AUTOIMMUNITY

Summary Immune metabolism is a rapidly moving field. While most of the research has been conducted to define the metabolism of healthy immune cells in the mouse, it is recognized that the overactive immune system that drives autoimmune diseases presents metabolic abnormalities that provide therapeutic opportunities, as well as a means to understand the fundamental mechanisms of autoimmune activation more clearly. Here, we review recent publications that have reported how the major metabolic pathways are affected in autoimmune diseases, with a focus on rheumatic diseases. Introduction The connection between cellular metabolism and immune activation was first established when it was shown that activation of T cells through the engagement of the co‐stimulatory receptor CD28 triggered glycolysis through the phosphoinositide‐3‐kinase (PI3K) pathway 1 (Fig. 1). A few years later, it was shown that the ligation of the B cell receptor activated the same pathway, and that glycolysis was required for B cell activation and differentiation 2 (Fig. 2). The next decade has seen an explosive growth of research in immunometabolism that has defined nutrient utilization and cellular processes to sustain the unique energy biosynthetic needs of immune cells to achieve rapid growth and to perform effector functions upon challenge with pathogens 3. While most of the research has characterized the metabolism of healthy immune cells, it has been rapidly recognized that the overactive immune system that drives autoimmune diseases presents metabolic abnormalities that provide therapeutic opportunities 4, 5. Exploiting the intricately dysregulated metabolic networks in cancer is being considered to develop therapeutic approaches that would either starve tumor cells or boost the immune system 6, 7. Reviews have been recently published focusing on rheumatic diseases 8, rheumatoid arthritis (RA) 9 and systemic lupus erythematosus (SLE) 10, two autoimmune diseases for which the field of immunometabolism is the most advanced. Here, we review recent publications that have reported how the major metabolic pathways are affected in autoimmune diseases, with a focus on rheumatic diseases. The yin and yang regulation of autoimmunity by MTOR and AMPK Mammalian target of rapamycin (mTOR) and AMP‐activated protein kinase (AMPK) are metabolic sensors that play a major role in immune cell functions 11. Activated AMPK inhibits adenosine triphosphate (ATP)‐consuming processes, such as protein synthesis while enhancing ATP‐producing processes, such as fatty acid oxidation (FAO) and glucose uptake 12. Activated AMPK inhibits mTOR activation, which promotes energy‐consuming processes. mTOR signaling is mediated by two multimeric complexes, mTOR complex 1 (mTORC1) and mTORC2, which share the catalytic subunit mTOR but are distinguished by the scaffold proteins regulatory‐associated protein of mTOR (RAPTOR) and rapamycin‐insensitive companion of mammalian target of rapamycin (RICTOR), respectively. Activation of both complexes promote glucose metabolism, linking mTORC and glycolysis. Direct modulation of glucose metabolism through over‐expression of glucose transporter 1 (Glut1) enhances follicular helper T (Tfh) cell differentiation and leads to autoimmunity 13. mTOR activation promotes the differentiation of T helper type 1 (Th1), Th17 14 and Tfh T cells 13, three effector subsets that are expanded in lupus 15, 16. mTOR activation plays a more complex role in regulatory T cell (Treg) differentiation and function by preventing the generation of long‐lived central Tregs, but promoting the generation of effector Tregs 17. Treg‐specific deletion of mTOR reduced their frequency, leading to spontaneous effector T cell activation and inflammation 18. Deletion of Lkb1 in Tregs, the upstream kinase of AMPK and a well‐known sensor of metabolic stress, resulted in dysfunctional Tregs and the development of a Th2‐dominant severe autoimmune phenotype 19, 20. Deletion of PP2A in Tregs, a serine‐threonine phosphatase involved in the development SLE by regulating the production of interleukin (IL)‐2 and IL‐17 in CD4+ T cells 21, resulted in increased mTORC1 activation and a severe, multi‐organ autoimmune disorder 22. These results indicated that while mTOR activity is required for Treg development and function, its level of activation