Protective and pathogenic functions of macrophage subsets

Tissue surveillance and immunosuppression

Mature macrophages are strategically located throughout the body and perform an important immune surveillance function. They constantly survey their immediate surroundings for signs of tissue damage or invading organisms and are poised to stimulate lymphocytes and other immune cells to respond when danger signals are phagocytosed and/or detected by cell surface receptors. For example, when a macrophage ingests a pathogen, the pathogen becomes trapped in a phagosome, which then fuses with a lysosome unless prevented from doing so by pathogen-specific mechanisms. Within the fused phagolysosome, enzymes and toxic free radicals digest and destroy the pathogen. In addition to fighting infections, resident tissue macrophages are involved in maintaining healthy tissues by removing dead and dying cells and toxic materials. For example, alveolar macrophages facilitate the removal of allergens from the lung, whereas Kupffer cells in liver participate in the clearance of pathogens and toxins from the circulation. Tissue macrophages also suppress inflammation mediated by inflammatory monocytes, thereby ensuring that tissue homeostasis is restored following infection or injury. Indeed, important homeostatic functions have been assigned to the mononuclear phagocytes in almost every tissue of the body (FIG. 1).

Macrophages function as sentinel cells in the tissues

Because normal cells of the body must not be mistakenly removed or compromised, macrophages are selective of the material that they phagocytose. During and following phagocytosis, PRRs (including TLRs, C-type lectin receptors (CLRs), scavenger receptors, retinoic acid-inducible gene 1 (RIG1)-like helicase receptors (RLRs) and NOD-like receptors (NLRs)) recognize signals associated with invading pathogens, foreign substances (for example, silica or asbestos) and dead or dying cells^1,5. Some PRRs (such as the mannose receptor, DC-specific ICAM3-grabbing non-integrin (DC-SIGN) and macrophage receptor with collagenous structure (MARCO)) function in pathogen binding and phagocytosis, whereas signalling PRRs (which include the TLRs, NLRs and RLRs) sense microbial products and aberrant self on the cell surface or in the cytoplasm of cells and activate transcriptional mechanisms that lead to phagocytosis, cellular activation and the release of cytokines, chemokines and growth factors^29–31. Macrophages also express numerous secreted molecules, including complement and Fc receptors that bind opsonin molecules, C3b and antibodies, which activate the complement cascade and enhance the process of phagocytosis by tagging the pathogen surface. Thus, macrophages use various surface receptors and secreted molecules to monitor and respond to changes in their environment.

Monocyte recruitment and subsequent macrophage fate in tissues

‘Emergency myelopoiesis’ is the process of generating large pools of monocytes and neutrophils from cells in the bone marrow beyond the normal requirements of a healthy person. Tissue stress, including acute and chronic infection, as well as sterile inflammation, drives the production of monocytes and neutrophils in a process that is dependent on cytokines such as granulocyte colony stimulating factor (G-CSF) and chemokines including CC-chemokine ligand 2 (CCL2) and CCL5 (REF. 33) (FIG. 2). The increased production of monocytes and neutrophils is found in many different types of stress and can therefore be considered a common, conserved pathway. Moreover, the production of circulating MDSCs increases in cancer, but also in Crohn’s disease³⁴, autoimmune disease³⁵, transplantation tolerance³⁶ and smouldering sepsis induced by caecal ligation and puncture (CLP)³⁷. CLP-mediated induction of MDSCs is dependent on myeloid differentiation primary response protein 88 (MYD88)³⁷ and therefore we might expect that the TLR and IL-1 receptor (IL-1R) common pathway, via MYD88, induces haematopoietic cytokines, such as G-CSF and granulocyte/macrophage colony stimulating factor (GM-CSF), that act on bone marrow precursors to increase the output of neutrophils and monocytes^38,39. The MDSC pool that exits the bone marrow comprises mature and immature mixtures of monocytic and granulocytic cells, suggesting that either the capacity of the bone marrow to mature the cells is compromised or the bone marrow receives signals to expel the haematopoietic cells at an increased rate. This pathway is an example of long-range communication between the damaged site and the bone marrow to generate increased numbers of tissue macrophages.

