Macrophage fusion induced by IL-4 alternative activation is a multistage process involving multiple target molecules

The Interleukin-13 Receptor-α1 Chain Is Essential for Induction of the Alternative Macrophage Activation Pathway by IL-13 but Not IL-4

Macrophages coexpress both the interleukin (IL)-2Rγ chain (γ(c)) and IL-13Rα1. These receptor chains can heterodimerize with IL-4Rα to form type I or type II IL-4 receptor complexes, respectively. We used macrophages derived from Il2rg and Il13ra1 knockout (KO) mice to evaluate the requirements for these receptor chains for induction of the alternative macrophage activation (AMA) pathway by IL-4 and IL-13. Absence of γ(c) significantly decreased activation of STAT6 by IL-4 but not IL-13. However, although activation of STAT6 by IL-4 was markedly reduced in γ(c) KO macrophages, it was not abolished, indicating that IL-4 can still signal through type II IL-4 receptors via the IL-13Rα1 chain. IL-13 failed to activate STAT6 in macrophages derived from Il13ra1 KO mice; however, these cells remained fully responsive to IL-4. The inability of IL-13 but not IL-4 to signal in Il13ra1(-/-) macrophages correlated with the inability of IL-13 but not IL-4 to induce expression of genes such as Arg1, Retnla and Ccl11 that are characteristically expressed by alternatively activated macrophages. In addition, IL-13 but not IL-4 failed to induce membrane fusion and giant cell formation by Il13ra1 KO macrophages. These findings demonstrate that the IL-13Rα1 chain is essential for induction of the AMA pathway by IL-13 but not IL-4.

Polarization of macrophages induced by Toxoplasma gondii and its impact on abnormal pregnancy in rats

Toxoplasma gondii infection is the leading cause of fetal intrauterine growth retardation among the five kinds of pathogens termed as TORCH, including Toxoplasma, Rubella virus, Cytomegalo virus, herpes virus and others during pregnancy. Pathogens infect the fetus through the placenta. T. gondii infection may result in congenital toxoplasmosis, miscarriage, stillbirth, and preemie, and increase pregnancy complications. Adaptive immune response induced by T. gondii infection stimulates T cells and macrophages to produce high levels of cytokines. Physiologically, the microenvironment of pregnancy was Th2-dominant. Here we set up a pregnant Sprague-Dawley rat model, and reported the polarization of macrophages induced by genotype Chinese 1 strain (Wh6) of Toxoplasma, and its adverse impact on pregnancy. The results showed that Wh6 infection pre- or in-gestation both led to abnormal pregnancy outcomes. Peritoneal macrophages in pre-gestation infection were polarized toward classically activated macrophages (M1), while in-gestation infection drove macrophages to polarize toward M2 activation. The Th2-dominant immune response in pregnant rat somewhat inhibits the excessive bias of the macrophages toward M1, and partially, toward M2. Infection of pre- and in-gestation may alter the physiological immune microenvironment in pregnant rats, giving rise to abnormal pregnancy outcomes.

The unappreciated intracellular lifestyle of Blastomyces dermatitidis

Blastomyces dermatitidis, a dimorphic fungus and the causative agent of blastomycosis, is widely considered an extracellular pathogen, with little evidence for a facultative intracellular lifestyle. We infected mice with spores, that is, the infectious particle, via the pulmonary route and studied intracellular residence, transition to pathogenic yeast, and replication inside lung cells. Nearly 80% of spores were inside cells at 24 h postinfection with 10(4) spores. Most spores were located inside of alveolar macrophages, with smaller numbers in neutrophils and dendritic cells. Real-time imaging showed rapid uptake of spores into alveolar macrophages, conversion to yeast, and intracellular multiplication during in vitro coculture. The finding of multiple yeast in a macrophage was chiefly due to intracellular replication rather than multiple phagocytic events or fusion of macrophages. Depletion of alveolar macrophages curtailed infection in mice infected with spores and led to a 26-fold reduction in lung CFU by 6 d postinfection versus nondepleted mice. Phase transition of the spores to yeast was delayed in these depleted mice over a time frame that correlated with reduced lung CFU. Spores cultured in vitro converted to yeast faster in the presence of macrophages than in medium alone. Thus, although advanced B. dermatitidis infection may exhibit extracellular residence in tissue, early lung infection with infectious spores reveals its unappreciated facultative intracellular lifestyle.

