T-cell immunoglobulin- and mucin-domain-containing molecule-4 maintains adipose tissue homeostasis by orchestrating M2 macrophage polarization via nuclear factor kappa B pathway

Obesity is generally associated with low-grade inflammation. Adipose tissue macrophages (ATMs) orchestrate metabolic inflammation. The classical (M1-like) or alternative (M2-like) activation of ATMs is functionally coupled with the metabolic status of fat tissues. It has been found that T-cell immunoglobulin- and mucin-domain-containing molecule-4 (Tim-4) inhibits inflammation by regulating macrophages. However, the exact role of Tim-4 in macrophage polarization and obesity remains unknown. Here, we identified Tim-4 as a critical switch governing macrophage M1/M2 polarization and energy homeostasis. Tim-4 deletion led to spontaneous obesity in elder mice and promoted obesity severity of db/db mice. Obesity microenvironment enhanced the expression of Tim-4 in white adipose tissue and ATMs. In vitro, we detected an increase in M1-like cells and decrease in M2-like cells in both peritoneal macrophages and bone marrow-derived macrophages from Tim-4 knockout mice. Mechanistically, we demonstrated that Tim-4 promoted M2-like macrophages polarization via suppressing nuclear factor kappa B (NF-κB) signaling pathway. In addition, we found that Tim-4 promoted TLR4 internalization, which might contribute to regulation of NF-κB signaling. Collectively, these results indicated that Tim-4 maintained adipose tissue homeostasis by regulating macrophage polarization via NF-κB pathway, which would provide a new target for obesity intervention.     

Adipose tissue macrophages: Unique polarization and bioenergetics in obesity

Macrophages comprise a majority of the resident immune cells in adipose tissue (AT) and regulate both tissue homeostasis in the lean state and metabolic dysregulation in obesity. Since the AT environment rapidly changes based upon systemic energy status, AT macrophages (ATMs) must adapt phenotypically and metabolically. There is a distinct dichotomy in the polarization and bioenergetics of in vitro models, with M2 macrophages utilizing oxidative phosphorylation (OX PHOS) and M1 macrophages utilizing glycolysis. Early studies suggested differential polarization of ATMs, with M2-like macrophages predominant in lean AT and M1-like macrophages in obese AT. However, recent studies show that the phenotypic plasticity of ATMs is far more complicated, which is also reflected in their bioenergetics. Multiple ATM populations exist along the M2 to M1 continuum and appear to utilize both glycolysis and OX PHOS in obesity. The significance of the dual fuel bioenergetics is unclear and may be related to an intermediate polarization, their buffering capacity, or the result of a mixed population of distinct polarized ATMs. Recent evidence also suggests that ATMs of lean mice serve as a substrate buffer or reservoir to modulate lipid, catecholamine, and iron availability. Furthermore, recent models of weight loss and weight cycling reveal additional roles for ATMs in systemic metabolism. Evaluating ATM phenotype and intracellular metabolism together may more accurately illuminate the consequences of ATM accumulation in obese AT, lending further insight into obesity-related comorbidities in humans.     

Adipose tissue macrophages

It is now broadly accepted that low-grade chronic inflammation associated with obesity leads to the onset of insulin resistance and type 2 diabetes mellitus. Obesity-associated inflammation is characterized by an increased abundance of macrophages in adipose tissue along with production of inflammatory cytokines. Adipose tissue macrophages (ATMs) are suspected to be the major source of inflammatory mediators such as TNF-alpha and IL-6 that interfere with adipocyte function by inhibiting insulin action. However, ATMs phenotypically resemble alternatively activated (M2) macrophages and are capable of anti-inflammatory mediator production challenging the concept that ATMs are simply the "bad guys" in obese adipose tissue. Triggers promoting ATM recruitment, ATM functions and dysfunctions, and stimuli and molecular mechanisms that drive them into becoming detrimental to their environment are subject to current research. Strategies to interfere with ATM recruitment and adverse activation could give rise to novel options for treatment and prevention of insulin resistance and type 2 diabetes mellitus.     

