Review

Nature Clinical Practice Rheumatology (2006) 2, 661-670
doi:10.1038/ncprheum0355  
Received 17 March 2006 | Accepted 16 October 2006

Mechanisms of Disease: pathogenesis and treatment of ANCA-associated vasculitides

Cees GM Kallenberg*, Peter Heeringa and Coen A Stegeman  About the authors

Correspondence *Department of Rheumatology and Clinical Immunology, University Medical Center Groningen and University of Groningen, PO Box 30.001, 9700 RB Groningen, The Netherlands

Email c.g.m.kallenberg@int.umcg.nl

Summary

Wegener's granulomatosis and microscopic polyangiitis are idiopathic systemic vasculitides strongly associated with antineutrophil cytoplasmic autoantibodies (ANCA). In Wegener's granulomatosis, ANCA are mostly directed against proteinase 3 (PR3), whereas in microscopic polyangiitis ANCA are directed against myeloperoxidase; increases in levels of these autoantibodies precede or coincide with clinical relapses in many cases. In vitro, ANCA can further activate primed neutrophils to release reactive oxygen species and lytic enzymes, and, in conjunction with neutrophils, can damage and lyse endothelial cells. Patients with Wegener's granulomatosis or microscopic polyangiitis have an increased percentage of neutrophils that constitutively express PR3 on their membrane. These neutrophils can be stimulated by ANCA, without priming. In vivo, transfer of splenocytes from myeloperoxidase-deficient mice immunized with mouse myeloperoxidase into wild-type mice resulted in pauci-immune systemic vasculitis. A similar experiment in PR3-deficient mice did not cause significant vasculitic lesions. Together, clinical, in vitro and in vivo experimental data support a pathogenic role for ANCA in Wegener's granulomatosis and microscopic polyangiitis, although this role is more evident for myeloperoxidase-specific ANCA than for PR3-specific ANCA. Several controlled trials have led to an evidence-based approach for the treatment of ANCA-associated vasculitis, and further studies, based on new insights into pathogenesis, are in progress.

Review criteria

PubMed and MEDLINE were searched from January 1983 (the first description of ANCA) until February 2006 for papers published in English-language journals. The following keywords were used: "ANCA", "proteinase 3/PR3-ANCA", "myeloperoxidase/MPO-ANCA", "ANCA-associated vasculitis", "Wegener's granulomatosis", "microscopic polyangiitis", "Churg–Strauss syndrome", "pathogenesis", and "treatment".

Keywords:

ANCA-associated vasculitis, MPO-ANCA, pathogenesis, PR3-ANCA, Wegener's granulomatosis

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Introduction

The idiopathic, or primary, vasculitides (Box 1) comprise a spectrum of diseases characterized by inflammation of blood vessels. Classification, according to a committee of the American College of Rheumatology,1 is based on the size of vessels predominantly involved, histopathologic findings, and clinical symptoms. Definitions for the various vasculitides have been proposed by the Chapel Hill consensus conference.2 The vasculitides differ not only in clinical and pathologic presentation, but also in their presumed pathogenesis. The large-vessel vasculitides are supposedly caused by cell-mediated immune reactions to poorly defined (auto) antigens.3 Immune complexes are involved in some of the small-vessel vasculitides. In Henoch–Schönlein purpura, IgA-containing immune deposits are observed in the target organs—in particular, the skin and kidneys. Cryoglobulinemic vasculitis, frequently associated with hepatitis C virus infection, is caused by deposition of type II cryoglobulins within small vessels, particularly in postcapillary venules in the skin. Biopsy samples from purpuric skin lesions in cutaneous leukocytoclastic angiitis frequently show leukocytoclastic vasculitis and deposition of IgM, IgG, and complement in dermal vessels. Vasculitic lesions that are caused by Wegener's granulomatosis, microscopic polyangiitis, and Churg–Strauss syndrome, however, frequently lack immune deposits (particularly in the kidneys)4 although deposits have been detected by electron microscopy in over half of renal biopsy samples.5

Box 1 Classification scheme for the idiopathic vasculitides, proposed by an international study group at the Chapel Hill Consensus Conference on the Nomenclature of Systemic Vasculitis.1

 

Large-vessel vasculitis

  • Giant cell (temporal) arteritis
  • Takayasu arteritis

 

Medium-sized-vessel vasculitis

  • Polyarteritis nodosa
  • Kawasaki disease

 