Molecular Characterization of Macrophage-Biomaterial Interactions

Implantation of biomaterials in vascularized tissues elicits the sequential engagement of molecular and cellular elements that constitute the foreign body response. Initial events include the non-specific adsorption of proteins to the biomaterial surface that render it adhesive for cells such as neutrophils and macrophages. The latter undergo unique activation and in some cases undergo cell-cell fusion to form foreign body giant cells that contribute to implant damage and fibrotic encapsulation. In this review, we discuss the molecular events that contribute to macrophage activation and fusion with a focus on the role of the inflammasome, signaling pathways such as JAK/STAT and NF-κB, and the putative involvement of micro RNAs in the regulation of these processes.

Phenotypic activation and pharmacological outcomes of spontaneously differentiated human monocyte-derived macrophages

Macrophage activation has been observed in vivo under physiological and pathological conditions, and may represent an attractive target for pharmacological modulation. This study tested the hypothesis that human blood-derived macrophages generated in vitro in the absence of specific macrophage growth factors respond flexibly to activation stimuli and pharmacological treatment. Monocytes were differentiated to macrophages for 7 days in culture in RPMI 1640 with 10% FCS. The resulting population showed predominance of the M2 over M1 phenotype as measured by flow cytometry and the expression of M1 vs. M2 markers was not mutually exclusive. Activation with LPS/IFN-γ for 48 h significantly increased the fraction of surface CD68-expressing cells, the CD14(+)/CD16(-)/CD68(+) subset and cell-bound TNF-α levels, whereas expression of the CC chemokine receptor (CCR)-2 was unchanged. Expression of the M2 markers CD206, CD163 and CX3CR1 was down-regulated following M1 activation compared with resting and after pre-exposure to M2-triggers. By contrast, alternative activation with IL-4/IL-13 for 48 h did not increase M2 markers, while CD206 up-regulation was observed after 7 days. Both activation signals induced changes in gene expression profiles as shown by Q-PCR. Treatment with 100 nM dexamethasone enhanced the M2 morphotype and CD163 expression while preventing LPS/IFN-γ-induced CD163 down-regulation. After 1-week dexamethasone treatment, virtually all cells acquired a CD163(+)/CD206(+)/CX3CR1(+) M2 phenotype. Therefore, these protocols appear to be useful to perform screens of pharmacological agents targeting human macrophage activation.

Pin1 regulates osteoclast fusion through suppression of the master regulator of cell fusion DC-STAMP

Cell fusion is a fundamental biological event that is essential for the development of multinucleated cells such as osteoclasts. Fusion failure leads to the accumulation of dense bone such as in osteopetrosis, demonstrating the importance of fusion in osteoclast maturity and bone remodeling. In a recent study, we reported that Pin1 plays a role in the regulation of bone formation and Runx2 regulation. In this study, we explored the role of Pin1 in osteoclast formation and bone resorption. Pin1 null mice have low bone mass and increased TRAP staining in histological sections of long bones, compared to Pin1 wild-type mice. In vitro osteoclast forming assays with bone marrow-derived monocyte/macrophage revealed that Pin1-deficient osteoclasts are larger than wild-type osteoclasts and have higher nuclei numbers, indicating greater extent of fusion. Pin1 deficiency also highly enhanced foreign body giant cell formation both in vitro and in vivo. Among the known fusion proteins, only DC-STAMP was significantly increased in Pin1(-/-) osteoclasts. Immunohistochemistry showed that DC-STAMP expression was also significantly increased in tibial metaphysis of Pin1 KO mice. We found that Pin1 binds and isomerizes DC-STAMP and affects its expression levels and localization at the plasma membrane. Taken together, our data indicate that Pin1 is a determinant of bone mass through the regulation of the osteoclast fusion protein DC-STAMP. The identification of Pin1 as a factor involved in cell fusion contributes to the understanding of osteoclast-associated diseases, including osteoporosis, and opens new avenues for therapeutic targets.

