TCR stimulation caused dynamic changes in the intracellular metabolism, including glucose, amino acid, intermediates of TCA cycle, and fatty acid metabolism to fuel biosynthetic processes.
To evaluate the changes in metabolic pathways during T cell differentiation, we first analyzed the amounts of intracellular metabolites and expression of metabolic proteins using a mass-spectrum-based metabolomics approach and previously established proteogenomic database19 (Fig. 1a). Naïve CD4+ T cells were stimulated with immobilized anti-TCR mAb and anti-CD28 mAb for 48 h under Th0, Th1, Th2, Th17, or iTreg cell culture conditions. Dead cell removal was performed before sample preparation for omics analysis. As reported previously, our omics analysis showed that TCR stimulation elicited a rapid induction of glycolysis at the level of metabolites and proteins6,7 (Fig. 1b). The intermediate of glycolysis, G3P, was decreased, while the end product, lactate, displayed a substantial increase upon TCR stimulation (Fig. 1b). Proteomics data revealed significant increases in the expression of glucose transporters and glycolytic enzymes, including GLUT1, GLUT3, HK1, GPI, PKM, and LDHA. Furthermore, we observed that TCR stimulation enhanced the utilization of glucogenic amino acids synthesis, which was synthesized at lower levels in resting naïve CD4+ T cells (Fig. 1b lower panel, and Supplementary Fig. 1a). While the amount of serine was reduced by 1.5-fold in Th0, Th1, and Th2 cells and increased by 1.5-fold in Th17 and iTreg cells, the amount of glycine and cysteine was significantly elevated in effector Th cell subsets (Fig. 1b, upper panel). In accordance with this result, protein expression of glucogenic amino acid synthase was highly upregulated. These amino acids serve as a primary source for the one carbon pathway, an essential component for the biosynthesis of both proteins and DNA. Thus, these findings suggest that activated Th cells acquire one carbon metabolism to support their rapid proliferation. Next, we focused on TCA cycle and its associated metabolites. Despite observed minimal variation in the intermediates of the TCA cycle, such as succinate, fumarate, malate, and citrate, between naïve CD4+ T cells and effector Th cell subsets, the amount of glutamine was greatly reduced following TCR stimulation (Fig. 1c). This finding was supported by the decrease in expression of the enzyme glutamine synthase (GLUL) and the concurrent increase of degradative enzyme (GLS2) in effector Th cell subsets, demonstrating the validity of our multi-omics analysis in reflecting the activation of glutaminolysis in effector Th cells20. In addition, the level of glutamate was increased in Th17 cells, whereas no significant change was observed in the amount of glutamate in the other effector Th cell subsets. Furthermore, there was a significant increase in the levels of several amino acids, including alanine, lysine leucine, threonine, aspartate, asparagine, phenylalanine, tyrosine, and proline. Correspondingly, we also noted an up-regulation in the expression of their synthase and transporters, indicating that amino acid biosynthesis and uptake programs were activated during T cell activation (Figs. 1b and 1c, and Supplementary Fig. 1b). It is noteworthy that we observed a marked increase in the amount of β-alanine, a crucial component for the production of acetyl-CoA, in effector Th cell subsets (Fig. 1d). Since acetyl-CoA was required for the fatty acid biosynthesis, we next investigated the effect of TCR-stimulation on fatty acid biosynthesis pathway. Initially, we detected minor alterations in the expression of enzymes associated with fatty acid elongation, such as ELOVL1, ELOVL5, ELOVL6, and ELOVL7 in Th cell subsets (Fig. 1e). In contrast, TCR stimulation resulted in a marked upregulation of proteins involved in fatty acid desaturation, including SCD1, SCD2, FADS1, and FADS2 (Fig. 1e). Subsequently, we sought to assess the levels of fatty acid saturation, focusing on saturated fatty acid (SFA), monounsaturated fatty acid (MUFA), and polyunsaturated fatty acid (PUFA). In quiescent naïve CD4+ T cells, SFA accounted for 92.5% of total fatty acids, while MUFAs and PUFAs comprised relatively smaller proportions (MUFA: 2.68%, PUFA: 4.85%) (Fig. 1f). Activated Th cells exhibited changes in the degree of fatty acid saturation, with the MUFA composition of activated Th cell subsets increasing to 26–37% (Fig. 1f). These findings are reflected in the observation that while most fatty acids were largely increased by TCR-stimulation, the amount of MUFAs increased the most (Fig. 1g). Taken together, these data indicate that TCR stimulation confers activated Th cells with an increased ability to engage in glycolysis and amino acid synthesis and generates a diversity of fatty acid composition.