Small-vessel vasculitis

  • Wegener's granulomatosis
  • Churg–Strauss syndrome
  • Microscopic polyangiitis
  • Henoch–Schönlein purpura
  • Essential cryoglobulinemic vasculitis
  • Cutaneous leukocytoclastic angiitis

These vasculitides are collectively designated as pauci-immune necrotizing vasculitides. The pauci-immune vasculitides are characterized by the presence of autoantibodies against neutrophil cytoplasmic constituents, known as antineutrophil cytoplasmic autoantibodies (ANCA);6 neutrophils are the main effector cells in the lesions of patients with Wegener's granulomatosis or microscopic polyangiitis. This article discusses new insights into the pathogenesis of ANCA-associated vasculitides, and briefly outlines data from recent trials of therapeutic treatments for these diseases.

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Clinical observations that suggest a pathogenic role for anca

ANCA, when detected by indirect immunofluorescence on ethanol-fixed neutrophils, can have a cytoplasmic (cANCA), perinuclear (pANCA), or atypical staining pattern (Figure 1). Cytoplasmic staining is in most cases caused by autoantibodies to proteinase 3 (PR3-ANCA). A perinuclear pattern can be produced by antibodies to myeloperoxidase (MPO-ANCA), but also by antibodies to various other antigens from myeloid cells. Incidentally, an atypical pattern can be generated by antibodies to either PR3 or myeloperoxidase, but in most cases the antigenic targets of these ANCA are not well defined.

Figure 1 Cytoplasmic components of ethanol-fixed neutrophils, stained by indirect immunofluorescence, in a serum sample from a patient with active Wegener's granulomatosis and antineutrophil cytoplasmic autoantibodies to proteinase 3
Figure 1 : Cytoplasmic components of ethanol-fixed neutrophils, stained by indirect immunofluorescence, in a serum sample from a patient with active Wegener's granulomatosis and antineutrophil cytoplasmic autoantibodies to proteinase 3 Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

(A) A characteristic granular pattern of fluorescence is seen. (B) This fluorescence pattern is different from the perinuclear pattern that can be seen in serum samples from patients with antineutrophil cytoplasmic autoantibodies to myeloperoxidase.

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As PR3-ANCA and MPO-ANCA are associated with ANCA-associated vasculitides, a positive test for ANCA by indirect immunofluorescence should be followed by PR3-specific and myeloperoxidase-specific autoantibody assays, which are mostly enzyme-linked immunosorbent assays.7 In the correct clinical context, the specificity of PR3-ANCA and/or MPO-ANCA for ANCA-associated vasculitides is as high as 98%.8, 9 Sensitivities of PR3-ANCA and MPO-ANCA for the ANCA-associated vasculitides are given in Table 1. Most patients (70–80%) with Wegener's granulomatosis are positive for PR3-ANCA, and few (10%) are positive for MPO-ANCA.6, 7, 8, 9 Conversely, the majority of patients with microscopic polyangiitis (60%) are positive for MPO-ANCA, and few (30%) are positive for PR3-ANCA. A few patients with either Wegener's granulomatosis or microscopic polyangiitis are ANCA-negative. In Wegener's granulomatosis, these ANCA-negative patients mostly have localized disease, although some of these patients might develop ANCA once the disease progresses into a generalized form. In Churg–Strauss syndrome, only 40% of patients are MPO-ANCA-positive. Interestingly, ANCA-positive patients with Churg–Strauss syndrome present with small-vessel vasculitis that manifests clinically as mononeuritis multiplex, purpura and glomerulonephritis, whereas the ANCA-negative patients with Churg–Strauss syndrome predominantly show tissue infiltration by eosinophils.10, 11 This observation suggests that Churg–Strauss syndrome exists as two distinct disease entities.12 PR3-ANCA-positive patients with ANCA-associated vasculitides differ from MPO-ANCA-positive patients in various ways: compared with MPO-ANCA-positive patients, PR3-ANCA-positive patients have much more granuloma formation in their lesions; involvement of more organs; a faster decline in renal function; and more frequent relapses.13 PR3-ANCA and MPO-ANCA are, therefore, markers for different phenotypes of disease.

