Conditional macrophage ablation in transgenic mice expressing a Fas-based suicide gene

Osteoclast depletion with clodronate liposomes delays fracture healing in mice

Osteoclasts are abundant within the fracture callus and also localize at the chondro-osseous junction. However, osteoclast functions during fracture healing are not well defined. Inhibition of osteoclast formation or resorptive activity impairs callus remodeling but does not prevent callus formation. Interestingly, though anti-osteoclast therapies differentially affect resolution of callus cartilage into bone. Treatments that inhibit osteoclast formation or viability tend to impair callus cartilage resolution, while treatments that target inhibition of bone resorption generally do not affect callus cartilage resolution. Here, we tested whether depletion of osteoclasts by systemic treatment with clodronate liposomes would similarly impair callus cartilage resolution. ICR mice were treated by intraperitoneal injections of clodronate-laden liposomes or control liposomes and subjected to closed femur fracture. Femurs were resected at multiple times after fracture and analyzed by radiography, histology, and mechanical testing to determine effects on healing. Clodronate liposome treatment did not prevent callus formation. However, radiographic scoring indicated that clodronate liposome treatment impaired healing. Clodronate liposome treatment significantly reduced callus osteoclast populations and delayed resolution of callus cartilage. Consistent with continued presence of callus cartilage, torsional mechanical testing found significant decreases in callus material properties after 28 days of healing. The results support a role for osteoclasts in the resolution of callus cartilage into bone. Whether the cartilage resolution role for osteoclasts is limited to simply resorbing cartilage at the chondro-osseous junction or in promoting bone formation at the chondro-osseous junction through another mechanism, perhaps similar to the reversal process in bone remodeling, will require further experimentation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1699-1706, 2017.

A systemic macrophage response is required to contain a peripheral poxvirus infection

The goal of the innate immune system is to reduce pathogen spread prior to the initiation of an effective adaptive immune response. Following an infection at a peripheral site, virus typically drains through the lymph to the lymph node prior to entering the blood stream and being systemically disseminated. Therefore, there are three distinct spatial checkpoints at which intervention to prevent systemic spread of virus can occur, namely: 1) the site of infection, 2) the draining lymph node via filtration of lymph or 3) the systemic level via organs that filter the blood. We have previously shown that systemic depletion of phagocytic cells allows viral spread after dermal infection with Vaccinia virus (VACV), which infects naturally through the skin. Here we use multiple depletion methodologies to define both the spatial checkpoint and the identity of the cells that prevent systemic spread of VACV. Subcapsular sinus macrophages of the draining lymph node have been implicated as critical effectors in clearance of lymph borne viruses following peripheral infection. We find that monocyte populations recruited to the site of VACV infection play a critical role in control of local pathogenesis and tissue damage, but do not prevent dissemination of virus. Following infection with virulent VACV, the subcapsular sinus macrophages within the draining lymph node become infected, but are not exclusively required to prevent systemic spread. Rather, small doses of VACV enter the bloodstream and the function of systemic macrophages, but not dendritic cells, is required to prevent further spread. The results illustrate that a systemic innate response to a peripheral virus infection may be required to prevent widespread infection and pathology following infection with virulent viruses, such as poxviruses.

Sphingosine 1-phosphate signaling in bone remodeling: multifaceted roles and therapeutic potential

Introduction: Sphingolipids belong to a complex class of lipid molecules that are crucially involved in the regulation of important biological processes including proliferation, migration and apoptosis. Given the significant progress made in understanding the sphingolipid pathobiology of several diseases, sphingolipid-related checkpoints emerge as attractive targets. Recent data indicate the multifaceted contribution of the sphingolipid machinery to osteoclast – osteoblast crosstalk, representing one of the pivotal interactions underlying bone homeostasis. Imbalances in the interplay of osteoblasts and osteoclasts might lead to bone-related diseases such as osteoporosis, rheumatoid arthritis, and bone metastases.

Areas covered: We summarize and analyze the progress made in bone research in the context of the current knowledge of sphingolipid-related mechanisms regulating bone remodeling. Particular emphasis was given to bioactive sphingosine 1-phosphate (S1P) and S1P receptors (S1PRs). Moreover, the mechanisms of how dysregulations of this machinery cause bone diseases, are covered.

Expert opinion: In the context of bone diseases, pharmacological interference with sphingolipid machinery may lead to novel directions in therapeutic strategies. Implementation of knowledge derived from in vivo animal models and in vitro studies using pharmacological agents to manipulate the S1P/S1PRs axes suggests S1PR2 and S1PR3 as potential drug targets, particularly in conjunction with technology for local drug delivery.

Macrophages and skeletal health

Bone is in a constant state of remodeling, a process which was once attributed solely to osteoblasts and osteoclasts. Decades of research has identified many other populations of cells in the bone that participate and mediate skeletal homeostasis. Recently, osteal macrophages emerged as vital participants in skeletal remodeling and osseous repair. The exact mechanistic roles of these tissue-resident macrophages are currently under investigation. Macrophages are highly plastic in response to their micro-environment and are typically classified as being pro- or anti-inflammatory (pro-resolving) in nature. Given that inflammatory states result in decreased bone mass, proinflammatory macrophages may be negative regulators of bone turnover. Pro-resolving macrophages have been shown to release anabolic factors and may present a target for therapeutic intervention in inflammation-induced bone loss and fracture healing. The process of apoptotic cell clearance, termed efferocytosis, is mediated by pro-resolving macrophages and may contribute to steady-state bone turnover as well as fracture healing and anabolic effects of osteoporosis therapies. Parathyroid hormone is an anabolic agent in bone that is more effective in the presence of mature phagocytic macrophages, further supporting the hypothesis that efferocytic macrophages are positive contributors to bone turnover. Therapies which alter macrophage plasticity in tissues other than bone should be explored for their potential to treat bone loss either alone or in conjunction with current bone therapeutics. A better understanding of the exact mechanisms by which macrophages mediate bone homeostasis will lead to an expansion of pharmacologic targets for the treatment of osteoporosis and inflammation-induced bone loss.

IL-1R Type 1-Deficient Mice Demonstrate an Impaired Host Immune Response against Cutaneous Vaccinia Virus Infection

The IL-1 superfamily of cytokines and receptors has been studied extensively. However, the specific roles of IL-1 elements in host immunity to cutaneous viral infection remain elusive. In this study, we applied vaccinia virus (VACV) by scarification to IL-1R1 knockout mice (IL-1R1^-/-) and found that these mice developed markedly larger lesions with higher viral genome copies in skin than did wild-type mice. The phenotype of infected IL-1R1^-/- mice was similar to eczema vaccinatum, a severe side effect of VACV vaccination that may develop in humans with atopic dermatitis. Interestingly, the impaired cutaneous response of IL-1R1^-/- mice did not reflect a systemic immune deficiency, because immunized IL-1R1^-/- mice survived subsequent lethal VACV intranasal challenge, or defects of T cell activation or T cell homing to the site of inoculation. Histologic evaluation revealed that VACV infection and replication after scarification were limited to the epidermal layer of wild-type mice, whereas lack of IL-1R1 permitted extension of VACV infection into dermal layers of the skin. We explored the etiology of this discrepancy and determined that IL-1R1^-/- mice contained significantly more macrophages and monocyte-derived dendritic cells in the dermis after VACV scarification. These cells were vulnerable to VACV infection and may augment the transmission of virus to adjacent skin, thus leading to larger skin lesions and satellite lesions in IL-1R1^-/- mice. These results suggest new therapeutic strategies for treatment of eczema vaccinatum and inform assessment of risks in patients receiving IL-1 blocking Abs for treatment of chronic inflammatory disorders.

