**5. Cellular immunometabolism of Th17 cells, their development and pathogenicity: how intracellular metabolism plays a fundamental role in determining plasticity of Th17 cells**

Within Th17 cell subset, depending on the presence of further local stimulatory cues (metabolites), there exists substantial functional and molecular heterogeneity

### *Th17/IL-17, Immunometabolism and Psoriatic Disease: A Pathological Trifecta DOI: http://dx.doi.org/10.5772/intechopen.102633*

determining the generation of pathogenic or non-pathogenic Th17 cells [18]. Due to the shared developmental requirement of TGF-β and due to functional and physical interaction of master transcriptional factors, i.e. RORγt and Foxp3 regulating Th17 and Treg respectively, these cells are capable of transdifferentiating into each other. The reciprocal metabolic cues are fundamental in shaping the relative proportions of Th17 *vs.* T reg cells and non-pathogenic *vs.* pathogenic Th17 cells, affecting Th17 cell plasticity and pathogenicity. Essentially, the active Th17 cells utilize the faster ATP-producing, oxygen-independent pathways, while the Treg cells utilize the more efficient, if slower, oxidative pathways.

Metabolically, Th17 cells are characterized by *"glycolytic-lipogenic-glutaminolytic"* anabolic phenotype with highly active PPP, ensuring the availability of biosynthetic precursors [50].

### **5.1 Glycolysis**

T cell receptor (TCR) ligation and CD28 co-stimulatory signals induce PI3K dependent phosphorylation of Akt that activates key metabolic regulator mTOR (selective role of mTORC1 but not mTORC2 in Th17 differentiation) leading to increased glycolysis (**Figure 3**). Under Th17-polarizing conditions, the PI3K-Akt/ mTORC1/HIF-1α/c-MYC axis activates a series of reactions shifting the Th17/Treg cell balance in favor of Th17 cells. HIF-1α drives Th17 differentiation while simultaneously suppressing Treg induction *via* its differential interaction with transcription factors RORγt and Foxp3 causing transactivation of the former and proteasomal degradation of the latter [51]. HIF-1α doubly enhances this response: firstly, it binds to hypoxia response element (HRE) located in the proximal region of the *RORC* gene promoter (**Figure 1**) and secondly, it might physically associate with RORγt transcription factor, serving as a coactivator for RORγt, thereby increasing *IL-17* gene expression without direct DNA binding on the *IL-17* gene locus [7]. HIF-1α is an essential facilitator of the acquisition of Th17 glycolytic metabolism as shown in **Figure 3** as it enhances expression of a series of glycolytic enzymes including GLUT1 (central glucose transporter on T cells) leading to robust glucose uptake, hexokinase 2 (HK), pyruvate kinase muscle enzyme (PKM2) and lactate dehydrogenase (LDH) causing a shift to aerobic glycolysis. The enzyme PDHK1 that inactivates the PDH enzyme, has been identified as an important player in selective regulation of Th17 cell differentiation and inflammation as evidenced by higher levels of PDHK1 expression on Th17 cells [52]. The transcription factor, inducible cAMP early repressor (ICER, an endogenous repressor of cAMP-responsive element {CRE})-mediated gene transcription, plays a vital role in deciphering Th17 cell biology. It has been shown to be overexpressed in Th17 cells, binds to and suppresses PDHPs, reducing PDH activity thereby enhancing glycolysis, and subsequently increasing Th17 differentiation [53]. Therefore, activation of glycolytic pathways contributes to the differentiation of pro-inflammatory Th17 cells that exhibit enhanced pathogenicity in the context of autoimmune responses.

