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Aims:CD40 and its ligand, CD40L, play a critical role in driving atherosclerotic plaque development. Disrupted CD40-signalling reduces experimental atherosclerosis and induces a favourable stable plaque phenotype. We recently showed that small molecule-based inhibition of CD40-tumour necrosis factor receptor associated factor-6 interactions attenuates atherosclerosis in hyperlipidaemic mice via macrophage-driven mechanisms. The present study aims to detail the function of myeloid CD40 in atherosclerosis using myeloid-specific CD40-deficient mice. Method and Results:Cd40flox/floxandLysM-cre Cd40flox/floxmice on anApoe−/−background were generated (CD40wtandCD40mac−/−, respectively). Atherosclerotic lesion size, as well as plaque macrophage content, was reduced inCD40mac−/−compared toCD40wtmice, and their plaques displayed a reduction in necrotic core size. Transcriptomics analysis of theCD40mac−/−atherosclerotic aorta revealed downregulated pathways of immune pathways and inflammatory responses. Loss of CD40 in macrophages changed the representation of aortic macrophage subsets. Mass cytometry analysis revealed a higher content of a subset of alternative or resident-like CD206+CD209b−macrophages in the atherosclerotic aorta ofCD40mac−/−compared toCD40wtmice. RNA-sequencing of bone marrow-derived macrophages ofCD40mac−/−mice demonstrated upregulation of genes associated with alternatively activated macrophages (includingFolr2,Thbs1,Sdc1, andTns1). Conclusions:We here show that absence of CD40 signalling in myeloid cells reduces atherosclerosis and limits systemic inflammation by preventing a shift in macrophage polarization towards pro-inflammatory states. Our study confirms the merit of macrophage-targeted inhibition of CD40 as a valuable therapeutic strategy to combat atherosclerosis.

Chronic low-grade inflammation, also called metaflammation, is associated with prevalent non-communicable diseases, such as atherosclerosis. Anti-inflammatory therapy provides a clinical benefit in patients, but the triggers that incite metaflammation remain largely unknown. To uncover non-genetic inflammatory factors influencing atherosclerosis severity, we performed an unbiased in vivo screen measuring thousands of host- and microbe-derived molecules in a mouse model of atherosclerosis. Machine learning-supported analyses identified, next to known pro- and anti-atherogenic factors, the medium fatty acid-chain sphingomyelin d18:1/14:0 (S14) as highly positively associated with atherogenesis. S14 activated macrophage innate immune signalling, immune-metabolic reprogramming and pro-inflammatory gene transcription. The inflammatory activity of S14 was TLR4- and MD-2-dependent, consistent with biochemical and molecular dynamics simulation analyses showing that S14 promoted the formation of active TLR4/MD-2 dimers. Human interventional and observational trials demonstrated that dietary changes altered concentrations of S14 and that increased circulating S14 was associated with carotid plaque development in obese individuals. Collectively, these findings identify an inducible endogenous ligand for TLR4 that drives metaflammation, providing the rationale for preventative approaches and pharmacological interventions that may curb detrimental inflammation secondary to Western-type lifestyle habits.

Supplementary Materials, Tables 1-4, Figures 1-10 from Different Tumor Microenvironments Contain Functionally Distinct Subsets of Macrophages Derived from Ly6C(high) Monocytes

Tumor-associated macrophages (TAM) form a major component of the tumor stroma. However, important concepts such as TAM heterogeneity and the nature of the monocytic TAM precursors remain speculative. Here, we show for the first time that mouse mammary tumors contained functionally distinct subsets of TAMs and provide markers for their identification. Furthermore, in search of the TAM progenitors, we show that the tumor-monocyte pool almost exclusively consisted of Ly6ChiCX3CR1low monocytes, which continuously seeded tumors and renewed all nonproliferating TAM subsets. Interestingly, gene and protein profiling indicated that distinct TAM populations differed at the molecular level and could be classified based on the classic (M1) versus alternative (M2) macrophage activation paradigm. Importantly, the more M2-like TAMs were enriched in hypoxic tumor areas, had a superior proangiogenic activity in vivo, and increased in numbers as tumors progressed. Finally, it was shown that the TAM subsets were poor antigen presenters, but could suppress T-cell activation, albeit by using different suppressive mechanisms. Together, our data help to unravel the complexities of the tumor-infiltrating myeloid cell compartment and provide a rationale for targeting specialized TAM subsets, thereby optimally “re-educating” the TAM compartment. Cancer Res; 70(14); 5728–39. ©2010 AACR.

