The MacBlue binary transgene (csf1r-gal4VP16/UAS-ECFP) provides a novel marker for visualisation of subsets of monocytes, macrophages and dendritic cells and responsiveness to CSF1 administration

Macrophage deficiency in CSF1R-knockout rat embryos does not compromise placental or embryo development

Macrophages are an abundant cell population in the placenta and developing embryo and appear to be involved in processes of vascularization, morphogenesis, organogenesis, and hematopoiesis. The proliferation, differentiation, and survival are dependent on signals from the macrophage colony-stimulating factor receptor, CSF1R. Aside from the role in macrophages, Csf1r mRNA is highly expressed in placental trophoblasts. To explore the function of macrophages and Csf1r in placental and embryonic development, we analyzed the impact of homozygous Csf1r null mutation (Csf1rko) in the rat. In late gestation, IBA1+ macrophages were abundant in control embryos in all tissues, including the placenta, and greatly reduced in the Csf1rko. CSF1R was also detected in stellate macrophage-like cells and in neurons using anti-CSF1R antibody but was undetectable in trophoblasts. However, the neuronal signal was not abolished in the Csf1rko. CD163 was most abundant in cells forming the center of erythroblastic islands in the liver and was also CSF1R dependent. Despite the substantial reduction in macrophage numbers, we detected no effect of the Csf1rko on development of the placenta or any organs, the relative abundance of vascular elements (CD31 staining), or cell proliferation (Ki67 staining). The loss of CD163+ erythroblastic island macrophages in the liver was not associated with anemia or any reduction in the proliferative activity in the liver, but there was a premature expansion of CD206+ cells, presumptive precursors of liver sinusoidal endothelial cells. We suggest that many functions of macrophages in development of the placenta and embryo can be provided by other cell types in their absence.

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

Monitoring Immune Cell Function Through Optical Imaging: a Review Highlighting Transgenic Mouse Models

Transgenic mouse models have facilitated research of human diseases and validation of therapeutic approaches. Inclusion of optical reporter genes (fluorescent or bioluminescent genes) in the targeting vectors used to develop such models makes in vivo imaging of cellular and molecular events possible, from the microscale to the macroscale. In particular, transgenic mouse models expressing optical reporter genes allowed accurately distinguishing immune cell types from trafficking in vivo using intravital microscopy or whole-body optical imaging. Besides lineage tracing and trafficking of different subsets of immune cells, the ability to monitor the function of immune cells is of pivotal importance for investigating the effects of immunotherapies against cancer. Here, we introduce the reader to state-of-the-art approaches to develop transgenics, optical imaging techniques, and several notable examples of transgenic mouse models developed for immunology research by critically highlighting the models that allow the following of immune cell function.

The influence of M-CSF on fracture healing in a mouse model

Macrophage colony-stimulating factor 1 (M-CSF) is known to play a critical role during fracture repair e.g. by recruiting stem cells to the fracture site and impacting hard callus formation by stimulating osteoclastogenesis. The aim of this experiment was to study the impact of systemic M-CSF application and its effect on bony healing in a mouse model of femoral osteotomy. Doing so, we studied 61 wild type (wt) mice (18-week-old female C57BL/6) which were divided into three groups: (1) femoral osteotomy, (2) femoral osteotomy + stabilization with external fixator and (3) femoral osteotomy + stabilization with external fixator + systemic M-CSF application. Further, 12 op/op mice underwent femoral osteotomy and served as proof of concept. After being sacrificed at 28 days bony bridging was evaluated ex vivo with µCT, histological and biomechanical testing. Systemic M-CSF application impacted osteoclasts numbers, which were almost as low as found in op/op mice. Regarding callus size, the application of M-CSF in wt mice resulted in significantly larger calluses compared to wt mice without systemic M-CSF treatment. We further observed an anabolic effect of M-CSF application resulting in increased trabecular thickness compared to wt animals without additional M-CSF application. Systemic M-CSF application did not alter biomechanical properties in WT mice. The impact of M-CSF application in a mouse model of femoral osteotomy was oppositional to what we were expecting. While M-CSF application had a distinct anabolic effect on callus size as well as trabecular thickness, this on bottom line did not improve biomechanical properties. We hypothesize that in addition to the well-recognized negative effects of M-CSF on osteoclast numbers this seems to further downstream cause a lack of feedback on osteoblasts. Ultimately, continuous M-CSF application in the absence of co-stimulatory signals (e.g. RANKL) might overstimulate the hematopoietic linage in favor of tissue macrophages instead of osteoclasts.

Visualizing murine breast and melanoma tumor microenvironment using intravital multiphoton microscopy

Intravital multiphoton imaging of the tumor milieu allows for the dissection of intricate and dynamic biological processes in situ. Herein, we present a step-by-step protocol for setting up an experimental cancer imaging model that has been optimized for solid tumors such as breast cancer and melanoma implanted in the flanks of mice. This protocol can be utilized for dissecting tumor-immune cell dynamics in vivo or other tumor-specific biological questions.

For complete details on the use of this protocol for intravital imaging of breast cancer, please refer to Tikoo et al. (2021a), and for intravital imaging of melanoma, please refer to Tikoo et al. (2021b).

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Detailed protocol for setting up high-resolution intravital imaging of murine tumors

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3D printing of custom stage inserts for tumor stabilization

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Procedures for cannulation of blood vessels

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Surgical preparation and tissue stabilization for imaging tumor milieu in vivo

Dermal bacterial LPS-stimulation reduces susceptibility to intradermal Trypanosoma brucei infection

Infections with Trypanosoma brucei sp. are established after the injection of metacyclic trypomastigotes into the skin dermis by the tsetse fly vector. The parasites then gain access to the local lymphatic vessels to infect the local draining lymph nodes and disseminate systemically via the bloodstream. Macrophages are considered to play an important role in host protection during the early stage of systemic trypanosome infections. Macrophages are abundant in the skin dermis, but relatively little is known of their impact on susceptibility to intradermal (ID) trypanosome infections. We show that although dermal injection of colony stimulating factor 1 (CSF1) increased the local abundance of macrophages in the skin, this did not affect susceptibility to ID T. brucei infection. However, bacterial LPS-stimulation in the dermis prior to ID trypanosome infection significantly reduced disease susceptibility. In vitro assays showed that LPS-stimulated macrophage-like RAW264.7 cells had enhanced cytotoxicity towards T. brucei, implying that dermal LPS-treatment may similarly enhance the ability of dermal macrophages to eliminate ID injected T. brucei parasites in the skin. A thorough understanding of the factors that reduce susceptibility to ID injected T. brucei infections may lead to the development of novel strategies to help reduce the transmission of African trypanosomes.