### **5.2 Amino acid metabolism**

Rapid AA import mediated by the amino acid transporters propels Th17 cell lineage specification by enhancing mTORC1 activity leading to enhanced protein biosynthesis and glycolysis. ICER binds to the *IL-17* gene promoter, enhancing its transcription. It enhances glutaminolysis through glutaminase induction and finally

#### **Figure 3.**

*Impact of glucose metabolism in Th17 cell differentiation: T-cell receptor (TCR) ligation and CD28 co-stimulation integrate phosphatidylinositol 3-kinase (PI3K)-Akt signaling, activating a kaleidoscope of metabolic pathways. mTORC1 activates glycolytic pathways in Th17 cells through activation of the transcription factors c-myc and hypoxia-inducible factor 1-alpha (HIF-1α), mediating multiple pathways. (1) HIF-1α enhances cellular glucose uptake by promoting membrane translocation of the glucose transporter 1 (GLUT1). (2) HIF-1α causes ubiquitination-mediated proteosomal degradation of Foxp3 thereby shifting the Th17/Treg cell balance towards Th17 cells. (3) HIF-1α induces IL-17 gene transcription. (4) HIF-1α enhances expression of genes encoding for key glycolytic enzymes hexokinase (HK2, rate-limiting enzyme of glycolysis), lactate dehydrogenase (LDH, converting pyruvate to lactate), and (4a) pyruvate kinase muscle enzyme 2 (PKM2), that catalyzes the final step of glycolysis producing pyruvate from phosphoenolpyruvate. PKM2 cab functionally exists as either a dimer or a tetramer, each exerting different functions: the cytoplasmic tetrameric configuration is associated with glycolytic activity while nuclear dimeric form interacts with transcription factors and histones, enabling post-translational modifications including the IL-17 gene. (5) HIF-1α also induces pyruvate dehydrogenase kinase, PDHK1 enzyme that phosphorylates and inactivates pyruvate dehydrogenase (PDH) that catalyzes the mitochondrial oxidative conversion of pyruvate to acetyl-coA for launching Kreb's cycle), thereby, nurturing aerobic glycolysis in Th17 cells.*

generates glutathione that supports Th17 cell steadiness by enhancing ROS-associated detoxification pathways, polarizing them towards a pathogenic phenotype [54]. Transamination of glutamate (catalyzed by the glutamate oxaloacetate transaminases (GOT)1/2), can epigenetically redirect Th17/T reg equilibrium towards Th17 cell destiny by generating epigenetic-regulating metabolites (α-KG and 2-HG). 2-HG, an inhibitor of α -KG-dependent histone/DNA demethylases, directly increases DNA methylation at CpG islands at the *Foxp3* gene locus leading to its transcriptional repression (**Figure 4**). This is how increased levels of 2-HG in Th17 cells lead to blockade of Treg cell lineage commitment. Th17 cells are characterized by an abundance of α-KG and 2-HG. This highlights the importance of the glutamate-GOT1/2-α-KG-2-HG axis in guiding Th17 cell destiny. GOT1 also contributes to increased mTORC1 signaling by suppressing AMPK activation [55].

Methionine-derived S-adenosyl methionine (SAM) plays a crucial role in chromatin remodeling by serving as a co-factor for epigenome-modifying enzymes, maintaining permissive H3K4me3 marks on *IL-17a*, *IFNG*, and *CSF2* genes promoting their transcription leading to increased pathogenic Th17 generation [56].

In this way, amino acids regulate energy metabolism, redox balance, and impact the epigenetic landscape, modulating Th17 lineage heterogeneity and plasticity [42].

*Th17/IL-17, Immunometabolism and Psoriatic Disease: A Pathological Trifecta DOI: http://dx.doi.org/10.5772/intechopen.102633*