Biomaterial Encapsulation Is Enhanced in the Early Stages of the Foreign Body Reaction During Conditional Macrophage Depletion in Transgenic Macrophage Fas-Induced Apoptosis Mice<sup/>

Macrophages are pivotal cells during the foreign body reaction (FBR), as they orchestrate the proinflammatory microenvironment inside and around biomaterials by secretion of inflammatory mediators. Furthermore, they are responsible for the degradation of biomaterials and are thought to instruct the fibroblasts that generate a fibrous capsule around implanted biomaterials. In this study, we investigated the events during the FBR when macrophages are not present. Hexamethylenediisocyanate crosslinked collagen scaffolds were implanted in "Macrophage Fas-Induced Apoptosis" mice, which allow "on demand" macrophage depletion. We observed that macrophage depletion completely inhibited inflammatory ingrowth into the scaffolds and resulted in an increased capsule size. Quantitative polymerase chain reaction analysis revealed decreased expression levels of proinflammatory mediators such as TNFα and IL1β, and increased expression levels of collagens and fibroblast-stimulating growth factors such as EGF, FGF1, FGF2, and TGFα. Our results indicate that macrophages are indeed crucial for the generation of a proinflammatory microenvironment inside implanted biomaterials, leading to inflammatory ingrowth. In contrast, macrophages do not appear to be important for the generation of a fibrous capsule around implanted biomaterials. In fact, our data suggest that the macrophages present in the capsule might instruct the surrounding fibroblasts to produce less fibroblast-stimulating factors and less collagens.

Mononuclear Phagocytes Are Dispensable for Cardiac Remodeling in Established Pressure-Overload Heart Failure

BACKGROUND

Although cardiac and splenic mononuclear phagocytes (MPs), i.e., monocytes, macrophages and dendritic cells (DCs), are key contributors to cardiac remodeling after myocardial infarction, their role in pressure-overload remodeling is unclear. We tested the hypothesis that these immune cells are required for the progression of remodeling in pressure-overload heart failure (HF), and that MP depletion would ameliorate remodeling.

METHODS AND RESULTS

C57BL/6 mice were subjected to transverse aortic constriction (TAC) or sham operation, and assessed for alterations in MPs. As compared with sham, TAC mice exhibited expansion of circulating LyC6hi monocytes and pro-inflammatory CD206- cardiac macrophages early (1 w) after pressure-overload, prior to significant hypertrophy and systolic dysfunction, with subsequent resolution during chronic HF. In contrast, classical DCs were expanded in the heart in a biphasic manner, with peaks both early, analogous to macrophages, and late (8 w), during established HF. There was no significant expansion of circulating DCs, or Ly6C+ monocytes and DCs in the spleen. Periodic systemic MP depletion from 2 to 16 w after TAC in macrophage Fas-induced apoptosis (MaFIA) transgenic mice did not alter cardiac remodeling progression, nor did splenectomy in mice with established HF after TAC. Lastly, adoptive transfer of splenocytes from TAC HF mice into naïve recipients did not induce immediate or long-term cardiac dysfunction in recipient mice.

CONCLUSIONS

Mononuclear phagocytes populations expand in a phasic manner in the heart during pressure-overload. However, they are dispensable for the progression of remodeling and failure once significant hypertrophy is evident and blood monocytosis has normalized.

Selective M2 Macrophage Depletion Leads to Prolonged Inflammation in Surgical Wounds

BACKGROUND

A prolonged inflammatory phase is seen in aberrant wound healing and in chronic wounds. Macrophages are central to wound healing. Distinct macrophage subtypes have differing roles both in initial inflammation and in later tissue repair. Broadly, these cells can be divided into M1 and M2 macrophages. M2 macrophage proliferation and differentiation is regulated by colony-stimulating factor 1 (CSF-1) signalling and can be blocked by GW2580, a competitive cFMS kinase inhibitor, thereby allowing for analysis of the effect of M2 blockade on progression of surgical wounds.

MATERIALS AND METHODS

Macrophage Fas-induced apoptosis (MaFIA) transgenic mice with a macrophage-specific promoter used to express green fluorescent protein (GFP) were used to allow for cell tracking. The animals were treated by oral gavage with GW2580. Surgical wounds were created and harvested after 2 weeks for analysis.

RESULTS

GW2580-treated mice had significantly more GFP+ cells in the surgical scar than vehicle-treated animals (GW2580, 68.0 ± 3.1%; vehicle, 42.8 ± 1.7%; p < 0.001), and GW2580 treatment depleted CD206+ M2 macrophages in the scar (GW2580, 1.4%; vehicle, 19.3%; p < 0.001). Treated animals showed significantly higher numbers of neutrophils (vehicle, 18.0%; GW2580, 51.3%; p < 0.01) and M1 macrophages (vehicle, 3.8%; GW2580, 12.8%; p < 0.01) in the scar compared to vehicle-treated animals. The total collagen content in the area of the scar was decreased in animals treated with GW2580 as compared to those treated with vehicle alone (GW2580, 67.1%; vehicle, 79.9%; p < 0.005).

CONCLUSIONS

Depletion of M2 macrophages in surgical wounds via CSF-1 signalling blockade leads to persistent inflammation, with an increase in neutrophils and M1 macrophages and attenuated collagen deposition.

Analysis of Thymocyte Migration, Cellular Interactions, and Activation by Multiphoton Fluorescence Microscopy of Live Thymic Slices

Thymocytes migrate through discrete compartments within the thymus, engaging in cellular interactions essential for their differentiation into functional and self-tolerant T cells. Thus, understanding the temporal and spatial behavior of thymocytes within an intact thymic microenvironment is critical for elucidating processes governing T cell development. Towards this end, we describe methods for preparing thymic explant slices, in which the migration of thymocytes through three-dimensional space can be probed using time-lapse, multiphoton fluorescence microscopy. Thymocytes, enriched for developmental subsets of interest, are labeled with cytoplasmic fluorescent dyes, and seeded onto live thymic slices that express an endogenous, stromal cell-specific fluorescent reporter. In response to chemotactic cues produced by thymic stromal cells, the labeled thymocytes migrate withinthymic microenvironments and engage in cellular interactions that recapitulate a physiological system, whichcan be readily imaged. Here we describe specimen preparation that maintains the integrity of thymic structures. We also describe imaging protocols for acquiring multiple fluorochrome channels to enable detection of thymocyte:stromal cell interactions and quantification of relative intracellular calcium levels to monitor T cell receptor activation. Parameters for quantifying motility and interaction behaviors during data analysis are also briefly described. The thymic slice is a versatile tool for probing live cell behaviors and developing novel hypotheses not readily apparent by static experimental methods.

Giant cells around bone biomaterials: Osteoclasts or multi-nucleated giant cells?

Recently accumulating evidence has put into question the role of large multinucleated giant cells (MNGCs) around bone biomaterials. While cells derived from the monocyte/macrophage lineage are one of the first cell types in contact with implanted biomaterials, it was originally thought that specifically in bone tissues, all giant cells were bone-resorbing osteoclasts whereas foreign body giant cells (FBGCs) were found associated with a connective tissue foreign body reaction resulting in fibrous encapsulation and/or material rejection. Despite the great majority of bone grafting materials routinely found with large osteoclasts, a special subclass of bone biomaterials has more recently been found surrounded by large giant cells virtually incapable of resorbing bone grafts even years after their implantation. While original hypotheses believed that a 'foreign body reaction' may be taking place, histological data retrieved from human samples years after their implantation have put these original hypotheses into question by demonstrating better and more stable long-term bone volume around certain bone grafts. Exactly how or why this 'special' subclass of giant cells is capable of maintaining long-term bone volume, or methods to scientifically distinguish them from osteoclasts remains extremely poorly studied. The aim of this review article was to gather the current available literature on giant cell markers and differences in expression patterns between osteoclasts and MNGCs utilizing 19 specific markers including an array of CD-cell surface markers. Furthermore, the concept of now distinguishing between pro-inflammatory M1-MNGCs (previously referred to as FBGCs) as well as wound-healing M2-MNGCs is introduced and discussed.

