Abstract IA02: Tumor microenvironment metabolism in T cell differentiation and dysfunction
Authors: Delgoffe Greg M. 
Published: January 2022

Abstract
Cancer thwarts immune destruction using several adaptations to suppress cancer immunity: upregulation of ligands for coinhibitory receptors like PD-1, recruitment of immunosuppressive populations like Treg cells, and withstanding cytolysis to generate a source of chronic antigen stimulation to drive T cell exhaustion. However, behind the veil of these molecular and cellular interactions is a unique metabolic landscape, driven by the energetic deregulation of the cancer cells themselves, that is toxic to infiltrating immune cells. Indeed, we and others have shown T cells infiltrating tumors succumb to severe metabolic insufficiency, characterized by a progressive loss of functional mitochondria. These metabolic states occur concomitant with the progression to terminal exhaustion. We hypothesized metabolic stress not only characterizes T cell exhaustion but plays a dominant role in driving the fate. We found that while T cells could withstand mitochondrial stressors like hypoxia in isolation, if T cells were experiencing persistent stimulation, they became extremely sensitive to oxygen tension. Indeed, continuous stimulation through the TCR under hypoxic conditions rapidly generated exhausted-like T cells in vitro. We used our in vitro system to dissect molecular mechanisms for this fate change, revealing mitochondrial stress produces intolerable levels of reactive oxygen species, which played a major role in driving exhaustion. Antioxidant chemicals and proteins as well as hypoxia mitigation could prevent T cell exhaustion in vivo and synergize with checkpoint blockade immunotherapy. Exhausted T cells are a major population within solid tumors, but what is their true function in cancer? Our metabolic and transcriptomic studies found the most terminally exhausted T cells bear striking similarity to tumor-infiltrating regulatory T cells. We thus asked whether exhausted CD8+ T cells harbored regulatory function using miniaturized suppression assays. Strikingly, when sorted from the same tumor, exhausted T cells carried similar suppressive capacity as CD4+ Treg cells. Unlike Foxp3+ Treg cells, which suppress through a myriad of mechanisms, exhausted CD8+ T cells suppress solely using the ectonucleotidase CD39, previously shown to define the most terminally exhausted cells. We found hypoxia exposure and HIF1α expression drive CD39 expression, which generates an adenosinergic local environment in which CD73+ immune cells create immunosuppressive adenosine. Deletion of CD39 within the CD8 lineage resulted in slowed tumor growth and heightened sensitivity to immunotherapy, highlighting a significant contribution by exhausted T cells to the tolerogenic environment in cancer. Our data highlight an unappreciated function of this terminally exhausted subset and suggest exhausted T cells are not merely dysfunctional, but in cancer become actively anti-functional, working against the immune response. By targeting adenosine, CD39, or the hypoxic nature of the tumor microenvironment, these deleterious functions may be limited to enable immunotherapy for cancer.
Citation Format: Greg M. Delgoffe. Tumor microenvironment metabolism in T cell differentiation and dysfunction [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2021 Oct 5-6. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(1 Suppl):Abstract nr IA02.