Granulomas are a source of interleukin-33 expression in pulmonary and extrapulmonary sarcoidosis

Sarcoidosis is a chronic inflammatory disease characterized by noncaseating epithelioid granulomas. These granulomas consist of highly differentiated mononuclear phagocytes--epithelioid cells and multinucleated giant cells (MNGCs)--surrounded by a proinflammatory infiltrate. Interleukin-33 (IL-33) is an inflammatory cytokine that is constitutively expressed in barrier tissues such as skin and lung and up-regulated in inflammation. Because sarcoidosis occurs most frequently in lung and skin, we studied the expression of this cytokine by immunohistochemistry in these tissues from patients with sarcoidosis, with foreign body granulomas, with other granulomatous diseases, and in corresponding normal tissues. We identified nuclear IL-33 staining of epithelioid cells and MNGCs in biopsies of skin (18/25 patients, 72%) and lung (10/19 patients, 53%) sarcoidosis. In contrast, sarcoidal granulomas in lymph nodes did not show IL-33 expression. Other granulomatous diseases showed only occasional and weak IL-33 expression. In sarcoidosis, we found a strong correlation between IL-33 expression and systemic disease, presence of MNGCs, and an M2-like macrophage phenotype as assessed by CD163 staining. Therefore, we propose that IL-33 plays a critical role in pathogenesis and disease progression of sarcoidosis. Because IL-33 is less commonly and only weakly expressed in other granulomatous diseases, the detection of IL-33 might serve as an adjunctive diagnostic marker. IL-33 expression in sarcoidosis seems to be dependent on the specific tissue microenvironment of sarcoidal granulomas and represents a novel biomarker for systemic involvement.

Resveratrol counteracts inflammation in human M1 and M2 macrophages upon challenge with 7-oxo-cholesterol: potential therapeutic implications in atherosclerosis

Macrophages consist of two main subsets: the proinflammatory M1 subset and the anti-inflammatory M2 one. 7-oxo-cholesterol, the most abundant cholesterol autoxidation product within atherosclerotic plaque, is able to skew the M1/M2 balance towards a proinflammatory profile. In the present study, we explored the ability of the polyphenolic compound resveratrol to counteract the 7-oxo-cholesterol-triggered proinflammatory signaling in macrophages. Resveratrol-pretreated human monocyte-derived M1 and M2 macrophages were challenged with 7-oxo-cholesterol and analyzed for phenotype and endocytic ability by flow cytometry, for metalloproteinase- (MMP-) 2 and MMP-9 by gelatin zymography, and for cytokine, chemokine, and growth factor secretome by a multiplex immunoassay. We also investigated the NF-κB signaling pathway. In the M1 subset, resveratrol prevented the downregulation of CD16 and the upregulation of MMP-2 in response to 7-oxo-cholesterol, whereas in M2 macrophages it prevented the upregulation of CD14, MMP-2, and MMP-9 and the downregulation of endocytosis. Resveratrol prevented the upregulation of several proinflammatory and proangiogenic molecules in both subsets. We identified modulation of NF-κB as a potential mechanism implicated in 7-oxo-cholesterol and resveratrol effects. Our results strengthen previous findings on the immunomodulatory ability of resveratrol and highlight its role as potential therapeutic or preventive compound, to counteract the proatherogenic oxysterol signaling within atherosclerotic plaque.

The M1 and M2 paradigm of macrophage activation: time for reassessment

Macrophages are endowed with a variety of receptors for lineage-determining growth factors, T helper (Th) cell cytokines, and B cell, host, and microbial products. In tissues, macrophages mature and are activated in a dynamic response to combinations of these stimuli to acquire specialized functional phenotypes. As for the lymphocyte system, a dichotomy has been proposed for macrophage activation: classic vs. alternative, also M1 and M2, respectively. In view of recent research about macrophage functions and the increasing number of immune-relevant ligands, a revision of the model is needed. Here, we assess how cytokines and pathogen signals influence their functional phenotypes and the evidence for M1 and M2 functions and revisit a paradigm initially based on the role of a restricted set of selected ligands in the immune response.

Gadolinium contrast agent-induced CD163+ ferroportin+ osteogenic cells in nephrogenic systemic fibrosis

Gadolinium-based contrast agents are linked to nephrogenic systemic fibrosis in patients with renal insufficiency. The pathology of nephrogenic systemic fibrosis is characterized by abnormal tissue repair: fibrosis and ectopic ossification. The mechanisms by which gadolinium could induce fibrosis and ossification are not known. We examined in vitro the effect of a gadolinium-based contrast agent on human peripheral blood mononuclear cells for phenotype and function relevant to the pathology of nephrogenic systemic fibrosis using immunofluorescence, flow cytometry, real-time PCR, and osteogenic assays. We also examined tissues from patients with nephrogenic systemic fibrosis, using IHC to identify the presence of cells with phenotype induced by gadolinium. Gadolinium contrast induced differentiation of human peripheral blood mononuclear cells into a unique cellular phenotype--CD163(+) cells expressing proteins involved in fibrosis and bone formation. These cells express fibroblast growth factor (FGF)23, osteoblast transcription factors Runt-related transcription factor 2, and osterix, and show an osteogenic phenotype in in vitro assays. We show in vivo the presence of CD163(+)/procollagen-1(+)/osteocalcin(+) cells in the fibrotic and calcified tissues of nephrogenic systemic fibrosis patients. Gadolinium contrast-induced CD163(+)/ferroportin(+)/FGF23(+) cells with osteogenic potential may play a role in systemic fibrosis and ectopic ossification in nephrogenic systemic fibrosis.

