Determining the role of CSF1R-dependent macrophages in of Paneth cells and the intestinal stem cell niche

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The epithelium which lines the intestine is self-renewing and is continually replaced every 5-7 d. In order to do this, intestinal stem cells are situated at the base of each finger-like villous. These stem cells produce highly proliferative daughter cells which can differentiate into distinct intestinal epithelial cell populations, eg: goblet cells, tuft cells and Paneth cells. The differentiated cells travel along the villus epithelium and perform their physiological functions before being shed at the tip. A particular cell type known as the Paneth cell, remains at the base of these crypts nestled between the intestinal stem cells. A primary function of the Paneth cells is release antimicrobial products such as lysozymes and defensins which help to protect the crypt from bacterial infection. Paneth cells also produce molecules which are essential to maintain the intestinal stem cells. When Paneth cells are lost, this also leads to a loss of the intestinal stem cells, ultimately affecting the gut epithelium. Macrophages are abundantly positioned throughout the gut wall, These phagocytic cells help to protect the intestine against infection, but also help support the gut epithelium. We have shown that when these macrophages are lost, this leads to the subsequent loss of Paneth cells and intestinal stem cells. Our studies reveal a previously-unrecognised, essential role for macrophages in the constitutive maintenance of Paneth cells and intestinal stem cells. The disturbances to the Paneth cells and intestinal stem cells caused by macrophage-depletion also adversely affected the subsequent formation and function of other types of epithelial cells in the gut epithelium. The differentiation and function of specialised antigen sampling M cells was impaired in the Peyer's patches of the small intestine. In contrast, the differentiation and abundance of the mucous-secreting goblet cells was enhanced. This suggests that modification of the phenotype or abundance of macrophages in the gut wall, for example after CSF1R-blockade, pathogen infection or inflammation, could dramatically affect the development of the intestinal epithelium and the ability to sample gut antigens. Paneth cell-dysfunction is evident in Crohn's disease patients with small intestinal involvement, and may be a consequence of inadequate provision of support from monocytes. In this project we will study the role of macrophages in the maintenance of Paneth cells and intestinal stem cells. We will also determine whether the effects of macrophage-depletion on M cells affect the ability of the mucosal immune system to sample foreign particles in the intestine and mount a specific immune response to them. We also aim to determine whether a specific class of molecules known as Wnts are the important factors which the macrophages produce to support the Paneth cells and intestinal stem cells. Finally, we will determine whether the adverse effects of macrophage-depletion on Paneth cells and intestinal stem cells can be restored by treatment with a molecule known as R-spondin1. Pharmacological CSF1R-blockade has been proposed as a means to modulate certain cancers, and inflammatory, autoimmune and bone diseases. However, CSF1R-blockade also depletes macrophages. Therefore, CSF1R-blockade could indirectly compromise Paneth cells and intestinal stem cells, cell differentiation in the gut epithelium and the ability of the mucosal immune system to sample Ag and pathogens from the gut lumen. A thorough analysis of the role of macrophages in the maintenance of Paneth cells and intestinal stem cells is essential to help identify the macrophage factors which help support these cells. The information from this project will help develop new therapeutics to treat certain intestinal diseases associated with disturbances to Paneth cells and intestinal crypts.