The main donor of free energy in cells is ATP 1, which is generated both by glycolysis and by oxidative phosphorylation (OXPHOS) 121314. Most cells break down glucose to pyruvate via cytosolic glycolysis, and then oxidize pyruvate to carbon dioxide in the mitochondrial tricarboxylic acid cycle, generating most of the ATP through OXPHOS at the electron transport chain 121314. Nutrients such as fatty acids and amino acids can also be degraded to pyruvate, acetyl-coenzyme A, or other intermediates of the tricarboxylic acid cycle to maintain ATP production 4. In contrast, in many cancer cells and activated T cells, pyruvate is preferentially converted into lactate that is secreted from the cells, rather than pyruvate being oxidized in the mitochondria 121314. By this process - known as aerobic glycolysis (also called the Warburg effect) - only two ATP molecules per molecule of glucose are yielded, compared with a maximum of 36 ATP molecules when glycolysis is coupled to OXPHOS 41214. Although it seems counterintuitive for cells to use a low-efficiency pathway to produce ATP under conditions of high energy demand, it has been proposed that aerobic glycolysis produces the requisite reducing equivalents and biosynthetic substrates that are required for proliferation 1214.

The serine/threonine kinases AKT1 to AKT3, AMP activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), and LKB1 (also known as STK11) are regarded as cellular nutrient sensors that help to maintain energy homeostasis by relaying signals that determine how cells respond to high or low levels of intracellular carbohydrates or amino acids 5. Activated AKT, also known as protein kinase B, is induced by phosphoinositide 3-kinase (PI3K) and represents the primary downstream mediator of the metabolic effects of insulin 15. In T cells, AKT is activated by T-cell receptor/CD28 co-stimulation and growth factors/cytokines such as IL-2 or IL-7 1617 (Figure 1). AKT increases glucose uptake by stimulating the localization of glucose transporters to the plasma membrane, and it can increase glycolysis by promoting the activities of the rate-limiting glycolytic enzymes hexokinase and phosphofructokinase 18. AKT activates mTOR, a key regulator of translation and major effector of cell growth and proliferation, which increases the expression of amino acid transporters 1920. mTOR forms two distinct complexes, mTORC1 and mTORC2, respectively. mTORC1 stimulates diverse metabolic pathways, including glycolysis, the oxidative arm of the pentose phosphate pathway, and de novo lipid biosynthesis 21.

Mammalian AMPK, another evolutionarily conserved protein kinase, which is a metabolic master switch and fuel gauge, is activated under conditions that increase the AMP⁄ATP ratio, including glucose deprivation and hypoxia 22. Once activated, these kinases mediate the upregulation of energy-producing catabolic processes, such as fatty acid oxidation and glycolysis, and down-regulate energy-consuming anabolic metabolism 2223. The phosphorylation of the mTORC1 component raptor by AMPK is required for the inhibition of mTORC1 and cell-cycle arrest induced by energy stress 24. There is complex crosstalk between the highly conserved nutrient sensors and the molecular clock of peripheral cells coordinating the circadian control of energy supply on a cellular level 2526.

Immune cells require energy for housekeeping functions as well as for specific immune functions (Table 1). The main housekeeping functions that use significant amounts of ATP are processes of ion transport and macromolecule synthesis. Specific immune functions include motor functions, antigen processing and presentation, activation and effector functions such as synthesis of antibodies, cytotoxicity, and regulatory functions 1 (Table 1). Calculations show that a quiescent leukocyte needs 1.1×10^-11 mol ATP/day for protein production and 5.5 × 10^-11 mol ATP/day in total 2. Activation of quiescent leukocytes leads to an increase of energy expenditure by a factor of 1.3 to 1.5 2, so an activated leukocyte needs 7.2 × 10^-11 to 8.3 × 10^-11 mol ATP/day.

The cellular energy metabolism is relevant to be considered in terms of diseases, for example in cells within the inflamed rheumatoid arthritic joint, because energy supply is limited 2728. Firstly, due to the invasion of immune cells and tissue swelling, the relative oxygen/cell ratio is decreased. Secondly, cell accumulation and inflammatory edema increase the distance between cells and oxygen-supplying arterial vessels. Thirdly, vasodilatation, as induced by inflammatory mediators such as prostaglandin E₂, lowers blood flow and thus the supply of oxygen and nutrients is reduced (such as glucose and amino acids) as well as the removal of metabolic waste (such as lactate and carbon dioxide) 2728.

