Investigation of the homeostasis regulation in the intestine1) Role of NO-sensitive guanylyl cyclase in the intestinal ecosystem 2) Characterization of PD-1 expression on intestinal innate lymphoid cells group 2 (ILC2)

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Mucosal sites encounter vast antigens including commensal bacteria, food environmental antigens and pathogens. To maintain homeostasis, unique mechanisms evolved to regulate mucosal immune response.The NO/cGMP signaling pathway plays a crucial role in the gastrointestinal (GI) tract as knockout mice (GCKO), deficient for its key enzyme nitric oxide (NO)-sensitive guanylyl cyclase (GC), suffer from a severe GI phenotype. They die a premature death due to ruptures and hemorrhages in colon and caecum, thus, a special fibre-reduced diet is crucial for survival.In the GI tract NO-GC expression has been shown in cells of the muscle layer and in unidentified cells of the lamina propria. Based on the GI phenotype, NO-GC expression in these cells potentially play a notable role in the maintenance of GI homeostasis.For verification, GCKO mice will be kept under germ-free (GF) and specific-pathogen-free (SPF) conditions. Analysis of bacterial composition and immune cell distribution of GCKO and control mice will identify the effect of NO-GC deficiency on the microbiota. Moreover, the mucus layer and the epithelial barrier will be evaluated to assess mucosal barrier integrity and thus identify early stages of inflammation.To bridge the time gap of GCKO establishment, function of PD-1/PD-L1 expression on intestinal innate lymphoid cells group 2 (ILC2) should be analyzed in an independent project. The negative regulator PD-1 and its ligand PD-L1 have been recently shown to be expressed on ILC2 of lung and small intestine. However, neither the expression of either both, receptor and ligand on a similar cell type, nor expression and function of PD-1/PD-L1 on colonic ILC2 has been determined yet. PD-1/PD-L1 expression will be analyzed by FACS and compared between GF and SPF mice, after antibiotics treatment and in disease models for intestinal inflammation and cancer.Finally, both projects will provide a better understanding of GI homeostasis regulation and may contribute to the improvement of therapies of highly prevalent diseases like GI inflammation and cancer.