Class IIa Histone Deacetylases Drive Toll-like Receptor-Inducible Glycolysis and Macrophage Inflammatory Responses via Pyruvate Kinase M2

Histone deacetylases (HDACs) drive innate immune cell-mediated inflammation. Here we identify class IIa HDACs as key molecular links between Toll-like receptor (TLR)-inducible aerobic glycolysis and macrophage inflammatory responses. A proteomic screen identified the glycolytic enzyme pyruvate kinase M isoform 2 (Pkm2) as a partner of proinflammatory Hdac7 in murine macrophages. Myeloid-specific Hdac7 overexpression in transgenic mice amplifies lipopolysaccharide (LPS)-inducible lactate and promotes a glycolysis-associated inflammatory signature. Conversely, pharmacological or genetic targeting of Hdac7 and other class IIa HDACs attenuates LPS-inducible glycolysis and accompanying inflammatory responses in macrophages. We show that an Hdac7-Pkm2 complex acts as an immunometabolism signaling hub, whereby Pkm2 deacetylation at lysine 433 licenses its proinflammatory functions. Disrupting this complex suppresses inflammatory responses in vitro and in vivo. Class IIa HDACs are thus pivotal intermediates connecting TLR-inducible glycolysis to inflammation via Pkm2.

Imaging resident and recruited macrophage contribution to Wallerian degeneration

Wallerian degeneration (WD) is a process of autonomous distal degeneration of axons upon injury. Macrophages (MPs) of the peripheral nervous system (PNS) are the main cellular agent controlling this process. Some evidence suggests that resident PNS-MPs along with MPs of hematogenous origin may be involved, but whether these two subsets exert distinct functions is unknown. Combining MP-designed fluorescent reporter mice and coherent anti–Stokes Raman scattering (CARS) imaging of the sciatic nerve, we deciphered the spatiotemporal choreography of resident and recently recruited MPs after injury and unveiled distinct functions of these subsets, with recruited MPs being responsible for efficient myelin stripping and clearance and resident MPs being involved in axonal regrowth. This work provides clues to tackle selectively cellular processes involved in neurodegenerative diseases.

Characterization of Conventional Dendritic Cells and Macrophages in the Spleen Using the CSF1R-Reporter Transgenic Chickens

The spleen is a major site for the immunological responses to blood-borne antigens that is coordinated by cells of the mononuclear phagocyte system (MPS). The chicken spleen is populated with a number of different macrophages while the presence of conventional dendritic cells (cDC) has been described. However, a detailed characterization of the phenotype and function of different macrophage subsets and cDC in the chicken spleen is limited. Using the CSF1R-reporter transgenic chickens (CSF1R-tg), in which cells of the MPS express a transgene under the control elements of the chicken CSF1R, we carried out an in-depth characterization of these cells in the spleen. Immunohistological analysis demonstrated differential expression of MRC1L-B by periarteriolar lymphoid sheaths (PALS)-associated CSF1R-tg+ cells. In the chicken's equivalent of the mammalian marginal zone, the peri-ellipsoid white-pulp (PWP), we identified high expression of putative CD11c by ellipsoid-associated cells compared to ellipsoid-associated macrophages. In addition, we identified a novel ellipsoid macrophage subset that expressed MHCII, CD11c, MRC1L-B, and CSF1R but not the CSF1R-tg. In flow cytometric analysis, diverse expression of the CSF1R-tg and MHCII was observed leading to the categorization of CSF1R-tg cells into CSF1R-tgdim MHCIIinter−hi, CSF1R-tghi MHCIIhi, and CSF1R-tghi MHCIIinter subpopulations. Low levels of CD80, CD40, MHCI, CD44, and Ch74.2 were expressed by the CSF1R-tghi MHCIIinter cells. Functionally, in vivo fluorescent bead uptake was significantly higher in the CSF1R-tghi MHCIIhi MRC1L-B+ cells compared to the CSF1R-tgdim and CSF1R-tghi MHCIIinter MRC1L-B+ subpopulations while LPS enhanced phagocytosis by the CSF1R-tghi MHCIIinter subpopulation. The analysis of bead localization in the spleen suggests the presence of ellipsoid-associated macrophage subsets. In addition, we demonstrated the functionality of ex vivo derived CSF1R-tg+ MRC1L-Bneg cDC. Finally, RNA-seq analysis of the CSF1R-tg subpopulations demonstrated that separating the CSF1R-tghi subpopulation into CD11chi and CD11cdim cells enriched for cDC and macrophage lineages, respectively, while the CSF1R-tghi MHCIIinter subpopulation was enriched for red pulp macrophages. However, our analysis could not define the cell lineage of the heterogeneous CSF1R-tgdim subpopulation. This detailed overview of the MPS in the chicken spleen will contribute to future research on their role in antigen uptake and presentation.

The equine mononuclear phagocyte system: The relevance of the horse as a model for understanding human innate immunity

The mononuclear phagocyte system (MPS) is a family of cells of related function that includes bone marrow progenitors, blood monocytes and resident tissue macrophages. Macrophages are effector cells in both innate and acquired immunity. They are a major resident cell population in every organ and their numbers increase in response to proinflammatory stimuli. Their function is highly regulated by a wide range of agonists, including lymphokines, cytokines and products of microorganisms. Macrophage biology has been studied most extensively in mice, yet direct comparisons of rodent and human macrophages have revealed many functional differences. In this review, we provide an overview of the equine MPS, describing the variation in the function and phenotype of macrophages depending on their location and the similarities and differences between the rodent, human and equine immune response. We discuss the use of the horse as a large animal model in which to study macrophage biology and pathological processes shared with humans. Finally, following the recent update to the horse genome, facilitating further comparative analysis of regulated gene expression between the species, we highlight the importance of future transcriptomic macrophage studies in the horse, the findings of which may also be applicable to human as well as veterinary research.