A recent investigation of the energy-adaptive potential of human stimulated CD4⁺ T-helper cells under conditions of impaired OXPHOS and/or low glucose (which inhibits glycolysis) demonstrated that OXPHOS and glycolysis are completely interchangeable in terms of ATP production 29. Furthermore, specific T-cell functions such as cytokine production and proliferation are unaffected in glucose-containing medium, even under complete OXPHOS suppression. Only when glucose is also absent are these functions significantly decreased 29. Human CD4⁺ T cells thus own a high adaptive potential to maintain specific functions even under severely impaired bioenergetic conditions. However, proliferation and cytokine production of human CD4⁺ T cells are - despite a high adaptive potential - dependent on a certain supply with energy, and glucose is obviously of crucial importance for proliferation not only from the bioenergetic point of view as a substrate for ATP synthesis, but also in terms of being either directly and/or indirectly and essentially involved (for example, as a precursor molecule for macromolecule synthesis) in key steps of cellular proliferation and cytokine synthesis 2729.

Another study on stimulated human CD4⁺ T cells examined the effect of limited oxygen availability and/or lack of glucose, and found both that cumulative hypoxia stimulates the secretion of proinflammatory and proangiogenic cytokines in the presence of glucose and that the lack of glucose resulted in lower cytokine concentrations 27. These observations support the view of hypoxia being a key driving factor in chronic inflammation. The first data indicating the hypoxic nature of the rheumatoid arthritis (RA) synovium were achieved in the 1970s by measuring oxygen tension by means of a Clark-type electrode in samples of synovial fluids of patients with RA 283031. Hypoxia has been demonstrated in patients with RA undergoing surgery for tendon rupture by Sivakumar and colleagues 32. Just recently, by means of a novel oxygen-sensing probe in vivo, even a direct relationship between tissue partial pressure of oxygen levels and joint inflammation (specifically T-cell and macrophage infiltrates and proinflammatory cytokine expression) could be demonstrated for the first time 33, and it was shown that hypoxia can be reversed by antiinflammatory treatment 34.

One principal regulator of the adaptive response to hypoxia is the transcription factor hypoxia inducible factor (HIF)-1 2835. HIF-1 is a heterodimeric protein that consists of an oxygen-sensitive α subunit and a constitutively expressed β subunit 2836. In nonhypoxic cells, HIF-1α is continuously tagged by oxygen-dependent hydroxylation and in this way targeted for proteasomal degradation 2836. Under hypoxic conditions, however, HIF-1 is stabilized. HIF-1α protein synthesis is upregulated mainly via the PI3K/mTOR pathway 37. HIF target genes promote erythropoiesis, angiogenesis and vasodilatation, and HIF is a master switch to a glycolytic cell metabolism, resolving and counteracting hypoxic conditions 28.

Several findings indicate that HIF is involved in the persistence of inflammation and progression of neovascularization during RA. HIF is abundantly expressed in the arthritic tissue 38. Deletion of HIF in macrophages and neutrophils resulted in a complete loss of the inflammatory response 39. Hypoxia might also play a central role in pathogenesis of systemic sclerosis by augmenting vascular disease and tissue fibrosis 4041. However, HIF-1 was found to be decreased in the epidermis of systemic sclerosis patients compared with healthy controls 42, perhaps due to an increased prolyl-hydroxylase activity resulting in faster degradation of HIF-1 41.

A recent article suggests a positive-feedback loop of HIF-1α and the proinflammatory cytokine macrophage migration inhibitory factor (MIF) in human CD4⁺ T cells 36. Hypoxia, and specifically HIF-1, is a potent and rapid inducer of MIF. In turn, MIF signaling via the MIF receptor CD74 is essential for hypoxia-mediated HIF-1α expression and HIF-1 target gene induction involving ERK/mTOR activity complemented by PI3K activation upon mitogen stimulation 36. MIF is also able to counter-regulate glucocorticoid-mediated suppression of MIF and HIF-1α expression 36. Targeting MIF and HIF may thus be effective in disrupting self-maintaining inflammation.

