MPO was significantly higher (1.367 ± 0.321 vs 1.135 ± 0.205, p = 0.023). Moreover, at the approximately physical concentration of H(2)O(2) (0.02 g/l), MPO-ANCA exhibited higher titer to post-catalyzing MPO than to pre-catalyzing MPO (3.91 ± 0.84 vs 3.57 ± 0.84, p < 0.001, expressed as the lgT). These data demonstrated that MPO-catalyzing reaction could potentially increase the antigenicity of MPO.

A case of a false-positive anti-myeloperoxidase antibody ELISA in a patient with hypothyroidism

We present a case of a false-positive anti-myeloperoxidase (MPO) antibody result on an ELISA in a patient with anti-thyroid microsomal antibody (TMA)-positive hypothyroidism. A 41-year-old woman presented with dyspnea on exertion. The initial evaluation revealed pericardial effusion associated with hypothyroidism. In addition, microscopic hematuria with normal renal function and positive cytoplasmic anti-neutrophil cytoplasmic antibodies (c-ANCA) on immunofluorescent assay were found. In further evaluation, elevated anti-TMA and MPO antibodies by ELISA. While no definite signs of vasculitis were present, the clinical state improved with thyroid hormone replacement and diuretics. Anti-MPO antibody was still positive in the follow-up tests, and microscopic hematuria persisted. On the basis of previous reports that thyroid peroxidase and MPO molecules contain cross-reactive epitopes that are exposed in denaturated molecules, we suggest that in a patient with anti-TMA-positive hypothyroidism, anti-MPO antibody might also be positive on ELISA without clinical signs of vasculitis.

Comparison of characteristics of natural autoantibodies against myeloperoxidase and anti-myeloperoxidase autoantibodies from patients with microscopic polyangiitis

OBJECTIVE

Natural autoantibodies (NAAs) against MPO exist in normal human plasma. In the current study, the immune characteristics of MPO-NAA and MPO-ANCA were examined and compared with the aim to investigate the pathogenesis of MPO-ANCA.

METHODS

MPO-NAAs were affinity purified from normal plasma of five healthy blood donors and one batch of IVIG. MPO-ANCAs were purified from plasma of 10 patients with MPA. Antigen specificity of the antibodies was tested by western blot analysis. The titre, the avidity, the Immunoglobulin G (IgG) subclasses and the effect of the antibodies towards the binding between ceruloplasmin and MPO were tested using ELISAs. The MPO-NAA-induced production of reactive oxygen species was assessed using oxidation of dihydrorhodamine (DHR) to rhodamine.

RESULTS

MPO-NAA recognized epitope(s) in the heavy chains of MPO with conformation-dependent structure, the same as MPO-ANCA. The median titre of MPO-NAA was lower than that of MPO-ANCA (1 : 40 vs 1 : 4800, P < 0.001). The median avidity of MPO-NAA was lower than that of MPO-ANCA (2.2 × 10(7) vs 8.7 × 10(7)/M, P = 0.014). The IgG subclasses of MPO-NAA were mainly restricted to IgG1 (100%) and lack of IgG3. The inhibition effect on the binding between ceruloplasmin and MPO was lower for MPO-NAA than MPO-ANCA (P = 0.046). The MPO-NAA-induced respiratory burst of neutrophils was significantly weaker than that of MPO-ANCA (P = 0.036).

CONCLUSION

The lower titre, lower avidity and lack of IgG3 subclass compared with MPO-ANCA may contribute to the non-pathogenic co-existence of MPO-NAA with MPO in serum.

[Pathogenic role of antimyeloperoxidase antibodies]

Antimyeloperoxidase antibodies are a variety of antineutrophil cytoplasm antibodies (Anca), which can be detected in systemic small-sized vessel vasculitides such as microscopic polyangiitis, Wegener's granulomatosis and Churg-Strauss syndrome. Antimyeloperoxidase antibodies have been also associated with the development of lung fibrosis. Their pathogenic role has been well established, both in vitro and in vivo. These autoantibodies can activate neutrophils and trigger their oxidative burst leading to the release of free oxygen species and cytotoxic proteins. The oxidative burst is deleterious for the endothelium. Another mechanism by which antimyeloperoxidase may act is the activation of myeloperoxydase leading to an increased production of hypochlorous acid, which is highly toxic for the endothelial cells. These mechanisms contribute to the development of vasculitis.

Clinical and pathophysiological significance of anti-neutrophil cytoplasmic autoantibodies in vasculitis syndromes

Necrotizing vasculitis of small blood vessels is a rare condition, but when it affects important organs it can lead to life-threatening organ damage and death. Thus, recognizing these conditions at an early stage before they spread to become systemic is a constant challenge to clinical medicine. The objectives of this review are: to give advice on clinical indications for ANCA diagnostics and laboratory procedures for highly specifically detecting the most important ANCA; to provide some data on the autoantigens involved in ANCA reactivity in small vessel vasculitides; and to discuss at the occurrence of ANCA in different vasculitic populations and at different stages of disease. One important task for the near future will be to standardize the assays used for ANCA detection/quantification and to harmonize the results given to clinicians by ensuring that international reference reagents are used by laboratories and the diagnostic industry. Finally, the author has attempted to summarize the role that ANCA are currently believed to play in the immuno-inflammatory events that take place in tissues and that affect small vessels in idiopathic vasculitis. The review concludes that the presence of ANCA is likely to become an important criterion for diagnosing idiopathic small vessel vasculitis.

Epitope-specific anti-neutrophil cytoplasmic antibodies: do they matter? Can they be detected?

Proteinase 3 (PR3)-anti-neutrophil cytoplasmic antibodies (ANCA) and myeloperoxidase (MPO)-ANCA are suggested to play a pathogenic role as they are closely related to the small-vessel vasculitis syndromes, Wegener's granulomatosis and microscopic polyangiitis. A large body of in vitro and animal experiments supports this concept. The mechanisms of action involve a direct interaction between ANCA and its antigen. The epitope specificity of ANCA may therefore influence the functional effects of ANCA and/or may reflect the mechanisms behind different disease manifestations or disease courses.

Antineutrophil cytoplasmic autoantibodies: how should the biologist manage them?

