The malate aspartate shuttle in T cell metabolic fitness: Glutamic oxaloacetic transaminase 1 antagonizes T cell exhaustion

CD8+ T cells play a crucial role in fighting infectious diseases and cancers, but it is frequently observed that CD8+ T cells are functionally compromised or “exhausted” in chronic infections and solid tumors. Exhausted T cells are characterized by the expression of immune checkpoint molecules, reduction in effector cytokine secretion, and alterations in cellular metabolism, such as loss of mitochondrial membrane potential (ΔΨm). Immune checkpoint blockade-based immunotherapies revive exhausted T cells, enhance effector cytokine production, and have achieved clinical success in several types of cancers. It is unknown whether and how ΔΨm regulates T cell exhaustion in chronic infection and cancer. Mitochondrial complexes accept electrons from tricarboxylic acid (TCA) cycle-derived electron carriers, transfer electrons, and pump protons to establish a membrane potential. The malate aspartate shuttle (MAS) produces and transports electron carriers into the mitochondrial matrix and donates electrons to the electron transport chain (ETC). We, therefore, hypothesize that MAS may regulate ΔΨm and antagonize T cell exhaustion. To test this hypothesis, we have generated a mouse model with T cell-specific deficiency of glutamic oxaloacetic transaminase 1 (Got1, encoding the MAS enzyme GOT1). We chose GOT1 because it is a key enzyme of the MAS, and we observed that GOT1 expression was induced by persistent antigenic stimulation in mouse CD8+ T cells and upregulated in CD8+ tumor-infiltrating lymphocytes (TILs) from human colon cancer. T cell-specific ablation of Got1 impaired tumor-specific effector CD8+ T cell accumulation, effector cytokine production and potential to sustain long term immune responses in a mouse melanoma model. Using the lymphocytic choriomeningitis virus (LCMV) Armstrong acute infection and the LCMV clone 13 chronic infection model, we found that Got1 deficiency affected the ongoing antiviral effector responses, but not the memory T cell formation after viral clearance, suggesting that CD8+ responses required GOT1 in the presence of persistent, but not transient antigenic stimulation. Further analysis revealed that Got1 deficiency alone did not affect CD8+ T cell metabolism or ΔΨm when cells were cultured in complete medium. Instead, Got1 deficiency became only catastrophic for CD8+ T cells when extracellular nutrients were restricted, and antigenic stimulation was persistent. Got1 deficiency reduced the NAD+/NADH ratio, increased reactive oxygen species (ROS) production and decreased ΔΨm. This study reveals, GOT1 selectively supports effector CD8+ T cells that are nutrient restricted and chronically stimulated. Summarized, the presented work revealed that persistent stimulation with tumor and viral antigens induced GOT1 expression on CD8+ T cells. GOT1 was shown to desensitize CD8+ T cells to environmental nutrient limitation and persistent antigenic stimulation-induced loss of ΔΨm and T cell exhaustion. These results present an example of how immune cells alter their metabolic pathways to adapt to nutrient and antigen availability