A guide to immunometabolism for immunologists

Abstract

In recent years a substantial number of findings have been made in the area of immunometabolism, by which we mean the changes in intracellular metabolic pathways in immune cells that alter their function. Here, we provide a brief refresher course on six of the major metabolic pathways involved (specifically, glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway, fatty acid oxidation, fatty acid synthesis and amino acid metabolism), giving specific examples of how precise changes in the metabolites of these pathways shape the immune cell response. What is emerging is a complex interplay between metabolic reprogramming and immunity, which is providing an extra dimension to our understanding of the immune system in health and disease.

Conflict of interest statement

statement The authors declare no competing interests.

Figures

Figure 1. Metabolic reprogramming by the immune system

Historically, oxygen levels and nutrient supply were seen as the key drivers of metabolic pathways. Normoxia supports the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, whereas hypoxia leads to the activation of hypoxia-inducible factor 1α (HIF1α) and the expression of glycolytic enzymes. More recently, it has become apparent that immune stimuli can also cause metabolic reprogramming in cells. For example, stimulation of cells with interleukin-4 (IL-4) can induce oxidative phosphorylation, whereas the activation of cells through pattern recognition receptors (PRRs), such as Toll-like receptor 4 (TLR4), induces HIF1α expression to promote glycolysis. Glycolysis also predominates in tumours under normoxia in the form of aerobic glycolysis, presumably giving tumours a growth advantage in oxygen-replete tissues.

Figure 2. Six major metabolic pathways

Glycolysis converts glucose into pyruvate, which can then either be converted into lactate, and secreted, or can enter the tricarboxylic acid (TCA) cycle in which it will generate NADH and FADH2 for the electron transport chain, leading to ATP production. Glycolysis also feeds the pentose phosphate pathway (PPP), which generates ribose for nucleotides, amino acids and NADPH. NADPH is used for fatty acid synthesis, which uses citrate withdrawn from the TCA cycle. Fatty acids can also be oxidized, generating NADH and FADH2, which again drive ATP production from the electron transport chain. Finally amino acid metabolism can feed the TCA cycle and is also important for cell growth and protein biosynthesis. In this figure, the pathways that require oxygen are indicated in green boxes, and pathways that are not oxygen dependent are indicated in blue boxes. In addition, inhibitors of metabolic pathways are indicated in grey boxes. BPTES, bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulphide.

Figure 3. Glycolysis and the pentose phosphate pathway in immunity

Glycolysis occurs in M1-like macrophages in response to hypoxia-inducible factor 1α (HIF1α) activation. HIF1α not only promotes glycolysis but also induces the expression of genes that encode inflammatory cytokines, notably interleukin-1β (IL-1β). The glycolytic enzyme hexokinase 1 has also been shown to directly interact with and activate the NLRP3 inflammasome, leading to caspase 1 activation and the processing of pro-IL-1 β. In T cells, the glycolytic enzyme glyceraldehyde 3 phosphate dehydrogenase (GAPDH) binds to mRNA encoding interferon-γ (IFNγ) and represses its translation; the switch to glycolysis that occurs in response to T cell activation leads to the dissociation of GAPDH allowing for translation of IFNγ. Glycolysis is also crucial for the functioning of natural killer (NK), T helper 1 (TH1), TH2, TH17 and peripherally induced regulatory T (pTreg) cells. The pentose phosphate pathway branches off glycolysis and generates ribose for nucleotides for DNA and RNA synthesis, but also NADPH, for NADPH oxidase or for glutathione biosynthesis, promoting an antioxidant response. In human pTreg cells, the glycolytic enzyme enolase has been shown to promote their suppressive functions by regulating the expression of FOXP3 splicing variants containing exon 2 (FOXP3-E2). CARKL, carbohydrate kinase-like protein; NLRP3, NOD-, LRR- and pyrin domain-containing 3; PKM2, pyruvate kinase isoenzyme M2.

Figure 4. The TCA cycle in macrophages

In M2-like macrophages (that is, interleukin-4 (IL-4)-activated macrophages) the tricarboxylic acid (TCA) cycle is intact and participates in oxidative phosphorylation, providing ATP for energy. In M1-like macrophages (that is, cells that have been activated by lipopolysaccharide (LPS) and interferon-γ), the TCA cycle is broken in two places — after citrate and after succinate. Citrate is used to generate fatty acids for membrane biogenesis and also for prostaglandin production. It also generates itaconic acid via the enzyme immune-responsive gene 1 (IRG1). Itaconic acid has direct antimicrobial activity against Mycobacterium tuberculosis and Salmonella sp. HIF1α, hypoxia-inducible factor 1α.

Figure 5. Fatty acid synthesis and oxidation in immunity

Inflammatory signals drive fatty acid synthesis, which is important for immune cell proliferation and inflammatory cytokine production. By contrast, tolerogenic stimuli from the immune system drive fatty acid oxidation, which is required for the production of suppressive cytokines leading to immune tolerance and the inhibition of inflammation. Effector T cells show enhanced fatty acid synthesis and this is needed for their growth. Memory T cells show fatty acid oxidation, which limits their growth and allows them to persist.

Figure 6. Amino acid metabolism in immunity

Amino acid metabolism plays an important role in mediating functionality of the innate and adaptive immune systems. In macrophages, the amino acids glutamine and arginine are crucial for immune functions including cytokine and nitric oxide production. The fate of arginine in macrophages is a key distinction between inflammatory and tolerant cell phenotypes. Tryptophan metabolism by macrophages may suppress the activity of the adaptive immune system. In T cells, glutamine and arginine promote robust responses to T cell receptor (TCR) stimulation, including proliferation and cytokine production. Sufficient availability is necessary for proper mechanistic target of rapamycin (mTOR) pathway signalling. Tryptophan has an important role in promoting T cell proliferation, and lack of availability may mediate failure to respond to infections or tumours. TCA, tricarboxylic acid; Treg, regulatory T.

Figure 7. Metabolism of immune cell subtypes

The various immune cell subsets exhibit a reliance on distinct metabolic pathways to promote cell survival, lineage generation and function. Inflammatory macrophages use glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway, fatty acid synthesis and amino acid metabolism to proliferate and to support the production of inflammatory cytokines. M2 macrophages, which exhibit a more tolerant phenotype, use the TCA cycle, fatty acid oxidation and arginine flux into the arginase pathway. Rapidly proliferating effector T cells, including T helper 1 (TH1), TH17 and cytotoxic CD8+ T cells, use glycolysis, fatty acid synthesis and amino acid metabolism to promote proliferation and cytokine production. Immunosuppressive regulatory T (Treg) cells use the TCA cycle and fatty acid oxidation. Similarly, memory CD8+ T cells also require the use of the TCA cycle and fatty acid oxidation to promote increased cell lifespan. ROS, reactive oxygen species.