Adipose tissue macrophages (ATMs) comprise resident macrophages present in adipose tissue. Besides adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells that includes pre-adipocytes, fibroblasts, vascular endothelial cells, and a large variety of immune cells. The latter ones are composed of mast cells, eosinophils, B cells, T cells and macrophages. [1] The number of macrophages within adipose tissue differs depending on the metabolic status. As discovered by Rudolph Leibel and Anthony Ferrante et al. in 2003 at Columbia University, the percentage of macrophages within adipose tissue ranges from 10% in lean mice and humans up to 50% in obese leptin deficient mice, and up to 40% in obese humans. [2] ATMs comprise nearly 50% of all immune cells in normal conditions, suggesting an important role in supporting normal functioning of the adipose tissue. [3] Increased number of adipose tissue macrophages may correlate with increased production of pro-inflammatory molecules and might therefore contribute to the pathophysiological consequences of obesity (e.g. insulin resistance, type 2 diabetes), [4] although is becoming recognized that in healthy conditions tissue-resident macrophages actively support a variety of critical physiological functions in nearly all organs and tissues, including adipose tissue. [5]
There exist several distinct subpopulations of adipose tissue macrophages that are different in terms of both origin and function. In healthy, lean mice, nearly all macrophages are located on the outer side of blood vessels, in tight contact with adipocytes and other cells in the tissue. [3] They can be readily distinguished by their expression of TIM4 and MHC class II markers in mice. It was shown that TIM4+ MHCII- macrophages originate in the embryo, even before adipocytes are fully formed, while TIM4- MHCII+ arise in the bone marrow. [6] The formation of ATM identity critically depends on the transcription factor c-Maf. [6] Macrophages from different depots (inguinal and epididymal) have recently been thoroughly characterized using single-cell RNA-sequencing in both mice and humans. [7]
Macrophages are remarkably plastic cells which in order to adapt to different tissue microenvironments can assume a range of different phenotypes. Accordingly, macrophages can exhibit either pro- or anti-inflammatory phenotypes and are routinely classified into M1 (classically activated) phenotype and M2 (alternatively activated) phenotype. [8] According to this classification, macrophages acquire M1 phenotype following in vitro stimulation with interferon gamma (IFN-γ) alone or in combination with TLR ligands (e.g. lipopolysaccharide (LPS)) whereas macrophages acquire M2 phenotype after in vitro exposure to IL-4 and IL-13. M1 macrophages secrete high levels of proinflammatory cytokines (e.g. tumor necrosis factor (TNF-α), IL-6, IL-1β) and generate reactive oxygen and nitrogen species such as nitric oxide via activation of inducible nitric oxide synthase (iNOS). Conversely, M2 macrophages activate arginase 1 (Arg1) that blocks iNOS activity and therefore inhibits nitric oxide production. They also secrete anti-inflammatory cytokines (e.g. IL-10, TGF-β, IL-4) essential for inflammatory response resolution. M1 macrophages are microbicidal and tumoricidal, and stimulate adaptive immune response. M2 macrophages are associated with anti-inflammatory and homeostatic functions linked to wound healing. However, in this classification system, M1 and M2 macrophages are regarded as two extreme phenotypes. For example, macrophages stimulated with IL-4 and IL-13 are defined as M2a, whereas macrophages stimulated with LPS and apoptotic cells as M2b and macrophages stimulated with IL-10, transforming growth factor-β (TGF-β) or glucocorticoids as M2c. [9] In adipose tissue, distinction between M1 and M2 macrophage polarization can be monitored by assessing the expression of selected markers. Macrophages displaying M1 phenotype have been characterized by expression of F4/80, CD11c and iNOS whereas macrophages displaying M2 phenotype have been characterized by expression of F4/80, CD301 and Arg1. [10] Adiopose tissue macrophage polarization was summarized in a review article Appari M et al., et al. [11]
It is important to note that M1/M2 polarization paradigm is primarily based on in vitro studies of cells outside of their tissue context. [12] Recent studies unveiled that tissue-specific resident macrophages generally cannot be classified into pure M1 or M2 polarization states. As a result, some functions described above may not be actually performed by ATMs, and additional states (e.g. "metabolic activated" ATMs) can also exist. [13]
Increased recruitment of macrophages into adipose tissue is multifactoral. [14] Adipocyte cell death observed within pathologically expanding adipose tissue is one of the factors. Macrophages are specialized phagocytes that remove dying or dead cells or cellular debris. Within adipose tissue, presence of dead adipocytes is a hallmark of obesity. Macrophages surrounding dying or dead adipocytes form crown-like structures (CLSs), identified by the absence of perilipin staining. [15]
