Macrophage plasticity and polarization in tissue repair and remodelling

Authors

  • Alberto Mantovani,

    Corresponding author
    1. Humanitas Clinical and Research Center, Rozzano, Milan, Italy
    2. Department of Biotechnology and Translational Medicine, University of Milan, Italy
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  • Subhra K Biswas,

    1. Singapore Immunology Network (SIgN), Agency for Science, Technology and Research (A*STAR), Singapore
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  • Maria Rosaria Galdiero,

    1. Humanitas Clinical and Research Center, Rozzano, Milan, Italy
    2. Division of Clinical Immunology and Allergy, University of Naples Federico II, Naples, Italy
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  • Antonio Sica,

    1. Humanitas Clinical and Research Center, Rozzano, Milan, Italy
    2. Department of Pharmaceutical Sciences, Università del Piemonte Orientale ‘Amedeo Avogadro’, Novara, Italy
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  • Massimo Locati

    1. Humanitas Clinical and Research Center, Rozzano, Milan, Italy
    2. Department of Biotechnology and Translational Medicine, University of Milan, Italy
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  • No conflicts of interest were declared.

Abstract

Mononuclear phagocyte plasticity includes the expression of functions related to the resolution of inflammation, tissue repair and remodelling, particularly when these cells are set in an M2 or an M2-like activation mode. Macrophages are credited with an essential role in remodelling during ontogenesis. In extraembryonic life, under homeostatic conditions, the macrophage trophic and remodelling functions are recapitulated in tissues such as bone, mammary gland, decidua and placenta. In pathology, macrophages are key components of tissue repair and remodelling that occur during wound healing, allergy, parasite infection and cancer. Interaction with cells bearing stem or progenitor cell properties is likely an important component of the role of macrophages in repair and remodelling. These properties of cells of the monocyte–macrophage lineage may represent a tool and a target for therapeutic exploitation.

Introduction

Mononuclear phagocytes are an essential element in the orchestration and expression of innate immunity and adaptive immune responses. These cells play a central role in inflammation and host defence [1-3]. Additionally, cells of the monocyte–macrophage lineage fulfil homeostatic functions beyond defence. Tissue remodelling during ontogenesis involves mononuclear phagocytes and, moreover, it is now apparent that cells of the monocyte–macrophage lineage play a role in the regulation of metabolism [1-3].

Diversity and plasticity have long been known to be characteristics of cells of the monocyte–macrophage lineage. In tissues, mononuclear phagocytes respond to environmental cues (microbial products, damaged cells, activated lymphocytes) with the acquisition of distinct functional phenotypes. In response to TLR ligands and IFNγ or IL-4/IL-13 macrophages undergo M1 (classical) or M2 (alternative) activation, which mirror the TH1–TH2 polarization and represent extremes of a continuum in a universe of activation states [2-5].

Heterogeneity and plasticity are hallmarks of mononuclear phagocytes [1-4, 6, 7]. Lineage-defined populations of mononuclear phagocytes have not been identified. However, already at the short-lived stage of circulating precursor monocytes, subsets characterized by differential expression of the FcγRIII receptor (CD16) or of chemokine receptors (CCR2, CX3CR1 and CCR8) and by different functional properties have been described [8]. Recent results have shed new light on the origin of macrophages and on mechanisms that sustain their levels in tissues. It has long been held that tissue macrophages originate from haematopoietic stem cells (HSCs). Recent evidence has put into question this long-held view. Macrophages in several tissues (eg liver, microglia) were found to originate in a Myb-independent HSC-independent way from yolk sac (YS) macrophages [9]. In parallel studies, the role of proliferation in sustaining macrophage numbers in inflammation has been revisited (see below). These data raise the issue of a division of labour between HSC- and YS-derived macrophages, the latter being natural candidates for fulfilling homeostatic function, and of their role in pathology. Once in tissues, macrophages acquire distinct morphological and functional properties, directed by the tissue (lung, bone, liver) and immunological microenvironment. In tissues, in addition to presenting a first line of resistance against pathogens, mononuclear phagocytes contribute to remodelling and repair under homeostatic and damage conditions (Figure 1). Recent evidence has shed new light on the repair and remodelling function of macrophages, including, for instance, interaction with cells with stem or precursor properties (Figure 1) [10-26]. These new vistas have broad implications for pathology.

