Cigarette smoking and inflammatory bowel disease : insights into microbial and immune homeostasis

For decades, cigarette smoking has been known to modulate the development and course of inflammatory bowel diseases (IBD). IBD include two major types of disease: Crohn’s disease (CD; affecting the ileum) and ulcerative colitis (UC; affecting the colon). Intriguingly, cigarette smoking exerts a dual effect on IBD: it has a detrimental effect on Crohn’s disease, while it has been shown to be protective for ulcerative colitis. Despite the epidemiological evidence, many of the cellular and molecular mechanisms underlying this differential effect remain to be revealed. Moreover, little is known of the effect cigarette smoking on microbial and immune homeostasis in the gut. Previous studies within our research group have shed light on the effect of cigarette smoke on the Peyer’s patches (main lymphoid organs in the gut) in the ileum of mice. It was demonstrated that chronic cigarette smoke exposure induces the recruitment of immune cells (CD4+ and CD8+ T cells, CD11b+ dendritic cells) to the subepithelial dome of the murine Peyer’s patches. In addition, chronic cigarette smoke exposure induced autophagy and apoptosis in the follicle-associated epithelium overlying these Peyer’s patches. The current thesis wants to elaborate further on these initial findings and aims to provide additional insights on the effect of cigarette smoke on the gut immune system and even the gut microbiome. In the first part of this thesis, we study the effect of chronic smoke exposure on the composition and activity of the gut microbiome and the production of immune factors in mice. A first finding is that the composition and activity of the microbiome shifts in the colon in response to chronic smoke exposure. Moreover, the activity of the Lachnospiraceae sp. is increased. Secondly, we find an increase in the expression of mucins, the building blocks of the mucus layer in response to smoking: MUC2 and MUC3 were increased in the ileum, while MUC4 increases in the colon. Thirdly, we find that although no microbial changes are induced by smoking in the ileum, the expression of immune factors is altered. The expression of ileal CXCL2 is increased and ileal IFN-γ is decreased after chronic smoke exposure. In the proximal colon, an increase in IL-6 and a decrease in TGF-β is denoted due to chronic smoke exposure. Interestingly, these findings highlight the dual effect of smoking on the gut: smoke exposure affects the immune system in the ileum, whereas it affects the microbiome in the colon. The local effect of smoking therefore strongly depends on the gut compartment. The second part of this thesis elaborates on the effect of smoking in the context of intestinal inflammation. We make use of two different Crohn-like inflammation mouse models: TNBS-induced colitis as a model for Crohn-like colitis and the TNFΔARE model for Crohn-like ileitis. In the TNBS colitis model, we show that smoke exposure further increased inflammation-induced cytokine/chemokine production, however does not exacerbate clinical disease parameters and histology. In the TNFΔARE model, cigarette smoke exposure induces no changes in cytokine/chemokine production, clinical disease parameters or histology. It seems that the effect of cigarette smoke is too subtle to further worsen full-blown intestinal inflammation, however, cigarette smoke does affect cytokine and chemokine production during ongoing inflammation. The third and final part of this thesis focuses on the effect of cigarette smoking on the production of cytokines and chemokines, especially IL-8, and the expression of the transient receptor potential channel TRPV1. A first finding is that cigarette smoking boosts IL-8 expression in healthy and inflamed gut of both human and mice. Using a human database of currently and never smoking Crohn’s disease patients, we demonstrate that IL-8 increases in currently smoking human subjects and this is often paralleled with an increase of TRPV1. TRP channels have been shown to modulate cigarette smoke-induced IL-8 expression in the airways. In mice experiments, we find a simultaneous increase of the mouse IL-8 homologue CXCL2 and TRPV1 in the ileum. In an IBD mouse model (TNBS-induced colitis), we also observe a parallel smoke-induced increase of CXCL2 and TRPV1 in the inflamed distal colon. The simultaneous increase of IL-8 and TRPV1 in the gut due to cigarette smoking suggests a link between both, however many more research is needed into this matter. In conclusion, this thesis provides important insights into the effect of cigarette smoke on the gut microbiome and immune system. In the colon, cigarette smoking mainly affects the microbiome, while in the ileum, the production of immune factors is induced. Furthermore, cigarette smoking increases cytokine and chemokine expression in healthy and inflamed gut of both human and mice, and this parallels smoke-induced expression of TRPV1