Immunohistochemical study of morphology and distribution of CD163+ve macrophages in the normal adult equine gastrointestinal tract

Intestinal macrophages are the largest group of mononuclear phagocytes in the body and play a role in intestinal innate immunity, neuroimmune interactions and maintaining intestinal homeostasis. Conversely, they also are implicated in numerous pathologies of the gastrointestinal tract, such as postoperative ileus and inflammatory bowel disease. As a result, macrophages could be potential therapeutic targets. To date, there are limited studies on the morphology and distribution of macrophages in the equine gastrointestinal tract (GIT). The aim of this study was to identify the location and abundance of resident macrophages in the equine GIT using CD163 as an immunohistochemical marker. Tissue samples were obtained post-mortem from 14 sites along the gastrointestinal tracts of 10 horses free from gastrointestinal disease; sample sites extended from the stomach to the small colon. CD163^+ve cells were present in all regions of the equine GIT from stomach to small colon. CD163^+ve cells were also identified in all tissue layers of the intestinal wall, namely, mucosa, submucosa, muscularis externa (ME), myenteric plexus and serosa. Consistent with a proposed function in regulation of intestinal motility, CD163^+ve cells were regularly distributed within the ME, with accumulations closely associated with the myenteric plexus and effector cells such as neurons and the interstitial cells of Cajal (ICC).

Screening for new macrophage therapeutics

Myeloid derived macrophages play a key role in many human diseases, and their therapeutic modulation via pharmacological means is receiving considerable attention. Of particular interest is the fact that these cells are i) dynamic phenotypes well suited to therapeutic manipulation and ii) phagocytic, allowing them to be efficiently targeted with nanoformulations. However, it is important to consider that macrophages represent heterogeneous populations of subtypes with often competing biological behaviors and functions. In order to develop next generation therapeutics, it is therefore essential to screen for biological effects through a combination of in vitro and in vivo assays. Here, we review the state-of-the-art techniques, including both cell based screens and in vivo imaging tools that have been developed for assessment of macrophage phenotype. We conclude with a forward-looking perspective on the growing need for noninvasive macrophage assessment and laboratory assays to be put into clinical practice and the potential broader impact of myeloid-targeted therapeutics.

Why are olfactory ensheathing cell tumors so rare?

The glial cells of the primary olfactory nervous system, olfactory ensheathing cells (OECs), are unusual in that they rarely form tumors. Only 11 cases, all of which were benign, have been reported to date. In fact, the existence of OEC tumors has been debated as the tumors closely resemble schwannomas (Schwann cell tumors), and there is no definite method for distinguishing the two tumor types. OEC transplantation is a promising therapeutic approach for nervous system injuries, and the fact that OECs are not prone to tumorigenesis is therefore vital. However, why OECs are so resistant to neoplastic transformation remains unknown. The primary olfactory nervous system is a highly dynamic region which continuously undergoes regeneration and neurogenesis throughout life. OECs have key roles in this process, providing structural and neurotrophic support as well as phagocytosing the axonal debris resulting from turnover of neurons. The olfactory mucosa and underlying tissue is also frequently exposed to infectious agents, and OECs have key innate immune roles preventing microbes from invading the central nervous system. It is possible that the unique biological functions of OECs, as well as the dynamic nature of the primary olfactory nervous system, relate to the low incidence of OEC tumors. Here, we summarize the known case reports of OEC tumors, discuss the difficulties of correctly diagnosing them, and examine the possible reasons for their rare incidence. Understanding why OECs rarely form tumors may open avenues for new strategies to combat tumorigenesis in other regions of the nervous system.

Phenotypic impacts of CSF1R deficiencies in humans and model organisms

Mϕ proliferation, differentiation, and survival are controlled by signals from the Mϕ CSF receptor (CSF1R). Mono-allelic gain-of-function mutations in CSF1R in humans are associated with an autosomal-dominant leukodystrophy and bi-allelic loss-of-function mutations with recessive skeletal dysplasia, brain disorders, and developmental anomalies. Most of the phenotypes observed in these human disease states are also observed in mice and rats with loss-of-function mutations in Csf1r or in Csf1 encoding one of its two ligands. Studies in rodent models also highlight the importance of genetic background and likely epistatic interactions between Csf1r and other loci. The impacts of Csf1r mutations on the brain are usually attributed solely to direct impacts on microglial number and function. However, analysis of hypomorphic Csf1r mutants in mice and several other lines of evidence suggest that primary hydrocephalus and loss of the physiological functions of Mϕs in the periphery contribute to the development of brain pathology. In this review, we outline the evidence that CSF1R is expressed exclusively in mononuclear phagocytes and explore the mechanisms linking CSF1R mutations to pleiotropic impacts on postnatal growth and development.

Deletion of a Csf1r enhancer selectively impacts CSF1R expression and development of tissue macrophage populations

The proliferation, differentiation and survival of mononuclear phagocytes depend on signals from the receptor for macrophage colony-stimulating factor, CSF1R. The mammalian Csf1r locus contains a highly conserved super-enhancer, the fms-intronic regulatory element (FIRE). Here we show that genomic deletion of FIRE in mice selectively impacts CSF1R expression and tissue macrophage development in specific tissues. Deletion of FIRE ablates macrophage development from murine embryonic stem cells. Csf1rΔFIRE/ΔFIRE mice lack macrophages in the embryo, brain microglia and resident macrophages in the skin, kidney, heart and peritoneum. The homeostasis of other macrophage populations and monocytes is unaffected, but monocytes and their progenitors in bone marrow lack surface CSF1R. Finally, Csf1rΔFIRE/ΔFIRE mice are healthy and fertile without the growth, neurological or developmental abnormalities reported in Csf1r-/- rodents. Csf1rΔFIRE/ΔFIRE mice thus provide a model to explore the homeostatic, physiological and immunological functions of tissue-specific macrophage populations in adult animals.

