Bcl-2 protein Noxa is required for metabolic reprogramming to glutamine dependence and for apoptosis in stimulated human CD8+ T cells

Naive or memory T cells reprogram their metabolism upon antigenic stimulation. They increase their glucose uptake, relying on aerobic glycolysis for generating biomass while switching to glutamine to fuel energy production. Here we have identified a requirement for human Bcl-2 family, Noxa, in the metabolic switch to glutamine dependence in activated CD8+ T cells, that is independent of its canonical role in apoptosis at the end of the immune response. Using an in vitro co-stimulation model, we demonstrate that Noxa is induced in CD8+ T cells and remains elevated during the proliferative and differentiation phases of the response and through the onset of apoptosis. Noxa protein induction requires glutamine, is mediated via mTOR, and is independent of glutaminolysis. Glutamine, in turn, requires Noxa to facilitate its conversion to glutamate. CD8+ T cells lacking Noxa showed reduced levels of intracellular glutamate but no impairment of mitochondrial or effector function, and decreased dependence on glutamine for both respiration and growth during the proliferative phase. NOXA knockout CD8 T cells also displayed significantly higher viability in the apoptotic phase of the immune response. CD8+ T cells from a human NOXA gene-replacement mouse responded normally to in vitro stimulation and in vivo acute infection. However, human Noxa-expressing murine CD8+ T cells displayed a distinctly proliferative gene signature in their transcriptome following activation, supporting an early growth-promoting role for this BH3-only protein. Our studies suggest that knocking out NOXA in human CD8+ T cells to increase their lifespan as well as their ability to survive and function in glutamine-poor microenvironments could be a promising immunotherapeutic strategy.