The differentiation of naïve CD4 cells into Th1 and Th17 subsets of T-helper cells is selectively regulated by signaling from mTORC1 that is dependent on the small GTPase Rheb 43. Interestingly, CD4⁺ T-cell subsets require distinct metabolic programs 44. Th1, Th2 and Th17 cells express high surface levels of the glucose transporter- Glut1 and switch on a highly glycolytic program. In contrast, regulatory T cells (Tregs) express low levels of Glut1 and have high lipid oxidation rates 44. In an asthma model, AMPK stimulation was sufficient to decrease Glut1 and increase Treg generation, indicating that the distinct metabolic programs can be modulated in vivo 44. Recently, persistent hypoxia and glycolysis were demonstrated to control the balance between inflammation-promoting Th17 cells and inflammation-restricting Tregs 945.

Hypoxia-induced HIF expression exerts a direct transcriptional activation of RORγt, a master regulator of Th17 cell differentiation, and recruitment to the IL-17 promoter via tertiary complex formation with RORγt and p300 (Figure 1) 945. Concurrently, HIF-1 attenuates induced Treg development by binding Foxp3, a key transcription factor that promotes the Treg lineage, via a proposed ubiquitination pathway 945. Mice with HIF-1α-deficient T cells are resistant to induction of Th17-dependent experimental autoimmune encephalitis, associated with diminished Th17 cells and increased Tregs, indicating the therapeutic potential of HIF modulation. Similar to these findings, another study suggested that HIF-1α is involved in differentiation of Th17 cells and Tregs, but ascribed the role of HIF-1a to upregulation of glycolysis and not as a direct effect of HIF-1a on RORγt and Foxp3 946. Supporting the concept that hypoxia and IL-17A are key mediators in inflammatory arthritis, patients with lower tissue partial pressure of oxygen <20 mmHg have been demonstrated to have increased IL-17A positive mononuclear cells 47. Tumor hypoxia appears to be different, as it has been reported to inhibit T-cell proliferation and cytokine secretion and to activate Tregs 48.

Glycolysis has been suggested to play a role in the pathogenesis of RA 49. The activity levels of two major enzymes of the glycolytic pathway - glyceraldehyde 3-phosphate dehydrogenase and lactate dehydrogenase - were increased in RA synovial cells 50. However, clear studies of a direct relationship of increased glycolytic activity and inflammation are lacking. It is striking that several glycolytic enzymes such as glucose-6-phosphate isomerase, enolase, aldolase and triose phosphate isomerase act as autoantigens 49; however, their role in RA remains unclear 495152.

Rapamycin (also known as sirolimus) is an mTOR inhibitor used in transplantation medicine. This inhibitor acts similar to the immunosuppressant FK506 (tacrolimus) by binding to the intracellular immunophilin FK-binding protein 12 (FKBP12). Unlike the FK506-FKBP12 complex that inhibits calcineurin, however, the rapamycin-FKBP12 complex interferes with the function of mTOR. Rapamycin has been found effective for systemic lupus erythematosus and systemic sclerosis in animal models and pilot clinical trials 5354555657. Tregs can be expanded by rapamycin in vitro 58 and were found to suppress colitis in an experimental mouse model 59.

Energy metabolism is not only a question for a single cell or a group of cells such as, for example, T cells or muscle cells, because provision and allocation of energy-rich fuels involves the entire body. Need for energy-rich substrates at a certain location in the body can induce a systemic response if local stores are not sufficient to provide necessary supplies. The systemic response redirects energy-rich fuels from stores to the site of action, the consumers 2.

Such a redirection program can be started by a voluntary act when an individual decides to use muscles during exercise. In such a situation, the central nervous system activates, among others, the sympathetic nervous system (adrenaline, noradrenaline), the hypothalamic-pituitary-adrenal axis (cortisol), and the hypothalamic-pituitary-somatic axis (growth hormone, insulin-like growth factor-1), which induce gluconeogenesis,- glycogenolysis, and lipolysis. This is supported by release of IL-6 from muscles into systemic circulation, which helps activate the redirection program 73. Redirection of energy-rich substrates from storage sites to consumer can be called the energy appeal reaction.

