Intrahepatic accumulation of CD8⁺ T cells following antigen-specific activation has been demonstrated in a number of transgenic models and also in extrahepatic viral infections. In some transgenic models, intrahepatic accumulation of cytotoxic T lymphocytes is associated with hepatitis. This observation suggests that hepatocellular damage may occur in some forms of immune-mediated hepatitis on the basis of a “bystander injury,” whereby cytotoxic T lymphocytes accumulating in the liver mediate injury to hepatocytes in a nonspecific manner. Mouse transgenic models were therefore developed to investigate whether bystander damage to non–antigen-bearing hepatocytes occurs in vivo.

T cell receptor transgenic T cells were adoptively transferred into transgenic mice ubiquitously expressing the specific antigen, or into bone marrow radiation chimeras in which hepatocytes did not express the antigen.

Selective accumulation of transgenic CD8⁺ T cells in the liver of intact recipients could be detected within 2 hours of transfer, despite ubiquitous antigenic expression. T cells retained in the liver were activated and induced hepatitis. Similar results were obtained using bone marrow chimeras, suggesting that antigen expression by hepatocytes was not required either for intrahepatic accumulation or for subsequent hepatitis. This “bystander hepatitis” was dependent on tumor necrosis factor α and interferon γ.

Intrahepatic accumulation of activated CD8⁺ T cells and subsequent hepatitis can result from primary activation of CD8⁺ T cells by liver resident bone marrow–derived cells, inducing bystander damage to non–antigen-bearing hepatocytes. This mechanism may play a role in some forms of biologically significant hepatitis, including autoimmune hepatitis and hepatitis associated with extrahepatic diseases.