Adipose tissue is not only a storage depot for excess energy but also an endocrine organ that regulates energy homeostasis through the secretion of adipokines (Rosen and Spiegelman 2006; Lee and Shao 2014). Adipose tissue macrophages (ATMs) are an essential component of the immune system within adipose tissue, playing a critical role in maintaining metabolic health (Li et al. 2020; Rahman and Jun 2022). ATMs are a heterogeneous population of cells that can either promote or inhibit inflammation, depending on their phenotype and activation state (Daemen and Schilling 2020).

Macrophages, in general, have a crucial role in initiating immune responses by detecting initial immune insults and releasing various inflammatory mediators, including cytokines and chemokines (Medzhitov 2008). Their biology is complex and influenced by multiple factors such as ontogeny, tissue environment, and their specific inflammatory status within their respective niches (Bouloumié et al. 2005; Wynn et al. 2013). In the context of metabolic disorders, adipose tissue macrophages (ATMs) play a critical role in obesity-related inflammation and diseases such as type 2 diabetes and cardiovascular disorders. Obesity-induced inflammation in adipose tissue is recognized as a major pathogenic factor contributing to insulin resistance (Heilbronn and Campbell 2008). During obesity, the accumulation of ATMs in adipose tissue becomes a significant source of pro-inflammatory cytokines (Coenen et al. 2007). This chronic inflammation not only expands adipose tissue but also contributes to systemic inflammation, ultimately leading to the development of insulin resistance, type 2 diabetes, and metabolic syndrome (Monteiro and Azevedo 2010; Esser et al. 2014). Given the critical role of ATMs in the development of metabolic disorders, targeting these cells has become a promising therapeutic strategy for treating obesity and related metabolic disorders (Herrada et al. 2021). Recent studies have demonstrated that modulating ATMs activation and polarization can improve metabolic health and reduce the risk of metabolic diseases (Vieira-Potter 2014).

In this review, we provide an overview of the current understanding of the role of ATMs in adipose tissue metabolism and consequently in metabolic health and disease. We also discuss recent advances in the development of therapeutic strategies targeting ATMs, including pharmacological interventions and targeted delivery. Finally, we highlight the opportunities for developing novel therapies targeting ATMs for treating metabolic disorders.

In this section, we provide an overview of the current understanding of heterogeneous ATMs and their role in metabolic health and disease, as also summarized in Fig. 1. We discuss the molecular mechanisms underlying their function and the implications of their activation in metabolic disorders. We also highlight the potential therapeutic opportunities for targeting ATMs subtypes in the treatment of these diseases.

Figure 1. (A) In obesity, TREM2+ macrophages increase and accumulate in dying adipocytes or crown-like structures (CLS). These macrophages display enhanced lipid metabolism and phagocytosis genes, which can improve obesity-induced dysfunction. (B) TIM4+ macrophages can uptake lipoproteins with CD36 and mediate lysosomal function, resulting in the release of high-density lipoprotein (HDL) cholesterol. Additionally, TIM4+ macrophages release PDGF-cc, which can induce adipocyte hypertrophy and increase lipid storage in adipocytes. (C) CD9+ macrophages compose lipid droplets through lysosome-dependent lipolysis after lipid uptake. CD9+ Lipid-laden macrophages are located in CLS and exhibit high proinflammatory genes. (D) CD11c+ CD64+ double-positive adipose tissue macrophages are called vascular-associated macrophages. These vascular-associated macrophages express anti-inflammatory genes and release VEGFA, which plays a role in regulating vascular homeostasis. (E) SAMs that inhibit lipolysis in adipocytes, caused by norepinephrine secreted by neurons, increase in obesity situations. These macrophages uptake norepinephrine through transporters, SLC1A2, and inhibit adrenergic signaling in adipocytes. TREM2, triggering receptor expressed on myeloid cells 2; TIM4, T-cell, immunoglobulin, mucin; PDGF-cc, platelet-derived growth factor; Abca1, ATP-binding cassette transporter ABCA1; VEGFA, vascular endothelial growth factor A; SAMs, sympathetic neuron-associated macrophages; NE, norepinephrine; SLC6A2, solute carrier family 6 member 2.

