Adipose tissue macrophages (ATMs) are abundant immune cells in human and murine AT [27,28]. ATMs, which have substantial functional heterogeneity, actively participate in various aspects of nutrient metabolism within AT [29]. Macrophages identified in lean AT are anti-inflammatory “M2”, or alternatively activated (M2-like) macrophages, which typically express the mannose receptor CD206 and secrete anti-inflammatory cytokines (Fig. 1) [30,31]. AT in lean mice and humans contains a higher proportion of M2/M1 macrophages, which are associated with local production of Th2 cytokines by eosinophils [18,32]. Initially, the involvement of the immune response in AT homeostasis was understood only in the context of type 1 immunity, represented by the increased proinflammatory M1 macrophages and production of IFN-γ and chemokines, such as CCL2 released from CD8⁺ and Th1-polarized lymphocytes, as well as their ability to exacerbate metabolic dysfunctions, such as type 2 diabetes mellitus and obesity [33].

M2 macrophages play a role in nutrient storage in adipocytes by promoting insulin sensitivity through the secretion of IL-10 [34], and suppress type 1 immune responses of leukocyte populations by processing excess iron, disposing of dead cells and debris, and promoting vascularization and tissue matrix remodeling [35,36]. The regulatory key mediators of macrophage polarization were recently identified [37]. M1 macrophages have huge metabolic demands depending on glycolysis [38,39]. M2 macrophages meet their energy requirements using mitochondrial oxidative phosphorylation (OXPHOS) pathways, and have increased ability to uptake free fatty acid and for fatty acid oxidation (FAO) to facilitate the tricarboxylic acid cycle (TCA) cycle, and drive inflammasome activation in inflammatory macrophages [37,40]. Inhibition of FAO in M2 macrophages with the carnitine palmitoyltransferase 1 (CPT1) inhibitor, etomoxir, limits M2 activation in response to IL-4 [34]. Peroxisome proliferator-activated receptor γ/δ (PPAR-γ/δ), which can be driven by adipocyte derived IL-4 and IL-13, is a crucial transcription factor in M2 macrophage polarization [41,42]. In contrast, M1 macrophages increase energy production through glycolysis. M1 macrophages also have a “broken” TCA cycle, resulting in accumulation of citrate and succinate. Increased succinate results in secretion of the proinflammatory cytokine, IL-1β, via activation of hypoxia-inducible factor 1α (HIF1α) (Fig. 1) [40,43,44]. Blocking oxidative metabolism not only impairs the development of an M2-like phenotype but also drives the cells toward an M1-like state [45]. In addition, failure of alternative M2 activation, which is associated with reduced oxidative function, leads to classical macrophage activation, elevated weight gain, and obesity with concurrent adipose inflammation and insulin resistance [46,47]. However, it is unclear whether the oxidative function of macrophages is reduced under these conditions, and it is not known whether treatments capable of increasing the oxidative function of macrophages would reverse insulin resistance and adipose inflammation.

ILC2s are a recently identified innate immune cell lineage related to lymphocytes and natural killer cells with emerging roles in mediating immune responses and regulating tissue homeostasis and inflammation [48]. The group 2 ILC population is comprised of ILC2s, which express IL-5 and IL-13 and require GATA-binding protein 3 (GATA3) and retinoic acid receptor-related orphan receptor α (ROR-α) for their development [49]. In a mouse model of intestinal damage and inflammation, the epithelial cytokine IL-33 can promote ILC2 amphiregulin (Areg) production, leading to the resolution of colitis and promoting epithelial repair [50]. Indeed, type 2 immune responses are known to promote wound repair and tissue regeneration following infection or injury [1], suggesting that ILC2s may be key organizers of these beneficial tissue responses. Although they have crucial roles in protective type 2 immunity, the role of these cells in WAT is quite distinct [51]. They function to promote WAT homeostasis through several defined mechanisms.

ILC2s are present in visceral adipose tissue (VAT), where they are the predominant producers of IL-5 and IL-13 following prolonged exposure to IL-33 [25,52]. In lean AT, IL-33 drives the recruitment and/or proliferation of ILC2, but the cellular origin of IL-33 and the mechanisms leading to its secretion in homeostasis remain poorly understood. Tissue ILC2s are key producers of systemic IL-5 required for homeostatic eosinophil maintenance [53]. In AT, secretion of IL-5 by ILC2 is essential for the recruitment and maintenance of eosinophils and is dependent on IL-33 [53]. In addition, the most well-studied mechanism involves their regulatory effect on the resident macrophage phenotype from M1 macrophages to M2 macrophages by IL-4 or IL-13 signaling [54]. The regulatory function of these Th2 cytokines is involved in systemic glucose homeostasis. IL-4 and IL-13, both induced by type 2 immune responses, reduce inflammation in AT and improve systemic glucose intolerance by inducing polarization of M2 macrophages [18,55,56]. Interestingly IL-33 has been shown to be competent to induce macrophage proliferation independently of IL-4Ra expression in other non-adipose macrophage populations [53], but whether IL-33 can directly activate ATMs remains to be investigated.

Eosinophils are a type of granulocyte that combat parasitic infection and allergic reactions. However, in AT their role is to maintain metabolic homeostasis [18]. Initial studies in WAT showed that eosinophils are associated with lean healthy WAT and promote M2-like polarization of WAT macrophages [18,25]. WAT eosinophils decline with weight gain in association with increased AT M1-like macrophages and metabolic impairments, and eosinophil-deficient mice are more vulnerable to the onset of insulin resistance [18]. Further research has shown that WAT eosinophil numbers are maintained by IL-5, which is largely secreted by local ILC2s [18,25]. One study showed that body weight and glucose control are improved by ILC2 and natural killer T cells in a model of high fat diet-induced obesity, which was attributed to the accumulation of eosinophils and M2-like macrophages in visceral WAT [57]. Recently, it was found that resident eosinophils expressing IL-4 and ILC2s expressing IL-13 are necessary to maintain an anti-inflammatory state in WAT [18,25]. During cold exposure, eosinophils may induce WAT beiging by polarizing M2-like macrophages [58,59]. These results suggest that ILC2s, eosinophils, and M2-like macrophages influence metabolic health through the regulation of AT inflammation, beiging capacity, and insulin resistance by facilitating a network of immune cell interactions (Fig. 1). In the simplest terms, AT M2-like macrophages, eosinophils, and ILC2s are positively correlated with AT health.

Treg cells are abundant in the visceral AT of lean mice, where they have a distinct transcriptome and antigen-receptor repertoire [60]. Similar to ILC2s, Treg cells rapidly populate AT after birth. AT Treg cells express high levels of CD25 and IL-10, as well as the transcription factors GATA3 and PPAR-γ, and the IL-33 receptor [15]. They uniquely depend on these molecules, since mice lacking IL-33, IL-33R, or PPAR-γ expression in Treg cells are deficient in Treg cells specifically within AT and exhibit evidence of increased AT inflammation [32,52]. In addition, IL-33-driven expansion of ILC2s and/or Treg cells has been shown to revert the chronic inflammatory processes that drive obesity and improve insulin resistance in mouse models [61].