New insights into the functional diversity of the hepatic antiviral T cell response during hepacivirus infection in vivo

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Inflammatory liver diseases such as chronic hepatitis C virus (HCV) infection can progress to liver fibrosis and hepatocellular carcinoma (HCC). Hepatic T cell immunity is closely linked to liver diseases since T cells are not only essential for the clearance of hepatotropic viruses like HCV but may also cause liver injury and contribute to disease progression. However, the specific role of functionally distinct CD4+ and CD8+ T cell subsets or innate-like T cells, such as natural killer T (NKT) cells, in mediating an effective antiviral immune response or regulating liver damage during inflammation are not well understood. Thus, gaining new insights into the functional biology of hepatic T cell subsets is a prerequisite for the development of HCV vaccine strategies and immunotherapeutic options for the treatment of progressive liver diseases. Limited access to human liver tissue and the lack of immune-competent small animal models has impeded studies of hepatic antiviral immune mechanisms. However, we have recently developed an immune-competent mouse model of an HCV-related virus, Norway rat hepacivirus (NrHV), which shares significant virological and immunological similarities with HCV infection in humans. This advance now allows for in-depth mechanistic studies of anti-hepaciviral immunity in vivo. Our initial analysis of hepatic lymphocyte subsets during acute, chronic and secondary NrHV infection revealed a significant induction of a diverse antiviral type 1 T cell response and T cell dependent viral clearance. However, we also made the unexpected observation that subsets of hepatic CD8+ T cells and NKT cells are polarized towards type 2 immunity and may contribute to immune-regulation. In this proposal, we will build on these preliminary data and elucidate the functional properties and potential crosstalk of distinct hepatic type 1 T cells and type 2 cytokine secreting lymphocyte subsets during NrHV infection. Specifically, we will use high- dimensional flow cytometry, in vivo functional approaches and/or transcriptome analysis to 1) define the functional role of different virus-specific effector and memory CD8+ T cell subsets, 2) determine the kinetic and impact of CD4+ T cells on the generation of virus-specific CD8+ T cells and protective immunity, 3) delineate the function of type 2 CD8+ and NKT cell subsets and define their impact on liver damage, virus-specific T cells and other hepatic immune cells during different stages of infection. Finally, we aim to translate our main findings from the mouse model to human HCV patients. Our data will provide novel biological insight into the role and interplay of distinct hepatic type 1 and type 2 T cell and NKT cell subsets in mediating viral clearance, protective immunity, tissue pathology or repair during a hepatotropic virus infection in vivo.