STATEMENT OF SIGNIFICANCE

This review article presents 19 specific cell-surface markers to distinguish between osteoclasts and MNGCs including an array of CD-cell surface markers. Furthermore, the concept of now distinguishing between pro-inflammatory M1-MNGCs (often previously referred to as FBGCs) as well as wound-healing M2-MNGCs is introduced and discussed. The proposed concepts and guidelines aims to guide the next wave of research facilitating the differentiation between osteoclast/MNGCs formation, as well as provides the basis for increasing our understanding of the exact function of MNGCs in bone tissue/biomaterial homeostasis.

Digesting the role of bone marrow macrophages on hematopoiesis

Tissue resident macrophages are found in various tissues like Langerhans cells in the skin or alveolar macrophages in the lung, and their main function is to regulate organ homeostasis. They have also been observed in the bone marrow and these cells in particular have been gaining importance in recent years as they are key players in hematopoiesis. However, as the characterization and classification of these putatively different bone marrow resident macrophages is far from established there is a need to generate an overview of tissue resident macrophages of the bone marrow. Here, we will review the current knowledge of bone marrow resident macrophages both in mouse and human. We will discuss the state of the art on the origin of bone marrow macrophages, specialized microenvironments where they reside and their unique characteristics. We will emphasize the two best studied examples of macrophage homeostatic function in the bone marrow, specifically within erythroblastic islands and the hematopoietic stem cell niche. Although increasing evidence shows that bone marrow resident macrophages are indispensable for hematopoietic stem cell function and bone marrow erythroid output, the field of bone marrow macrophages is in its infancy. This field is in dire need for a unified nomenclature to support functional experiments, model systems, and the identification of niches.

Bone marrow macrophages support prostate cancer growth in bone

Resident macrophages in bone play important roles in bone remodeling, repair, and hematopoietic stem cell maintenance, yet their role in skeletal metastasis remains under investigated. The purpose of this study was to determine the role of macrophages in prostate cancer skeletal metastasis, using two in vivo mouse models of conditional macrophage depletion. RM-1 syngeneic tumor growth was analyzed in an inducible macrophage (CSF-1 receptor positive cells) ablation model (MAFIA mice). There was a significant reduction in tumor growth in the tibiae of macrophage-ablated mice, compared with control non-ablated mice. Similar results were observed when macrophage ablation was performed using liposome-encapsulated clodronate and human PC-3 prostate cancer cells where tumor-bearing long bones had increased numbers of tumor associated-macrophages. Although tumors were consistently smaller in macrophage-depleted mice, paradoxical results of macrophage depletion on bone were observed. Histomorphometric and micro-CT analyses demonstrated that clodronate-treated mice had increased bone volume, while MAFIA mice had reduced bone volume. These results suggest that the effect of macrophage depletion on tumor growth was independent of its effect on bone responses and that macrophages in bone may be more important to tumor growth than the bone itself. In conclusion, resident macrophages play a pivotal role in prostate cancer growth in bone.

Role of bone marrow macrophages in controlling homeostasis and repair in bone and bone marrow niches

Macrophages, named for their phagocytic ability, participate in homeostasis, tissue regeneration and inflammatory responses. Bone and adjacent marrow contain multiple functionally unique resident tissue macrophage subsets which maintain and regulate anatomically distinct niche environments within these interconnected tissues. Three subsets of bone-bone marrow resident tissue macrophages have been characterised; erythroblastic island macrophages, haematopoietic stem cell niche macrophages and osteal macrophages. The role of these macrophages in controlling homeostasis and repair in bone and bone marrow niches is reviewed in detail.

Neonatal Pulmonary Macrophage Depletion Coupled to Defective Mucus Clearance Increases Susceptibility to Pneumonia and Alters Pulmonary Immune Responses

Resident immune cells (e.g., macrophages [MΦs]) and airway mucus clearance both contribute to a healthy lung environment. To investigate interactions between pulmonary MΦ function and defective mucus clearance, a genetic model of lysozyme M (LysM) promoter-mediated MΦ depletion was generated, characterized, and crossed with the sodium channel β subunit transgenic (Scnn1b-Tg) mouse model of defective mucus clearance. Diphtheria toxin A-mediated depletion of LysM(+) pulmonary MΦs in wild-type mice with normal mucus clearance resulted in lethal pneumonia in 24% of neonates. The pneumonias were dominated by Pasteurella pneumotropica and accompanied by emaciation, neutrophilic inflammation, and elevated Th1 cytokines. The incidence of emaciation and pneumonia reached 51% when LysM(+) MΦ depletion was superimposed on the airway mucus clearance defect of Scnn1b-Tg mice. In LysM(+) MΦ-depleted Scnn1b-Tg mice, pneumonias were associated with a broader spectrum of bacterial species and a significant reduction in airway mucus plugging. Bacterial burden (CFUs) was comparable between Scnn1b-Tg and nonpneumonic LysM(+) MΦ-depleted Scnn1b-Tg mice. However, the nonpneumonic LysM(+) MΦ-depleted Scnn1b-Tg mice exhibited increased airway inflammation, the presence of neutrophilic infiltration, and increased levels of inflammatory cytokines in bronchoalveolar lavage fluid compared with Scnn1b-Tg mice. Collectively, these data identify key MΦ-mucus clearance interactions with respect to both infectious and inflammatory components of muco-obstructive lung disease.

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.

Resident corneal c-fms(+) macrophages and dendritic cells mediate early cellular infiltration in adenovirus keratitis

The cornea contains a heterogeneous population of antigen-presenting cells with the capacity to contribute to immune responses. Adenovirus keratitis is a severe corneal infection with acute and chronic phases. The role of resident corneal antigen-presenting cells in adenovirus keratitis has not been studied. We utilized transgenic MaFIA mice in which c-fms expressing macrophages and dendritic cells can be induced to undergo apoptosis, in a mouse model of adenovirus keratitis. Clinical keratitis and recruitment of myeloperoxidase and CD45(+) cells were diminished in c-fms depleted, adenovirus infected mice, as compared to controls, consistent with a role for myeloid-lineage cells in adenovirus keratitis.

Endothelial Cells Use a Formin-Dependent Phagocytosis-Like Process to Internalize the Bacterium Listeria monocytogenes

Vascular endothelial cells act as gatekeepers that protect underlying tissue from blood-borne toxins and pathogens. Nevertheless, endothelial cells are able to internalize large fibrin clots and apoptotic debris from the bloodstream, although the precise mechanism of such phagocytosis-like uptake is unknown. We show that cultured primary human endothelial cells (HUVEC) internalize both pathogenic and non-pathogenic Listeria bacteria comparably, in a phagocytosis-like process. In contrast with previously studied host cell types, including intestinal epithelial cells and hepatocytes, we find that endothelial internalization of Listeria is independent of all known pathogenic bacterial surface proteins. Consequently, we exploited the internalization and intracellular replication of L. monocytogenes to identify distinct host cell factors that regulate phagocytosis-like uptake in HUVEC. Using siRNA screening and subsequent genetic and pharmacologic perturbations, we determined that endothelial infectivity was modulated by cytoskeletal proteins that normally modulate global architectural changes, including phosphoinositide-3-kinase, focal adhesions, and the small GTPase Rho. We found that Rho kinase (ROCK) is acutely necessary for adhesion of Listeria to endothelial cells, whereas the actin-nucleating formins FHOD1 and FMNL3 specifically regulate internalization of bacteria as well as inert beads, demonstrating that formins regulate endothelial phagocytosis-like uptake independent of the specific cargo. Finally, we found that neither ROCK nor formins were required for macrophage phagocytosis of L. monocytogenes, suggesting that endothelial cells have distinct requirements for bacterial internalization from those of classical professional phagocytes. Our results identify a novel pathway for L. monocytogenes uptake by human host cells, indicating that this wily pathogen can invade a variety of tissues by using a surprisingly diverse suite of distinct uptake mechanisms that operate differentially in different host cell types.