Macrophage fusion is controlled by the cytoplasmic protein tyrosine phosphatase PTP-PEST/PTPN12

Macrophages can undergo cell-cell fusion, leading to the formation of multinucleated giant cells and osteoclasts. This process is believed to promote the proteolytic activity of macrophages toward pathogens, foreign bodies, and extracellular matrices. Here, we examined the role of PTP-PEST (PTPN12), a cytoplasmic protein tyrosine phosphatase, in macrophage fusion. Using a macrophage-targeted PTP-PEST-deficient mouse, we determined that PTP-PEST was not needed for macrophage differentiation or cytokine production. However, it was necessary for interleukin-4-induced macrophage fusion into multinucleated giant cells in vitro. It was also needed for macrophage fusion following implantation of a foreign body in vivo. Moreover, in the RAW264.7 macrophage cell line, PTP-PEST was required for receptor activator of nuclear factor kappa-B ligand (RANKL)-triggered macrophage fusion into osteoclasts. PTP-PEST had no impact on expression of fusion mediators such as β-integrins, E-cadherin, and CD47, which enable macrophages to become fusion competent. However, it was needed for polarization of macrophages, migration induced by the chemokine CC chemokine ligand 2 (CCL2), and integrin-induced spreading, three key events in the fusion process. PTP-PEST deficiency resulted in specific hyperphosphorylation of the protein tyrosine kinase Pyk2 and the adaptor paxillin. Moreover, a fusion defect was induced upon treatment of normal macrophages with a Pyk2 inhibitor. Together, these data argue that macrophage fusion is critically dependent on PTP-PEST. This function is seemingly due to the ability of PTP-PEST to control phosphorylation of Pyk2 and paxillin, thereby regulating cell polarization, migration, and spreading.

Genetic basis of cell-cell fusion mechanisms

Cell-cell fusion in sexually reproducing organisms is a mechanism to merge gamete genomes and, in multicellular organisms, it is a strategy to sculpt organs, such as muscle, bone, and placenta. Moreover, this mechanism has been implicated in pathological conditions, such as infection and cancer. Studies of genetic model organisms have uncovered a unifying principle: cell fusion is a genetically programmed process. This process can be divided in three stages: competence (cell induction and differentiation); commitment (cell determination, migration, and adhesion); and cell fusion (membrane merging and cytoplasmic mixing). Recent work has led to the discovery of fusogens, which are cell fusion proteins that are necessary and sufficient to fuse cell membranes. Two unrelated families of fusogens have been discovered, one in mouse placenta and one in Caenorhabditis elegans (syncytins and F proteins, respectively). Current research aims to identify new fusogens and determine the mechanisms by which they merge membranes.

Interleukin-4-induced β-catenin regulates the conversion of macrophages to multinucleated giant cells

The cytokine interleukin-4 (IL-4) exerts pleiotropic effects on macrophages as it plays a key role in the immune response to infectious agents, allergens, and vaccines. Macrophages exposed to IL-4 drastically change their gene expression and metabolic state to adjust to new functional requirements. IL-4 also induces macrophages to fuse together and form multinucleated giant cells (MGCs). MGC formation is associated with chronic inflammation resulting from persistence of pathogenic microorganisms or foreign materials in tissues. Very little is known, however, about the mechanisms regulating IL-4-induced macrophage-to-MGC conversion. We observed a dramatic increase in β-catenin protein but not mRNA amount in mouse macrophages following exposure to IL-4. To investigate the role of β-catenin in macrophages, we generated mice with a myeloid cell-specific deletion of the β-catenin gene. Ablation of β-catenin expression did not affect the viability of macrophages or impair expression of known IL-4-inducible genes. Intriguingly, β-catenin-deficient macrophages incubated with IL-4 formed MGCs with markedly greater efficiency than wild-type macrophages. Similar increases in multinucleated cell formation were detected in the peritoneal cavity of myeloid cell-specific β-catenin knockout mice injected with chitin, which is known to induce endogenous IL-4 production. Our findings reveal β-catenin as a novel regulator of macrophage responses to IL-4, and suggest that therapeutic modulation of its expression or function may help enhance the effectiveness or ameliorate the pathology of IL-4-driven immune responses.