Antineutrophil cytoplasmic antibodies (ANCA) are directed against enzymes found in the granules of the polymorphonuclear (PMN) leukocytes. They are detected by indirect immunofluorescence microscopy assays on human ethanol fixed neutrophils. Three different fluorescence patterns can be distinguished: a cytoplasmic pattern (cANCA), a perinuclear pattern (pANCA), and an atypical pattern (aANCA). The use of other fixatives, e.g., formalin and methanol, allows differentiation between the pANCA and the antinuclear antibodies. ANCA specificity is determined by solid phase assays (ELISA, immunodot, and multiplex assay). ANCA with high titres and defined specificities (antiproteinase 3 [PR 3] or antimyeloperoxidase [MPO]) are proven to be good serological markers of active primary systemic vasculitis: c/PR 3-ANCA for Wegener's granulomatosis and p/MPO-ANCA for microscopic polyangiitis. The former have higher sensitivity and specificity for Wegener's granulomatosis than the latter for microscopic polyangiitis. ANCA with low titres and unknown specificity have been detected in a wide range of inflammatory and infectious diseases leading to a critical reappraisal of the diagnostic significance of ANCA testing. Physicians must keep in mind the possible occurrence of infectious diseases like subacute endocarditis that could be dramatically worsened by irrelevant immunosuppressive therapy. ANCA findings in certain manifestations, such as the pulmonary-renal syndrome in which massive pulmonary hemorrhage can quickly be life-threatening, warrant ANCA testing as an emergency test for patient care.

Natural and disease associated anti-myeloperoxidase (MPO) autoantibodies

Myeloperoxidase (MPO) is a cationic protein present in primary azurophilic granules of neutrophils and monocytes. MPO produces a highly deleterious reactive oxygen species, the hypochlorous acid (HOCl), using hydrogen peroxide (H(2)O(2)) and chloride ions as substrate. Anti-MPO antibodies (Abs) are present in 70% of the cases in patients with microscopic polyangiitis (MPA), a small-sized vessel vasculitis. Anti-MPO Abs from patients with MPA can trigger the release of MPO by neutrophils and monocytes. Anti-MPO Abs can activate MPO to generate an oxidative stress deleterious for the endothelium. Thus, we recently demonstrated that MPA sera with anti-MPO Abs activated MPO in vitro, and generated hypochlorous acid, whereas sera from MPA patients with no anti-MPO Abs or healthy individuals did not. Both hypochlorous acid production and endothelial lysis were abrogated by N-acetylcysteine (NAC), an antioxidant molecule. Thus, anti-MPO Abs could play a pathogenic role in vivo by triggering an oxidative burst leading to severe endothelial damages.

Inhibition of oxidation activity of myeloperoxidase (MPO) by propylthiouracil (PTU) and anti-MPO antibodies from patients with PTU-induced vasculitis

Propylthiouracil (PTU) could induce antineutrophil cytoplasmic antibody (ANCA) associated vasculitis. This study aimed to investigate the inhibitory effects on MPO oxidation activity by PTU and MPO-ANCA from patients with primary microscopic polyangiitis (MPA) and PTU-induced vasculitis. IgG preparations were purified from MPO-ANCA-positive sera from seven patients with PTU-induced vasculitis and ten patients with primary MPA. The oxidation activity of MPO was measured in the presence of PTU and MPO-ANCA-positive IgG preparations from patients with PTU-induced vasculitis and primary MPA respectively. PTU could competitively inhibit the oxidation activity of MPO dose dependently. MPO-ANCA-positive IgG preparations from 6/7 patients with PTU-induced vasculitis and only 3/10 from patients with primary MPA could inhibit the MPO activity in a dose-dependent manner. In conclusions, the oxidation activity of MPO could be inhibited by PTU and PTU-induced MPO-ANCA in a dose-dependent manner, which might be involved in the pathogenesis PTU-induced vasculitis.

A specific method for measurement of equine active myeloperoxidase in biological samples and in in vitro tests

An original method called SIEFED (specific immunological extraction followed by enzymatic detection) was developed for the specific detection of the activity of equine myeloperoxidase (MPO). The method consists of the extraction of MPO from aqueous solutions by immobilized anti-MPO antibodies followed by washing (to eliminate proteins and interfering molecules) and measurement of MPO activity using a detection system containing a fluorogenic substrate, hydrogen peroxide, and nitrite as reaction enhancer. The SIEFED technique was applied to study active MPO in horse biological fluids and the effects of 2 polyphenolic molecules, curcumin and resveratrol, on MPO activity. The detection limit of the SIEFED was 0.23 mU/ml. The SIEFED exhibited good precision with intra-assay and interassay coefficients of variation below 10% and 20%, respectively, for MPO activities ranging from 0.25 to 6.4 mU/ml. The activity of MPO was generally higher than 1 mU/ml in the fluids collected from horses with inflammatory diseases. Curcumin and resveratrol exerted a dose-dependent inhibition on MPO activity and, as they were removed before the enzymatic detection of MPO, the results suggest a direct drug-nzyme interaction or an enzyme structure modification by the drug. The SIEFED is a new tool that would be useful for specific detection of active MPO in complex media and for selection of MPO activity modulators.

Autoantibodies to intracellular autoantigens and their B-cell epitopes: molecular probes to study the autoimmune response

A common laboratory finding in systemic autoimmune diseases is the presence of autoantibodies against intracellular autoantigens. Although their pathogenesis is not fully understood, autoantibodies are important tools for establishing diagnosis, classification, and prognosis of autoimmune diseases. Autoantibodies mainly target multicomponent complexes containing both protein antigens and (ribo)-nucleic acid(s), such as the spliceosome or Ro/La RNPs. In this review, we address the main characteristics and the clinical value of the main autoantibody types with respect to their disease association, and we describe the corresponding autoantigens, their biologic function, and their B-cell antigenic determinants (epitopes). The structural characteristics and clinical associations of these epitopes, and their utility as tools to investigate the autoimmune response, are discussed in detail. New insights into the pathogenetic role of epitopes in systemic autoimmunity are also examined. In this regard, using the defined structures of the B-cell antigenic epitopes, complementary epitopes can be designed according to the "molecular recognition" theory. These complementary epitopes can be used as probes to study pathogenetic and immunoregulatory aspects of the anti-idiotypic response. The origin of humoral autoimmunity and the spreading of the epitopes in systemic lupus erythematosus are also discussed. Finally, the ability of post-translational modifications to induce autoreactive immune attack via the generation of neo-epitopes is summarized.