Validation of a device for the active manipulation of the tumor microenvironment during intravital imaging

The tumor microenvironment is recognized as playing a significant role in the behavior of tumor cells and their progression to metastasis. However, tools to manipulate the tumor microenvironment directly, and image the consequences of this manipulation with single cell resolution in real time in vivo, are lacking. We describe here a method for the direct, local manipulation of microenvironmental parameters through the use of an implantable Induction Nano Intravital Device (iNANIVID) and simultaneous in vivo visualization of the results at single-cell resolution. As a proof of concept, we deliver both a sustained dose of EGF to tumor cells while intravital imaging their chemotactic response as well as locally induce hypoxia in defined microenvironments in solid tumors.

Mechanical Stabilization of Mouse Carotid Artery for In Vivo Intravital Microscopy Imaging of Atherogenesis

We present here a procedure that allows real-time high-resolution multichannel imaging of early atherosclerotic lesions of live mice, by dramatically reducing the respiratory and pulsatile movements of the athero-susceptible carotid artery, without significantly altering blood flow dynamics. This surgical preparation can be combined with the use of various fluorescent probes and reporter mice to simultaneously visualize the dynamics of inflammatory leukocytes, platelets, or even subcellular structures. Stabilization of the tissue renders it suitable for two-photon laser scanning microscopic imaging and allows tracking the behavior of inflammatory cells in three dimensions.

Emerging Roles for CSF-1 Receptor and its Ligands in the Nervous System

The colony-stimulating factor-1 receptor (CSF-1R) kinase regulates tissue macrophage homeostasis, osteoclastogenesis, and Paneth cell development. However, recent studies in mice have revealed that CSF-1R signaling directly controls the development and maintenance of microglia, and cell autonomously regulates neuronal differentiation and survival. While the CSF-1R-cognate ligands, CSF-1 and interleukin-34 (IL-34) compete for binding to the CSF-1R, they are expressed in a largely non-overlapping manner by mature neurons. The recent identification of a dominantly inherited, adult-onset, progressive dementia associated with inactivating mutations in the CSF-1R highlights the importance of CSF-1R signaling in the brain. We review the roles of the CSF-1R and its ligands in microglial and neural development and function, and their relevance to our understanding of neurodegenerative disease.

Targeting Macrophages Sensitizes Chronic Lymphocytic Leukemia to Apoptosis and Inhibits Disease Progression

The role of monocytes/macrophages in the development and progression of chronic lymphocytic leukemia (CLL) is poorly understood. Transcriptomic analyses show that monocytes/macrophages and leukemic cells cross talk during CLL progression. Macrophage depletion impairs CLL engraftment, drastically reduces leukemic growth, and favorably impacts mouse survival. Targeting of macrophages by either CSF1R signaling blockade or clodrolip-mediated cell killing has marked inhibitory effects on established leukemia also. Macrophage killing induces leukemic cell death mainly via the TNF pathway and reprograms the tumor microenvironment toward an antitumoral phenotype. CSF1R inhibition reduces leukemic cell load, especially in the bone marrow, and increases circulating CD20(+) leukemic cells. Accordingly, co-targeting TAMs and CD20-expressing leukemic cells provides a survival benefit in the mice. These results establish the important role of macrophages in CLL and suggest therapeutic strategies based on interfering with leukemia-macrophage interactions.

Occlusive lung arterial lesions in endothelial-targeted, fas-induced apoptosis transgenic mice

Pulmonary arterial hypertension (PAH) is a lethal disease that is characterized by functional and structural abnormalities involving distal pulmonary arterioles that result in increased pulmonary vascular resistance and ultimately right heart failure. In experimental models of pulmonary hypertension, endothelial cell (EC) apoptosis is a necessary trigger for the development of obliterative lung arteriopathy, inducing the emergence of hyperproliferative and apoptosis-resistant vascular cells. However, it has not been established whether EC apoptosis is sufficient for the induction of complex lung arteriolar lesions. We generated a conditional transgenic system in mice to test the hypothesis that lung endothelial cell apoptosis is sufficient to induce a PAH phenotype. The Fas-induced apoptosis (FIA) construct was expressed under the control of endothelial-specific Tie2 promoter (i.e., EFIA mice), and administration of a small molecule dimerizing agent, AP20187, resulted in modest pulmonary hypertension, which was associated with obliterative vascular lesions localized to distal lung arterioles in a proportion of transgenic mice. These lesions were characterized by proliferating cells, predominantly CD68 macrophages. Although endothelial cell apoptosis was also seen in the kidney, evidence of subsequent arteriopathy was seen only in the lung. This model provides direct evidence that lung endothelial cell apoptosis acts as a trigger to initiate a PAH phenotype and provides initial insight into the potential mechanisms that underlie a lung-specific arterial response to endothelial injury.

Macrophage Ablation Reduces M2-Like Populations and Jeopardizes Tumor Growth in a MAFIA-Based Glioma Model

Monocytes/macrophages are an influential component of the glioma microenvironment. However, understanding their diversity and plasticity constitute one of the most challenging areas of research due to the paucity of models to study these cells' inherent complexity. Herein, we analyzed the role of monocytes/macrophages in glioma growth by using a transgenic model that allows for conditional ablation of this cell population. We modeled glioma using intracranial GL261-bearing CSF-1R–GFP⁺ macrophage Fas-induced apoptosis (MAFIA) transgenic mice. Conditional macrophage ablation was achieved by exposure to the dimerizer AP20187. Double immunofluorescence was used to characterize M1- and M2-like monocytes/macrophages during tumor growth and after conditional ablation. During glioma growth, the monocyte/macrophage population consisted predominantly of M2 macrophages. Conditional temporal depletion of macrophages reduced the number of GFP⁺ cells, targeting mainly the repopulation of M2-polarized cells, and altered the appearance of M1-like monocytes/macrophages, which suggested a shift in the M1/M2 macrophage balance. Of interest, compared with control-treated mice, macrophage-depleted mice had a lower tumor mitotic index, microvascular density, and reduced tumor growth. These results demonstrated the possibility of studying in vivo the role and phenotype of macrophages in gliomas and suggested that transitory depletion of CSF-1R⁺ population influences the reconstitutive phenotypic pool of these cells, ultimately suppressing tumor growth. The MAFIA model provides a much needed advance in defining the role of macrophages in gliomas.

OsteoMacs: Key players around bone biomaterials

Osteal macrophages (OsteoMacs) are a special subtype of macrophage residing in bony tissues. Interesting findings from basic research have pointed to their vast and substantial roles in bone biology by demonstrating their key function in bone formation and remodeling. Despite these essential findings, much less information is available concerning their response to a variety of biomaterials used for bone regeneration with the majority of investigation primarily focused on their role during the foreign body reaction. With respect to biomaterials, it is well known that cells derived from the monocyte/macrophage lineage are one of the first cell types in contact with implanted biomaterials. Here they demonstrate extremely plastic phenotypes with the ability to differentiate towards classical M1 or M2 macrophages, or subsequently fuse into osteoclasts or multinucleated giant cells (MNGCs). These MNGCs have previously been characterized as foreign body giant cells and associated with biomaterial rejection, however more recently their phenotypes have been implicated with wound healing and tissue regeneration by studies demonstrating their expression of key M2 markers around biomaterials. With such contrasting hypotheses, it becomes essential to better understand their roles to improve the development of osteo-compatible and osteo-promotive biomaterials. This review article expresses the necessity to further study OsteoMacs and MNGCs to understand their function in bone biomaterial tissue integration including dental/orthopedic implants and bone grafting materials.