has to be kept in check by protein phosphatase 2 (PP2A), and possibly other mechanisms. The function and differentiation of follicular regulatory T cells (Tfr), a Treg subset that suppresses germinal center (GC) B cells and Tfh cells, is also mTORC1‐dependent 23. These results suggest that T cell differentiation of most T cell subsets is mTOR‐dependent and aberrant expression of mTOR might lead to autoimmunity. CD4+ T cells from lupus patients present a high level of mTOR activation that is directly implicated in the disease process 24. Indeed, treatment with sirolimus, an mTOR inhibitor, reduced disease activity in refractory lupus patients 25. Intriguingly, the therapeutic response in these patients was best associated with a reduced number of effector memory CD8+ T cells, a subset whose role in lupus pathogenesis is as yet undefined. Tfh cells in the B6.Sle1.Sle2.Sle3 (TC) model of lupus show a high level of mTORC1 activation, which was reduced by the inhibition of glucose metabolism 26. This reduction was associated with a decreased frequency of Tfh cells, GC B cells and autoantibody production. This effectively linked glycolysis, mTORC1 activation and Tfh expansion in lupus. mTOR also plays an essential role in B cell differentiation. In the Roquin mouse model of lupus, activation of AMPK and inhibition of mTOR limited B cell differentiation into GC B and plasma cells, which was associated with a reduced disease activity 27. In SLE patients, high mTOR activation in CD19+ B cells correlates with plasmablast numbers and disease activity 28 (Fig. 2). Conversely, treatment with metformin, which activates AMPK 29, has beneficial effects in lupus patients 30 and in mouse models of lupus 31, 32. Overall, these studies showed that mTOR plays a central role in lupus by affecting multiple cell types. However, these findings should not be generalized to other autoimmune diseases without further studies, in which the AMPK/mTOR pathway has not been explored in detail. Glycolysis Glycolysis refers to the metabolic pathway by which glucose is metabolized. The first common phase of glycolysis is the production of pyruvate. Pyruvate is then either oxidized in the Krebs cycle, leading to the production of up to 38 molecules of ATP per molecule of glucose, or reduced into lactate in either hypoxic conditions or when metabolite intermediates are needed over ATP production, which in this case is limited to two molecules. Glycolysis commonly refers to this lactate end‐point branch of glycolysis, while the other is referred to as glucose oxidative or mitochondrial metabolism. Activation of CD4+ T cells from lupus‐prone mice and SLE patients occurs with high levels of oxygen consumption and oxidation 31, 33. Lupus T cells also display a high level of glycolysis 31, with oxidation representing a major part of glucose utilization 32. Glucose transporters provide the ‘primary first step’ of glycolysis by importing glucose into the cell. The major glucose transporter expressed by T cells is Glut1, which is significantly up‐regulated upon T cell receptor and co‐stimulator CD28 signaling 34. Over‐expression of Glut1 in mice led to the accumulation of activated CD4+ T cells, the production of autoantibodies and a modest immune complex deposition in the glomeruli of aged mice 35. Furthermore, these mice showed increased Tfh and GC B cell numbers, with elevated IL‐21 and immunoglobulin (Ig)A production 13. The combination of 2‐deoxy‐D‐glucose (2DG), a glycolysis inhibitor, and metformin, which inhibits complex I of the mitochondrial electron transport chain 36, reversed lupus pathogenesis in mice 31. While treatment with either metformin or 2DG alone could prevent the development of the disease 32, these results indicate that targeting cellular metabolism could be a potential therapy for lupus and other autoimmune diseases 37. Among the subsets of T cells, Tfh cells from lupus mice are highly glycolytic (Fig. 1), and their expansion as well as that of GC B cells was abrogated by 2DG treatment 26. This glycolytic requirement is restricted to autoreactive Tfh cells, as Tfh cells induced by immunization with a nominal antigen or by infection with influenza virus were not affected by 2DG 26. This suggests that the metabolic requirements of autoreactive CD4+ T cells are unique, which may provide a window of opportunity for their selective elimination. The expansion of Tfh cells was also kept in check by 2DG in the K/BXN model of RA, which indicated that the high glucose requirement of autoreactive Tfh cells is not model‐dependent 38. 