A widely accepted view is that monocytes adopt two distinct fates after bone marrow exit¹. One type of monocyte — which is defined by high expression of CX₃C-chemokine receptor 1 (CX₃CR1) and low expression of the myeloid marker lymphocyte antigen 6C (LY6C) (TABLE 1) — has a ‘patrolling’ function in and around the vascular endothelium¹. Importantly, patrolling monocytes lack the expression of the chemokine receptor CC-chemokine receptor 2 (CCR2) and cannot respond to CCL2. A recent study has shown that the transcription factor NUR77 (encoded by nuclear receptor subfamily 4, group A, member 1 (Nr4a1)) is required for the development of patrolling monocytes⁴⁰. By contrast, the LY6C^hi monocyte pool is linked to inflammation, expresses CCR2 and can be rapidly mobilized¹. The spleen harbours large numbers of LY6C^hi monocytes in the subcapsular red pulp that rapidly emigrate to inflammatory sites⁴¹.

Multiple types of acute infections cause monocyte mobilization, including infection with influenza, Listeria monocytogenes, Toxoplasma gondii and fungi^42–45. Recent results have revealed a surprising complexity to chemokine-induced monocyte recruitment. For example, in acute Citrobacter rodentium infection in the gut (a mouse model of severe Escherichia coli infection), the NLR protein nucleotide-binding oligomerization domain protein 2 (NOD2) in non-haematopoietic cells of the gut lamina propria is responsible for CCL2 production and the subsequent recruitment of large numbers of monocytes that flood the colon and become inflammatory macrophages⁴⁶. This process is essential for bacterial clearance and for the restoration of tissue homeostasis because NOD2-deficient mice cannot clear the bacteria efficiently and thereby have increased bacterial loads and tissue damage.

CCL2 also drives monocyte recruitment in other settings. For example, when the protozoan parasite Leishmania major infects macrophages, it does not induce a strong inflammatory response and few, if any, chemokines and cytokines are made⁴⁷. Nevertheless, L. major induces a strong inflammatory response at the infection site; it was shown that complement deposition on parasites induces platelets to accumulate at the infection site and release platelet-derived growth factor (PDGF), which stimulates local CCL2 production and thus creates a chemokine gradient to induce monocyte recruitment⁴⁸ (FIG. 2). Moreover, monocyte recruitment can be initiated by low circulating amounts of TLR agonists that induce CCL2 production in bone marrow mesenchymal cells and drive inflammatory monocytes into circulation⁴⁹. This mechanism presumably bypasses the splenic reservoir and is thus an example of the diverse mechanisms the body uses to produce sufficient monocytes and get them into circulation, and ultimately into tissues where they terminally differentiate into macrophages.

Differentiation of the recruited monocytes in situ

The fate of the recruited monocytes and their subsequent differentiation into macrophages is a key issue because inflammatory monocytes have the potential to cause tissue damage or even promote metastasis⁵⁰. Monocytes quickly differentiate into macrophages and DCs at the site, and it remains unclear how their inflammatory activity is constrained, although IL-10 is likely to have an irreplaceable effect in suppressing activated macrophages at the damage site⁵¹.

We cannot assume that the circulating LY6C^hi monocyte population is uniform. It is possible that the LY6C^hi monocyte population consists of both inflammatory and regulatory populations that counter-balance each other, or the LY6C^hi monocytes might convert into regulatory macrophages upon exposure to the non-inflammatory tissue mononuclear phagocytes. In this regard, it was recently shown that pre-emptive CSF1 treatment reduced graft-versus-host disease by expanding suppressive or regulatory macrophages⁵².

The gut has been fertile ground for research into the fate of recruited monocytes. Several groups have established that a population of gut macrophages is exclusively derived from the circulating monocyte pool, whereas another gut mononuclear phagocyte population, which is characterized by the expression of CD103, is a distinct population of resident gut DCs that have their own functional specializations in terms of promoting immune responses^53,54.