Effector Th cell subsets altered the composition of cellular lipids compared to naïve CD4 + T cells.
Since activated Th cells dramatically altered fatty acid metabolism, we next analyzed lipidomic data to assess the changes in cellular lipid profiles after TCR stimulation. Our comprehensive cellular lipidomics analysis identified a total of 567 lipid species. These lipids were further classified based on their structure into cholesterol, free fatty acids, glycerolipids, lysophospholipids, phospholipids, and sphingolipids, with 27, 37, 132, 66, 196, and 109 species, respectively (Fig. 2a). We first observed that activated Th cells increased the overall quantity of these lipid species to over twice that of naïve CD4+ T cells (Fig. 2b). A deeper analysis revealed that the amount of most lipid species was largely increased in Th cell subsets as compared to naive CD4+ T cells, the numbers of which were 504, 504, 487, 522, and 511 in Th0, Th1, Th2, Th17, and iTreg cells, respectively (Figs. 2c and 2d). A principal-component analysis (PCA) demonstrated that TCR-mediated activation had a significant impact on cellular lipid profiles (Fig. 2e). Additionally, Th17 and iTreg cells formed a cluster that was distinct from other activated Th cells including Th0, Th1, and Th2 cells. The stimulation of TCR not only led to the upregulation of lipid biosynthesis, but also resulted in a change in cellular lipid composition as well as free fatty acid levels as shown in Fig. 1g. The TCR stimulation increased the ratio of glycerolipids with values of 15.4% for naïve CD4+ T cells, 28% for Th0 cells, 28.3% for Th1 cells, 27.1% for Th2 cells, 23.9% for Th17 cells, and 24.7% for iTreg cells (Fig. 2f). In a group of glycerolipid, effector Th cell subsets increased the proportion of triacylglycerol (TAG), which are incorporated into lipid droplet as energy storage with values of 26.2% for naïve CD4+ T cells, 54.4% for Th0 cells, 52.3% for Th1 cells, 57.1% for Th2 cells, 44.9% for Th17 cells, and 51.0% for iTreg cell (Fig. 2g). The stimulation of TCR has only a minor impact on the composition of cholesteryl ester (ChE) (as depicted in Fig. 2h, with the percentages of 91.2% for naïve CD4+ T cells, 83.1% for Th0 cells, 84.0% for Th1 cells, 88.3% for Th2 cells, 88.3% for Th17 cells, and 86.0% for iTreg cells). The percentage of phosphatidylcholine (PC) was nearly comparable across each T cell subset (as depicted in Fig. 2i, with the percentages of 37.5% for naïve CD4+ T cells, 46.0% for Th0 cells, 46.6% for Th1 cells, 45.8% for Th2 cells, 46.2% for Th17 cells, and 46.3% for iTreg cells). In comparison to naive CD4+ T cells, activated Th cells were found to alter the composition of phosphatidylethanolamine (PE) and phosphatidylserine (PS) from 27.0% to 14–16% or from 11.1% to 6–9%, respectively (Fig. 2i). We also observed substantial changes in the category of lysophospholipids and sphingolipids. Naïve CD4+ T cells exhibited the greatest proportion of LPC, yet TCR stimulation resulted in the most prominent lipid shifting from LPC to LPI. In naïve CD4+ T cells, LPC and LPI comprised 53.0% and 7.83%, respectively. In activated Th cell subsets, LPC and LPI occupied 23–34% or 27–39% of total lysophospholipids, respectively. TCR stimulation also largely changed the composition of cellular sphingolipids, with the most abundant lipid shifting from sphingomyelin (SM) to ceramide (Cer). In naïve CD4+ T cells, SM and Cer comprised 58.0% and 8.34%, respectively, while in activated Th cells subsets, SM and Cer occupied 14–20% or 34–43% of total sphingolipids, respectively (Fig. 2k). In addition, TCR-stimulation led to the production of sphingoglycolipids, including ganglioside GM1 (GM1) and ganglioside GD1 (GD1) (Fig. 2k). These findings demonstrate that T-cell activation dynamically changes the quantity and composition of cellular lipids, which suggests the establishment of a metabolic environment favorable to the regulation of T cell homeostasis.
