The bacterial microflora has always been regarded as beneficial for the host but recent studies have shown that this symbiosis has risks as well as benefits. Although active mechanisms allow tolerating the commensal flora, the physiological stress that is associated with the symbionts’ metabolism can exhaust the intestinal barrier resulting in serious effects on the health of the host. Protracted immune deregulations can lead to severe disorders including diabetes, cancer and inflammatory bowel disease (IBD). Several mechanisms and players are involved in the maintenance of intestinal immune homeostasis, including T regulatory cells and Immunoglobulin (Ig)-A. In this proposal we focus our attention on dendritic cells (DCs) for their ability to induce both tolerance and immunity by regulating B and T cell responses. We have recently shown that DC function is controlled by intestinal epithelial cell (EC) derived factors and in particular by Thymic stromal lymphopoietin (TSLP). EC-conditioned DCs acquire a ‘mucosal’ phenotype as they are prone to activate T regulatory cells and IgA responses. Three major issues related to the maintenance and disruption of intestinal immune homeostasis will be explored in this project: 1) What are the mediators and mechanisms that regulate the interaction between intestinal epithelial cells and dendritic cells? What is the function of TSLP? 2) Which are the sites and players for the activation of an IgA response to pathogenic and commensal bacteria? Can we visualize them in vivo? 3) Can prolonged infections or bacterial products promote intestinal tumour development? Are there different bacterial constituents acting as inducers or protectors of carcinogenesis? What is the role of Toll-like receptors?