Figure 1.

A schematic representation of the role of macrophages in tissue repair and remodelling. LM, lipid metabolites (eg lipoxins and maresins); CK, chemokines; GF, growth factors (eg EGF); MMP, matrix metalloproteinases; aa metabolism, amino acid metabolism (eg arginase polyamine pathway).

Macrophage plasticity and polarization

In response to signals derived from microbes, damaged tissues or activated lymphocytes [2, 7], monocytes/macrophages undergo a reprogramming which leads to the emergence of a spectrum of distinct functional phenotypes. Mirroring the Th1/Th2 nomenclature, macrophages undergo two different polarization states: the classically activated M1 phenotype and the alternatively activated M2 phenotype [4, 27]. Classically activated M1 macrophages have long been known to be induced by IFNγ alone or in concert with microbial stimuli (eg LPS) or cytokines (eg TNF and GM-CSF). IL-4 and IL-13 were subsequently found to be more than simple inhibitors of macrophage activation and to induce an alternative M2 form of macrophage activation [6]. It is now known that many other cytokines can govern M2 polarization. IL-33 is a cytokine of the IL-1 family associated with TH2 and M2 polarization [28, 29]. IL-33 amplifies IL-13-induced polarization of alveolar macrophages to an M2 phenotype characterized by the up-regulation of YM1, arginase 1, CCL24 and CCL17, which mediate lung eosinophilia and inflammation [29]. IL-21 is another TH2-associated cytokine shown to drive M2 activation of macrophages [30]. Colony-stimulating factors are also important drivers of macrophage activation and polarization [31, 32].

Macrophages can also be polarized into an ‘M2-like’ state, which shares some but not all signature features of M2 cells. In fact, various stimuli, such as immune complexes together with LPS or IL-1, glucocorticoids, transforming growth factor-β (TGFβ), Wnt5a [33] and IL-10, result in M2-like functional phenotypes that share properties with IL-4- or IL-13-activated macrophages (such as high expression of mannose receptor, IL-10 and angiogenic factors) [34]. Interestingly, resolvin D1 and 12/15 lipooxygenase induce an M2-like activation state [35, 36].

Variations on the theme of M2 polarization are also found in vivo (eg in the placenta and embryo, and during helminth infection, Listeria infection, obesity and cancer) [37-41]. M1 and M2 or M2-like polarized macrophages represent extremes of a continuum of functional states [4, 5, 42].

In general, M1 cells: have an IL-12high, IL-23high, IL-10low phenotype; are efficient producers of effector molecules (reactive oxygen and nitrogen intermediates) and inflammatory cytokines (IL-1β, TNF, IL-6); participate as inducer and effector cells in polarized Th1 responses; and mediate resistance against intracellular parasites and tumours. In contrast, the various forms of M2 macrophages share an IL-12low, IL-23low, IL-10high phenotype, with variable capacity to produce inflammatory cytokines, depending on the signal utilized. M2 cells generally have high levels of scavenger, mannose and galactose-type receptors, and the arginine metabolism is shifted to ornithine and polyamines. Polarized macrophages also present differential regulation of components of the IL-1 system [43], with low IL-1β and low caspase I, high IL-1ra and high decoy type II receptor in M2 cells.

In general, M2 cells take part in polarized Th2 responses, parasite clearance, [44], the dampening of inflammation, the promotion of tissue remodelling [45], angiogenesis, tumour progression and immunoregulation [2]. In this general context, IL-4-induced M2 polarized macrophages have recently been shown to regulate adaptive thermogenesis by producing catecholamines [46, 47]. Moreover, polarization of neutrophil functions has also been reported [48-50].

M1 and M2 macrophages have distinct chemokinome profiles, with M1 macrophages expressing TH1 cell-attracting chemokines, such as CXCL9 and CXCL10, and M2 macrophages expressing the chemokines CCL17, CCL22 and CCL24 [51, 52]. Chemokines can also influence macrophage polarization, with CCL2 and CXCL4 driving macrophages to an M2-like phenotype [53, 54].