During C. rodentium and T. gondii infection, the recruited monocyte population is essential to resolve acute inflammation, but must rapidly convert to an anti-inflammatory phenotype following interaction with the gut-resident macrophages in order to restrain excessive responses to the gut flora. Moreover, in the brain, it was recently shown that recruited pro-inflammatory immature LY6C^hi myeloid cells convert in situ to regulatory populations that suppress T cell response⁵⁵. Be it the gut or any other organ system, it remains unclear if and how monocytes differentiate at the damage site and how the overall number of mononuclear cells in an organ are controlled after homeostasis is re-established. It seems likely the underlying plasticity in myeloid lineages and conversion between pro- and anti-inflammatory activities will be a paradigm uncovered in numerous pathological scenarios.

In situ proliferation of macrophages

The textbook picture of macrophage differentiation from recruited monocytes was recently challenged by a study that demonstrated that tissue macrophages undergo massive proliferation in T_H2-mediated inflammation²⁴. In this scenario, IL-4 produced by T_H2 cells is sufficient to cause local macrophage proliferation during helminth infections, resulting in increased numbers of M2 effector macrophages, which expel worms (FIG. 3). Furthermore, recruited M1 macrophages were induced to proliferate as long as sufficient IL-4 was present²⁴. The signalling mechanism regulated by IL-4 to push macrophages into the cell cycle remains unclear, but may be related to the expression of macrophage-activating factor (MAF; also known as c-MAF) and MAFB transcription factors that suppress macrophage proliferation⁵⁶.

Self-renewal of tissue macrophages is an appealing concept because it would bypass the requirement for bone marrow-generated monocytes and thus allow local sites to develop an anti-inflammatory milieu that allows for wound repair. Presumably, the expanded population of M2 macrophages would also restrain excessive T cell responses by l-arginine depletion⁵⁷. However, many questions about tissue macrophage self-renewal remain unanswered. It is unclear whether tissue macrophage self-renewal occurs generally in T_H2-dominated inflammation. For example, do alveolar macrophages proliferate in asthma and allergic lung inflammation? Similarly, do tissue-resident macrophages proliferate at the sites of deep tissue T_H2 responses, such as at sites of schistosome egg deposition in the liver and Trichinella spiralis worm invasion in muscle? Finally, as the gut harbours the largest population of macrophages in the body, do these cells self-renew to perpetuate the necessary numbers of anti-inflammatory macrophages, or do most originate from bone marrow-derived monocytes?

Macrophage activity in response to tissue injury or infection

Following tissue injury or infection, the first-responder macrophages usually exhibit an inflammatory phenotype and secrete pro-inflammatory mediators such as tumour necrosis factor (TNF), nitric oxide (NO) and IL-1, which participate in the activation of various antimicrobial mechanisms, including oxidative processes that contribute to the killing of invading organisms^51,58. Other mediators produced by activated macrophages include IL-12 and IL-23, which are decisive in influencing the polarization of T_H1 and T_H17 cells, which further drive inflammatory responses forward. Activated macrophages produce reactive oxygen and nitrogen intermediates, including NO and super-oxide, that are highly toxic for microorganisms but can also be highly damaging to neighbouring tissues and lead to aberrant inflammation³². Indeed, M1 macrophages are believed to participate in various chronic inflammatory and autoimmune diseases⁵⁹ (FIG. 4). Therefore, pro-inflammatory and antimicrobial M1 macrophage responses must be controlled to prevent extensive collateral tissue damage to the host.

Regulators of tissue repair

In addition to their innate phagocytic activity and role in antimicrobial immunity, macrophages are intimately involved in wound repair^60,61 (FIG. 4). In contrast to pro-inflammatory and antimicrobial M1 macrophage responses, M2 macrophages exhibit potent anti-inflammatory activity and have important roles in wound healing and fibrosis^62,63. They also antagonize M1 macrophage responses, which may be crucial for the activation of the wound healing response and for tissue homeostasis to be restored⁵⁹. Recent studies have also shown that M1 macrophages can themselves ‘convert’ into anti-inflammatory macrophages with an M2 wound-healing phenotype^64,65.