Contains fulltext : 232648.pdf (Publisher’s version ) (Open Access) An increasing body of evidence shows a role for macrophages and monocytes (as their precursors) in hypertension, but with conflicting results with regard to whether they are protective or harmful. Therefore, we systematically reviewed the effect of macrophage interventions on blood pressure in animal models, to explore which factors determine the blood pressure increasing vs. decreasing effect. A search in PubMED and EMBASE yielded 9620 records, 26 of which were included. Eighteen studies (involving 22 different experiments (k = 22)) performed macrophage depletion, whereas 12 studies specifically deleted certain macrophage proteins. The blood pressure effects of macrophage depletion were highly various and directed toward both directions, as expected, which could not be reduced to differences in animal species or methods of hypertension induction. Prespecified subgroup analysis did reveal a potential role for the route in which the macrophage-depleting agent is being administrated (intraperitoneal vs intravenous subgroup difference of P = 0.07 (k = 22), or P < 0.001 in studies achieving considerable (ie, >50%) depletion (k = 18)). Along with findings from specific macrophage protein deletion studies-showing that deletion of one single macrophage protein (like TonEBP, endothelin-B, EP(4), NOX-2 and the angiotensin II type 1 receptor) can alter blood pressure responses to hypertensive stimuli-the indication that each route has its specific depletion pattern regarding targeted tissues and macrophage phenotypes suggests a determinative role for these features. These hypothesis-generating results encourage more detailed depletion characterization of each technique by direct experimental comparisons, providing a chance to obtain more knowledge on which macrophages are beneficial versus detrimental in hypertension development.

Cellular metabolism regulates immune cell activation, differentiation and effector functions, but current metabolic approaches lack single-cell resolution and simultaneous characterization of cellular phenotype. In this study, we developed an approach to characterize the metabolic regulome of single cells together with their phenotypic identity. The method, termed single-cell metabolic regulome profiling (scMEP), quantifies proteins that regulate metabolic pathway activity using high-dimensional antibody-based technologies. We employed mass cytometry (cytometry by time of flight, CyTOF) to benchmark scMEP against bulk metabolic assays by reconstructing the metabolic remodeling of in vitro-activated naive and memory CD8+ T cells. We applied the approach to clinical samples and identified tissue-restricted, metabolically repressed cytotoxic T cells in human colorectal carcinoma. Combining our method with multiplexed ion beam imaging by time of flight (MIBI-TOF), we uncovered the spatial organization of metabolic programs in human tissues, which indicated exclusion of metabolically repressed immune cells from the tumor–immune boundary. Overall, our approach enables robust approximation of metabolic and functional states in individual cells.

Contains fulltext : 251502.pdf (Publisher’s version ) (Open Access) Macrophages undergo extensive metabolic rewiring upon activation which assist the cell in roles beyond energy production and synthesis of anabolic building blocks. So-called immunometabolites that accumulate upon immune activation can serve as co-factors for enzymes and can act as signaling molecules to modulate cellular processes. As such, the Krebs-cycle-associated metabolites succinate, itaconate and alpha-ketoglutarate (αKG) have emerged as key regulators of macrophage function. Here, we describe that 2-hydroxyglutarate (2HG), which is structurally similar to αKG and exists as two enantiomers, accumulates during later stages of LPS-induced inflammatory responses in mouse and human macrophages. D-2HG was the most abundant enantiomer in macrophages and its LPS-induced accumulation followed the induction of Hydroxyacid-Oxoacid Transhydrogenase (HOT). HOT interconverts αKG and gamma-hydroxybutyrate into D-2HG and succinic semialdehyde, and we here identified this enzyme as being immune-responsive and regulated during the course of macrophage activation. The buildup of D-2HG may be further explained by reduced expression of D-2HG Dehydrogenase (D2HGDH), which converts D-2HG back into αKG, and showed inverse kinetics with HOT and D-2HG levels. We tested the immunomodulatory effects of D-2HG during LPS-induced inflammatory responses by transcriptomic analyses and functional profiling of D-2HG-pre-treated macrophages in vitro and mice in vivo. Together, these data suggest a role for D-2HG in the negative feedback regulation of inflammatory signaling during late-stage LPS-responses in vitro and as a regulator of local and systemic inflammatory responses in vivo. Finally, we show that D-2HG likely exerts distinct anti-inflammatory effects, which are in part independent of αKG-dependent dioxygenase inhibition. Together, this study reveals an immunometabolic circuit resulting in the accumulation of the immunomodulatory metabolite D-2HG that can inhibit inflammatory macrophage responses.