If the immune system needs energy-rich fuels in the context of infection or other forms of activation, a similar energy appeal reaction is prompted 2. The response is a concerted action of the neuroendocrine immune network. But does the activated immune system need a lot of energy? Table 2 presents the energy demand of the entire body, systems, and organs. Obviously, the immune system needs a lot of energy, particularly in an activated state. In an inflammatory situation, the energy appeal reaction is driven by cytokine-induced stimulation of the central nervous system, endocrine organs, and energy storage organs such as the liver, muscles, and fat tissue 2. IL-6 is a classical candidate that can activate these remote places (but also IFNγ, IFNα, IL-2, TNF, and others 2).

The question remains whether this seemingly adaptive program has been positively selected in the context of CIDs such as RA or systemic lupus erythematosus.

The evolutionary principle of replication with variation and selection is undeniably fundamental and has history. This is a successful history of positive selection, which can only happen under circumstances of unrestricted gene transfer to offspring. The hypothesis is that genes which play a specific role in CIDs were not positively and specifically selected for a CID because unrestricted gene transfer was not possible in CIDs 274. If this is correct, regulatory mechanisms of the neuroendocrine immune network did not evolve to cope with CIDs. Instead, the neuroendocrine immune network was positively selected in the context of nonlife-threatening transient inflammatory episodes such as, for example, infection or wound healing. These episodes are usually short lived and do not last longer than 3 to 6 weeks. No prolonged adaptive program specifically exists for CIDs.

Similarly, the abovementioned energy appeal reaction as a consequence of systemic cytokine stimulation has been positively selected for transient nonlife-threatening inflammatory episodes 274.

Furthermore, genes that are associated with CIDs have been positively selected independent of CIDs. The theory of antagonistic pleiotropy - formulated by Williams in the 1950s - similarly applies to CIDs 275. This theory suggests that genes associated with CIDs have been positively selected to improve survival at younger ages and to stimulate reproduction independent of CIDs. Recent delineation shows that several CID risk genes have a pleiotropic meaning outside CIDs at younger ages 76.

Organisms evolved under conditions that favored the development of complex mechanisms for obtaining food and for storage and allocation of energy-rich fuels. Energy regulation and cellular bioenergetics take the highest position in the hierarchy of homeostatic control. The main supporters of energy-rich fuel storage in liver, muscle, and adipose tissue are insulin, insulin-like growth factor 1, androgens/estrogens, and the parasympathetic nervous system. We can call them storing factors. In contrast, provision of energy-rich fuels to the entire body in the form of glucose, protein, and fatty acids is mainly supported by mediator substances of the sympathetic nervous system, the hypothalamic-pituitary-hormonal axes (cortisol and growth hormone), and the pancreas (glucagon). We can call them provision factors. Table 3 describes particular aspects of the neuroendocrine immune response linking it to the energy appeal reaction.

The energy appeal reaction is not an unspecific fight-or-flight response in the sense of Hans Selye, but an adaptive program. If the adaptive program is used too long, real problems can appear that are a consequence of worn-out regulation. That exhausted regulation really exists is substantiated by the fact that patients on ICUs with severe activation of the stress system sometimes suffer from lifelong adrenal insufficiency even after overall recovery 77.

A longstanding reallocation program can thus lead to acute and chronic disease sequelae as mentioned in Table 3. The framework explains that CID sequelae are a consequence of a continuous energy appeal reaction. The systemic response of the body - the energy appeal reaction - is important to support the immune system during short-lived inflammatory episodes, but its continuous use in CIDs is highly unfavorable.

Since disease sequelae are a significant part of clinical CID, etiology of disease sequelae is also part of CID etiology. It becomes understandable that long-term changes of the neuroendocrine immune network as a consequence of a chronic energy appeal reaction are also part of etiological considerations. We conclude that among genetic issues, environmental factors (microbes, toxins, drugs, injuries, radiation, cultural background, and geography), exaggerated immune and wound responses, and irrecoverable tissue destruction, changes of the neuroendocrine immune network in the context of a prolonged energy appeal reaction become a fifth factor of CID etiology 78.