Project 1: High resolution profiling of protective HCV-specific CD4 T cells Abstract Hepatitis C virus (HCV) is a virus that infects the liver and is transmitted through contaminated blood. HCV infection is still a public health priority and a major cause of liver disease worldwide, especially in developing countries like Egypt. New HCV infections are also on the rise in North America in association with the opioid epidemic. While new antiviral drugs can cure >95% of infected individuals, many do not know that they are infected and remain at risk of developing liver disease and an active source of new infections. Furthermore, successful treatment does not prevent reinfection if the individual is re-exposed to the virus. Hence, there is an urgent need for vaccines that can protect against this virus. Unfortunately, despite our knowledge about the immune response against HCV, we still do not have an effective vaccine. Recent results from the only vaccine candidate that made it into a large scale (Phase 2) clinical trial did not show any protection in people who inject drugs (PWID) at risk of HCV infection. This means that we need to understand better the immune factors and cells that protect against HCV in cohorts of patients who are able to clear the virus spontaneously. The overarching goal of this proposal t is to define these factors that are essential to achieve protective immunity. The aim of this specific project (Project 1) is to characterize the role and functions of a subset of white blood cells known as helper CD4 lymphocytes in a group of PWID who are constantly being exposed to the virus where some are able to clear it several times and others that cannot. We will identify the differences in those who clear the virus repeatedly and those who cannot. We will also compare the immune response in individuals who clear the virus spontaneously versus those who clear it post treatment to determine if they are less protected against HCV if they are re-exposed to it. We will use two unique cohorts of subjects, a PWID cohort from Montreal, Canada and a cohort of subjects undergoing antiviral therapy from Egypt, the country with the highest prevalence of HCV. The results obtained in this project will be complementary to Project 2 that will examine the antibody response to HCV in the same subjects. We will use the latest technology to examine the immune response at the functional and genomic level and identify cellular pathways and key molecules that are implicated in the development and maintenance of a protective immune response. We will attempt to use chemical compounds to correct the defects observed in those who are unable to clear the virus. Such compounds may be combined with different vaccine candidates, as described in project 3, to enhance their immunogenicity and protective capacity.