As humans, we are continuously exposed to pathogens. Our innate immune system must be able to differentiate pathogenic from nonpathogenic organisms, and it must be able to tailor an immune response to respond to that pathogenic organism. This problem is particularly acute at mucosal surfaces, an area of the body in which the surface cells are in direct contact with bacteria, fungi and viruses. A number of inflammatory disorders, including Crohn's Disease, are initiated at these mucosal surfaces when the initial innate immune response is not adequately down-regulated after the pathogen is eradicated. In this grant, we study the mechanisms that control this down-regulation at mucosal surfaces. We have found that a key anti-inflammatory protein, A20, is phosphorylated and activated by the central kinase in the NF-?B signaling pathway (IKK2). We mapped the site of phosphorylation and have shown that it is required for full A20 inhibitory activity. We generated a phospho-specific antibody against this site, and we have shown that this phosphorylation occurs in vivo in response to a number of inflammatory stimuli. Our central hypothesis is that the IKK-dependent phosphorylation of A20 leads to a novel feedback mechanism to inhibit the NF-?B response such that too much inflammation does not occur at mucosal surfaces. Failure of IKK to phosphorylate A20 may lead to inflammatory pathology such as that seen in Crohn's Disease. This grant is designed to test this hypothesis. Mucosal immunity regulates the initial immune response to a variety of viral, bacterial and fungal pathogens. Dysregulation of mucosal immunity is an initiating event in a variety of inflammatory disorders including Inflammatory Bowel Disease, Asthma, Pyelonephritis and a number of primary immunodeficiencies. Understanding how this dysregulation occurs will have relevance both for understanding the pathophysiology of chronic inflammatory diseases and for preventing this dysregulation from occurring after exposure to pathogens. PUBLIC HEALTH RELEVANCE Mucosal immunity regulates the initial immune response to a variety of viral, bacterial and fungal pathogens. Dysregulation of mucosal immunity is an initiating event in a variety of inflammatory disorders including Inflammatory Bowel Disease, Asthma, Pyelonephritis, and a number of primary immunodeficiencies. Understanding how this dysregulation occurs will have relevance both for understanding the pathophysiology of chronic inflammatory diseases and for preventing this dysregulation from occurring after exposure to pathogens.