The “Big Bang” in obese fat: Events initiating obesity‐induced adipose tissue inflammation

Obesity is associated with the accumulation of pro‐inflammatory cells in visceral adipose tissue (VAT), which is an important underlying cause of insulin resistance and progression to diabetes mellitus type 2 (DM2). Although the role of pro‐inflammatory cytokines in disease development is established, the initiating events leading to immune cell activation remain elusive. Lean adipose tissue is predominantly populated with regulatory cells, such as eosinophils and type 2 innate lymphocytes. These cells maintain tissue homeostasis through the excretion of type 2 cytokines, such as IL‐4, IL‐5, and IL‐13, which keep adipose tissue macrophages (ATMs) in an anti‐inflammatory, M2‐like state. Diet‐induced obesity is associated with the loss of tissue homeostasis and development of type 1 inflammatory responses in VAT, characterized by IFN‐γ. A key event is a shift of ATMs toward an M1 phenotype. Recent studies show that obesity‐induced adipocyte hypertrophy results in upregulated surface expression of stress markers. Adipose stress is detected by local sentinels, such as NK cells and CD8⁺ T cells, which produce IFN‐γ, driving M1 ATM polarization. A rapid accumulation of pro‐inflammatory cells in VAT follows, leading to inflammation. In this review, we provide an overview of events leading to adipose tissue inflammation, with a special focus on adipose homeostasis and the obesity‐induced loss of homeostasis which marks the initiation of VAT inflammation.     

CD4+ T cells regulate glucose homeostasis independent of adipose tissue dysfunction in mice

Obesity is frequently associated with a chronic low-grade inflammation in the adipose tissue (AT) and impaired glucose homeostasis. Adipose tissue macrophages (ATMs) have been shown to accumulate in the inflamed AT either by means of recruitment from the blood or local proliferation. ATM proliferation and activation can be stimulated by TH2 cytokines, such as IL-4 and IL-13, suggesting involvement of CD4-positive T cells in ATM proliferation and activation. Furthermore, several studies have associated T cells to alterations in glucose metabolism. Therefore, we sought to examine a direct impact of CD4-positive T cells on ATM activation, ATM proliferation and glucose homeostasis using an in vivo depletion model. Surprisingly, CD4 depletion did not affect ATM activation, ATM proliferation, or insulin sensitivity. However, CD4 depletion led to a significant improvement of glucose tolerance. In line with this, we found moderate disturbances in pancreatic endocrine function following CD4 depletion. Hence, our data suggest that the effect on glucose metabolism observed after CD4 depletion might be mediated by organs other than AT and independent of AT inflammation.     

Adipokines and redox signaling: impact on fatty liver disease

Adipokines (adipose tissue cytokines) are polypeptide factors secreted by adipose tissue in a highly regulated manner. The 'classical' adipokines (leptin, adiponectin, and resistin) are expressed only by adipocytes, but other adipokines have been shown to be released by resident and infiltrating macrophages, as well as by components of the vascular stroma. Indeed, adipose tissue inflammation is known to be associated with a modification in the pattern of adipokine secretion. Several studies indicate that adipokines can interfere with hepatic injury associated with fatty infiltration, differentially modulating steatosis, inflammation, and fibrosis. Moreover, plasma levels of adipokines have been investigated in patients with nonalcoholic fatty liver disease in order to establish correlations with the underlying state of insulin resistance and with the type and severity of hepatic damage. In this Forum article, we provide a review of recent data that suggest a significant role for oxidative stress, reactive oxygen species, and redox signaling in mediating actions of adipokines that are relevant in the pathogenesis of nonalcoholic fatty liver disease, including hepatic insulin resistance, inflammation, and fibrosis.     

Epigenetic mechanisms of macrophage activation in type 2 diabetes

The alarming rise of obesity and type 2 diabetes (T2D) has put a tremendous strain on global healthcare systems. Over the past decade extensive research has focused on the role of macrophages as key mediators of inflammation in T2D. The inflammatory environment in the obese adipose tissue and pancreatic β-cell islets creates and perpetuates imbalanced inflammatory macrophage activation. Consequences of this chronic low-grade inflammation include insulin resistance in the adipose tissue and pancreatic β-cell dysfunction. Recently, the emerging field of epigenetics has provided new insights into the pathogenesis of T2D, while also affording potential new opportunities for treatment. In macrophages, epigenetic mechanisms are increasingly being recognized as crucial controllers of their phenotype. Here, we first describe the role of macrophages in T2D. Then we elaborate on epigenetic mechanisms that regulate macrophage activation, thereby focusing on T2D. Next, we highlight how diabetic conditions such as hyperlipidemia and hyperglycemia could induce epigenetic changes that promote an inflammatory macrophage phenotype. In conclusion we discuss possible therapeutic interventions by targeting macrophage epigenetics and speculate on future research directions.     

