The diagnosis and classification of WG and related vasculitides were advanced considerably by characterization of serum antibodies that react with PR-3 (c-ANCA) [17
]. It is not yet known whether ANCA are directly involved in the pathogenesis of WG or are merely an epiphenomenon [10
]. One of the unresolved issues is the inability to explain the nonrandom, selected organ injury that defines the WG vasculitis and the concurrent, seemingly random, nature of injury within 'targeted' organs [18
]. Little is known about the expression and distribution of PR-3 in the normal lung and in the lung tissue of patients with WG.
To contribute to this little-known issue, we examined the PR-3 expression in normal lung and lung tissue of WG patients, as the lung is the most frequent organ involved in WG [15
]. In normal lung tissue, mainly pneumocytes type I and type II and just a few granulocytes and macrophages express PR-3. We could detect the PR-3 mRNA expression in pneumocytes especially at sites with an increased number of infiltrating macrophages. Speculatively, these cells could be responsible for initiating PR-3 mRNA expression; probably through changing the microenvironmental cytokine levels. Cytokines like interleukin-1 and tumor necrosis factor-α, secreted from macrophages, can induce PR-3 mRNA in nonhematopoietic cells [19
The different expression of PR-3 in pneumocytes could also be due to variations of transcription factors. One possible candidate gene could be PU.1, which regulates the PR-3 transcription in B cells and macrophages. The expression in other nonhematopoietic cells, in particular pneumocytes, has not been extensively investigated [20
]. Although expressed in a very small amount in normal lung tissue, a question concerning the function of PR-3 in normal lung arose, since the occurrence of a proteolytic enzyme like PR-3 should not be favorable in this tissue [22
]. It is tempting to speculate that PR-3 could also play a role in microbiologic defense, since the enzyme also has an antimicrobial function [6
]. Further studies are clearly needed to elucidate the function of PR-3 in pneumocytes.
After we had proven that PR-3 mRNA is expressed in normal lung tissue, we questioned whether the expression pattern is different in the lung tissue of WG patients. The examined specimens of patients with WG showed strong PR-3 mRNA expression, with a double to threefold PR-3 mRNA increase compared with normal lung tissue. Similar to our findings in normal lung tissue, we could demonstrate in lung tissue of WG patients that PR-3 is mostly expressed in pneumocytes type I and type II. PR-3 expression was pronounced at sites of inflammatory infiltration, vasculitis and granulomas. In comparison with normal lung tissue, however, there was a higher number of infiltrating PR-3 expressing cells, which were not pneumocytes.
Apart from some PR-3 expressing granulocytes, most of the infiltrating PR-3 expressing cells were macrophages. One explanation for the occurrence of PR-3-positive macrophages in inflammatory tissue is the invasion of circulating monocytes to inflammatory sites. Just et al
. showed an upregulation of PR-3 mRNA expression especially in circulating monocytes, but not in neutrophils, in patients with cystic fibrosis [23
]. We found PR-3 mRNA expression in and around vascular structures and vasculitic lesions, where most of the infiltrating cells were characterized as macrophages. However, a few cells seemed to be endothelial cells. This is in line with our recent findings of de novo
synthesis of PR-3 in endothelial cells [24
]. On the contrary, most endothelial cells (even at multiple sites of acute vasculitis with inflammatory cells, invading the vessel and the surrounding tissue), were negative for PR-3 mRNA. Recent studies on the kinetics and stability of PR-3 transcripts revealed PR-3 mRNA expression is only transiently upregulated in endothelial cells [25
The histological features within the WG specimens varied considerably with different inflammatory infiltrates and with or without granulomas. The only apparent issue the WG specimens have in common is the upregulation of PR-3 throughout the whole specimen. The upregulation of PR-3 in the lung of WG patients may therefore reflect the selected organ injury, whereas the histological heterogeneity may represent a multiplicity of concurrent immune responses to a unique disease precipitant like PR-3.
As it is most probable that pneumocytes, vascular endothelial cells and renal epithelial cells are no longer only innocent bystanders but active participants in inflammatory reactions of autoimmune vasculitides such as WG, it is very important to study the expression pattern of PR-3 in other organs or tissues that are involved in the manifestation pattern of WG [26
]. Schwarting et al
. showed the in vitro
expression of PR-3 mRNA in tubular epithelial cells and glomerular epithelial cells of the kidney [25
]. In vivo
experiments in the kidney revealed a PR-3 expression in distal tubular epithelial cells and in the glomerulum. The PR-3 mRNA expressed by human glomerular epithelial cells correlates with crescent formation in patients with WG [25
We thus envision that the upregulation of PR-3 in parenchymal lung tissue in patients with WG, probably through an altered cytokine pattern, can lead to PR-3/ ANCA-mediated lung damage. Further in vitro experiments and functional studies with pneumocytes will show whether the interaction of anti-PR-3 antibodies with pneumocytes will also activate signal transduction events or the expression of chemokines or adhesion molecules.
In conclusion, we report for the first time that PR-3 mRNA, the target antigen for c-ANCA, is expressed by normal lung parenchymal cells and is upregulated in lung tissue of WG patients. PR-3 mRNA may therefore contribute to lung damage in WG and other ANCA-associated diseases via direct interaction.