In the present study, we found that IL-17A was overexpressed in CD strictures compared with non-strictured CD areas and control gut. We also found that myofibroblasts from CD strictures, which express the IL-17A receptor(IL-17RC), responded to IL-17A by producing more collagen and TIMP-1, and had reduced migratory ability. By contrast, IL-17E was not upregulated in CD strictures, and it did not influence collagen and TIMP-1 production or migration of myofibroblasts from CD strictures.
IL-17A, which is produced mainly by Th1 and Th1/Th17 cells [18, 19], mediates autoimmunity and immune defense against pathogens , and is increased in the intestinal mucosa of patients affected by chronic inflammatory bowel disorders, such as celiac disease , CD, and ulcerative colitis [22, 23]. Moreover, experimental studies have shown an important role for IL-17A in tissue remodeling and fibrosis in a number of different tissues. In particular, IL-1β-induced and bleomycin-induced lung fibrosis seems to depend on the action of IL-17A, and in addition, IL-17A−/− mice are less susceptible to experimental skin fibrosis [7, 24]. As opposed to acute experimental colitis, which is mainly characterized by a Th1 immune response, chronic trinitrobenzenesulfonic acid (TNBS)-induced murine colitis, which is accompanied by intestinal fibrosis, is driven predominantly by IL-17A-producing Th17 cells . The observation that targeting IL-23 (a key cytokine in Th17 cell development) with a p40 peptide-based vaccine ameliorates chronic TNBS-induced colitis,and reduces IL-17A, TGF-β1 levels, and collagen deposition in the bowel wall , highlights the importance of IL-17A in intestinal experimental fibrosis.
In the current study, we found, using immunoblotting and ELISA, that IL-17A was overexpressed in vivo in strictured CD gut compared with non-strictured CD and control gut. We decided to collect samples from uninflamed areas of patients with fibrostenosing CD so that any confounding effect of inflammation on IL-17A was minimized. Similar to the in vivo data, ex vivo tissue explants from CD strictures produced more IL-17A than explants from non-strictured CD areas and control gut. In keeping with our previous results , collagen content and TGF-β1 transcripts in tissue explants from uninflamed CD strictures were also increased compared with uninflamed non-strictured CD areas and control gut. The recent observation that IL-17A expression is higher in long-standing CD mucosa compared with early mucosal lesions  further strengthens our findings, because intestinal fibrosis is a late-stage process in CD.
On the basis of our findings showing that CD myofibroblasts expressed the IL-17A receptor IL-17RC we decided to stimulate these cells with IL-17A in in vitro experiments. In keeping with the study of Bamba et a l. , we found that IL-17A increased MMP-3 and MMP-12 production by myofibroblasts from CD strictured and non-strictured areas. However, in parallel, IL-17A upregulated TIMP-1 and collagen release, and reduced the migration ability of CD myofibroblasts. In a murine model of intestinal fibrosis, TIMP-1, which is upregulated, effectively inhibited ECM degradation by MMPs , and TIMP-1 was found to be increased in collagenous colitis and in CD strictures [16, 29]. Taken together, our data support a pro-fibrogenic role for IL-17A in CD intestinal fibrosis.
In parallel to studying the role of IL-17A, we also investigated the influence of IL-17E in the fibrogenic process in CD. IL-17E exerts two distinct immunological functions: on the one hand, it promotes Th2 response in allergic diseases including asthma , while on the other hand, it dampens the inflammatory process in immune-mediated disorders including IBD . A pro-fibrogenic role for IL-17E in experimental fibrotic disorders has also been shown. In particular, IL-17E mediates pulmonary collagen deposition in mice exposed to house dust mite , and intestinal TNBS-induced fibrosis was found to be associated in the early phase by a marked increase in IL-17E . When we investigated IL-17E in vivo, we found no difference in the expression of IL-17E in uninflamed CD strictures, uninflamed non-strictured CD areas, and control gut. Likewise, ex vivo tissue explants from the same three groups of patients produced comparable amounts of IL-17E. The IL-17E receptor IL-17RB was expressed at a similar level in strictured, non-strictured CD, and control gut. IL-17RB was found to be expressed by CD myofibroblasts, and stimulation with IL-17E increased MMP-3 and MMP-12 production by myofibroblasts from CD strictured and non-strictured areas and from control subjects. However, we did not observe any influence of IL-17E on collagen or TIMP-1 production by CD myofibroblasts or on their migration ability.