Innate and acquired immune responses in the intestine must promote homeostasis in the presence of large numbers of commensal microorganisms, while maintaining the capacity to defend the body against invasive pathogens. The adult human intestinal tract is inhabited by 100 trillion microorganisms, 10 times more than the number of human cells in the entire body. Commensal bacteria provide multiple benefits to the host, including processing of essential nutrients, regulation of energy balance, protection against pathogens and maintenance of epithelial integrity. However, inappropriate immune responses to colonic bacteria can lead to chronic inflammatory bowel disease and life-threatening colitis. Host cells recognize microorganisms through pattern recognition molecules including Toll-like receptors (TLRs), which bind microbial cell wall constituents, nucleic acids and other byproducts. MyD88 is a cytoplasmic adaptor protein that transduces signals emanating from most TLRs, as well as members of the IL-1R family. Recent studies have demonstrated that mice genetically deficient in MyD88 expression are more sensitive to experimental colitis than are wild-type mice, suggesting a key role for TLR and/or IL-1 R signaling in regulation of intestinal inflammation. Our preliminary data demonstrate that MyD88-deficient mice have severely depressed expression of the polymeric immunoglobulin receptor (plgR), a key anti-inflammatory molecule that mediates epithelial transport of protective IgA antibodies. Recent work by others has demonstrated that plgR-deficient mice are, like MyD88-deficient mice, particularly sensitive to chemically-induced colitis. The goal of the proposed research is to test the hypothesis that expression of MyD88 by epithelial cells is crucial for regulation of plgR gene expression and protection against experimental colitis. As a consequence of these studies we will generate novel chimeric and transgenic mouse models for studying epithelial-specific MyD88 signaling within the intact intestine. First, we will analyze the susceptibility of these mice to experimental colitis. Second, we will analyze patterns of pro- and anti-inflammatory gene expression in the colon. These experiments should increase our knowledge of epithelial-specific MyD88 signaling, as well as cross-talk between epithelial and immune cells, which is key to the identification of normal and dysregulated responses to colonic bacteria.