In vivo depletion and genetic targeting of mouse intestinal CX3CR1(+) mononuclear phagocytes

Mononuclear phagocytes (MPs) are an essential component of the intestinal immune system. They are comprised of a few dendritic cell and macrophage subsets, all with the common ability to sample extracellular milieu and to discriminate between dangerous and innocuous signals. Despite the commonality, each MP subset acquires distinct developmental pathways and unique functions, likely to fulfill needs of the tissue in which they reside. Some MP subsets develop from monocytes and are distinguished by their expression of CX3C-chemokine receptor 1 (CX3CR1). This manuscript summarizes our expertise in vivo targeting of intestinal CX3CR1(+) MP subsets. The described tools might be useful for studies of CX3CR1(+) MP function in various murine experimental models, particularly under non-inflammatory conditions.

Injured sensory neuron-derived CSF1 induces microglial proliferation and DAP12-dependent pain

Although microglia are implicated in nerve injury-induced neuropathic pain, how injured sensory neurons engage microglia is unclear. Here we demonstrate that peripheral nerve injury induces de novo expression of colony-stimulating factor 1 (CSF1) in injured sensory neurons. The CSF1 is transported to the spinal cord where it targets the microglial CSF1 receptor (CSF1R). Cre-mediated sensory neuron deletion of Csf1 completely prevented nerve injury-induced mechanical hypersensitivity and reduced microglia activation and proliferation. In contrast, intrathecal injection of CSF1 induces mechanical hypersensitivity and microglial proliferation. Nerve injury also upregulated CSF1 in motoneurons, where it is required for ventral horn microglial activation and proliferation. Downstream of CSF1R, we found that the microglial membrane adapter protein DAP12 is required for both nerve injury- and intrathecal CSF1-induced upregulation of pain-related microglial genes and the ensuing pain, but not for microglia proliferation. Thus, both CSF1 and DAP12 are potential targets for the pharmacotherapy of neuropathic pain.

Macrophages: Their Emerging Roles in Bone

Macrophages are present in nearly all tissues and are critical for development, homeostasis, and regeneration. Resident tissue macrophages of bone, termed osteal macrophages, are recently classified myeloid cells that are distinct from osteoclasts. Osteal macrophages are located immediately adjacent to osteoblasts, regulate bone formation, and play diverse roles in skeletal homeostasis. Genetic or pharmacological modulation of macrophages in vivo results in significant bone phenotypes, and these phenotypes depend on which macrophage subsets are altered. Macrophages are also key mediators of osseous wound healing and fracture repair, with distinct roles at various stages of the repair process. A central function of macrophages is their phagocytic ability. Each day, billions of cells die in the body and efferocytosis (phagocytosis of apoptotic cells) is a critical process in both clearing dead cells and recruitment of replacement progenitor cells to maintain homeostasis. Recent data suggest a role for efferocytosis in bone biology and these new mechanisms are outlined. Finally, although macrophages have an established role in primary tumors, emerging evidence suggests that macrophages in bone support cancers which preferentially metastasize to the skeleton. Collectively, this developing area of osteoimmunology raises new questions and promises to provide novel insights into pathophysiologic conditions as well as therapeutic and regenerative approaches vital for skeletal health.

Oxysterols and EBI2 promote osteoclast precursor migration to bone surfaces and regulate bone mass homeostasis

The mechanisms guiding cells toward bone surfaces are generally unknown. Here, Nevius et al. show that the Gαi protein–coupled receptor EBI2 is expressed in mouse osteoclast precursors to guide these cells toward bone surfaces. Defective EBI2 signaling increased bone mass in male mice and protected female mice from age- and estrogen deficiency–induced osteoporosis.

Bone surfaces attract hematopoietic and nonhematopoietic cells, such as osteoclasts (OCs) and osteoblasts (OBs), and are targeted by bone metastatic cancers. However, the mechanisms guiding cells toward bone surfaces are essentially unknown. Here, we show that the Gαi protein–coupled receptor (GPCR) EBI2 is expressed in mouse monocyte/OC precursors (OCPs) and its oxysterol ligand 7α,25-dihydroxycholesterol (7α,25-OHC) is secreted abundantly by OBs. Using in vitro time-lapse microscopy and intravital two-photon microscopy, we show that EBI2 enhances the development of large OCs by promoting OCP motility, thus facilitating cell–cell interactions and fusion in vitro and in vivo. EBI2 is also necessary and sufficient for guiding OCPs toward bone surfaces. Interestingly, OCPs also secrete 7α,25-OHC, which promotes autocrine EBI2 signaling and reduces OCP migration toward bone surfaces in vivo. Defective EBI2 signaling led to increased bone mass in male mice and protected female mice from age- and estrogen deficiency–induced osteoporosis. This study identifies a novel pathway involved in OCP homing to the bone surface that may have significant therapeutic potential.

Systemic and Cardiac Depletion of M2 Macrophage through CSF-1R Signaling Inhibition Alters Cardiac Function Post Myocardial Infarction

The heart hosts tissue resident macrophages which are capable of modulating cardiac inflammation and function by multiple mechanisms. At present, the consequences of phenotypic diversity in macrophages in the heart are incompletely understood. The contribution of cardiac M2-polarized macrophages to the resolution of inflammation and repair response following myocardial infarction remains to be fully defined. In this study, the role of M2 macrophages was investigated utilising a specific CSF-1 receptor signalling inhibition strategy to achieve their depletion. In mice, oral administration of GW2580, a CSF-1R kinase inhibitor, induced significant decreases in Gr1^lo and F4/80^hi monocyte populations in the circulation and the spleen. GW2580 administration also induced a significant depletion of M2 macrophages in the heart after 1 week treatment as well as a reduction of cardiac arginase1 and CD206 gene expression indicative of M2 macrophage activity. In a murine myocardial infarction model, reduced M2 macrophage content was associated with increased M1-related gene expression (IL-6 and IL-1β), and decreased M2-related gene expression (Arginase1 and CD206) in the heart of GW2580-treated animals versus vehicle-treated controls. M2 depletion was also associated with a loss in left ventricular contractile function, infarct enlargement, decreased collagen staining and increased inflammatory cell infiltration into the infarct zone, specifically neutrophils and M1 macrophages. Taken together, these data indicate that CSF-1R signalling is critical for maintaining cardiac tissue resident M2-polarized macrophage population, which is required for the resolution of inflammation post myocardial infarction and, in turn, for preservation of ventricular function.

Real-Time Imaging Reveals Local, Transient Vascular Permeability, and Tumor Cell Intravasation Stimulated by TIE2hi Macrophage-Derived VEGFA

Dissemination of tumor cells is an essential step in metastasis. Direct contact between a macrophage, mammalian-enabled (MENA)-overexpressing tumor cell, and endothelial cell [Tumor MicroEnvironment of Metastasis (TMEM)] correlates with metastasis in breast cancer patients. Here we show, using intravital high-resolution two-photon microscopy, that transient vascular permeability and tumor cell intravasation occur simultaneously and exclusively at TMEM. The hyperpermeable nature of tumor vasculature is described as spatially and temporally heterogeneous. Using real-time imaging, we observed that vascular permeability is transient, restricted to the TMEM, and required for tumor cell dissemination. VEGFA signaling from TIE2(hi) TMEM macrophages causes local loss of vascular junctions, transient vascular permeability, and tumor cell intravasation, demonstrating a role for the TMEM within the primary mammary tumor. These data provide insight into the mechanism of tumor cell intravasation and vascular permeability in breast cancer, explaining the value of TMEM density as a predictor of distant metastatic recurrence in patients.