Mapping of myeloperoxidase epitopes recognized by MPO-ANCA using human-mouse MPO chimers

Myeloperoxidase (MPO) is one of the major target antigens of antineutrophil cytoplasmic autoantibodies (ANCA) found in patients with small-vessel vasculitis and pauci-immune necrotizing glomerulonephritis. To date, the target epitopes of MPO-ANCA remain poorly defined. Human MPO-ANCA do not typically bind mouse MPO. We utilized the differences between human and mouse MPO to identify the target regions of MPO-ANCA. We generated five chimeric MPO molecules in which we replaced different segments of the human or mouse molecules with their homologous counterpart from the other species. Of serum samples from 28 patients screened for this study, 43 samples from 14 patients with MPO-ANCA-associated vasculitis were tested against recombinant human and mouse MPO and the panel of chimeric molecules. Sera from 64 and 71% of patients bound to the carboxy-terminus of the heavy chain, in the regions of amino acids 517-667 or 668-745, respectively. No patient serum bound the MPO light chain or the amino-terminus of the heavy chain. All sera bound to only one or two regions of MPO. Although the pattern of MPO-ANCA binding changed over time (4-27 months) in 6 of 10 patients with several serum samples, such changes were infrequent. Other target regions of MPO-ANCA may not have been detected due to conformational differences between the native and recombinant forms of MPO. MPO-ANCA do not target a single epitope, but rather a small number of regions of MPO, primarily in the carboxy-terminus of the heavy chain.

Anti-myeloperoxidase antibodies in sera from patients with propylthiouracil-induced vasculitis might recognize restricted epitopes on myeloperoxidase molecule

Myeloperoxidase (MPO) is one of the major target antigens of antineutrophil cytoplasmic antibodies (ANCA) in primary systemic vasculitis. It is known that propylthiouracil (PTU) could induce MPO-ANCA-positive vasculitis. The production of anti-MPO antibodies in patients with PTU-induced vasculitis may be different from that in patients with primary microscopic polyangiitis (MPA). One possible reason for this may be differences in epitope recognition. The aim of this study is to compare the epitopes of antibodies to MPO in sera from patients with PTU-induced vasculitis (n = 10) and MPA (n = 10). The sera were collected and used to inhibit monoclonal antibodies against human MPO (3D8 and 6B9) and affinity purified, horseradish peroxidase conjugated human anti-MPO antibodies (Pab1-HRP, Pab2-HRP) in a competitive inhibition enzyme-linked immunosorbent assay (ELISA) system using soluble human MPO as solid phase ligand. The Pab1-HRP and Pab2-HRP were affinity purified from plasma exchanges of a patient with PTU-induced vasculitis and a patient with MPA, respectively. The inhibition rates were evaluated and compared between the PTU and primary MPA groups. In the PTU group all 10 sera could inhibit 3D8: the average inhibition rate was 44.7% +/- 5.0%; 9/10 sera could inhibit 6B9: the average inhibition rate was 35.6% +/- 6.0%. However, in the MPA group all 10 sera could inhibit 3D8 and 6B9; the average inhibition rates were 68.4% +/- 16.1% (P < 0.01) and 62.2% +/- 17.2% (P < 0.01), respectively. Sera in both the PTU and MPA groups could inhibit Pab1-HRP and the inhibition rates were 81.4% +/- 9.4%versus 86.6% +/- 17.2% (P > 0.05). However, the average inhibition rate for Pab2-HRP in the MPA group was significantly higher than that in the PTU group (76.3% +/- 7.8%versus 58.9% +/- 15.5%, P < 0.01). We conclude that anti-MPO antibodies from patients with PTU-induced vasculitis and from patients with primary MPA could recognize more than one epitope on the native MPO molecule. Although the epitopes overlapped between the two groups, the epitopes of anti-MPO antibodies from patients with PTU-induced vasculitis might be more restricted.

B cell epitope specificity in ANCA-associated vasculitis: does it matter?

Pauci-immune idiopathic small-vessel vasculitis is strongly associated with the presence of antineutrophil cytoplasm autoantibodies (ANCA). Antibodies to PR3 predominate in patients with Wegener's granulomatosis; antibodies to myeloperoxidase (MPO) are found more frequently in patients with microscopic polyangiitis. There is increasing in vivo and in vitro evidence for a pathogenic role of ANCA in systemic vasculitis based on associations of ANCA with disease activity. If ANCA are pathogenic, why is the course of disease different from one patient to another? Antibodies can recognize different binding sites (epitopes) on their corresponding antigens. Differences in binding specificity may influence the pathogenic potential of the antibodies. Differences between epitope specificity of ANCA between patients or changes in epitope specificity of ANCA in time in an individual patient may, accordingly, result in differences in disease expression. This review will focus on epitope specificity of autoantibodies in systemic autoimmune diseases and especially on the epitope specificity of PR3- and MPO-ANCA. We will discuss whether PR3-ANCA or MPO-ANCA recognize different epitopes on PR3 and MPO, respectively, and whether the epitopes recognized by ANCA change in parallel with the disease activity of ANCA-associated vasculitis. Finally, we will speculate if the direct pathogenic role of ANCA can be ascribed to one relapse- or disease-inducing epitope. Characterization of relapse- or disease-inducing epitopes bound by PR3-ANCA and MPO-ANCA is significant for understanding initiation and reactivation of ANCA-associated vasculitis. Elucidating a disease-inducing epitope bound by ANCA may lead to the development of epitope-specific therapeutic strategies.

