The Dr. Jekyll and Mr. Hyde complexity of the macrophage response in disease

Review of concepts related to the complexity of macrophage biology, with emphasis on their origin and transcriptional cues that dictate their roles in disease.

Abstract

Macrophages comprise a highly diverse cell population expressing a continuum of biologic activities dictated by exposure to a plethora of inflammatory cues. Moreover, in contrast to most other hematopoietic populations, macrophages can arise from multiple sites—namely, the bone marrow or yolk sac, adding to the complexity of macrophage biology during health and disease. Nonetheless, it is this very type of diversity that is indispensable for macrophages to respond effectively to pathologic insults. Most of the interest in macrophage biology has been devoted to bone marrow-derived populations, but it is now becoming clearer that tissue-resident populations, which arise from distinct hematopoietic compartments, serve critical roles in host defense, including protection against neoplastic disease. Depending on the inflammatory milieu, macrophages can behave as a “two-edged sword,” playing either host-protective (i.e., antitumor) or host-destructive (i.e., protumor) roles. Accordingly, we review herein the mechanisms that instruct macrophage functional diversity within their microenvironments, with special emphasis on transcriptional regulation, which is less understood. Given their polarizing positions in disease processes, we will also provide an overview of strategies that target macrophages or their effector mechanisms for therapeutic purposes.

Introduction

Ordinarily, macrophages are indispensable for host defense, acting directly as effectors or indirectly by engaging elements of the innate and adaptive immune systems. However, this highly heterogeneous class of cells continues to be a challenge to fully understand and appreciate, in large part because of their unique capacity to respond to a variety of environmental and inflammatory cues in a multitude of ways. Expanding our knowledge of macrophages is complicated by the fact that they originate from multiple sites and that bone marrow–derived populations, for example, are biologically distinct from tissue-resident subsets, each playing unique roles in host defense or maintenance and preservation of tissue and organ integrity.

In this article, we seek to provide an overview of the major macrophage cell types that contribute to health and disease: bone marrow-derived and tissue-resident. It is important to emphasize that most studies of macrophages have focused on the bone marrow–derived population and, therefore, much more is known about their biology and roles in disease, especially cancer. In contrast, less is known about tissue-resident macrophages, including the complexity of their phenotypes, signaling events in response to inflammatory or pathologic episodes, and importance in the TME. Therefore, we provide a review of concepts related to macrophage biology, including those associated with phenotype, functionality, or transcriptional regulation and roles in disease, mainly pertaining to the bone marrow–derived population. We review the tissue-resident populations in a context-dependent manner, in accordance with how these cells have usually been studied. Overall, we focus on inflammatory signals and transcriptional regulators that govern the biology of these two populations of cells, as well as strategies that potentially target these cells or their effector functions to alter, direct, and improve their participation in host defense, including neoplastic disease.

MACROPHAGE POLARIZATION

Macrophages, both bone marrow-derived and tissue-resident, comprise a functionally heterogeneous class of myeloid cells that are critical in maintaining tissue integrity and host defense during inflammatory or pathologic episodes. However, such studies in macrophage biology have largely focused on BMDMs. Therefore, this section focuses on concepts derived mainly from studies on BMDMs. It is reasonable to posit that tissue-resident macrophages may adopt many of these same characteristics, although more studies are necessary. Macrophages play numerous roles in tissue homeostasis, thwarting infections and pathogens, and in wound healing, accompanied by tissue repair and remodeling. To achieve these tasks, macrophage subsets have been defined and broadly grouped into two major functional subsets. The ability of macrophages to assume such broad functional states has been termed “polarization,” a process guided by their reaction to diverse inflammatory signals generated during the particular physiologic or pathologic event (Fig. 1).

Figure 1. Complex spectrum of macrophage polarization states.

Macrophages derived from bone marrow or yolk sac compartments are initially naive and lack specific functional characteristics until exposed to distinct inflammatory signals. On one side of the spectrum, M1, or classically activated macrophages, arise in response to IFN-γ and/or LPS exposure, possessing a phenotype that is proinflammatory and tumoricidal characterized by the expression or production of the indicated mediators. On the other side of the spectrum, different M2 macrophage subtypes can arise depending on the nature and type of stimulus. M2 or alternatively activated macrophage subtypes emerge in response to the indicated stimuli, possessing an anti-inflammatory or immune regulatory phenotype characterized by the expression or production of the indicated mediators supporting angiogenesis, vascularization and immune suppression. ROS, reactive oxygen species; RNS, reactive nitrogen species, MMP matrix metalloproteinase.