SIGNIFICANCE

Tumor vasculature is abnormal with increased permeability. Here, we show that VEGFA signaling from TIE2(hi) TMEM macrophages results in local, transient vascular permeability and tumor cell intravasation. These data provide evidence for the mechanism underlying the association of TMEM with distant metastatic recurrence, offering a rationale for therapies targeting TMEM.

Immunomodulatory Peptide IDR-1018 Decreases Implant Infection and Preserves Osseointegration

BACKGROUND

Innate defense regulator peptide-1018 (IDR-1018) is a 12-amino acid, synthetic, immunomodulatory host defense peptide that can reduce soft tissue infections and is less likely to induce bacterial resistance than conventional antibiotics. However, IDRs have not been tested on orthopaedic infections and the immunomodulatory effects of IDR-1018 have only been characterized in response to lipopolysacharide, which is exclusively produced by Gram-negative bacteria.

QUESTIONS/PURPOSES

We sought (1) to more fully characterize the immunomodulatory effects of IDR-1018, especially in response to Staphylococcus aureus; and (2) to determine whether IDR-1018 decreases S aureus infection of orthopaedic implants in mice and thereby protects the implants from failure to osseointegrate.

METHODS

In vitro effects of IDR-1018 on S aureus were assessed by determining minimum inhibitory concentrations in bacterial broth without and with supplementation of physiologic ion levels. In vitro effects of IDR-1018 on macrophages were determined by measuring production of monocyte chemoattractant protein-1 (MCP-1) and proinflammatory cytokines by enzyme-linked immunosorbent assay. In vivo effects of IDR-1018 were determined in a murine model of S aureus implant infection by quantitating bacterial burden, macrophage recruitment, MCP-1, proinflammatory cytokines, and osseointegration in nine mice per group on Day 1 postimplantation and 20 mice per group on Day 15 postimplantation.

RESULTS

IDR-1018 demonstrated antimicrobial activity by directly killing S aureus even in the presence of physiologic ion levels, increasing recruitment of macrophages to the site of infections by 40% (p = 0.036) and accelerating S aureus clearance in vivo (p = 0.008) with a 2.6-fold decrease in bacterial bioburden on Day 7 postimplantation. In vitro immunomodulatory activity of IDR-1018 included inducing production of MCP-1 in the absence of other inflammatory stimuli and to potently blunt excess production of proinflammatory cytokines and MCP-1 induced by lipopolysaccharide. Higher concentrations of IDR-1018 were required to blunt production of proinflammatory cytokines and MCP-1 in the presence S aureus. The largest in vivo immunomodulatory effect of IDR-1018 was to reduce tumor necrosis factor-α levels induced by S aureus by 60% (p = 0.006). Most importantly, IDR-1018 reduced S aureus-induced failures of osseointegration by threefold (p = 0.022) and increased osseointegration as measured by ultimate force (5.4-fold, p = 0.033) and average stiffness (4.3-fold, p = 0.049).

CONCLUSIONS

IDR-1018 is potentially useful to reduce orthopaedic infections by directly killing bacteria and by recruiting macrophages to the infection site.

CLINICAL RELEVANCE

These findings make IDR-1018 an attractive candidate to explore in larger animal models to ascertain whether its effects in our in vitro and mouse experiments can be replicated in more clinically relevant settings.

DC-SIGN(+) Macrophages Control the Induction of Transplantation Tolerance

Tissue effector cells of the monocyte lineage can differentiate into different cell types with specific cell function depending on their environment. The phenotype, developmental requirements, and functional mechanisms of immune protective macrophages that mediate the induction of transplantation tolerance remain elusive. Here, we demonstrate that costimulatory blockade favored accumulation of DC-SIGN-expressing macrophages that inhibited CD8(+) T cell immunity and promoted CD4(+)Foxp3(+) Treg cell expansion in numbers. Mechanistically, that simultaneous DC-SIGN engagement by fucosylated ligands and TLR4 signaling was required for production of immunoregulatory IL-10 associated with prolonged allograft survival. Deletion of DC-SIGN-expressing macrophages in vivo, interfering with their CSF1-dependent development, or preventing the DC-SIGN signaling pathway abrogated tolerance. Together, the results provide new insights into the tolerogenic effects of costimulatory blockade and identify DC-SIGN(+) suppressive macrophages as crucial mediators of immunological tolerance with the concomitant therapeutic implications in the clinic.

Macrophages promote osteoblastic differentiation in-vivo: implications in fracture repair and bone homeostasis

Macrophages are activated in inflammation and during early phases of repair processes. Interestingly, they are also present in bone during development, but their function during this process is unclear. Here, we explore the function of macrophages in bone development, growth, and repair using transgenic mice to constitutively or conditionally deplete macrophages. Depletion of macrophages led to early skeletal growth retardation and progressive osteoporosis. By 3 months of age, macrophage-deficient mice displayed a 25% reduction in bone mineral density and a 70% reduction in the number of trabecular bone compared to control littermates. Despite depletion of macrophages, functional osteoclasts were still present in bones, lining trabecular bone and the endosteal surface of the cortical bone. Furthermore, ablation of macrophages led to a 60% reduction in the number of bone marrow mesenchymal progenitor cells and a decrease in the ability of these cells to differentiate to osteoblasts. When macrophages were depleted during fracture repair, bone union was impaired. Calluses from macrophage-deficient animals were smaller, and contained less bone and more fibrotic tissue deposition. Taken together, this shows that macrophages are crucial for maintaining bone homeostasis and promoting fracture repair by enhancing the differentiation of mesenchymal progenitors.

Size- and shape-dependent foreign body immune response to materials implanted in rodents and non-human primates

The efficacy of implanted biomedical devices is often compromised by host recognition and subsequent foreign body responses. Here, we demonstrate the role of the geometry of implanted materials on their biocompatibility in vivo. In rodent and non-human primate animal models, implanted spheres 1.5 mm and above in diameter across a broad spectrum of materials, including hydrogels, ceramics, metals and plastics, significantly abrogated foreign body reactions and fibrosis when compared with smaller spheres. We also show that for encapsulated rat pancreatic islet cells transplanted into streptozotocin-treated diabetic C57BL/6 mice, islets prepared in 1.5-mm alginate capsules were able to restore blood-glucose control for up to 180 days, a period more than five times longer than for transplanted grafts encapsulated within conventionally sized 0.5-mm alginate capsules. Our findings suggest that the in vivo biocompatibility of biomedical devices can be significantly improved simply by tuning their spherical dimensions.

Hyaluronan in the healthy and malignant hematopoietic microenvironment

The fate of both endogenous and transplanted stem cells is dependent on the functional status of the regulatory local microenvironment, which is compromised by disease and therapeutic intervention. The glycosaminoglycan hyaluronan (HA) is a critical component of the hematopoietic microenvironment. We summarize recent advances in our understanding of the role of HA in regulating mesenchymal stem cells, osteoblasts, fibroblasts, macrophages, and endothelium in bone marrow (BM) and their crosstalk within the hematopoietic microenvironment. HA not only determines the volume, hydration, and microfluidics of the BM interstitial space, but also, via interactions with specific receptors, regulates multiple cell functions including differentiation, migration, and production of regulatory factors. The effects of HA are dependent on the polymer size and are influenced by the formation of complexes with other molecules. In healthy BM, HA synthases and hyaluronidases form a molecular network that maintains extracellular HA levels within a discrete physiological window, but HA homeostasis is often perturbed in pathological conditions, including hematological malignancies. Recent studies have suggested that HA synthases may have functions beyond HA production and contribute to the intracellular regulatory machinery. We discuss a possible role for HA synthases, intracellular and extracellular HA in the malignant BM microenvironment, and resistance to therapy.

