An investigation of human regulatory T cell metabolic pathways
Regulatory T cells (Tregs) are essential to homeostatic immune tolerance. Clinical trials of autologous Treg cell therapy have demonstrated promising preliminary results, but there remain challenges to overcome including the problem of recipient-dependent expansion variability. Enhancing the suppressive function of expanded Tregs to reduce the required Treg dose would present a potential solution to this problem. Cellular metabolism regulates immune cells through two interacting aspects: bioenergic and biosynthetic activities, and non-canonical activities of metabolic enzymes and intermediates. Early in vitro work has revealed distinct metabolic profiles between mouse conventional T cells (Tconvs) and Tregs. However, it remains unclear how the metabolism of human Tregs is regulated. A better understanding of the contrasting metabolic demands and regulation of function between human Tconvs and Tregs may uncover routes to target Treg-specific metabolic pathways in order to tune their activity as desired. In this work, the immunometabolism of human Treg is investigated and new strategies revealed to harness elements of their metabolism for therapeutic use. In Chapters 3 and 4, I show that Treg display the ‘Warburg effect’ after T cell receptor stimulation alongside CD28 co-stimulation via the mTOR signalling pathway. Subsequently, I show that only naïve and central memory Tregs preferentially utilise glucose; suggesting that human Treg metabolism may differ due to their anatomical location. In contrast, effector memory Tregs and Tconvs have distinct metabolic characteristics: whilst Tconvs negatively alter effector function through limiting glucose metabolism, Tregs positively regulate their suppressive function by limiting mitochondrial energy metabolism. Furthermore, this metabolic regulation allows Tregs to enhance their suppressive activity towards Treg-resistant Tconvs which are characterised by their low mitochondrial mass. In Chapters 5 and 6, I investigate the mechanism of this phenomenon, and show that Tregs generate immunosuppressive extracellular vesicles (exosomes) by mitochondrial ATP synthase inhibition. Metabolomics data suggests that epigenetic alteration in Tregs through a change in mitochondrial metabolism intermediates. The donor-independent function of these small extracellular vesicles suggests a new potential therapeutic strategy in which Treg-derived extracellular vesicles could be used ‘off-the-shelf’. The immunometabolic experiments presented in this thesis shine new light on the differential metabolic regulation of human Tregs and Tconvs, and reveal novel and potent mode of Treg immunosuppresive action that can be induced by modulating metabolism. By defining these pathways, new therapeutic strategies to immune regulation may be potentially revealed.