The classically activated, or “M1,” macrophage, is characterized by the production of high levels of proinflammatory cytokines and the ability to initiate antimicrobial and tumoricidal programs, in part through the production of reactive nitrogen and oxygen species [1–3]. Historically, the antimicrobial activity of macrophages lends itself to this characterization, as Martinez and Gordon [3] and Mackaness and colleagues [4] used it to define the macrophage response to Bacillus Calmette-Guerin. Thus, hallmark features of M1 macrophages reflect high expression of proinflammatory cytokines—namely, IL-12, -23, and -1 [2, 3, 5] or the enzyme iNOS, which generates NO, a potent cytotoxic agent. Classically activated macrophages arise in response to IFN-γ produced from T_h1 cells, from macrophages themselves or from NK cells [1, 3, 6, 7], or in response to other inflammatory stimuli such as bacteria-derived LPS [3, 6, 8, 9].

In contrast, the alternatively activated, or “M2,” macrophage phenotypes are induced by a spectrum of signals, such as IL-4, -10, or -13 produced by T_h2 cells, macrophages themselves, or basophils [3, 6, 7, 10, 11] (Fig. 1). Specifically, M2a macrophages arise in response to IL-4 and -13 and are thought to be the hallmark M2 macrophage, responsible for type II inflammation and killing of parasites. M2b macrophages arise in response to immune complexes and TLR ligands, which activate an immune regulatory program. These macrophages were first described by Mantovani [1, 2] and other laboratories [3]. M2c macrophages that arise in response to IL-10 are involved in tissue remodeling and matrix production. The ability of these different macrophage phenotypes to assume such different characteristics is enabled by the complex transcriptional networks activated in the presence of these inflammatory stimuli. The nature of the complex transcriptional programming of macrophages is discussed later in this review. Hallmark features of the M2 macrophage response include the production of anti-inflammatory cytokines, such as IL-10 and TGF-β. Moreover, M2 macrophages—specifically, the M2a subtype—metabolize arginine to ornithine through the expression of Arg-1 and increase expression of scavenging molecules that are necessary for their phagocytic potential [3, 6, 11, 12]. Degradation of the amino acid arginine by Arg-1 leads to immune suppression, as arginine is necessary for productive T cell activation. M2 macrophages (the M2c subtype) aid in tissue remodeling and vascularization through the production of MMPs and VEGF [13, 14]. The ability of macrophages to achieve such diverse functional states is indeed a great strength of the immune system, as it permits macrophages to meet the demands of an appropriate host defense response.

In summary, M1 and M2 macrophages profoundly differ functionally. Recent advances in this field have broadened this dichotomous view of macrophage polarization patterns. The 2-state view of macrophage function and behavior is thought to comprise a multitude or continuum of activation and functional states in vitro and in vivo [3, 6, 15]. Nonetheless, the M1 vs. M2 discovery provides both seminal and critical insights into the complexity of macrophage biology during vastly different pathologic conditions. Again, it is important to note that tissue-resident macrophages may adopt this M1/M2 dichotomy, although this area of macrophage biology has been understudied. The next sections will focus briefly on the ontogeny and functional roles of tissue-resident macrophages.

ONTOGENY OF TISSUE-RESIDENT MACROPHAGE POPULATIONS

When discussing tissue-resident macrophages it is important to review their ontogeny, as it is distinct from that of BMDMs. Although originally thought to be derived from monocytes that settle and differentiate into tissues [16], it is now established that tissue-resident macrophages are derived from the yolk sac [17] and the fetal liver based on elegant studies in mouse models [18] (Fig. 2). It remains unclear how these macrophages are maintained, but several reports have suggested that it is a combination of self-renewing proliferation within tissues [19] and perhaps potential repopulation from the bone marrow during times of increased stress or inflammation [20]. Common to both BMDMs and tissue-resident macrophage populations, however, is their reliance on signaling through the CSF1R, as ablation of this chemokine–chemokine receptor axis causes depletion of macrophages in various organs, including the brain, skin, and bone [21–23]. Exceptions have been noted wherein CSF1R-null mice still preserve tissue-resident macrophages, such as in the spleen [24]. Note that in humans, these tissue-resident macrophages demonstrate phenotypes consistent with phagocytic cells, such as expression of HLA-DR, CD11b, CD14, and CD86 molecules [25].