2DG‐treated K/BXN mice also showed a reduced disease severity, in association with a decreased T and B cell metabolism and a reduced activation of both adaptive and innate immune cells 38. In the same RA model, reducing glycolysis by targeting hexokinase (HK) showed beneficial effects by decreasing the activation of fibroblast‐like synoviocytes 39. Contrary to murine RA T cells and synoviocytes, glycolysis is reduced in the CD4+ T cells from RA patients, which develop a hyper‐reduced state due to an over‐active pentose phosphate pathway (PPP) 40. The pathogenicity of these T cells can be reduced by diverting the glucose flux away from PPP 41 or with oxidative agents 42. Lactate accumulation has been reported in the synovia of RA patients, which may be secondary to the hypoxic conditions in the inflamed joint. The excess is responsible for the ‘entrapment’ of CD4+ and CD8+ T cells. The expression of lactate transporters on these cells correlates with the clinical T cell score in the synovia of RA patients. Lactate directly inhibits CD4+ T cell motility by interfering with glycolysis activated upon engagement of the chemokine receptor C‐X‐C motif chemokine receptor 3 (CXCR3) with C‐X‐C motif chemokine ligand 10 (CXCL10) 43. These results suggest that blocking lactate production in the RA joint may decrease T cell infiltrates and present therapeutic benefits. Pyruvate dehydrogenase (PDH) promotes the oxidative phosphorylation of pyruvate over the lactate glycolytic pathway. Pyruvate dehydrogenase phosphatase catalytic subunit 2 (PDP2) converts the inactive PDH to its active form. PDP2 expression was decreased in memory Th17 cells from patients with SLE and forced expression of PDP2 in CD4+ T cells from lupus‐prone MRL/lpr mice and patients with SLE‐suppressed Th17 differentiation. This may be due at least partly to the direct control of energy production by the transcription factor‐inducible cAMP early repressor/cAMP response element modulator (ICER/CREM) at the PDH metabolism bifurcation level 44. These results are consistent with the glycolytic requirements of Th17 cells 45 and the expansion of Th17 cells in SLE patients 15. Finally, dimethyl fumarate, a derivative of the Krebs cycle intermediate fumarate that inactivates glyceraldehyde 3‐phosphate dehydrogenase (GAPDH) and therefore inhibits both branches of glycolysis, altered the differentiation and function of Th1 and Th17 cells, attenuating disease in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis 46, stressing again the potentials of glucose metabolic inhibitors to target pathogenic autoreactive T cells. HIF1α, a transcription factor that controls the cellular response to hypoxia, activates the glycolytic pathway and, as such, promotes inflammation 47. HIF1α expression is required for the differentiation of Th17 cells 45, a T cell subset expanded in many autoimmune and inflammatory diseases. Mice with a B cell‐specific deletion of HIF1α have reduced numbers of IL‐10‐producing B cells, which exacerbate collagen‐induced arthritis and EAE 48. HIF1α‐inhibitor echinomycin reduced Th1 and Th17 responses, and attenuated a mouse model of acute graft‐versus‐host disease 49. Moreover, hypoxia induced HIF1α expression as well as signal transducer and activator of transcription (STAT)‐1 and STAT‐3 activation in human RA‐fibroblast‐like synoviocytes (FLS). STAT‐3 knock‐down inhibited HIF1α expression and the hypoxia‐induced cell invasion, migration and cytokine production, indicating a functional link between HIF1α and STAT‐3 in the regulation of proinflammatory mechanisms in RA 50. Prolyl hydroxylase domain (PHD) are enzymes that regulate HIFα levels by promoting its degradation. PHD‐2 is the major hydroxylase regulating HIF levels and the expression of angiogenic genes in RA‐FLS, illustrating the major role played by hypoxia in the RA joint 51. Finally, knocking down HIF1α in myeloid cells ameliorated induced colitis 52. Taken together, targeting glycolysis directly and indirectly may present promising therapeutic venues to treat autoimmune diseases; however, detailed disease‐specific analyses of preclinical