Macrophages are considered a critical component of innate immunity against intracellular pathogens. Although macrophages have historically been viewed as monocyte-derived and terminally differentiated cells, recent progress has revealed that many tissue-resident macrophages are embryonically seeded, self-renewed, and perform homeostatic functions associated with M2-like activation programs. There is evidence that tissue-resident macrophages (TRMs) maintain their M2-like phenotype even in an infection-driven pro-inflammatory environment. In this regard, several intracellular pathogens are shown to exploit M2-like TRMs as replicative niches to evade pathogen-specific immunity. This knowledge provides a new perspective to understand the chronicity of infections and develop therapeutic strategies which can selectively target TRMs.
In 1893, Metchnikoff  first described macrophages and their phagocytic function during tissue inflammation. Since then, macrophages have been considered as central innate immune components to eliminate pathogens. All macrophages were long considered to be terminally differentiated cells that originate from blood monocytes, and together, these cells constitute the mononuclear phagocyte system . Several early reports, however, indicated that tissue macrophages were not terminally differentiated [3–6], and it is now clear that some macrophages can proliferate enabling their long-term self-renewal in situ without input from monocytes. The first conceptual advance pertained not to macrophages but to human Langerhans cells, which were found to be of donor origin several years after a hand transplant . This self-renewing capacity was confirmed in murine Langerhans cells [8,9], followed by similar observations of in vivo proliferation of various tissue-resident macrophages (TRMs), including those found in the brain [10,11], peritoneum [12,13], alveoli , red pulp , adipose tissue , and heart .
Fate-mapping studies have also revealed the origin of many TRMs to be the yolk sac or fetal liver [17,18]. These TRMs are thus prenatally seeded into the peripheral tissues and locally maintained. A notable exception is the gastrointestinal tract, for which the major TRMs appear to be blood monocytes — derived in the steady state . Embryonic-derived TRMs can also be replaced by monocyte-derived cells after severe inflammation, which can assume markers and transcriptional profiles similar to TRMs [20,21].
With regard to dermal TRMs, the first paper to properly address their ontogeny reported that a substantial fraction of these cells originated from the parabiotic partner [22,23], implicating an important contribution from blood monocytes. Our own data employing multiple approaches, including parabiosis, BM chimeras, and adoptive transfer of monocytes, indicated that the TRMs in the mouse ear dermis are largely a radio-resistant, self-renewing population that is minimally replaced by blood or adult bone marrow precursors, even in inflammatory conditions . Our studies also highlighted IL-4-dependent turnover of dermal TRMs during Leishmanial major infection. IL-4-induced local proliferation of TRMs was originally reported in the pleural cavity of helminth-infected mice, again with minimal recruitment of adult bone marrow-derived cells . A key distinction of our studies, however, is that the self-renewal of dermal TRMs occurred not within the context of a helminth-driven, polarized type 2 response, but within the strong pro-inflammatory environment of the L. major-loaded dermis.
Macrophages are now classified into two broadly defined phenotypic populations: (i) TRMs that perform homeostatic functions related to the resolution of inflammation and to tissue repair and (ii) activated monocyte-derived macrophages (MDMs) that are recruited into tissues in a CCR2-dependent manner in response to homeostatic disturbances, such as infectious agents . MDMs perform the classical roles of macrophages in microbial capture and killing, and there is a huge literature devoted to understanding the nature of these killing mechanisms and the strategies evolved by intracellular pathogens to remodel or escape from the phagolysosomal compartments of these cells . In contrast, there is little information regarding the antimicrobial functions of TRMs. Here, we discuss several recent findings about the role of TRMs in providing important replicative niches for intracellular pathogens and in restraining infection-driven inflammatory responses in various tissues.