Table 1 Disease associations of antineutrophil cytoplasmic autoantibodies against proteinase 3 and myeloperoxidase
Table 1 - Disease associations of antineutrophil cytoplasmic autoantibodies against proteinase 3 and myeloperoxidase
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There are no available data on the presence of these autoantibodies before clinical expression of the associated diseases occurs. Although not all studies are congruent, prospective data have shown that rising levels of PR3-ANCA, particularly when assessed by enzyme-linked immunosorbent assay, preceded the onset of clinical relapse of ANCA-associated vasculitides in 26 out of 33 cases (79%).14 However, 12 out of 38 rises in PR3-ANCA levels were not followed by a relapse.14 Surveillance of alterations in ANCA titers is considered helpful in prediction of disease relapse.15 Partly because of the much lower frequency of relapse in patients with MPO-ANCA-associated vasculitis compared with PR3-ANCA-associated vasculitis, comparable data are not available for MPO-ANCA-associated vasculitis.

Taken together, data from clinical studies suggest, but certainly do not prove, that ANCA are directly pathogenic in ANCA-associated vasculitides. Some patients with ANCA-associated vasculitides are ANCA-negative, and the correlation between ANCA titers and disease activity is limited.

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In vitro evidence for a pathogenic role for anca

Lesions in ANCA-associated vasculitides are characterized by neutrophil infiltration within the blood-vessel wall. Neutrophils have left the bloodstream and become fully activated in the vascular wall. How do these neutrophils become activated?

ANCA-induced neutrophil activation

Falk et al.16 demonstrated that neutrophils express PR3 and myeloperoxidase on their surface when primed with low-dose proinflammatory cytokines such as tumor necrosis factor (TNF). When primed neutrophils are incubated with IgG fractions from sera that contain PR3-ANCA or MPO-ANCA, they are fully activated to produce reactive oxygen species and release lytic enzymes including elastase and PR3.16 These effects could not be induced by IgG fractions from healthy control sera, but they were induced by monoclonal antibodies to PR3 and myeloperoxidase. Neutrophil activation by ANCA occurs only when neutrophils are attached to a surface, and not in solution17—circulating neutrophils, therefore, are not activated. Furthermore, neutrophils are even more strongly activated by ANCA in the presence of arachidonic acid,18 which indicates a potential for further activation in an already inflammatory in vivo milieu.

The signal transduction routes by which neutrophils are activated by ANCA have largely been characterized, although there is still some uncertainty about the fine details.19 F(ab')2-fragments (dimers of the antigen-binding fragment) of ANCA have been shown by some authors to activate primed neutrophils,16, 20 but this finding has not been confirmed by others.21, 22 Fc gamma receptor (FcgammaR) interactions certainly have a role in neutrophil activation by ANCA. Blockade of FcgammaRIIa with nonstimulating monoclonal antibodies prevents ANCA-mediated neutrophil activation.21, 22 Further studies have indicated a role for the FcgammaRIIIb as well.23 FcgammaR ligation is involved in ANCA-induced neutrophil activation, so signal transduction is expected to occur via pathways known to be involved in FcgammaR ligation. Other pathways in addition to those mediated by G proteins have, however, also been shown to be involved.19, 24

Membrane expression of proteinase 3 and myeloperoxidase

After isolation from the bloodstream, even without priming, neutrophils from different individuals differ in their expression of membrane PR3 (mPR3). Neutrophils from some individuals do not express mPR3 at all, whereas other individuals show uniform mPR3 expression on their neutrophils.25 Most individuals, however, have a heterogeneous population of neutrophils that includes cells that are positive and cells that are negative for mPR3 expression. The proportion of each type is stable within an individual and seems to be genetically determined.26 Interestingly, patients with Wegener's granulomatosis or other inflammatory conditions have an elevated proportion of mPR3-positive neutrophils.25 Also, in patients with Wegener's granulomatosis, an elevated proportion of neutrophils that express mPR3 has been associated with an increased frequency of relapse.27 Neutrophils that express mPR3 can be activated by anti-PR3 antibodies with (or possibly without), priming.28

Studies have clearly demonstrated that myeloperoxidase expression is essential for MPO-ANCA-mediated neutrophil activation, because neutrophils from myeloperoxidase-deficient individuals are unresponsive to MPO-ANCA.29 Whether MPO-ANCA-mediated neutrophil activation involves myeloperoxidase translocation to the surface of primed neutrophils has, however, yet to be resolved.16, 17, 30, 31 Other mechanisms might be involved in MPO-ANCA-mediated neutrophil activation. Hess and colleagues have shown that resting human neutrophils exposed to supernatants from degranulated autologous neutrophils expressed myeloperoxidase, but not PR3, on their cell surface and became responsive to antimyeloperoxidase autoantibodies.30 Besides neutrophils, monocytes also express PR3 and myeloperoxidase. A few studies have shown that monocytes can also be activated, after priming, by ANCA.32, 33