models are necessary to understand the cellular targets and specific pathways in glucose flux that are responsible for pathogenesis. Oxidative phosphorylation and fatty acid oxidation In order to produce ATP, enzymes in the electron transport chain (ETC) transfer electrons from donor metabolites to electron acceptors. This is known as oxidative phosphorylation (OXPHOS) 53. In this process, complex I of the ETC, nicotinamide adenine dinucleotide hydrogenase (NADH), oxidizes NADH to nicotinamide adenine dinucleotide (NAD) 54. Succinate dehydrogenase, complex II, oxidizes succinate to fumarate, reducing ubiquinone 55. Ubiquinone is then oxidized by complex III, cytochrome c reductase, which reduces cytochrome c allowing for its oxidation by complex IV, cytochrome oxidase 56, 57. Thus, the design of OXPHOS is to produce ATP by the oxidation and reduction of key metabolites. A number of studies have demonstrated alterations in the OXPHOS and FAO metabolic pathways in autoimmune diseases. As mentioned earlier, T cells from RA patients function in a hyper‐reduced phenotype due to a hyperactive PPP, probably in response to the deficiency of necessary intermediates and the hypoxic environment 5, 9, 58. This is probably a result of oxidative stress from macrophages and other reactive oxygen species (ROS) releasing cells in the symptomatic tissue 59. Contrary to the RA phenotype, SLE T cells are hyper‐oxidative 33. These changes in oxidative profiles may be due in part to genetic predisposition. This hypothesis is supported by the finding that the murine lupus risk allele Esrrg encodes for a nuclear receptor, estrogen‐related receptor gamma (ERRγ), which regulates mitochondrial function and OXPHOS 60. Directly relevant to lupus, type 1 interferon (IFN) induces OXPHOS and FAO in plasmacytoid dendritic cells (pDCs), and this metabolic reprogramming is critical for their activation and amplification of IFN production 61. B cells and other myeloid cells are less well metabolically characterized, and it is not understood if these cell types also have altered OXPHOS or FAO metabolism in autoimmune disease 62. Notably, long‐lived plasma cells, which are a critical component of many antibody‐mediated autoimmune diseases, require mitochondrial import of pyruvate 63. A recent study utilizing several mice models of lupus nephritis demonstrated that kidney‐infiltrating T cells (KITs) present an exhausted phenotype similar to that observed in tumor‐infiltrating lymphocytes 64. Both CD4+ and CD8+ KITs demonstrated substantial loss of their spare respiratory capacity and little to no oxidative phosphorylation. The significance of this finding remains unclear, as CD4+ T cell infiltrates have been associated with lupus nephritis in mouse models 65, 66 and patients 67, 68. Another unexpected finding was the identification of C1q, a member of the complement pathway involved in efferocytosis, as a modulator of mitochondrial metabolism in CD8+ T cells in the chronic graft‐versus‐host‐disease (cGVHD)‐induced model of SLE 69. C1q‐deficiency reduced cGVHD induction and, mechanistically, resulted in the reduction of mitochondrial function in effector memory CD8+ T cells (Fig. 1). These results indicated a novel link between C1q, mitochondrial metabolism and CD8+ T cell effector functions in autoimmunity. OXPHOS may also play an important role in the pathogenesis of RA, according to a recent study showing that the Krebs cycle metabolite succinate is under‐utilized in the adjuvant‐induced arthritis (AIA) model of RA, possibly leading to increased joint inflammation 70. Suncr1, the cognate receptor succinate receptor 1, is an active participant in the innate immune response 71-73, and increased amounts of succinate correlate with disease in AIA 70, 72. Accordingly, Suncr1 deficiency reduced the expansion of pathogenic Th17 cells and AIA severity 70. Succinate is largely used by OXPHOS, which has a reduced activity in the T cells of RA patients and in other autoimmune diseases 74. This reduced OXPHOS probably results in an under‐utilization of succinate which would, in turn, create a microenvironment that increases disease activity. A recent study found, in a small subset of CD4+CXCR3+CXCR5–PD‐1hi, T cells in the blood of SLE patients who have increased ROS due to reverse ETC regulated by succinate 75. These cells