The intestinal epithelium mediates the absorption of life-sustaining nutrients while protecting against the large and diverse population of bacteria that colonize this surface. While, classically, the gut epithelium was thought to manage these tasks primarily by functioning as a passive, albeit highly selective, barrier, research over the last dozen years has demonstrated that, in fact, the gut epithelium plays a very active role in protecting the host from both the commensal and potentially pathogenic bacteria that can colonize this surface. Consequently, understanding the interactions between bacteria, epithelial cells, and immune cells is important to understanding bacterial pathogenesis in the gut, systemic infection by food-borne pathogens, and chronic inflammatory diseases of the intestine in which there is a seemingly inappropriate immune response directed at the commensal enteric microflora. Our in vitro modeling of bacterial epithelial-interactions performed under this proposal revealed that ligation of toll-like receptor 5 (TLR5) by bacterial flagellin, the primary structural component of flagella, is a dominant means of activating innate immune signaling in intestinal epithelial cells. Flagellin is released by a number of pathogens and commensal microbes but TLR5 expression is polarized to the basolateral surface of the epithelium thus minimizing activation of pro-inflammatory signaling by flagellin released from commensal bacteria. Conversely, flagellated pathogens that can breech the epithelium or alter vesicular trafficking in epithelial cells activates TLR5-signaling in model epithelium. Such TLR5 signaling utilizes the NF-?B, MAPK and JaK-STAT signal transduction pathways result in a transient burst of gene expression seemingly designed to directly repel bacteria, recruit/activate immune cells, and prevent apoptosis. Such induction of gene expression can be envisaged to protect the host from pathogens but may contribute to the inflammatory sequellae associated with many enteric infections. Furthermore, that some pathogens such as Salmonella typhimurium have a specific mechanism that mediates release of flagellin monomers upon sensing of epithelial cells suggest that some pathogens might 'deliberately' trigger TLR5 activation to aid in their dissemination. The in vivo studies performed to date are consistent with both of these possibilities. However, such studies have been far too limited in scope to provide a reasonable sense of the importance of the flagellin-TLR5 interaction in vivo let alone elucidate the mechanisms by which flagellin-induced signaling might affect the outcomes of host-bacteria interactions in health and disease. Thus, to fill this gap of knowledge, we outline a series of experiments to determine the mechanisms by which innate immune responses to flagellin affect the outcomes of host-bacterial interactions in vivo.
Salmonella remain amongst the most prolific and ubiquitous microbial causes of disease in both developed and developing countries. Human disease caused by Salmonella can be divided into 2 primary categories: 1) gut-restricted self-limiting gastroenteritis and Typhoid fever. Our research indicates that the interaction of the Salmonella protein flagellin with a host protein toll-like receptor 5 plays a key role in both disease processes. Thus, we propose to understand the role of the interaction in mouse models of disease.
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