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

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

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‐ crotising glomerulonephritis has been described in patients treated with penicillamine[34] and hydralazine.[35] The rarity of these phenomena has prevented us from learning more about disease pathogenesis.

also had antibodies to a complementary PR3. Complementary PR3 is the protein sequence resulting from transcription of the antisense DNA strand of the PR3 gene. Subsequently it was found that mice immunized with complementary PR3 also developed PR3-ANCA, suggesting a form of molecular mimicry. Pendergraft et al proposed that loss of tolerance, with the development of autoantibodies, could develop as a consequence of immune responses directed

The Pathogenesis of Antineutrophil Cytoplasmic Antibody Renal Vasculitis

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

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Both of these studies utilized human samples and elegant rodent models to propose infections as initiators of autoimmunity and renal vasculitis. Further work in this field is required to facilitate a better understanding of how molecular mimicry functions in humans and what

Infections activate and ligate Toll-like receptors (TLRs). These receptors are innate pattern and danger recognition receptors, ubiquitously expressed on immune cells, and resident tissue cells. which heighten innate and adaptive immune responses in response to infection or danger signals. Ligation of TLRs after infection can stimulate host immune responses, promoting autoinflammatory and auto-immune responses. Furthermore TLR ligation can stimulate endothe‐ lial cells and other resident kidney cells to generate a cytokine milieu conducive to the

Since their description in the 1980s antibodies directed against MPO and PR3 have formed the diagnostic hallmark of AAV. While not entirely specific there is a strong association between MPO-ANCA and MPA, while PR3 is commonly associated with GPA. Clinical and experi‐ mental studies have supported the notion that ANCA are pathogenic. Furthermore therapies targeting (humoral immunity and) ANCAs, including plasma exchange[47] and the anti-CD20 monoclonal antibody Rituximab,[48-49] have been successful in clinical practice. Most of the experimental evidence has supported a role for MPO in disease, but more recently an animal model of PR3-associated vasculitis has also been developed. This represents a significant advance and it is anticipated that this model will facilitate an improved understanding of the pathogenesis of PR3-AAV. In this section, we will also discuss other roles for B cells including their function as antigen presenting cells (APCs) and as potential regulators of disease.

Are ANCAs pathogenic? There has been increasing evidence supporting a pathogenic role for ANCAs. Results from *in vitro* studies demonstrate that ANCAs activate primed neutrophils which degranulate and deposit autoantigens in glomeruli. Similarly results from *in vivo* studies, including an expanding number of animal models, have confirmed a pathogenic role for ANCAs. *In vitro* studies have consistently demonstrated that neutrophils from patients with AAV express increased amounts of the target antigens (MPO/PR3) on their cell surface. [50] These auto-antigens are targets for ANCA binding. Furthermore, several cytokines

**5. The role of adaptive immunity in the development of ANCA**

against a complementary protein to the autoantigen.[46]

organisms could be involved.

recruitment of inflammatory leukocytes.

**autoimmunity and glomerulonephritis**

**5.1. The role of humoral immunity in AAV pathogenesis**

Links between infection and ANCA vasculitis have been suggested for some time, with seasonal variation in disease presentation suggesting a correlation with microbial infection. [36] Moreover results from several studies suggested that infection(s) may predate disease initiation and/or relapse in GPA, MPA and pulmonary vasculitis.[37-40] It must be noted that these results are contentious and other studies have not confirmed them.[30] However, nasal colonization with *Staphylococcus Aureus* is significantly increased in patients with GPA and increases the relative risk of relapse over 7 fold.[37] In a key study, published more than 15 years ago, it was shown that prophylactic antibiotic therapy (co-trimoxazole) successfully decreased disease relapses in ANCA vasculitis. This effect was presumed to result from decreased nasal carriage of *Staphylococcus Aureus.*[41] Interestingly, despite this finding longterm maintenance therapy with co-trimoxazole is not the standard of care in many centres, which may reflect concerns about the long-term safety of the drug. Consistent with an infective trigger to the development of AAV; features of vasculitis have been described in patients with bacterial endocarditis.[42-43] Despite the strong evidence linking infection with the develop‐ ment of autoimmunity (MPO/ PR3) and the ensuing organ injury few mechanistic links have been provided, until recently.

Several mechanisms have been proposed to link infection with the development of AAV, including the use of complementary proteins, molecular mimicry and the ligation of Toll like receptors (TLRs) which heighten innate and adaptive immune responses as well as activating resident kidney cells. A series of clinical and experimental studies have supported each of these concepts, however it is likely that these mechanisms act, at least partially, in combination.

Molecular mimicry refers to the development of antibodies to host proteins after (repeat‐ ed) exposure to foreign antigens, this occurs due to structural similarities between host and foreign proteins. Molecular mimicry has been proposed as a reason for the loss of tolerance to self and the subsequent development of autoimmunity.[44] In a series of ele‐ gant experiments it was demonstrated that antibodies to the lysosomal associated mem‐ brane protein-2 (LAMP-2) were highly prevalent in patients with ANCA vasculitis. Furthermore LAMP-2 was pathogenic and administration of polyclonal LAMP-2 to ro‐ dents resulted in a characteristic pattern of AAV, with focal necrotising glomerulonephri‐ tis, similar to that observed in human renal vasculitis. We will discuss LAMP-2 in more detail later in this chapter. There is homology between the immunodominant LAMP-2 epitope and the peptide of FimH, which is a component of the fimbriae of Gram nega‐ tive bacteria. It is hypothesized that certain patients infected with Gram negative bacteria would generate antibodies to LAMP-2 and develop vasculitis, through the process of mo‐ lecular mimicry.[45] This highly plausible theory provides one explanation for the clinical association between infection and the development of ANCAs or LAMP-2 antibodies.

An earlier study reported that a form of molecular mimicry could link *Staphylococcus Aureus* infection with the development of AAV. This process was more complex and involved the use of complementary proteins. The authors observed that patients who were PR3-ANCA positive also had antibodies to a complementary PR3. Complementary PR3 is the protein sequence resulting from transcription of the antisense DNA strand of the PR3 gene. Subsequently it was found that mice immunized with complementary PR3 also developed PR3-ANCA, suggesting a form of molecular mimicry. Pendergraft et al proposed that loss of tolerance, with the development of autoantibodies, could develop as a consequence of immune responses directed against a complementary protein to the autoantigen.[46]

Both of these studies utilized human samples and elegant rodent models to propose infections as initiators of autoimmunity and renal vasculitis. Further work in this field is required to facilitate a better understanding of how molecular mimicry functions in humans and what organisms could be involved.

Infections activate and ligate Toll-like receptors (TLRs). These receptors are innate pattern and danger recognition receptors, ubiquitously expressed on immune cells, and resident tissue cells. which heighten innate and adaptive immune responses in response to infection or danger signals. Ligation of TLRs after infection can stimulate host immune responses, promoting autoinflammatory and auto-immune responses. Furthermore TLR ligation can stimulate endothe‐ lial cells and other resident kidney cells to generate a cytokine milieu conducive to the recruitment of inflammatory leukocytes.
