Upon activation, quiescent, naïve T cells have an increased need for cellular energy in the form of adenosine triphosphate (ATP) and biomass to meet proliferative demands and production of effector molecules. As such, the rapidly dividing cells utilize aerobic glycolysis, by which pyruvate is used to produce lactate in the presence of oxygen by cells with a great enough mitochondrial capacity to perform oxidative phosphorylation (OXPHOS), known as the Warburg effect (11, 12). T cell receptor (TCR) signals induce c-Myc, which initiates the expression of metabolic genes to induce glycolysis and mitochondrial metabolism necessary for T cell activation (13, 14). Both c-Myc and mammalian target of rapamycin (mTOR) are positive regulators of glycolysis in T cells (13, 15). mTOR exists as two complexes, mTORC1 and mTORC2. mTORC1 enhances glycolytic metabolism and can regulate other metabolic programs such as lipid synthesis and amino acid metabolism (16). mTOR can partially mediate TCR sensitivity (17), and co-stimulatory signals from CD28 have been shown to upregulate glucose utilization in T cells by recruitment of the phosphatidylinositol 3-kinase (PI3K) catalytic subunit p110γ which activates AKT and mTORC1 (18, 19).

AMP-activated protein kinase (AMPK) is another central node controlling metabolic remodeling during T cell activation. It is activated by TCR signaling via two different pathways. The first pathway is LKB1-dependent: AMPK can sense the cellular AMP-to-ATP ratio and control the progression of cells to complete activation (20). The second pathway of AMPK activation is calcium-dependent, mediated by activation of calcineurin and downstream kinase CAMKK (21) ( Figure 1 ). T cells that lack AMPK are completely dependent on glycolysis and do not re-engage mitochondrial respiration upon glucose depletion (22). This suggests that activation of AMPK in early T cell activation limits mTOR signaling and prevents premature engagement of glycolysis that is associated with the high proliferative demands of clonal expansion (23). In the mitochondria, the tricarboxylic acid (TCA) cycle creates reducing equivalents that are fed into the electron transport chain (ETC) to generate ATP via OXPHOS. Substrates are derived from multiple cytosolic and mitochondrial metabolic pathways including glycolysis and the oxidation of amino acids and fatty acids. FAO translocates free fatty acids (FFA) into the mitochondrial matrix to be broken down and produce acetyl-CoA, which enters the TCA. Acetyl-CoA can also act as a substrate for epigenetic modifications, thereby illustrating how metabolism can directly influence gene expression (24). During the first 24-48 hours after T cell activation, T cells are dependent on ATP production from ATP synthase to become fully activated and meet proliferative demands (25). This reliance on ATP is demonstrated by limited T cell activation after knockdown of the ETC complex IV subunit COX10 (26), and by the inhibition of antigen-specific T cell activation upon loss of ETC complex III subunit RISP (27).

After the initial metabolic reprogramming that occurs with T cell activation, T_eff cells use glycolysis to proliferate and maintain their effector functions (28–30). However, Th cell subsets are supported by unique metabolic needs. For example, Th1 cells rely on glycolysis and glutaminolysis to support their growth and proliferation (31), and activating naïve CD4⁺ T cells in a glutamine deficient environment generates FOXP3⁺ T_regs even in the presence of Th1 polarizing cytokines (32). Further, the by-product of glutaminolysis, α-ketoglutarate, might serve as the metabolic determinant in Th1 differentiation by promoting expression of TBET and mTORC1 signaling (32). mTORC1 has been shown to phosphorylate TBET, and inhibition of mTORC1 signaling reduces production of interferon gamma (IFN)-γ (33), highlighting the importance of glucose uptake and aerobic glycolysis in the production of IFN-γ (25, 34, 35). Th1 expression of IFN-γ production is regulated transcriptionally and post-transcriptionally (22, 25), and when activated under glucose-depleted conditions, production of IFN-γ is inhibited (25, 35–38). IFN-γ production is limited by the binding of GAPDH to the 3’ UTR of ifng mRNA, which is reversed when GAPDH is engaged in glycolysis (25). Epigenetically, glycolysis has been shown to promote IFN-γ production in Th1 cells via lactate dehydrogenase (LDHA) (35). LDHA is required to sustain aerobic glycolysis that supports Th1 differentiation, and loss of LDHA leads to reduced H3K9 acetylation at the ifng locus that is associated with active transcription and reduced glycolytic flux (35) ( Figure 2 ). Moreover, a recent study has demonstrated a role for mitochondrial metabolism in Th1 effector function by showing that mitochondrial metabolism uncouples Th1 proliferation and effector functions. The TCA cycle, and specifically, succinate dehydrogenase activity (ETC complex II), is required for Th1 effector functions including expression of IFN-γ, but suppresses proliferation and histone acetylation. Conversely, complex I of the ETC, the malate-aspartate shuttle, and mitochondrial citrate export maintain Th1 proliferation and promote histone acetylation to regulate T cell activation genes (39).

