Intrahepatic Cross-Presentation and Hepatocellular Antigen Presentation Play Distinct Roles in the Induction of Hepatitis B Virus-Specific CD8 + T Cell Responses.

CD8( )(+)T cells are the key cellular effectors mediating the clearance of hepatitis B virus (HBV) infections. However, early immunological events surrounding the priming of HBV-specific CD8(+) T cell responses remain poorly understood. This study examined the importance of priming location and the relative contribution of endogenous antigen presentation by hepatocytes versus cross presentation by bone marrow-derived cells to the induction of functional HBV specific CD8(+) T cell responses using the animal models of acute and chronic HBV infection. Functional HBV-specific CD8(+) T cell responses could be induced to intrahepatically expressed HBV even when T cell homing to the lymphoid tissues was severely suppressed, suggesting that functional priming could occur in the liver. The expansion of HBV-specific CD8(+) T cells was significantly reduced in the mice whose major histocompatibility complex (MHC) class I expression was mostly restricted to nonhematopoietic cells, suggesting the importance of cross-presentation by hematopoietic cells in the induction of HBV-specific CD8(+) T cells. Strikingly, the expansion and cytolytic differentiation of HBV-specific CD8(+) T cells were reduced even more severely in the mice whose MHC class I expression was restricted to hematopoietic cells. Collectively, these results indicate that cross-presentation is required but relatively inefficient in terms of inducing the cytolytic differentiation of HBV-specific CD8(+) T cells by itself. Instead, the expansion and functional differentiation of HBV-specific CD8(+) T cells are primarily dependent on hepatocellular antigen presentation. IMPORTANCE Hepatitis B virus (HBV) causes acute and chronic hepatitis. Approximately 260 million people are chronically infected with HBV and under an increased risk of developing cirrhosis and hepatocellular carcinoma. Host immune responses, particularly HBV-specific CD8 T cell responses, largely determine the outcome of HBV infection. It is widely accepted that antigen inexperienced CD8(+ )T cells should be initially activated by professional antigen-presenting cells (pAPCs) in lymphoid tissues to differentiate into effector CD8(+) T cells. However, this notion has not been tested for HBV-specific CD8(+) T cells. In this study, we show that HBV-specific CD8(+) T cell responses can be induced in the liver. Surprisingly, antigen presentation by hepatocytes is more important than cross-presentation by hematopoietic cells for the induction of HBV-specific CD8(+) T cell responses. These results revealed a previously unappreciated role of antigen presentation by hepatocytes in the induction of HBV-specific CD8(+) T cell responses.