Metabolic reprogramming is a hallmark of activated T cells. The switch from oxidative phosphorylation to aerobic glycolysis provides energy and intermediary metabolites for the biosynthesis of macromolecules to support clonal expansion and effector function. Here, we show that glycolytic reprogramming additionally controls inflammatory gene expression via epigenetic remodeling. We found that the glucose transporter GLUT3 is essential for the effector functions of Th17 cells in models of autoimmune colitis and encephalomyelitis. At the molecular level, we show that GLUT3-dependent glucose uptake controls a metabolic-transcriptional circuit that regulates the pathogenicity of Th17 cells. Metabolomic, epigenetic, and transcriptomic analyses linked GLUT3 to mitochondrial glucose oxidation and ACLY-dependent acetyl-CoA generation as a rate-limiting step in the epigenetic regulation of inflammatory gene expression. Our findings are also important from a translational perspective because inhibiting GLUT3-dependent acetyl-CoA generation is a promising metabolic checkpoint to mitigate Th17-cell-mediated inflammatory diseases.

ATP-citrate lyase (Acly) is the target of the new class low-density lipoprotein-cholesterol (LDL-C)-lowering drug bempedoic acid (BA). Acly is a key metabolic enzyme synthesizing acetyl-CoA as the building block of cholesterol and fatty acids. Treatment with BA lowers circulating lipid levels and reduces systemic inflammation, suggesting a dual benefit of this drug for atherosclerosis therapy. Recent studies have shown that targeting Acly in macrophages can attenuate inflammatory responses and decrease atherosclerotic plaque vulnerability. Therefore, it could be beneficial to extend the application of Acly inhibition from solely lipid-lowering by liver-specific inhibition to also targeting macrophages in atherosclerosis. Here, we outline the possibilities of targeting Acly and describe the future needs to translate these findings to the clinic.

As key cells of the immune system, macrophages coordinate the activation and regulation of the immune response. Macrophages present a complex phenotype that can vary from homeostatic, proinflammatory, and profibrotic to anti-inflammatory phenotypes. The factors that drive the differentiation from monocyte to macrophage largely define the resultant phenotype, as has been shown by the differences found in M-CSF– and GM-CSF–derived macrophages. We explored alternative inflammatory mediators that could be used for in vitro differentiation of human monocytes into macrophages. IFN-γ is a potent inflammatory mediator produced by lymphocytes in disease and infections. We used IFN-γ to differentiate human monocytes into macrophages and characterized the cells at a functional and proteomic level. IFN-γ alone was sufficient to generate macrophages (IFN-γ Mϕ) that were phagocytic and responsive to polarization. We demonstrate that IFN-γ Mϕ are potent activators of T lymphocytes that produce IL-17 and IFN-γ. We identified potential markers (GBP-1, IP-10, IL-12p70, and IL-23) of IFN-γ Mϕ and demonstrate that these markers are enriched in the skin of patients with inflamed psoriasis. Collectively, we show that IFN-γ can drive human monocyte to macrophage differentiation, leading to bona fide macrophages with inflammatory characteristics.

Simple Summary Tumor cells show different metabolism compared to normal tissue. For instance, they increase ATP-citrate lyase (Acly) expression for increased proliferation. Acly is therefore currently being targeted for cancer therapy. However, targeting this enzyme may have side effects on other cells within the tumor microenvironment, including tumor-associated macrophages (TAMs) which are immune cells crucial for cancer progression. As Acly deletion previously showed increased pro-inflammatory responses of macrophages in vitro, we hypothesized that Acly-deficient TAMs may better control tumor growth. Here, we determined the effect of Acly deficiency specifically in pro-cancer anti-inflammatory macrophages and TAMs in two mouse models of cancer. While Acly is crucial for anti-inflammatory macrophage activation, Acly-deficient TAMs do not affect tumor growth. Together, our results indicate that targeting Acly as anti-cancer therapy has no adverse effects on TAMs. Abstract Cancer cells rely on ATP-citrate lyase (Acly)-derived acetyl-CoA for lipid biogenesis and proliferation, marking Acly as a promising therapeutic target. However, inhibitors may have side effects on tumor-associated macrophages (TAMs). TAMs are innate immune cells abundant in the tumor microenvironment (TME) and play central roles in tumorigenesis, progression and therapy response. Since macrophage Acly deletion was previously shown to elicit macrophages with increased pro- and decreased anti-inflammatory responses in vitro, we hypothesized that Acly targeting may elicit anti-tumor responses in macrophages, whilst inhibiting cancer cell proliferation. Here, we used a myeloid-specific knockout model to validate that absence of Acly decreases IL-4-induced macrophage activation. Using two distinct tumor models, we demonstrate that Acly deletion slightly alters tumor immune composition and TAM phenotype in a tumor type-dependent manner without affecting tumor growth. Together, our results indicate that targeting Acly in macrophages does not have detrimental effects on myeloid cells.