Priming in vivo by infection

How leukocytes are primed in vivo remains unknown, but might result from low-grade infection. Chronic nasal carriage of Staphylococcus aureus has been observed in 63% of patients with Wegener's granulomatosis (compared to 20% of control individuals), and nasal carriage is strongly associated with the occurrence of relapse.34 The essential role of infection-related neutrophil priming might explain the reduction of relapse observed in patients with Wegener's granulomatosis who receive maintenance treatment with co-trimoxazole.35

ANCA-induced neutrophil–endothelial interaction

In vitro studies have also addressed the interaction between endothelial cells and neutrophils in the presence of ANCA. The presence of ANCA resulted in detachment and lysis of endothelial cells.36 In an endothelial-cell-coated flow system, ANCA could convert rolling neutrophils into neutrophils that stably adhered to the endothelial layer.37 So, data from in vitro studies suggest that ANCA can induce neutrophils and monocytes to adhere to the endothelial layer, where these cells become activated within (or in close proximity to) the vessel wall, which results in small-vessel necrotizing and leukocytoclastic vasculitis (Figure 2). On the basis of the presence of PR3 within (and on) the membrane of endothelial cells, it has been suggested that PR3-ANCA can upregulate the expression of adhesion molecules on endothelial cells.38, 39 Other groups, however, have been unable to confirm PR3 expression on endothelial cells.40

Figure 2 Schematic representation of the neutrophil responses that are putatively involved in the pathogenesis of ANCA-associated small vessel vasculitis
Figure 2 : Schematic representation of the neutrophil responses that are putatively involved in the pathogenesis of ANCA-associated small vessel vasculitis Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

(A) Proinflammatory cytokines and chemokines (e.g. tumor necrosis factor) that are released as a result of local or systemic infection cause upregulation of the expression of endothelial adhesion molecules (e.g. selectins, intercellular adhesion molecule 1 and vascular cell adhesion molecule 1), and prime neutrophils. (B) Neutrophil priming causes upregulation of the expression of neutrophil adhesion molecules (CD11b) and translocation of the ANCA antigens from their lysosomal compartments to the cell surface. (C) Engagement of dimers of the antigen-binding portion of ANCA with ANCA antigens on the cell surface, and interaction of the Fc part of the antibody with Fc receptors, activates neutrophils and causes increased transmigration and adherence of neutrophils to vessel walls. (D) ANCA-mediated neutrophil activation also triggers production of reactive oxygen radicals and possibly causes neutrophil degranulation. The consequent release of proteolytic enzymes leads to vasculitis. Reproduced with permission from reference64 © (2005) Elsevier Ltd. Abbreviations: ANCA, antineutrophil cytoplasmic autoantibodies; CD11b, a beta2-integrin cell-surface adhesion molecule involved in neutrophil adherence to and migration through vascular endothelial cells; ICAM-1, intercellular adhesion molecule 1; O2 dot oxygen radicals.

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Role of T cells

The role of T cells in ANCA-associated vasculitides has been less well defined.41 Although T cells are involved in the immune response to ANCA and probably also participate in granuloma formation in Wegener's granulomatosis, their pathogenic role still requires further study.

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In vivo evidence for a pathogenic role of anca

Models for MPO-ANCA-associated vasculitis

The first experimental model to prove the pathogenic role of ANCA in ANCA-associated vasculitides was described by Brouwer et al.42 They immunized rats with human myeloperoxidase, which resulted in the development of antibodies against human myeloperoxidase that cross-reacted with rat myeloperoxidase. Vasculitis did not, however, develop in these rats. When the team perfused these rats' kidneys with the products of activated neutrophils—that is, myeloperoxidase, lysosomal enzymes and hydrogen peroxide—the myeloperoxidase-immunized rats developed necrotizing crescentic glomerulonephritis without immune deposits, which the control-immunized rats did not develop. A comparable model was introduced for antimyeloperoxidase-associated pulmonary vasculitis, in which the lungs of myeloperoxidase-immunized rats were perfused with neutrophil products.43 The proinflammatory potential of MPO-ANCA was also demonstrated in a rat model of glomerulonephritis induced by antiglomerular-basement-membrane antibodies. In rats immunized with human myeloperoxidase (these rats developed antibodies to human myeloperoxidase that cross-reacted with rat myeloperoxidase), injection of a subnephritogenic dose of heterologous antiglomerular-basement-membrane antibodies caused severe necrotizing glomerulonephritis.44 The presence of antimyeloperoxidase antibodies, possibly because of their neutrophil-activating potential, augmented the inflammatory reaction.