Obesity and dysregulated central and peripheral macrophage–neuron cross‐talk

The involvement of macrophages in the pathogenesis of obesity has been recognized since 2003. Early studies mostly focused on the role of macrophages in adipose tissue (AT) and in obesity‐associated chronic low‐grade inflammation. Lately, AT macrophages were shown to undergo intrinsic metabolic changes that affect their immune function (i.e., immunometabolism), corresponding to their unique properties along the range of pro‐ versus anti‐inflammatory activity. In parallel, recent studies in mice revealed critical neuronal–macrophage interactions, both in the CNS and in peripheral tissues, including in white and brown AT. These intercellular activities impinge on energy and metabolic homeostasis, partially by also engaging adipocytes in a neuronal–macrophage–adipocyte ménage à trois. Finally, neuropeptides (NP), such as NPY and appetite‐reducing NPFF, may prove as mediators in such intercellular network. In this concise review, we highlight some of these recent insights on adipose macrophage immunometabolism, as well as central and peripheral neuronal–macrophage interactions with emphasis on their impact on adipocyte biology and whole‐body metabolism. We also discuss the expanding view on the role of the NP, NPY and NPFF, in obesity.     

An update on immune dysregulation in obesity‐related insulin resistance

Obesity is associated with chronic low‐grade inflammation of the adipose tissue (AT) that might develop into systemic inflammation, insulin resistance (IR) and an increased risk of type 2 diabetes mellitus (T2DM) in severe obese rodents and humans. In the lean state, small normal adipocytes and AT macrophages interact with each other to maintain metabolic homeostasis but during obesity, enlarged adipocytes secrete inflammatory mediators and express immune receptors to recruit immune cells and aggravate the inflammation. The better understanding of the obesity‐related inflammatory milieu and the sequential events leading to IR could be helpful in designing new preventive and therapeutic strategies. The present review will discuss the cellular and molecular abnormalities participating in the pathogenesis of obesity in obese individuals as well as high‐fat diet (HFD)‐fed mice, a mouse model of obesity.     

Macrophages, inflammation, adipose tissue, obesity and insulin resistance

Obesity is associated with a complex systemic inflammatory reaction that has been associated with the development of atherosclerosis and insulin resistance. Obesity also induces macrophage accumulation in adipose tissue. Macrophages produce many of the pro inflammatory molecules released by adipose tissue and have been implicated in the development of obesity-induced adipose tissue inflammation. Monocyte chemoattractant proteins (MCPs) and their receptors play key roles in the development of inflammatory responses and are crucial for the recruitment of immune cells towards inflammation sites. Adipose tissue expression of at least 1 MCP, C-C motif chemokine ligand-2 (CCL2 or MCP1), increases in proportion to adiposity. The C-C motif chemokine receptor-2 (CCR2) regulates monocyte and macrophage recruitment and is necessary for macrophage-dependent inflammatory responses and the development of atherosclerosis. Because CCR2 regulates monocyte and macrophage chemotaxis and local inflammatory responses, it has been hypothesized that monocyte chemoattractant molecules acting through CCR2 might regulate obesity-induced inflammation in adipose tissue. Our study focuses on the molecular and genetic mechanisms that recruit and retain macrophages in adipose tissue.     

Development and function of tissue resident macrophages in mice

Macrophages are important for tissue development, homeostasis as well as immune response upon injury or infection. For a long time they were only seen as one uniform group of phagocytes with a common origin and similar functions. However, this view has been challenged in the last decade and revealed a complex diversity of tissue resident macrophages. Here, we want to present the current view on macrophage development and tissue specification and we will discuss differences as well as common patterns between heterogeneous macrophage subpopulations.     

Metabolic Regulation of Macrophage Activation

Macrophages, the central mediators of innate immune responses, being in the first-line of defense, they have to readily respond to pathogenic or tissue damage signals to initiate the inflammatory cascade. Such rapid responses require energy to support orchestrated production of pro-inflammatory mediators and activation of phagocytosis. Being a cell type that is present in diverse environments and conditions, macrophages have to adapt to different nutritional resources. Thus, macrophages have developed plasticity and are capable of utilizing energy at both normoxic and hypoxic conditions and in the presence of varying concentrations of glucose or other nutrients. Such adaptation is reflected on changes in signaling pathways that modulate responses, accounting for the different activation phenotypes observed. Macrophage metabolism has been tightly associated with distinct activation phenotypes within the range of M1-like and M2-like types. In the context of diseases, systemic changes also affect macrophage metabolism, as in diabetes and insulin resistance, which results in altered metabolism and distinct activation phenotypes in the adipose tissue or in the periphery. In the context of solid tumors, tumor-associated macrophages adapt in the hypoxic environment, which results in metabolic changes that are reflected on an activation phenotype that supports tumor growth. Coordination of environmental and pathogenic signals determines macrophage metabolism, which in turn shapes the type and magnitude of the response. Therefore, modulating macrophage metabolism provides a potential therapeutic approach for inflammatory diseases and cancer.     