Isocitrate dehydrogenase (IDH) mutations produce high levels of the ‘oncometabolite’ R-2-hydroxyglutarate (R-2-HG) and play a key role in the initiation and progression of glioma tumors in the brain. A recent study in Nature Cancer by Friedrich et al. describes how IDH-mutant-derived R-2-HG elicits an immunosuppressive phenotype in glioma-associated macrophages. As such, the authors uncovered a new vulnerability that can be exploited for therapy.

As age is the greatest risk factor for the development of most prevalent chronic diseases, there is an enormous interest in understanding the process of aging, with the hope of delaying or preventing age-related comorbidities. Along these lines, a recent study by Minhas et al. (2021) describes how aged macrophages downregulate glycolysis and mitochondrial oxidative phosphorylation (OXPHOS), inducing an energy-deficient state that compromises macrophage function and supports maladaptive inflammation that together cause brain dysfunction.

Macrophages define a key component of immune cells present in atherosclerotic lesions and are central regulators of the disease. Since epigenetic processes are important in controlling macrophage function, interfering with epigenetic pathways in macrophages might be a novel approach to combat atherosclerosis. Histone H3K27 trimethylation is a repressive histone mark catalyzed by polycomb repressive complex with EZH2 as the catalytic subunit. EZH2 is described to increase macrophage inflammatory responses by supressing the suppressor of cytokine signaling, Socs3. We previously showed that myeloid deletion of Kdm6b, an enzymes that in contrast to EZH2 removes repressive histone H3K27me3 marks, results in advanced atherosclerosis. Because of its opposing function and importance of EZH2 in macrophage inflammatory responses, we here studied the role of myeloid EZH2 in atherosclerosis. A myeloid-specific Ezh2 deficient mouse strain (Ezh2del) was generated (LysM-cre+ x Ezh2fl/fl) and bone marrow from Ezh2del or Ezh2wt mice was transplanted to Ldlr-/- mice which were fed a high fat diet for 9 weeks to study atherosclerosis. Atherosclerotic lesion size was significantly decreased in Ezh2del transplanted mice compared to control. The percentage of macrophages in the atherosclerotic lesion was similar, however neutrophil numbers were lower in Ezh2del transplanted mice. Correspondingly, the migratory capacity of neutrophils was decreased in Ezh2del mice. Moreover, peritoneal Ezh2del foam cells showed a reduction in the inflammatory response with reduced production of nitric oxide, IL-6 and IL-12. In Conclusion, myeloid Ezh2 deficiency impairs neutrophil migration and reduces macrophage foam cell inflammatory responses, both contributing to reduced atherosclerosis.

Inflammasome-independent release of IL-1β in the tumor microenvironment recruits T cell–suppressive neutrophils that support cancer progression.

Summary Dendritic cells (DCs) are key in the initiation of the adaptive T cell responses to tailor adequate immunity that corresponds to the type of pathogen encountered. Oppositely, DCs control the resolution phase of inflammation and are able to induce tolerance after receiving anti-inflammatory cytokines or upon encounter of self-associated molecular patterns, such as α2-3 linked sialic acid (α2-3sia). Objective: We here investigated whether α2-3sia, that bind immune inhibitory Siglec receptors, would alter signaling and reprogramming of LPS-stimulated human monocyte-derived DCs (moDCs). Methods and Results: Transcriptomic analysis of moDCs stimulated with α2-3sia-conjugated dendrimers revealed differentially expressed genes related to metabolic pathways, cytokines, and T cell differentiation. An increase in genes involved in ATPase regulator activity, oxidoreductase activity, and glycogen metabolic processes was detected. Metabolic extracellular flux analysis confirmed a more energetic moDC phenotype upon α2-3sia binding as evidenced by an increase in both glycolysis and mitochondrial oxidative phosphorylation. TH1 differentiation promoting genes IFNL and IL27, were significantly downregulated in the presence of α2-3sia. Functional assays confirmed that α2-3sia binding to moDCs induced phosphorylation of Siglec-9, reduced production of inflammatory cytokines IL-12 and IL-6, and increased IL-10. Surprisingly, α2-3sia-differentiated moDCs promoted FoxP3+CD25+/-CD127- regulatory T cell differentiation and decreased FoxP3-CD25-CD127- effector T cell proliferation. Conclusions: In conclusion, we demonstrate that α2-3sia binding to moDCs, phosphorylates Siglec-9, alters metabolic pathways, cytokine signaling, and T cell differentiation processes in moDCs and promotes regulatory T cells. The sialic acid-Siglec axis on DCs is therefore, a novel target to induce tolerance and to explore for immunotherapeutic interventions aimed to restore inflammatory processes.