Antineutrophil cytoplasmic antibodies in patients with Graves' disease: association of antimyeloperoxidase autoantibodies with propylthiouracil therapy

Abstract Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitic disorders have been reported in patients with Graves' disease during propylthiouracil (PTU) therapy. To investigate whether ANCA are found in serum samples from patients with Graves' disease, and whether PTU therapy is associated with ANCA positivity, levels of serum ANCA were examined in Graves'-disease patients receiving either PTU (n = 49) or 1-methyl-2-mercaptoimidazole (methimazole, MMI) (n = 50), and in untreated Graves'-disease patients (n = 32) by enzyme-linked immunosorbent assay (ELISA). Serum samples from patients with Hashimoto's thyroiditis (n = 46) were also analyzed. Antimyeloperoxidase (MPO) autoantibodies (MPO-ANCA) were present in 10 (20.4%) of 49 Graves'-disease patients receiving PTU therapy, whereas MPO-ANCA were not detected in Graves'-disease patients receiving MMI, in untreated Graves'-disease patients, or in Hashimoto's thyroiditis patients. The MPO-ANCA-positive sera showed a perinuclear staining pattern which was detected by indirect immunofluorescence microscopy using a human polymorphonuclear leukocyte-cytospin preparation. Furthermore, Western blot analysis revealed that MPO-ANCA in the Graves'-disease patients, as well as MPO-ANCA in patients with idiopathic pauci-immune necrotizing and crescentic glomerulonephritis, recognize the 105-kD protein of native MPO. These results indicate that MPO-ANCA in Graves'-disease patients are strongly associated with PTU therapy, and not simply related to the autoimmune thyroid disease. This study also suggests that the presence of MPO-ANCA alone may not be sufficient for the development of vasculitic disorders.

Detection of anti-neutrophil cytoplasmic antibodies under actual clinical testing conditions

Anti-neutrophil cytoplasmic antibodies (ANCA) are a useful diagnostic tool for Wegener's granulomatosis (WG) and microscopic polyangiitis (MPA). To maximize diagnostic utility, current guidelines recommend dual testing by standard indirect immunofluorescence (IIF) and target antigen-specific assays. Most published data come from specialized research laboratories, not reflecting the performance of assays under routine clinical conditions. Therefore, we compared the performance of standard IIF, PR3-, and MPO-ANCA-specific direct ELISA, and a PR3-ANCA-specific capture ELISA used alone and in combination under routine clinical conditions. Consecutive serum samples (615) submitted for routine ANCA testing over a 10-month period were assayed. Diagnoses were WG/MPA (n = 86), other autoimmune disease (n = 118), and various others (n = 411). The combination of PR3-ANCA and MPO-ANCA ELISA had the highest sensitivity (72.1%), and C-ANCA determination using IIF, the highest specificity (99.6%). While maintaining maximal diagnostic accuracy, significant labor savings are achieved by screening for WG/MPA by ELISA followed by confirmatory IIF.

Are antineutrophil cytoplasmic antibodies pathogenic? Experimental approaches to understand the antineutrophil cytoplasmic antibody phenomenon

Antineutrophil cytoplasmic antibodies (ANCA) directed against the neutrophil enzymes PR3 and MPO are tightly associated with the development of small vessel vasculitis. This article reviews the large body of data derived from in vitro experiments documenting many different proinflammatory effects of these ANCA on neutrophils, monocytes, and endothelial cells. Taken in conjunction with clinical observations and data from animal models, a concept of the pathogenicity of ANCA emerges.

Clinical use of serological tests for antineutrophil cytoplasmic antibodies. What do the studies say?

Trustworthy and clinically useful laboratory test results depend on high quality laboratory knowledge and performance and on close collaboration with experienced clinicians. Just like a diagnosis can only be made by contrasting it to a number of differential diagnoses, a prototype autoantibody profile can only be recognized if autoantibodies that partially mimic the former are just as well-known. In antineutrophil cytoplasmic antibody (ANCA)-associated small vessel vasculitides the characteristic autoantibody profile can only be demonstrated by use of at least two different methodologies and an assay-specific cut-off selection that clearly distinguishes between strongly positive and disease control low positive to negative values. Local serum banks containing samples from patients with well-characterized chronic immunoinflammatory diseases and ANCA-associated vasculitides are needed for such important studies whether in-house or commercial assays are used.

Neutrophil functions of patients with vasculitis related to myeloperoxidase-specific anti-neutrophil antibody

Neutrophils are hypothesized to cause tissue damage resulting in the development of vasculitis and glomerulonephritis, although they are known to primarily take part in host defense functions. The infiltration of inflammatory cells. notably neutrophils and macrophages, is observed in the progression of vasculitis. Neutrophils with activated status and anti-neutrophil cytoplasmic antibodies (ANCAs), especially myeloperoxidase-specific (MPO)-ANCA, have been implicated in the development of vasculitis. The target molecule of MPG-ANCA is a lysosomal enzyme MPO that usually acts to kill bacteria, viruses, and fungi and that causes damage to the tissue due to the toxicity of its product, hypochlorite (OCl-). To elucidate the role of MPO-ANCA in the progression of vasculitis, a set of MPO-peptide fragments has been developed, and the corresponding epitope site for the specific monoclonal and/or oligoclonal antibody resulting in vasculitis has been determined. Recently some mouse models have been used for analyzing the correlation between MPO and MPO-ANCA in relation to damage of blood vessels followed by the development of vasculitis. This review focuses on the role of activated neutrophils in the development of vasculitis associated with MPO-ANCA and the target molecules of ANCA. In addition, the reactivities of ANCA and inflammatory cytokines involving leukocyte-derived chemotaxin 2 (LECT2) are also discussed.

Absence of cross-reactivity to myeloperoxidase of anti-thyroid microsomal antibodies in patients with autoimmune thyroid diseases

BACKGROUND

Thyroperoxidase is the major antigen of the thyroid microsomal antibodies (TMA) detected in autoimmune thyroid diseases. Its amino acid sequence has 44% homology with myeloperoxidase (MPO), an enzyme present in the primary granules of neutrophils and one of the major antineutrophil cytoplasmic antibodies (ANCA) antigens. The objective of the present study was to investigate the presence of cross-reactivity to MPO of TMA.

METHODS

We studied sera from 51 patients with autoimmune thyroid diseases, all of them TMA-positive. The presence of ANCA was investigated by indirect immunofluorescence and by capture enzyme-linked immunosorbent assay.

RESULTS

ANCA were positive in 3.9% of the TMA-positive sera and none of them reacted with MPO. In contrast, the ANCA-positive sera revealed antielastase activity. None of the ANCA-positive cases presented clinical signs of vasculitis. However, these 2 patients had been on prolonged treatment with propylthiouracil.

CONCLUSIONS

We conclude that there is no cross-reactivity to MPO of TMA in patients with autoimmune thyroid diseases, possibly because of difference in the spatial configuration of the immunodominant region. The presence of ANCA in patients with autoimmune thyroid diseases without evidence of vasculitis might result from propyIthiouracil-induced polyclonal activation.