In correlation with enhanced glycolytic metabolism, in vitro-generated Th17 cells express higher levels of pyruvate dehydrogenase kinase 1 (PDK1), which is essential for Th17 differentiation in vitro (40, 41). Th17 cells differentiated in vitro utilize both OXPHOS and glycolysis, and Th17 cells from steady state and inflamed tissues rely on OXPHOS for cytokine production (40). In support of these data, the OXPHOS inhibitor oligomycin reduces Th17 pro-inflammatory cytokine production and improves pathology in mouse models of colitis (40). Evidence suggests that lipid metabolism plays an important role in the function of Th17 cells under normal and stressed conditions (42, 43), and Th17 cells rely on de novo fatty acid synthesis, rather than acquisition of extracellular fatty acids (42). While the reasons for this are unclear, it has been shown that Th17 cells are particularly dependent on cholesterol synthesis, as intermediates of the pathway can bind to the RORγt promoter and promote interleukin (IL)-17 expression, whereas inhibition of cholesterol biosynthesis blocks Th17 development (44, 45). The ratio of intracellular saturated to polyunsaturated fatty acids has been shown to control CD5L expression, which can determine whether or not Th17 cells will adopt a pro- or anti-inflammatory phenotype (46). Saturated to polyunsaturated FFA ratios also affect the expression of metabolic genes cyp51 and sc4mol, which synthesize ligands for RORγt (43, 46). Given the intricacies of Th17 and T_reg development, studies are unveiling a role for metabolic intermediates in skewing T cell lineages alongside cytokine polarization. For example, acetyl-CoA carboxylase-1 (ACC1)-deficient T cells default to the T_reg lineage (6), and mice fed a high fat diet increases the expression of ACC1 in CD4⁺ T cells, which upregulates the expression of the Th17 master regulator, RORγt (7) ( Figure 2 ). The increase of ACC1 expression in CD4⁺ T cells is mimicked in obese individuals and proposed to contribute to associated Th17 pathologies (7).

Th2 cells have been reported to display high glycolytic activity (31, 47). Ras homologue enriched in brain (Rheb) is a small GTPase that activates mTORC1. Rheb-deficient mice have impaired mTORC1 function and fail to generate Th1 and Th17 cells without alterations in Th2 differentiation. Conversely, rapamycin-insensitive companion of TOR (Rictor) is a component of the mTORC2 complex, and Rictor-deficient mice have impaired mTORC2 signaling, with differentiation into Th2 cells, but intact Th1 and Th17 generation (48). Further, downstream targets of mTORC2, SGK1 and the GTPase RhoA, have been shown to play important roles in the commitment to, and function of Th2 cells, respectively (49, 50). These data suggest mTORC2 is the signaling determinant of the Th2 lineage, not mTORC1, and that glycolytic engagement differs between Th2 and Th1 and Th17 lineages. Genome-wide transcriptional profiling of human Th2 cells from allergic asthma patients demonstrate a positive correlation between the expression of c-Myc and disease state further supporting glycolysis as a marker of Th2 pathogenicity (51). Lipids might also play a role in Th2 metabolism, as expression of peroxisome proliferator-activated receptor (PPAR)-γ is critical for Th2 responses against house dust mite antigens, and PPARγ-deficient mice display Th2 cells unable to produce IL-5 and IL-13 (51, 52) ( Figure 2 ).