Figure 2. Ontogeny and function of tissue-resident macrophages.

Such macrophage populations, which arise from yolk sac erythroid–myeloid progenitors, are found in numerous tissues and organs of the body and exhibit a multitude of functions during health and disease. They maintain tissue homeostasis and integrity through the indicated immune system–dependent and –independent mechanisms. As with BMDMs, tissue-resident macrophages can be co-opted to exacerbate the pathogenesis of various disease states, including neoplastic progression. AD, Alzheimer’s disease; PD, Parkinson’s disease.

TISSUE-RESIDENT MACROPHAGES IN DISEASE

The role of tissue-resident macrophages in disease is largely understudied and has been overshadowed by studies focusing on BMDMs. Although the contribution of inflammatory monocytes in disease has proven to be highly important [26], many studies are now emerging to suggest that tissue-resident populations play essential roles in a variety of disorders as well, including autoimmune disease, cancer, and host–pathogen responses. As there are tissue-resident macrophage populations in nearly every organ/tissue which contributes to disease, we will focus on major sites of the lung (alveolar macrophages), liver (Kupffer cells), epidermis (Langerhans cells), and brain (microglia) because recent fate-mapping experiments have demonstrated that these cells have a common erythro-myeloid progenitor in the yolk sac [17, 27].

Alveolar macrophages

The tissue-resident macrophages of the lung, known as alveolar macrophages, are characterized in mice by F4/80 and CD11c expression with intermediate to low expression of CD11b. These cells play a large role in several lung-related diseases. Animals that have no alveolar macrophage populations succumb rapidly to influenza viral infection [28] because they lack CD8⁺ T cell responses and have a subsequent inability to clear viral load, suggesting that alveolar macrophages play an important role in mounting an innate immune response to infectious lung insults.

Indeed, several studies have also highlighted the role of alveolar macrophages in immune surveillance during various aspects of tumorigenesis [29], suggesting that they can have early antitumor roles. Moreover, alveolar macrophages have been shown to facilitate neoplastic progression, both in primary lung cancer and secondary lung metastasis. A recent study demonstrated that alveolar macrophage depletion significantly reduced tumor burden in a murine model of epidermal growth factor receptor–dependent lung adenocarcinoma [30]. This finding was attributed to the fact that these macrophages decreased Ag presentation via down-regulation of MHC class II molecules and promoted inflammation via the production of CXCL1, CXCL2, and IL-1RA. The study further revealed that an activated alveolar macrophage signature (as defined by 72 proinflammatory genes by cluster analysis) in patients correlated with a poorer prognosis. This highlights the important role of alveolar macrophages in primary lung cancer progression in both animal models and patients. In addition, the role of alveolar macrophages in metastasis to the lung was underscored in a study that demonstrated that depletion of alveolar macrophage populations in a murine mammary carcinoma model leads to a significant decrease in lung metastasis as compared to control mice [31]. The study further demonstrated that alveolar macrophages in tumor-bearing settings inhibited DC accumulation and T_h1 responses in the lungs, contributing to tumor progression. These recent studies emphasize the need for continued examination of alveolar macrophages, as they are a population that is often overlooked in both preclinical and clinical studies.

Kupffer cells

Kupffer cells, which express CD11b, F4/80, and LRP-1 in mice, function in the liver as sentinels for pathogens, tumor cells, and other danger-associated molecular patterns. However, this macrophage population has been strongly implicated in the pathogenesis of liver tumors, both primary and metastatic [32]. They have been known to contribute to tumor growth via modulation of the extracellular matrix (i.e., via MMP secretion) and to enhance tumor seeding and growth through angiogenesis [33]. Kupffer cells have also been shown to increase both hepatocellular carcinoma incidence and growth in patients via the secretion of IL-6 [34], which is thought to lead to a chronic inflammatory response supportive of tumor growth. TNF-α signaling in Kupffer cells has been shown to contribute to both alcoholic and nonalcoholic fatty liver disease, resulting in fat accumulation and fibrosis within the liver and ultimately leading to liver dysfunction [35, 36].