M1- and M2-polarized macrophages have distinct features in terms of the metabolism of iron, folate and glucose [42, 55-57]. Indeed, recent evidence shows the importance of metabolism in shaping the functional phenotype of macrophages in response to distinct polarizing stimuli in the tissue microenvironment, under normal as well as pathological settings. There is a bidirectional crosstalk between metabolism and macrophages: macrophages not only exert ‘extrinsic’ effects to regulate metabolism (via release of soluble mediators such as inflammatory cytokines), but also exhibit ‘intrinsic’ effects, wherein the metabolic status of these cells shape their functional phenotype.

Polarized macrophages show a distinct regulation of glucose metabolism. Macrophages, in response to M1 stimuli, display a metabolic shift towards the anaerobic glycolytic pathway, while exposure to M2 stimuli such as IL-4 show a minor effect [56]. The use of specific metabolic pathways can be functionally related to different purposes. M1-activated macrophages are often associated with acute infection; these cells need to quickly trigger microbicidal activity as well as keep up with the hypoxic tissue microenvironment [55]. In this context, an anaerobic process such as glycolysis is best suited to meet their rapid energy requirements. In contrast, M2 polarization-related functions, such as tissue remodelling, repair and healing, require a sustained supply of energy. This request is fulfilled by oxidative glucose metabolism (fatty acid oxidation), which is believed to be the metabolic pathway of choice in M2 macrophages [57].

The amino acid metabolism is closely linked to the functional phenotype of myelomonocytic cells. M1 macrophages are characterized by the expression of NOS2 and the production of NO, which is an important effector for their microbicidal activity [58]. In contrast, M2 macrophages do not produce NO, but express high levels of ArgI, which catalyses polyamines production, which is necessary for collagen synthesis, cell proliferation, fibrosis and other tissue-remodelling functions [59]. Interestingly, polyamine production per se has been reported to be a driver of M2 polarization [60].

Recent studies in mouse as well as human macrophages show striking differences in iron metabolism between M1- and M2-polarized cells [61, 62]. M1 macrophages express high levels of proteins involved in iron storage, such as ferritin, while expressing low levels of ferroportin, an iron exporter. In contrast, M2 macrophages show low levels of ferritin but high levels of ferroportin. This divergent iron metabolism can be related to functional outcomes. Sequestration of iron by M1 cells would have a bacteriostatic effect (since iron is essential for supporting growth) and thus support host protection to infection. Conversely, iron release from M2 cells would favour tissue repair as well as tumour growth, consistent with the functional phenotype of these cells. Based on the facts presented above, it is clear that divergent iron management seems to be an important metabolic signature in polarized macrophages [63]. Collectively, these facts highlight that metabolic adaptation is an integral aspect of macrophage polarization and their functional diversity.

Under physiological and pathological conditions, macrophages are confronted by an oxygen gradient. Mononuclear phagocytes adapt to hypoxia by shifting their metabolic setting to glycolysis [4]. In addition, activation of hypoxia-inducible factor (HIF)-1 and -2 orchestrates profound functional changes, including expression of chemokines and chemokine receptors (CXCR4 and CXCL12) [64] and angiogenic factor (VEGF). Thus, macrophages contribute to the orchestration of the tissue response to hypoxic conditions [65]. Progress has been made in defining the molecular mechanisms underlying macrophage polarization [7]. These include members of the IRF/Stat families, Myc, NFκB hetero- and homo-dimers, KLF4, PPARγ, as well as miRNA and epigenetic modifications, A review of this aspect of macrophage polarization is beyond the scope of the present paper and the reader is referred to recent reviews [7, 66]

Interaction with stem/precursor cells

There is scattered evidence supporting the hypothesis that mononuclear phagocytes interact with cells with progenitor or bona fide stem cell properties, and that this interplay may contribute to repair and remodelling. As discussed below, macrophages play an important role in the organogenesis of the mammary gland, orchestrating ductal branching (for review, see [67]). A model of transplantation of mammary/stem progenitor cells in the mammary fat pad has recently been used to investigate the significance of the interplay with macrophages. Using CSF1 op/op mice genetically deficient in macrophages, it was found that mononuclear phagocytes exert an essential trophic function on mammary stem cells [24].

Cells of the monocyte–macrophage lineage can exert neuroprotective activity [10, 68]. Following glutamate intoxication of the eye, macrophages infiltrate the retina. Here they were shown to skew the retinal milieu towards anti-inflammatory and neuroprotective functions [68]. In particular, macrophages were found to promote neural progenitor cell renewal.

