Thymic Tregs (tTregs) originate during T cell ontogeny in the thymus and are selected for high-affinity recognition of self-MHC molecules without crossing the threshold for deletion. Peripheral Tregs (pTregs) arise from conventional CD4⁺ T cells in vivo under tolerogenic conditions, while induced Tregs (iTregs) are generated in vitro from naive CD4⁺ T cells cultured in the presence of IL-2 and TGF-β (21, 22). Interestingly, tTregs and iTregs display distinct metabolic signatures (Figures 1A, B). Indeed, mouse tTregs cultured in IL-2 for three days exhibit high glycolytic activity, whereas iTregs induced with IL-2 and TGF-β display reduced glycolysis. Enhanced glycolysis in tTregs is associated with increased expression of glycolytic regulators such as HK2, Glut1, and Glut3. Addition of TGF-β to tTregs suppresses glycolysis, as indicated by reduced extracellular acidification rate (ECAR), decreased expression of glycolytic enzymes, and inhibition of the PI3K/Akt/mTOR pathway, which collectively impair their proliferation and suppressive capacity (23). In contrast, CD4-specific Glut1 deletion reduces conventional Foxp3⁻ CD4⁺ T cells but has little effect on Foxp3⁺ Tregs, suggesting that tTregs can rely on alternative metabolic pathways (24). These findings highlight the complexity of Treg metabolic requirements.

Glycolysis provides rapid ATP and biosynthetic intermediates necessary for cell growth. Previous studies have emphasized the importance of mTOR activation as an evolutionarily conserved requirement for T cell development and differentiation (25, 26). TLR1 and TLR2 signaling induce the PI3K–Akt–mTORC1 axis, which enhances mouse iTreg cells’ proliferation and glycolytic activity, as measured by increased ECAR and Glut1 expression (Figure 1B). However, these highly glycolytic iTregs show reduced suppressive capacity (27). Foxp3 itself appears to antagonize glycolysis (17). Ectopic expression of Foxp3 in naive T cells reduces glucose uptake, GLUT1 and HK2 expression, and Akt/mTOR phosphorylation, while increasing oxygen consumption and pyruvate oxidation. This metabolic shift diverts cells toward OXPHOS, supporting suppressive function rather than effector activity (27). Moreover, HIF-1α, a transcription factor stabilized under hypoxic and glycolytic conditions, favors Th17 differentiation while inhibiting Treg generation, illustrating the reciprocal balance between metabolic and immunological fates (28). While mTOR signaling is dispensable for iTreg differentiation in vitro (29), in vivo studies reveal its necessity for maintaining pTregs. It also promotes activation of peripheral tTreg to effector Treg status through regulation of IRF4, particularly in mucosal tissues which further promote glycolysis and nucleotide metabolism (30)(Figure 1A). In human Tregs, glycolysis is also critical. Freshly isolated, unmanipulated human Tregs demonstrate a higher dependence on glycolysis and express higher levels of mTOR compared to Tconv cells. Inhibition of mTOR using 2-DG impairs the suppressive function of Treg and reduces Foxp3 expression level in human Treg (31). Deletion of mTORC1 in Tregs leads to severe inflammatory disease, reduced mitochondrial fitness, and impaired metabolic reprogramming (32). These findings underscore that glycolysis can support Treg proliferation but may compromise suppressive function unless carefully balanced by other pathways.

Beyond glycolysis, Tregs rely heavily on mitochondrial metabolism (Figure 1C). Deficiency in respiratory chain complex III within Treg cells leads to the accumulation of succinate and 2-hydroxyglutarate (2-HG) (33). These metabolites competitively inhibit TET enzymes by displacing their cofactor, α-ketoglutarate, causing hypermethylation and subsequent silencing of key immunosuppressive genes like PD-1, CD73, and TIGIT (34, 35). Ultimately, this epigenetic silencing impairs the cell’s ability to produce critical co-inhibitory molecules and ectoenzymes, severely decreasing the Treg’s immunosuppressive function. Foxp3 itself directly links transcriptional programming to mitochondrial metabolism. By promoting OXPHOS and FAO, Foxp3 enhances spare respiratory capacity and prevents fatty acid-induced apoptosis. Inhibition of fatty acid binding protein 5 (FABP5) disrupts mitochondrial integrity, reduces OXPHOS, and skews Treg metabolism toward glycolysis, compromising their suppressive function (36).