Macrophages are highly plastic immune cells that can adopt several activation states. Often, macrophages are stimulated in vitro with lipopolysaccharide (LPS) to elicit pro-inflammatory macrophages or interleukin (IL)-4 to obtain a homeostatic phenotype. Fundamental to these functional activation states is the regulation of cellular metabolic processes. The metabolic alterations underlying LPS- and IL-4-induced phenotypes have been described thoroughly with several -omics techniques. However, methods allowing for time- and cost-effective screening of cellular metabolism are rather limited. Here, we establish a comprehensive toolbox to assess cellular metabolism in a semi-high-throughput 96-well-plate-based format. We validated the approach on LPS- and IL-4-activated mouse and human macrophages by measuring extracellular flux, SCENITH, mitochondrial mass and membrane potential, glucose and lipid uptake, and mitochondrial substrate utilization. By discussing the experienced strengths, limitations and complementarities of the distinct techniques, we guide readers for efficient application of the toolbox in their immune subsets of interest.

Macrophages are innate phagocytic leukocytes that are highly present in solid tumors, where they are referred to as tumor-associated macrophages (TAMs). In solid tumors, the microenvironment is often immunosuppressive and hypoxic regions are prevalent. These hypoxic conditions impose tumor cells to reprogram their metabolism, shifting from oxidative phosphorylation to anaerobic glycolysis. This so-called glycolytic switch enables hypoxic tumor cells to survive, proliferate, and eventually to outcompete untransformed cells. The hypoxia-induced change in tumor cell metabolism leads to the production of oncometabolites, among which are the glycolytic end-metabolite lactate and the tricarboxylic acid cycle intermediate succinate. TAMs can react to these oncometabolites, resulting in an altered maturation and the adoption of pro-angiogenic features. These angiogenesis-promoting TAMs have been reported to cooperate with tumor cells in the formation of new vessels, and even have been considered an important cause of resistance against anti-angiogenic therapies. For a long time, the mechanisms by which lactate and succinate activated pro-angiogenic TAMs were not understood. Researchers now start to unravel and understand some of the underlying mechanisms. Here, the importance of microenvironmental cues in inducing different macrophage activation states is discussed, as well as the role of hypoxia in the recruitment and activation of pro-angiogenic macrophages. In addition, the latest findings on the oncometabolites lactate and succinate in the activation of angiogenesis supporting macrophages are reviewed. Finally, various oncometabolite-targeting therapeutic strategies are proposed that could improve the response to anti-angiogenic therapies. SIGNIFICANCE STATEMENT: Tumor-associated macrophages (TAMs) are known promotors of tumor neovascularization, and significantly contribute to the emergence of resistance to anti-angiogenic therapies. Recent evidence suggests that the angiogenesis promoting phenotype of TAMs can be activated by hypoxic tumor cell-derived oncometabolites, including lactate and succinate. Here, the latest findings into the lactate- and succinate-mediated mechanistic activation of pro-angiogenic TAMs are reviewed, and therapeutic strategies that interfere with this mechanism and may delay or even prevent acquired resistance to anti-angiogenic agents are discussed.

Macrophages represent a major immune cell population in atherosclerotic plaques and play central role in the progression of this lipid-driven chronic inflammatory disease. Targeting immunometabolism is proposed as a strategy to revert aberrant macrophage activation to improve disease outcome. Here, we show ATP citrate lyase (Acly) to be activated in inflammatory macrophages and human atherosclerotic plaques. We demonstrate that myeloid Acly deficiency induces a stable plaque phenotype characterized by increased collagen deposition and fibrous cap thickness, along with a smaller necrotic core. In-depth functional, lipidomic, and transcriptional characterization indicate deregulated fatty acid and cholesterol biosynthesis and reduced liver X receptor activation within the macrophages in vitro. This results in macrophages that are more prone to undergo apoptosis, whilst maintaining their capacity to phagocytose apoptotic cells. Together, our results indicate that targeting macrophage metabolism improves atherosclerosis outcome and we reveal Acly as a promising therapeutic target to stabilize atherosclerotic plaques., Inhibition of the metabolic enzyme ATP-citrate lyase can attenuate atherosclerosis by preventing dyslipidemia and potentially also by reducing macrophage-mediated inflammation. Here, the authors show that specific targeting of ACLY in macrophages results in more stable atherosclerotic plaques.

