PROJECT SUMMARY Hepatitis C virus (HCV) infect ~1% of the world population, predisposing the infected individuals to increased risk of liver cirrhosis and liver cancer. New infections in the US has risen in the past decade and it is estimated that only 20% of the individuals with hepatitis C are aware of their infection, and only 15% of those have been treated. Although highly effective direct-acting antivirals (DAAs) can cure over 95% chronic HCV patients, reinfections have been reported in individuals who have cleared a previous infection either spontaneously or after a successful treatment. The observations underscore that immune memory to HCV acquired through infection is not effective against reinfection. To guide the development of a vaccine and public health strategy to protect high-risk populations, it will be important to understand immune memory to HCV in DAA-cured patients particularly cross-neutralizing antibody response. In this project we will investigate how human memory B cell response is altered by chronic infection and its recovery in chronic HCV patients undergoing DAA treatment. We will investigate why HCV-immune lymphocytes do not maintain an effective memory against HCV after successful DAA treatment. We will also track the functions and recovery of memory B cells encoding for antibodies targeting conserved virus neutralizing epitopes in the patients. These cells will be crucial for protection against HCV both in reinfection and in vaccination. This study will not only provide an excellent opportunity to learn about human immune response impacted by chronic viral infection but will also generate critical information for the protection against HCV infection.