provide help to B cells via IL‐10 and succinate, independently of IL‐21. These studies emphasize the importance of the metabolite succinate in rheumatic diseases. Itaconate inhibits succinate dehydrogenase, regulating succinate oxidation to fumarate. Itaconate is one of the most highly produced metabolites in lipopolysaccharide (LPS)‐activated macrophages, in which it has anti‐inflammatory effects 76. Mechanistically, itaconate derivatives can regulate nuclear factor kappa B inhibitor zeta (IκBζ) via activating transcription factor 3 (ATF3), ameliorating IL‐17 IκBζ‐driven inflammation 76. Another study identified that activation of nuclear factor‐like 2 (Nrf2) via Kelch‐like ECH‐associated protein 1 (KEAP1) alkylation could explain the anti‐inflammatory effects exhibited by itaconate 77. Further, cell‐permeable itaconate derivatives decreased cytokine production in macrophages and limited the type I IFN response, indicating a possible therapeutic use for itaconate and its derivatives in IFN‐driven diseases such as SLE 77, 78. FAO represents a major energy source for Treg and memory T cells 79, although a genetic rather than a pharmacological approach has recently challenged this tenet 80. The contribution to FAO to autoimmune diseases has not been well characterized. Elevated levels of adipocyte‐fatty acid binding protein (FABP), which is responsible for facilitating the transport of fatty acids to specific organelles for oxidation, correlate with disease activity in patients with multiple sclerosis (MS) and clinical isolated syndrome 81. Inhibition of FABP prevented EAE in mice 82, providing a possible treatment option to correct this specific metabolic defect that is contributing to immune cell dysfunction in MS‐related diseases. Finally, D‐mannose was found to induce Treg differentiation and decrease the production of inflammatory cytokines, indicating that D‐mannose has the potential to improve autoimmune pathology 83. Mechanistically, D‐mannose promotes transforming growth factor (TGF)‐β production, which is mediated at least in part through ROS produced by increased FAO, an indirect consequence of the inhibitory effect of D‐mannose on glycolysis 83. This study illustrates the complex relationships that exist between metabolic networks and their consequences on immune activation. Finally, glycolysis is required by Treg cells to migrate to the site of inflammation 84. Mechanistically, PI3K‐mTORC2 activation in Treg cells induces the expression of glycokinase, which binds actin and promotes the cytoskeletal remodeling that is necessary for cell migration. This latter finding emphasizes that specific metabolic pathways sustain different aspects of immune function. Branch chain amino acid and glutamine metabolism Amino acids are metabolites used for the biosynthesis of lipids, nucleotides, glutathione, glucosamine and polyamines, as well as in the anaplerosis of the Krebs cycle 85. Moreover, branch chain amino acids (BCAA) and glutamine function as signaling molecules of the cellular metabolic state to activate mTORC1 86. Here, we will focus on BCAAs, especially leucine and glutamine metabolism. The potential contribution of leucine metabolism to autoimmune diseases is largely attributed to its role as a critical checkpoint of mTORC1 signaling and glycolysis, which then control the function of T cells and monocytes/macrophages 87-89. Leucine influx is mediated by solute carrier family 7 member 5 (SLC7A5). Intracellular leucine undergoes reversible transamination by branched‐chain aminotransferase (BCAT) to form α‐ketoisocaproate, which is further metabolized into acetoacetate, then acetyl‐co‐enzyme A (CoA), to be subsequently oxidized in the Krebs cycle. BCAT exists in two isoforms, mitochondrial BCAT2 and cytosolic BCAT1. In human monocyte‐derived macrophages, BCAT1 is the most abundantly expressed BCAT isoform 90. Circulating monocytes from RA patients express higher SLC7A5 levels than cells from healthy controls, and these levels correlate with clinical parameters. Furthermore, blockade of SLC7A5 reduced IL‐1β production in these monocytes 87. Treatment with a selective inhibitor of BCAT1 reduced inflammation and macrophage infiltration in target organs in murine models of RA and crescentic glomerulonephritis 90. Glutamine (Gln) uptake is mediated by ASCT2/SLC1A5. It is converted