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Key Points Mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that is present in two complexes: mTORC1 and mTORC2. mTORC1 is the main energy and nutrient sensor of the cell: it senses the presence of amino acids, glucose, lipids and ATP to enable efficient activation of the network in response to growth factors, Toll-like receptor (TLR) ligands and cytokines. Activation of the mTOR pathway usually promotes an anabolic response that induces the synthesis of nucleic acids, proteins and lipids. In addition, it stimulates glycolysis and mitochondrial respiration. Emerging data suggest that this metabolic reconfiguration is required for specific effector functions in myeloid cells. Translational control of gene expression in myeloid immune cells has emerged as one way in which mTORC1 controls cellular processes such as migration, expression of type I interferon and pro-inflammatory or anti-inflammatory cytokines, and metabolic reprogramming. Counterintuitively, inhibition of mTORC1 during TLR triggering generally promotes interleukin-12 (IL-12) production and inhibits expression of IL-10 and type I interferon by dendritic cells (DCs); it also augments their T cell-stimulatory capacity. Inhibition of mTORC2 enhances a pro-inflammatory response and IL-12 production in DCs. Inhibition of mTORC1 in macrophages promotes autophagy, which is important for intracellular pathogen killing and clearance of ingested complex lipids such as low-density lipoprotein (LDL) cholesterol. mTORC2 is especially important for cell polarity and chemotaxis in neutrophils and mast cells. mTORC2 controls the leading edge as well as tail retraction during chemotactic migration. Activation of mTORC1 in NK cells by IL-15 triggers a glycolytic response, which is important for their proliferation and acquisition of cytotoxicity. The rapid response of innate immune cells requires metabolic reprogramming to support their specific effector functions. As discussed here, mTOR is a key regulator of this process: it senses the environmental and intracellular nutritional status of innate immune cells to dictate and optimize the inflammatory response. The innate immune system is central for the maintenance of tissue homeostasis and quickly responds to local or systemic perturbations by pathogenic or sterile insults. This rapid response must be metabolically supported to allow cell migration and proliferation and to enable efficient production of cytokines and lipid mediators. This Review focuses on the role of mammalian target of rapamycin (mTOR) in controlling and shaping the effector responses of innate immune cells. mTOR reconfigures cellular metabolism and regulates translation, cytokine responses, antigen presentation, macrophage polarization and cell migration. The mTOR network emerges as an integrative rheostat that couples cellular activation to the environmental and intracellular nutritional status to dictate and optimize the inflammatory response. A detailed understanding of how mTOR metabolically coordinates effector responses by myeloid cells will provide important insights into immunity in health and disease.

Macrophages are sentinel cells of the innate immune system and have important functions in development, tissue homeostasis, and immunity. These phylogenetically ancient cells also developed a variety of mechanisms to control erythropoiesis and the handling of iron. Red pulp macrophages in the spleen, Kupffer cells in the liver, and central nurse macrophages in the bone marrow ensure a coordinated metabolism of iron to support erythropoiesis. Phagocytosis of senescent red blood cells by macrophages in the spleen and the liver provide a continuous delivery of recycled iron under steady-state conditions and during anemic stress. Central nurse macrophages in the bone marrow utilize this iron and provide a cellular scaffold and niche to promote differentiation of erythroblasts. This review focuses on the role of the distinct macrophage populations that contribute to efficient iron metabolism and highlight important cellular and systemic mechanisms involved in iron-regulating processes.

Metabolic reprogramming is rapidly gaining appreciation in the etiology of immune cell dysfunction in a variety of diseases. Tuberculosis, schistosomiasis, and sarcoidosis represent an important class of diseases characterized by the formation of granulomas, where macrophages are causatively implicated in disease pathogenesis. Recent studies support the incidence of macrophage metabolic reprogramming in granulomas of both infectious and non-infectious origin. These publications identify the mechanistic target of rapamycin (mTOR), as well as the major regulators of lipid metabolism and cellular energy balance, peroxisome proliferator receptor gamma (PPAR-γ) and adenosine monophosphate-activated protein kinase (AMPK), respectively, as key players in the pathological progression of granulomas. In this review, we present a comprehensive breakdown of emerging research on the link between macrophage cell metabolism and granulomas of different etiology, and how parallels can be drawn between different forms of granulomatous disease. In particular, we discuss the role of PPAR-γ signaling and lipid metabolism, which are currently the best-represented metabolic pathways in this context, and we highlight dysregulated lipid metabolism as a common denominator in granulomatous disease progression. This review therefore aims to highlight metabolic mechanisms of granuloma immune cell fate and open up research questions for the identification of potential therapeutic targets in the future.