Fatty acids in tumor-derived exosomes induce a metabolic switch resulting in immune dysfunction in dendritic cells. Dendritic cells (DCs) are central regulators of antitumor immunity but are often suppressed within tumors. This results in poor T cell activation and limits the efficacy of checkpoint blockade therapy, which aims to reactivate antitumor immunity. Defining the factors that induce DC dysfunction within the immunosuppressive tumor microenvironment is therefore expected to enable improved outcomes after immunotherapy. One such proposed factor is the release of exosomes by tumor cells. Exosomes are extracellular vesicles that contain distinct components of the cells that secrete them and are important mediators of intercellular crosstalk. Tumor-derived exosomes (TDEs) are of particular interest, because the nucleotides, proteins, metabolites, and lipids they contain can suppress antitumor immunity. Yin et al. aimed to unravel the effect of TDEs on immunity with a focus on TDE-derived lipids. First, they demonstrated that TDEs not only induce lipid accumulation in DCs but also negatively affect their function in vitro. TDE-exposed DCs expressed more inhibitory checkpoint proteins and suppressive cytokines, were less capable of activating antitumor CD8+ T cells, and induced generation of immunosuppressive regulatory T cells. Next, they generated fluorescently labeled TC-1 cervical cancer cells that secrete traceable TDEs to demonstrate that TDEs were mainly captured by lipid-rich dysfunctional DCs within the tumor microenvironment in vivo. Lipidomic analysis of TDEs in comparison with non-cancer cell-derived exosomes revealed that the former contained fatty acids that directly induce lipid accumulation and immune dysfunction in DCs. Further transcriptomic analysis revealed that TDE-treated DCs displayed enriched expression of lipid metabolism genes. In particular, they were enriched for downstream targets of the ligand-activated transcription factor peroxisome proliferator-activated receptor α (PPARα). Using PPARα-knockout mice, the authors demonstrated that PPARα deficiency resulted in decreased DC lipid content and improved CD8+ T cell function, resulting in slower tumor growth in the MC38 colon carcinoma mouse model. Combination therapy of PPARα and immune checkpoint inhibition further inhibited MC38 tumor growth and improved survival. Mechanistically, the authors demonstrated that fatty acids within TDEs induced a switch from glycolysis toward mitochondrial fatty acid oxidation. Yet the molecular mechanisms by which this metabolic shift inhibits DC function remain to be investigated. An important future line of investigation is whether the observed TDE-induced metabolic changes and immune dysfunction are also at play in human tumor-infiltrating DCs. This is especially important from a translational perspective because it could help to predict response or resistance to checkpoint inhibition in the clinic and could pave the way to better designed combination immunotherapies.

Simple Summary Tumors often include many immune cells that are theoretically able to combat tumor growth. Yet, tumors can induce immune functions that support tumor growth via different routes. In this review, we discuss how cancer cell metabolism regulates the activity of macrophages within tumors and how this affects tumor progression. This is particularly relevant as metabolic pathways in both cancer and immune cells can serve as targets to improve cancer treatment. Abstract Tumors consist of a wide variety of cells, including immune cells, that affect tumor progression. Macrophages are abundant innate immune cells in the tumor microenvironment (TME) and are crucial in regulating tumorigenicity. Specific metabolic conditions in the TME can alter the phenotype of tumor-associated macrophages (TAMs) in a direction that supports their pro-tumor functions. One of these conditions is the accumulation of metabolites, also known as oncometabolites. Interactions of oncometabolites with TAMs can promote a pro-tumorigenic phenotype, thereby sustaining cancer cell growth and decreasing the chance of eradication. This review focuses on the metabolic cancer-macrophage crosstalk in the TME. We discuss how cancer cell metabolism and oncometabolites affect macrophage phenotype and function, and conversely how macrophage metabolism can impact tumor progression. Lastly, we propose tumor-secreted exosome-mediated metabolic signaling as a potential factor in tumorigenesis. Insight in these processes may contribute to the development of novel cancer therapies.

The Krebs cycle–derived immunometabolite itaconate constrains NLRP3 inflammasome activation and IL-1β release.