Langerhans cells

Although Langerhans cells, which express CD1a and CD207 in mice, have traditionally been considered to be DCs, recent profiling studies have shown that they are yolk sac–derived and rely on CSF-1 signaling for their differentiation and, therefore, may require reclassification as macrophages [37]. These cells line the skin and mucosal tract, and it has been demonstrated that Langerhans cells may be the first site of infection for viruses, such as HIV [38] and human papilloma virus (HPV) [39]. Langerhans cells may also play an active role in exacerbating inflammation in psoriatic lesions through increased Ag presentation to T cells [40]. Langerhans cell–associated histiocytosis is characterized by the clonal expansion of these cells which intensify the severity of disease in patients depending on where the growth is located [41]. The cause of this disease is an active area of study, although recent work supports that it may involve BRAF mutations [42].

Microglia

Microglia cells constitute the tissue-resident macrophages of the brain of mice, which express CD11b and, uniquely, have a low expression of CD45. These cells are essential to normal neuronal development and signaling [43], but can stimulate an inflammatory cascade in pathologic conditions, which can eventually lead to disease progression. Microglia express abundant TLRs and NODs to serve their primary function as sentinels for infections in the brain [44]. However, TLR and NOD signaling can be co-opted to cause chronic inflammation and contribute to disease progression. For example, while the role of microglia in the development of Alzheimer’s disease is unclear, their activation to proinflammatory states leads to increased pathologic changes in the brain such as the decrease in cognition that characterizes this type of disease [45]. Similarly, in Parkinson’s disease, microglia produce large amounts of TNF-α, IL-1β, and IFN-γ, chemokines and reactive oxygen species, which perpetuate symptoms and promote disease progression [46]. Indeed, decreasing the activation of microglia in rodents using CD163-targeted glucocorticoids decreased the death of dopaminergic cells and increased motor function [47]. Microglia also nurture the tumor cell microenvironment in both low-grade gliomas and in glioblastoma multiforme. Recent studies have shown that gliomas cause reduced caspase-3 activity in microglia via an iNOS-based mechanism, resulting in tumor growth. In addition, it is thought that microglia directly support cancer stem cell growth in glioblastoma multiforme, contributing to its aggressive progression and resistance to treatment [48].

TRANSCRIPTIONAL REGULATION OF MACROPHAGE ACTIVATION

The “plasticity” of macrophages in adopting different functional states or transitioning between different functional states is mediated by transcriptional events that activate specific gene programs (Fig. 3A, B). It is again important to note that these mechanisms are more often investigated in bone marrow–derived compared to tissue-resident populations; therefore, this section focuses on concepts mainly derived from studies on BMDMs. As previously stated, M1 macrophages can arise in response to different inflammatory stimuli that activate transcription factors that instruct specific functional behaviors. In response to IFN-γ, an IRF/STAT signaling pathway is activated [2, 3, 49]. Localization of JAK1 and -2 at the IFN-γ receptor activates STAT1 which binds to STAT-binding regions in IRF1 and -8 promoters [3]. IRFs have been shown to play diverse roles in myeloid phenotype and function, especially in macrophages [50, 51]. IRF1 and -8 have been shown to be master regulators of macrophage differentiation and function, particularly for M1 macrophages [50]. IRF1 and -8 activate transcription of iNOS, which is produced by M1 macrophages as part of their pathogen-killing arsenal [52, 53]. IL-12p40 is a direct transcriptional target of IRF8, and its expression by M1 macrophages is integral for the production of bioactive IL-12 which, in turn, is critical for the activation of the adaptive immune response [53–56].

Figure 3. Transcriptional regulation of macrophage biology.

The M1 or M2 macrophage functional state can be achieved through the indicated transcriptional pathways highlighted by engagement of the different signaling pathways in (A) and (B). Both M1 and M2 phenotypes are tightly regulated by STATs, for example, resulting in transcription of genes that instruct their unique functional profiles. (C) Monocytes recruited to the TME are polarized into protumor TAM phenotypes characterized by the expression of the indicated cytokines, enzymes, and other mediators in a complex cycle broadly illustrated in 4 steps. Left: tumor or stromal cells secrete various cytokines, which promote mobilization of bone marrow–derived circulating monocytes (step 1). Monocytes differentiate into macrophages once they are embedded in tumor tissue (step 2). Right: image of macrophage in the left panel is amplified for purposes of discussion. Exposure of macrophages to various tumor- or stromal-derived factors renders them protumorigenic (step 3). Such macrophages are known as TAMs and express CD206, CD163, and/or TIE2. Under hypoxic conditions, TAMs up-regulate expression of HIF-1α or -2α. The activation of such transcriptional networks instructs TAMs to produce mediators that favor angiogenesis, invasion, and immune suppression (step 4).

