PROJECT SUMMARY/ABSTRACT: Dendritic cells (DCs) are sentinels of the immune system that operate at interface of the innate and adaptive immune response. DC-driven responses require their migration from peripheral tissue to the draining lymph node in order prime naïve T cells. Understanding and discovering novel targets that can manipulate DC localization will be beneficial for developing new therapeutic interventions, particularly in the context of vaccination where DC-driven responses are critical for initiation of memory T cell responses. Programmed death ligand-1 (PD-L1) and CD80 are cell-surface protein ligands that belong to the B7 family of proteins and possess the ability to bind to the CD28 family of receptors on T cells in order to provide coinhibitory or costimulatory signals. In addition to their roles in cell-cell communication between DCs and T cells, how PD-L1 and CD80 regulate intracellular signals within migratory DCs remains understudied. Recent published work from the laboratory of my mentor, Dr. Beth Tamburini, demonstrated that PD-L1 intracellular signaling plays an important role in regulating DC migration by controlling chemokine receptor signaling in the context of a type I immune response. My preliminary data suggest that blocking extracellular interactions of PD- L1 with CD80, but not PD-1, decreases DC migration during an inflammatory response within the skin. Additionally, I also found that increased amounts of pathogen-specific tissue resident memory T cells (TRM) persist in the skin after resolution of vaccinia infection in mice with impaired PD-L1 intracellular signaling within cross-presenting DCs. With these preliminary findings in mind, we hypothesize that PD-L1 cis interactions with CD80 regulate intracellular signaling cascades needed to promote migration of DCs. We also hypothesize that while PD-L1-mediated DC retention during cutaneous infection leads to enhanced formation of TRM cells, persistent antigen presentation leads to T cell exhaustion and impaired cell-mediated immunity upon rechallenge. Our aims are as follows: 1. Define which extracellular interactions between PD-L1 and CD80 regulate DC migration and 2. Determine how PDL1 intracellular signals facilitate memory T cell phenotype and function during a viral infection. These studies present an opportunity to investigate the mechanism by which PD-L1 governs migration of cDCs and characterize how retention of migratory DCs in the skin can alter T cell responsiveness to viral antigen. These findings are especially significant in the context of PD-L1 immunotherapy, currently used in the clinic, which prohibits both cis and trans PD-L1 interactions. Loss of these interactions by PD-L1 binding partners could explain resulting off-target effects within nonlymphoid organs such the skin in patients on PD-L1 immunotherapy.