Claudin-1, claudin-2 and claudin-11 genes differentially associate with distinct types of anti-inflammatory macrophages in vitro and with parasite- and tumour-elicited macrophages in vivo

Macrophages altered by various Th2-associated and anti-inflammatory mediators--including IL-4 and IL-13 [inducing alternatively activated macrophages (AAMs)], IL-10 and TGF-β--were generically termed M2. However, markers that discriminate between AAMs and other M2 remain scarce. We previously described E-cadherin as a marker for AAMs, permitting these macrophages to fuse upon IL-4 stimulation. To identify novel potential contributors to macrophage fusion, we assessed the effect of IL-4 on other adherens and tight junction-associated components. We observed an induction of claudin-1 (Cldn1), Cldn2 and Cldn11 genes by IL-4 in different mouse macrophage populations. Extending our findings to other stimuli revealed Cldn1 as a mainly TGF-β-induced gene and showed that Cldn11 is predominantly associated with IL-4-induced AAMs. Cldn2 is upregulated by diverse stimuli and is not associated with a specific macrophage activation state in vitro. Interestingly, different claudin genes preferentially associate with M2 from distinct diseases. While Cldn11 is predominantly expressed in AAMs from helminth-infected mice, Cldn1 is the major macrophage claudin during chronic trypanosomiasis and Cldn2 dominates in tumour-associated macrophages. Overall, we identified Cldn1, Cldn2 and Cldn11 as genes that discriminate between diverse types of M2.

Senescence and quiescence induced compromised function in cultured macrophages

Implants are predisposed to infection even years after implantation, despite ostensibly being surrounded by innumerable macrophages as part of the host foreign body response. The local implant environment could adversely influence the implant-associated macrophage phenotype, proliferative capacity, activation states, and ability to neutralize pathogens. This study monitored cultured macrophage proliferative states and phagocytotic competence on tissue culture plastic to address the hypothesis that extended contact with foreign materials alters macrophage phenotype. That such macrophage alterations might also occur around implants has significance to the foreign body response, infection, cancer, autoimmune and other diseases. Specifically, multiple indicators of macrophage proliferation in various culture conditions, including cell confluence, long-term culture (21 days), lipopolysaccharide (LPS) stimulation, passaging, and mitogenic stimulation are reported. Importantly, primary murine macrophages became quiescent at high confluence and senescent during long-term culture. Senescent macrophages significantly reduced their ability to phagocytose particles, while quiescent macrophages did not. Cell senescence and quiescence were not observed with repeated passaging. Primary macrophage stimulation with LPS delayed senescence but did not eliminate it. These results prompt the conclusion that both cell quiescence and senescence are observed under common macrophage culture conditions and could alter macrophage behavior and phenotypes in extended in vitro culture, such as the ability to phagocytose. Such macrophage transitions around foreign bodies in vivo are not documented: quiescence and senescence reported here in macrophage culture could be relevant to macrophage behavior both in vitro in bioassays and in vivo in the foreign body response and implant-centered infection.

Multinucleated giant cells from fibroblast cultures

Many multinucleated giant cells are well-known to form from macrophage origin. Those formed from other cell types are less described, but may be as prevalent in pathological tissue. Giant multinucleated cells derived from secondary and primary fibroblast sources in various cultures with similar characteristics to foreign body giant cells are reported. Secondary-transformed NIH 3T3 fibroblasts rapidly fuse within 24 h in contact co-cultures with RAW 264.7 immortalized macrophages, while 3T3 monocultures, non-contact (transwell) co-cultures, and macrophage-conditioned media-treated 3T3 monocultures all do not fuse. Primary-derived murine fibroblasts also form multinucleated cells, both in the presence or absence of co-cultured macrophages that increase during long-term culture (5-30 days). In contrast to 3T3 fusion, this primary cell phenomenon is not due to fibroblast fusion, but rather to nuclear division without cytokinesis. That these multinucleated fibroblasts can originate via different mechanisms may influence and distinguish their behaviors in conditions under which they may arise, including various in vitro culture assays, and in certain fibroblastic pathologies such as the foreign body response, fibrosis, cancer and aged tissue.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