Recently infiltrating MAC387(+) monocytes/macrophages a third macrophage population involved in SIV and HIV encephalitic lesion formation

Monocytes/macrophages are critical components of HIV and SIV encephalitic lesions. We used in vivo BrdU labeling and markers specific to stages of macrophage differentiation or inflammation to define macrophage heterogeneity and to better define the role of macrophage populations in lesion formation and productive infection. Lesions were heterogeneously composed of resident macrophages (CD68(+)HAM56(+)), perivascular macrophages (CD163(+) CD68(+)MAC387(-)), and recently infiltrated MAC387(+) CD68(-)CD163(-) monocytes/macrophages. At 24 and 48 hours after BrdU inoculation, 30% of MAC387(+) monocytes/macrophages were BrdU(+), consistent with their being recently infiltrated. In perivascular cuffs with low-level SIV replication, MAC387(+) monocytes/macrophages outnumbered CD68(+) macrophages. Conversely, lesions with numerous SIV-p28(+) macrophages and multinucleated giant cells had fewer MAC387(+) monocytes/macrophages. The MAC387(+) cells were not productively infected nor did they express detectable CCR2, unlike perivascular macrophages. Overall, we found that the proportion of MAC387(+) cells tends to be higher than the proportion of CD68(+) macrophages in the brain of animals with mild encephalitis; the ratio was reversed with more severe encephalitis. These results suggest that development of SIV and HIV encephalitis is an active and ongoing process that involves the recruitment and accumulation of: i) nonproductively infected MAC387(+) monocytes/macrophages that are present with inflammation (potentially M1-like macrophages), ii) CD163(+) perivascular macrophages (consistent with M2-like macrophages), and iii) CD68(+) or HAM56(+) resident macrophages. The latter two populations are cellular reservoirs for productive infection.     

Aging and cancer: The role of macrophages and neutrophils

Impaired immune function has been implicated in the declining health and higher incidence of cancer in the elderly. However, age-related changes to immunity are not completely understood. Neutrophils and macrophages represent the first line of defence yet their ability to phagocytose pathogens decrease with aging. Cytotoxic T lymphocytes are critical in eliminating tumors, but T cell function is also compromised with aging. T cell responses can be regulated by macrophages and may depend on the functional phenotype macrophages adopt in response to microenvironmental signals. This can range from pro-inflammatory, anti-tumorigenic M1 to anti-inflammatory, pro-tumorigenic M2 macrophages. Macrophages in healthy elderly adipose and hepatic tissue exhibit a more pro-inflammatory M1 phenotype compared to young hosts whilst immunosuppressive M2 macrophages increase in elderly lymphoid tissues, lung and muscle. These M2-like macrophages demonstrate altered responses to stimuli. Recent studies suggest that neutrophils also regulate T cell function and, like macrophages, neutrophil function is modulated with aging. It is possible that age-modified tissue-specific macrophages and neutrophils contribute to chronic low-grade inflammation that is associated with dysregulated macrophage-mediated immunosuppression, which together are responsible for development of multiple pathologies, including cancer. This review discusses recent advances in macrophage and neutrophil biology in healthy aging and cancer.     

Regulation of metabolic health and adipose tissue function by group 2 innate lymphoid cells

Adipose tissue (AT) is home to an abundance of immune cells. With chronic obesity, inflammatory immune cells accumulate and promote insulin resistance and the progression to type 2 diabetes mellitus. In contrast, recent studies have highlighted the regulation and function of immune cells in lean, healthy AT, including those associated with type 2 or “allergic” immunity. Although traditionally activated by infection with multicellular helminthes, AT type 2 immunity is active independently of infection, and promotes tissue homeostasis, AT “browning,” and systemic insulin sensitivity, protecting against obesity‐induced metabolic dysfunction and type 2 diabetes mellitus. In particular, group 2 innate lymphoid cells (ILC2s) are integral regulators of AT type 2 immunity, producing the cytokines interleukin‐5 and IL‐13, promoting eosinophils and alternatively activated macrophages, and cooperating with and promoting AT regulatory T (Treg) cells. In this review, we focus on the recent developments in our understanding of group 2 innate lymphoid cell cells and type 2 immunity in AT metabolism and homeostasis.     

