Chronic HCV infection is associated with persistent innate immune activation and dampened cellular immune responses. Interferons (IFNs) are key mediators of the antiviral innate immune response, initiating the expression of interferon-stimulated genes (ISG) with numerous host protective effector functions. However, in chronic HCV, high pre-therapy expression of ISG and persistent activation of the innate immune system negatively predicts the response to IFN-based therapies and failure of viral clearance8. In addition, HCV persistence is also associated with HCV-specific-CD8+ T cell exhaustion. HCV-specific CD8+ T cell exhaustion is characterized by diminished ex vivo polyfunctionality, upregulation of negative costimulatory cell modulators and, decreased cellular proliferation and IFN production10,12. This phenotype is associated with the development of short-term effector CD8+ T cells rather than durable, long-term memory CD8+ T cells. Chronic bystander infections (e.g. chronic HCV), characterized by persistent inflammation have been linked to bystander (non-HCV specific) CD8+ T cell dysfunction. Bystander CD8+ T cell dysfunction significantly impairs the expansion of memory CD8+ T cells and could prevent the development of secondary immunological memory to new antigens and/or vaccines11,13. Clinically, chronic HCV has been associated with impaired immune response to Hepatitis B vaccination13,14,15. Only 40% to 60% of individuals with chronic HCV achieve seroprotective titers following HBV vaccination versus 90% to 95% in healthy subjects13,14. Specific immune defects responsible for HBV vaccine failure in HCV-infected patients are unknown at present. However, some studies have suggested that the blunting of the immune response to HBV vaccination is associated with lymphocyte dysfunction and upregulation of PD-1 expression on CD4+ T cells in HBV vaccine non-responders13,15.