Immune responses to implants - a review of the implications for the design of immunomodulatory biomaterials

A key for long-term survival and function of biomaterials is that they do not elicit a detrimental immune response. As biomaterials can have profound impacts on the host immune response the concept emerged to design biomaterials that are able to trigger desired immunological outcomes and thus support the healing process. However, engineering such biomaterials requires an in-depth understanding of the host inflammatory and wound healing response to implanted materials. One focus of this review is to outline the up-to-date knowledge on immune responses to biomaterials. Understanding the complex interactions of host response and material implants reveals the need for and also the potential of "immunomodulating" biomaterials. Based on this knowledge, we discuss strategies of triggering appropriate immune responses by functional biomaterials and highlight recent approaches of biomaterials that mimic the physiological extracellular matrix and modify cellular immune responses.

M2 polarized macrophages and giant cells contribute to myofibrosis in neuromuscular sarcoidosis

The etiopathogenesis of sarcoidosis, a systemic granulomatous disease, still remains obscure. A multitude of organs have been described to be affected in systemic sarcoidosis. Skeletal muscles may also be affected, leading to myalgia and weakness. A workup of the specific immune response with emphasis on the macrophage response is provided herein. Affected muscle tissue from seven patients with systemic sarcoidosis was analyzed and compared with that from seven patients with other myopathies containing macrophagocytic infiltration. Monocytes/macrophages and giant cells in granulomas of muscle tissue from patients with sarcoidosis show a status of alternative activation (M2) based on their expression of CD206, CD301, arginase-1, and suppressor of cytokine signaling-1 as a consequence of a functionally type 2 helper T cell (Th2)-biased cytokine profile. Significant fibrosis and up-regulation of CCL18 were associated with the M2 phenotype of macrophages. Conversely, up-regulated Th1 cytokines did not result in significant classical activation of macrophages (M1), with poor inducible nitric oxide synthase and cyclooxygenase-2 production. Giant cell formation was further associated with up-regulated expression of DNAX-activating protein of 12 kDa (DAP12; gene symbol TYROBP). Functionally, alternative activation of macrophages on the basis of a Th2-biased immune response may induce clinical symptoms and chronic evolution of neuromuscular sarcoidosis. This is the first characterization of Th2-mediated immune mechanisms in neuromuscular sarcoidosis suggesting that a specific macrophage activation status leading to myofibrosis may be a key event in the pathogenesis of this disease.

Advanced microscopy: laser scanning confocal microscopy

Fluorescence microscopy is an important and fundamental tool for biomedical research. Optical microscopy is almost non-invasive and allows highly spatially resolved images of organisms, cells, macromolecular complexes, and biomolecules to be obtained. Generally speaking, the architecture of the observed structures is not significantly modified and the environmental conditions can be kept very close to physiological reality. The development of fluorescence microscopy was revolutionized with the invention of laser scanning confocal microscopy (LSCM). With its unique three-dimensional representation and analysis capabilities, this technology gives us a more real view of the world.This chapter introduces the reader to the methodology of setting up basic experiments for use with a laser scanning confocal microscope. There are practical guidelines about sample preparation for both fixed and living specimens, as well as examples of some of the applications of confocal microscopy.

Alternative activation of tumor-associated macrophages by IL-4: priming for protumoral functions

Although macrophages were originally recognized as major immune effector cells, it is now appreciated that they also play many important roles in the maintenance of tissue homeostasis, and are involved in a variety of pathological conditions including cancer. Several studies have demonstrated the contributions of tumor-associated macrophages (TAMs) to tumor initiation, progression, and metastasis. However, the detailed mechanisms underlying how TAMs differ molecularly from their normal counterparts and how the conversion to TAMs occurs have only just begun to be understood. TAMs have been proposed to exhibit phenotypes of 'alternatively activated' macrophages, though there has been limited evidence directly linking the phenotypes of TAMs to the alternative activation of macrophages. This review will focus on IL-4, the prototypic cytokine that induces the alternative activation of macrophages, and review current knowledge regarding the contributions of IL-4 to the phenotypes of TAMs and its effects on tumorigenesis.

