ABSTRACT The incidence of immune mediated diseases, including inflammatory bowel disease, has grown exponentially over the last decades. This has been linked to a variety of factors, including dietary preservatives and processed food. Interest has recently turned to salt (NaCl) is a potential culprit. Abundant data show that high salt diets can activate intestinal mucosal inflammation and increase severity up to a variety of experimental disorders, including inflammatory bowel disease and multiple sclerosis, and experimental animals. Further, some epidemiologic studies have correlated salt intake with frequency of immune-media disease. We recently discovered that genetic or pharmacological inhibition of claudin-2, which forms a paracellular Na+ channel within the intestinal epithelium, is an effective intervention in experimental, immune-mediated inflammatory bowel disease. Our additional studies indicate that claudin-2 overexpression in transgenic mice promotes Th17 polarization of mucosal immune cells while claudin-2 deletion promotes Tregs development. A high salt diet also activates mucosal IL-17 production in claudin-2 transgenic, but not knockout, mice. Thus, expression of the paracellular Na+ channel protein claudin-2 synergizes with dietary salt to promote mucosal inflammation. This is relevant to wide variety of inflammatory intestinal disorders, especially inflammatory bowel disease, because claudin-2 expression is markedly upregulated in these settings. Thus, this is the potential of creating a vicious cycle where claudin-2 and dietary salt promote inflammation and, in turn, the inflammation signals back to the epithelium and causes further upregulation of claudin-2. High salt diets are also well-recognized to affect the microbiota. We found that the effect of high salt diet on the microbiome was modified by claudin-2 overexpression or deletion. This suggests that the immune changes induced by claudin-2 expression or deletion also modify the microbiome or, alternatively, microbial changes feedback to alter the immune system. Examples of both mechanisms have been reported in other settings. Finally, is possible that claudin-2 and dietary salt affect immune system and microbiota separately. Regardless of which affect his primary, these data provide strong support for the conclusion that claudin-2 inhibitors should be developed as potential therapeutic agents. Unfortunately, structural data and general understanding of how claudin-2 paracellular channels are created and regulated are limited. Thus, the second aim of this proposal seeks to define structure-function relationships in order to understand the molecular mechanisms that determine claudin-2 biology and to identify potential molecular interfaces to be targeted therapeutically.