In response to TLR4 ligands such as LPS, the canonical pathway that is generally activated is NF-κB, mediated by MyD88 and other IRF family members [3, 49]. This process results in the production of potent proinflammatory cytokines, such as IFN-γ, IL-12, and TNF-α and the induction of chemokines, such as CCL2 and CXCL10, which attract more macrophages to the site of pathologic insult [3]. Upregulation of MHC molecules, especially MHC class II, and the costimulatory molecules CD80 and CD86 accompany this response, enabling the macrophages to efficiently present antigens to the adaptive immune system. It should be noted that these responses are not exclusive to macrophages only activated by TLR agonists, but also extends to the response seen in the presence of IFN-γ. GM-CSF activates a STAT5/IRF5 program, which passes through the NF-κB signaling pathway, leading to the enhancement of antigen presentation by M1 macrophages. In addition, GM-CSF-activated macrophages produce additional proinflammatory cytokines, including IL-6 and TNF-α, and also adopt antimicrobial abilities.

Transcription factors involved in M2 polarization and their activation include STAT6, MYC, and IRF4 [49]. In response to IL-4 binding to its cognate receptor, IL-4Rα1, JAK1, and JAK3 are recruited, creating a docking site for phosphorylation and nuclear translocation of STAT6 [2, 3]. Peroxisome proliferator-activated receptor-γ, in coordination with STAT6 and Kruppel-like factor 4, induces M2-associated genes, such as Arg1, Mrc1, and Fizz1, while inhibiting M1 gene products, such as iNOS and TNF-α [2]. M2b macrophages, a second polarized M2 subset, are activated by immune complexes and TLR ligands that use their Fcγ receptors to recognize IgG [3]. This signaling involves spleen tyrosine kinase and PI3K, which results in the production of cytokines, such as IL-10 and TNF-α, subsequently essential for T_h2 activation and function [3]. M2c macrophages, a third polarized M2 subset, are activated through an IL-10/STAT3 axis, which involves STAT3-mediated induction of genes such as Tgfb1 and Mrc1 [2, 3, 49].

STAT-mediated activation and polarization of macrophages also involves feedback regulation by members of the SOCS family [2]. For example, IFN-γ activates SOCS1, which feedback inhibits STAT1 activation, whereas IL-4 activates the SOCS3 program, which feedback inhibits STAT6 activation [2]. Several studies have investigated the nature and basis of these regulatory mechanisms in cytokine biology, and can be reviewed in detail elsewhere [57].

Although tissue-resident macrophages may adopt these same transcriptional programs [58], they may also exhibit separate transcriptional programs specific to the tissues that they inhabit. Although tissue-resident macrophages up-regulate CSF-1R and F4/80 in a manner similar to their bone-marrow-derived counterparts, they can be distinguished by gene expression that is thought to aid them in their tissue homeostatic functions. In brief, it is now known that specific gene expression can distinguish these cell types, such as high expression of Sall1 by microglia, Celc4f in Kupffer cells and Car4 in lung macrophages [59]. The role of these genes requires further study, but it is clear that these cell types may have unique transcriptional profiles that aid them in their tissue-specific abilities.

TUMOR-ASSOCIATED MACROPHAGES

The role of the TME in promoting tumor progression to metastasis has revealed pivotal roles for stromal cells and, most important, of immune cells, including macrophages. The terminology of TAM often refers to bone marrow-derived populations as they are recruited to the TME. Therefore, this section will focus on the TAM concept based on studies of BMDMs (Fig. 3C). It is reasonable to suggest that tissue-resident macrophages may also be called TAMs if they provide protumor activities within the particular TME, as highlighted in earlier sections.