#### **Figure 4.**

*Glutamine and methionine potentiating Th17 cell differentiation by impacting epigenetic landscape: (1) glutamine imported by the amino acid transporter, (alanine-serine-cysteine transporter, ASCT2) activated mTORC1/c-Myc signaling axis, enhancing glycolysis. (2) Glutaminase, transactivated by ICER binding, generates glutamate and finally glutathione. (3) glutathione causes reactive oxygen species (ROS) neutralization, enhancing pathogenic Th17 cell production. (4) glutamate is metabolized to α-KG [via glutamate oxaloacetate transaminases, GOT1/2]. (5) GOT1/2 inhibits the AMPK pathway leading to enhanced mTORC1 signaling. (6) α-KG generates 2-HG via. Isocitrate dehydrogenases 1/2(IDH1/2). (7) 2-HG impacts DNA methylation by inhibiting histone/DNA demethylases, e.g., Jumonji domain-containing demethylase (Jmjc) and ten-eleven translocation (TET1/2) methylcytosine dioxygenases (that demethylate the CpG islands on the Foxp3 promoter) (8). This leads to heightened DNA methylation at CpG islands of the Foxp3 gene locus and its transcriptional repression. (9) methionine-derived S-adenosyl methionine (SAM), by serving as a methyl donor, causes chromatin remodeling and helps maintain permissive H3K4me3 marks on IL-17, IFNG, and CSF2 genes promoting their transcription, leading to increased pathogenic Th17 generation.*

#### **5.3 Lipogenesis**

Rather than utilizing already-available exogenous FA for their lipid requirements, Th17 cells primarily engage in the ATP-costly process of *de novo* FAS for their proliferation and differentiation [45]. *De novo* FA and cholesterol synthesis promote activationinduced proliferation and differentiation of Th17 cells (**Figure 5**). Cholesterol precursors, as well as its derivatives, are essential for Th17 cell lineage commitment [57]. They enhance the transcriptional activity of RORγt by increasing co-activator recruitment leading to enhanced *IL-17* and *IL-23* gene transcription (**Figure 5** Inset) CYP51 and CYP27A1, key mediators of the cholesterol biosynthesis pathway are the most highly upregulated genes in Th17 cells [15]. An upregulation of cholesterol biosynthesis and simultaneous downregulation of cholesterol metabolism and efflux during Th17 differentiation leads to the accumulation of the cholesterol precursors desmosterol and its sulphate conjugates. Th17-polarizing milieu upregulates expression of *SREBP1* and *SREBP2*, *FASN*, *3- hydroxy-3-methylglutaryl-CoA reductase (HMGCR*, the rate-limiting enzyme in the mevalonate–cholesterol pathway) as well as expression of enzymes involved in citrate-pyruvate shuttle system leading to enhanced cholesterol synthesis and rapid *de novo* FAS from glucose. This metabolic alteration is again under

### **Figure 5.**

*Lipid metabolism as a central controller of Th17 cell differentiation: T-cell receptor (TCR) ligation and CD28 co-stimulation activate PI3K-Akt/mTORC1 signaling affecting lipid metabolism. (1) increased expression of sterol response element-binding proteins (SREBP2), CYP51 and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) enhances cholesterol biosynthesis. (2) this leads to accumulation of the cholesterol precursors desmosterol, its sulphate conjugates as well as oxysterols [with participation of CYP27A1]. (3) these precursors and derivatives serve as endogenous RORγt agonists, enhancing pathogenic Th17 cell production. The ligand-binding domain (LBD) of RORγt binds to these agonistic ligands including CD5L-dependent displacing the co-repressors and recruiting co-activator proteins, causing enhanced IL-17 gene expression. DNA-binding domain (DBD) of RORγt binds to ROR response elements (RORE) located in CNS2 (conserved non-coding sequences) of IL-17 gene, and globally controls its transcription. (4) HMGCR also contributes towards protein geranylation by generating mevalonate. (5) increased expression of SREBPI stimulates de novo fatty acid synthesis by enhancing the expression of acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FASN) genes. (6) CD5 antigen-like (CD5L) protein inhibits the de novo synthesis of saturated and monounsaturated fatty acids (SFA/MUFA) causing elevation of polyunsaturated fatty acids (PUFA) that results in enhanced RORγt binding to the IL-10 locus leading to its transactivation and production of non-pathogenic Th17 cells. (7) loss of CD5L elevates intracellular SFA/MUFA levels causing enhanced RORγt binding to the IL-17α and IL-23 loci leading to their transactivation and production of pathogenic Th17 cells.*

partial control of mTORC1 activation. In addition, to enhancing glycolysis, mTOR signaling fabricates a closely-interacting loop between glycolysis and lipogenesis, "the glycolytic-lipogenic pathway" in Th17 development [58].