In contrast to T_eff cells, T_regs appear to exhibit a greater capacity for metabolic flexibility. Murine T_regs have been described to preferentially rely on OXPHOS rather than aerobic glycolysis for ATP production (31). However, metabolic adaptations of T_reg cells are context-dependent and influenced by T_reg origin and anatomical distribution (53). It remains unclear how thymic-derived T_regs (tT_regs) and T_regs differentiated in the periphery from conventional T_eff cells, termed peripheral T_regs (pT_regs) differ metabolically. However, it appears that T_reg proliferation and effector functions are maintained through different metabolic programs. Proteomic analysis of human T_reg and T_eff cells suggests that freshly isolated T_regs are highly glycolytic and proliferative, while in vitro proliferating T_regs engage in both glycolysis and FAO (53). Conversely, T_eff cells switch from OXPHOS to aerobic glycolysis upon in vitro activation, and FAO appears to be indispensable for proliferation and effector functions (53). One explanation for the discrepancy between human and murine models is that mice are typically housed in pathogen-free environments, whereas human immune cells are constantly engaging with pathogens that requires T_regs to meet higher proliferative and functional demands (54).

AMPK and mTORC1 signals are important determinants of T_reg fitness. T_reg-specific mTOR deletion impairs T_reg fitness and homeostasis resulting in a reduction of T_regs in the tissues and T_eff cell activation and heightened inflammation at barrier sites (55). Inhibiting mTORC1 by rapamycin along with IL-2 signaling greatly expands T_reg numbers (56). Conversely, AMPK, which activates FAO and OXPHOS, provides critical signals required for T_reg differentiation (57). Activation of AMPK by metformin induces T_reg differentiation and, in murine models, was sufficient to limit progression of EAE and inflammatory bowel disease (31, 58, 59).

FOXP3 is involved in the fine tuning of metabolic cues to maintain T_reg function. Inflammatory stimuli and FOXP3 oppose each other through regulation of mTORC1 and glucose metabolism as T_reg treatment with TLR1/TLR2 agonists enhances mTORC1 activity and T_reg proliferation but reduces their suppressive capacity (60). In contrast, FOXP3 inhibits mTORC1 signaling and glycolysis while promoting OXPHOS and reduces the rate of proliferation (60), suggesting that mTORC1 activity and glycolysis are adequate for T_reg proliferation in inflammatory environments. However, excessive mTORC1 activity impairs T_reg function and indeed T_regs suppress mTORC1 activity via PP2A (48, 61, 62). Murine T_regs induced via transforming growth factor (TGF)-β express low levels of GLUT1 compared to T_eff cells (31), and in human T_regs, GLUT1 expression is also thought to be limited by FOXP3 via AKT inhibition (63). T_regs that overexpress GLUT1 exhibit reduced CD25 and FOXP3 expression and fail to suppress colitis (60). Expression of FOXP3 allows T_regs to survive in lactate rich environments by increasing the ratio of NAD to NADH (64). T_reg proliferation and suppressive functions can be uncoupled metabolically, which is likely advantageous in an inflammatory setting. For example, it might be advantageous for T_regs to engage in glycolysis to rapidly proliferate and deprive proliferating T_eff cells of an essential nutrients, then switch to an OXPHOS program for optimal suppressor functions. Of note, PI3K-mTORC2 mediated activation of glucokinase, which activates glycolysis and mediates actin cytoskeletal rearrangements, was found to be important for proper migration of T_regs in skin allograft models (65). Therefore, although glycolysis seems to play a role in T_reg migration and proliferation, utilization of OXPHOS appears to be the main metabolic program utilized for T_reg function, which is dependent on FAO (31).

Cytokines that promote T_reg differentiation such as TGF-β (66), activate AMPK (67) and promote FAO to skew naïve T cells into a T_reg phenotype instead of pro-inflammatory Th17 cells (31, 68). In support of this observation, T_reg differentiation and suppressive functions are reduced by the FAO inhibitor etomoxir (31), and suppression of PI3K activity by phosphatase and tensin homolog (PTEN) drives T_reg differentiation (69). PTEN-deficient T_regs have elevated glycolytic activity and lose FOXP3 expression and suppressive function (70). These data suggest that the balance between PTEN and AKT functions can regulate T_reg differentiation. Data also suggest a reciprocal relationship between FOXP3 and glycolysis as FOXP3 has been shown to interfere with c-Myc expression to dampen glycolytic activity (64) and forced expression of FOXP3 suppresses glycolysis-related genes while inducing lipid and oxidative metabolic genes required for maximum suppression (60). In murine models, mitochondrial complex III is required for T_reg suppression through the active metabolism of 2-HG and succinate, resulting in de-repressed transcription of T_reg suppressive genes, including PD-1, CD73, and TIGIT ( Figure 2 ). Moreover, loss of mitochondrial complex III function accumulates 2-HG and succinate, which inhibits demethylases and represses transcription of T_reg suppressive genes PD-1, CD73, Nrp1, and TIGIT. In addition, mice containing a RISP knockout, an essential component of mitochondrial complex III, die after one month exhibiting lethal inflammation as characterized by enlargement of lymph nodes and spleens along with lymphocytic infiltration into multiple organs (71).