Essential role of PU.1 in maintenance of mixed lineage leukemia-associated leukemic stem cells

Acute myeloid leukemia is a clonal malignant disorder derived from a small number of leukemic stem cells (LSCs). Rearrangements of the mixed lineage leukemia (MLL) gene are found in acute myeloid leukemia associated with poor prognosis. The upregulation of Hox genes is critical for LSC induction and maintenance, but is unlikely to support malignancy and the high LSC frequency observed in MLL leukemias. The present study shows that MLL fusion proteins interact with the transcription factor PU.1 to activate the transcription of CSF-1R, which is critical for LSC activity. Acute myeloid leukemia is cured by either deletion of PU.1 or ablation of cells expressing CSF-1R. Kinase inhibitors specific for CSF-1R prolong survival time. These findings indicate that PU.1-mediated upregulation of CSF-1R is a critical effector of MLL leukemogenesis.

Commensal-dendritic-cell interaction specifies a unique protective skin immune signature

The skin represents the primary interface between the host and the environment. This organ is also home to trillions of microorganisms that play an important role in tissue homeostasis and local immunity. Skin microbial communities are highly diverse and can be remodelled over time or in response to environmental challenges. How, in the context of this complexity, individual commensal microorganisms may differentially modulate skin immunity and the consequences of these responses for tissue physiology remains unclear. Here we show that defined commensals dominantly affect skin immunity and identify the cellular mediators involved in this specification. In particular, colonization with Staphylococcus epidermidis induces IL-17A(+) CD8(+) T cells that home to the epidermis, enhance innate barrier immunity and limit pathogen invasion. Commensal-specific T-cell responses result from the coordinated action of skin-resident dendritic cell subsets and are not associated with inflammation, revealing that tissue-resident cells are poised to sense and respond to alterations in microbial communities. This interaction may represent an evolutionary means by which the skin immune system uses fluctuating commensal signals to calibrate barrier immunity and provide heterologous protection against invasive pathogens. These findings reveal that the skin immune landscape is a highly dynamic environment that can be rapidly and specifically remodelled by encounters with defined commensals, findings that have profound implications for our understanding of tissue-specific immunity and pathologies.

Macrophage targeted theranostics as personalized nanomedicine strategies for inflammatory diseases

Inflammatory disease management poses challenges due to the complexity of inflammation and inherent patient variability, thereby necessitating patient-specific therapeutic interventions. Theranostics, which integrate therapeutic and imaging functionalities, can be used for simultaneous imaging and treatment of inflammatory diseases. Theranostics could facilitate assessment of safety, toxicity and real-time therapeutic efficacy leading to personalized treatment strategies. Macrophages are an important cellular component of inflammatory diseases, participating in varied roles of disease exacerbation and resolution. The inherent phagocytic nature, abundance and disease homing properties of macrophages can be targeted for imaging and therapeutic purposes. This review discusses the utility of theranostics in macrophage ablation, phenotype modulation and inhibition of their inflammatory activity leading to resolution of inflammation in several diseases.

Liver-resident macrophage necroptosis orchestrates type 1 microbicidal inflammation and type-2-mediated tissue repair during bacterial infection

Kupffer cells, the phagocytes of fetal origin that line the liver sinusoids, are key contributors of host defense against enteroinvasive bacteria. Here, we found that infection by Listeria monocytogenes induced the early necroptotic death of Kupffer cells, which was followed by monocyte recruitment and an anti-bacterial type 1 inflammatory response. Kupffer cell death also triggered a type 2 response that involved the hepatocyte-derived alarmin interleukin-33 (IL-33) and basophil-derived interleukin-4 (IL-4). This led to the alternative activation of the monocyte-derived macrophages recruited to the liver, which thereby replaced ablated Kupffer cells and restored liver homeostasis. Kupffer cell death is therefore a key signal orchestrating type 1 microbicidal inflammation and type-2-mediated liver repair upon infection. This indicates that beyond the classical dichotomy of type 1 and type 2 responses, these responses can develop sequentially in the context of a bacterial infection and act interdependently, orchestrating liver immune responses and return to homeostasis, respectively.

Visualization of bone marrow monocyte mobilization using Cx3cr1gfp/+Flt3L-/- reporter mouse by multiphoton intravital microscopy

Monocytes are innate immune cells that play critical roles in inflammation and immune defense. A better comprehension of how monocytes are mobilized and recruited is fundamental to understand their biologic role in disease and steady state. The BM represents a major "checkpoint" for monocyte homeostasis, as it is the primary site for their production and release. Our study determined that the Cx3cr1(gfp/+) mouse strain is currently the most ideal model for the visualization of monocyte behavior in the BM by multiphoton intravital microscopy. However, we observed that DCs are also labeled with high levels of GFP and thus, interfere with the accuracy of monocyte tracking in vivo. Hence, we generated a Cx3cr1(gfp/+)Flt3L(-/-) reporter mouse and showed that whereas monocyte numbers were not affected, DC numbers were reduced significantly, as DCs but not monocytes depend on Flt3 signaling for their development. We thus verified that mobilization of monocytes from the BM in Cx3cr1(gfp/+)Flt3L(-/-) mice is intact in response to LPS. Collectively, our study demonstrates that the Cx3cr1(gfp/+)Flt3L(-/-) reporter mouse model represents a powerful tool to visualize monocyte activities in BM and illustrates the potential of a Cx3cr1(gfp/+)-based, multifunctionality fluorescence reporter approach to dissect monocyte function in vivo.

Impaired eukaryotic translation initiation factor 2B activity specifically in oligodendrocytes reproduces the pathology of vanishing white matter disease in mice

Vanishing white matter disease (VWMD) is an inherited autosomal-recessive hypomyelinating disease caused by mutations in eukaryotic translation initiation factor 2B (eIF2B). eIF2B mutations predominantly affect the brain white matter, and the characteristic features of VWMD pathology include myelin loss and foamy oligodendrocytes. Activation of pancreatic endoplasmic reticulum kinase (PERK) has been observed in oligodendrocytes in VWMD. PERK activation in response to endoplasmic reticulum stress attenuates eIF2B activity by phosphorylating eIF2α, suggesting that impaired eIF2B activity in oligodendrocytes induced by VWMD mutations or PERK activation exploit similar mechanisms to promote selective white matter pathology in VWMD. Using transgenic mice that allow for temporally controlled activation of PERK specifically in oligodendrocytes, we discovered that strong PERK activation in oligodendrocytes during development suppressed eIF2B activity and reproduced the characteristic features of VWMD in mice, including hypomyelinating phenotype, foamy oligodendrocytes, and myelin loss. Notably, impaired eIF2B activity induced by PERK activation in oligodendrocytes of fully myelinated adult mice had minimal effects on morphology or function. Our observations point to a cell-autonomous role of impaired eIF2B activity in myelinating oligodendrocytes in the pathogenesis of VWMD.