to glutamate by glutaminase (GLS), followed by the conversion to α‐ketoglutaric acid (αKG) by transaminases or glutamate dehydrogenase 1 (GLUD1), after which αKG enters the Krebs cycle. Isocitrate dehydrogenase (IDH1 and IDH2) produce (D)‐2‐hydroxyglutarate (2HG) from αKG. In the SKG zymosan‐induced model of RA, the proliferation of RA‐FLS was decreased by glutamine but not glucose deprivation 91. Furthermore, Gls1 expression was higher in RA‐FLS than in osteoarthritis‐derived FLS. Accordingly, the administration of a Gls1 inhibitor ameliorated disease symptoms by suppressing FLS proliferation. Similarly, blocking glutaminolysis with the Gls1 inhibitor BPTES [bis‐2‐(5‐phenylactamido‐1,2,4‐thiadiazol‐2‐yl)ethyl sulfide] 92 or with the transaminase inhibitor aminooxyacetic acid (AOA) 93 prevented disease in the EAE model. The regulation of the balance between Th17 and Treg differentiation may underline the role of glutaminolysis in the pathogenesis of autoimmune diseases. Indeed, glutaminolysis is a major source of energy for the generation of Th17 cells 6. Gln deprivation led to the differentiation of activated naïve CD4+ T cells 6 and Th1‐polarized cells 94 into Treg cells. Gln transporter deficiency or depletion of Gln in culture media inhibited both Th1 and Th17 differentiation 95, but blocking the conversion of Gln to glutamate preferentially suppressed Th17 over other T cell subsets 6, 92. Blocking glutamate oxaloacetate transaminase 1 (Got1) also altered the balance between Th17 and Treg cells without affecting Th1 cells 93. However, Gls1 inhibition did not alter the number of Th17 cells and decreased the number of Treg cells in SKG mice 91. The reason for these discrepancies is as yet unclear, but it may include differences between healthy versus inflammatory or autoimmune strains. It may also be due to the role of some metabolic enzymes or metabolites in non‐metabolic pathways. The latter includes a greater accumulation of 2‐hydroxyglutarate (2‐HG) in differentiating Th17 than Treg cells, which promotes the methylation of the Foxp3 locus and silences its expression 91. Limiting glutamate production reversed this process and ameliorated EAE disease by expanding Treg and reducing Th17 cell numbers 92. This highlights the role of glutamine metabolism in epigenetic regulation in addition to energy production. Finally, the role of glutamine metabolism has been recently examined in Tfh cells in the TC model of SLE. Slc1a5 expression was lower in TC Tfh than in congenic non‐autoimmune B6 Tfh cells 26. Contrary to glycolysis, glutaminolysis inhibition greatly reduced immunization‐induced as well as autoimmune humoral responses. This diminished Tfh function, in both lupus‐prone and non‐autoimmune mice 26, indicating that it required for the development of germinal centers. Lipid synthesis De‐novo FA synthesis starts from the generation of malonyl‐CoA from acetyl‐CoA by the rate‐limiting enzyme acetyl‐CoA carboxylase (ACC1) 96. Subsequent steps and FA elongation require fatty acid synthase (FAS), stearoyl‐CoA desaturase (SCD) and the FA‐co‐enzyme A ligase family to generate diacetyl‐ and triacetyl‐glycerols and long‐chained FA 97. Most of the de‐novo synthesized FA are incorporated into phospholipids for membrane biogenesis and localize to lipid rafts to participate in crucial membrane‐based processes 98, while some form lipid droplets 99. Pathogenic T cells in RA patients are characterized by increased FA synthesis leading to the formation of cytoplasmic lipid droplets. The inhibition of FA synthesis corrected the pro‐arthritogenic tissue‐invasiveness of these T cells 100. FA, in the form of triglycerides, phosphoglycerides or sphingolipids, are directly involved in T cell activation and proliferation as key components of cell membranes, signaling molecules and energy‐yielding substrates. T cell‐specific deletion of ACC1 severely impaired the accumulation of antigen‐specific CD8+ T cell due to activation‐induced cell death. Furthermore, exogenous FA rescued defective blasting and survival of ACC1‐deficient CD8+ T cells in vitro 101. These results may provide insights for therapeutic targets to either increase or decrease CD8+ T cell activity. Th17 cell induction results in increased levels of mRNA and phosphorylated ACC1 and ACC2 proteins. Pharmacological inhibition or T cell‐specific