Interestingly, in human T_regs, glycolytic activity has been linked to suppressive function via enolase-1, which mediates expression of a specific splice variant of FOXP3 that is necessary for suppression (72). However, this study also showed that the relationship between FOXP3 and metabolic intermediates changes depending on which metabolic program the cells are engaging. Upon inhibition of glycolysis with 2-DG, the non-glycolytic activity of enolase-1 represses FOXP3 expression, whereas the generation of T_regs under suboptimal TCR stimulation promotes FOXP3 expression via the glycolytic activity of enolase-1 (72) ( Figure 2 ). Lack of mTOR activity does not decrease FOXP3 expression, but it does diminish cholesterol biosynthesis that has been shown to be required for suppressive function in vivo (73). Therefore, T_regs seem to be partially dependent on mTOR and glycolytic signals that are more oscillatory in nature (74), or perhaps for maintenance of metabolic intermediates that feed into lipid related pathways, as seen with cholesterol.

Fatty acids can induce cell death in T cells (75–78). It is suggested that T_regs have specifically evolved FAO dependence to combat fatty-acid-induced cell death via FOXP3 (79). For example, long chain fatty acids like palmitate, are pro-apoptotic through various mechanisms, including depolarization of mitochondrial action potential and reactive oxygen species generation (80). This shows that FOXP3 uniquely enables T_regs to utilize fatty acids as fuel by upregulating enzymes to engage in FAO and become resistant to apoptosis (79). This evolved anti-apoptotic strategy is likely even more useful during steady state or inflammation as T_regs are important in the maintenance of homeostasis in lipid-rich tissue environments. Undoubtedly, the metabolic-functional axis in these cells is incredibly complex, and is essential to our complete understanding of disease pathology and resolution.

Overall, it is clear that T cell metabolic programs are flexible and highly dynamic. T cells can utilize different substrates based on metabolic needs, and tailor metabolic programs to support effector functions. Studies investigating Th-subset differentiation show that specific metabolic engagements promote and suppress specific Th-lineage differentiation and functions.

Lipids provide the foundation of both cellular and organelle membranes, serve as fuel for cells, and are the precursors of bioactive lipid mediators. Lipids are incredibly diverse, just like proteins; however, because of limitations in technologies to study lipids, we are just beginning to understand the diverse effects lipids exert on biological processes such as gene expression and cellular function. It is understood that the storage and secretion of lipids from the adipose tissue is one mechanism by which the body can communicate hunger and the need for energy (81, 82). However, during lipolysis and even inflammation, FFAs are exposed to immune cells that reside in the tissues and in circulation. How immune cells sense and interpret these signals remains an active area of research, as evidence is emerging that not all lipids are created equal and perform the same functions on specific cell types.

Cells obtain fatty acids from lipid species in the diet, fats stored in cells as lipid droplets, and lipids synthesized in one organ for export to another. Lipid species from the diet are ingested as triglycerides, which travel into the small intestine where they are emulsified by bile salts, and form hydrophobic structures called micelles. Together, bile salts and pancreatic lipases breakdown triglycerides into FFAs. FFAs in the intestine are transported across intestinal cell membranes, where they are converted back into triglycerides, and packaged with cholesterol and other apolipoproteins into vesicles called chylomicrons. Chylomicrons move freely from the intestines, through the lymphatic system, and into the bloodstream where they either enter the liver, or the adipose tissue. In the adipose, they will either be oxidized for fuel, or stored until they are needed. When hormones in the body signal the need for metabolic fuel, triglycerides that are stored in adipose tissue are mobilized and transported to other tissues throughout the body where the fatty acids may be oxidized for energy (81).