Mesenchymal stem (or stromal) cells (MSCs) are candidates for cellular therapies targeted to the promotion of tissue repair or immunoregulation [69]. MSCs engage in a bidirectional interaction with cells of the monocyte–macrophage lineage. M2-like macrophages and their mediators promoted the growth of human MSCs [26]. Moreover, macrophages have been reported to stimulate MSC motility [13]. Conversely, MSCs profoundly influence the function of macrophages. MSCs have been reported to induce an IL-10high IL-12low alternative (M2) activation phenotype in macrophages [18]. Injection of MSCs was associated with promotion of functional recovery after spinal cord injury, including axonal preservation and reduced scar formation [10]. Neuroprotection in this system was attributed to a MSC-induced shift in macrophage polarization from M2 to M1 [10]. Thus, MSCs engage in a bidirectional interaction with macrophages that may be relevant to therapeutic effects. The interplay between macrophages and cells with stem/progenitor properties is relevant to the tumour microenvironment, as discussed below.

Homeostatic tissue remodelling

Macrophages participate in homeostatic tissue remodelling during oncofetal life and in selected tissues during adulthood. Transcriptional profiling revealed that macrophages isolated from mouse embryos and from human fetal liver had an M2-like gene expression pattern [19, 39], consistent with the trophic and tissue remodelling function which has long been attributed to mononuclear phagocytes during development [39]. Development of mononuclear phagocytes can be obtained from embryonic stem cells and, consistent with the above finding, mononuclear phagocytes derived from human embryonic stem cells have an M2-like phenotype [19].

Macrophages are present at the interface between fetus and mother in decidua and placenta [37, 70-73]. Macrophages in decidua and placenta have an immunosuppressive, M2-like phenotype [37, 70, 73, 74]. Intriguingly, rat resident testicular macrophages also have an IL-10high phenotype [75]. Macrophages in decidual tissues are in close proximity to NK cells [74, 76] that orchestrate the remodelling of the vascular bed. NK cells interact differentially with polarized macrophages [77]. The interplay between macrophages and NK cells results in the induction of Treg cells and immunosuppression [78]. Thus, macrophages present at the fetal–maternal interface likely contribute to the homeostatic remodelling and tolerogenic milieu.

The developmental functions of mononuclear phagocytes include remodelling of the extracellular matrix (ECM), epithelial proliferation, development and organization of the vascular tree and shaping of tissue organization [67]. The genetic evidence formally demonstrating the role of cells of the monocyte–macrophage lineage in tissue development and organization is essentially based on genetic inactivation of the CSF1–CSF1 receptor axis [39, 67], which results in a substantial reduction, although not complete, of many tissue macrophages. Organs for which there is evidence for a role of macrophages in tissue development and shaping include bone, mammary gland, nervous system and pancreas [67]. CSF1 drives the differentiation of osteoclasts from monocytic precursors and increases osteoclast-mediated bone resorption [67]. Interestingly, IL-33 has been reported to skew differentiation/activation from osteoclasts to alternatively activated macrophages and to protect against TNF-caused bone resorption [79]. Macrophages have a trophic function in the mammary gland, promoting outgrowth of ductal structures and their branching [67]. Post partum, the mammary gland undergoes involution, returning to a non-lactating state ready to respond to subsequent pregnancies. Post-partum involution of the mammary gland is associated with high levels of IL-4 and IL-13, macrophage infiltration and skewed M2 polarization [80]. The trophic and remodelling function of macrophages in the mammary gland is mirrored by their function in breast cancer ([67]; and see below).

Resolution of inflammation, remodelling and tissue repair

Macrophages are essential contributors towards the resolution of inflammation, which has emerged as an active, orchestrated process. Interestingly, in addition to recruitment, it has now been demonstrated that local proliferation is an important determinant of macrophage accumulation in peritoneal and allergic airway inflammation [81, 82]. A fundamental macrophage function in resolution is phagocytosis of debris and apoptotic neutrophils [83]. During resolution, macrophages produce ‘anti-inflammatory’ cytokines (eg IL-10, IL-1ra and the IL-1 type II decoy receptor), which are part of the M2 or M2-like repertoire [42]. In addition, mononuclear phagocytes are a source of lipid mediators involved in resolution [35, 36]. In a model of peritoneal inflammation, resolution phase macrophages have been shown to express a unique transcriptional profile, controlled by cAMP [83]. These and other findings [7] caution against oversimplified views of M1/M2 polarization.