M2 macrophages produce growth factors that stimulate epithelial cells and fibroblasts, including transforming growth factor-β1 (TGFβ1) and PDGF⁶⁶. Macrophage-derived TGFβ1 contributes to tissue regeneration and wound repair by promoting fibroblast differentiation into myofibroblasts, by enhancing expression of tissue inhibitors of metalloproteinases (TIMPs) that block the degradation of extracellular matrix (ECM) and by directly stimulating the synthesis of interstitial fibrillar collagens in myofibroblasts^67,68. Macrophage-derived PDGF also stimulates the proliferation of activated ECM-producing myofibroblasts⁶⁹.

M2 macrophages can also regulate wound healing independently of their interactions with myofibroblasts. Indeed, they produce matrix metalloproteinases (MMPs) and TIMPs that control ECM turnover⁷⁰, they engulf and digest dead cells, debris and various ECM components that would promote tissue-damaging M1 macrophage responses^66,71, and they secrete specific chemokines that recruit fibroblasts, T_H2 cells and regulatory T (T_Reg) cells^72,73. Moreover, M2 macrophages produce factors that induce myofibroblast apoptosis⁷⁴, serve as antigen-presenting cells (APCs) that propagate antigen-specific T_H2 and T_Reg cell responses (which promote wound healing while limiting the development of fibrosis^75,76) and express immunoregulatory proteins (such as IL-10, resistin-like molecule-α (RELMα; also known as RETNLα or FIZZ1), chitinase-like proteins and arginase 1 (ARG1)) that have been shown to decrease the magnitude and duration of inflammatory responses and promote wound healing^57,77–81 (FIG. 4).

Adipose tissue macrophages in metabolic disorders

M2 macrophages have been found to regulate important metabolic functions⁸². These macrophages are induced by peroxisome proliferator activated receptor-γ (PPARγ) signalling and maintain adipocyte function, insulin sensitivity and glucose tolerance, which can prevent the development of diet-induced obesity and type 2 diabetes^83,84. A recent paper suggested that IL-4-producing eosinophils are required to maintain M2 macrophages in healthy non-obese mice¹⁷⁶. These studies suggest that as obesity progresses, adipose tissue-associated macrophages switch from an M2-like phenotype to a classically activated M1-like phenotype with potent pro-inflammatory activity⁸², with the NLRP3 inflammasome serving as the molecular switch by sensing obesity-associated danger signals⁸⁵ (see REF. 86 for a review).

The role of M2 macrophages in allergy and asthma

M2 macrophages were originally described as suppressive cells because they inhibit the production of a wide variety of pro-inflammatory mediators^87,88. However, the definition and function of M2 macrophages has been expanded, particularly in regards to their role in regulating T_H2-type inflammatory responses, as in addition to downregulating pro-inflammatory responses, M2 macrophages are involved in the development of T_H2-dependent immunity to some extracellular parasites and fungi^89,90.

Numerous studies have identified roles for M2 macrophages in allergic responses driven by IL-4 and IL-13 (REF. 91). However, their function in allergy and asthma remains controversial, with some studies suggesting that M2 macrophages promote allergic inflammation and others indicating a suppressive role for these cells. A recent study suggested that M2 macrophages are required for the development of airway disease following infection with Sendai virus, which is a mouse parainfluenza virus⁹². The authors found that M2 macrophages secrete IL-13 and that their depletion significantly attenuated T_H2-driven inflammation in the lung. M2 macrophages induced during rhinovirus infection have also been shown to exacerbate eosinophilic airway inflammation by producing the chemokine CCL11 (also known as eotaxin 1), which recruits eosinophils⁹³. The epithelial-derived cytokine IL-33 has also been hypothesized to function as a major driver of eosinophilic airway inflammation because it promotes the differentiation of airway macrophages towards an M2 phenotype^94,95.