deletion of ACC1 restrained the formation of human and mouse Th17 cells and promoted the induction of Treg cells, which attenuated Th17 cell‐mediated EAE 102. Inhibition of fatty acid synthase (FASN) also reduced EAE severity, with direct evidence from adaptive transfer that this effect is Th17‐dependent. However, FASN inhibition also promoted IFN‐γ production by Th1 and Th1‐like Th17 cells, which is different from the effect of ACC inhibition 103. The specific mechanisms by which FA synthesis regulates CD8+ T cell survival or inflammatory CD4+ T cell polarization have not yet been elucidated. Glycophingolipids (GSLs) are a key constituent of lipid rafts that have been implicated in lupus pathogenesis. CD4+ T cells from SLE patients present elevated GSL levels due to increased synthesis and altered trafficking 104. Normalization of GSL metabolism in these T cells normalized their function. In addition, activated T cells from SLE patients show a reduced induction of BTLA, an inhibitory receptor similar to cytotoxic T lymphocyte antigen (CTLA)‐4 and programmed death 1 (PD1) 105. The capacity of BTLA to restrain T cell activation in SLE is impaired, which could be due to poor BTLA recruitment to the immunological synapse. In support of this hypothesis, a glucosylceramide synthase inhibitor that normalized lipid metabolism restored BTLA inhibition. The dissociation of preclustered T cell receptor (TCR) molecules in the presence of this inhibitor allowed BTLA recruitment to TCR clusters directly linking GSL homeostasis to intracellular trafficking of components of the TCR 105. Cholesterol homeostasis Cholesterol homeostasis is maintained through the sophisticated balance between its synthesis, import and elimination of excessive cholesterol from the cells. Cholesterol is synthesized by endoplasmic reticulum‐bound 3‐hydroxy‐3‐methylglutaryl‐CoA reductase (HMGCR), which is controlled by the transcription factor sterol regulatory element‐binding protein (SREBP) 106. Apolipoprotein E (ApoE) and low‐density lipoprotein receptor (LDLR) contribute to the recycling and intercellular transport of cholesterol. ApoA1 and high‐density lipoproteins (HDL) mediate the efflux of cholesterol from immune cells via liver X receptor (LXR)‐regulated genes, such as ABCA1 and ABCG1. Cholesterol plays a key role in the regulation of immune responses through at least three mechanisms. First, cholesterol is required for membrane synthesis during cell expansion. Secondly, cholesterol is a key constituent of lipid rafts; thus, any changes in cholesterol content modify raft‐dependent signaling of major immune pathways such as Toll‐like receptors (TLRs), major histocompatibility complex (MHC), TCRs and B cell receptors (BCRs) 107-109. Finally, type I IFN reprograms lipid metabolism, including a decreased synthetic and increased import of cholesterol 110. Interestingly, ApoE is bound not only to plasma lipoproteins but is also present on immune cell membranes 111. ApoE deficiency in DCs impaired the removal of cholesterol from the membrane, resulting in lipid raft accumulation, enhanced MHC‐II clustering on the membrane and increased antigen presentation. This, in turn, expanded proinflammatory CD4+ T cells and enhanced skin allograft rejection, independently of dyslipidemia 112. Atherogenic dyslipidemia caused by a Western diet in ApoE or LDLR‐deficient mice increased the production of autoantibodies and the severity of lupus by expanding the number of Tfh cells 113. A novel mechanism was identified by which dyslipidemia induces IL‐27 production by DCs, which in turn expands Tfh cell responses and GC reactions to lupus‐associated self‐antigens 113. The levels of IL‐27 were increased in patients with hypercholesterolemia, while blocking IL‐27 in atherogenic mice reduced autoantibodies and Tfh cells to levels similar to those of control mice. An almost complete absence of HDL due to ApoA1 deletion induced autoimmune phenotypes, characterized by cholesterol‐engorged, enlarged lymph nodes, anti‐dsDNA autoantibodies and increased T cell activation 114, suggesting a link between cellular cholesterol accumulation and autoimmunity. Mice lacking LXRs exhibit an age‐dependent systemic autoimmune disease, and pharmacological activation of LXRs attenuated disease progression in a mouse model of lupus 115. Transcriptional induction