Since the discovery of a role for PAR
2 in inflammation [
28,
33] and the induction or inhibition of inflammatory signs in response to PAR
2 agonists [
7,
10,
34,
35], there have been controversies over its role as a pro- or an anti-inflammatory message, depending on the cell types that were activated. The studies outlined here provide evidence that PAR
2 activation is a pro-inflammatory signal in the context of animal models of IBD, and provide further understanding on the source of PAR
2-expressing cells that are playing or not a critical role in colitis. We showed here that deficiency of PAR
2 expression on bone marrow-derived cells was not sufficient to protect from DSS colitis, although it seemed sufficient in the other model of IBD: the TNBS model. However, PAR
2-deficiency on recipient tissues other than bone marrow-derived cells was sufficient in all models to induce strong resistance to the development of colitis. While the mechanisms that underlie the pro-inflammatory effects of PAR
2 activation seemed to involve leukocyte recruitment, PAR
2 activation on tissue resident cells rather than leukocytes seemed to play the most critical role in colitis.
PAR
2 is widely expressed in almost all cell types present in the gut, including endothelium, epithelium, enteric neurons, fibroblasts, smooth muscle cells, and immune cells [
14‐
17]. Various in vitro studies have shown that the activation of PAR
2 in these cells may interplay with the inflammatory process. Studies have indicated that the activation of PAR
2 promotes the release of pro-inflammatory cytokines and recruitment of immune cells [
36‐
41], but can also protect from inflammatory insults, through the release by epithelial cells of prostaglandins or neuropeptides by primary afferents. Because the expression of PAR
2 is changed in tissues from IBD patients, the idea that PAR
2 could contribute to the pathogenesis of the disease has been evoked [
42‐
46]. However, which cellular source for PAR
2 expression plays a central role in colitis, and whether this role is pro- or anti-inflammatory depending on the cell types, were still open questions. We focused our attention on the expression of PAR
2 on bone marrow-derived cells, as a major role for PAR
2 activation on leukocyte recruitment has been suggested by many studies [
28,
34,
40,
47]. PAR
2
−/−
chimeric mice were protected from DSS-induced colitis, irrespective of the source of bone marrow cells. Thus, in DSS challenge, PAR
2 expressed on the recipient tissues appears to play a major role in the development of colitis. In TNBS challenge also, we observed that PAR
2
−/−
chimeric mice were protected from developing colitis irrespective of the source of bone marrow cells. We also observed that PAR
2
−/−
bone marrow cells conferred protection in PAR
2
+/+
mice challenged with TNBS. Thus, in the TNBS model, PAR
2 expressed on both recipient and bone marrow derived cells appears to play a major role in the development of colitis. Different mechanisms involved in the initiation and maintenance of the inflammatory response in these two models may be responsible for these differences. The current concept of pathogenic mechanism of TNBS involves the binding of TNBS to colonic or microbial proteins to generate hapten–protein complex. This complex is then thought to be recognized by the immune system mainly involving macrophages and T cells that induce inflammatory response [
48,
49]. In contrast, the pro-inflammatory effects of DSS is generally thought to be initiated by its cytotoxic effects on the colonic epithelial cells, and therefore loss of epithelial barrier integrity, which leads to an increased exposure of the innate immune system to bacteria and their proteins [
49,
50]. The pro-inflammatory signal thereby initiated leads to the recruitment of leukocytes, mainly neutrophils [
49,
51]. Thus, epithelial cell response is generally more extensive for the pathogenic mechanism of DSS- rather than TNBS-induced colitis. The results of the present study could suggest that PAR
2 activation plays a severe pro-inflammatory role in the response of intestinal epithelial cells to DSS injury. Indeed, previous studies have shown that PAR
2 activation on intestinal epithelial cells can promote the release of IL-8, a potent chemotactic factor for immune cells (especially neutrophils) [
41], and increased intestinal epithelial cell permeability [
19]. Considering this evidence, the importance of PAR
2 on the recipient tissues during the development of DSS-induced colitis may be due, at least in part, to its presence in intestinal epithelial cells. Our results suggest also that PAR
2 activation on epithelial cells is a crucial and necessary event to the development of DSS colitis. However, it is also important to point out that PAR
2
−/−
-recipient mice were protected from the development of colitis in both models: DSS and TNBS. TNBS-induced colitis also strongly depends on the loss of epithelial barrier functions, and on the presentation to the innate immune system of luminal microbial antigens, as it has been well established that TNBS colitis did not develop in the absence of colonic flora [
52]. The activation of PAR
2 on intestinal epithelial cells could also be a crucial event of TNBS-induced colitis. However, our results also show that PAR
2 activation on bone marrow-derived cells is a step limiting the event of TNBS colitis. In transmural inflammatory damage such as the ones observed in the TNBS colitis model and that are common in that way to CD characteristics, regulation of PAR
2 activation specifically on bone marrow cells may therefore represent a potential novel form of therapy.
In this study, we have demonstrated a strong correlation between reduction in inflammatory response and a reduction in leukocyte adherence/MPO activity. The process of leukocyte adherence is mainly mediated through interaction between adhesion molecules expressed on the endothelium (intracellular adhesion molecule, ICAM-1, or vascular cell adhesion molecule, VCAM-1) and leukocytes (integrins; composed of alpha and beta subunits) [
53,
54]. Previous studies have shown adhesion molecules to mediate the development of colitis. In DSS-induced colitis, antibodies against various adhesion molecules (E-selectin, MAdCAM-1, VCAM-1, and ICAM-1) have been shown to attenuate the intestinal inflammatory responses in animals [
32,
55,
56]. In TNBS-induced colitis, antibodies (anti-VCAM-1 or anti-MAC-1) or leukocytes (anti-MAC-1) have been shown to decrease tissue damage and monocyte infiltration [
57,
58]. We and others have also shown that PAR
2 plays an important role in up-regulating the expression of adhesion molecules including VCAM-1 and ICAM-1 in inflammatory conditions [
29,
38,
59]. Of particular note, these adhesion molecules (with the exception of MAC-1) are commonly found on endothelial cells, which could suggest that PAR
2 activation on this cell type constitutes the major signal for leukocyte recruitment. PAR
2 activation on endothelial cells has been shown to promote the release of IL-8 and increase the expression of various adhesion molecules [
36,
38,
60]. PAR
2 activation on endothelial cells therefore appears as a potential major player to promote leukocyte recruitment upon colitis.