Cell-cell signaling in co-cultures of macrophages and fibroblasts

The foreign body response (FBR) comprises a general, ubiquitous host tissue-based reaction to implanted materials. In vitro cell-based models are frequently employed to study FBR mechanisms involving cell signaling responses to materials. However, these models often study only one cell type, identify only limited signals, and cannot accurately represent the complexity of in vivo inflammatory signaling. To address this issue, a cell co-culture system involving two primary effector cells of the FBR, macrophages and fibroblasts, was employed. Cell-cell signaling systems were monitored between these cell types, including long-term 1) culture of one cell type in conditioned media from the other cell type, 2) non-contacting cell co-cultures (paracrine signaling), and 3) contact co-cultures (juxtacrine signaling) of primary- and secondary-derived cells. Cell culture media and cell images were collected on Days 1, 2, 3, 7, 14, and 21 and changes in soluble protein secretion, cellular behavior, and morphology were assessed. Primary- and secondary-derived cells responded uniquely during each signaling scenario and to one another. In general higher in vitro fidelity to FBR-like responses was found in primary cell co-cultures compared to their mono-cultures and all secondary cell cultures.

MT1-MMP is required for myeloid cell fusion via regulation of Rac1 signaling

Cell fusion is essential for fertilization, myotube formation, and inflammation. Macrophages fuse under various circumstances, but the molecular signals involved in the distinct steps of their fusion are not fully characterized. Using null mice and derived cells, we show that the protease MT1-MMP is necessary for macrophage fusion during osteoclast and giant-cell formation in vitro and in vivo. Specifically, MT1-MMP is required for lamellipodia formation and for proper cell morphology and motility of bone marrow myeloid progenitors prior to membrane fusion. These functions of MT1-MMP do not depend on MT1-MMP catalytic activity or downstream pro-MMP-2 activation. Instead, MT1-MMP null cells show a decreased Rac1 activity and reduced membrane targeting of Rac1 and the adaptor protein p130Cas. Retroviral rescue experiments and protein binding assays delineate a signaling pathway in which MT1-MMP, via its cytosolic tail, contributes to macrophage migration and fusion by regulating Rac1 activity through an association with p130Cas.

Cholate-containing high-fat diet induces the formation of multinucleated giant cells in atherosclerotic plaques of apolipoprotein E-/- mice

OBJECTIVE

To determine the role of multinucleated giant cells (MGCs) in cardiovascular diseases.

METHODS AND RESULTS

MGCs are a hallmark of giant cell arteritis. They are also described in atherosclerotic plaques from aortic aneurysms and carotid and coronary arteries. Herein, we demonstrate that the cholate-containing Paigen diet yields many MGCs in atherosclerotic plaques of apolipoprotein E-/- mice. These mice revealed a 4-fold increase in MGC numbers when compared with mice on a Western or Paigen diet without cholate. Most of the MGCs stained intensively for cathepsin K and were located at fibrous caps and close to damaged elastic laminae, with associated medial smooth muscle cell depletion. During in vitro experiments, MGCs demonstrated a 6-fold increase in elastolytic activity when compared with macrophages and facilitated transmigration of smooth muscle cells through a collagen-elastin matrix. An elastin-derived hexapeptide (Val-Gly-Val-Ala-Pro-Gly [VGVAPG]) significantly increased the rate of macrophage fusion, providing a possible mechanism of in vivo MGC formation. Comparable to the mouse model, human specimens from carotid arteries and aortic aneurysms contained cathepsin K-positive MGCs.

CONCLUSIONS

Apolipoprotein E-/- mice fed a Paigen diet provide a model to analyze the tissue-destructive role of MGCs in vascular diseases.

Multi-nucleated giant cell formation from human cord blood monocytes in vitro, in comparison with adult peripheral blood monocytes

Multi-nucleated giant cells (MGCs; Langhans-type cell), formed from macrophage fusion, are recognized as a hallmark histological feature in chronic inflammation. However, their precise pathological role is still poorly understood, especially for microorganism pathogens in the neonatal immune system, which are capable of surviving intracellularly in phagocytes. To conduct a partial evaluation of the monocyte function of neonates, we investigated the ability of human cord blood monocytes to form MGCs in vitro by stimulating various cytokines and comparing them with adult peripheral blood monocytes. Monocytes from cord blood and adult peripheral blood were isolated and cultured for 14 days with cytokines known to induce MGC in vitro. The fusion index in experiments with a combination of interleukin (IL)-4 and macrophage colony-stimulating factor (M-CSF) and a combination of IL-4 and granulocyte-macrophage colony-stimulating factor (GM-CSF) was significantly lower in cord blood than in adult blood monocytes (P = 0.0018 and P = 0.0141, respectively). The number of nuclei per MGC was significantly lower in cord blood than in adult blood monocytes in experiments with IL-4 alone, the combination of IL-4 and M-CSF, and the combination of IL-4 and GM-CSF (P < 0.0001). These results suggest the possibility that the susceptibility of newborns to mycobacterium infection is due partly to impaired MGC formation.

