Differential target cell sensitivity to CTL-activated death pathways in hepatitis B virus transgenic mice

The current study was designed to explore the relative contribution of Fas ligand (FasL), perforin, IFN-gamma, and TNF-alpha-induced death pathways in the pathogenesis of CTL-induced liver disease. Hepatitis B virus-specific CTL that are genetically unable to produce either FasL, perforin, or IFN-gamma were injected into Fas-competent and Fas-deficient hepatitis B virus transgenic mice that are either sensitive or resistant to the cytopathic effects of IFN-gamma based on the extent to which their hepatocytes retain hepatitis B surface Ag (HBsAg). The results of these experiments indicate that FasL- and perforin-dependent signals are primarily responsible for the induction of liver disease in the absence of HBsAg retention, but both signaling pathways must be activated simultaneously by the CTL in order to kill the hepatocyte in vivo. In contrast, neither FasL nor perforin are required to kill hepatocytes that retain HBsAg as long as the CTL secrete IFN-gamma on antigen recognition. Finally the results indicate that, irrespective of their HBsAg content, hepatocytes are much less sensitive to destruction by TNF-alpha than by the other death pathways. While all of these death pathways appear to be operative during a normal CTL response, the current experiments suggest that the target cell determines which pathway is dominant and selects its mode of execution.