Liver kinase B1 (LKB1), also known as STK11, is a key metabolic sensor in Tregs. LKB1 deficiency leads to reduced mitochondrial oxygen consumption, ATP production, and mitochondrial mass, resulting in impaired Treg survival and function both in vitro and in vivo (37, 38). Metabolomic profiling shows that TCA cycle intermediates and fatty acid oxidation (FAO) products are markedly reduced upon LKB1 deletion (39). LKB1 also regulates cholesterol biosynthesis via the mevalonate pathway. Tregs lacking LKB1 or the key enzyme HMGCR show impaired lipid metabolism, increased inflammatory cytokine production, and systemic autoimmunity, which can be partially rescued by mevalonate supplementation (38). These results emphasize that lipid metabolism is not only a source of bioenergetics but also provides critical intermediates for maintaining immune tolerance. CD36, a fatty acid transporter, has been identified as another important metabolic adaptation in intratumoral Tregs. By facilitating uptake of long-chain fatty acids and oxidized lipoproteins, CD36 enhances PPAR-β signaling, mitochondrial fitness, and NAD⁺/NADH balance, thereby promoting Tregs survival in nutrient-poor tumor microenvironments (40). Together, these findings demonstrate that Tregs require intact mitochondrial and lipid metabolic pathways for long-term survival, stability, and function.

Treg metabolism is highly sensitive to external cues. Nutrient availability, oxygen tension, and microbial metabolites all shape Tregs function. Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate produced by gut microbiota, promote Tregs differentiation via epigenetic regulation of Foxp3 and enhanced FAO (26). Propionate facilitates Treg differentiation in vitro via the GPR43-HDAC6 axis, which correlates with an elevated Treg/Th17 ratio in vivo in mice (41). Similarly, butyrate and its derivatives exert potent pro-regulatory effects. Treatment with methyl butyrate fosters Treg development and IL-10 secretion in the draining lymph nodes of EAE mice and also directly promotes Treg differentiation in vitro (42). Furthermore, evidence indicates that butyrate itself helps regulate the Th17/Treg balance by activating PPARγ, a process which subsequently promotes the upregulation of CD36 and Foxp3 in vitro (43) (Figure 1C). Tryptophan metabolites, generated by commensals through the indoleamine-2,3-dioxygenase (IDO) pathway, also favor Treg stability while suppressing effector T cell responses. These findings highlight that Treg metabolism is not cell-autonomous but is deeply influenced by interactions with the microbiome and tissue-specific niches. Such insights raise the possibility that microbiome-targeted therapies could modulate Treg metabolism to treat inflammatory diseases.

Metabolic dysregulation of Tregs contributes to autoimmune pathology (Figures 2A–C). For example, constitutive Glut1 activation in Tregs reduces Foxp3 expression and impairs suppression in colitis models (27), whereas Glut1 deletion has little effect on Treg function (24). Obesity is associated with increased mTOR activation, glycolytic gene expression, and reduced Treg frequency in adipose tissue, contributing to insulin resistance and chronic inflammation (44). In multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), Tregs display impaired mitochondrial metabolism, reduced spare respiratory capacity, and altered lipid handling, correlating with disease severity. Importantly, evidence from human studies highlights a distinct role for glycolysis in maintaining Treg stability. De Rosa et al. demonstrated that glycolysis controls the induction of human Tregs by modulating FOXP3 exon 2 splicing variants (45). Impaired glycolysis and reduced Foxp3 exon 2 (Foxp3-E2) expression were also observed in iTregs from patients with relapsing-remitting multiple sclerosis (RRMS) and type 1 diabetes (T1D) (45). Studies have shown that an increase of metabolic sensors such as HIF-1α is responsible for Th17/Treg imbalance that causes disease pathogenesis in EAE (46). Finally, mice models with Treg-specific deletion of LKB1 further demonstrating the relationship between metabolic dysregulation in Treg, Treg stability, and function in inducing immune imbalance (47). These observations underscore the potential of targeting Treg metabolism to restore tolerance in autoimmunity.

