Nutritional impact on IL-15 induced dysbiosis

Inflammatory bowel disease (IBD), as cancer, is a multigenic disorder that requires multiple hits. The very high rate at which the prevalence of IBD is increasing implies that environmental factors such as diet are at play. Dysregulation in the composition and function of the intestinal microbiota (dysbiosis) is viewed as being a relatively early event that alters intestinal immune homeostasis and sets the stage for the development of IBD in genetically-predisposed individuals. However the mechanisms leading to dysbiosis are still in the process of being defined and likely involve multiple host and environmental factors. Here we will dissect whether and how IL-15, a cytokine that induces differentiation of proinflammatory dendritic cells (DCs) and is upregulated in affected and unaffected areas of IBD patients impacts the composition and function of the microbiota, and how this impact can be influenced by diet. The Jabri lab has recently shown that high milk fat (MF), but not high polyunsaturated fat (PUFA), diet induces the production of sulfur-containing bile acids that then leads to a bloom of the Deltaproteobacteria Bilophila wadsworthia (Bw) which in turn promotes colitis in genetically susceptible animals. However, we now have observed in D d -IL-15 transgenic mice, in which increased levels of IL-15 are expressed in the lamina propria similarly to celiac disease and IBD patients, that a PUFA diet can actually promote a bloom of Proteobacteria and a corresponding decrease in protective butyrate producing bacteria. This observation suggests that a diet that is non-proinflammatory in wild-type mice promotes an expansion of pathobionts and a decrease of symbionts in the presence of IL-15. In addition, presence of IL-15 further aggravates MF-induced dysbiosis. In the first specific aim of this proposal we will study the multi-hit induction of dysbiosis, in particular we will determine the effect of the interplay of IL-15 and high-fat diets on the microbiota. In the second specific aim, we will determine the pathogenic potential of dietary/IL-15 altered microbiota in models of colitis (IL-10-/- and CD45RBhigh transfer mouse models) and the ability of therapeutic interventions to restore eubiosis (healthy microbiota) and prevent colitis. This proposal will test the hypothesis that dysregulated expression of IL-15 induces dysbiosis that can be aggravated by high fat diet. We will identify mechanisms by which IL-15 promotes dysbiosis and determine the pathologic potential of IL-15/diet-altered microbiota. In particular we will study the impact on intestinal immune homeostasis and colitis. The knowledge gained from these studies will provide fundamental insights into how the interactions between the triad host, microbiota and diet can promote IBD and help to define therapeutic strategies to ameliorate or even prevent IBD through manipulation of the enteric microbiota.

Complex disorders, such as inflammatory bowel disease (IBD) and cancer, result from the interaction between environmental factors, the microbiota, and the state of the host. Numerous studies suggest that an imbalance between harmful and protective intestinal bacteria (dysbiosis) is largely responsible for the rising incidence of IBD, but what causes dysbiosis remains poorly understood. An important immune regulatory role played by the microbiota is linked to its ability to produce short chain fatty acids, in particular butyrate, derived from dietary fibers. A reduction in butyrate concentration in the intestine has been associated with higher severity of IBD. Furthermore, treatment of IBD patients with butyrate reduces signs of inflammation. The proinflammatory cytokine IL-15 is upregulated in a number of autoimmune disorders such as psoriasis, rheumatoid arthritis and celiac disease. It is also overexpressed in IBD, both in the small and large intestine, in affected and unaffected areas, suggesting that it is an early and predisposing event. Furthermore, the natural killer receptor NKG2D expressed on intraepithelial lymphocytes (IELs), which is critically regulated by IL-15 was shown to play a role in IBD pathogenesis. In the course of my FWF funded Erwin Schrödinger Fellowship, at the University of Chicago, I have been investigating if changes in the host, associated with IL-15 upregulation in the intestinal epithelium (EP), promote dysbiosis and whether the IL-15-induced dysbiosis leads to disruption of immune homeostasis and increases the risk of IL-15-medaited immunopathology. We used villin-IL-15 transgenic mice (v-IL-15tg) as an experimental model organism, which similar to patients with IBD, overexpress IL-15 in the intestinal EP. We identified that the overexpression of IL-15 in the intestinal EP leads to both the significantly reduced production of butyrate and the reduction of butyrate producing bacterial phyla, especially Clostridium cluster IV and XIVa. We have determined that the IL-15-mediated expansion of cytotoxic NKG2D+ IELs in the EP is suppressed by butyrate treatment. By performing fecal transplantations, we identified that in contrast to the microbiota of v-IL-15tg mice, the fecal microbiota of healthy mice is able to suppress the IL-15-mediated expansion of cytotoxic NKG2D+ IELs. These results strongly suggest that IL-15-induced dysbiosis may play a role in the pathogenesis of intestinal inflammatory disorders. Results of this study will allow us to gain a better understanding of the mechanisms underpinning the development of dysbiosis and define its impact on intestinal immune homeostasis. Furthermore, we will have a fundamental basis for developing successful strategies that can be used in the identification, prevention and treatment of patients with IBD.