**2. Antineutrophil cytoplasmic antibody associated vasculitis**

#### **2.1. Background and chapter overview**

Glomerulonephritis is a common cause of renal failure both worldwide and in Australia. Rapidly progressive or crescentic glomerulonephritis represents the most severe form of the disease and antineutrophil cytoplasmic antibody (ANCA) associated vasculitis (AAV) accounts for >50% and more likely up to 80% of all cases of rapidly progressive glomerulo‐ nephritis. The AAVs are considered a heterogenous group of systemic autoimmune conditions characterised by necrotising inflammation of small to medium sized arteries, capillaries and venules. The disease is diagnosed by detecting ANCA in the serum which characteristically is directed against myeloperoxidase (MPO) or proteinase 3 (PR3). The two most severe clinical manifestations of disease are rapidly progressive glomerulonephritis and pulmonary hae‐ morrhage due to pulmonary capillaritis. These syndromes are associated with significant morbidity and untreated have a mortality that approaches 100%. Renal vasculitis occurs in more than 50% of patients at presentation but in 70-85% of patients with AAV during the course of their disease [9]. While current treatments for active ANCA vasculitis are often life-saving they are toxic and more than 1 in 3 patients will suffer a significant treatment related adverse event.[10] A better understanding of the *critical molecular events* which underlie the disease process will help identify more specific targeted therapies.

In the early 1980s two Australian groups based in Melbourne, from St Vincent's Hospi‐ tal[11] and the Austin Hospital[12] described the association of antibodies directed against the neutrophil cytoplasm in patients with rapidly progressive glomerulonephritis. These reports represented key advances in our understanding of the pathogenesis of au‐ toimmune small vessel vasculitis. Subsequent work by a Dutch group helped establish the correlation between ANCAs and the three clinical syndromes; Wegener's granuloma‐ tosis, microscopic polyangiits and Churg-Strauss syndrome.[13] More recently these syn‐ dromes have been renamed to generate nomenclature free from the use of eponyms. [14-16] The new nomenclature proposed and adopted into the literature and clinical prac‐ tice in 2011 is as follows; Microscopic Polyangiitis (MPA), Granulomatosis with polyangii‐ tis, (GPA), formally known as Wegener's, Allergic Granulomatosis and Angiitis (AGA) formally known as Churg Strauss Disease and Renal Limited Vasculitis (RLV).[14] This new terminology will be adopted for the remainder of this chapter.

In this chapter, we will concentrate on renal injury resulting from AAV which has formed the basis for clinical and experimental studies. For both MPA and GPA target autoantigens have been identified which are constituents of neutrophils. For MPA, myeloperoxidase (MPO) is usually the target autoantigen, while antibodies to proteinase 3 (PR3) are usually detectable in patients with clinical features of GPA. In both clinical and experimental AAV (GPA or MPA) two separate key steps are required for the development of glomerulonephritis and renal injury. The first critical step involves the development of systemic autoimmunity to the target antigen, MPO or PR3. The second step involves antigen specific nephritogenic immune responses driving glomerular injury and renal disease.

#### **2.2. The development of systemic autoimmunity in MPA and GPA**

**Classification Disease Name** Large Vessel Vasculitis Giant Cell (Temporal) Arteritis

Medium Sized Vessel Vasculitis Polyarteritis Nodosa

\*These diseases have subsequently been renamed.

34 Updates in the Diagnosis and Treatment of Vasculitis

**2.1. Background and chapter overview**

**Table 1.** The Chapel Hill Conference on the Nomenclature of Systemic Vasculitis

process will help identify more specific targeted therapies.

**2. Antineutrophil cytoplasmic antibody associated vasculitis**

Glomerulonephritis is a common cause of renal failure both worldwide and in Australia. Rapidly progressive or crescentic glomerulonephritis represents the most severe form of the disease and antineutrophil cytoplasmic antibody (ANCA) associated vasculitis (AAV) accounts for >50% and more likely up to 80% of all cases of rapidly progressive glomerulo‐ nephritis. The AAVs are considered a heterogenous group of systemic autoimmune conditions characterised by necrotising inflammation of small to medium sized arteries, capillaries and venules. The disease is diagnosed by detecting ANCA in the serum which characteristically is directed against myeloperoxidase (MPO) or proteinase 3 (PR3). The two most severe clinical manifestations of disease are rapidly progressive glomerulonephritis and pulmonary hae‐ morrhage due to pulmonary capillaritis. These syndromes are associated with significant morbidity and untreated have a mortality that approaches 100%. Renal vasculitis occurs in more than 50% of patients at presentation but in 70-85% of patients with AAV during the course of their disease [9]. While current treatments for active ANCA vasculitis are often life-saving they are toxic and more than 1 in 3 patients will suffer a significant treatment related adverse event.[10] A better understanding of the *critical molecular events* which underlie the disease

In the early 1980s two Australian groups based in Melbourne, from St Vincent's Hospi‐ tal[11] and the Austin Hospital[12] described the association of antibodies directed against the neutrophil cytoplasm in patients with rapidly progressive glomerulonephritis. These reports represented key advances in our understanding of the pathogenesis of au‐

Small Vessel Vasculitis Wegener's Granulomatosis\*

Takayasu's Arterits

Kawasaki's disease

Churg Strauss Syndrome\* Microscopic Polyangiitis\* Henoch Schonlein Purpura Essential Cryoglobulinaemic Vasculitis Cutaneous Leukocytoclastic Angiitis

> The development of autoimmunity is a complex process, multifactorial in origin, which involves the loss of tolerance and enhanced cellular and humoral activity.[17] In AAV, disease is defined and characterized by antibodies detected against MPO or PR3. While antibodies form the diagnostic hallmark of disease, cellular immunity is critical and is required for the development of humoral immunity and the subsequent generation of B cells and production of ANCAs. A role for cellular immunity has been defined in both clinical and experimental ANCA vasculitis. In addition to adaptive immune cells, innate immune cells contribute to the generation of autoimmunity with evidence for involvement of different cell types in this disease process.