In the tumor microenvironment (TME), Tregs adapt their metabolism to survive under hypoxic and nutrient-poor conditions (Figures 2A, B). Unlike effector T cells, which suffer from nutrient competition and exhaustion, mouse Tumor-infiltrating Tregs (TI-Tregs) upregulate lactate transporters (MCT1) and lactate dehydrogenase (LDh) to metabolize extracellular lactate into pyruvate for OXPHOS (48). Additionally, TI-Tregs also utilize Lactate to generate Phosphoenolpyruvate (PEP), a precursor of pyruvate suggesting increased gluconeogenesis to fuel their proliferation (48). Interestingly, mouse TI-Tregs upregulate HIF-1α due to hypoxic conditions, which enhances their glycolytic-driven migration but compromises their suppressive function (49, 50). To maintain suppressive function, mouse TI-Tregs utilize fatty acid and lipid metabolism; indeed, TI-Tregs upregulate CD36 expression, which enhances fatty acid oxidation and the PPAR signaling pathway responsible for mitochondrial fitness and adaptation to lipid metabolism (40). In humans, TI-Tregs also displayed a gene signature oriented to glycolysis and lipid biosynthesis (51). Additionally, mouse studies also show that TI-Tregs upregulate SCAP/SREBP signaling, leading to de novo lipid biosynthesis, including fatty acids and cholesterols (52, 53). To withstand the inevitable ROS build-up associated with increased lipid metabolism, TI-Tregs upregulate antioxidant molecule synthesis such as serine-dependent glutathione (GSH) synthesis (50, 54). This mechanism is conserved in humans, as pharmacological inhibition of GSH synthesis in human iTregs led to loss of suppression and reduced FoxP3 expression (54). Furthermore, TI-Tregs compete with effector T cell to uptake the limited glucose in the TME by upregulating GLUT1 and GLUT3 receptors (50). These adaptations allow them to maintain suppressive capacity and dampen anti-tumor immunity, representing a major barrier to immunotherapy. Lipid metabolism also plays a role in TI-Tregs; TI-Tregs display a metabolic preference for lipid utilization compared to their circulating counterparts in different types of human cancer. Functional studies also confirm that TI-Tregs uptake more fatty acids in mouse cancer models (40). Targeting Treg metabolism, for example, through inhibition of CD36 or MCT1, is therefore being explored as a strategy to selectively impair tumor-infiltrating Tregs while sparing systemic tolerance.

In chronic viral infections such as HIV and HCV, Tregs expand and display altered metabolic signatures, contributing to viral persistence by suppressing effector T cell responses (Figure 2C). In chronic HIV patients undergoing AntiRetroviral Therapy (ART), a study revealed that gut Treg cells from patients had upregulated glycolysis-related genes while downregulating genes associated with OXPHOS and FAO, leading to Treg cells expansion with decreased function (55). A study on Tregs of Long-term non-progressor (LTNPs), a small group of HIV-infected individuals with decreased viral load without ART, has increased glycolytic genes such as Myc and HIF-1α and glycolytic receptors such as Glut1, while having decreased FAO and PPAR signaling, which suggests increased expansion but decreased Treg suppressive capacity (56). Conversely, in acute infection, Tregs may prevent immunopathology by limiting excessive inflammation (57). Yet, there is a large gap in knowledge regarding establishing a direct relationship between alteration of Treg metabolism under chronic and acute infections. Such investigations can provide bases for strategic manipulation of Treg metabolism to balance pathogen clearance with tissue protection.

The field of Treg metabolism is rapidly evolving, with several promising directions. Therapeutic modulation of key metabolic pathways, such as mTOR, AMPK, and FAO, offers the potential to enhance or suppress Treg function depending on the disease context; for instance, metformin, an AMPK activator, promotes FAO and supports Treg survival in autoimmune settings. However, systemic manipulation of metabolism poses risks due to widespread effects, highlighting the need for precision targeting strategies that are tissue-specific or context-dependent. Ultimately, these insights hold strong potential for clinical translation, informing the design of adoptive Treg therapies for autoimmunity and transplantation, as well as strategies for selectively targeting Tregs to improve cancer immunotherapy.