Antiproteinase 3- and antimyeloperoxidase-associated vasculitis

Antiproteinase 3- and antimyeloperoxidase-associated vasculitis. Wegener's granulomatosis, microscopic polyangiitis, and idiopathic pauci-immune necrotizing crescentic glomerulonephritis (NCGN) are strongly associated with antineutrophil cytoplasmic autoantibodies (ANCAs) directed against either proteinase 3 (anti-PR3) or myeloperoxidase (anti-MPO). This has led some investigators to prefer combining these diseases under the common heading of ANCA-associated vasculitides. However, it is increasingly recognized that there are characteristic differences between patients with anti-PR3 and those with anti-MPO-associated vasculitis. This review focuses on the clinical, histopathologic, and possibly pathophysiologic differences between anti-PR3- and anti-MPO-associated vasculitis. Although there is considerable overlap, the anti-PR3- and anti-MPO-associated vasculitides are each characterized by particular clinical and histopathological findings. Extrarenal organ manifestations and respiratory tract granulomas occur more frequently in patients with anti-PR3 than in those with anti-MPO. Anti-PR3-positive patients with NCGN generally have a more dramatic deterioration of their renal function compared with anti-MPO-positive patients. The term "ANCA-associated vasculitis" is considered as a useful concept in the presence of systemic vasculitis. Likewise, in the presence of vasculitis, the terms "anti-PR3-associated vasculitis" and "anti-MPO-associated vasculitis" are useful concepts.

Antineutrophil cytoplasmic antibodies (ANCA) and systemic vasculitis: update of assays, immunopathogenesis, controversies, and report of a novel de novo ANCA-associated vasculitis after kidney transplantation

OBJECTIVES

To characterize antineutrophil cytoplasmic antibodies (ANCA), their major autoantigens, disease associations, and pathophysiology in systemic vasculitides. To describe a patient with a novel de novo ANCA-associated vasculitis after kidney transplantation.

METHODS

We reviewed and compiled the literature on ANCA-related topics and systemic vasculitis. Laboratory and clinical data from a cadaveric kidney transplant patient who developed necrotizing vasculitis involving glomerular capillaries, with crescent formation associated with P-ANCA and myeloperoxidase, were analyzed.

RESULTS

Large-scale multi-center testing of patient and normal sera by the European ANCA Assay Standardization Project using immunofluorescence assays and enzyme immunoassays indicate the assays have good sensitivity and specificity, and diagnostic utility for ANCA-associated vasculitis. A few investigations covering basic and clinical research with ANCA remain controversial: whether endothelial cells do or do not express a 29-kd neutral serine protease termed proteinase-3 (PR-3), the target of ANCA in most individuals with Wegener's granulomatosis, and whether anti-myeloperoxidase (MPO) ANCAs recognize a restricted number of epitopes on MPO. This issue has relevance for using monoclonal antibodies to treat patients with vasculitis who have adverse effects from immunosuppressive drugs. The two allelic forms of FcgammaRIIa (H131/R131) and the two of FcgammaRIIlb (NA1/NA2) are discussed as possible inheritable genetic elements for vasculitic disorders and for signaling responses. Stimulatory and costimulatory molecules, and cytokine profiles of T lymphocytes are characterized to show that these cells are actively involved in the ANCA-associated vasculitides. The patient described had a de novo ANCA associated small vessel vasculitis which developed after renal transplantation.

CONCLUSIONS

There have been significant advances in the development of sensitive and specific ANCA assays. The immunopathogenetic mechanism of ANCA involves the constitutive FcgammaRs, ligands, and signaling responses to activate cytokine-primed neutrophils. This may lead to the generation of reactive oxygen intermediates, degranulation, and secretion of intracellular granule contents, and ultimately inflammation and vasculitis.

Pathogenesis of ANCA-associated systemic vasculitis

The aetiology of primary systemic vasculitides remains unknown. Recent advances have been made in the understanding of relevant mechanisms of inflammation, particularly the role of the endothelium and interactions with inflammatory mediators and immune effector cells. In Wegener's granulomatosis and microscopic polyangiitis the evidence suggests an autoimmune inflammatory process, characterized by an early lesion involving neutrophils and endothelial cells as both targets and active participants; priming of neutrophils and endothelial cells allows ANCA to activate neutrophils with damage localized to the endothelium. In the absence of immune complex deposition, the role of the ANCA is particularly intriguing. Endothelial cell damage and activation produces pro-inflammatory mediators with influx monocytes and T cells intensifying damage. Increased understanding of the pathogenesis of systemic vasculitis is likely to provide the basis for the use of more selective immunomodulatory therapies in the future.

The inhibition of myeloperoxidase by ceruloplasmin can be reversed by anti-myeloperoxidase antibodies

BACKGROUND

The purpose of this study was to characterize the recently reported inhibition of myeloperoxidase (MPO) by ceruloplasmin and to determine whether this may be disturbed in the presence of anti-MPO antibodies.

METHODS

Specificity of the binding between ceruloplasmin and MPO was confirmed by Western blotting and enzyme-linked immunosorbent assay (ELISA), and the enzymatic activity of MPO was measured in the presence of ceruloplasmin, affinity-purified anti-MPO antibodies, or both. The affinity of the binding between MPO and ceruloplasmin and MPO and the anti-MPO antibodies was measured using a biosensor, with the results confirmed by chaotrope ELISA.

RESULTS

Affinity-purified anti-MPO antibodies from patients with microscopic polyangiitis and florid renal vasculitis inhibited the binding between ceruloplasmin and MPO to a maximum of 72.9 +/- 12.8%, whereas those from patients with Wegener's granulomatosis and only minimal renal involvement inhibited the binding to a maximum of only 36.8 +/- 10.9% (P < 0. 001), with comparable reversal of the ceruloplasmin-mediated inhibition of MPO activity. Measurement of the affinity of the interactions demonstrated that binding between MPO and the anti-MPO antibodies is stronger than that between MPO and ceruloplasmin (1.61 x 107 to 1.33 x 108 vs. 7.46 x 106 m-1), indicating that binding to the autoantibody would be favored in vivo.