Lipid mediators play a key role in the orchestration of inflammation and its resolution [84, 85]. In particular, the arachidonic acid pathway synthesizes pro-inflammatory lipid mediators such as prostaglandins (PGs) E2 and D2, which support acute inflammation. During the resolution phase, the same molecules trigger a switch in this pathway to give rise to lipid mediators, such as lipoxins, as well as the biosynthesis of ω3-unsaturated fatty acid-derived resolvins and protectins that have anti-inflammatory and pro-resolving characteristics [84, 86]. Some of these lipid mediators, PGE2 and maresins, and their related enzymes, such as cyclooxygenase-2 (COX2) and 12/15-lipoxygenase (LO), are also expressed by macrophages [52, 85-87]. In fact, differential gene regulation of arachidonate metabolism-related enzymes has been reported in M1- and M2-polarized human macrophages [52]. M1 macrophages show a marked induction of COX2, with down-regulation of COX1, leukotriene A4 hydrolase, thromboxane A synthase 1 and arachidonate 5-lipoxygenase (ALOX5). Conversely, M2 macrophages show up-regulation of arachidonate 15-lipoxygenase and COX1. Further, PGE synthase (PGES) is the terminal enzyme in the pathway for PGE2 production. The microsomal isoform (mPGES) is induced in macrophages by inflammatory M1 signals such as LPS and is functionally coupled to COX2 expression. In contrast, M2 stimuli such as IL-4 and IL-13 down-regulate the expression of mPGES in macrophages [88].

Resolvin D1 has been shown to elicit an M2-like activation state [35]. In a mouse peritonitis model, two types of macrophages have been described: the CD11bhigh M1-like macrophages expressing COX2 and the CD11blow pro-resolving macrophages, expressing 12/15-LO, a precursor of protectins and resolvins [87]. Thus, under conditions of polarized inflammation, macrophages profoundly alter their lipid profile and the production of lipid mediators. The latter play a key role in the induction, regulation and resolution of inflammation.

Macrophages undergo dynamic changes during different phases of wound healing. M1 polarized macrophages mediate tissue damage and initiate inflammatory responses [1, 2]. During the early stages of the repair response after wounding the skin, infiltrating macrophages expressed an M2 phenotype and their depletion inhibited the formation of a highly vascularized, cellular granulation tissue, and of scar tissues [89]. Under these conditions, efferocytosis [90, 91] and TGFβ [92] may skew macrophage function, although demonstration of their actual in vivo relevance is lacking. In a peritoneal model of inflammation, resolution-phase macrophages expressed a unique mixed M1/M2 phenotype, with cAMP being essential to restrain M1 activation [83].

In humans, chronic venous ulcers (CVUs) represent a failure to resolve a chronic inflammatory condition [93]. Correlative analysis in patients and a mouse model suggested that in CVUs the infiltrating macrophages fail to switch from an M1 to an M2 phenotype [93]. Notably, iron metabolism is differentially regulated in polarized macrophages [63]. In CVUs, the iron overload sustained M1 activation with ROS-mediated DNA damage, fibroblast cellular senescence and defective tissue repair [93]. It is tempting to speculate that similar mechanisms may underlie M2 skewing in severe burns patients, where haemorrhage and tissue damage may result in high iron levels in tissues [94].

Dynamic changes in the phenotype of recruited mononuclear phagocytes have been observed in models of ischaemic heart and kidney disease [95-99], suggesting that this is a general feature of the natural history of repair processes. In models of acute ischaemic heart and kidney pathology, monocytes are recruited in the tissue and here their activation state undergoes dynamic changes from a predominantly M1 to a predominantly M2 phenotype [96-98].

In a murine model of hindlimb ischaemia, haplodeficiency of the oxygen sensor prolyl-hydroxylase (PHD2) induced the canonical NF-κB pathway in macrophages, which promoted their M2 polarization and pro-arteriogenic phenotype [100]. Thus, although the macrophage phenotype in resolution need not be a phenocopy of in vitro-generated M2 cells, preclinical and clinical evidence supports the long-held view of a key role of polarized macrophages in tissue repair.