The well-established concept regarding classical (‘M1’) or alternative (‘M2’) macrophage activation states is based largely on studies employing M-CSF-grown murine bone marrow-derived or human CD14+ MDMs exposed to defined exogenous factors in vitro. The plasticity of macrophages and their ability to integrate multiple and diverse signals from different tissue environments have demanded refinement of the simple M1 vs. M2 dichotomy [27,28]. The heterogeneity of ‘M2’ populations, in particular, can be substantial. Thus, while the original M2 phenotype was defined by macrophages acted on by IL-4 and IL-13, macrophages responding to other exogenous factors, such as IL-10, immune complexes, TGF-β, or glucocorticoids, can show M2 overlapping but still quite distinctive activation programs. For the purposes of this discussion, and as recommended in a recent perspective article , we will refer to macrophages as ‘M2-like’ when they share a consensus collection of markers consistent with IL-4/IL-13 macrophages. Adhering to this model, TRMs are often classified as M2-like cells with anti-inflammatory and reparatory roles for maintaining tissue homeostasis . Since, as discussed above, most TRMs are embryonically seeded in peripheral tissues, they will encounter a changing inflammatory microenvironment from the start to the resolution of infection. A key question, therefore, has been whether or not they maintain their M2-like activation program in various infection-driven inflammatory settings. Rückerl et al.  recently studied the functional reprogramming of TRMs in the immune response against a sequential infection model involving TH2-inducing nematodes and TH1-inducing bacteria. They reported that the functional adaptation of TRMs appeared limited compared with newly recruited MDMs, indicating that the embryonic origin of TRMs probably imposes a restriction on their plasticity and reprogramming. In this regard, we also demonstrated that embryonic-derived, dermal TRMs showed M2-like characteristics in the steady state, and so long as they remained locally conditioned by IL-4 and IL-10 during Leishmania infection, their number and M2-like functionality were maintained despite a strong pro-inflammatory response that activated the MDMs that were recruited to the site . The up-regulated expression of TGFR2, IL-4Rα, and IL-10Rα on TRMs in the steady-state dermis may explain how they remain highly sensitive to the anti-inflammatory cytokines produced in the tissue microenvironment . This result is in agreement with a recent report by Barreiro et al.  showing that a unique subset of dermal TRMs, termed skin trans-endothelial radio-resistant anti-inflammatory macrophages (STREAM), are of embryonic-origin and committed to perform anti-inflammatory functions, even in the presence of a potent pro-inflammatory stimulus such as LPS.
We identified dermal TRMs in the steady-state skin by combining mannose receptor (MR) and Ly6C staining on CD11b+Ly6G−SiglecF− cells and comparing them with other markers that have been used to define dermal macrophages (MRhiCD64hiLy6CintCCR2loMHCII+/−) [22,24]. MR up-regulation on macrophages in response to type 2 cytokines provided one of the first markers for their alternative activation state [33,34]. MR functions as a scavenger receptor, including the uptake of blood-circulatory glycoproteins that are elevated in inflammation and wound healing . We showed that MRhi dermal macrophages rapidly scavenged high molecular mass dextran from blood lumen, which was reported to be MR-dependent [24,36]. Importantly, MR on dermal TRMs mediated more efficient uptake of a strain of L. major (LmSd) that produces nonhealing cutaneous lesions in conventionally resistant C57BL/6 mice compared with a healing strain (LmFn), and genetic ablation of MR produced a healing phenotype in LmSd-infected mice . We conclude that MRhi dermal TRMs actively maintain M2-characteristics and remain permissive for parasite growth even in a strong TH1 immune environment, and the preferential infection of these cells through MR plays a crucial role in the severity of cutaneous disease. MR-mediated infection of TRMs is a possible immune evasive strategy used by other intracellular pathogens. For example, the MR is engaged in the phagocytosis of a virulent but not an attenuated strain of Mycobacterium tuberculosis .
This concept of TRMs as a permissive niche for infection is supported by another report that identified lung alveolar TRMs as being most susceptible to Coxiella burnetii infection [38,39]. This population was responsible for the majority of early cellular uptake of C. burnetii, and while infiltrating MDMs were capable of killing phagocytosed bacteria, this was not the case for alveolar TRMs. IL-4 was proposed as essential to maintain the M2-like activation program of the alveolar TRMs because in its absence, alveolar TRMs became significantly more resistant to infection in vitro.
