Biliary atresia results from an inflammatory and fibrosing obstruction of extrahepatic bile ducts in young infants. Despite prompt diagnosis and surgical treatment, the disease progresses and leads to end-stage cirrhosis in most infants. The etiology and pathogenic mechanisms of disease are largely undefined. In the first period of this award, we searched for prominent molecular processes in livers of infants with biliary atresia and found a dominant interferon-gamma (IFN?)-rich proinflammatory circuit at the time of diagnosis. We then used a novel mouse model of experimental atresia to directly examine the role of IFN? in regulating the biliary atresia phenotype. This model displayed a similar proinflammatory process in the liver. Most notably, the in vivo loss of IFN?, completely prevented duct obstruction and improved long-term outcome. In this competing renewal application, we propose an overriding hypothesis that the pathogenic mechanisms of biliary atresia begin with an epithelial injury by the innate immune system and progress to duct obstruction by the activation of an exuberant adaptive immune response. This hypothesis will be tested in three closely related but independent aims. In Aim 1, we will determine the role of hepatic CD8+ T cells in neonatal injury of bile ducts. This will be done by investigating how CD8+ T cells engage cholangiocytes through recognition and binding to the MHC-I complex, and by dissecting the intracellular signals trigged by IFN? to render cholangiocytes susceptible to apoptosis induced by tumor necrosis factor-alpha. In Aim 2, we will establish the mechanisms by which hepatic NK cells injure the neonatal duct epithelium. Using a similar experimental approach, we will examine the mechanisms of NK cell-mediated cytotoxicity and how NK cells work in synergy with CD8+ T cells to induce epithelial injury and duct obstruction. And in Aim 3, we will define how hepatic macrophages trigger the innate immune response following a neonatal viral challenge that targets the bile ducts. In this aim, we will determine whether hepatic macrophages are targeted by rotavirus. In related experiments, we will also investigate the molecular mechanisms by which infected macrophages induce chemotaxis to neutrophils and, possibly, cytotoxicity to cholangiocytes. Upon completion, the proposed experiments will advance our understanding of the biological basis for experimental atresia and potentially identify new therapeutic targets to stop progression of disease and improve long-term outcome in children with biliary atresia. Project Narrative: This project studies biliary atresia, the most common cause of chronic liver disease in children and the leading indicator for pediatric liver transplantation in the United States. With the aim to determine key cellular and molecular mechanisms of disease pathogenesis, the proposed experiments will use a unique mouse model of neonatal biliary injury to dissect the biological processes regulating the injury and obstruction of extrahepatic bile ducts.