Nevertheless, other studies have questioned the importance of macrophages in the development of allergic airway disease and instead support a role for another type of mononuclear phagocyte, CD11c⁺ DCs, in the development of eosinophilic inflammation and T_H2-associated cytokine production in the lung⁹⁶. Additional reports have also identified a suppressive role for M2 macrophages in allergy and asthma. Indeed, by facilitating the uptake and removal of fungal conidia, M2 macrophages have been shown to inhibit asthma symptoms associated with chronic fungal infections⁹⁰. In contrast with M2 macrophages in mice infected with Sendai virus, M2 macrophages producing IL-13 mediated the resolution of respiratory syncitial virus-induced lung injury by reducing inflammation and epithelial damage⁹⁷. Chitinase proteins expressed by M2 macrophages have also been proposed to suppress allergic inflammation by degrading or sequestering chitin, a potent and highly abundant allergen in the airway⁸⁰. RELMα, which is expressed by M2 macrophages, eosinophils and epithelial cells, inhibits T_H2-driven inflammation in the lung^79,98. However, the specific contribution of M2 macrophages and the proteins they express to airway inflammation remains unclear, as the expression of many of these proteins is not exclusive to T_H2 cytokine-stimulated macrophages. These studies emphasize the need to elucidate the functions of molecules expressed specifically by M2 macrophages (TABLE 2).

The role of macrophages in tumorigenesis

Distinct macrophage subsets have been linked with either protective or pathogenic roles in cancer⁹⁹. A protective role in tumorigenesis has been described for M1 macrophages, which activate tumour-killing mechanisms and antagonize the suppressive activities of TAMs, MDSCs, M2 macrophages, regulatory macrophages and immature myeloid cells (which have all been shown to suppress adaptive tumour-specific immune responses and promote tumour growth, invasion, metastasis, stroma remodelling and angiogenesis^100–105). M1 macrophages also amplify T_H1 responses, providing a positive feedback loop in the antitumour response⁶⁴.

By contrast, TAMs isolated from solid and metastatic tumours have a suppressive M2-like phenotype. Furthermore, accumulating evidence from many tumour models suggests that macrophages contribute to tumour progression, with increasing numbers of TAMs, MDSCs and immature monocytes correlating with poor outcomes^106–108. These observations are also consistent with the tumour-promoting activities of IL-4 and IL-13, which also promote M2 macrophage differentiation^{109– 112}. A novel population of forkhead box P3 (FOXP3)-expressing macrophages was also shown to display immunosuppressive properties and promote tumour growth¹¹³.

Importantly, IFNγ was recently shown to reverse the immunosuppressive and pro-tumoural properties of TAMs. So, IFNγ could potentially be administered locally to combat the generation and maintenance of immunosuppressive TAMs and thus boost protective M1 macrophage and T cell responses within the tumour microenvironment¹¹⁴. Moreover, blocking nuclear factor-κB (NF-κB) signalling can switch TAMs to an M1-like phenotype that is cytotoxic against tumour cells¹¹⁵. Natural killer T cells can also kill TAMs directly, providing an additional approach for targeting TAMs and promoting tumour-specific immunity¹¹⁶.

A major problem in the analysis of TAMs concerns how the cells are phenotyped and thus categorized. Diverse phenotypes have been attributed to TAMs, and this stems partly from differences in tumour types, donors and isolation techniques. Therefore, TAM phenotyping should rely on defining gene and protein expression profiles in vivo and ex vivo and on comparison of these profiles with the gene expression profiles of conventional macrophage subsets. Moreover, TAMs should be expected to exhibit the same plasticity as other macrophages following cytokine stimulation ex vivo. Undoubtedly, comprehensive profiling of TAMs from both mouse cancer models and human samples will be a key part of understanding the tumorigenesis process, as cancer researchers have increasingly recognized ‘inflammation’ as being inseparable from cancer itself¹¹⁷.

