Immunometabolism revealed the crucial role of cellular metabolism in controlling immune cell phenotype and functions. Macrophages, key immune cells that support progression of numerous inflammatory diseases, have been well described as undergoing vast metabolic rewiring upon activation. The immunometabolite succinate particularly gained a lot of attention and emerged as a crucial regulator of macrophage responses and inflammation. Succinate was originally described as a metabolite that supports inflammation via distinct routes. Recently, studies have indicated that succinate and its receptor SUCNR1 can suppress immune responses as well. These apparent contradictory effects might be due to specific experimental settings and particularly the use of distinct succinate forms. We therefore compared the phenotypic and functional effects of distinct succinate forms and receptor mouse models that were previously used for studying succinate immunomodulation. Here, we show that succinate can suppress secretion of inflammatory mediators IL-6, tumor necrosis factor (TNF) and nitric oxide (NO), as well as inhibit Il1b mRNA expression of inflammatory macrophages in a SUCNR1-independent manner. We also observed that macrophage SUCNR1 deficiency led to an enhanced inflammatory response without addition of exogenous succinate. While our study does not reveal new mechanistic insights into how succinate elicits different inflammatory responses, it does indicate that the inflammatory effects of succinate and its receptor SUCNR1 in macrophages are clearly context dependent.

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.

Macrophages are dynamic immune cells that can adopt several activation states. Fundamental to these functional activation states is the regulation of cellular metabolic processes. Especially in mice, metabolic alterations underlying pro-inflammatory or homeostatic phenotypes have been assessed using various techniques. However, researchers new to the field may encounter ambiguity in choosing which combination of techniques is best suited to profile immunometabolism. To address this need, we have developed a toolbox to assess cellular metabolism in a semi-high-throughput 96-well-plate-based format. Application of the toolbox to activated mouse and human macrophages enables fast metabolic pre-screening and robust measurement of extracellular fluxes, mitochondrial mass and membrane potential, and glucose and lipid uptake. Moreover, we propose an application of SCENITH technology for ex vivo metabolic profiling. We validate established activation-induced metabolic rewiring in mouse macrophages and report new insights into human macrophage metabolism. By thoroughly discussing each technique, we hope to guide readers with practical workflows for investigating immunometabolism.

The accumulation of foamy macrophages is a pathological hallmark of demyelinating brain disorders. Perturbed metabolism and efflux of intracellular lipids underlie the development of a harmful foamy macrophage phenotype in these disorders, yet, the molecular mechanisms underlying this dysregulation are poorly understood. Here, we show that the ubiquitin-proteasome system controls the turnover of the cholesterol efflux transporter ATP-binding cassette A1 (ABCA1) in lipid-loaded macrophages in the brain. We report that accumulation of myelin-derived lipids promotes the abundance and activity of ubiquitin-protein E3 ligase A (UBE3A) in macrophages, which stimulates ABCA1 ubiquitination and subsequent degradation. This boosts cellular lipid accumulation and induces an inflammatory macrophage phenotype that impairs remyelination. We further establish Tat-interacting protein 30 (TIP30), an inhibitor of importin β-mediated nuclear import, as an essential regulator of cytosolic UBE3A levels. Together, our findings identify UBE3A as a driver of foam cell formation and indicate that targeting UBE3A-mediated ABCA1 degradation is a promising strategy to enhance central nervous system repair.

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.

Microglia are essential regulators of myelin integrity and repair, yet their regenerative capacity declines with ageing and in neurodegenerative diseases such as multiple sclerosis (MS). Neonatal microglia retain a uniquely reparative program that may offer insight into restoring lost functions in the adult CNS. Here we show that transplantation of neonatal microglia ameliorates disability, reduces leukocyte infiltration, and promotes remyelination in both inflammatory (EAE) and non-inflammatory (cuprizone) models, and reverses cognitive decline in aged mice. These benefits persisted even when transplanted cells remained confined to the meninges and were reproduced by the neonatal microglia secretome, indicating a paracrine mechanism. Multi-omic profiling revealed that the neonatal secretome is enriched in trophic factors and membrane-building lipids compared to adult microglia, while transcriptomic analyses of treated aged brains showed reactivation of developmental repair pathways and suppression of inflammatory signatures. Together, these results demonstrate that neonatal microglia re-engage rejuvenation-like programs in the adult CNS and highlight the importance of multifactorial strategies, integrating trophic, metabolic, and immunomodulatory cues, over single-target approaches. Our findings establish early microglial programs as a paradigm for designing new regenerative therapies for CNS disorders.