Similar to amino acids, there are both essential and non-essential fatty acids. Essential fatty acids comprise groups of lipids that must be acquired from the diet, as they cannot be synthesized in the body. These include omega-3 and omega-6 fatty acids. Omega-3 fats are derived from fish, eggs, and other plant based sources and are typically associated with anti-inflammatory effects (82). In contrast, omega-6 fatty acids are found various oils and animal meats and are associated with the production of inflammatory bioactive mediators (83). Of interest, these two groups of fatty acids can compete with each other for insertion into cellular membranes, where they are cleaved and converted into lipid mediators and other signaling molecules (84). Further, studies have shown due to this competition, fats in the diet reflect the overall lipid composition in cellular membranes (84–87), highlighting an important link between diet and inflammatory status. Therefore, fat sources enriched in the Westernized diet may partially explain rises in some inflammatory associated diseases.

FFAs are further characterized according to length and saturation. SCFA are typically classified as having fewer than six carbons, whereas medium chain fatty acids (MCFA) contain between six to twelve carbons, and LCFA are those with more than twelve carbons. Saturated fatty acids contain no double bonds in their carbon tail, whereas monounsaturated and polyunsaturated fatty acids contain one double bond, and more than one double bond, respectively. SCFAs are able to freely move into the mitochondria for β-oxidation, whereas LCFAs must first enter the carnitine shuttle. Of note, the majority of FFAs obtained from the diet or released from the adipose tissue are MCFA or LCFAs. On the other hand, most of the SCFA studied are those derived from bacterial metabolism and associated byproducts.

There is great focus on understanding factors that drive and regulate immune cell phenotypes and functions in tissues. Recent studies have described contributions from cytokines and hormones secreted from tissue-resident cells and other neighboring immune cells (88–91), the presence of local antigens driving immune cell persistence in the tissues (88, 92, 93), and the changes in nutrients that prompt metabolic reprogramming in specific microenvironments, such as tumors (94, 95). Evidence has shown that CD4⁺ T cell fate is driven by availability of metabolic substrates and cell-intrinsic programs that determine metabolic requirements (96). Metabolic reprogramming is critical for T cell proliferation, differentiation, and function, however, how specific substrates such as lipids, affect T cells remains unanswered. T_regs rely mainly on FAO-driven OXPHOS for survival and function (31), so lipid species in the tissues likely provide a critical signal for maintenance of T_reg survival and homeostatic functions within tissues as well. It is likely that altered lipid profiles in disease settings can impact T cell function and is of great interest to fully understand how tissue microenvironments shape T cell metabolism.

Recent data have shown that the length of fatty acids can differentially affect immune cell phenotypes. In the colon, resident gut bacteria produce SCFAs as a byproduct of fermentation (97, 98), which shapes the colonic T_reg population (99). The addition of the SCFA propionate to the drinking water of germ-free mice increases T_reg numbers in the colon, but not spleen or thymic T_reg numbers. Further, antibiotic-mediated depletion of gut bacteria reduces T_reg numbers that can be rescued with the addition of propionate to the drinking water (99). SCFAs promote FOXP3 expression and are dependent on fatty acid receptor, GPR43 (100, 101). In support of these data, the addition of SCFAs to naïve CD4⁺ T cells increases the proportion of FOXP3⁺ cells upon activation. However, the addition of LCFAs results in CD4⁺ T cells that produce more pro-inflammatory cytokines including IFN-γ and IL-17a that exacerbate EAE (5).

Unsaturated fatty acids like oleic acid and linoleic acid have been shown to modulate cytokine production in T cells (102, 103), but only saturated fatty acids induce cytokine secretion in the absence of T cell activation in a dose-dependent manner (102). In T cells, oleic acid has been shown to induce proliferation in the spleen and lymph nodes, but to inhibit Jurkat cell proliferation and IL-2 and IFN-γ production (104). In CD4⁺ T cells, fatty acid synthesis blockade by the inhibitor TOFA, reduces proliferation, which can be rescued by the addition of oleic acid specifically (105). Moreover, oleic acid can also rescue proliferation of CD4⁺ T cells cultured in fatty acid-free media (105). Interestingly, oleic acid has also been shown to decrease the content of arachidonic acid in cell membranes that result in decreased arachidonic acid-derived pro-inflammatory lipid mediators (86, 87), and to inhibit palmitic-induced inflammation in type 2 diabetes (106). Mechanistically, attenuation of palmitic acid-induced inflammation by oleic acid is linked to increases in CPT1a expression and FAO driven by AMPK activation. In adipocytes, oleic acid induces IL-10 and adiponectin expression which can activate AMPK (106). We have also shown that oleic acid is one of the most prevalent fatty acids in human adipose tissue, and is significantly decreased in the adipose tissue from MS patients (4).