Lipid-associated macrophages in adipose tissue

Lipid-associated macrophages (LAMs) are a unique population of tissue-resident macrophages that are found in close proximity to lipid-laden cells in various tissues, including adipose tissue, liver, and brain (van Eijk and Aerts 2021). LAMs have been shown to play a critical role in regulating tissue homeostasis and function in both health and disease (Wculek et al. 2022). The importance of LAMs has been highlighted in a growing body of literature, with studies demonstrating their role in the pathogenesis of metabolic disorders, such as obesity, insulin resistance, non-alcoholic fatty liver disease (NAFLD), and atherosclerosis (Hou et al. 2021; Endo-Umeda et al. 2022).

1) Trem2+ macrophages

Single-cell transcriptomic analysis of gonadal white adipose tissue isolated macrophages revealed that the prominent changes induced by obesity (diet-induced obesity and db/db obesity mouse models) include an expansion of the adipose tissue macrophages and a reduction in regulatory T cells and type 2 innate lymphoid cells (ILCs) (Jaitin et al. 2019). A specific subset of macrophages exhibited a high expression of genes related to lipid metabolism (lipase A (Lipa), lipoprotein lipase (Lpl), cathepsin L (Ctsl), fatty acid binding protein 4 (Fabp4) and fatty acid binding protein (Fabp5)) and phagocytosis (triggering receptor expressed on myeloid cells 2 (Trem2), Cd9 and Cd36). In this study, Trem2 was identified as a marker of the LAM subset (Jaitin et al. 2019). Similar to findings from mouse obesity models, human LAMs expressing TREM2 were detected in adipose tissue of patients with obesity and formed a distinct cluster with a highly conserved gene signature (Jaitin et al. 2019) (Fig. 1A). Further, genetic deletion of Trem2 causes an increase in adipose tissue hypertrophy and induced systemic hypercholesterolemia and glucose intolerance in obese mice, suggesting that LAMs like Trem2+ macrophages affect lipid metabolism in adipose tissue (Jaitin et al. 2019).

2) Tim4+ Macrophages

T cell immunoglobulin and mucin domain containing 4 (Tim4)+ ATMs have been identified and their critical roles in adipose tissue remodeling have been demonstrated by several studies (Cox et al. 2021; Magalhaes et al. 2021) (Fig. 1B). Magalhaes et al. (2021) investigated the effect of high lipid-content food consumption on ATMs subtypes and conducted single-cell RNA-seq profiling of ATMs from gonadal WAT from mice after 24 hours of high-fat diet (HFD). This analysis identified subtypes of macrophages expressing lymphatic vessel endothelial hyaluronan receptor 1 (Lyve1), Tim4, and these ATMs showed high expression of ATP binding cassette subfamily A member 1 (ABCA1) and improved lysosomal lipolysis capacity. Inhibition of the phosphatidylserine receptor, T-cell immunoglobulin mucin protein 4 (Tim4) resulted in a deficiency in lysosomal-lipolysis activation and impaired release of post-prandial high-density lipoprotein (HDL) cholesterol (Magalhaes et al. 2021). This suggests that the Tim4 blockade resulted in the deregulation of post-prandial cholesterol trafficking as well as ATM lysosomal function suppression. In the other study, Tim4+ ATMs express platelet-derived growth factor-cc (Pdgf-cc) induced by diet, which regulates the lipid storages and size of adipocytes through leptin and C-C chemokine receptor type 2- independent manner (Cox et al. 2021). Cox et al. (2021) indicated that Tim4+ATMs can facilitate the lipogenesis pathway by regulating Pdgf-cc which interferes with the expression of negative lipogenic regulators.

3) Cd9+ Macrophages

To evaluate the different subtypes of ATMs in lean and obese groups, macrophages were sorted by flow cytometry (Hill et al. 2018). In obese mice, Cd11b+Ly6c+ ATMs were derived from monocytes that were accumulated outside the crown-like structures (CLS) and exhibited high adipogenesis-related gene expression (Hill et al. 2018). In contrast, Cd9+ ATMs were located in CLS and expressed proinflammatory genes (Fig. 1C). Two subtypes of macrophages, Cd11b+Ly6c+ATMs, and Cd9+ATMs, also differed in their lipid content, Cd9+ATMs are lipid-rich while Cd11b+Ly6c+ATMs are not. Furthermore, Cd9+ATMs located in CLS exhibited lipid accumulation in lysosome-like structure and highly expressed genes related to lysosomal-dependent lipid metabolism (Hill et al. 2018). Other studies have also demonstrated that Cd9+ ATMs serve as a marker for lipid-loaded macrophages and improved the function of phagocytosis and lipid metabolism (Jaitin et al. 2019).

