Chronic viral infections are characterized by a state of T cell dysfunction that is associated with expression of the PD-1 (programmed cell death 1) inhibitory receptor. A better understanding of the mechanisms that regulate CD8 T cell responses during chronic viral infection is required to improve immunotherapies that restore function in exhausted CD8 T cells. We have recently identified a novel population of virus specific CD8 T cells that act as stem cells to maintain T cell responses during chronic infection of mice with lymphocytic choriomeningitis virus (LCMV). These stem-like LCMV specific CD8 T cells are found in the lymphoid tissues where they reside predominantly in the T cell zones along with nave CD8 T cells. They act as stem cells by undergoing a slow self-renewal and can also differentiate to give rise to the terminally differentiated CD8 T cells that are found at the major sites of infection in lymphoid as well as non-lymphoid tissues. Most importantly, the proliferative burst of T cells that is seen after PD-1 blockade comes almost exclusively from this stem-like CD8 T cell subset. Our studies have been confirmed and extended by others who have found similar CD8 T cells in other chronic viral infections of mice and also in chronic infections of non-human primates and humans. The studies proposed in the application are now focused on understanding how these virus specific stem-like CD8 T cells are generated and maintained during chronic infection and how this information can be used to develop rational approaches for optimizing PD-1 directed immunotherapy. The following specific aims are proposed to achieve these goals: Specific aim 1: To understand how the generation and maintenance of virus specific stem-like CD8 T cells is regulated during chronic viral infection. Specific aim 2: To develop strategies for enhancing virus specific stem-like CD8 T cells and optimizing PD-1 directed immunotherapy.