Contrasting roles for macrophage subsets in autoimmunity

M1-like macrophage-derived TNF, IL-18, IL-12 and IL-23 have been identified as important mediators in several chronic inflammatory and autoimmune diseases, including Crohn’s disease, rheumatoid arthritis, multiple sclerosis and autoimmune hepatitis^118–120. For example, during experimental colitis, a subset of CX₃CR1^int LY6C^hi GR1⁺ (glutathione reductase 1⁺) macrophages expressing TLR2, CCR2 and TNF was shown to promote inflammation in the colon¹²¹. Similarly, in patients with Crohn’s disease, researchers identified a population of CD14⁺ macrophages that are distinct from the normal intestinal macrophage pool and produce large amounts of pro-inflammatory cytokines, including IL-23 and TNF^122,123. Because IL-23 and TNF mediate pathology in Crohn’s disease, these inflammatory macrophages have been hypothesized to contribute to pathogenesis of the disease. Nevertheless, other studies have shown that impaired pro-inflammatory cytokine production by macrophages can also contribute to Crohn’s disease by diminishing the capacity of macrophages to clear potentially pathogenic commensal bacteria from the lining of the bowel¹¹⁹.

Resident tissue macrophages also maintain homeostasis in the intestine by clearing apoptotic cells and debris, by promoting epithelial repair and by producing IL-10, which has been shown to maintain expression of FOXP3 in colonic T_Reg cells^124,125. In a pathology as complex as Crohn’s disease, it is important to bear in mind that the principles of macrophage heterogeneity and plasticity also apply, and thus multiple macrophage populations are likely to have flexible pro- and anti-inflammatory (or homeostatic) effects in the gut and are subject to both temporal and anatomical effects.

Contrasting roles for different macrophage subsets have also been described in the pathogenesis of rheumatoid arthritis. For example, TNF produced by M1-like macrophages was shown to trigger cytokine production by synovial cells, leading to the development of chronic polyarthritis¹²⁰. By contrast, macrophages producing reactive oxygen species were found to protect mice from arthritis by inhibiting T cell activation¹²⁶.

Macrophages have also been identified as key regulators in demyelinating diseases of the central nervous system (CNS). Indeed, infiltrating M1-like macrophages are thought to contribute to axonal loss in multiple sclerosis and in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis¹²⁷. Macrophages recruited to the CNS prime T cells to execute a T_H1 effector programme in EAE¹²⁸, whereas recruited myeloid cells producing IL-23 stimulate the production of GM-CSF by helper T cells, which regulates disease development and severity^129,130. These observations suggest that macrophages could be targeted to prevent or reduce axonal loss in multiple sclerosis¹³¹. However, macrophages also have protective roles in multiple sclerosis by promoting T cell apoptosis and by expressing anti-inflammatory cytokines such as TGFβ1 and IL-10, which contribute to the termination of inflammation¹³². Moreover, a subset of macrophages expressing the inhibitory receptor CD200 (also known as OX2) has also been shown to prevent the onset of EAE in mice¹³³. Finally, a population of monocyte-derived macrophages was shown to inhibit inflammation in a model of spinal cord injury, providing further evidence for a protective role for macrophages in the CNS¹³⁴. Thus, macrophages have both protective and pathogenic roles in a wide variety of autoimmune and inflammatory diseases.

Macrophage subsets in atherosclerosis

It has been appreciated for quite some time that atherosclerosis is both a lipid disorder and inflammatory disease, with macrophages having a central role¹³⁵. In atherosclerosis, it is thought that macrophages lodge in the intima and subintima of arteries, eventually leading to the formation of obstructive atherosclerotic plaques that are prone to rupture, leading to thrombosis, myocardial infarction or stroke. Studies have suggested that T_H1 cells contribute to the development of atherosclerosis by producing IFNγ¹³⁶, which stimulates the differentiation of highly activated macrophages, termed foam cells, that promote the formation of unstable lesions¹³⁷. These pathogenic macrophages also express higher levels of scavenger receptors and CD36, which augments the uptake of modified forms of low-density lipoprotein^138–140.

