Abstract Anti-microbial T cell responses play a major role in determining the outcome of infection. Chronic infections are often distinguished by T cell responses that are not able to fully eliminate the pathogen. The mechanisms that explain this failure of T cell effector responses are only beginning to be understood. The regulation of T cell responses to infection reflects a delicate balance between effector functions needed to eliminate the microbe and the potential to cause immunopathology. Regulating the immune response to avoid tissue damage may be particularly important in the setting of chronic infection. During the current funding period, we have used the lymphocytic choriomeningitis virus (LCMV) model to investigate how inhibitory pathways in the B7:CD28 family regulate T cell responses during chronic infection. Our studies indicate that PD-1 and its ligands, PD-L1 and PD-L2, contribute directly to T cell exhaustion and lack of viral control during chronic LCMV infection. In vivo blockade of PD-1:PD-L1 interactions in chronically infected mice restores T cell function and leads to a substantial reduction in virus levels. Thus, these studies identify a specific mechanism of T cell exhaustion, and suggest that blockade of this pathway may provide a new therapeutic approach for chronic infections. Further studies are needed to determine how to best modulate PD-1 and its ligands to activate anti-viral T cells while minimizing the risk of immunopathology and autoimmunity, since PD-1 and its ligands also have key roles in regulating tolerance. The discovery of the PD-L1:B7-1 pathway leads us to ask whether PD-L1:PD-1 and PD- L1:B7-1 interactions have unique or overlapping roles in controlling chronic infection, T cell exhaustion, and immunopathology. Our main hypothesis is that the newly discovered PD-L1:B7-1 pathway, as well as PD-L1:PD-1 and PD-L2:PD-1 interactions, regulate virus-specific T cell responses and viral control during chronic infection. PD-L1 may trigger more profound inhibitory effects than PD-1 because PD-L1 has the potential to trigger two inhibitory interactions. To test this hypothesis, our Specific Aims are to: 1) Analyze the functional significance of the newly defined PD-L1:B7-1 pathway, and the relative contributions of the PD-L1:B7-1 and PD-L1:PD-1 pathways in controlling the balance between virus-specific immunity and immunopathology. 2) Analyze the role of PD-L1 on specific cell types in regulating T cell responses, viral clearance and immunopathology;and 3) Determine how PD-L2 controls the balance between viral immunity and immunopathology. Our goal is to determine the best therapeutic modality for enhancing viral clearance while minimizing immunopathology