CONCLUSIONS

This study confirms a role for ceruloplasmin as a physiological inhibitor of MPO, and demonstrates how the inhibition may be disrupted in the presence of anti-MPO antibodies. Because a majority (16 of 21) of the antibodies did not themselves inhibit MPO activity, their interference with the inhibition mediated by ceruloplasmin may be brought about by steric hindrance consequent upon the binding of the antibody to a dominant epitope at or near the active site.

A comparison of commercial and in-house ELISAs for antineutrophil cytoplasmic antibodies directed against proteinase 3 and myeloperoxidase

This study compares the concordance of results in different ELISAs for antineutrophil cytoplasmic antibodies (ANCA) directed against proteinase 3 (PR3) or myeloperoxidase (MPO). Sera were considered "true positives" if they were positive according to the manufacturer's criteria in a least three of the five PR3-ANCA ELISAs, or in at least four of the six MPO-ANCA ELISAs. Of the 26 sera that demonstrated cytoplasmic fluorescence (C-ANCA), 23 (89%) contained PR3-ANCA and three (11%) had MPO-ANCA. Two sera that were negative by indirect immunofluorescence (IIF) contained PR3-ANCA. Of the 26 sera with perinuclear fluorescence (P-ANCA), 19 (73%) contained MPO-ANCA, and one (4%) had PR3-ANCA. Six sera with P-ANCA did not have PR3- or MPO-ANCA. No serum that was negative by IIF contained MPO-ANCA. For the different PR3-ANCA ELISAs, sensitivities ranged from 88 to 100%, and specificities from 91 to 100%. For the MPO-ANCA ELISAs, sensitivities varied from 59 to 100% and specificities from 83 to 100%. The highest sensitivity and specificity for both the PR3- and MPO-ANCA ELISAs were obtained with the IBL and Eurodiagnostica assays. The in-house PR3-ANCA ELISA performed slightly less well than the commercial assays, but the performance of the in-house MPO-ANCA assay was comparable or better.

A comparison of the characteristics of circulating anti-myeloperoxidase autoantibodies in vasculitis with those in non-vasculitic conditions

Although circulating anti-neutrophil cytoplasmic antibodies (ANCA) specific for myeloperoxidase (MPO) are strongly associated with the presence of vasculitis, they have been described in sera from patients with other conditions. High levels of anti-MPO antibodies can also persist in sera from patients with vasculitis despite the achievement of clinical remission. One possible interpretation is that a potentially pathogenic subset of anti-MPO antibodies exists, which is only present in patients with active vasculitis. We therefore compared the characteristics of anti-MPO antibodies in sera from patients with active vasculitis (n = 18) with those present in remission (n = 9) and in a disease control group (n = 10) without clinical evidence of vasculitis. The class, subclass and ability of anti-MPO antibodies from the three groups of patients to recognize three different conformational epitopes were analysed using ELISA-based techniques. The expression of an idiotope, designated 9G4, was also examined. Epitope recognition by anti-MPO antibodies from all patients tested was found to be similar. Sera from patients with active vasculitis showed an over-representation of IgG4 subclass anti-MPO antibodies and a more frequent presence of IgM class anti-MPO antibodies. In disease controls, IgG1 anti-MPO antibodies were predominant. In vitro, neutrophil activation by ANCA has been shown to be dependent on engagement of neutrophil FcgammaRIIa receptors following binding of these autoantibodies to surface-expressed ANCA antigens. We found that active vasculitis may be associated with the presence of circulating anti-MPO antibodies which do not significantly bind this receptor, suggesting that mechanisms other than those dependent on FcgammaRIIa binding should be explored. In addition, the expression of the 9G4 idiotope on anti-MPO antibodies in 60% (12/18) of patients with active vasculitis and 20% (2/10) of disease control patients may indicate a common origin for anti-MPO antibodies in different individuals.

Myeloperoxidase autoantibodies distinguish vasculitis mediated by anti-neutrophil cytoplasm antibodies from immune complex disease in MRL/Mp-lpr/lpr mice: a spontaneous model for human microscopic angiitis

Anti-neutrophil cytoplasm antibodies (ANCA) with specificity for myeloperoxidase (MPO) occur in the sera of patients with microscopic angiitis, an autoimmune disease characterized by necrotizing vasculitis and crescentic glomerulonephritis. These autoantibodies have been shown to stimulate neutrophil degranulation and are believed to participate in pathogenesis. A neutrophilic vasculitis has been reported in MRL-lpr mice which has histological appearances similar to microscopic angiitis. In the present study we show that 22% of female MRL-lpr mice develop MPO autoantibodies. These animals develop a clinical syndrome of vasculitis and glomerulonephritis that is distinct from immune complex disease. Anti-MPO monoclonal antibodies derived from these mice are polyreactive and react with double-stranded DNA. They bind a conformational epitope on human MPO which is also expressed by activated human neutrophils. The results suggest that a subset of MRL-lpr mice develop ANCA-related vasculitis rather than systemic lupus erythematosus and may be used as a model for human microscopic angiitis.

A panel set for epitope analysis of myeloperoxidase (MPO)-specific antineutrophil cytoplasmic antibody MPO-ANCA using recombinant hexamer histidine-tagged MPO deletion mutants

A major target protein of antineutrophil cytoplasmic antibody with a perinuclear staining pattern (P-ANCA) has been identified as myeloperoxidase (MPO). Recombinant deletion mutants of MPO, eight fragments of the heavy-chain subunit, and two fragments of the light chain subunit were expressed in E. coli using a pQE expression vector. The recombinant hexamer histidine-tagged fragments were partially purified as the denatured proteins on a Ni2+-charged nitrirotriacetic acid column. The recombinant fragments were reacted with a rabbit polyclonal antibody to human MPO in Western blotting. In addition, the reactivities of the proteins with MPO-ANCA-positive sera of four patients with renal diseases were examined by Western blotting. The profile of the reactivity showed that different sera recognized different sets of fragments of the heavy chain, whereas no serum reacted with the fragments of the light chain. These results indicate that the sera of patients with MPO-ANCA-positive diseases showed varied reactivities with the different fragments. Furthermore, an ELISA system using a set of the fragments completely purified by Sephacryl S-200HR column chromatography was established. The panel set is useful for subclassification of MPO-ANCA-related diseases.