Vasculature-associated ATMs

Silva et al. (2019) identified a specific subset of visceral ATMs characterized by Cd11c+Cd64+ double positivity and association with vasculature in response to a chronic HFD (Fig. 1D). This subpopulation manifests increased expression levels of genes related to anti-inflammatory responses and detoxification, while endocytic function decreased (Silva et al. 2019). Another study investigated Cd11c+ATMs that are located in close proximity (< 1 µm) to the blood vasculature in obese mice (Chen et al. 2021). Additionally, Cd11c+ATMs expressed vascular endothelial growth factor A (Vegfa), which is known as a vascular homeostatic factor (Chen et al. 2021). These studies suggested that vasculature-associated ATMs contribute to maintaining adipose tissue homeostasis in metabolic stress and inflammation conditions.

Sympathetic neuron-associated macrophages (SAMs)

Sympathetic neuron-associated macrophages (SAMs) have been identified as a macrophage group that mediates the clearance of norepinephrine (NE) (Pirzgalska et al. 2017). They demonstrated that transporter inhibition, such as solute carrier family 6 member 2 (Slc6a2), interferes with the ability of SAMs to remove NE (Pirzgalska et al. 2017) (Fig. 1E). In mice, the genetic deletion of Slc6a2 in SAMs increased brown adipose tissue (BAT) ratio, browning of white fat, and a boost in thermogenesis, resulting in significant and sustained weight loss (Pirzgalska et al. 2017).

Mitochondrial transfer by macrophages

To support an effective inflammatory response, cells require reprogramming of mitochondrial metabolism to provide them with the necessary energy and intermediates (Marchi et al. 2023). Disrupted mitochondrial function has been linked to the pathophysiology of type 2 diabetes, obesity, dyslipidemia, and cardiovascular disease (Maassen et al. 2004). A recent investigation showed that macrophages residing in the healthy heart actively ingested material, including mitochondria, produced from cardiomyocytes (Nicolás-Ávila et al. 2020). The role of mitochondrial uptake by macrophages has been investigated in ATMs as well (Brestoff et al. 2021; Rosina et al. 2022).

The study on adipose tissue revealed that a specific type of macrophage acquires mitochondria from adipocytes in vivo, and this process is influenced by diet-induced obesity (Brestoff et al. 2021). The acquisition of mitochondria by macrophages is facilitated by heparan sulfates (HS), as indicated by a genome-wide CRISPR-Cas9 knockdown screen. Brestoff et al. (2021) demonstrated that macrophages acquiring mitochondria from adipocytes exhibited an increase in mitochondrial reactive oxygen species (ROS) production. Additionally, these macrophages displayed transcriptomic profiles enriched with genes involved in the hypoxia-inducible factor 1a/tissue factor pathway and showed upregulation of mtDNA-encoded genes as well as anti-inflammatory genes (Brestoff et al. 2021). The study revealed that a lower HS content was observed on epidydimal white adipose tissue (eWAT) macrophages, along with a reduction in intercellular mitochondrial translocation from adipocytes to macrophages in HFD-induced obese mice (Brestoff et al. 2021). Furthermore, the deletion of HS synthesizing gene exostosis glycosyltransferase 1 (Ext1) in myeloid cells decreased mitochondrial transport to eWAT macrophages, increased eWAT mass, reduced energy expenditure, and aggravated HFD-induced obesity (Brestoff et al. 2021).

A recent study has shown that mitochondria can be exchanged between cells to maintain metabolic homeostasis in adipose tissue (Rosina et al. 2022). This study demonstrated that extracellular vesicles (EVs) containing fragments of oxidatively damaged mitochondrial are released from brown adipocytes due to cold stress. When ATMs were deficient, these damaged mitochondria fragments were re-uptaken by surrounding brown adipocytes, resulting in a decrease in mitochondria protein levels in adipose tissue and impairment in thermogenic response (Rosina et al. 2022). Preserving BAT physiology depends on the phagocytic activity of macrophages residing in BAT, which plays a crucial role in eliminating extracellular damaged mitochondrial vesicles (Rosina et al. 2022).

ATM-mediated regulation of lipid metabolism in adipose tissue

Dysregulated lipid metabolism is linked to conditions such as obesity, diabetes, heart disease, and inflammation (Athyros et al. 2018). ATMs play a crucial role in controlling lipid metabolism in adipose tissue, as identified in numerous studies (Dahik et al. 2020).