In EAE models, docosahexaenoic acid (DHA) downregulates Th1 and Th17-related cytokines (107). Both DHA and eicosapentaenoic acid (EPA) have been shown to modulate the JAK-STAT pathway and IL-2 signaling in T cells (108). In Jurkat cell lines DHA exerts immunosuppressive effects by increasing calcium concentrations in cells (109–111). Similarly in vivo, mice fed a DHA- and EPA-enriched diet exhibited reduced T cell proliferation and IL-2 signaling (77, 112, 113). DHA has been also shown to induce dose-dependent reductions in the ability of T_regs to inhibit effector T cell proliferation. In contrast, DHA can upregulate FOXP3 mRNA and other immunosuppressive cytokines such as IL-10 (114).

Saturated fatty acids like palmitic acid have been shown to drive T cell activation via the PI3K/AKT pathway (5, 115, 116). Palmitic acid treatment also induces the expression of cytokines related to T cell activation such as IL-2, IL-6, IL-17A, TNF-α, and IFN-γ (102, 116). Similarly, lauric acid has been shown to have some pro-inflammatory properties, as it increases IL-2 and can promote Th17 differentiation in models of EAE (117).

The PPAR family of lipid receptors has been implicated in T cell biology, through regulation of IL-2 production (118–120), influencing Th17 and T_reg differentiation (121), and regulation of inflammation (121–126). Similarly, SREBPs are transcription factors that regulate gene expression related to lipogenesis and cholesterol synthesis and uptake (127). SREBPs are important for lipid membrane synthesis that allows rapid expansion of proliferating T cells (128). However, in the absence of fatty acids or upon blockade of FAO, T_reg development is inhibited in vitro. T_regs rely on exogenous fatty acids as their metabolic substrate (31, 42), which seems to impart a survival advantage for T_regs as FOXP3 alone upregulates enzymes associated with FAO and OXPHOS (79). Interestingly, triglyceride storage in T_regs limits protein kinase C activity to promote FOXP3 expression, potentially highlighting a feedback mechanism between exogenous fatty acid uptake in T_regs (79). Much of these data support the idea that fatty acids can differentially effect T_eff cells and T_reg subsets, but further, that specific fatty acid species have the potential to differentially affect T_reg biology. However, the mechanism driving these changes remains unknown.

Lipids play many roles in the CNS including signaling, structural support, mediating inflammation, and membrane biogenesis (9). Therefore, during inflammation changes in the availability and profiles of FFA species might alter disease pathogenesis by influencing immune cell function. In multiple sclerosis, immune cells attack the myelin sheath around nerves that are rich in lipids, containing approximately 700 different lipid species (11, 15, 16), and as a result many studies have found altered lipid profiles in the MS patients and EAE models (8, 166), that could potentially serve as biomarkers of the disease. At the time of diagnosis, the CSF of MS patients harbor differences in many lipid species, including pro-inflammatory arachidonic acid, compared to the non-MS control group (167). In support of these data, lipid profiling of mouse EAE brains showed that during acute inflammation, lipid metabolism is shifted from the pathway that creates common lipid substrates into the pathway producing the pro-inflammatory arachidonic acid (168). Studies profiling the phospholipids of serum samples from patients with MS have also demonstrated that patients with MS have a phospholipidomic signature different from that of healthy controls (169).

As with infections and metabolic syndromes, the inflammation induced in autoimmunity triggers metabolic alterations locally and systemically (166). In the blood of patients with MS and other autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus, there are elevated levels of IL-1β, IL-6, TGF-β, and IL-23, which promote pathogenic Th17 cell differentiation and block T_reg development in vivo and in vitro (170–176), proving that there is a disturbance in the balance between the induction and regulation of pro-inflammatory T_eff cells. Although the metabolic cues that shift this balance are still being defined, there is evidence that glycolysis and OXPHOS are impaired during T cell activation in relapsing-remitting MS patients (177).