BackgroundPD1 immune checkpoint blockade (αPD1 ICB) has shown unparalleled success in treating many types of cancer. However, response to treatment does not always lead to tumor rejection. While αPD1 ICB relies on cytotoxic CD8+ T cells, antigen-presenting cells (APCs) at the tumor site are also needed for costimulation of tumor-infiltrating lymphocytes (TILs). It is still unclear how these APCs develop and function before and during αPD1 ICB or how they are associated with tumor rejection.MethodsHere, we used B16 mouse melanoma and MC38 colorectal carcinoma tumor models, which show differential responses to αPD1 ICB. The immune composition of ICB insensitive B16 and sensitive MC38 were extensively investigated using multi-parameter flow cytometry and unsupervised clustering and trajectory analyses. We additionally analyzed existing single cell RNA sequencing data of the myeloid compartment of patients with melanoma undergoing αPD1 ICB. Lastly, we investigated the effect of CD40 agonistic antibody on the tumor-infiltrating monocyte-derived cells during αPD1 ICB.ResultsWe show that monocyte-derived dendritic cells (moDCs) express high levels of costimulatory molecules and are correlated with effector TILs in the tumor microenvironment (TME) after αPD1 ICB only in responding mouse tumor models. Tumor-resident moDCs showed distinct differentiation from monocytes in both mouse and human tumors. We further confirmed significant enrichment of tumor-resident differentiated moDCs in patients with melanoma responding to αPD1 ICB therapy compared with non-responding patients. Moreover, moDCs could be targeted by agonistic anti-CD40 antibody, supporting moDC differentiation, effector T-cell expansion and anti-tumor immunity.ConclusionThe combined analysis of myeloid and lymphoid populations in the TME during successful and non-successful PD1 ICB led to the discovery of monocyte-to-DC differentiation linked to expanding T-cell populations. This differentiation was found in patients during ICB, which was significantly higher during successful ICB. The finding of tumor-infiltrating monocytes and differentiating moDCs as druggable target for rational combination therapy opens new avenues of anti-tumor therapy design.

CD8 + T cells accumulate lipids that drive metabolic dysfunction in tumors.

The increased incidence of cancer has been attributed to raised numbers of individuals with obesity/overweight worldwide. Different types of cancers in obese individuals have poor prognosis, high remission rate and resistance to traditional therapies. Literature has identified that lipid metabolism in lipid-laden immune cells, including macrophages, abrogates anti-tumoral responses. In this review we highlight the main metabolic features observed in macrophages in obesity and cancer, and discuss how metabolic changes during metabolic diseases elicits a phenotype in macrophages that could promote malignant progression. This perspective could support new diagnosis and treatment strategies for cancer patients in immunometabolic predisposed condition.

Summary Emerging research has identified metabolic pathways that are crucial for the proper regulation of immune cells and how, when deranged, they can cause immune dysfunction and disease progression. However, due to technical limitations such insights have relied heavily on bulk measurements in immune cells, often activated in vitro. But with the emergence of single-cell applications, researchers can now estimate the metabolic state of individual immune cells in clinical samples. Here, we review these single-cell techniques and their ability to validate common principles in immunometabolism, while also revealing context-dependent metabolic heterogeneity within the immune cell compartment. We also discuss current gaps and limitations, as well as identify future opportunities to move the field forward toward the development of therapeutic targets and improved diagnostic capabilities.

Metformin reverses an aging-associated inflammatory immune profile. Age is a major risk factor for many life-threatening diseases including cardiometabolic disease, cancer, and neurodegeneration. Although age is non-adjustable, the aging-associated inflammation that underlies these prevalent diseases is potentially targetable. Yet the pathways that control this so-called inflammaging and their probable contribution to adverse disease progression remain poorly understood. Bharath et al. set out to define the immune profile during aging to better understand which treatments may be beneficial to promote healthy aging by comparing immune cells isolated from the blood of healthy 30-year-old and 60-year-old people. After activation outside the body, T cells from the older group produced more cytokines associated with Th17, a subset of T lymphocytes that promotes inflammation in many diseases. Because the glycemic control drug metformin is known to suppress inflammation and may dampen inflammaging, the authors tested whether this type 2 diabetes medication could reduce age-related T cell inflammation. Although metformin effectively ameliorated the Th17 inflammaging profile toward a young profile in cultured T cells, it remains to be demonstrated whether such switch also occurs in vivo. Next, the authors demonstrated that the Th17 profile coincided with increased oxidative phosphorylation and mitochondrial mass in aged T cells. This is rather surprising because oxidative phosphorylation is generally believed to be anti-inflammatory, and boosting mitochondrial fitness is considered to improve healthy aging. One could speculate that the increased mitochondrial abundance compensates for the accumulation of defective mitochondria. Metformin not only restored mitochondrial bioenergetics and the associated production of reactive oxygen species, it also improved the blunted autophagy in T cells of 60-year-old subjects. Autophagy is a major protein recycling pathway, and blocking this homeostatic process was enough to induce an inflammaging Th17 profile in T cells of young people. Is there enough evidence to take metformin to rejuvenate our immune system and promote healthy aging? Not yet, because its effects on chronic low-grade inflammation in vivo and also the contribution of monocytes and macrophages as major drivers of inflammaging were not studied. Nevertheless, this article justifies the establishment of clinical trials that test the effects of metformin on health span.