In diet-induced obese mice, exogenous treatment of interleukin 25 (IL-25) reduces lipid accumulation in adipose tissues, along with increased anti-inflammatory gene expression such as arginase 1 (Arg1) and interleukin 13 (Il-13) in the eWAT resident ATMs (Feng et al. 2018). Consistent with these in vivo results, in vitro IL-25 treatment on RAW 264.7 macrophages co-cultured with adipocytes upregulates genes involved in lipolysis and mitochondrial oxidative metabolism and downregulates genes involved in lipogenesis in adipocytes. These findings indicate that the macrophages polarized to M2 by treating IL-25 functions to regulate the lipid metabolism in eWAT (Feng et al. 2018).

Lipolysis induced by catecholamines in adipose tissue decreases with age (Farrell and Howlett 2008). Camell et al. showed that Cd11b+ macrophages decline in visceral adipose tissue with age, and these aged ATMs inhibit the effects of non-adrenaline-induced lipolysis (Camell et al. 2017). Through global transcriptomic analysis of ATMs from visceral WAT, they demonstrated that aged ATMs upregulate genes involved in the activation of nod-like receptor (NLR) family pyrin domain containing 3 (Nlrp3) inflammasome and it directly inhibits lipolysis. They showed that Nlrp3 gene deficiency inhibited decreasing Cd11b+ ATMs in aging and recovered the aging-related loss of catecholamine-induced lipolysis (Camell et al. 2017). Furthermore, it was shown to restore noradrenaline-degrading genes growth differentiation factor 3 (Gdf3) and monoamine oxidase A (Maoa) (Camell et al. 2017). This study suggests activation of Nlrp3 inflammasome in aged ATMs can modulate catecholamine-induced lipolysis.

Analysis of the mouse model indicated that the ATMs in obese models induced by diet have elevated levels of growth differentiation factor 3 (GDF3) in Cd11c+ macrophages surrounding adipocytes (Bu et al. 2018). GDF3, secreted by CD11c+macrophages, acts as an activin receptor-like kinase 7 (ALK7) ligand in adipocytes, inhibiting lipolysis and leading accumulation of fat and insulin resistance (Bu et al. 2018). In ex vivo experiments, insulin treatment to isolated epididymal Cd11c -ATMs indicated increased expression of both Cd11c and GDF3. Furthermore, insulin treatment in vivo indicated increased WAT mass and declined lipases such as adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) transcripts by GDF3-ALK7 signaling (Bu et al. 2018). These findings suggest that GDF3 secreted by ATMs may be involved in insulin-induced regulation of adipose tissue.

The gene, hypoxia-inducible lipid droplet-associated protein (Hilpda), which is related to the lipid droplet deposition in other tissues, is found to be upregulated in ATMs from gonadal WAT under HFD (van Dierendonck et al. 2020). Deletion of Hilpda in macrophage led to reduced lipid lead in BMDM after treatment with oleate and palmitic acid (van Dierendonck et al. 2020). In vivo study indicated that Hilpda KO reduced lipid droplet accumulation ATMs under HFD feeding without affecting the inflammatory cytokines (van Dierendonck et al. 2020). The study revealed the Hilpda gene plays a role in enhancing lipids in ATMs by inhibiting ATGL-mediated lipolysis.

Regulatory roles of autophagy in ATM

Autophagy is a cellular process that degrades and recycles damaged organelles and proteins (Su et al. 2020). In macrophages, autophagy plays a crucial role in maintaining their immune functions (Wu and Lu 2019). Recent studies have shown that autophagy regulation in ATMs can also impact adipose tissue metabolism (Ferhat et al. 2019). The inhibition of macrophage autophagy by macrophage-specific autophagy related 7 (Atg7) KO has been linked to the development of obesity (Kang et al. 2016). Increased ROS due to autophagy inhibition in macrophages induced M1 macrophage polarization and secretion of interleukin-1beta (Il-1β), interleukin-18 (Il-18), and tumor necrosis factor-alpha (Tnf-α) cytokines and interfered with insulin signaling in adipose tissue (Kang et al. 2016). These findings suggest that the modulation of autophagy in ATMs could be a potential therapeutic strategy for treating metabolic disorders associated with adipose tissue dysfunction.

A suggestion has been made that insulin resistance is caused by systemic inflammation that triggers the innate immune system’s activation (da Cruz Nascimento et al. 2022). Fig. 2 illustrates the recently discovered therapeutic targets of ATMs in relation to inflammatory responses.

