Our PPG renewal is driven by the fundamental and therapeutic importance of PD-1 and TIGIT in regulating T cell tolerance and T cell dysfunction in cancer and autoimmunity and builds on significant progress we have made in our previous grant cycle. Our productivity is highlighted by 43 primary publications and 27 reviews. Working together, we previously demonstrated that PD-1:PD-L1 interactions regulate T cell tolerance while TIGIT has T cell intrinsic inhibitory effects that mediate Treg function. Based on new preliminary data showing PD-1 deletion increases the frequency of TIGIT+ Tregs and TIGIT agonist reversed in vitro defects of Treg function in MS, we examined if TIGIT agonism could control pathogenic T cells and ameliorate autoimmunity exacerbated by PD-1 blockade using the EAE model. Remarkably, TIGIT agonist diminished EAE severity in mice given anti- PD-1 blocking mAb while not impairing PD-1 efficacy in controlling tumors. Thus, TIGIT is a potential target for treating immune-related adverse events (irAEs) associated with PD-1 blockade therapy. We also found that both PD-L1 and CD155 are upregulated in CNS myeloid cells in states of chronic neurological inflammation and the TIGIT expressed on Tregs promotes oncogenesis and immune evasion by directly signaling via its ligand CD155 expressed on tumor cells. These discoveries demonstrate ongoing synergy within our PPG with sharing of unpublished data that have inspired the hypotheses and experiments which drive the focus of this application. The hypothesis underlying this PPG is that bi-directional signaling through TIGIT/CD155 and PD-1/PD- L1 balance Treg and T effector function and regulate tissue inflammation in autoimmunity and cancer. Our PPG will consist of three highly integrated and interactive Projects, supported by three Cores. Project 1 will elucidate bidirectional interactions between TIGIT and CD155 in autoimmunity and cancer. Project 2 will determine cellular and molecular underpinnings of TIGIT/CD155 and PD-1/PD-L1 interactions that regulate Treg and CD4+Foxp3– function. Project 3 will investigate the roles of TIGIT/CD155 and PD-1/PD-L1 in regulating human CNS inflammation and cancer. Core A (Hafler) will provide administrative and scientific coordination, Core B (Sharpe/Kuchroo) will provide novel mouse strains and gene perturbation technologies, and Core C (Zhang/Fan) will provide spatial multi-omics and data integration of all three projects. Our major goals are to: 1) deeply examine the TIGIT/CD226-CD155 and PD-1/PD-L1 pathways in autoimmunity in relationship to their opposing roles in cancer; 2) identify the cellular and molecular circuits underlying interactions between the TIGIT/CD155 and PD-1/PD-L1 pathways to regulate Treg and T effector cell function; 3) determine how reverse signaling of CD155/PD-L1 alone or in combination into myeloid cells and tumors regulates their function; 4) develop novel strategies for treating autoimmunity and irAEs. These Projects exempl...