The long term goal of these studies is to define the innate host-virus interactions that occur during hepatitis C virus (HCV) infection with the hope that this will lead to novel therapeutic approaches to this disease. HCV is a noncytopathic, positive-strand RNA virus that causes acute and chronic hepatitis and hepatocellular carcinoma. Approximately 2-4 million persons are chronically infected by HCV in the USA and 170 million people are chronically infected worldwide, many of whom will die from liver failure and hepatocelluar carcinoma. The recent development of a robust cell culture model of HCV infection permits analysis of the entire HCV life cycle and facilitates analysis of the host-virus interactions that determine the outcome of HCV infection. Using that model we have recently shown that that the viral NS3/4A protease blocks double stranded (ds) RNA signaling and type 1 interferon (IFN) induction by cleaving a key intermediate in the dsRNA signaling pathway, and we discovered an entirely novel NS3/4A-independent mechanism whereby HCV evades the innate host response in infected cells. We also discovered that IFN rapidly modulates the expression of numerous cellular microRNAs (miRs) several of which have sequence-predicted targets within the HCV genome, that synthetic miR-mimics corresponding to several of these IFN-induced miRs reproduces the antiviral effects of IFN on HCV infection, and that neutralization of these miRs significantly reduces the antiviral effects of IFN against HCV. Finally, we showed that IFN inhibits the expression of the liver-specific miR-122 which is known to be essential for HCV replication. These findings identify a previously unsuspected effector arm of the innate host response that contributes to the control of HCV infection. In Specific Aim 1, we will use molecular and biochemical approaches to define the viral protein(s) that mediate NS3/4A-independent suppression of dsRNA-induced IFN expression (subaim 1a), identify the cellular dsRNA signaling molecule that is targeted by that (those) protein(s) (subaim 1b), and determine the molecular basis of this novel host evasion mechanism (subaim 1c). These results will enrich our understanding of the basis for viral persistence in HCV infection and, potentially, identify a previously unappreciated target(s) in HCV for the development of antiviral drugs. In Specific Aim 2, we will use microarray technology to define the full complement of cellular microRNAs that are regulated by type 1 and type 2 interferons in human liver cells (subaim 2a); determine whether any of these IFN-regulated miRs inhibit HCV infection (subaim 2b) and mediate IFN[unreadable]s antiviral effects (subaim 2c); and identify the viral and cellular targets of these miRs (subaim 2d), and the step(s) in the virus life cycle they interrupt when they inhibit HCV infection (subaim 2e). Collectively, these experiments will provide insight into an entirely new innate defense mechanism that protects the host against HCV infection and, potentially, lead to the development of novel antiviral strategies that exploit this mechanism to terminate chronic HCV infection.