By contrast, T_H2-associated cytokines, particularly IL-10, seem to have a protective role, as they block the formation of pathogenic M1-like macrophages in atherosclerotic plaques¹⁴¹. Although hypercholesterolaemia was initially hypothesized to be the primary stimulus for the recruitment of macrophages into the arterial wall, immunological and mechanical injuries, as well as bacterial and viral infections, are likely to contribute to the pathogenesis of atherosclerosis¹³⁷. Toxic blood lipids, such as oxidized low-density lipoproteins (cholesterol) are removed by macrophages as part of their general homeostatic scavenging function¹³⁹. Therefore, because macrophages facilitate the clearance of cholesterol, they could be viewed as having a protective role in atherosclerosis and lipid homeostasis.

However, hypercholesterolaemic mice that are deficient in macrophages were found to be highly resistant to developing atherosclerosis, suggesting that macrophages primarily have a pathogenic role in the disease¹⁴². Depletion of CD11b⁺ macrophages after plaque formation is, by contrast, less protective, suggesting that monocytes and macrophages are involved in the genesis but not maintenance of atherosclerosis¹⁴³. Nevertheless, some reports have suggested that decreases in plaque size and regression of atherosclerosis correlates with macrophages emigrating from the plaque^135,144. Thus, devising strategies that facilitate the depletion or inactivation of pathogenic M1-like macrophages from actively growing plaques could emerge as a useful therapy for atherosclerosis^145,146.

Macrophage subsets in the pathogenesis of fibrosis

Studies have suggested that progressive fibrotic diseases, such as idiopathic pulmonary fibrosis (IPF), hepatic fibrosis and systemic sclerosis, are tightly regulated by macrophages⁶¹. ‘Pro-fibrotic’ macrophages produce various mediators, including TGFβ1, PDGF and insulin-like growth factor 1, that directly activate fibroblasts, and therefore these cells are intimately involved in wound healing (FIG. 4). These secreted proteins regulate the proliferation, survival and activation status of myofibroblasts, which control ECM deposition^147–149. Pro-fibrotic macrophages also produce their own MMPs and TIMPs, which regulate inflammatory cell recruitment and ECM turnover⁷⁰. In addition, they secrete various pro-fibrotic cytokines and chemokines, including IL-1β, which was identified as a potent pro-fibrotic mediator in the lung^150,151. IL-1β stimulates T_H17 cells to produce IL-17, which was identified as an important inducer of bleomycin-induced pulmonary fibrosis, a fibrotic disorder with characteristics that are similar to those of IPF¹⁵². Furthermore, macrophages function as APCs and promote T_H2 responses¹⁵³, which have been shown to induce and activate the pro-fibrotic cytokine TGFβ1 in macrophages through an IL-13- and MMP9-dependent mechanism^62,154.

Nevertheless, although macrophages are clearly required for the initiation and maintenance of fibrosis, other studies have suggested that they are also involved in the suppression, resolution and reversal of fibrosis¹⁵⁵. Indeed, macrophages phagocytose dead cells and cellular debris, which can help to reduce the danger signals that contribute to the production of pro-inflammatory and pro-fibrotic mediators. Moreover, they engulf and digest ECM components and stimulate the production of collagen-degrading MMPs in other inflammatory cells, including myofibroblasts and neutrophils⁷⁰. The production of IL-10, RELMα and ARG1 by M2-like macrophages has been shown to suppress fibrosis^57,79,156. Thus, with their potential to both induce and inhibit fibrosis, macrophages and the factors they express are integrated into all stages of the fibrotic process (FIG. 4). To better understand the pathogenesis of fibrosis, we therefore need to identify the specific macrophage subsets that promote, inhibit and reverse fibrosis and elucidate the contributions of the unique mediators that are expressed by each population.

Together, these examples illustrate how inflammatory and suppressive macrophages are crucially involved in the progression and resolution of disease. They also demonstrate the complex and often opposing roles of different macrophage subsets in health and disease. A more detailed understanding of the mechanisms that regulate the activation and deactivation of human macrophages is likely to lead to the development of more effective strategies for treating various important inflammatory diseases¹⁵⁷.