The immunology of hepatitis C virus (HCV) infection should be studied in the context of HCV antigen expression in the liver, because HCV primarily infects this organ. Indeed, the nature, function, and fate of T cells primed after antigen expression in the liver might differ from those primed when antigens are expressed systemically or in other organs, because the nature of the antigen-presenting cells (APCs) involved may be different. In addition, the normal liver contains a resident population of lymphocytes that differ from those present at other sites. Thus, we investigated whether HCV-specific CD8 ؉ cytotoxic T cells (CTLs) could be elicited following portal vein (PV) injection of plasmid DNA in mice whose hepatic veins were transiently occluded. We show that PV injection of mice with ''naked'' DNA expressing the HCV-NS5a protein, under the control of a liver-specific enhancer/promoter, resulted in NS5a expression in the liver and the priming of HCVspecific CTLs. These results suggested that such a model might be relevant to the study of HCV-specific immune responses primed during natural infection. (HEPATOLOGY 2000;31:1327-1333.)Hepatitis C virus (HCV), a positive-strand RNA virus member of the Flaviviridae family, 1 is recognized as a major public health concern. In addition to being one of the leading causes of chronic liver disease, 2 chronic HCV infection has been associated with autoimmune syndromes, immune complex disorders, and mixed cryoglobulinemia. 3,4 One of the most striking features of HCV is that it seems able, in most instances, to circumvent eradication by the immune system. It is estimated that up to 75% of patients infected with HCV become chronically infected, 2,5-7 despite the fact that most patients generate HCV-specific antibody, [8][9][10][11][12] as well as CD4 ϩ and CD8 ϩ T-cell responses. [13][14][15][16] On the other hand, there is evidence that humoral and T-cell-mediated immune responses to HCV infection can, at least in some instances, determine the outcome of HCV infection and disease. [17][18][19][20] In spite of recent progress in the management of chronic HCV disease, the current therapies for chronic HCV infection often do not result in viral clearance. 21,22 Thus, there is a growing interest in developing immune-based therapies for HCV. To develop such therapies, one must understand the immune responses to HCV infection, because they should provide insight into the mechanisms involved in limiting HCV replication in infected hosts. Despite the fact that substantial efforts have been made to decipher the immunology of HCV infection, it remains poorly understood. The main factors responsible for the relatively slow progress in this field are the unavailability of a small animal model for infection and disease, and inefficient systems for cultivating the virus. Because acute hepatitis in humans has been difficult to study, most attempts aimed at clarifying the immune responses to HCV have been conducted in patients with chronic hepatitis or in chimpanzees, the only reliable a...