Project Summary Colorectal cancer (CRC) represents the third most diagnosed malignancy in the world (1). More than two-fold of all cases and deaths are associated with modifiable environmental risk factors (2). In particular, CRC incidence has been increasing at younger ages (<50 years) (3). Therefore, novel target-based approaches that link both genetic and environmental factors need to be established for the treatment of this neoplasm (4, 5). Intestinal epithelial homeostasis is maintained by a complex interplay between epithelial cells, gut microorganisms, and immune cells (6, 7). Disruptions of these orchestrated interactions results in inflammatory disorders such as colitis and CRC. One genetic factor that modulates the interactions between gut epithelium, microbiota and immune cells is sirtuin 1 (SIRT1), the most conserved mammalian member of a family of highly conserved NAD+- dependent histone deacetylases and/or ADP-ribosyltransferases called sirtuins (8). We and others have shown that intestinal epithelial SIRT1 regulates intestinal stress response and tissue homeostasis, deficiency of this factor leads to intestinal inflammation, disruption of gut microbial composition, and altered susceptibility to environmentally CRC (9-13). Particularly, we found that deletion of intestinal epithelial SIRT1 results in hyperactivation of Paneth cells (11), intestine-originated innate immune cells important for gut microbiota regulation and intestinal stem cell niche maintenance (14). Recently we generated a Paneth-cell specific SIRT1 KO mouse model (SIRT1 PKO) by breeding SIRT1 floxed allele with Defa6-Cre line (15), and found elevated expression of Paneth cell markers, increased Paneth cell number, and enhanced protection against chemical induced inflammation. This study will investigate the role of SIRT1 in Paneth cells and how deficiency of SIRT1 in Paneth cells impacts environmental influences on intestinal epithelial homeostasis. Since gut microbiota plays a fundamental role on the host immune system (16), as well as the efficacy of anti-tumor immunotherapies (17-20), it will also seek to parse out SIRT1 regulation of gut microbiome and how this interaction can alter intestinal and systemic immune function, which can impact anti-tumor immunity and the efficacy of anti-tumor immunotherapies. Thus, understanding the role of SIRT1 in intestinal epithelial in the context of Paneth cells will shed light on to how disruptions on Paneth cells can result in changes of gut microbiome, altered immune functions, and disrupts epithelial homeostasis. The aims of the study are: Aim 1: To determine if SIRT1 deletion in Paneth cells disrupts gut microbiota and alters gut immunity. Aim 2: To explore if SIRT1 deletion in Paneth Cells alters anti-tumor immunity. Aim 3: To investigate if the differential anti-tumor immunity in Flox and PKO mice affects the efficacy of anti- tumor immunotherapies.