Mechanisms of the pathogenesis of cell injury and viral persistence in the woodchuck model of hepatitis B

Hepatitis B virus (HBV) persistently infects approximately 400 million people worldwide. It has several natural histories including silent infection, acute hepatitis and a serologically detectable chronic carrier state, which commonly leads to liver cirrhosis and hepatocellular carcinoma. Woodchucks infected with woodchuck hepatitis virus (WHV) have remarkably similar spectra of progression of infection and liver disease. This model is, therefore, invaluable for in vivo studies that are difficult or impossible to conduct in humans. The studies that comprise this thesis were aimed at determining some of the viral and host factors that may influence the outcome of experimental hepadnavirus infection of adult woodchucks. However, due to the relatively poor characterization of the woodchuck model, we first established a number of gene sequences relevant to the woodchuck immune system. This led to a gene discovery program that identified 14 novel woodchuck genes. Our first set of experiments studied Fas and perforin-mediated lymphocyte cytotoxicity to see if differences in the types of immune cell activation could predict the outcome of viral hepatitis. Our results demonstrate that lymphoid cells from woodchucks with acute WHV infection have an augmented capacity to elicit perforin-dependent cell killing when compared to woodchucks with chronic hepatitis (CH). This suggests that nonspecific cellular immunity, presumably NK cells, may play a role in early recovery from WHV infection. A second set of experiments investigated the hepatic and splenic expression of MHC class I in acute and chronic WHV infections. We have found that CH is accompanied by severely diminished hepatocyte and lymphoid cell MHC class I surface expression despite upregulated transcription of affiliated genes. This provides evidence that the virus posttranscriptionally disrupts MHC class I display and, this may protect infected cells from T cell-mediated immune clearance. This would contribute to viral persistence and possibly deregulate the MHC class I-dependent functions of the host's immune system. Our final set of experiments studied intrahepatic cytokine expression and T cell influx in the course of, or after, experimental WHV infection. This work demonstrated a positive correlation between recovery from adult WHV hepatitis and upregulated interferon γ (IFNγ). tumor necrosis factor α (TNFα) and CD3 gene expression. In addition, the same markers of immune activation were found to endure for years after resolution of acute infection. This suggests that antiviral cytokines, such as IFNγ and TNFα, may play a central role in recovery from acute hepatitis, as well as in the long term control of occult hepadnavirus persistence. The processes identified in the present studies could be critical for perpetuation of liver damage and evasion of anti-viral immunological surveillance in chronic hepadnavirus infection. The data obtained and the investigative tools generated should enable a better understanding of the immunopathogenesis of HBV infection and aid in the development of more effective anti-viral agents