As mentioned above, glycolysis is a common feature of Th1 and Th17 CD4⁺ T_eff cells (13, 47). Inhibiting glucose metabolism can inhibit differentiation of Th1 and Th17 lineages even in the presence of polarizing cytokine (31, 47). Further, blocking glycolysis using 2-deoxyglucose (2-DG) improves clinical outcomes in EAE models (47) ( Figure 3 ). A direct link between dysregulated glucose metabolism in lymphocytes and autoimmunity was shown in mice that over-express GLUT1 in T cells (178). Increased glucose uptake drives IFN-γ and IL-2 expression in T cells upon in vitro stimulation, and deletion of GLUT1 reduces glycolysis in CD4⁺ T cells, and effector differentiation. These cells are also unable to provide protection in a model of colitis (34). Further, CD4⁺ T cells in lupus-prone mice show increases in glycolytic engagement compared to healthy control cells (179).

Some studies have reported alterations in T cell lipid metabolism as a contributor to MS pathogenesis (180–183). Activated T cells have higher cholesterol and fatty acid concentrations in their membrane (184, 185), therefore altered lipid-mediated signaling could contribute to MS pathogenesis (186). Specific deletion of ACC1 in T cells protects mice from EAE by reducing the number of IFN-γ⁺ Th17 cells and increases the number of FOXP3⁺ T_regs in the spinal cord (42). Conditional deletion of LDHA in T cells protects susceptible mice from Th1-mediated autoimmunity (35). Simvastatin, an inhibitor of cholesterol biosynthesis, was used in a clinical trial to treat MS and was able to attenuate brain atrophy and disease progression (187). In support, during the chronic phase of EAE, linoleic and cholesterol metabolism have been found to be altered (9, 10), and inhibition of cholesterol synthesis via statins, prevented EAE progression by blocking Th17 differentiation (188). However, statins broadly affect the immune system, including promotion of Th2 cells and inhibition of the cytokines that induce Th1 and Th17 differentiation, and therefore their effect on MS might not be directly related to their effect on cholesterol (187–189).

Long chain fatty acids (LCFAs) and short chain fatty acids (SCFAs) have been shown to exert opposing effects on T cell differentiation. LCFAs promote IFN-γ and IL-17 production and exacerbate EAE, whereas SCFAs induce FOXP3 expression and provide protection (5). We have recently shown that T_regs isolated from the adipose tissue of MS patients exhibit a pro-inflammatory transcriptome that resembles the transcriptome of T_regs treated in vitro with the pro-inflammatory LCFA arachidonic acid (4). T_regs isolated from the adipose of healthy subjects and T_regs exposed to oleic acid in vitro do not show an inflammatory transcriptome, but rather exhibit increased FAO-driven OXPHOS metabolism, creating a positive feedback mechanism that induces the expression of FOXP3 and enhances phosphorylation of STAT5, which acts to stabilize the T_reg lineage and increase suppressive function (4). Finally, treating dysfunctional MS T_regs with oleic acid can partially restore their suppressive function, highlighting a role for dietary lipids in shaping T_reg function and identity in MS (4, 92, 190, 191) ( Figure 3 ). Further studies must address how metabolic perturbations generate and sustain pathogenic T cells in the context of MS and other autoimmune diseases. Metabolic signatures of T cells are intricately linked to their differentiation and activation status. However, under inflammatory conditions such as infection or during metabolic syndromes, autoimmunity, and cancer, the microenvironment drastically changes in regard to nutritional availability, forcing T cells to adapt to these changing environments.

Future studies focusing on how nutrient depletion or metabolite availability shapes metabolic utilization and differentiation of T_eff and T_reg cells will be critical to our understanding and implementation of targeted therapies. For example, Shi et al. have shown that blocking glycolysis improves clinical outcomes of EAE (47), and disease-modulatory effects of feeding mice both LCFAs and SCFAs have been observed (5). However, to realistically translate these data to human therapies we must first develop more reliable metabolic profiles of T cells and other cells involved in disease pathogenesis and progression during active disease states. Once established, we can couple these data to cytokine profiles or other functional readouts, e.g. suppressive capacity of T_regs, in order to more precisely define relevant metabolic targets. One current limitation of metabolic therapies is that the target molecules are often the regulators of central metabolic nodes that integrate many signals and crosstalk between multiple pathways, such as mTOR. However, the ability to fine tune T cell functions and phenotypes that are more favorable for disease outcomes will depend on specifically targeting certain metabolic intermediates.