Tracking Monocytes and Macrophages in Tumors With Live Imaging

In most cancers, myeloid cells represent the major component of the immune microenvironment. Deciphering the impact of these cells on tumor growth and in response to various anti-tumor therapies is a key issue. Many studies have elucidated the role of tumor-associated monocytes and tumor-associated macrophages (TAM) in tumor development, angiogenesis, and therapeutic failure. In contrast, tumor dendritic cells (DC) are associated with tumor antigen uptake and T-cell priming. Myeloid subpopulations display differences in ontogeny, state of differentiation and distribution within the neoplastic tissue, making them difficult to study. The development of high-dimensional genomic and cytometric analyses has unveiled the large functional diversity of myeloid cells. Important fundamental insights on the biology of myeloid cells have also been provided by a boom in functional fluorescent imaging techniques, in particular for TAM. These approaches allow the tracking of cell behavior in native physiological environments, incorporating spatio-temporal dimensions in the study of their functional activity. Nevertheless, tracking myeloid cells within the TME remains a challenging process as many markers overlap between monocytes, macrophages, DC, and neutrophils. Therefore, perfect discrimination between myeloid subsets remains impossible to date. Herein we review the specific functions of myeloid cells in tumor development unveiled by image-based tracking, the limits of fluorescent reporters commonly used to accurately track specific myeloid cells, and novel combinations of myeloid-associated fluorescent reporters that better discriminate the relative contributions of these cells to tumor biology according to their origin and tissue localization.

Characterization of Subpopulations of Chicken Mononuclear Phagocytes That Express TIM4 and CSF1R

The phosphatidylserine receptor TIM4, encoded by TIMD4, mediates the phagocytic uptake of apoptotic cells. We applied anti-chicken TIM4 mAbs in combination with CSF1R reporter transgenes to dissect the function of TIM4 in the chick (Gallus gallus). During development in ovo, TIM4 was present on the large majority of macrophages, but expression became more heterogeneous posthatch. Blood monocytes expressed KUL01, class II MHC, and CSF1R-mApple uniformly. Around 50% of monocytes were positive for surface TIM4. They also expressed many other monocyte-specific transcripts at a higher level than TIM4^- monocytes. In liver, highly phagocytic TIM4^hi cells shared many transcripts with mammalian Kupffer cells and were associated with uptake of apoptotic cells. Although they expressed CSF1R mRNA, Kupffer cells did not express the CSF1R-mApple transgene, suggesting that additional CSF1R transcriptional regulatory elements are required by these cells. By contrast, CSF1R-mApple was detected in liver TIM4^lo and TIM4^- cells, which were not phagocytic and were more abundant than Kupffer cells. These cells expressed CSF1R alongside high levels of FLT3, MHCII, XCR1, and other markers associated with conventional dendritic cells in mice. In bursa, TIM4 was present on the cell surface of two populations. Like Kupffer cells, bursal TIM4^hi phagocytes coexpressed many receptors involved in apoptotic cell recognition. TIM4^lo cells appear to be a subpopulation of bursal B cells. In overview, TIM4 is associated with phagocytes that eliminate apoptotic cells in the chick. In the liver, TIM4 and CSF1R reporters distinguished Kupffer cells from an abundant population of dendritic cell-like cells.

The Mononuclear Phagocyte System: The Relationship between Monocytes and Macrophages

The mononuclear phagocyte system (MPS) is defined as a cell lineage in which committed marrow progenitors give rise to blood monocytes and tissue macrophages. Here, we discuss the concept of self-proscribed macrophage territories and homeostatic regulation of tissue macrophage abundance through growth factor availability. Recent studies have questioned the validity of the MPS model and argued that tissue-resident macrophages are a separate lineage seeded during development and maintained by self-renewal. We address this issue; discuss the limitations of inbred mouse models of monocyte-macrophage homeostasis; and summarize the evidence suggesting that during postnatal life, monocytes can replace resident macrophages in all major organs and adopt their tissue-specific gene expression. We conclude that the MPS remains a valid and accurate framework for understanding macrophage development and homeostasis.

The expression of ecdysteroid UDP-glucosyltransferase enhances cocoon shell ratio by reducing ecdysteroid titre in last-instar larvae of silkworm, Bombyx mori

Ecdysteroid UDP glucosyltransferase (EGT) is a baculovirus-encoded protein which can hinder the normal molting of insects by inactivating 20-hydroxyecdysone (20E). Here we expressed EGT in the last-instar silkworm larvae using the GAL4/ UAS system. Compared with the control, for the EGT overexpressed silkworm, the hemolymph 20E content was significantly decreased, the feeding and spinning periods of the last-instar silkworm larvae were extended, the cocoon shell ratio was significantly increased, and the transformation from silkworm larvae to pupa was blocked. Increasing EGT expression resulted in the decrease of 20E content in the hemolymph of silkworm larvae, treating the EGT overexpressed male silkworm with 20E decreased the larval weight and cocoon shell ratio, confirming that the increase in the availability of nutrients to the cocoon and an increase in the cocoon shell weight in the hybrid transgenic silkworms is because of the EGT-induced reduction in active 20E content. Furthermore, though the sericin and flavonoid contents were increased in the cocoon of the EGT overexpressing silkworm, the production of silk fibroin didn’t change.

Macrophages of distinct origins contribute to tumor development in the lung

Tissue-resident macrophages can self-maintain without contribution of adult hematopoiesis. Herein we show that tissue-resident interstitial macrophages (Res-TAMs) in mouse lungs contribute to the pool of tumor-associated macrophages (TAMs) together with CCR2-dependent recruited macrophages (MoD-TAMs). Res-TAMs largely correlated with tumor cell growth in vivo, while MoD-TAMs accumulation was associated with enhanced tumor spreading. Both cell subsets were depleted after chemotherapy, but MoD-TAMs rapidly recovered and performed phagocytosis-mediated tumor clearance. Interestingly, anti-VEGF treatment combined with chemotherapy inhibited both Res and Mod-TAM reconstitution without affecting monocyte infiltration and improved its efficacy. Our results reveal that the developmental origin of TAMs dictates their relative distribution, function, and response to cancer therapies in lung tumors.