Macrophages are critical for granuloma formation, but the cell-intrinsic mechanisms remain unknown. Weichhart and colleagues demonstrate that chronic mTOR activity leads to macrophage-dependent granuloma formation, which may have relevance to sarcoidosis. The aggregation of hypertrophic macrophages constitutes the basis of all granulomatous diseases, such as tuberculosis or sarcoidosis, and is decisive for disease pathogenesis. However, macrophage-intrinsic pathways driving granuloma initiation and maintenance remain elusive. We found that activation of the metabolic checkpoint kinase mTORC1 in macrophages by deletion of the gene encoding tuberous sclerosis 2 ( Tsc2 ) was sufficient to induce hypertrophy and proliferation, resulting in excessive granuloma formation in vivo . TSC2-deficient macrophages formed mTORC1-dependent granulomatous structures in vitro and showed constitutive proliferation that was mediated by the neo-expression of cyclin-dependent kinase 4 (CDK4). Moreover, mTORC1 promoted metabolic reprogramming via CDK4 toward increased glycolysis while simultaneously inhibiting NF-κB signaling and apoptosis. Inhibition of mTORC1 induced apoptosis and completely resolved granulomas in myeloid TSC2-deficient mice. In human sarcoidosis patients, mTORC1 activation, macrophage proliferation and glycolysis were identified as hallmarks that correlated with clinical disease progression. Collectively, TSC2 maintains macrophage quiescence and prevents mTORC1-dependent granulomatous disease with clinical implications for sarcoidosis.

Sarcoidosis is a complex disease that belongs to the vast group of autoinflammatory disorders, but the etiological mechanisms of which are not known. At the crosstalk of environmental, infectious, and genetic factors, sarcoidosis is a multifactorial disease that requires a multidisciplinary approach for which genetic research, in particular, next generation sequencing (NGS) tools, has made it possible to identify new pathways and propose mechanistic hypotheses. Codified treatments for the disease cannot always respond to the most progressive forms and the identification of new genetic and metabolic tracks is a challenge for the future management of the most severe patients. Here, we review the current knowledge regarding the genes identified by both genome wide association studies (GWAS) and whole exome sequencing (WES), as well the connection of these pathways with the current research on sarcoidosis immune-related disorders.

The metabolic master regulator mTORC1 is shown to promote alternative macrophage activation via Semaphorin 6D reverse signaling and reprogramming of lipid metabolism.

Gastrointestinal bacteria interact with the host and each other through various mechanisms, including the production of extracellular vesicles (EVs). However, the composition and potential roles of EVs released by gut archaea are poorly understood. Here, we study EVs produced by four strains of human gut-derived methanogenic archaea: Methanobrevibacter smithii ALI, M. smithii GRAZ-2, M. intestini , and Methanosphaera stadtmanae . The size (~130 nm) and morphology of these EVs are comparable to those of bacterial EVs. Proteomic and metabolomic analyses reveal that the archaeal EVs are enriched in putative adhesins or adhesin-like proteins, free glutamic and aspartic acid, and choline glycerophosphate. The archaeal EVs are taken up by macrophages in vitro and elicit species-specific responses in immune and epithelial cell lines, including production of chemokines such as CXCL9, CXCL11, and CX3CL1. The EVs produced by M. intestini strongly induce pro-inflammatory cytokine IL-8 in epithelial cells. Future work should examine whether archaeal EVs play roles in the interactions of archaea with other gut microbes and with the host. The composition and potential roles of extracellular vesicles released by gut archaea are poorly understood. Here, Weinberger et al. show that extracellular vesicles produced by human gut-derived methanogenic archaea are enriched in adhesin-like proteins, can be taken up by macrophages, and elicit responses in immune and epithelial cells.

