HCV Vaccine Program Project 1 Summary Evidence from chimpanzee and human studies support critical roles for CD4 and CD8 T lymphocyte responses in the control HCV infection, yet defining the correlates of protective HCV immunity has been challenging. Identifying CD4 and CD8 T cell memory that protects against infection from multiple different HCV genotypes and subtypes is a cornerstone in prophylactic HCV vaccine design. Our groups at Johns Hopkins University and Massachusetts General Hospital have contributed many key findings to the current understanding of HCV- specific T cell immunity, based on the combination of large and well documented patient cohorts with robust and sensitive experimental tools for the assessment of both CD8 and CD4 responses targeting HCV. We will build on this expertise and employ newly developed approaches to study the metabolism and transcriptional landscape of HCV-specific CD4 and CD8 T cells in order to assess T cell immunity in unique patient cohorts. We previously demonstrated that clearance occurs more often in reinfection than in primary infection, with reduced peak and duration of viremia associated with broadened T cell responses vs. initial infection of the same person. This suggests induction of memory that protects against infection from multiple different HCV genotypes and subtypes - a central goal of prophylactic HCV vaccine design. Using a population of patients who have cleared as many as six distinct HCV infections, we plan to better define the specific correlates of T cell mediated protection from chronic HCV infection. In addition, we will leverage access to research specimens from the only prophylactic HCV vaccine regiment tested in an at-risk human population. This regimen, consisting of a ChAd3NS prime followed by an MVA-NS boost, did not protect against chronic infection, but did induce T cells specific for vaccine NS antigens and suppressed geometric mean peak HCV RNA vs. placebo recipients. Specimens from this trial provide a unique opportunity to study the effect of vaccine- induced T cells on incident HCV infection, allowing us to further validate T cell properties associated with control of HCV, but also to identify the reasons for failure of this potent T cell based vaccine. Specifically, we propose: Aim 1: To define the critical characteristics of HCV-specific CD8 T-cell responses in spontaneous recovery from repeated HCV infection as determinants of protective immunity. Aim 2 To determine if specific CD8 T cell characteristics are associated with vaccine induced suppression of peak viremia in subsequent HCV infection. Aim 3: To define effective CD4 T cell responses in acute infection and HCV-specific CD4 memory T cells in patients with multiple episodes of successfully controlled HCV infection. Aim 4: To characterize the HCV-specific CD4 memory T cell population post vaccination and its response after HCV exposure. We build on our combined extensive expertise in studying both CD8 and CD4 T...