In addition to increased number of macrophages within adipose tissue, obesity also induces a phenotypic switch in these cells toward the pro-inflammatory phenotype. [4] Moreover, some inflammatory cytokines such as tumor necrosis factor is mostly derived from macrophages rather than adipocytes. [2] It has been proposed that their presence contributes to the development of insulin resistance and type 2 diabetes. Early and late stages of diet-induced obesity can also induce macrophage populations that are not representative of M1 or M2 phenotypes, including metabolically activate macrophages (MMe) and oxidized macrophages (Mox), both associated with insulin resistance. [16]
Adipose tissue macrophages isolated from obese patients express growth factors, cytokines, chemokines, and proteolytic enzymes involved in the regulation of tumor growth, angiogenesis, invasion, and metastatic spread, and resemble macrophages present in tumor stroma. [17]
Acute weight loss is also associated with increased, yet transient recruitment of macrophages into adipose tissue. However the recruited macrophages do not promote inflammatory response but rather regulate lipolysis. Recruited macrophages are characterized by higher expression of scavenger receptors (i.e. CD36 and macrophage scavenger receptor 1 (MSR1)) and lipid-handling genes (i.e. adipose differentiation-related protein (Adfp), fatty acid-binding protein 4 (Fabp4), ApoE and ABCA1), and increased accumulation of Oil Red O-positive lipids. In this case, release of free fatty acids (FFAs) serves as a signal for macrophage recruitment. [10] [18]
It has been shown in mice that adipose tissue macrophages regulate the age-related reduction of adipocyte lipolysis during ageing by lowering the bioavailability of noradrenaline. Inhibition of MAOA, an enzyme known to degrade noradrenaline, reversed the reduction in noradrenaline concentration and restored lipolysis in mice. [19]
Macrophages within tumor stroma, so called tumor-associated macrophages (TAMs) promote tumor growth and metastasis. [20] Tumor-associated macrophage infiltration correlates with poor prognosis in patients with breast, cervix, bladder and brain cancers. [21] Pathophysiological interaction between tumor-associated macrophages and surrounding cells, such as endothelial cells promote tumor progression. In 1971, Judah Folkman proposed that angiogenesis plays essential role in tumor growth. [22] Macrophages secrete many pro-angiogenic factors including vascular endothelial growth factor (VEGF), TNF-α, granulocyte macrophage colony-stimulating factor (GM-CSF) and IL-1 and IL-6. [23] Additionally it has been shown that adipose tissue surrounding certain tumors or metastases to the lymph nodes, which are embedded in adipose tissue, fuels tumor growth by serving as a depot for adipose tissue macrophages that stimulate angiogenesis and resemble TAMs. [24] [25] [26] [27]
Macrophages are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris, and foreign substances, which do not have proteins that are specific to healthy body cells on their surface. This process is called phagocytosis, which acts to defend the host against infection and injury.
Tumor necrosis factor is an adipokine and a cytokine. TNF is a member of the TNF superfamily, which consists of various transmembrane proteins with a homologous TNF domain.
Adipose tissue is a loose connective tissue composed mostly of adipocytes. It also contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular endothelial cells and a variety of immune cells such as adipose tissue macrophages. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.
Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. In cell culture, adipocyte progenitors can also form osteoblasts, myocytes and other cell types.
Stromal cells, or mesenchymal stromal cells, are differentiating cells found in abundance within bone marrow but can also be seen all around the body. Stromal cells can become connective tissue cells of any organ, for example in the uterine mucosa (endometrium), prostate, bone marrow, lymph node and the ovary. They are cells that support the function of the parenchymal cells of that organ. The most common stromal cells include fibroblasts and pericytes. The term stromal comes from Latin stromat-, "bed covering", and Ancient Greek στρῶμα, strôma, "bed".
Perilipin, also known as lipid droplet-associated protein, perilipin 1, or PLIN, is a protein that, in humans, is encoded by the PLIN gene. The perilipins are a family of proteins that associate with the surface of lipid droplets. Phosphorylation of perilipin is essential for the mobilization of fats in adipose tissue.
Interleukin 19 (IL-19) is an immunosuppressive protein that belongs to the IL-10 cytokine subfamily.
Group XVI phospholipase A2 also commonly known as adipocyte phospholipase A2 (AdPLA) is an enzyme that in humans is encoded by the PLA2G16 gene. This enzyme has also been identified as PLA2G16, HRASLS3, HREV107, HREV107-3, MGC118754 or H-REV107-1 from studies on class II tumor suppression but not on its enzymatic properties. AdPLA is encoded by a 1.3 kilobase AdPLA messenger RNA and is an 18 kDa protein. It belongs to a superfamily of phospholipase A2 (PLA2) enzymes and is found primarily in adipose tissue. AdPLA regulates adipocyte lipolysis and release of fatty acids through a G-protein coupled pathway involving prostaglandin and EP3. It has also been reported to play a crucial role in the development of obesity in mouse models.
Cell death activator CIDE-A is a protein that in humans is encoded by the CIDEA gene. Cidea is an essential transcriptional coactivator regulating mammary gland secretion of milk lipids.
