Glutathione restricts serine metabolism to preserve regulatory T cell function. (Doctoral thesis)

Regulatory T cells (Tregs) play an indispensable role in maintaining immune tolerance and preventing autoimmunity. Tregs are known to rely on oxidative metabolism in their quiescent state while engaging a complex orchestration of glycolysis, lipid oxidation, and amino acids metabolism upon activation. Mitochondrial oxidative phosphorylation (OXPHOS) results in the generation of reactive oxygen species (ROS), which are important cellular signaling molecules. Nevertheless, ROS have to be tightly balanced as increased concentrations result in cellular damage.
A group of molecules termed antioxidant are instrumental in the ROS-detoxifying process, of which glutathione (GSH) is the most abundant one. The tripeptide GSH is synthesized in a two-step process. In the initial rate-limiting step, L-glutamate and cysteine are converted to γ-glutamylcysteine by the enzyme glutamate-cysteine ligase. The enzyme GSH synthetase, then adds glycine to the C-terminal of γ-glutamylcysteine to form GSH.
Serine, when converted to glycine, fuels GSH synthesis and supports the one-carbon metabolic network which is important for effector T cell (Teff) responses. However, the role of serine in Treg metabolism and Treg specific stress responses is unknown. Using a genetic mutant mouse model harboring a Treg-specific deletion of the glutamate cysteine ligase catalytic subunit (Gclc), we show that loss of GSH and increased ROS level in Tregs altered serine synthesis and uptake, and that this feedback loop was critical for Treg function. Although Gclc deletion did not impair Treg differentiation and number, the mutant mice displayed severe sign of spontaneous autoimmunity, infiltration of various immune cells in different organs and a reduced lifespan.
At the molecular level, Gclc-deficient Tregs showed increased serine uptake via alanine/serine/cysteine/threonine transporter 1 (ASCT1) and serine synthesis via phosphoglycerate dehydrogenase (PHGDH). These led to enhanced mTOR activity and proliferation, but downregulation of FoxP3 expression, which is critical for proper Treg function. Limiting intracellular serine uptake and synthesis in vitro restored FoxP3 expression and Treg suppressive capacity. In vivo, serine deprivation reduced spontaneous T and B cell activation, inflammatory cytokine production and rescued mutant mice from their lethal inflammatory disease.
Interestingly, Tregs specific Gclc-deficient mice exhibited superior anti-tumor immunity, which was solely dependent on Treg dysfunction. Importantly, pharmacological blockade of GSH synthesis in human Tregs confirmed our findings derived from the genetically altered Gclc mutant mice. Our work has shed light on an unexpected role of ROS and GSH in modulating serine metabolism to conserve Treg functionality.