PROJECT SUMMARY/ABSTRACT Current immunotherapies, including immune checkpoint blockage therapy and adoptive T cell therapy, have resulted in promising results in certain types of cancer patients. However, these immunotherapies have so far been insufficient to reproducibly eliminate tumors. It is clear that tumor-reactive T cells are suppressed and dysfunctional in the suppressive tumor microenvironment that is a major obstacle for successful tumor immunotherapy. Thus, dissecting the distinct mechanisms responsible for T cell dysfunctional states within the suppressive tumor microenvironment should provide novel avenues for tumor immunotherapy. We discovered that induction of T cell senescence is an important T cell dysfunctional state and a novel suppressive mechanism utilized by both human naturally occurring and tumor-derived regulatory T (Treg) cells in the tumor microenvironment. In fact, significant accumulation of senescent CD8+ T cells has also been found in the tumor-infiltrating T cells (TILs) from various types of cancer patients. Importantly, we found that these senescent T cells are functionally suppressive and molecularly distinct from anergic and exhausted T cellsand that they are a critical mediator and amplifier for immune suppression within the tumor microenvironments. Therefore, a better understanding of this novel suppressive mechanism and the molecular processes in responder T cells suppressed by Treg cells is essential for the development of effective strategies to treat human cancers. Cellular energy metabolism directs T cell survival, proliferation and their specific functions. Different T cell subsets have different metabolic profiles. We have more recently identified that Treg-induced senescent T cells exhibit active lipid metabolism, resulting in upregulation of lipid metabolic enzymes and secretory lipid species, and accumulation of lipid droplets (LDs). The central hypotheses of this proposal are that: 1) Excessive lipid metabolism is critical for senescence development and immunosuppression of effector T cells mediated by Treg cells; 2) Senescent and dysfunctional tumor-specific T cells can be rejuvenated via lipid reprogramming for enhanced anti-tumor immunity. Specific Aim 1 seeks to identify whether the excessive lipid metabolism is involved in senescence development and immunosuppression of T cells induced by Treg cells. Specific Aim 2 will explore the novel concept and develop effective strategies to overcome senescent and exhausted tumor-specific T cells via lipid metabolism reprogramming combined with selective checkpoint blockage therapy of anti-PDL1 for enhanced anti-tumor efficiency in the adoptive T cell transfer therapy tumor models. The positive outcome of these studies should lead to novel strategies to reprogram lipid metabolism and effector functions of tumor-specific T cells for cancer treatments.