Hypoxia Inducible Factors: Redefining Metabolic Regulation of CD8 T cell Differentiation and Function
T cell responses are initiated by detection of cognate peptide presented by antigen presenting cells in secondary lymphoid tissues and result in migration of T cells to various tissues with disparate local conditions. Hypoxia Inducible Factors (HIFs) are uniquely poised to integrate physiological and disease-derived signals to modulate T cell responses making them an attractive target for further study. Understanding how T cells have taken advantage of the HIF pathway to interpret local conditions and integrate these with the array of mitogenic signals produced during infection is essential for the advancement of our understanding of T cell biology and critical for the development of novel therapeutic approaches. This dissertation takes advantage of genetic models to modulate the oxygen sensing HIF pathway to define how HIFs regulate CD8+ T cell differentiation and function.
By employing genetic deletion of the von Hippel Lindau tumor suppressor protein (VHL), a crucial negative regulator of HIF signaling, we identfied a novel role for HIF activity in modulating effector CD8+ T cell function following chronic viral infection. We demonstrated that sustained HIF signaling not only enhances effector function in the face of persistent antigen, but drives resistance to immune exhaustion in antigen-specific CD8+ T cells. Constitutive HIF signaling also drove enhanced glycolytic metabolism and pharmacological inhibition of glycolysis resulted in partial rescue of HIF-dependent alterations in expression of effector molecules, activation-associated receptors, and critical transcription factors suggesting that modulating cellular metabolism can directly impact T cell function and possibly fate decisions.
Extensive metabolic changes accompany T cell activation including a switch to glycolytic energy production and increased biosynthesis. Recent studies suggest that a subsequent return to reliance on fatty acid oxidation to fuel oxidative phosphorylation and increasing spare respiratory capacity are essential for the differentiation of memory CD8+ T cells. Constitutive HIF activity in CD8+ sustains glycolytic metabolism and suppresses oxidative phosphorylation providing a powerful model by which to define the role of metabolic pathway choice in CD8+ T cell differentiation. Here we demonstrate that surprisingly, constitutive glycolytic metabolism does not inhibit the formation of long-lived memory CD8+ T cells, but predominantly supports the formation of effector memory (TEM) CD8+ cells. Moreover, the departure from a reliance on oxidative phosphorylation also drives accelerated emergence of memory precursor cells following the peak of the response to acute viral infection. Importantly, these long-lived memory CD8+ T cells upregulated IL-7 receptor, expressed critical transcription factors at similar levels to wildtype cells, and demonstrated a heightened response upon secondary challenge demonstrating that restricting CD8+ T cells to glycolytic metabolism does not inhibit the differentiation of a protective memory pool.
While constitutive HIF signaling has provided a powerful model to define targets of HIF activity in CD8+ T cells, we also sought to understand how loss of HIF signaling and HIF-mediated glycolytic metabolism may impact CD8+ T cell differentiation and function. In this dissertation we begin to explore how deletion of HIF-1a and HIF-2a impact CD8+ T cell metabolism and function following acute viral infection and place these findings in the context of CD8+ T cell immunity as well as propose areas of future study.