All homeotherms utilize thermogenesis to maintain core body temperature, ensuring that cellular functions and physiologic processes can ensue in cold environments1-3. In the prevailing model, when the hypothalamus senses cold temperatures, it triggers sympathetic discharge, resulting in the release of noradrenaline in brown adipose tissue (BAT) and white adipose tissue (WAT)4,5. Acting via the β3-adrenergic receptors, noradrenaline induces lipolysis in white adipocytes6, whereas it stimulates the expression of thermogenic genes, such as PPARγ coactivator 1a (Ppargc1a), uncoupling protein 1 (Ucp1), and acyl-CoA synthetase long-chain family member 1 (Acsl1), in brown adipocytes7-9. However, the precise nature of all the cell types involved in this efferent loop is not well established. Here we report an unexpected requirement for the interleukin 4 (IL4)-stimulated program of alternative macrophage activation in adaptive thermogenesis. Cold exposure rapidly promoted alternative activation of adipose tissue macrophages, which secrete catecholamines to induce thermogenic gene expression in BAT and lipolysis in WAT. Absence of alternatively activated macrophages impaired metabolic adaptations to cold, whereas administration of IL4 increased thermogenic gene expression, fatty acid mobilization, and energy expenditure, all in a macrophage-dependent manner. We have thus discovered a surprising role for alternatively activated macrophages in the orchestration of an important mammalian stress response, the response to cold.

SUMMARY Beige fat, which expresses the thermogenic protein UCP1, provides a defense against cold and obesity. Although a cold environment is the physiologic stimulus for inducing beige fat in mice and humans, the events that lead from the sensing of cold to the development of beige fat remain poorly understood. Here, we identify the efferent beige fat thermogenic circuit, consisting of eosinophils, type 2 cytokines interleukin (IL)-4/13 and alternatively activated macrophages. Genetic loss of eosinophils or IL-4/13 signaling impairs cold-induced biogenesis of beige fat. Mechanistically, macrophages recruited to cold-stressed subcutaneous white adipose tissue (scWAT) undergo alternative activation to induce tyrosine hydroxylase expression and catecholamine production, factors required for browning of scWAT. Conversely, administration of IL-4 to thermoneutral mice increases beige fat mass and thermogenic capacity to ameliorate pre-established obesity. Together, our findings have uncovered the efferent circuit controlling biogenesis of beige fat and provide support for its targeting to treat obesity.

SUMMARY Type 2 innate lymphoid cells (ILC2s), an innate source of the type 2 cytokines interleukin (IL)-5 and -13, participate in the maintenance of tissue homeostasis. Although type 2 immunity is critically important for mediating metabolic adaptations to environmental cold, the functions of ILC2s in beige or brown fat development are poorly defined. We report here that activation of ILC2s by IL-33 is sufficient to promote the growth of functional beige fat in thermoneutral mice. Mechanistically, ILC2 activation results in the proliferation of bipotential adipocyte precursors (APs) and their subsequent commitment to the beige fat lineage. Loss- and gain-of-function studies reveal that ILC2-and eosinophil-derived type 2 cytokines stimulate signaling via the IL-4Rα in PDGFRα+ APs to promote beige fat biogenesis. Together, our results highlight a critical role for ILC2s and type 2 cytokines in the regulation of adipocyte precursor numbers and fate, and as a consequence, adipose tissue homeostasis.

Circadian clocks have evolved to regulate physiologic and behavioral rhythms in anticipation of changes in the environment. Although the molecular clock is present in innate immune cells, its role in monocyte homeostasis remains unknown. Here, we report that Ly6Chi inflammatory monocytes exhibit diurnal variation, which controls their trafficking to sites of inflammation. This cyclic pattern of trafficking confers protection against Listeria monocytogenes and is regulated by the repressive activity of the circadian gene BMAL1. Accordingly, myeloid cell-specific deletion of BMAL1 induces expression of monocyte-attracting chemokines and disrupts rhythmic cycling of Ly6Chi monocytes, predisposing mice to development of pathologies associated with acute and chronic inflammation. These findings have unveiled a critical role for BMAL1 in controlling the diurnal rhythms in Ly6Chi monocyte numbers.

Obesity is associated with metabolic inflammation and endoplasmic reticulum (ER) stress, both of which promote metabolic disease progression. Adipose tissue macrophages (ATMs) are key players orchestrating metabolic inflammation, and ER stress enhances macrophage activation. However, whether ER stress pathways underlie ATM regulation of energy homeostasis remains unclear. Here, we identified inositol-requiring enzyme 1α (IRE1α) as a critical switch governing M1-M2 macrophage polarization and energy balance. Myeloid-specific IRE1α abrogation in Ern1; Lyz2-Cre mice largely reversed high-fat diet (HFD)-induced M1-M2 imbalance in white adipose tissue (WAT) and blocked HFD-induced obesity, insulin resistance, hyperlipidemia and hepatic steatosis. Brown adipose tissue (BAT) activity, WAT browning and energy expenditure were significantly higher in Ern1; Lyz2-Cre mice. Furthermore, IRE1α ablation augmented M2 polarization of macrophages in a cell-autonomous manner. Thus, IRE1α senses protein unfolding and metabolic and immunological states, and consequently guides ATM polarization. The macrophage IRE1α pathway drives obesity and metabolic syndrome through impairing BAT activity and WAT browning.