In contrast to the origin of tissue-resident macrophages, BMDMs arise from circulating Ly6C⁺ monocytes [60, 61] and are recruited to the TME generally by circulating levels of various chemokines, such as CSF-1, secreted by the growing malignancy or other stromal cell types. Upon mobilization to the TME, these monocytes undergo differentiation into macrophages. In the TME, macrophages are the most abundant cells of the CD45⁺ leukocyte population. This CSF-1 gradient is very important, as in many different tumor models, a high concentration of CSF-1 in tumors correlates with a poor prognosis. As such, CSF-1 being a key component of macrophage biology makes it an important target for immunotherapeutic intervention in neoplastic disease. Pollard and his group [62] showed that overexpressing CSF-1 in the mammary epithelium of the MMTV-PyMT mouse model of spontaneous mammary carcinoma increased macrophage recruitment and tumor progression to metastasis. Accordingly, deletion of CSF-1 using neutralizing antibodies, RNA or genetic strategies impaired recruitment of TAMs to the primary site, which translated to diminished tumor progression to metastasis [61]. It is important to note that in certain tumor-bearing contexts, TAMs can also be derived from myeloid-derived suppressor cells [63]. In these settings, hypoxic conditions within the TME induce the expression of HIF-1α in monocytic myeloid-derived suppressor cells, resulting in a rapid differentiation into macrophages. Furthermore, these immature myeloid cells can also differentiate into DCs in the presence of all-trans retinoic acid [64].

Other factors secreted by the tumor, also recruit monocytes to the TME (Fig. 3C). CCR2 expressed on the surface of circulating monocytes responds to its cognate ligand CCL2 produced by the growing tumor to draw monocytes into the TME [65]. This recruitment has been shown to inundate both the primary tumor and metastatic sites with monocytes. VEGF-A also plays a major role in the recruitment of monocytes into the TME [66]. This is facilitated through the IL-4 axis and loss of VEGF-A results in decreased tumor angiogenesis and invasion [66].

As discussed earlier, circulating Ly6C⁺ monocytes arise from the bone marrow that home to tissues in response to chemokine signaling, typically through CSF-1R and CCR2 engagement [67]. Within tissues, these Ly6C⁺ monocytes may undergo differentiation into macrophages under the direction of the transcription factor Kruppel-like factor 4 [68]. However, further phenotypic clarification is required as Ly6C⁺ monocytes may also differentiate in tissues into either monocyte-derived DCs or conventional DCs [69]. Whereas macrophages are PU.1^lo Zbtb46⁻, monocyte-derived DCs are PU.1^hiZbtb46⁻ and conventional DCs are PU.1^hiZbtb46⁺. It is important to note that whereas Zbtb46 can be used to distinguish conventional DCs from macrophages in mice, recent studies in humans suggest that DCs can be distinguished by high expression of Dngr-1 [70]. This complexity of phenotypic overlap lends itself to careful classification of macrophages within tissues, both in healthy and disease states.

Altogether, it is now becoming clear that a dichotomous view of two macrophage functional states in the TME is perhaps oversimplified. Macrophages are highly plastic cells that can arise from multiple precursors and respond to environmental or inflammatory cues to adopt diverse phenotypes. The TME is a complex landscape adapted to entice the mobilization of host cells, especially monocytes/macrophages to nurture tumor growth and subsequent metastasis. The resultant TAM phenotype thus represents a continuum of subpopulations exhibiting various functional states, which are still being dissected.

Also, as previously stated, macrophages can be broadly grouped into M1 or M2 subtypes depending on whether they arise from LPS/IFN-γ or IL-4/IL-10/IL-13 exposure, respectively. M1 macrophages arising from exposure to LPS/IFN-γ exhibit tumoricidal functions [55]. This classification is accomplished by the generation of reactive oxygen species and RNS (via expression of iNOS), the production of proinflammatory cytokines such as IL-1 and -12, the latter of which is critically important for T cell activation. In contrast, M2 macrophages have the traditional role of being involved in tissue repair and tissue remodeling through the production of MMPs [71]. M2 macrophages play pivotal roles in angiogenesis and vascularization through the production of VEGF family members, which, as previously stated, is a chemoattractant for TAMs. A major part of the angiogenic switch that allows tumor cells to undergo neovascularization, involves TIE2⁺ macrophages, which express ANG2 (the TIE2 ligand). Genetic studies that target TIE2 or its ligand result in a loss of angiogenesis, as illustrated in various mouse tumor models including the MMTV-PyMT model [72]. VEGF production is also increased by the up-regulation of HIF-1α or -2α, inducible transcription factors activated by the hypoxic TME [55, 71]. These transcription factors bestow metabolic adaptation to the macrophage in the TME which is poor in oxygen, thus creating a loop whereby more macrophages are attracted to the TME [55, 71]. TDFs or those derived from stroma, such as IL-4, IL-10, or glucocorticoids, activate macrophages to adopt a protumor M2 phenotype [55], characterized by an increase in Arg-1 and TGF-β expression [55, 61, 71]. TAMs in many tumors have been found to up-regulate mannose and scavenger receptors, as well as increasing their production of the prototypic anti-inflammatory cytokine, IL-10 [55, 61, 71].

