Project Summary/Abstract The gut microbiota influences many aspects of host physiology, spanning metabolism, gastrointestinal function, neuroactivity, and immune homeostasis. However, the molecular mechanisms that govern the symbiotic relationship between the host and gut microbiome remains unclear. Current evidence suggests that immunological tolerance in the intestine is essential for maintaining healthy host-microbe interactions by dampening immune responses to beneficial gut microbes and thus, promote commensalism. Interestingly, recent studies suggest that specific microbes that preferentially reside in the colon can promote immune tolerance by enhancing the development and function of regulatory T cells (Treg), a T helper (Th) subset important for tolerance. For example, polysaccharide A from Bacteroides fragilis can increase the suppressive capacity of Tregs, mediate the conversion of CD4+ Th cells to Tregs, and enhance microbe colonization onto host mucosa in the colon. Furthermore, Clostridial species (Clostridial spp), an abundant member of the proximal colon, produce short-chain fatty acids (SCFAs) that can also promote colonic Treg development and function. In the absence of SCFAs, Clostridial spp maintain some capacity to induce Treg development, suggesting an undescribed alternative mechanism. Interestingly, Clostridial spp are also potent inducers of host serotonin synthesis by colonic enteroendocrine cells. While commonly associated with neurotransmission in the brain regulating mood and behavior, serotonin in the periphery can also regulate diverse processes including gut motility and platelet activity. Furthermore, serotonin has immunomodulatory properties and can signal through the variety of serotonin receptors expressed by immune subsets. In the gut, serotonin receptor 7 (5-HT7) is highly expressed by innate immune cells, such as dendritic cells (DCs) and macrophages, that are important for Treg induction and maintenance. Since DCs are essential for T cell activation, we hypothesize that serotonin signaling through 5-HT7+ colonic DCs contribute to immunological tolerance that support the symbiotic relationship with gut microbes. To test this hypothesis, I will leverage novel transgenic mice and intersectional genetic approaches to determine i) the spatiotemporal regulation of 5-HT7 expression on colonic DCs, ii) the cellular consequences of 5-HT7-mediated signaling on colonic DCs, iii) the functional effects of DC-specific 5- HT7-mediated signaling on immune tolerance to the gut microbiota. Moreover, immunological tolerance is essential for dampening inflammatory responses during immune resolution to prevent chronic activation of proinflammatory pathways. In the gastrointestinal tract, chronic inflammatory disorders, such as irritable bowel disease (IBD), is associated with dysbiosis and dysregulated serotonergic signaling, suggesting a relationship between IBD, gut microbiota, and the enteric serotonergic system. Thus, resul...