Together with IL-4, phagocytosis itself may help to maintain the M2-like activation program of TRMs. Using a parabiosis-based approach, TRMs from multiple tissues were shown to phagocytose cellular material, a process that imprinted a distinct activation program on the TRMs . The efficient and nonimmunogenic clearance of apoptotic cells (ACs) and apoptotic bodies is crucial to dispose of self-antigens and to maintain self-tolerance . This homeostatic process can help to establish and maintain the anti-inflammatory program of TRMs by engaging deactivating receptors and by inducing the production of TGF-β/IL-10 [42,43]. We showed that dermal TRMs strongly express phagocytic receptors, such as Mer and Tim-4 involved in AC clearance, and they rapidly take up ACs from the skin . It was recently reported that recognition of ACs along with signals from type 2 cytokines maintains the reparative function of TRMs during inflammation in the lung and the gut . Genetic ablation of receptor tyrosine kinases (Mer and Axl) involved in the clearance of ACs impaired the proliferation of TRMs and the induction of reparatory genes in the lungs after helminth infection or in the gut after the induction of colitis.
Sustaining the anti-inflammatory properties of TRMs during infection, and especially in the context of infection-driven TH1 responses, may also be conditioned by multiple tissue-specific microenvironmental factors. Alveolar TRMs express high amounts of the negative regulator CD200 receptor (CD200R), which is up-regulated by TGF-β and IL-10 produced by the lung epithelium . During influenza infection, the absence of CD200 ligand altered the phenotype of alveolar TRMs and made them more responsive to activating stimuli, which led to delayed resolution of inflammation and ultimately death. In this case, it was not the ability of the TRMs to serve as a replicative niche that influenced the infection outcome, but their critical role in the resolution of inflammation and tissue repair. Similarly, in a murine malaria infection model using Plasmodium berghei ANKA (PbA) , it was demonstrated that TRMs that are strategically positioned at the tissue entry sites of infected red blood cells (iRBCs), such as the splenic marginal zone or liver sinusoids, phagocytosed the iRBCs and controlled their propagation into different organs. CD169-DTR mice lacking TRMs developed an earlier parasitemia, which was followed by the premature death of the infected mice due to the increased infiltration of inflammatory cells, vascular leakage, and deposition of hemozoin in multiple organs, all associated with a low IL-10/TNF ratio. Thus, the highly phagocytic and anti-inflammatory properties of CD169+ TRMs were essential to restrain the systemic spreading of the parasite and excessive immunopathology in blood stage malaria.
In conclusion, M2-like TRMs can be preferential host cells for intracellular pathogens to evade pathogen-specific immunity. Maintaining an M2-like phenotype may represent an evolutionary strategy for TRMs to limit inflammation, particularly in the lung and skin exposed to an abundance of innocuous antigens . We have emphasized the cell-intrinsic and tissue-specific factors that help TRMs maintain their reparative, anti-inflammatory programs even in a strong Th1 environment (summarized schematically in Figure 1). Critically, the evidence that TRMs serve as replicative niches for intracellular pathogens needs to be extended to human infections. Substantial clinical evidence already exists to indicate that tumor-associated macrophages that are implicated in tumor progression maintain an M2-like phenotype , and possess many of the properties that have been ascribed to TRMs. Therefore, the tools that have been developed to selectively target tumor-associated macrophages, such as anti-CSF1R antibody and derivatives of FDA-approved Tilmanocept [49,50], might be used as an alternative or adjunct therapeutic strategy to lessen the replicative niches of intracellular pathogens. Any antimicrobial benefits would, of course, need to be carefully balanced with the possible consequences of removing TRMs as a component of the homeostatic response.
Dermis-resident macrophages maintain an M2-like phenotype and serve as replicative niches for intracellular pathogens.
Most tissue-resident macrophages (TRMs) are embryonically seeded and self-maintained.
TRMs maintain M2-like activation programs even during infection.
Intracellular pathogens exploit TRM as replicative niches.
This work was supported, in part, by the Intramural Research Program of the National Institute of Allergy and Infectious Diseases, National Institutes of Health.
The Authors declare that there are no competing interests associated with the manuscript.