Reprogramming mitochondrial metabolism in macrophages as an anti‐inflammatory signal

Mitochondria are master regulators of metabolism. Mitochondria generate ATP by oxidative phosphorylation using pyruvate (derived from glucose and glycolysis) and fatty acids (FAs), both of which are oxidized in the Krebs cycle, as fuel sources. Mitochondria are also an important source of reactive oxygen species (ROS), creating oxidative stress in various contexts, including in the response to bacterial infection. Recently, complex changes in mitochondrial metabolism have been characterized in mouse macrophages in response to varying stimuli in vitro. In LPS and IFN‐γ‐activated macrophages (M1 macrophages), there is decreased respiration and a broken Krebs cycle, leading to accumulation of succinate and citrate, which act as signals to alter immune function. In IL‐4‐activated macrophages (M2 macrophages), the Krebs cycle and oxidative phosphorylation are intact and fatty acid oxidation (FAO) is also utilized. These metabolic alterations in response to the nature of the stimulus are proving to be determinants of the effector functions of M1 and M2 macrophages. Furthermore, reprogramming of macrophages from M1 to M2 can be achieved by targeting metabolic events. Here, we describe the role that metabolism plays in macrophage function in infection and immunity, and propose that reprogramming with metabolic inhibitors might be a novel therapeutic approach for the treatment of inflammatory diseases.     

Adipose invariant natural killer T cells

Adipose tissue is a dynamic organ that makes up a substantial proportion of the body; in severe obesity it can account for 50% of body mass. Details of the unique immune system resident in human and murine adipose tissue are only recently emerging, and so it has remained a largely unexplored and unappreciated immune site until now. Adipose tissue harbours a unique collection of immune cells, which often display unusual functions compared with their counterparts elsewhere in the body. These resident immune cells are key to maintaining tissue and immune homeostasis, yet in obesity their chronic aberrant stimulation can contribute to the inflammation and pathogenesis associated with obesity. Anti‐inflammatory adipose‐resident lymphocytes are often depleted in obesity, whereas pro‐inflammatory immune cells accumulate, leading to an overall inflammatory state, which is a key step in the development of obesity‐induced metabolic disease. A good example is invariant natural killer T (iNKT) cells, which make up a large proportion of lymphocytes in human and murine adipose tissue. Here, they are unusually poised to produce anti‐inflammatory or regulatory cytokines, however in obesity, iNKT cells are greatly reduced. As iNKT cells are potent transactivaors of other immune cells, and can act as a bridge between innate and adaptive immunity, their loss in obesity represents the loss of a major regulatory population. Restoring iNKT cells, or activating them in obese mice leads to improved glucose handling, insulin sensitivity, and even weight loss, and hence represents an exciting therapeutic avenue to be explored for restoring homeostasis in obese adipose tissue.     

Human macrophages primed with angiogenic factors show dynamic plasticity, irrespective of extracellular matrix components

Macrophages are important in inflammation as well as in tissue repair processes. They can be activated by various stimuli and classified into two major groups: M1 (classically activated) or M2 (alternatively activated). Inflammation, angiogenesis and matrix remodeling play a major role in tissue repair. Here, we investigate the combined influence of a pro-angiogenic microenvironment and specific extracellular matrix (ECM) components or tissue culture polystyrene (TCPS) on the dynamics of human macrophage polarization. We established that human angiogenically primed macrophages cultured on different ECM components exhibit an M2-like polarization. These M2-like macrophages polarized to M1 and M2 macrophages with classical (LPS and IFNγ) stimuli and alternative (IL-4 and IL-13) stimuli respectively. Moreover, these M1 and M2 (primary) polarized macrophages rapidly underwent a secondary (re)polarization to M2 and M1 with conditioned media from M2 and M1 primary polarized macrophages respectively. In these initial priming and later (re)polarization processes the soluble factors had a dominant and orchestrating role, while the type of ECM (collagen I, fibronectin, versus tissue culture polystyrene) did not play a crucial role on the polarization of macrophages.