The activation of a specific Th cell subset resulted in a significant change of metabolites and proteins involved in the biosynthesis pathway of sphingolipids and glycosphingolipids
Subsequently, we seek to determine the species of lipids that influence the generation of effector Th cell subsets. To this end, we conducted an initial evaluation of the expression of 269 proteins involved in lipid metabolism in naïve CD4+ T cells, Th0, Th1, Th2, Th17, and iTreg cells. Our proteomic analysis revealed that protein expression levels related to lipid metabolism underwent dynamic changes following TCR stimulation (Figs. 3a and 3b). As indicated by the PCA plots of lipid metabolites (Fig. 2e), Th0, Th1, and Th2 cells displayed similar protein profiles, whereas Th17 and iTreg cells exhibited greater similarity to each other than to the other cell states within the PCA space. We then sought to identify proteins that were commonly changed among the various T cell subsets. Our observations indicated that the expression of diacylglycerol kinase (DGKA, DGKB, DGKZ), phosphatidylinositol kinase (PIK3CB, PIK3CD, PIP4K2A, PIP4K2B), and phospholipase (PLBD1, PLCB2, PLCB3, PLCL1) was decreased in a common manner among effector Th cell subsets (Supplementary Fig. 2a). Each Th cell subset commonly upregulated proteins that are essential for the biosynthesis of fatty acids (ACC1, FASN), elongation of fatty acids (ELOVL1, ELOVL6), desaturation of fatty acids (FADS1, FADS2, SCD2), fatty acid binding protein (FABP5), and cholesterol synthesis (HMGCS1 and HMGCR) (Fig. 3c). Especially, the proteins predominantly associated with the biosynthesis of lipids, primarily sphingolipids, that were commonly upregulated include CERS5, DEGS1, and B4GALT5 (Fig. 3c). In accordance with upregulation of these proteins, the quantity of ceramides (Cer) was considerably increased in effector T cell subsets (Fig. 3d). Lipidomics analysis further revealed substantial elevations in the amounts of cardiolipin (CL) and phosphatidylglycerol (PG) in glycerophospholipid, bis monoacylglycero phosphate (BMP), LPI, and LPG in lysophospholipid, TAG in glycerolipid (Supplementary Figs. 2b-2d). Nevertheless, the synthases of these lipids were not among the commonly altered proteins in effector Th cell subsets except for LPGAT1. Thus, we next focused on the sphingolipid synthesis pathway for the following reasons: Proteomic analysis indicated that proteins related to the sphingolipid synthesis pathway were upregulated in a general manner, and lipidomic analysis showed substantial increase in the quantity of Cer and significant changes in the composition of sphingolipids. Subsequently, we conducted a detailed evaluation of the changes in sphingolipids during T-cell activation. We first observed that the amounts of ceramides and their precursor, sphinganine, were greatly increased (Fig. 3d). Cer serves as the hub of sphingolipid metabolism and is responsible for the generation of sphingomyelin, sphingosine, and glycosphingolipids. Although the amount of SM remained largely unchanged, there were increases in the amounts of hexosyl Cer (HexCer) and di-hexosyl Cer (DiHexCer), which are types of glycosphingolipids (Fig. 3d). In addition, although many gangliosides, which are metabolites of these glycosphingolipids, were barely detectable in naive CD4+ T cells, effector Th cell subsets increased the levels of ganglioside, such as GM2, GM3 and GT1a (Fig. 3e). Taken together, these data suggest that TCR activation enhances the utilization of the sphingolipid synthesis pathway and confers the regulation of glycosphingolipid metabolism.