Chemerin, also known as retinoic acid receptor responder protein 2 (RARRES2), tazarotene-induced gene 2 protein (TIG2), or RAR-responsive protein TIG2 is a protein that in humans is encoded by the RARRES2 gene.
An inflammatory cytokine or proinflammatory cytokine is a type of signaling molecule that is secreted from immune cells like helper T cells (Th) and macrophages, and certain other cell types that promote inflammation. They include interleukin-1 (IL-1), IL-6, IL-12, and IL-18, tumor necrosis factor alpha (TNF-α), interferon gamma (IFNγ), and granulocyte-macrophage colony stimulating factor (GM-CSF) and play an important role in mediating the innate immune response. Inflammatory cytokines are predominantly produced by and involved in the upregulation of inflammatory reactions.
A myokine is one of several hundred cytokines or other small proteins and proteoglycan peptides that are produced and released by skeletal muscle cells in response to muscular contractions. They have autocrine, paracrine and/or endocrine effects; their systemic effects occur at picomolar concentrations.
Tumor-associated macrophages (TAMs) are a class of immune cells present in high numbers in the microenvironment of solid tumors. They are heavily involved in cancer-related inflammation. Macrophages are known to originate from bone marrow-derived blood monocytes or yolk sac progenitors, but the exact origin of TAMs in human tumors remains to be elucidated. The composition of monocyte-derived macrophages and tissue-resident macrophages in the tumor microenvironment depends on the tumor type, stage, size, and location, thus it has been proposed that TAM identity and heterogeneity is the outcome of interactions between tumor-derived, tissue-specific, and developmental signals.
Innate lymphoid cells (ILCs) are the most recently discovered family of innate immune cells, derived from common lymphoid progenitors (CLPs). In response to pathogenic tissue damage, ILCs contribute to immunity via the secretion of signalling molecules, and the regulation of both innate and adaptive immune cells. ILCs are primarily tissue resident cells, found in both lymphoid, and non- lymphoid tissues, and rarely in the blood. They are particularly abundant at mucosal surfaces, playing a key role in mucosal immunity and homeostasis. Characteristics allowing their differentiation from other immune cells include the regular lymphoid morphology, absence of rearranged antigen receptors found on T cells and B cells, and phenotypic markers usually present on myeloid or dendritic cells.
Regulatory macrophages (Mregs) represent a subset of anti-inflammatory macrophages. In general, macrophages are a very dynamic and plastic cell type and can be divided into two main groups: classically activated macrophages (M1) and alternatively activated macrophages (M2). M2 group can further be divided into sub-groups M2a, M2b, M2c, and M2d. Typically the M2 cells have anti-inflammatory and regulatory properties and produce many different anti-inflammatory cytokines such as IL-4, IL-33, IL-10, IL-1RA, and TGF-β. M2 cells can also secrete angiogenic and chemotactic factors. These cells can be distinguished based on the different expression levels of various surface proteins and the secretion of different effector molecules.
Macrophage polarization is a process by which macrophages adopt different functional programs in response to the signals from their microenvironment. This ability is connected to their multiple roles in the organism: they are powerful effector cells of the innate immune system, but also important in removal of cellular debris, embryonic development and tissue repair.
Pathophysiology of obesity is the study of disordered physiological processes that cause, result from, or are otherwise associated with obesity. A number of possible pathophysiological mechanisms have been identified which may contribute in the development and maintenance of obesity.
Dermal macrophages are macrophages in the skin that facilitate skin homeostasis by mediating wound repair, hair growth, and salt balance. Their functional role in these processes is the mediator of inflammation. They can acquire an M1 or M2 phenotype to promote or suppress an inflammatory response, thereby influencing other cells' activity via the production of pro-inflammatory or anti-inflammatory cytokines. Dermal macrophages' ability to acquire pro-inflammatory properties also potentiates them in cancer defence. M1 macrophages can suppress tumour growth in the skin by their pro-inflammatory properties. However, M2 macrophages support tumour growth and invasion by the production of Th2 cytokines such as TGFβ and IL-10. Thus, the exact contribution of each phenotype to cancer defence and the skin's homeostasis is still unclear.
Immune system contribution to regeneration of tissues generally involves specific cellular components, transcription of a wide variety of genes, morphogenesis, epithelia renewal and proliferation of damaged cell types. However, current knowledge reveals more and more studies about immune system influence that cannot be omitted. As the immune system exhibits inhibitory or inflammatory functions during regeneration, the therapies are focused on either stopping these processes or control the immune cells setting in a regenerative way, suggesting that interplay between damaged tissue and immune system response must be well-balanced. Recent studies provide evidence that immune components are required not only after body injury but also in homeostasis or senescent cells replacement.
Apoptosis inhibitor of macrophage (AIM) is a protein produced by macrophages that regulates immune responses and inflammation. It plays a crucial role in key intracellular processes like lipid metabolism and apoptosis.