#### **2.3. The initiation and progression of rapidly progressive glomerulonephritis and renal injury in AAV**

Enhanced cellular autoimmunity and innate cells stimulate B cells resulting in the production of antigen specific ANCAs. These auto-antibodies bind to and activate circulating neutrophils. These activated neutrophils are recruited to glomerular capillaries,[18] where they degranulate and initiate renal injury. Degranulating neutrophils release their noxious constituents and also deposit MPO [19] and probably PR3 in the glomerulus. Later, CD4+ T cells recognise the autoantigen (MPO/PR3) in the glomerulus and attract additional immune effector cells; this results in severe renal injury. In both clinical and experimental settings cellular nephritogenic immunity, humoral immunity and innate immune cells are critical for the development of rapidly progressive glomerulonephritis.[20-24] Our current treatment regimes were designed to target these cells, or combinations of them.

In this chapter we will focus on the pathogenesis of the ANCA associated vasculitides, focussing on AAV attributable to MPA and GPA. We will pay attention to the develop‐ ment of autoimmunity and concentrate on end organ injury in the kidney, a critical tar‐ get of the small vessel vasculitides. Interestingly, while GPA and MPA share many diagnostic and clinical features and patients with these diseases have been grouped to‐ gether in many clinical trials, more recent evidence including a landmark genetic study, suggests that GPA and MPA represent two different diseases. While we will discuss GPA and MPA separately, there is stronger experimental evidence linking MPO with disease. This includes several small animal studies which have confirmed pathogenic roles for cellular and humoral autoimmunity, directed against MPO, which closely re‐ semble human disease. Our discussion will focus on the disease pathogenesis of AAV and attempt to define future directions for study which ultimately may lead to therapeu‐ tic interventions. Information has been made available from human studies assessing mechanisms of disease as well as experimental studies, utilizing rodent models of vascu‐ litis. Further insights into disease pathogenesis can be gained from clinical trials, includ‐ ing those with negative results.

many of these genes display aberrant expression in several autoimmune diseases. This is not surprising considering several of these genes encode proteins critical for maintenance of the immune system, including the function of innate immune cells, T lymphocytes, B lymphocytes and regulatory cells. There are several limitations to these studies. Some studies which linked aberrant gene expression with AAV included patients with only one form of the disease (i.e. GPA, MPA, RLV or AGA), while other studies were less specific and included all patients who had detectable ANCA levels. Furthermore several of these associations were not confirmed when assessing disease in different population groups and hence results from these early studies suggested that there was, at best, a modest link between genetic background and

The Pathogenesis of Antineutrophil Cytoplasmic Antibody Renal Vasculitis

http://dx.doi.org/10.5772/54637

37

In a genome wide association study with over 10 000 patients (including controls), not only was a genetic component confirmed but the antigenic specificity for AAV, i.e. for MPO or PR3 was found to have distinct genetic associations. For patients with ANCA directed against PR3, there was a strong genetic association with *HLA-DP and* genes encoding *α1-AT-SERPINA1* and *PTN3*. Conversely patients with antibodies directed against MPO showed a strong association with *HLA-DQ*.[28] The observation that there were different genetic associations for MPO-ANCA and PR3-ANCA strengthens the proposal that these diseases represented two different clinical entities. Furthermore the stronger genetic component to PR3 related disease identified

An epigenetic basis for disease has also been proposed. Neutrophil levels of the chroma‐ tin modification protein complex, H3K27me3, required for gene silencing were decreased in patients with AAV, at both the MPO and PR3 loci. This phenomenon was dependent on the transcription factor encoding gene, RUNX3. Interestingly RUNX3 message was found to be decreased in patients with AAV compared to healthy controls. These studies provided the first evidence that epigenetic modifications present in AAV patients could impair gene silencing and result in aberrant expression of the target auto-antigens, MPO and PR3.[29] These recently published genetic and epigenetic studies have added consid‐

In addition to genetic factors, environmental factors contribute to the loss of tolerance, the development of autoimmunity (to MPO or PR3) and subsequent organ injury. Environmental triggers that have been implicated in disease pathogenesis include environmental toxins,

Epidemiological studies have demonstrated increased incidence of ANCA vasculitis, and more specifically MPA, is increased in patients exposed to a variety of environmental toxins,[30] in particular silica.[31] This is thought to result from environmental toxins serving as adjuvants to the immune system.[32] The development of ANCAs, in particu‐ lar those reactive to MPO, is not uncommon after treatment with propylithiouracil,[33] although systemic disease following treatment is uncommon. Overt MPA with focal ne‐

pharmacological therapies and infections, for which there is the strongest evidence.

**4. Environmental factors driving disease in ANCA vasculitis**

disease.[26-27]

in earlier studies was substantiated.

erably to our understanding of AAV.