Activated sphingolipid metabolism is required for the proper generation of Th17 and iTreg cells
Next, to explore the influence of sphingolipid biosynthesis on the differentiation of effector Th cell subsets, we inhibited sphingolipid production through the utilization of myriocin, a molecule known to inhibit the initial step of sphingolipid biosynthesis. Our observations indicate that such inhibition of sphingolipid biosynthesis resulted in a moderate decline in the number of iTreg cells and a substantial reduction in the number of Th17 cells (Fig. 4a). However, no significant changes were observed in the number of Th0, Th1, and Th2 cells (Supplementary Fig. 3a). These finding are in consistent with our subsequent observations, which showed myriocin-treated Th17 cells were unable to undergo TCR stimulation-induced cell division (Figs. 4b and 4c). In addition, the inhibition of sphingolipid biosynthesis caused a moderate suppression of cell proliferation in iTreg cells (Figs. 4b and 4c). Subsequently, the effect of myriocin on cell cycle progression was analyzed via the evaluation of BrdU incorporation into Th17 and iTreg cells. As a consequence, myriocin treatment prevented Th17 and iTreg cells from entering the S-phase following TCR stimulation (Figs. 4d and 4e). Furthermore, treatment of myriocin resulted in the inhibition of the cytokine production of Th17 cells, with no effect observed in Th0, Th1, and Th2 cells (Figs. 4f and 4g, and Supplementary Figs. 3b-3d). Our findings also indicated that the inhibition of sphingolipid biosynthesis led to a decrease in PD-1 expression on iTreg cells (Figs. 4h and 4i). The genetic deletion of Sptlc1 and Sptlc2, which represent a pharmacological target of myriocin, moderately suppressed the differentiation of Th17 cells and the expression of PD1 on iTreg cells (Figs. 4j-4m, Supplementary Figs. 3e and 3f). Collectively, our combined analysis demonstrates that effector Th cell subsets exhibit enhanced sphingolipid and glycosphingolipid metabolism. Additionally, the generation of both Th17 and iTreg cells was inhibited by the ceramide biosynthesis inhibitor.
Glycosphingolipid metabolism was facilitated in Th17 and iTreg cells compared to Th0, Th1, and Th2 cells.
The inhibition of sphingolipid biosynthesis selectively suppressed the generation of Th17 and iTreg cells, suggesting characteristic differences in cellular lipid metabolism between these effector Th cell subsets and Th0, Th1, and Th2 cells. To further investigate this cellar metabolic profile, we examined global lipidomic profiling and analyzed protein expression related to lipid metabolism among the various effector Th cells subsets. Results from PCA revealed that Th0, Th1, and Th2 cells showed similar lipid and protein profiles, while Th17 and iTreg cells were more alike to each other than to the other cell states in PCA space (Figs. 5a and 5b). The analysis also revealed elevated amounts of sphingolipids in Th17 and iTreg cells compared to non-polarizing Th0 cells, as well as decreased levels of 3-ketosphinganine, an intermediate in sphingolipid synthesis, in Th17 and iTreg cells (Figs. 5c and 5d). These results led us to focus on sphingolipids and their downstream metabolic pathways, where we observed increased levels of HexCer, DiHexCer, GD1, and GD1[O-acetyl NeuAc] (AcGD1) (Fig. 5e). Although total amount of ceramide lipid levels did not significantly differ in Th17 and iTreg cells compared to Th0 cells, a closer examination revealed that the proportion of individual ceramide species in the most of HexCer, DiHexCer, GD1, and AcGD1 was altered in Th17 and iTreg cells (Figs. 5f-5h). Furthermore, inhibition of glycosphingolipid through pharmacological inhibitor, Genz-123346, suppressed the number of Th17 and iTreg cells as well as myriocin treatment (Figs. 5i-5k and Supplementary Figs. 4a and 4b). In accordance with these data, Genz-123346 inhibited the cytokine production of Th17 cells and PD1 expression on Treg cells, further demonstrating the importance of the glycospingolipid biosynthesis pathway in the differentiation of Th17 and iTreg cells (Figs. 5l and 5m, and Supplementary Figs. 4c and 4d).