A discussion on the role of macrophages and microglia in the nervous system is beyond the scope of this review (refer to [101]). However, it is noteworthy that, as discussed above, in a model of retinal neuropathy, mononuclear phagocyte infiltration generated a neuroprotective microenvironment promoting retinal progenitor cell survival [68]. The actual tissue protective significance of polarized macrophages in degenerative diseases [7] and involvement of stem/progenitor cells remains to be determined. Thus, the interplay of polarized macrophages with stem and progenitor cells is likely a key component of their role in repair and remodelling (see above).

Parasites represent a paradigm for the role of Th2-driven antimicrobial resistance and for tissue remodelling and fibrosis as an encapsulation strategy [45, 102]. In experimental and human parasite infections, macrophages generally undergo a dynamic switch toward M2 polarization [44, 103]. The early and late phases of Taenia crassiceps infection are characterized by TH1-driven M1- and TH2-driven IL-4-mediated M2 polarization of macrophages, respectively [102, 104]. A similar M1–M2 switch has been reported during Schistosoma mansoni and T. congolense infection [105]. The p50 NF-κB subunit (see above) provides protective conditions in the chronic stage of T. crassiceps infection [106]. The recurrent association of M2 polarization with parasite infections does not necessarily imply relevance in pathogenesis. For instance, lineage selective ablation of the IL-4α chain causes a dramatic increase in susceptibility to S. mansoni, but has no discernible effect on Nippolostrongylus infection [107].

Together with parasite infection, allergy represents a paradigm for IL-4/IL-13-driven type 2 inflammation [108-111]. It has generally been held that alternative (M2) activation of macrophages occurs in allergy and its major clinical manifestation, asthma [108, 109, 111]. Indeed, tissue remodelling, including collagen deposition and goblet cell hyperplasia, are key features of asthma. IL-4/IL-13-driven M2 polarization is likely to play a key role in orchestrating these processes [110], although cells with mixed phenotypes are present in situ [111].

Fibrosis is a common feature of lung and other parenchymal organ diseases. Alternatively activated M2 macrophages can produce profibrotic mediators. There is evidence that macrophages indeed play a critical role in lung fibrosis, in particular in TGFβ-orchestrated fibrotic responses [112-115]. Involvement of TGFβ and alternatively M2 activated macrophages has also been observed in fibrosis in Duchenne muscular dystrophy [116]. Here, intriguingly, prime movers are fibrinogen and IL-1. Induction of TGFβ production in macrophages per se stimulates collagen production and TGFβ-stimulated M2-like macrophages have profibrotic activity [116]. Serum amyloid P component (SAP) is an acute phase protein in the mouse with a pentraxin structure [117]. SAP is present in the circulation and ECM. SAP has been shown to inhibit fibrosis in different models by regulating macrophage function [112-115]. Thus, positive and negative regulators control the profibrotic function of macrophages. Elucidation of its regulation and function may shed new light on diverse human pathologies.

