Upregulation of indoleamine 2,3-dioxygenase in hepatocyte during acute hepatitis caused by hepatitis B virus-specific cytotoxic T lymphocytes in vivo

Background/Aims: Indoleamine-2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme inducing suppression of T-cell function and immune tolerance. In hepatitis B virus (HBV) transgenic (Tg) mice, the adoptive transfer of HBV-specific cytotoxic T lymphocytes (CTL) causes a necroinflammatory liver disease that is histologically similar to acute viral hepatitis in man. The present study aimed to determine IDO expression in the liver and hepatocytes during an acute hepatitis model. Methods: Serum L-kynurenine (L-Kyn) concentration in HBV Tg mice administered with HBV-specific CTL was measured over time, together with serum levels of alanine aminotransferase (ALT). Furthermore, we examined the expression of IDO in the total liver and isolated hepatocytes of HBV Tg mice after CTL injection using immunohistochemical analysis and reverse-transcription polymerase chain reaction (PCR). Results: In HBV Tg mice, HBV-specific CTL induced, over the course of several days, a chronic increase in serum L-Kyn levels, which was associated with a sustained enhancement of liver IDO activity. In particular, IDO expression was enhanced in the liver parenchymal cells (hepatocytes) after HBV-specific CTL injection both in immunohistochemical analysis and in reverse-transcription PCR. Moreover, murine recombinant interferon-γ (IFN-γ) directly increased the IDO expression in primary hepatocytes in vitro. Conclusions: Cytotoxic T lymphocytes transduction results in the upregulation of IDO, which might downregulate T-cell responsiveness. Our findings provide evidence that hepatocyte itself expresses IDO and increases levels of l-Kyn in the blood in acute lethal hepatitis of mice. These data indicate that HBV infection facilitates the induction of IDO in response to proinflammatory cytokines, particularly IFN-γ.