Immune regulation of metabolic homeostasis in health and disease

Abstract

Obesity is an increasingly prevalent disease worldwide. While genetic and environmental factors are known to regulate the development of obesity and associated metabolic diseases, emerging studies indicate that innate and adaptive immune cell responses in adipose tissue have critical roles in the regulation of metabolic homeostasis. In the lean state, type 2 cytokine-associated immune cell responses predominate in white adipose tissue and protect against weight gain and insulin resistance through direct effects on adipocytes and elicitation of beige adipose. In obesity, these metabolically beneficial immune pathways become dysregulated, and adipocytes and other factors initiate metabolically deleterious type 1 inflammation that impairs glucose metabolism. This review discusses our current understanding of the functions of different types of adipose tissue and how immune cells regulate adipocyte function and metabolic homeostasis in the context of health and disease and highlights. We also highlight the potential of targeting immuno-metabolic pathways as a therapeutic strategy to treat obesity and associated diseases.

Figure 1. White, beige and brown adipocytes are developmentally and functionally distinct cell populations

White and beige adipocytes arise from a Myf5⁻ precursor cell population that is bipotent. These pre-adipocytes give rise to white or beige adipocytes depending on the stimulus and physiologic setting. White adipocytes are promoted by high fat diet feeding or obesity and by thermoneutrality (30°C in mice). Beige adipocytes are elicited by β₃ adrenergic receptor agonists such as norepinephrine or epinephrine, and are recruited within white adipose tissue in the settings of chronic exercise or exposure to cold environmental temperatures. Although white and beige adipocytes emerge from pre-adipocytes via cell differentiation, mature white and beige adipocytes might undergo a process called transdifferentiation, in which one cell type acquires phenotypic characteristics of the other. There is uncertainty about whether transdifferentiation occurs. In contrast, brown adipocytes arise from a Myf5⁺ precursor cell population and are present in discrete brown adipose tissue depots. Despite being developmentally distinct cell populations, beige and brown adipocytes are activated by similar physiologic stimuli, including exercise- and cold temperature-induced hormones and metabolites.

Figure 2. Healthy white adipose tissue (WAT) is enriched in type 2 cytokine-associated immune cells

In the lean state, adipocytes and endothelial cells in WAT constitutively produce interleukin (IL)-33 that can act on Group 2 innate lymphoid cells (ILC2s) to induce production of IL-5 and IL-13 that sustain eosinophil (Eos) and alternatively activated macrophage (AAMac) responses, respectively, in WAT. In addition, eosinophils produce IL-4 that is necessary to maintain AAMac responses in WAT. AAMacs have multiple functions to maintain metabolic homeostasis, including storing large amounts of iron, leading to sequestration of this pro-oxidative metal cation from adipocytes to prevent lipid peroxidation, oxidative damage to proteins and mitochondrial dysfunction. In addition, AAMacs produce norepinephrine (NE) that acts on both white and beige adipocytes via the β₃ adrenergic receptor to stimulate lipolysis and elicit beige adipocytes that increase metabolic rate. ILC2s can also promote beiging through production of methionine-enkpehalin (MetEnk) peptides. In addition, adipocytes present lipid antigen to invariant natural killer T (iNKT) cells to elicit a unique tissue-specific anti-inflammatory phenotype characterized by increased production of IL-4 and IL-13, which may act on AAMacs, and IL-10. Lipids from adipocytes are also believed to promote regulatory T (T_reg) responses in WAT to induce production of IL-10. iNKT cells are critical sources of IL-2 and are necessary to sustain T_regs in WAT. IL-10 production by iNKT cells, T_regs and other cell types promote insulin action in white adipocytes to facilitate maintenance of an insulin-sensitive state. Together, these pathways contribute to metabolically healthy WAT.

Figure 3. Immunologic mechanisms that regulate beiging

In the context of chronic exposure to cold environmental temperatures or chronic exercise, white adipose tissue (WAT) and muscle produce the adipokine/myokine meteorin-like. This hormone promotes eosinophil (Eos) accumulation in WAT. Whether other factors contribute to increased Eos in WAT to promote beiging remains unknown. Meteorin-like induces interleukin (IL)-4 and IL-13 production by Eos and possibly other cell types. IL-4 and perhaps IL-13 act on AAMacs to stimulate norepinephrine (NE) production. NE acts to stimulate beiging via differentiation and/or transdifferentiation pathways and to activate existing beige adipocytes, resulting in mitochondrial biogenesis, Uncoupling protein 1 (UCP1) upregulation and UCP1-dependent increases in energy expenditure. In addition, WAT produces the cytokine IL-33 that is critical for maintaining Group 2 innate lymphoid cell (ILC2) responses in WAT. IL-33 stimulates ILC2s to produce IL-5 and IL-13 that sustain the eosinophil/AAMac pathways that can contribute to beiging. In addition, IL-13 can act on pre-adipocytes to promote their proliferation and induce differentiation to beige adipocytes. Further, IL-33 stimulates ILC2s to produce methionine-enkephalin (MetEnk) peptides that can directly promote beiging. Therefore AAMacs and ILC2s both contribute to beiging through production of distinct effector molecules.

Figure 4. Obese white adipose tissue (WAT) is characterized by type 1 cytokine-associated immune responses

In obese WAT, adipocyte hypertrophy is associated with hypoxia and adipocyte cell death. Dead and neighboring adipocytes produce pro-inflammatory signals such as Monocyte chemotractant protein 1 (MCP1), C-X-C motif chemokine 12 (CXCL12), Retinol binding protein 4 (RBP4) and Resistin that are associated with the recruitment of classically activated macrophages (MP) into WAT and MP activation. MP accumulation in WAT is also mediated by pro-inflammatory invariant natural killer T (iNKT) cells that exhibit impaired production of interleukin (IL)-10 and upregulated production of Tumor necrosis factor-α (TNF-α), and by CD8⁺ cytotoxic T cells that produce Interferon (IFN)-γ. These factors also promote MP activation and upregulate Major histocompatibility complex class II (MHC II) on MP and adipocytes. MHC II-mediated antigen presentation by MP and adipocytes stimulates polarization of CD4⁺ T cells towards a T helper type 1 (Th1) phenotype. Supporting this process are TNF-α, IFN-γ, IL-1β and IL-6 produced by MP cells, dysregulated alternatively activated macrophages (AAMacs), iNKT cells and CD8⁺ T cells. In addition, leptin is upregulated in obese WAT, and this factor also promotes Th1 cell polarization. MP cells, CD8⁺ T cells and Th1 cells collectively interact to form crown-like structures (CLS) to facilitate phagocytosis of dead adipocytes. This process further promotes antigen presentation and type 1 immune responses, establishing a vicious cycle. Type 1 cytokines such as TNF-α and IFN-γ act directly on adipocytes to impair insulin action, leading to insulin resistance. Dysregulated AAMacs also produce type 1 cytokines in the setting of obesity to contribute to insulin resistance. AAMacs also lose their capacity to store iron, resulting in redistribution of iron to adipocytes. This results in iron-initiated lipid peroxidation that causes reactive oxygen species (ROS) production, insulin resistance and mitochondrial dysfunction. In addition, AAMacs in obese WAT promote collagen deposition in WAT and fibrosis, leading ultimately to exacerbated hypoxia and inflammation and potentiation of the type 1 immune response. These processes occur in the setting of decreased abundance of regulatory T cell (T_reg), Group 2 innate lymphoid cell (ILC2) and eosinophils (Eos) that promote insulin sensitivity and metabolic homeostasis in WAT in the steady state.