The etiopathogenesis of autoimmune hepatitis (AIH) remains poorly understood. In this study, we sought to develop an animal model of human AIH to gain insight into the immunological mechanisms driving this condition. C57BL/6 mice were iv injected with adeno-associated viral vectors encoding murine IL-12 or luciferase under the control of a liver-specific promoter. Organ histology, response to immunosuppressive therapy, and biochemical and immunological parameters, including Ag-specific humoral and cellular response, were analyzed. Mechanistic studies were carried out using genetically modified mice and depletion of lymphocyte subpopulations. Adeno-associated virus IL-12–treated mice developed histological, biochemical, and immunological changes resembling type 1 AIH, including marked and persistent liver mononuclear cell infiltration, hepatic fibrosis, hypergammaglobulinemia, anti-nuclear and anti–smooth muscle actin Abs, and disease remission with immunosuppressive drugs. Interestingly, transgenic IL-12 was short-lived, but endogenous IL-12 expression was induced, and both IL-12 and IFN-γ remained elevated during the entire study period. IFN-γ was identified as an essential mediator of liver damage, and CD4 and CD8 T cells but not NK, NKT, or B cells were essential executors of hepatic injury. Furthermore, both MHC class I and MHC class II expression was upregulated at the hepatocellular membrane, and induction of autoreactive liver-specific T cells was detected. Remarkably, although immunoregulatory mechanisms were activated, they only partially mitigated liver damage. Thus, low and transient expression of transgenic IL-12 in hepatocytes causes loss of tolerance to hepatocellular Ags, leading to chronic hepatitis resembling human AIH type 1. This model provides a practical tool to explore AIH pathogenesis and novel therapies.