More direct proof of a pathogenic effect of the antimyeloperoxidase response was obtained in a mouse model of MPO-ANCA-associated systemic vasculitis. Xiao et al.45 immunized myeloperoxidase-deficient mice with mouse myeloperoxidase. They transferred spleen cells from these mice into immune-deficient mice and observed the development of pauci-immune necrotizing crescentic glomerulonephritis, pulmonary capillaritis, and systemic vasculitis in the recipients. Transfer of IgG alone from myeloperoxidase-immunized, myeloperoxidase-deficient mice into wild-type mice resulted in focal necrotizing glomerulonephritis in the recipients. Pretreatment of the recipients with lipopolysaccharide induced more-diffuse necrotizing and crescentic glomerulonephritis.46 Priming of neutrophils and endothelial cells with lipopolysaccharide is thought to prepare neutrophils for the activating effects of MPO-ANCA. Little et al.47 induced an immune response to autologous myeloperoxidase in rats by immunization with human myeloperoxidase (in complete Freund's adjuvant), which caused the rats to develop pauci-immune systemic vasculitis. The researchers showed (by intravital microscopy) that MPO-ANCA-containing IgG could induce firm adhesion and transmigration of leukocytes after topical application of CXC-chemokine ligand (CXCL) 1, a proinflammatory stimulus. Control IgG did not induce the leukocyte adhesion or transmigration that underlies microvascular damage. The essential role of neutrophils in MPO-ANCA-associated glomerulonephritis was also demonstrated by Xiao et al.48 Mice depleted of neutrophils were completely protected from antimyeloperoxidase-induced glomerulonephritis.

All of the experiments described above demonstrate that MPO-ANCA are pathogenic and cause pauci-immune systemic vasculitis, by their promotion of leukocyte–microvascular interactions. Interestingly, a recent observation of congenital systemic vasculitis in a baby born to a mother with MPO-ANCA underscores the pathogenic potential of MPO-ANCA, as transplacental transfer of IgG-class MPO-ANCA from the mother into the fetus might have produced neonatal vasculitis.49

Modeling PR3-ANCA-associated vasculitis

In contrast to the several animal models of MPO-ANCA-associated vasculitis, a satisfying model for PR3-ANCA-associated vasculitis that shows characteristics similar to human Wegener's granulomatosis is not yet available. Pfister et al.50 aimed to develop such a model by immunization of mice deficient for PR3 and elastase with mouse PR3. These mice developed anti-PR3 antibodies that could bind to primed mouse neutrophils. Transfer of PR3-ANCA-containing IgG from these mice into wild-type mice did not, however, induce (systemic) vasculitis. After subcutaneous injections of TNF, only a local increase of inflammation was observed in recipients of PR3-ANCA-containing IgG compared to recipients of control IgG. The structural and functional homology between murine PR3 and human PR3 is, however, far lower than between murine and human myeloperoxidases.51

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Are mpo-anca and pr3-anca pathogenic?

The data presented strongly suggest that MPO-ANCA are, indeed, pathogenic. In vitro and in vivo experiments demonstrate the pathogenic pathways by which pauci-immune necrotizing systemic small-vessel vasculitis originates. In addition, the clinical presentation of MPO-ANCA-associated microscopic polyangiitis lesions in humans is compatible with that of lesions observed in experimental animals. The occurrence of congenital MPO-ANCA-associated microscopic polyangiitis underscores the pathogenic role of MPO-ANCA.

The pathogenic role of PR3-ANCA is less clear. Although clinical–serologic associations and in vitro experimental data strongly support a pathogenic role for PR3-ANCA, in vivo experimental models are lacking. In addition, the clinical and histopathologic differences between MPO-ANCA-associated vasculitides (among which microscopic polyangiitis predominates), and PR3-ANCA-associated vasculitides (among which Wegener's granulomatosis predominates) should be explained in terms of specific pathogenic differences.

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Recent therapeutic developments in ANCA-associated vasculitides

Cyclophosphamide or azathioprine for maintenance of remission?