Undoubtedly, it is not surprising that TAMs are termed M2-like and are synonymous with protumorigenic behavior, based on the fact that they can be polarized by cytokines and factors such as IL-4, IL-10, IL-13, and glucocorticoids and are involved in wound healing, up-regulate scavenger receptors and in turn produce anti-inflammatory cytokines. However, Franklin and colleagues [73] reported on a paradigm-shifting study that TAMs are distinct from M2 macrophages. Using the MMTV-PyMT mouse mammary cancer model, they observed that TAMs isolated from tumors of these mice lacked expression of Ym1 (chitinase3-like 3), Fizz1 (resistin-like α) and Mrc1 (CD206), genes and scavenger receptors considered to be hallmark features of M2 macrophages [55, 73]. This study further adds to the complexity of TAM biology and the challenges the field faces when it comes to their classification and importance in neoplastic and other inflammatory diseases and disorders.

THERAPEUTIC INTERVENTIONS

On the forefront of therapy in virtually every disease, from infection to autoimmunity to cancer, are agents that target the immune system. Agents, such as TNF-α antagonists have had overwhelming success in rheumatoid arthritis and ulcerative colitis [74, 75]. Antibodies that block engagement of the inhibitory receptor programmed death-1 (PD-1) on T cells have also rapidly increased in use and popularity following success in several late stage cancer types [75]. However, agents that directly target macrophage function are not yet widely used despite strong evidence that macrophages may represent a significant hurdle to immunotherapeutic or even chemotherapeutic efficacy [76]. Gene therapy using macrophages as “cellular vehicles” for gene delivery has been explored in atherosclerosis and arthritis [77]. However, strategies that target macrophages in disease have been most sought after in cancer therapy. For example, Klug et al. [78] demonstrated that radiation may polarize macrophages toward a more antitumor phenotype, contributing to tumor regression. Therapies using trabectedin, a liposome agent used to induce apoptosis in macrophages, have resulted in decreased tumor burden in murine models of ovarian cancer and sarcoma [79]. Recent work has shown promising efficacy in cancer patients with diffuse-type giant-cell tumors using a neutralizing antibody to CSF-1R [80]. However, these studies are limited, and several have failed in clinical trials or resulted in significant toxicities [76]. As a result, macrophages (especially tissue-resident macrophages) remain an elusive, yet highly desirable target in cancer therapy, as well as in other diseases in which macrophages play pathologic roles.

CONCLUSIONS

We have provided a broad perspective of the contributions of macrophage populations, both bone marrow-derived and tissue-resident, and their responses during disease. Their diversity is highlighted in their ability to uniquely assume various functional states in response to a world of inflammatory cues, but whether this represents the same cell truly undergoing functional transition or distinct cells that emerge in reaction to distinct stimuli remains an important area of investigation. Nonetheless, this is likely dictated at a transcriptional level, which allows for a level of plasticity necessary to meet the demands of the pathologic challenge. Therefore, this critical population of cells is uniquely poised to play key roles both in disease prevention through immune system–dependent and –independent mechanisms. On the other hand, under conditions in which macrophages fuel pathologic processes, such as chronic diseases characterized by inflammation that fails to resolve [81], the macrophages themselves, the drivers of the undesired macrophage response, or their effector mechanisms may represent additional or alternative targets for therapeutic intervention.

AUTHORSHIP

D.Y.F.T. and L.B.-M. conceived of and wrote the article. S.I.A. conceived of and wrote the manuscripts. All authors reviewed and revised the manuscripts.

Glossary

BMDM

bone marrow-derived macrophage

CSF

colony-stimulating factor

CSFR

colony stimulating factor receptor

DC

dendritic cell

GBM

glioblastoma multiforme

HIF

hypoxia-inducible factor

HPV

human papilloma virus

IRF

interferon regulatory factor

MMP

matrix metalloproteinases

MMTV-PyMT

mouse mammary tumor virus polyoma middle T-antigen

SOCS

suppressor of cytokine signaling

TAM

tumor-associated macrophage

TIE2

aniopoietin-2 receptor

TME

tumor microenvironment

DISCLOSURE

The authors declare no conflicts of interest.