Figure 2. (A) Dietary uptake of BGD improves adipose tissue inflammation through upregulated secretion of IL-10 and reducing inflammation agents. (B) The inhibitor of TACE, ATS-9R indicated capabilities that reduce inflammation, and improved glucose tolerance and insulin sensitivity in HFD mice. (C) Small-molecule inhibitor called extracellular capthesin D, CTD-002 reduces liver inflammation, improves cholesterol metabolism, and induces a decrease in plasma insulin levels. (D) In cold conditions, SLIT3 secreted by ATMs binds to ROBO1 of sympathetic neurons and activates the neuron to induce adipocyte thermogenesis. (E) Protein Meteorin-like protein (METRNL) activates adipose tissue macrophages (ATMs), induces the release of IL-4, and enhances thermogenesis. (F) Near-infrared fluorophore (IR-61) promotes macrophage mitochondria oxidative phosphorylation and improves chronic inflammation and adipose tissue and hypertrophy. BDG, (1,3) (1,6)-beta-D-glycans; ATS-9R, tumor necrosis factor-alpha (TNF-α) converting enzyme (TACE) inhibitor; SILT3, slit guidance ligand 3; ROBO1, roundabout guidance receptor 1; Metrnl, meteorin-like protein; IR-61, near-infrared fluorophore.

Polysaccharide

Studies on animals suggest that particular dietary fibers, such as (1,3) (1,6)-beta-D-glycans (BDG), can significantly affect immune activity, for example, increasing the production of the anti-inflammatory cytokine interleukin-10 (Il-10) and decreasing the secretion of inflammatory agents (Kohl et al. 2009) (Fig. 2A). Moreover, changes in pro- and anti-inflammatory markers in adipose tissue and serum after 4 weeks of consuming BDG in obese people showed the same results as in animal studies (Kohl et al. 2009).

Peptides

Yong et al. (2017) developed a novel gene delivery system that utilizes oligopeptide (ATS-9R) to inhibit the activity of the tumor necrosis factor-alpha (TNF-α) converting enzyme (TACE) activity in visceral adipose tissue (Fig. 2B). This silencing of TACE activity reduced the inflammatory effects of TNF-α, as well as other inflammatory cytokines such as interleukin-6 (Il-6), monocyte chemoattractant Protein-1 (Mcp-1), Cd11c, and Il-1β. Furthermore, inhibiting TACE activity was found to improve blood glucose tolerance and insulin sensitivity in diet-induced obese mice. Additionally, the inhibition of TACE activity induced ATMs to form CLS around adipocytes in visceral adipose tissue (Yong et al. 2017).

The study investigated the effects of a small-molecule inhibitor called extracellular cathepsin D (CTSD) (CTD-002), which has demonstrated high selectivity and efficacy, on NAFLD disease (Khurana et al. 2019) (Fig. 2C). Previous studies have shown that CTSD is associated with cholesterol metabolism and inflammation, and Khurana et al. further identified what role CTSD plays in NAFLD (Moallem et al. 2011). The results indicated that the incubation of hepatic HepG2 cells with conditioned media from macrophages treated with CTD-002 resulted in decreased inflammation and improvement in cholesterol metabolism (Khurana et al. 2019). Additionally, CTD-002 treatment reduced hepatic steatosis in rats fed HFD and showed a significant decline in plasma levels of insulin and hepatic transaminases (Khurana et al. 2019).

According to a recent study, the macrophage cytokine slit guidance ligand 3 (SLIT3) is released by inguinal white adipose tissue (iWAT) macrophages and helps mice adapt to cold weather by enhancing sympathetic innervation and thermogenesis (Wang et al. 2021b) (Fig. 2D). SLIT3 stimulates Ca2+/calmodulin-dependent protein kinase II signaling and NE release through binding to the roundabout guidance receptor 1 (ROBO1) receptor on sympathetic neurons, which increases adipocyte thermogenesis. Mice lacking slit guidance ligand 3 (Slit3) in myeloid cells are more susceptible to the effects of cold and tend to gain weight, while adoptive transfer of Slit3-overexpressing M2 macrophages to iWAT enhances thermogenesis and browning (Wang et al. 2021b).

Protein meteorin-like (METRNL) is a circulating factor that is induced in muscle after exercise and in the adipose tissue upon cold exposure (Rao et al. 2014) (Fig. 2E). Elevated METRNL levels increase energy expenditure and improved glucose tolerance, and promote alternative activation of ATMs (Rao et al. 2014). Conversely, blocking METRNL in vivo significantly attenuates cold-exposure-induced alternative macrophage activation and thermogenic gene responses. These findings suggest that METRNL possesses therapeutic potential for metabolic and inflammatory diseases (Rao et al. 2014).