Caspase-3 mediates the pathogenic effect of Yersinia pestis YopM in liver of C57BL/6 mice and contributes to YopM's function in spleen

The virulence protein YopM of the plague bacterium Yersinia pestis has different dominant effects in liver and spleen. Previous studies focused on spleen, where YopM inhibits accumulation of inflammatory dendritic cells. In the present study we focused on liver, where PMN function may be directly undermined by YopM without changes in inflammatory cell numbers in the initial days of infection, and foci of inflammation are easily identified. Mice were infected with parent and ΔyopM-1 Y. pestis KIM5, and effects of YopM were assessed by immunohistochemistry and determinations of bacterial viable numbers in organs. The bacteria were found associated with myeloid cells in foci of inflammation and in liver sinusoids. A new in-vivo phenotype of YopM was revealed: death of inflammatory cells, evidenced by TUNEL staining beginning at d 1 of infection. Based on distributions of Ly6G⁺, F4/80⁺, and iNOS⁺ cells within foci, the cells that were killed could have included both PMNs and macrophages. By 2 d post-infection, YopM had no effect on distribution of these cells, but by 3 d cellular decomposition had outstripped acute inflammation in foci due to parent Y. pestis, while foci due to the ΔyopM-1 strain still contained many inflammatory cells. The destruction depended on the presence of both PMNs in the mice and YopM in the bacteria. In mice that lacked the apoptosis mediator caspase-3 the infection dynamics were novel: the parent Y. pestis was limited in growth comparably to the ΔyopM-1 strain in liver, and in spleen a partial growth limitation for parent Y. pestis was seen. This result identified caspase-3 as a co-factor or effector in YopM's action and supports the hypothesis that in liver YopM's main pathogenic effect is mediated by caspase-3 to cause apoptosis of PMNs.

Fracture healing via periosteal callus formation requires macrophages for both initiation and progression of early endochondral ossification

The distribution, phenotype, and requirement of macrophages for fracture-associated inflammation and/or early anabolic progression during endochondral callus formation were investigated. A murine femoral fracture model [internally fixed using a flexible plate (MouseFix)] was used to facilitate reproducible fracture reduction. IHC demonstrated that inflammatory macrophages (F4/80(+)Mac-2(+)) were localized with initiating chondrification centers and persisted within granulation tissue at the expanding soft callus front. They were also associated with key events during soft-to-hard callus transition. Resident macrophages (F4/80(+)Mac-2(neg)), including osteal macrophages, predominated in the maturing hard callus. Macrophage Fas-induced apoptosis transgenic mice were used to induce macrophage depletion in vivo in the femoral fracture model. Callus formation was completely abolished when macrophage depletion was initiated at the time of surgery and was significantly reduced when depletion was delayed to coincide with initiation of early anabolic phase. Treatment initiating 5 days after fracture with the pro-macrophage cytokine colony stimulating factor-1 significantly enhanced soft callus formation. The data support that inflammatory macrophages were required for initiation of fracture repair, whereas both inflammatory and resident macrophages promoted anabolic mechanisms during endochondral callus formation. Overall, macrophages make substantive and prolonged contributions to fracture healing and can be targeted as a therapeutic approach for enhancing repair mechanisms. Thus, macrophages represent a viable target for the development of pro-anabolic fracture treatments with a potentially broad therapeutic window.

Prolonged antigen presentation following an acute virus infection requires direct and then cross-presentation

Antiviral CD8(+) T cell recognition of MHC class I-peptide complexes on the surface of professional APCs is a requisite step in an effective immune response following many potentially lethal infections. Although MHC class I-peptide production is thought to be closely linked to the continued presence of virus, several studies have shown that the persistence of Ag presentation occurs for an extended period of time following the clearance of RNA viruses. However, the mechanism responsible for Ag presentation persistence following viral clearance was unknown until now. In this study, we used a recombinant DNA virus expressing different forms of a model Ag to study the mechanism of prolonged Ag presentation in mice. We determined that the persistence of Ag presentation consists of three distinct mechanistic phases, as follows: ongoing viral replication, persistence of virally infected cells, and cross-presentation of Ag. These data will allow manipulation of the form of Ag contained within viral vectors to produce the most effective and protective CD8(+) T cell response to be generated following vaccination.

IκB kinase activity drives fetal lung macrophage maturation along a non-M1/M2 paradigm

In preterm infants, exposure to inflammation increases the risk of bronchopulmonary dysplasia, a chronic, developmental lung disease. Although macrophages are the key cells that initiate lung inflammation, less is known about lung macrophage phenotype and maturation. We hypothesized that fetal lung macrophages mature into distinct subpopulations during mouse development, and that activation could influence macrophage maturation. Expression of the fetal macrophage markers CD68, CD86, CD206, Ym1, fibrinogen-like protein 2, and indolamine-2, 3-dioxygenase was developmentally regulated, with each marker having different temporal patterns. Flow cytometry analysis showed macrophages within the fetal lung were less diverse than the distinctly separate subpopulations in newborn and adult lungs. Similar to adult alveolar macrophages, fetal lung macrophages responded to the TLR4 agonist LPS and the alternative activation cytokines IL-4 and IL-13. Using a macrophage-specific constitutively active IκB Kinase transgenic model (IKFM), we demonstrated that macrophage activation increased proinflammatory gene expression and reduced the response of fetal lung macrophages to IL-4 and IL-13. Activation also increased fetal lung macrophage proliferation. Fetal IKFM lungs contained increased percentages of more mature, CD11b(low)F4/80(high) cells that also expressed higher levels of the alternative activation markers CD204 and CD206. Development of fetal lung macrophages into mature alveolar macrophages may therefore include features of both proinflammatory and alternative activation paradigms.

Macrophages in cardiac homeostasis, injury responses and progenitor cell mobilisation

Macrophages are an immune cell type found in every organ of the body. Classically, macrophages are recognised as housekeeping cells involved in the detection of foreign antigens and danger signatures, and the clearance of tissue debris. However, macrophages are increasingly recognised as a highly versatile cell type with a diverse range of functions that are important for tissue homeostasis and injury responses. Recent research findings suggest that macrophages contribute to tissue regeneration and may play a role in the activation and mobilisation of stem cells. This review describes recent advances in our understanding of the role played by macrophages in cardiac tissue maintenance and repair following injury. We examine the involvement of exogenous and resident tissue macrophages in cardiac inflammatory responses and their potential activity in regulating cardiac regeneration.

Vascular endothelial growth factor promotes fibrosis resolution and repair in mice

BACKGROUND & AIMS

Vascular endothelial growth factor (VEGF)-induced angiogenesis is implicated in fibrogenesis and portal hypertension. However, the function of VEGF in fibrosis resolution has not been explored.

METHODS

We developed a cholecystojejunostomy procedure to reconstruct biliary flow after bile duct ligation in C57BL/6 mice to generate a model of fibrosis resolution. These mice were then given injections of VEGF-neutralizing (mcr84) or control antibodies, and other mice received an adenovirus that expressed mouse VEGF or a control vector. The procedure was also performed on macrophage fas-induced apoptosis mice, in which macrophages can be selectively depleted. Liver and blood samples were collected and analyzed in immunohistochemical, morphometric, vascular permeability, real-time polymerase chain reaction, and flow cytometry assays.

RESULTS

VEGF-neutralizing antibodies prevented development of fibrosis but also disrupted hepatic tissue repair and fibrosis resolution. During fibrosis resolution, VEGF inhibition impaired liver sinusoidal permeability, which was associated with reduced monocyte migration, adhesion, and infiltration of fibrotic liver. Scar-associated macrophages contributed to this process by producing the chemokine (C-X-C motif) ligand 9 (CXCL9) and matrix metalloproteinase 13. Resolution of fibrosis was impaired in macrophage fas-induced apoptosis mice but increased after overexpression of CXCL9.

CONCLUSIONS

In a mouse model of liver fibrosis resolution, VEGF promoted fibrogenesis, but was also required for hepatic tissue repair and fibrosis resolution. We observed that VEGF regulates vascular permeability, monocyte infiltration, and scar-associated macrophages function.