TNF-α has been clearly identified as central mediator of T cell activation-induced acute hepatic injury in mice, e.g., Con A hepatitis. In this model, liver injury depends on both TNFRs, i.e., the 55-kDa TNFR1 as well as the 75-kDa TNFR2. We show in this report that the hepatic TNFRs are not transcriptionally regulated, but are regulated by receptor shedding. TNF directly mediates hepatocellular death by activation of TNFR1 but also induces the expression of inflammatory proteins, such as cytokines and adhesion molecules. Here we provide evidence that resistance of TNFR1−/− and TNFR2−/− mice against Con A hepatitis is not due to an impaired production of the central mediators TNF and IFN-γ. Con A injection results in a massive induction of ICAM-1, VCAM-1, and E-selectin in the liver. Lack of either one of both TNFRs did not change adhesion molecule expression in the livers of Con A-treated mice, presumably reflecting the fact that other endothelial cell-activating cytokines up-regulated adhesion molecule expression. However, treatment of TNFR1−/− and TNFR2−/− mice with murine rTNF revealed a predominant role for TNFR1 for the induction of hepatic adhesion molecule expression. Pretreatment with blocking Abs against E- and P-selectin or of ICAM−/− mice with anti-VCAM-1 Abs failed to prevent Con A hepatitis, although accumulation of the critical cell population, i.e., CD4+ T cells was significantly inhibited. Hence, up-regulation of adhesion molecules during acute hepatitis unlikely contributes to organ injury but rather represents a defense mechanism.

Con A administration results in dose-dependent immune-mediated liver injury. Cytokines are important to determine the outcome of liver failure in this model, and especially TNF-α and IFN-γ directly contribute to hepatocyte damage. The intracellular pathways of these two cytokines, which eventually result in tissue destruction, are not well defined. Here we used anti-IFN-γ Abs and adenoviral vectors that express molecules inhibiting distinct TNF-α-dependent pathways in hepatocytes to better understand the relevance of specific intracellular signaling cascades for Con A-induced liver failure. We show that activation of TNF-α- and IFN-γ-dependent intracellular pathways occurs prior to the influx of immune-activated cells into the liver and that anti-TNF-α and anti-IFN-γ neutralizing Abs cannot block infiltration of these cells. Blocking experiments with Abs and adenoviral vectors showed that NF-κB activation and the Fas-associated death domain protein/caspase 8 cascade in hepatocytes during Con A-induced liver failure have no impact on tissue injury. Additionally, STAT1 activation alone after Con A injection in liver cells does not result in liver damage. In contrast, IFN-γ-dependent expression of IFN regulatory factor-1 and TNF-α-dependent activation of c-Jun N-terminal kinase in liver cells correlates with liver cell damage after Con A injection. Therefore, our experiments indicate that IFN regulatory factor-1 and the c-Jun N-terminal kinase pathway are involved in determining hepatocyte damage during Con A-induced liver failure and thus may provide new targets for therapeutic intervention.

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