Wounds that do not heal: cancer

In a seminal review, Hal Dvorak crystallized the concept that cancer and wound healing share cellular and molecular pathways [118]. Epidemiological and experimental evidence demonstrates that chronic inflammation is one of the consistent features of the tumour microenvironment [119-121]. Tumour-associated macrophages (TAMs) are the major components of the infiltrate of most tumours [122, 123] and are key regulators of the link between inflammation and cancer. In the tumour microenvironment macrophages can express pro- and anti-tumour functions. TAMs in mouse and human tumours generally display an M2-like phenotype [2] which is oriented to promoting tumour growth, remodelling tissues, promoting angiogenesis and suppressing adaptive immunity. Signals derived from tumour-infiltrating T cells or from the tumour cells (including M-CSF, IL-10 and TGFβ) [124] might induce this M2-like phenotype of macrophages that have been recruited into tumours [124]. There is now evidence that TAMs in murine tumours consist of populations with substantial differences [125]. The immunosuppressive cytokines IL-10 and TGFβ are produced by many types of cancer cells and TAMs and a gradient of tumour-derived IL-10 may account for differentiation along the dendritic cells (DC) versus the macrophage pathway in different microanatomical locations within a tumour [126]. In a transplanted mammary carcinoma, TAMs were reported to have an M1-like and M2-like phenotype in normoxic and hypoxic areas, respectively [8]. In detail, TAMs become polarized based not on their location but on distinct signals deriving from the particular microenvironment in which they reside [127-129]. Thus, integration of distinct signals can result in production of a wide spectrum of TAM phenotypes with characteristic tumour-regulating properties [130]. An additional degree of diversity of mechanisms responsible for TAM generation takes place at the level of the tumours and organs involved [128, 131]. TAM products can influence many aspects of tumour growth and progression [119]. In particular, they can: regulate senescence; interact with and contribute to the remodelling of the ECM; promote cancer cell proliferation [132] invasion and metastasis [133]; and sustain angiogenesis and lymphangiogenesis [133, 134]. TAMs express low levels of the major histocompatibility complex class II and reduced antimicrobial and tumouricidal activity; finally, they suppress anti-tumoural adaptive immunity. TAMs produce enzymes and proteases, which regulate the digestion of the ECM, such as matrix metalloproteinases (MMPs), plasmin, urokinase-type plasminogen activator (uPA) and the uPA receptor [135]. TAM-derived MMP-9 can also induce the release of heparin-bound growth factors, thus promoting the angiogenic switch [136]. Moreover, macrophages exposed to IL-4, IL-10, TGFβ and tumour cell supernatants selectively express the fibronectin isoform migration-stimulating factor (MSF), a potent motogen for monocytes. However, its role in ontogeny and immunopathology remains to be defined [137].

TAMs have been reported to promote angiogenesis and lymphangiogenesis by releasing pro-angiogenic growth factors, such as TGFβ, VEGFA, VEGFC, EGF and thymidine phosphorylase (TP), and chemokines, such as CCL2 and CXCL8 [133, 138-140], favouring subsequent dissemination of cancer cells [141]. Macrophages act as ‘bridge cells’ or ‘cellular chaperones’ that guide the fusion of endothelial tip cells (vascular anastomosis) and facilitate vascular sprouting [139, 142].

Hypoxic regions of tumours trigger a pro-angiogenic programme in macrophages accumulated in these areas, through the up-regulation of HIF-1 and HIF-2. In fact, these factors enhance the expression of VEGF, FGF-2, CXCL8 and glycolytic enzymes [143]. Therefore, in addition to tumour-derived angiogenic molecules, macrophages are recruited in situ and programmed to amplify the angiogenesis.

TAMs interact with so-called cancer stem cells (CSCs) or cancer initiating cells, a function that mirrors interaction with normal stem/progenitor cells (see above). Studies with TAMs and CSCs have focused mainly on mammary carcinoma and gliomas [12, 14, 15, 22]. In a recent careful study, TAMs were found to promote CSC tumourigenicity and drug resistance by releasing milk fat globule-E8 (MFG-E8) [17]. MFG-E8 acted on CSCs via the Notch and Stat3 pathway. In glioma, a bidirectional interplay between CSF and TAMs may occur. TAMs are recruited in gliomas via chemokines [22, 144]. Glioma CSCs shape the function of macrophages/microglial cells which acquire immunosuppressive activity [14]. In turn, TAMs and resident microglial cells have been reported to promote glioma invasion [15]. Thus, as for normal stem/progenitor cells, a reciprocal interaction may occur between CSCs and macrophages.

Concluding remarks

Resolution of inflammation, tissue repair and remodelling involve the function of cells of the mononuclear phagocytes. Frequently in these settings, macrophages are set in an M2 or M2-like mode, although there are exceptions [83]. In adults, in organs such as the mammary gland, macrophages exert a trophic function. The trophic and remodelling function of macrophages mirrors their action in ontogenesis. The tissue remodelling activity of macrophages is part of diverse pathological conditions ranging from allergy to cancer. The trophic and remodelling functions of macrophages may present both a tool and a target for therapeutic intervention.

Author contributions

AM designed the review, drafted the figure and references and finalized its contents; MRG drafted section 5; AS and SB drafted the section on Homeostatic tissue remodelling; and ML helped finalize the paper.

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