PROJECT SUMMARY/ABSTRACT – PROJECT 1 The CD226/TIGIT pathway is a novel co-stimulatory/co-inhibitory pathway that closely parallels the CD28/CTLA- 4 pathway. Similar to CD28 and CTLA-4, CD226 and TIGIT share ligands (CD112 and CD155) and ligand engagement of CD226 co-stimulates T cell activation whereas engagement of TIGIT inhibits T cell responses. Over the last few years, TIGIT has emerged as an important co-inhibitory receptor. Although initial experiments indicated that TIGIT inhibits T cell responses indirectly by promoting a tolerogenic phenotype in dendritic cells (DCs), we and others have shown that TIGIT has T cell-intrinsic inhibitory functions. We have now found that TIGIT is co-expressed with other co-inhibitory receptors (PD-1, Tim-3, Lag-3) on CD8+ T cells that exhibit a dysfunctional/exhausted phenotype in chronic diseases such as cancer and chronic viral infection. In addition to its role in effector T cells, conditional deletion of TIGIT on various cell types suggests that TIGIT plays a very critical role in the function of Foxp3+ Tregs both in autoimmunity and cancer. Importantly, we find that TIGIT+ Tregs interact directly with CD155 expressed on tumors to promote stemness/oncogenesis and on DCs to induce a tolerogenic phenotype. The bi-directional signaling between TIGIT on T cells and CD155 on target cells has been poorly explored. In this grant cycle we hypothesize that TIGIT, in addition to inhibiting effector T cell responses, promotes Treg function and affects target cells by inducing inhibitory signal transduction by CD155, the high affinity ligand for TIGIT, thereby promotes tissue tolerance and tumor growth. To address this hypothesis, we propose the following aims: 1) To elucidate the mechanisms by which TIGIT strengthens Tregs function by interacting with its ligand CD155. The effect of TIGIT signaling into T cells and role on effector T cells (CD4 and CD8) will be directly explored in both autoimmune and tumor models; 2) To determine how TIGIT/CD155 signaling into myeloid cells/tumor cells regulates their function in a cell intrinsic manner. We will specifically determine how TIGIT signals back via CD155 into tumor cells and professional antigen presenting cells using proteomic and genomic analyses. By undertaking a comprehensive bi-directional analysis of interactions between TIGIT on T cells and CD155 on target cells (APCs and tumor cells), we will begin to identify the mechanisms by which the TIGIT-CD155 interaction induces tissue tolerance and promotes tumor growth.