Recording the wild lives of immune cells

Intravital microscopic imaging can uncover fundamental aspects of immune cell behavior in real time in both healthy and pathological states. Here, we discuss approaches for single-cell imaging of adaptive and innate immune cells to explore how they migrate, communicate, and mediate regulatory or effector functions in various tissues throughout the body. We further review how intravital single-cell imaging can be used to study drug effects on immune cells.

Macrophage colony-stimulating factor increases hepatic macrophage content, liver growth, and lipid accumulation in neonatal rats

Signaling via the colony-stimulating factor 1 receptor (CSF1R) controls the survival, differentiation, and proliferation of macrophages. Mutations in CSF1 or CSF1R in mice and rats have pleiotropic effects on postnatal somatic growth. We tested the possible application of pig CSF1-Fc fusion protein as a therapy for low birth weight (LBW) at term, using a model based on maternal dexamethasone treatment in rats. Neonatal CSF1-Fc treatment did not alter somatic growth and did not increase the blood monocyte count. Instead, there was a substantial increase in the size of liver in both control and LBW rats, and the treatment greatly exacerbated lipid droplet accumulation seen in the dexamethasone LBW model. These effects were reversed upon cessation of treatment. Transcriptional profiling of the livers supported histochemical evidence of a large increase in macrophages with a resident Kupffer cell phenotype and revealed increased expression of many genes implicated in lipid droplet formation. There was no further increase in hepatocyte proliferation over the already high rates in neonatal liver. In conclusion, treatment of neonatal rats with CSF1-Fc caused an increase in liver size and hepatic lipid accumulation, due to Kupffer cell expansion and/or activation rather than hepatocyte proliferation. Increased liver macrophage numbers and expression of endocytic receptors could mitigate defective clearance functions in neonates. NEW & NOTEWORTHY This study is based on extensive studies in mice and pigs of the role of CSF1/CSF1R in macrophage development and postnatal growth. We extended the study to neonatal rats as a possible therapy for low birth weight. Unlike our previous studies in mice and pigs, there was no increase in hepatocyte proliferation and no increase in monocyte numbers. Instead, neonatal rats treated with CSF1 displayed reversible hepatic steatosis and Kupffer cell expansion.

Csf1r-mApple Transgene Expression and Ligand Binding In Vivo Reveal Dynamics of CSF1R Expression within the Mononuclear Phagocyte System

CSF1 is the primary growth factor controlling macrophage numbers, but whether expression of the CSF1 receptor differs between discrete populations of mononuclear phagocytes remains unclear. We have generated a Csf1r-mApple transgenic fluorescent reporter mouse that, in combination with lineage tracing, Alexa Fluor 647–labeled CSF1-Fc and CSF1, and a modified ΔCsf1–enhanced cyan fluorescent protein (ECFP) transgene that lacks a 150 bp segment of the distal promoter, we have used to dissect the differentiation and CSF1 responsiveness of mononuclear phagocyte populations in situ. Consistent with previous Csf1r-driven reporter lines, Csf1r-mApple was expressed in blood monocytes and at higher levels in tissue macrophages, and was readily detectable in whole mounts or with multiphoton microscopy. In the liver and peritoneal cavity, uptake of labeled CSF1 largely reflected transgene expression, with greater receptor activity in mature macrophages than monocytes and tissue-specific expression in conventional dendritic cells. However, CSF1 uptake also differed between subsets of monocytes and discrete populations of tissue macrophages, which in macrophages correlated with their level of dependence on CSF1 receptor signaling for survival rather than degree of transgene expression. A double ΔCsf1r-ECFP-Csf1r-mApple transgenic mouse distinguished subpopulations of microglia in the brain, and permitted imaging of interstitial macrophages distinct from alveolar macrophages, and pulmonary monocytes and conventional dendritic cells. The Csf1r-mApple mice and fluorescently labeled CSF1 will be valuable resources for the study of macrophage and CSF1 biology, which are compatible with existing EGFP-based reporter lines.

Barrier-tissue macrophages: functional adaptation to environmental challenges

Macrophages are found throughout the body, where they have crucial roles in tissue development, homeostasis and remodeling, as well as being sentinels of the innate immune system that can contribute to protective immunity and inflammation. Barrier tissues, such as the intestine, lung, skin and liver, are exposed constantly to the outside world, which places special demands on resident cell populations such as macrophages. Here we review the mounting evidence that although macrophages in different barrier tissues may be derived from distinct progenitors, their highly specific properties are shaped by the local environment, which allows them to adapt precisely to the needs of their anatomical niche. We discuss the properties of macrophages in steady-state barrier tissues, outline the factors that shape their differentiation and behavior and describe how macrophages change during protective immunity and inflammation.

Dendritic Cell Subsets in Asthma: Impaired Tolerance or Exaggerated Inflammation?

Asthma is a prevalent chronic heterogeneous inflammatory disease of the airways, leading to reversible airway obstruction, in which various inflammatory responses can be observed. Mild to moderate asthma patients often present with a Th2-mediated eosinophilic inflammation whereas in severe asthma patients, a Th17-associated neutrophilic or combined Th2 and Th17-mediated eosinophilic/neutrophilic inflammation is observed. The differentiation of these effector Th2 and Th17-cells is induced by allergen-exposed dendritic cells (DCs) that migrate toward the lung draining lymph node. The DC lineage comprises conventional DCs (cDCs) and plasmacytoid DCs (pDCs), of which the cDC lineage consists of type 1 cDCs (cDC1s) and cDC2s. During inflammation, also monocytes can differentiate into so-called monocyte-derived DCs (moDCs). These DC subsets differ both in ontogeny, localization, and in their functional properties. New identification tools and the availability of transgenic mice targeting specific DC subsets enable the investigation of how these different DC subsets contribute to or suppress asthma pathogenesis. In this review, we will discuss mechanisms used by different DC subsets to elicit or hamper the pathogenesis of both Th2-mediated eosinophilic asthma and more severe Th17-mediated neutrophilic inflammation.

