The PI3K/AKT signaling pathway is an essential node in mammalian cells that controls cell growth, migration, proliferation, and metabolism. During the last decade, a number of works have demonstrated an important role for the PI3K/AKT pathway in regulatory T cell development, function, and stability. This review summarizes our current knowledge of how the PI3K/AKT pathway regulates thymic and peripheral Treg generation and function, with an emphasis on translation of these observations to therapies targeting Tregs in several pathologies.

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Cellular metabolic remodeling is intrinsically linked to the development, activation, differentiation, function, and survival of T cells. T cells transition from a catabolic, naïve state to an anabolic effector state upon T cell activation. Subsequently, specialization of T cells into T helper (Th) subsets, including regulatory T cells (Treg), requires fine-tuning of metabolic programs that better support and optimize T cell functions for that particular environment. Increasingly, studies have shown that changes in nutrient availability at both the cellular and organismal level during disease states can alter T cell function, highlighting the importance of better characterizing metabolic-immune axes in both physiological and disease settings. In support of these data, a growing body of evidence is emerging that shows specific lipid species are capable of altering the inflammatory functional phenotypes of T cells. In this review we summarize the metabolic programs shown to support naïve and effector T cells, and those driving Th subsets. We then discuss changes to lipid profiles in patients with multiple sclerosis, and focus on how the presence of specific lipid species can alter cellular metabolism and function of T cells.

25FoxP3 positive regulatory T cells (T regs ) rely on fatty acid b-oxidation (FAO)-driven OXPHOS for 26 differentiation and function. Recent data have demonstrated a role for T regs in the maintenance of 27 tissue homeostasis with tissue-resident T regs possessing tissue-specific transcriptomes. However, 28 specific signals that establish these tissue-resident T regs programs are largely unknown. As T regs 29 metabolically rely on FAO, and considering the lipid-rich environments of tissues, we 30 hypothesized that environmental lipids drive T reg homeostasis. Using human adipose tissue as a 31 model for tissue residency, we identify oleic acid as the most prevalent free fatty acid in human 32 adipose tissue. Mechanistically, oleic acid amplifies T reg FAO-driven OXPHOS metabolism, 33 creating a positive feedback mechanism that induces the expression of Foxp3 and enhances 34 phosphorylation of STAT5, which acts to stabilize the T reg lineage and increase suppressive 35 function. Comparing the transcriptomic program induced by oleic acid to that of the pro-36 inflammatory arachidonic acid, we find that T regs sorted from peripheral blood and adipose of 37 healthy donors transcriptomically resemble the oleic acid in vitro treated T regs , whereas T regs 38 obtained from the adipose tissue of relapsing-remitting MS patients more closely resemble an 39 arachidonic acid profile. Finally, we find that oleic acid concentrations are reduced in the fat tissue 40 of MS patients, and exposure of dysfunctional MS T regs to oleic acid restores defects in their 41 suppressive function. These data demonstrate the importance of fatty acids in regulating tissue 42 inflammatory signals. 43 44 45 Metabolic signatures of T cells are intricately linked to their differentiation and activation 46 status. Metabolic pathways facilitate cellular functions, therefore linking metabolic remodeling to 47 the development, activation, differentiation, and survival of T cells. Upon activation, quiescent, 48 naïve T cells become rapidly dividing effector T (T eff ) cells and switch their metabolic program 49 from oxidative phosphorylation (OXPHOS) to aerobic glycolysis, known as the Warburg effect, in 50order to meet the increase in demand for cellular energy and biomass (1, 2). However, despite 51 having similar developmental origins, regulatory T cells (T regs ) rely predominantly on a fatty acid 52 b-oxidation (FAO)-driven OXPHOS metabolic program to maintain their suppressive phenotype, 53 which is further promoted by the expression of FoxP3 (3-5). Forced expression of FoxP3 in T cells 54 suppresses glycolysis-related genes while inducing lipid and oxidative metabolic-related genes 55 that are required for maximum suppression (4). In addition, cytokines that promote T reg 56 differentiation, such as TGF-b (6), activate AMPK (7) and promote FAO to skew naïve T cells into 57 a T reg phenotype (3, 8). Furthermore, T reg differentiation and suppression are reduced by inhibiting 58 FAO (3), highlighting the importance of FAO-driven OXPHOS in the in...