RNA-based therapeutics

In obese mice, siRNA encapsulated within the glucan shell was found to selectively silence genes in epididymal ATMs, without affecting other organ macrophages, including subcutaneous adipose tissue (Aouadi et al. 2013). Treatment of glucan-encapsulated siRNA particles (GeRPs) silenced epididymal ATMs- specifically inflammatory cytokines, TNF-α, or osteopontin, which led to improvements in glucose tolerance (Aouadi et al. 2013).

A polymer, lipid hybrid high-density lipoprotein-mimicking nanoparticle (HNP) loaded with anti-miR155 was developed for simultaneous anti-atherogenic effects on macrophages (Lu et al. 2017). The HNP exhibited macrophage-specific targeting with high transfection efficiency and could circumvent the endolysosomal pathways through transcription endocytosis. In vitro experiments demonstrated that the anti-miR155 loading HNP improved the biological function of preventing atherosclerosis, displaying antioxidative and cholesterol efflux-facilitating properties (Lu et al. 2017). To enhance short interfering RNA (siRNA) delivery to M2 polarized macrophages, α-mannose nanohydrogel particles (ManNP) with mannose residues were synthesized (Kaps et al. 2020). Immunosuppressive M2 macrophages have increased in diseases such as liver fibrosis and cancer, and ManNP is used to target the mannose receptor, CD206, which is highly expressed in M2 macrophages (Kaps et al. 2020).

To assess their in vivo targeting capabilities, double-labelled siRNA-loaded ManNP with distinct infrared tags were tested in an experimental mice model of liver fibrosis, characterized by an increase in M2-type macrophages (Kaps et al. 2020). The results indicated that siRNA-ManNP displayed strong colocalization CD206+ M2-type macrophages, while untargeted counterparts (NonNP) exhibited weak colocalization and non-specific uptake by other liver cells. Additionally, ManNP showed good biocompatibility and did not cause renal or hepatic irritation, as confirmed by serological analysis (Kaps et al. 2020).

Another study has shown that chemically modified mannose-siRNA conjugates are used to efficiently deliver contents to macrophages and dendritic cells (DCs) via the selective targeting CD206 (Uehara et al. 2022). Moreover, in vivo gene silencing experiments with CD206-expressing cells showed the conjugates to have substantial gene silencing potential with long-lasting effects and protein downregulation (Uehara et al. 2022). These findings offer new possibilities for the targeted delivery of siRNAs and may help to enhance the therapeutic potential of siRNA technology in ATMs (Uehara et al. 2022).

Small molecules

According to a recent study, the near-infrared fluorophore (IR-61) was found to target the mitochondria of ATMs and promote oxidative phosphorylation by enhancing the production and activity of mitochondrial complexes (Wang et al. 2021a) (Fig. 2F). This effect is mediated through the ROS- Protein kinase B (AKT) pathway in ATMs, resulting in an improvement in chronic inflammation and hypertrophy of adipose tissue. The study suggests that IR-61 may be a useful small-molecule drug for improving obesity-related disorders by selectively targeting the mitochondria capacity of ATMs (Wang et al. 2021a).

In conclusion, ATMs are a heterogeneous population that plays a critical role in lipid metabolism and systemic inflammation (Daemen and Schilling 2020). The different subtypes of ATMs are associated with distinct functions and phenotypes, including LAMs (Trem2+macrophages, Tim4+ macrophages, Cd9+ macrophages), vasculature-associated macrophages and SAMs (Pirzgalska et al. 2017; Hill et al. 2018; Jaitin et al. 2019; Silva et al. 2019; Cox et al. 2021; Magalhaes et al. 2021). Recent studies have shown that manipulating the activity of ATMs can have beneficial effects on metabolic health, such as improving insulin sensitivity and reducing inflammation (Jaitin et al. 2019; Rosina et al. 2022). Therefore, targeting ATMs could represent a promising approach for the prevention and treatment of metabolic disorders such as obesity and type 2 diabetes (da Cruz Nascimento et al. 2022). Future research should focus on elucidating the mechanisms underlying ATM regulation and identifying novel therapeutic targets for the modulation of ATM activity to improve metabolic health.

This research was supported by the National Research Foundation of Korea (NRF) grants (NRF-2019R1C1C1002014 and NRF-2018R1A5A2024425 to Y.-H.L.).

The authors declare that they have no conflict of interest.