Osteomacs and Bone Regeneration

PURPOSE OF REVIEW

Mounting evidence supporting the critical contribution of macrophages, in particular osteal macrophages, to bone regeneration is reviewed. We specifically examine the potential role of macrophages in the basic multicellular units coordinating lifelong bone regeneration via remodelling and bone regeneration in response to injury. We review and discuss the distinctions between macrophage and osteoclast contributions to bone homeostasis, particularly the dichotomous role of the colony-stimulating factor 1-colony-stimulating factor 1 receptor axis.

RECENT FINDINGS

The impact of inflammation associated with aging and other hallmarks of aging, including senescence, on macrophage function is addressed in the context of osteoporosis and delayed fracture repair. Resident macrophages versus recruited macrophage contributions to fracture healing are also discussed. We identify some of the remaining knowledge gaps that will need to be closed in order to maximise benefits from therapeutically modulating or mimicking the function of macrophages to improve bone health and regeneration over a lifetime.

Transcriptional mechanisms that control expression of the macrophage colony-stimulating factor receptor locus

The proliferation, differentiation, and survival of cells of the macrophage lineage depends upon signals from the macrophage colony-stimulating factor (CSF) receptor (CSF1R). CSF1R is expressed by embryonic macrophages and induced early in adult hematopoiesis, upon commitment of multipotent progenitors to the myeloid lineage. Transcriptional activation of CSF1R requires interaction between members of the E26 transformation-specific family of transcription factors (Ets) (notably PU.1), C/EBP, RUNX, AP-1/ATF, interferon regulatory factor (IRF), STAT, KLF, REL, FUS/TLS (fused in sarcoma/ranslocated in liposarcoma) families, and conserved regulatory elements within the mouse and human CSF1R locus. One element, the Fms-intronic regulatory element (FIRE), within intron 2, is conserved functionally across all the amniotes. Lineage commitment in multipotent progenitors also requires down-regulation of specific transcription factors such as MYB, FLI1, basic leucine zipper transcriptional factor ATF-like (BATF3), GATA-1, and PAX5 that contribute to differentiation of alternative lineages and repress CSF1R transcription. Many of these transcription factors regulate each other, interact at the protein level, and are themselves downstream targets of CSF1R signaling. Control of CSF1R transcription involves feed–forward and feedback signaling in which CSF1R is both a target and a participant; and dysregulation of CSF1R expression and/or function is associated with numerous pathological conditions. In this review, we describe the regulatory network behind CSF1R expression during differentiation and development of cells of the mononuclear phagocyte system.

The role of monocytes in models of infection by protozoan parasites

The confirmation of developmental differences between tissue macrophages and peripheral monocytes has changed our view of the functions and dynamics of these two important components of the innate immune system. It has been demonstrated conclusively that homeostasis of tissue resident macrophages is maintained by a low proliferative turn over. During an inflammatory response, bone marrow derived monocytes enter the tissue in large numbers and take part in the defense against the pathogens. After the destruction of invading pathogens, these cells disappear and tissue resident macrophages can be detected again. This new appreciation of the innate immune response has not only answered many outstanding questions regarding the role of the different myeloid cell types in inflammation, but also opened up new areas of research relating to the tissue- and pathogen-specific fate of the inflammatory macrophages or dendritic cells (DCs), and the transfer of this knowledge from mouse models to the human immune system. Nevertheless, there is still confusion in infection models, and especially in studies of human infections, as to what extent these recent observations and findings influence previous interpretations of data. This review will focus on insights from mouse models, summarize the literature on the ontogeny of macrophages and monocytes, explain the role of frequently used monocyte markers and effector molecules, and finally, discuss the role of inflammatory monocytes/macrophages/DCs in two experimental parasitic diseases.

Transcriptional Regulation and Macrophage Differentiation

Monocytes and macrophages are professional phagocytes that occupy specific niches in every tissue of the body. Their survival, proliferation, and differentiation are controlled by signals from the macrophage colony-stimulating factor receptor (CSF-1R) and its two ligands, CSF-1 and interleukin-34. In this review, we address the developmental and transcriptional relationships between hematopoietic progenitor cells, blood monocytes, and tissue macrophages as well as the distinctions from dendritic cells. A huge repertoire of receptors allows monocytes, tissue-resident macrophages, or pathology-associated macrophages to adapt to specific microenvironments. These processes create a broad spectrum of macrophages with different functions and individual effector capacities. The production of large transcriptomic data sets in mouse, human, and other species provides new insights into the mechanisms that underlie macrophage functional plasticity.

Imaging Leukocyte Responses in the Kidney

The kidney can be negatively affected by a range of innate and adaptive immune responses, resulting in alterations in the functions of the kidney and, in some cases, progression to renal failure. In many of these responses, infiltration of blood-borne leukocytes into the kidney is central to the response. In addition, a large population of mononuclear phagocytes resident in the kidney can modulate these responses. A great deal of research has investigated both the mechanisms of leukocyte recruitment to the kidney and the actions of immune cells resident within the kidney. Because of the dynamic nature of the processes whereby leukocytes enter sites of inflammation, in vivo imaging has been one of the key approaches used for understanding leukocyte recruitment as it occurs throughout the body, and this is also true for kidney. However, imaging this organ and its complicated microvasculature during different forms of renal pathology presents a unique set of challenges. In this review, we examine the approaches used for intravital imaging of the kidney and summarize the insights gained from these studies regarding the mechanisms of leukocyte entry into the kidney during inflammation and the actions of immune cells within this organ.

