Tumor Cells Dictate Anti-Tumor Immune Responses by Altering Pyruvate Utilization and Succinate Signaling in Cd8 ⁺ T Cells

The tumor microenvironment (TME) is a unique metabolic niche that can inhibit CD8+ T cell metabolism and cytotoxic activity. It has been a challenge to dissect the metabolic interplay between tumors and T cells. Consequently, the mechanisms whereby the TME regulates CD8+ T cell function are not completely understood. Here we establish an in vitro system that recapitulates the metabolic niche of the TME and allows us to define cell-specific metabolism. We identify tumor-derived lactate as a direct inhibitor of  CD8+ T cell cytotoxicity, revealing an unexpected metabolic shunt in the tricarboxylic acid (TCA) cycle in CD8+ T cells. Metabolically fit cytotoxic T cells shunt succinate out of the TCA cycle to promote autocrine signaling via the succinate receptor (SUCNR1), a G-protein coupled cell surface receptor.  This leaves CD8+ T cells reliant on pyruvate carboxylase (PC) to replenish TCA cycle intermediates (anaplerosis). In contrast, lactate reduces PC-mediated anaplerosis, thus depleting the TCA cycle and reducing SUCNR1 signaling. Notably, inhibition of pyruvate dehydrogenase (PDH) is sufficient to override lactate-mediated reprogramming, and to restore PC activity, succinate secretion, and autocrine activation of SUCNR1 both in vitro and in vivo using syngeneic tumor models. Thus, dissecting the CD8+ T cell-specific metabolic adaptation to the TME unexpectedly revealed that a TCA cycle succinate shunt and plasticity in pyruvate flux is a crucial determinant of CD8+ T cell anti-tumor activity. Furthermore, we identify PDH as a potential drug target to allow CD8+ T cells to retain cytotoxicity and overcome a lactate-rich TME. These studies demonstrate that the TME remodels CD8+ T cell pyruvate metabolism, leading to a reduction in CD8+ T cell cytotoxicity and anti-tumor immunity.