Background Cancer‐associated cachexia (CAC) is a wasting syndrome drastically reducing efficacy of chemotherapy and life expectancy of patients. CAC affects up to 80% of cancer patients, yet the mechanisms underlying the disease are not well understood and no approved disease‐specific medication exists. As a multiorgan disorder, CAC can only be studied on an organismal level. To cover the diverse aetiologies of CAC, researchers rely on the availability of a multifaceted pool of cancer models with varying degrees of cachexia symptoms. So far, no tumour model syngeneic to C57BL/6 mice exists that allows direct comparison between cachexigenic‐ and non‐cachexigenic tumours. Methods MCA207 and CHX207 fibrosarcoma cells were intramuscularly implanted into male or female, 10–11‐week‐old C57BL/6J mice. Tumour tissues were subjected to magnetic resonance imaging, immunohistochemical‐, and transcriptomic analysis. Mice were analysed for tumour growth, body weight and ‐composition, food‐ and water intake, locomotor activity, O2 consumption, CO2 production, circulating blood cells, metabolites, and tumourkines. Mice were sacrificed with same tumour weights in all groups. Adipose tissues were examined using high‐resolution respirometry, lipolysis measurements in vitro and ex vivo, and radioactive tracer studies in vivo. Gene expression was determined in adipose‐ and muscle tissues by quantitative PCR and Western blotting analyses. Muscles and cultured myotubes were analysed histologically and by immunofluorescence microscopy for myofibre cross sectional area and myofibre diameter, respectively. Interleukin‐6 (Il‐6) was deleted from cancer cells using CRISPR/Cas9 mediated gene editing. Results CHX207, but not MCA207‐tumour‐bearing mice exhibited major clinical features of CAC, including systemic inflammation, increased plasma IL‐6 concentrations (190 pg/mL, P ≤ 0.0001), increased energy expenditure (+28%, P ≤ 0.01), adipose tissue loss (−47%, P ≤ 0.0001), skeletal muscle wasting (−18%, P ≤ 0.001), and body weight reduction (−13%, P ≤ 0.01) 13 days after cancer cell inoculation. Adipose tissue loss resulted from reduced lipid uptake and ‐synthesis combined with increased lipolysis but was not associated with elevated beta‐adrenergic signalling or adipose tissue browning. Muscle atrophy was evident by reduced myofibre cross sectional area (−21.8%, P ≤ 0.001), increased catabolic‐ and reduced anabolic signalling. Deletion of IL‐6 from CHX207 cancer cells completely protected CHX207IL6KO‐tumour‐bearing mice from CAC. Conclusions In this study, we present CHX207 fibrosarcoma cells as a novel tool to investigate the mediators and metabolic consequences of CAC in C57BL/6 mice in comparison to non‐cachectic MCA207‐tumour‐bearing mice. IL‐6 represents an essential trigger for CAC development in CHX207‐tumour‐bearing mice.

Preeclampsia (PE) is a complex disease with unclear etiology. It is the most dangerous human pregnancy disease, causing morbidity and mortality in thousands of women and newborns worldwide. The soluble fms-like tyrosine kinase-1 (sFlt-1) to placental growth factor (PlGF) ratio is currently the best and only predictive biomarker. The higher the ratio, the more likely the pregnant women will develop PE. The molecular mechanism underlying the increased sFlt-1/PlGF ratio is not known. Here, we show that amino acid transporter LAT1 ( SLC7A5 ) and transcription factor NRF2 regulate this ratio via a previously unknown mechanism to produce sFlt-1 and PlGF in an anti-angiogenic ratio as observed in PE. In addition, we show that PE-associated oxidative stress, whose origin was unknown, is a secondary phenomenon caused by reduced NRF2 and LAT1 activity. The interdependence of the involved proteins, including also ATF4, Flt-1 and Akt, indicates that any disruption of the interaction would ultimately lead to a PE-like phenotype. Reduced placental angiogenesis is suspected to cause preeclampsia. Using placental in vitro models and an in vivo model, the authors uncover the key role of an amino acid transporter and related molecular interactions that together induce an anti-angiogenic state, as observed in preeclampsia.

Dairy cattle experience a profound nutrient deficit postpartum that is associated with immune dysfunction characterized by heightened inflammation and reduced pathogen clearance. The activation of the central nutrient-sensing mTOR pathway is comparatively reduced in leukocytes of early postpartum dairy cows during this time of most pronounced nutrient deficit. We assessed the effect of pharmacological mTOR inhibition (Torin-1, rapamycin) on differentiation of monocyte derived classically (M1) and alternatively (M2) activated macrophages (MPh) and dendritic cells (moDC) from 12 adult dairy cows. Treatment with mTOR inhibitors generated M1 MPh with increased oxidative burst and expression of IL12 subunits but decreased phagocytosis and expression of IL1B, IL6, and IL10. In M2 MPh, treatment inhibited expression of regulatory features (CD163, ARG2, IL10) skewing the cells toward an M1-like phenotype. In moDC, mTOR inhibition increased expression of pro-inflammatory cytokines ( IL12A, IL12B, IL1B, IL6 ) and surface CD80. In co-culture with mixed lymphocytes, mTOR-inhibited moDC exhibited a cytokine profile favoring a Th1 response with increased TNF and IFNG production and decreased IL10 concentrations. We conclude that mTOR inhibition in vitro promoted differentiation of inflammatory macrophages with reduced regulatory features and generation of Th1-favoring dendritic cells. These mechanisms could contribute to immune dysregulation in postpartum dairy cows.