Chronic infections by hepatotropic viruses such as hepatitis B and C are generally associated with an impaired CD8 T-cell immune response that is unable to clear the virus. The liver is increasingly recognized as an alternative site in which primary activation of CD8 T cells takes place, a property that might explain its role in inducing tolerance. However, the molecular mechanism by which intrahepatically activated T cells are tolerance activated is unknown. Here, we investigated the phenotype and fate of naïve CD8 T cells activated by hepatocytes in vivo.

Transgenic mouse models in which the antigen is expressed in lymph nodes and/or in the liver were adoptively transferred with naïve CD8⁺ T cells specific for the hepatic antigen.

Liver-activated CD8 T cells displayed poor effector functions and a unique CD25^low CD54^low phenotype. This phenotype was associated with increased expression of the proapoptotic protein Bim and caspase-3, demonstrating that these cells are programmed to die following intrahepatic activation. Importantly, we show that T cells deficient for Bim survived following intrahepatic activation.

This study identifies Bim for the first time as a critical initiator of T-cell death in the liver. Thus, strategies inhibiting the up-regulation of this molecule could potentially be used to rescue CD8 T cells, clear the virus, and reverse the outcome of viral chronic infections affecting the liver.