CX3CR1-dependent endothelial margination modulates Ly6Chigh monocyte systemic deployment upon inflammation in mice

Two subsets of blood monocytes are commonly described in mice and humans: the classical inflammatory monocytes, which are rapidly mobilized upon inflammation in a CC-chemokine receptor 2-dependent manner, and the nonclassical blood resident monocyte subset that patrols the intraluminal side of the endothelium. Old reports suggest that blood monocytes are distributed into circulating and marginating pools, but no direct evidence of the latter has been obtained so far. Using a combination of in vivo real-time imaging and blood/tissue partitioning by intravascular staining of leukocytes, we showed that both inflammatory and resident monocytes are retained in the bone marrow vasculature, representing an important reservoir of marginated monocytes. Upon lipopolysaccharide or cecal ligation and puncture-induced peritonitis, these marginated cells are rapidly released and recruited to the peritoneum membrane lumen vasculature where they reside through CX3C-chemokine receptor 1 (CX3CR1)-dependent adherence. At a later time point, inflammatory monocytes infiltrate the spleen parenchyma but remain mainly intravascular in the vicinity of the lungs and the peritoneum. Our results show that this monocyte deployment is controlled by a CX3CR1-dependent balance between marginating and circulating monocytes and highlight that tissue infiltration is not a mandatory fate for inflammatory monocytes.

Macrophage colony-stimulating factor (CSF1) controls monocyte production and maturation and the steady-state size of the liver in pigs

Macrophage colony-stimulating factor (CSF1) is an essential growth and differentiation factor for cells of the macrophage lineage. To explore the role of CSF1 in steady-state control of monocyte production and differentiation and tissue repair, we previously developed a bioactive protein with a longer half-life in circulation by fusing pig CSF1 with the Fc region of pig IgG1a. CSF1-Fc administration to pigs expanded progenitor pools in the marrow and selectively increased monocyte numbers and their expression of the maturation marker CD163. There was a rapid increase in the size of the liver, and extensive proliferation of hepatocytes associated with increased macrophage infiltration. Despite the large influx of macrophages, there was no evidence of liver injury and no increase in circulating liver enzymes. Microarray expression profiling of livers identified increased expression of macrophage markers, i.e., cytokines such as TNF, IL1, and IL6 known to influence hepatocyte proliferation, alongside cell cycle genes. The analysis also revealed selective enrichment of genes associated with portal, as opposed to centrilobular regions, as seen in hepatic regeneration. Combined with earlier data from the mouse, this study supports the existence of a CSF1-dependent feedback loop, linking macrophages of the liver with bone marrow and blood monocytes, to mediate homeostatic control of the size of the liver. The results also provide evidence of safety and efficacy for possible clinical applications of CSF1-Fc.

A Csf1r-EGFP Transgene Provides a Novel Marker for Monocyte Subsets in Sheep

Expression of Csf1r in adults is restricted to cells of the macrophage lineage. Transgenic reporters based upon the Csf1r locus require inclusion of the highly conserved Fms-intronic regulatory element for expression. We have created Csf1r-EGFP transgenic sheep via lentiviral transgenesis of a construct containing elements of the mouse Fms-intronic regulatory element and Csf1r promoter. Committed bone marrow macrophage precursors and blood monocytes express EGFP in these animals. Sheep monocytes were divided into three populations, similar to classical, intermediate, and nonclassical monocytes in humans, based upon CD14 and CD16 expression. All expressed EGFP, with increased levels in the nonclassical subset. Because Csf1r expression coincides with the earliest commitment to the macrophage lineage, Csf1r-EGFP bone marrow provides a tool for studying the earliest events in myelopoiesis using the sheep as a model.

Enforced Expression of Hoxa3 Inhibits Classical and Promotes Alternative Activation of Macrophages In Vitro and In Vivo

The regulated differentiation of macrophages (mφs) and their subsequent activation into proinflammatory or prohealing subtypes is critical for efficient wound healing. Chronic wounds such as diabetic (db) ulcers are associated with dysregulation of macrophage function. Whereas non-db mφs polarize to an M2-like, prohealing phenotype during the late stages of healing, db-derived mφs continue to display an M1-like, proinflammatory, or a mixed M1-like/M2-like phenotype. We have previously shown that sustained expression of Hoxa3 reduces the excessive number of leukocytes within the db wound; however, the effect of Hoxa3 on mφ polarization was unknown. In this study, we show that Hoxa3 protein transduction of mφs in vitro enhances macrophage maturation, inhibits M1 polarization, and promotes M2 polarization, in part via regulation of Pu.1/Spi1 and Stat6. Sustained expression of Hoxa3 in vivo in db wounds reduces the number of Nos2(+) (M1-like) mφs, increases the number of Arg1(+) and VEGF(+) (M2-like) mφs, and accelerates healing in a DNA-binding independent manner. Our findings suggest a role for Hox protein activity in promoting M1-to-M2-like phenotypic switching via interactions with myeloid transcription factors and provide insight into mechanisms regulating this process in db wound healing.

Live cell imaging to understand monocyte, macrophage, and dendritic cell function in atherosclerosis

Ley et al. provide a review of the technology and accomplishments of dynamic imaging of myeloid cells in atherosclerosis.

Intravital imaging is an invaluable tool for understanding the function of cells in healthy and diseased tissues. It provides a window into dynamic processes that cannot be studied by other techniques. This review will cover the benefits and limitations of various techniques for labeling and imaging myeloid cells, with a special focus on imaging cells in atherosclerotic arteries. Although intravital imaging is a powerful tool for understanding cell function, it alone does not provide a complete picture of the cell. Other techniques, such as flow cytometry and transcriptomics, must be combined with intravital imaging to fully understand a cell's phenotype, lineage, and function.

Origin, Localization, and Immunoregulatory Properties of Pulmonary Phagocytes in Allergic Asthma