In the 1970s, Fauci et al. introduced oral cyclophosphamide in combination with corticosteroids as an effective treatment for ANCA-associated vasculitides. After induction of remission, steroids were tapered and cyclophosphamide was continued for at least 18 months. Retrospective analysis of a cohort of 158 patients treated according to this regimen and followed for a median of 8 years reported that 91% of patients showed marked improvement and 75% achieved complete remission, but 50% of the 75% experienced at least one relapse within 5 years. Additionally, 13% of the cohort died of ANCA-associated vasculitides or treatment-related causes, and serious morbidity caused by irreversible effects of the disease (in 86% of the cohort) or of the treatment (in 42% of patients) was observed.52 Cyclophosphamide toxicity was the most prominent adverse effect, which manifested as hemorrhagic cystitis (in 15–43% of cases after oral treatment), bladder cancer (a 30-fold increased incidence compared to the background population during prolonged treatment), and opportunistic infections, particularly during cyclophosphamide-induced leukopenia.52, 53 In a study of 154 patients that compared the efficacy of cyclophosphamide and azathioprine treatment in the maintenance of remission, azathioprine proved as effective as cyclophosphamide in maintaining remission over 18 months (16% versus 17% of patients relapsed, respectively), but was less toxic (10% versus 18% of patients experienced serious adverse events, respectively).54 With a follow-up period of over 18 months, maintenance therapy with azathioprine might be associated with a somewhat higher relapse rate than cyclophosphamide therapy,55 especially for patients who are still PR3-ANCA-positive at the time of the switch from cyclophosphamide (induction) to azathioprine (maintenance) therapy.56 At present, in the absence of long-term, prospective studies, azathioprine can be advocated for maintenance treatment of patients with ANCA-associated vasculitides (instead of cyclophosphamide) particularly in patients who become ANCA-negative after induction of remission. In patients who remain persistently ANCA-positive, maintenance treatment might need to be extended for a prolonged period.

Plasmapheresis for life-threatening cases of ANCA-associated vasculitis

Although generally effective, cyclophosphamide and corticosteroids alone are considered inadequate for treating life-threatening cases of ANCA-associated vasculitis, in which one or more organ systems are failing. Additional therapy, therefore, seems required. The MEPEX (methylprednisolone versus plasma exchange as additional therapy for severe, ANCA-associated glomerulonephritis) study from the European Vasculitis Study Group (EUVAS) compared the additional value of plasma exchange with that of pulses of intravenous methylprednisolone (1 g for 3 consecutive days) in patients with severe ANCA-associated vasculitis at presentation (who had severe renal involvement, defined as serum creatinine >500 micromol/l or being dialysis-dependent). Superior survival free of renal-replacement therapy was seen in patients who underwent plasma exchange and received pulse methylprednisolone, compared with those treated with pulse methylprednisolone only (69% versus 49%). Mortality at 1 year in both groups was 25%, but significantly more patients in the plasma-exchange plus methylprednisolone group were dialysis-free at 1 year than in the methylprednisolone-only group (81% versus 61%, P <0.01).57 The addition of plasma exchange to the standard regimen is, therefore, advocated in cases of severe ANCA-associated vasculitis, in line with the supposed pathogenic role of ANCA in these diseases.

Induction of remission in ANCA-associated vasculitides

The Wegener's Granulomatosis Etanercept Trial (WGET) has investigated whether anti-TNF treatment with etanercept, given in addition to standard treatment, increases remission rate and decreases time to remission and number of disease flares.58 Surprisingly, the addition of etanercept had no beneficial effects. On the contrary, in this study of 174 patients with a follow-up of 27 months, solid cancers were seen in six patients treated with etanercept compared with none treated with placebo,58 although patients were not screened for tumors before the study. Infliximab might be more effective than etanercept in Wegener's granulomatosis, as shown by its beneficial effect in patients with refractory Wegener's granulomatosis and microscopic polyangiitis.59 Treatment with intravenous immunoglobulin, in addition to immunosuppressive therapy, has been evaluated in patients who experience persistent disease activity despite conventional treatment.60 Although a single course of intravenous immunoglobulin (12 g/kg) reduced disease activity, its effect was shortlived.