of Abca1 expression by LXRs promotes cholesterol efflux and alters plasma membrane cholesterol distribution. Mice with Abca1/g1 deficiency presented lymphadenopathy and glomerulonephritis 116, 117. Interestingly, autoimmune activation in these mice was due to the specific deficiency of these transporters in DCs (Fig. 1). Abca1/g1‐deficient DCs showed an activated inflammasome, increased proliferation and cytokine secretion. These led to the expansion of Th1, Th17 and Tfh cells, GC B cells and plasma cells, all changes which have been implicated in the pathogenesis of SLE 117. Macrophages lacking the ABC transporters Abca1 and Abcg1 showed an increased TLR cell surface expression and enhanced inflammatory responses to LPS 118, while Abcg1‐deficient T cells show enhanced TCR signaling and proliferation 119. None of these studies in macrophages and T cells have reported autoimmune phenotypes. The mechanisms responsible for cell‐specific consequences of cholesterol efflux are not clear at present. Conclusions Immune metabolism is a rapidly moving field. Earlier studies have proposed a simple model in which activated effector immune cells switched from mitochondrial respiration to aerobic glycolysis relying on an increased glucose import 120. More recent studies have demonstrated that glutamine and fatty acid utilization provided additional layers of metabolic reprogramming with functional consequences in immune cells. Additional levels of complexity have been uncovered. Altered metabolic pathways in one immune‐mediated disease cannot be automatically expanded to another immune‐mediated disease, as outlined in this review, with CD4+ T cells in SLE and RA at opposite poles of the oxidation spectrum. Metabolic reprogramming is cell type‐specific; for example, pDC activation by type I IFN relying on FAO instead of glycolysis 61, a process that has not been found in other types of DCs. Some metabolic enzymes may regulate immune function through mechanisms only indirectly linked to metabolism, such as GAPDH regulating IFN‐γ production 121. In addition, some metabolites such as succinate or itaconate show previously unsuspected anti‐inflammatory effects 122. Finally, mitochondria emerge as central players in immune responses, not only through the generation of these metabolite intermediates, but also by regulating ATP and ROS production through the fusion and fission processes, providing a platform for retinoic acid‐inducible gene I protein (RIG‐I) and nucleotide‐binding oligomerization domain‐like receptor family, pyrin domain‐containing‐3 (NRLP‐3) inflammasome signaling. Directly related to autoimmunity, mitochondria are a source of highly immunogenic mitochondrial DNA 123. The majority of these ground‐breaking studies have been conducted in normal mice, with only a few in models of autoimmune diseases, and even fewer in patients with autoimmune diseases as reported in this review. There are, however, reasons to believe that research in immunometabolism will have a significant long‐lasting impact in the field of autoimmunity. First, a clearer understanding of the specific nature of metabolic dysregulation in autoimmunity may provide much‐needed novel mechanistic insights into the nature of autoimmune activation. Secondly, these studies may lead to additional therapeutic tools, either alone or in combination with standard of care approaches. The metabolism of immune cells is already targeted by a drug commonly used by rheumatologists, methotrexate, which interferes with 1‐Carbon metabolism 124. Furthermore, mTOR inhibitors and metformin show promising results in SLE 25, 30. It would also be of great interest to explore the metabolic consequences of current treatments used or tested by rheumatologists to have a clearer understanding of the mechanisms by which drugs or biologics modulate the immune system effectively or not through altered metabolism. Preclinical studies guided by the results obtained in normal mice, as well as in‐depth characterization on the metabolic signatures of effector cells in patients with autoimmune diseases, are urgently needed to make significant strides in this direction. Acknowledgements This publication is supported by grants from the NIH (R01AI045050 and R01 AI128901) and from the Alliance for Lupus Research (TIL‐416522) to L. M. Disclosures None. References