Allergic asthma is a chronic inflammatory disease of the airways that is driven by maladaptive T helper 2 (Th2) and Th17 immune responses against harmless, airborne substances. Pulmonary phagocytes represent the first line of defense in the lung where they constantly sense the local environment for potential threats. They comprise two distinct cell types, i.e., macrophages and dendritic cells (DC) that differ in their origins and functions. Alveolar macrophages quickly take up most of the inhaled allergens, yet do not deliver their cargo to naive T cells sampling in draining lymph nodes. In contrast, pulmonary DCs instruct CD4(+) T cells develop into Th2 and Th17 effectors, initiating the maladaptive immune responses toward harmless environmental substances observed in allergic individuals. Unraveling the mechanisms underlying this mistaken identity of harmless, airborne substances by innate immune cells is one of the great challenges in asthma research. The identification of different pulmonary DC subsets, their role in antigen uptake, migration to the draining lymph nodes, and their potential to instruct distinct T cell responses has set the stage to unravel this mystery. However, at this point, a detailed understanding of the spatiotemporal resolution of DC subset localization, allergen uptake, processing, autocrine and paracrine cellular crosstalk, and the humoral factors that define the activation status of DCs is still lacking. In addition to DCs, at least two distinct macrophage populations have been identified in the lung that are either located in the airway/alveolar lumen or in the interstitium. Recent data suggest that such populations can exert either pro- or anti-inflammatory functions. Similar to the DC subsets, detailed insights into the individual roles of alveolar and interstitial macrophages during the different phases of asthma development are still missing. Here, we will provide an update on the current understanding of the origin, localization, and function of the diverse pulmonary antigen-presenting cell subsets, in particular with regard to the development and regulation of allergic asthma. While most data are from mouse models of experimental asthma, we have also included available human data to judge the translational value of the findings obtained in experimental asthma models.

Myelin and macrophages in the PNS: An intimate relationship in trauma and disease

Macrophages of the peripheral nervous system belong to the so-called tissue macrophages, with multiple functions during injury and disease. Their origin during ontogeny has not yet been completely resolved, but it is clear that upon injury and disease conditions, they are supplemented by hematopoietic derivatives. In the peripheral nervous system, the most abundantly investigated scenario in which resident and infiltrating macrophages are involved is the so-called "Wallerian degeneration", a complex degenerative process where macrophages exhibit mostly beneficial functions by phagocytosing myelin and axonal remnants. Of special interest is the implication of macrophages in inflammatory nerve diseases, like acute Guillain-Barré syndromes and its permanent variant, chronic inflammatory demyelinating polyneuropathy, where macrophages are supposed to be substantial (co-)mediators of the diseases. In inherited peripheral neuropathies nerve macrophages possess a clear disease-amplifying function. In the corresponding animal models, a coordinated interplay between mutant Schwann cells, macrophages, endoneurial fibroblasts and the target structure, myelin, emerged. Along this process, a newly discovered disease mechanism mediated by macrophages is the dedifferentiation of myelinating Schwann cells. As macrophages are amplifiers of the genetically-mediated, non-curable diseases, targeting the mechanisms of their activation might be a promising strategy to treat these disorders. This article is part of a Special Issue entitled SI: Myelin Evolution.

Immune surveillance of the lung by migrating tissue monocytes

Monocytes are phagocytic effector cells in the blood and precursors of resident and inflammatory tissue macrophages. The aim of the current study was to analyse and compare their contribution to innate immune surveillance of the lung in the steady state with macrophage and dendritic cells (DC). ECFP and EGFP transgenic reporters based upon Csf1r and Cx3cr1 distinguish monocytes from resident mononuclear phagocytes. We used these transgenes to study the migratory properties of monocytes and macrophages by functional imaging on explanted lungs. Migratory monocytes were found to be either patrolling within large vessels of the lung or locating at the interface between lung capillaries and alveoli. This spatial organisation gives to monocytes the property to capture fluorescent particles derived from both vascular and airway routes. We conclude that monocytes participate in steady-state surveillance of the lung, in a way that is complementary to resident macrophages and DC, without differentiating into macrophages.

DOI:http://dx.doi.org/10.7554/eLife.07847.001

White blood cells form part of the immune system, which protects the body against infectious diseases and other harmful agents. Some of these cells, including ‘mononuclear phagocytes’, can reside within different tissues of the body, such as the lungs. Other less specialized cells, called monocytes, circulate in the bloodstream. It had long been thought that once these monocytes had taken up residence in a tissue, they could only develop into tissue-resident phagocytes. Several researchers, however, recently reported that monocytes can also reside within tissues without becoming more specialized. Nevertheless, it remained unclear what these cells did when they were in these tissues.

Rodero, Poupel, Loyher et al. investigated the activities of tissue-resident monocytes found in the lungs of mice. First, mice were genetically engineered to produce fluorescent markers that meant that their monocytes could be easily distinguished from the mononuclear phagocytes in their lungs when viewed under a microscope. Rodero, Poupel, Loyher et al. then showed that the monocytes and the other mononuclear phagocytes localized to different regions of the lung. Further experiments showed that these two groups of cells also moved around the lungs in different ways. The tissue-resident monocytes surveyed both the blood vessels and airways, while the other tissue-resident mononuclear phagocytes only surveyed the airways.

These findings show that lung-resident monocytes perform a different role to those found in the bloodstream. The findings also open the way to improving our understanding of what tissue-resident monocytes do in other organs, and in healthy or diseased animals.

DOI:http://dx.doi.org/10.7554/eLife.07847.002

Ly6Chigh Monocytes Protect against Kidney Damage during Sepsis via a CX3CR1-Dependent Adhesion Mechanism

Monocytes have a crucial role in both proinflammatory and anti-inflammatory phenomena occurring during sepsis. Monocyte recruitment and activation are orchestrated by the chemokine receptors CX3CR1 and CCR2 and their cognate ligands. However, little is known about the roles of these cells and chemokines during the acute phase of inflammation in sepsis. Using intravital microscopy in a murine model of polymicrobial sepsis, we showed that inflammatory Ly6C(high) monocytes infiltrated kidneys, exhibited altered motility, and adhered strongly to the renal vascular wall in a chemokine receptor CX3CR1-dependent manner. Adoptive transfer of Cx3cr1-proficient monocyte-enriched bone marrow cells into septic Cx3cr1-depleted mice prevented kidney damage and promoted mouse survival. Modulation of CX3CR1 activation in septic mice controlled monocyte adhesion, regulated proinflammatory and anti-inflammatory cytokine expression, and was associated with the extent of kidney lesions such that the number of lesions decreased when CX3CR1 activity increased. Consistent with these results, the pro-adhesive I249 CX3CR1 allele in humans was associated with a lower incidence of AKI in patients with sepsis. These data show that inflammatory monocytes have a protective effect during sepsis via a CX3CR1-dependent adhesion mechanism. This receptor might be a new therapeutic target for kidney injury during sepsis.