The EUVAS group has also compared intravenous pulse cyclophosphamide (15 mg/kg every 2–3 weeks) with daily oral cyclophosphamide (2 mg/kg/day) for induction of remission in patients with ANCA-associated vasculitides,61 with the aim of reducing the toxic effects of cyclophosphamide treatment. During a follow-up period of 18 months, no differences were observed between these two regimens in cumulative survival, times to remission and to subsequent relapse, or disease-free periods. The cumulative dose of cyclophosphamide received by patients in the oral-treatment group was twice as high as that received by patients in the intravenous-treatment group, which suggests that patients treated with the intravenous regimen are likely to experience fewer adverse events related to cyclophosphamide toxicity. Another study tested whether cyclophosphamide treatment was necessary in patients who had mild disease. In the NORAM (nonrenal Wegener's granulomatosis treated alternatively with methotrexate) study, which included 100 patients, cyclophosphamide (2 mg/kg daily) was compared with methotrexate (20–25 mg weekly)—both in combination with corticosteroids—as induction treatment for patients with newly diagnosed, non-life-threatening ANCA-associated vasculitides, who had serum creatinine levels <150 micromol/l.62 These induction regimens were stopped at 12 months. Although remission rates did not differ between patients who received cyclophosphamide and methotrexate treatments (93.5% versus 89.8%), by 18 months, relapses had occurred more frequently in the methotrexate-treated group (69.5% versus 46.5%, P <0.05).

In the past few years, much attention has been given to B-cell depletion with rituximab, a chimeric human–mouse monoclonal antibody to the B-cell surface antigen CD20. Initial, noncontrolled studies of rituximab therapy in patients with ANCA-associated vasculitides who could not tolerate cyclophosphamide or whose disease was refractory to cyclophosphamide treatment showed favorable responses: all 11 patients entered remission.63 Remission persisted in a few patients even after their B cells had regenerated. Currently, the large, controlled RAVE (rituximab for ANCA-associated vasculitis) study is investigating whether B-cell depletion with rituximab can induce remission and, in addition, can tolerize patients to the ANCA antigens myeloperoxidase and PR3—the effective induction of such tolerance could potentially cure ANCA-associated vasculitides.

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Conclusions

Since ANCA were described in the early 1980s, the clinical association between these autoantibodies and the pauci-immune necrotizing small-vessel vasculitides has been firmly established. The relevant antigenic targets of ANCA have been identified as PR3 and myeloperoxidase. Despite some overlap, PR3-ANCA are particularly associated with Wegener's granulomatosis and MPO-ANCA with microscopic polyangiitis. In vitro studies have demonstrated that ANCA interact with neutrophils and endothelial cells to cause endothelial damage. Studies in experimental animals strongly support a direct pathogenic role for MPO-ANCA in causing necrotizing vasculitis and glomerulonephritis, as seen in microscopic polyangiitis. A direct pathogenic role is less clear for PR3-ANCA. As such, the occurrence of granulomatous inflammation in Wegener's granulomatosis is not well explained. This type of lesion suggests an involvement of T cells, but the precise role and antigenic specificities of effector T cells await further studies. Nevertheless, a huge increase in our understanding of the pathogenesis of ANCA-associated vasculitides has been seen in recent years. In addition, several empirically designed, controlled trials have resulted in modified treatment regimens that are less toxic and, possibly, more effective than those previously used. With the upcoming availability of biologic treatments that target specific immune effector cells and molecules, the insights that have been gained into the pathogenesis of ANCA-associated vasculitides could be translated into additional targeted therapies.

Key points

  • In patients with suspected vasculitis, the presence of antineutrophil cytoplasmic autoantibodies (ANCA) to proteinase 3 (PR3-ANCA) or myeloperoxidase (MPO-ANCA) strongly suggests one of the ANCA-associated vasculitides: Wegener's granulomatosis, microscopic polyangiitis, or Churg–Strauss syndrome

  • Rising ANCA titers should alert physicians to the possibility of clinical relapse of ANCA-associated vasculitides, although the correlation between rising ANCA titers and disease relapse is not perfect

  • A pathogenic role for MPO-ANCA is strongly supported by experimental studies, particularly in animal models, but the role of PR3-ANCA is less clear

  • Wegener's granulomatosis is associated with PR3-ANCA, and microscopic polyangiitis is associated with MPO-ANCA; these observations, together with experimental findings and the observed clinical differences between these two conditions, suggest a distinct pathogenesis for these diseases

  • Data from recent, randomized, controlled trials allow an evidence-based approach to the treatment of patients with ANCA-associated vasculitides

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Competing interests

The authors declared no competing interests.

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Subject areas under which this article appears: Vasculitis syndromes