Regulation of cellular metabolism in tumor infiltrating CD8+ T cells and its role in their dysfunction

Abstract
Tumor infiltrating CD8+ T cells (CD8+ TIL) play pivotal role in fighting cancers. Their accumulation within tumors has been associated with better tumor control in mouse models of cancers and favorable clinical outcomes in human patients. However, it has been well-understood that these cells undergo progressive loss of function and often fail to eradicate tumor cells. While significant progress has been made in understanding and targeting the mechanisms underlying CD8+ TIL dysfunction, the molecular details of the dysfunction remain to be fully elucidated. It has been widely appreciated that elevated glycolysis and oxidative phosphorylation (OXPHOS) are essential to support the generation and function of effector T cells. Here, we demonstrate the perturbation of energy metabolism as a biochemical basis of CD8+ TIL dysfunction. We found that both glycolytic metabolism and OXPOHS were attenuated in melanoma CD8+ TIL. Glycolysis was repressed by impaired activity of enolase 1, a key glycolytic enzyme responsible for the synthesis of phosphoenolpyruvate that is essential for the effector function of T cells. While this enzyme is highly expressed in CD8+ TIL, its activity was post-translationally regulated by mechanism that involved immune checkpoint signals from PD-1, CTLA-4, and TIM-3. The details of this mechanism remain to be elucidated, and we have developed a robust reporter of enolase activity to aid future investigation in this area. We also found impaired enolase activity in the CD8+ TIL that infiltrated human melanoma tumors and different types of murine tumor models. In addition to glycolysis, impaired enolase activity also limited the OXPHOS capability of CD8+ TIL. This was at least partly mediated by inability of glycolysis to produce sufficient pyruvate to feed into the mitochondrial metabolism. However, CD8+ TIL also had low mitochondrial mass and membrane potential that may be a major contributor to the OXPHOS deficiency of these cells. Importantly, we demonstrated that bypassing the enolase inactivity through provision of metabolites produced downstream of it significantly improved the glycolytic metabolism, OXPOHS, and effector function of CD8+ TIL. Furthermore, we showed that a combination of immune checkpoint blockade therapy that slowed tumor growth in mouse model generated CD8+ TIL with stronger enolase active that was essential for their function. Our studies demonstrated that metabolic dysfunction mediated by impaired enolase activity is a major contributor to the functional impairment of CD8+ TIL, and that reactivating this enzyme may reinvigorate antitumor immunity.