Antineutrophil cytoplasmic autoantibodies (ANCA) recognizing a recombinant myeloperoxidase subunit

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.

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.

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.

Drug-induced vasculitis

DIV is a relatively common cause of inflammatory vasculitis. Drugs from almost every pharmacologic class have been implicated in causing vasculitis in sporadic cases. The level of certainty and quality of evidence for these associations between specific agents and vasculitis vary greatly. The clinical manifestations of DIV range from single organ involvement (most commonly, skin) to life-threatening multiorgan disease. The recently described subset of cases of DIV associated with positive tests for ANCA are an interesting subset of DIV. The diagnosis of DIV is usually one of exclusion. The treatment of DIV is dependent on the severity of disease activity but should always include withdrawal of the suspected drug. If no agent can be implicated, as many drugs as feasible should be discontinued. The necessity of prescribing glucocorticoids or immunosuppressive agents depends on the disease severity and other case-specific information. Increasing understanding of the pathophysiologic characteristics of all inflammatory vasculitides should lead to better diagnostic and therapeutic approaches to DIV.

Perinuclear antineutrophilic cytoplasmic antibody-positive cutaneous polyarteritis nodosa associated with minocycline therapy for acne vulgaris

Minocycline is an oral antibiotic widely used for the long-term treatment of acne vulgaris. Unusual side effects of this medication include two overlapping autoimmune syndromes: drug-induced lupus and autoimmune hepatitis. In addition, in a few patients livedo reticularis or subcutaneous nodules have developed in association with arthritis and serum perinuclear antineutrophil cytoplasmic antibodies (P-ANCA) during long-term minocycline therapy. We report the cases of two young women receiving long-term minocycline therapy (>3 years) in whom P-ANCA-positive cutaneous polyarteritis nodosa developed. Both patients presented with a violaceous reticulated pattern on the lower extremities. Histologic examination of biopsy specimens from a reticulated area and a subcutaneous nodule showed necrotizing vasculitis of medium-sized arteries in the deep dermis, consistent with the diagnosis of polyarteritis nodosa. The cutaneous lesions rapidly resolved on discontinuation of minocycline and initiation of prednisone therapy. A high index of suspicion and testing for antineutrophil cytoplasmic antibody in addition to the standard antinuclear antibody panel can facilitate diagnosis of minocycline-related autoimmune disorders.

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.

Wegener's granulomatosis

Clinical manifestations of Wegener's granulomatosis are nonspecific and indistinguishable from a variety of neoplastic, infectious, and inflammatory diseases. Ophthalmic disease is the presenting feature in nearly one sixth of patients with Wegener's granulomatosis and will ultimately develop in a majority. The discovery of antineutrophil cytoplasmic antibodies, particularly antiproteinase-3, has changed the clinical approach to evaluating patients suspected of having Wegener's granulomatosis. These antibodies are distinguished from other related autoantibodies because they produce a coarse granular pattern of cytoplasmic staining on indirect immunofluorescence with ethanol-fixed neutrophils. Treatment of Wegener's granulomatosis with oral cyclophosphamide and corticosteroids has decreased morbidity and improved survival, but side effects from long-term immunosuppressive therapy are common and sometimes serious. The effectiveness of trimethoprim-sulfamethoxazole in decreasing the number and severity of recurrences of Wegener's granulomatosis is being investigated. It remains to be determined if wide use of trimethoprim-sulfamethoxazole in limited Wegener's granulomatosis could further improve the quality of life for some patients.

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.