Macrophage-derived meteorin-like promotes skeletal muscle repair. Macrophages are professional phagocytes that secure host defense. In parallel, these innate immune cells aid healing responses. The crucial importance of macrophages during muscle repair has recently received a lot of attention. Yet, macrophage functions during healing are so diverse that it has remained difficult to delineate precise mechanisms. Indeed, macrophages play crucial roles at both the initial inflammatory phase of muscle repair to clear injury-related debris and during the subsequent regenerative phase by promoting myofiber expansion. Unraveling the factors that coordinate this macrophage phenotype switching could not only reveal new targets to promote muscle repair but could also provide new ways to therapeutically reshape deranged macrophage activation in other settings. In a recent study, Bath et al. describe meteorin-like (METRNL) as an important regulator of skeletal muscle regeneration. METRNL is a protein that was originally shown to be produced in response to physiological stimuli to stimulate thermogenesis. Now, the authors show that METRNL is induced in damaged human muscle after resistance training and use whole-body and cell-specific Metrnl knock-out mice to further delineate the role of METRNL during muscle repair. In performing parabiosis experiments, bone marrow transplantations, and single-cell RNA sequencing, as well as using recombinant METRNL, the authors surprisingly found that mainly macrophage-derived METRNL mediated muscle repair in mice. Within macrophages, METRNL induced a proresolving phenotype and elicited insulin-like growth factor 1 production. This growth factor increased muscle stem cell proliferation and, as such, macrophage-mediated METRNL induction was shown to facilitate successful muscle regeneration. The potential translational relevance of this study lies in the observation that METRNL is induced in injured human muscle; the main caveat of this study is that the findings obtained in genetic models still need to be validated in humans. Another outstanding question is which stimuli trigger Metrnl expression in macrophages, and whether also other inflammatory insults can induce Metrnl. Last, these new findings raise the question whether METRNL could be employed to reprogram pro-inflammatory into anti-inflammatory macrophages and, as such, serve as a new therapeutic target for inflammatory diseases not directly related to muscle repair.

Tissue resident intestinal macrophages are known to exhibit an anti-inflammatory phenotype and produce little pro-inflammatory cytokines upon TLR ligation, allowing symbiotic co-existence with the intestinal microbiota. However, upon acute events such as epithelial damage and concomitant influx of microbes, these macrophages must be able to quickly mount a pro-inflammatory response while more inflammatory macrophages are recruited from the blood stream simultaneously. Here, we show that dietary intake of vitamin A is required for the maintenance of the anti-inflammatory state of tissue resident intestinal macrophages. Interestingly, these anti-inflammatory macrophages were characterized by high levels of Dectin-1 expression. We show that Dectin-1 expression is enhanced by the vitamin A metabolite retinoic acid and our data suggests that Dectin-1 triggering might provide a switch to induce a rapid production of pro-inflammatory cytokines. In addition, Dectin-1 stimulation resulted in an altered metabolic profile which is linked to a pro-inflammatory response. Together, our data suggests that presence of vitamin A in the small intestine enhances an anti-inflammatory phenotype as well as Dectin-1 expression by macrophages and that this anti-inflammatory phenotype can rapidly convert toward a pro-inflammatory state upon Dectin-1 signaling.

Macrophages are often prominently present in the tumor microenvironment, where distinct macrophage populations can differentially affect tumor progression. Although the metabolism of macrophages influences their function, little is known about the metabolic characteristics of tumor-associated macrophage (TAM) subsets. Using transcriptomic and metabolomic analyses, we now reveal that pro-inflammatory MHC-IIhi TAMs display a hampered TCA cycle, while reparative MHC-IIlo TAMs show a higher oxidative and glycolytic metabolism. Both TAM populations preferred lactate over glucose as a carbon source to fuel the TCA cycle. However, while lactate supported the oxidative metabolism in MHC-IIlo TAMs, it decreased the metabolic activity of MHC-IIhi TAMs. Lactate subtly affected the transcriptome of MHC-IIlo TAMs, increased L-arginine metabolism and enhanced T-cell suppressive capacity of these TAMs. Overall, our data uncover the metabolic intricacies of distinct TAM subsets and identify lactate as an important carbon source, and metabolic and functional regulator of TAMs.

Within atherosclerotic plaques, macrophages can take up cholesterol and thereby become lipid-laden cells. For long it has been thought that these foamy macrophages are the main contributors to chronic inflammatory responses in plaques. However, several publications lately highlighted that foamy plaque macrophages are less inflammatory than their non-foamy counterparts. Interestingly, a very recent EMBO Molecular Medicine paper by Wu et al. demonstrate that accumulation of lipids in tumor-associated macrophages (TAMs) elicits an immunosuppressive phenotype.