Studies of epitope restriction on myeloperoxidase (MPO), an important antigen in systemic vasculitis

Anti-neutrophil cytoplasmic antibodies are important components of the inflammatory response in patients with systemic vasculitis. Their role in the pathogenesis of these conditions remains incompletely defined. Several antigens have been identified, and MPO is one of the most important. To gain more understanding of the immune mechanisms involved, we were keen to see if the antibody response to MPO was restricted, or whether there was a general loss of tolerance to the whole surface of the molecule. To study the epitopes we employed both ELISA and biosensor technology, and were able to demonstrate restriction both in the number and localization of the epitopes being recognized.

Some anti-thyroperoxidase antibodies positive sera give a pANCA pattern on ethanol-fixed human neutrophils: cross-reactivity or false positives?

Fifty-six sera from patients with autoimmune thyroiditis and 33 sera from patients with MPO-ANCA were examined in order to ascertain whether a cross reactivity between MPO-ANCA and anti-thyroperoxidase (aTPO) was present. Sera from 20 healthy donors aTPO and aMPO negative were used as control. About 95% of heat inactivated sera from patients with autoimmune thyroiditis and from controls gave positive results (atypical pANCA pattern) on ethanol-fixed neutrophils. The prevalence of positive results was significantly lower when unheated aTPO positive sera were used (17.8%). On the other hand, only 9% of sera with MPO-ANCA were positive on cryostatic sections of human thyroid. Indirect immunofluorescence tests (IF) on human neutrophils with MPO defect were negative with sera from patients with MPO-ANCA, but uninactivated sera with aTPO and positive for pANCA on normal neutrophils showed a very high prevalence of positive results (90%). According to our data only few sera positive for aTPO recognize "normal" MPO, but the majority of sera from patients with autoimmune thyroiditis and positive for pANCA on normal neutrophils recognize also an "abnormal" MPO. On the other hand MPO-ANCA usually recognize epitopes presently only on the normal enzyme, a small proportion of these autoantibodies can react with TPO. Heat inactivated sera give false positive results for pANCA on ethanol fixed human neutrophils.

Analysis of immunoglobulin variable region genes of a human IgM anti-myeloperoxidase antibody derived from a patient with vasculitis

Circulating antibodies to myeloperoxidase (MPO) are associated primarily with pauci-immune glomerulonephritis and systemic vasculitis. Anti-MPO antibodies belong to a group of autoantibodies, anti-neutrophil cytoplasmic antibodies, that may play a pathogenic role in vasculitis. We have generated a human monoclonal anti-MPO antibody (E3-MPO) using peripheral blood lymphocytes from a patient with microscopic polyarteritis. Variable region gene analysis of E3-MPO showed that the VH region had 90% homology with the germ line gene VH4-21. E3-MPO was also shown to carry the 9G4 idiotope, which so far has been associated only with human antibodies that utilize the VH4-21 gene. The 9G4 idiotope was also expressed on anti-MPO antibodies in sera from the donor patient and from 4/7 additional patients with active, untreated vasculitis. The nucleotide sequences of both the variable heavy and light chains of E3-MPO showed evidence of an antigen-driven response.

Epitope mapping of anti-proteinase 3 and anti-myeloperoxidase antibodies

Anti-proteinase 3 (PR3) and anti-myeloperoxidase (MPO) autoantibodies are present in many patients with Wegener's granulomatosis (WG) and microscopic polyarteritis. The aim of this study was to determine whether these antibodies bound to linear peptide sequences on their target antigens. If common linear epitopes were demonstrated, then these could be manufactured and used in diagnostic ELISAs for anti-PR3 and anti-MPO antibodies. In addition, any homology between these epitopes and bacterial or viral sequences might implicate those microorganisms in the development of these antibodies and the pathogenesis of the associated diseases. The presence of linear epitopes on PR3 and MPO was suggested by the binding of the corresponding autoantibodies to these proteins after they had been reduced with beta-mercaptoethanol (beta-ME) and denatured with SDS or boiling, and digested with proteases. Four of the 22 sera with anti-PR3 antibodies bound to PR3 in Western blots after treatment with SDS, beta-ME and boiling for 5 min. Thermal denaturation reduced the amount of binding more than other forms of denaturation. One serum with anti-PR3 antibodies bound to Lys-C and Glu-C-digested PR3 in dot blots. Linear epitopes could not be further defined by their binding in an ELISA using overlapping peptides corresponding to the PR3 molecule because of non-specific binding. Three of the five sera with anti-MPO antibodies bound to MPO in Western blots after treatment with SDS, beta-ME and boiling for 5 min. One serum with anti-MPO antibodies bound to Lys-C and Glu-C-digested MPO in dot blots. Again, linear epitopes could not be further defined using an ELISA with overlapping peptides because of non-specific binding. Some anti-PR3 and anti-MPO antibodies are likely to recognize linear epitopes, but these cannot be defined by use of a PIN ELISA system.

Significance of myeloperoxidase in rapidly progressive glomerulonephritis

Antineutrophil cytoplasmic autoantibodies (ANCAs) are found in the sera of patients with systemic vasculitis and crescentic glomerulonephritis. We developed monoclonal antibodies against myeloperoxidase (MPO), which is a major perinuclear-ANCA antigen, and examined the presence of MPO in renal tissue from patients with rapidly progressive glomerulonephritis (RPGN) using these monoclonal antibodies. Myeloperoxidase was found in the glomeruli in four of five cases of perinuclear ANCA-positive RPGN, and the distribution pattern was different from that of immunoglobulin G. In perinuclear ANCA-negative crescentic glomerulonephritis, MPO also was detected in two of three cases. Myeloperoxidase was not detected in the capillary walls or mesangium in other nephropathies, such as minimal change disease, membranous nephropathy, and mesangial proliferative glomerulonephritis, or in normal controls. Myeloperoxidase activity was elevated and inversely correlated with the titer of MPO-ANCAs in the sera of perinuclear ANCA-associated RPGN. These data suggest that MPO plays an important role in the glomerular injury of RPGN.

Antineutrophil cytoplasmic autoantibodies, autoantigens, and systemic vasculitis

Antineutrophil cytoplasmic antibodies (ANCA) encompass a heterogeneous group of autoantibodies targeting antigens in neutrophils (PMN), monocytes, and endothelial cells. ANCA are routinely detected by the indirect immunofluorescence technique (IFT) and at least three different patterns of fluorescence can be distinguished which have been assigned the acronyms cANCA, pANCA and aANCA. cANCA is mostly induced by proteinase 3 (PR3) antibodies (PR3-ANCA), and pANCA by myeloperoxidase (MPO) antibodies (MPO-ANCA), while aANCA has unidentified subspecificity. Over the past decade, ANCA have been the subject of extensive investigation. They have proved to be of significant value both as diagnostic tools and for follow-up in several forms of systemic vasculitis (e.g. Wegener's granulomatosis, WG; microscopic polyarteritis, MPA; Churg-Strauss syndrome, CSS) which are now termed 'ANCA-associated vasculitides'. Furthermore, it is suspected that the presence of ANCA is an important factor in the pathogenesis of these disease groups. Data regarding the detection of ANCA and their diagnostic value and role in the pathogenesis of vasculitic disorders will be discussed in this review. Growing evidence points to a pathophysiological and diagnostic relevance of the distribution of the ANCA target antigens PR3 and MPO (presence in the circulation, on cell membranes, and in tissue extracellularly). An autoimmune process has been implicated in the pathogenesis of ANCA-associated vasculitis, but it is uncertain which mechanism underlies the induction of the ANCA-related immunoresponse. In this paper mechanisms such as antigenic cross-reactivity between human PMN proteins and extrinsic antigens by molecular mimicry, idiotypic immunoglobulin regulation, and T-cell reactivity to PR3 and MPO will be discussed.

Circulating myeloperoxidase may cause false negative findings in the analysis of myeloperoxidase antibodies in systemic vasculitis

In systemic vasculitis reliable detection of myeloperoxidase antibodies (MPO-Abs) is of great clinical importance in the diagnosis and follow-up of patients. We have studied whether circulating myeloperoxidase (MPO) could have an effect on MPO-Ab findings. Serum MPO and MPO-Abs were measured in 50 healthy individuals, 35 patients and in the follow-up samples from two patients with Wegener's granulomatosis. Heating the sera at 56 degrees C for 30 min reduced the concentration of immunoreactive MPO both in control and patient sera. In 71% of the patient sera heating made initially negative MPO-Abs detectable. In a few cases with severe vasculitis the antibody findings remained totally negative. These results, together with the data from the follow-up samples from two patients with Wegener's granulomatosis, revealed that the serological diagnosis of vasculitis may be considerably delayed if only native samples are analysed for MPO-Abs. These findings are of considerable clinical significance for the interpretation of MPO-Ab results. Circulating myeloperoxidase affects MPO-Ab measurements, causing false negative findings in MPO-Ab assays. Therefore, it is recommended to denaturate circulating MPO by heating the sera before the analysis of MPO-Abs and to re-evaluate the cut off-values.

Some patients with anti-myeloperoxidase autoantibodies have a C-ANCA pattern

Rapidly progressive glomerulonephritis with or without other signs of systemic vasculitis is often accompanied by antibodies to myeloperoxidase. Such antibodies normally produce a perinuclear pattern on ethanol-fixed neutrophils (perinuclear anti-neutrophil cytoplasm antibodies (P-ANCA)) at indirect immunofluorescence. We report here sera from three patients that are anti-myeloperoxidase-positive in ELISA that instead produce a cytoplasmic pattern (classical anti-neutrophil cytoplasmic antibodies (C-ANCA)), a pattern normally seen in conjunction with antibodies to proteinase 3. These sera did not react with proteinase 3. For two of the sera the specificity of the anti-myeloperoxidase reaction was confirmed with inhibition-ELISA experiments and with immunoblotting. A mouse anti-myeloperoxidase MoAb that produces a cytoplasmic pattern is also described. Competition ELISA experiments show that this antibody and anti-myeloperoxidase sera with cytoplasmic pattern recognize epitopes that are separate from epitopes recognized by another perinuclear pattern producing anti-myeloperoxidase MoAb. 'Cytoplasmic pattern' epitopes as well as 'perinuclear pattern' epitopes can be found on all three major myeloperoxidase isoforms, after separation by ion exchange chromatography. Affinity chromatography, using the cytoplasmic pattern producing anti-myeloperoxidase monoclonal antibody, shows that the epitope recognized by this MoAb is present on all myeloperoxidase molecules. This epitope is not confined to any special subpopulation. These findings indicate that all myeloperoxidase do not relocate after ethanol fixation, and that C-ANCA and P-ANCA epitopes exist simultaneously on the same myeloperoxidase molecule. We propose that the two immunofluorescence patterns arise due to different availabilities of the epitopes in the microenvironment where myeloperoxidase is present.

Anti-neutrophil cytoplasmic antibodies (ANCA): their detection and significance: report from workshops

Anti-neutrophil cytoplasmic antibodies (ANCA) are antibodies directed against enzymes that are found mainly within the azurophil or primary granules of neutrophils. There are 3 types of ANCA that can be distinguished by the patterns they produce by indirect immunofluorescence when tested on normal ethanol-fixed neutrophils. Diffuse fine granular cytoplasmic fluorescence (cANCA) is typically found in Wegener's granulomatosis, in some cases of microscopic polyarteritis and Churg Strauss syndrome, and in some cases of crescentic and segmental necrotising glomerulonephritis, but it is rare in other conditions. The target antigen is usually proteinase 3. Perinuclear fluorescence (pANCA) is found in many cases of microscopic polyarteritis and in other cases of crescentic and segmental necrotising glomerulonephritis. These antibodies are often directed against myeloperoxidase but other targets include elastase, cathepsin G, lactoferrin, lysozyme and beta-glucuronidase. The third group designated "atypical" ANCA includes neutrophil nuclear fluorescence and some unusual cytoplasmic patterns, and while a few of the target antigens are shared with pANCA, the others have not been identified. Sera that produce a pANCA or atypical ANCA pattern on alcohol-fixed neutrophils result in cytoplasmic fluorescence when formalin acetone fixation is used. pANCA or atypical ANCA occur in about 2/3 of all individuals with ulcerative colitis or primary sclerosing cholangitis, and they are found in a third of patients with Crohn's disease. The reported incidence of ANCA in rheumatoid arthritis and SLE varies considerably but the patterns are predominantly pANCA and atypical ANCA.(ABSTRACT TRUNCATED AT 250 WORDS)