Molecular mediators of macrophage fusion

Fusion of macrophages leads to the formation of osteoclasts in bone and of multinucleated giant cells in granulomas. The precise function of granuloma-associated multinucleates giant cells is not clear but substantial progress has recently been made in identifying the molecular machinery involved in macrophage fusion. Signaling processes mediated by DAP12 and STAT6 induce a fusion-competent status. Chemotaxis through CCL2, cell-cell adhesion mediated by E-cadherin, exposure of phosphatidylserine, lipid recognition by CD36 and cytoskeletal rearrangements depending on RAC1 are prerequisites for successful macrophage fusion. We review current knowledge on the molecular mediators of giant cell formation, compare giant cells with osteoclasts and highlight key target areas for future research and medical relevance.

Macrophages: regulators of sex differences in asthma?

Females are more susceptible to development of asthma than are males. In a mouse model of ovalbumin-induced airway inflammation, with aggravated disease in females compared with males, we studied interactions between immune and resident lung cells during asthma development to elucidate which processes are affected by sex. We studied numbers of regulatory T cells (Tregs), effector T cells, myeloid dendritic cells (mDCs), and alternatively activated macrophages (AAMPhi), and their functional capabilities. Male and female mice had comparable Treg numbers in lung tissue and comparable Treg function, but effector T cells had expanded to a greater extent in lungs of females after ovalbumin exposure. This difference in T cell expansion was therefore not the result of lack of Treg control, but appeared to be driven by a greater number of inflammatory mDCs migrating from the lungs to lymph nodes in females. Resident lung cells can influence mDC migration, and AAMPhi in lung tissue were found to be involved. Artificially elevating the number of AAMPhi in lung tissue increased the migration of mDCs and airway inflammation. We found greater numbers of AAMPhi in female lungs than in males; we therefore postulate that AAMPhi are involved in increased airway inflammation found in female mice.

Alternative activation of macrophages: an immunologic functional perspective

Macrophages are innate immune cells with well-established roles in the primary response to pathogens, but also in tissue homeostasis, coordination of the adaptive immune response, inflammation, resolution, and repair. These cells recognize danger signals through receptors capable of inducing specialized activation programs. The classically known macrophage activation is induced by IFN-gamma, which triggers a harsh proinflammatory response that is required to kill intracellular pathogens. Macrophages also undergo alternative activation by IL-4 and IL-13, which trigger a different phenotype that is important for the immune response to parasites. Here we review the cellular sources of these cytokines, receptor signaling pathways, and induced markers and gene signatures. We draw attention to discrepancies found between mouse and human models of alternative activation. The evidence for in vivo alternative activation of macrophages is also analyzed, with nematode infection as prototypic disease. Finally, we revisit the concept of macrophage activation in the context of the immune response.

Quantitative in vivo cytokine analysis at synthetic biomaterial implant sites

To further elucidate the foreign body reaction, investigation of cytokines at biomaterial implant sites was carried out using a multiplex immunoassay and ELISA. Macrophage activation cytokines (IL-1beta, IL-6, and TNFalpha), cytokines important for macrophage fusion (IL-4 and IL-13), antiinflammatory cytokines (IL-10 and TGFbeta), chemokines (GRO/KC, MCP-1), and the T-cell activation cytokine IL-2 were quantified at biomaterial implant sites. Empty cages (controls) or cages containing synthetic biomedical polymer (Elasthane 80A (PEU), silicone rubber (SR), or polyethylene terephthalate (PET)) were implanted subcutaneously in Sprague-Dawley rats for 4, 7, or 14 days, and cytokines in exudate supernatants and macrophage surface adhesion and fusion were quantified. The presence of a polymer implant did not affect the levels of IL-1beta, TGFbeta, and MCP-1 in comparison to the control group. IL-2 was not virtually detected in any of the samples. Although the levels of IL-4, IL-13, IL-10, and GRO/KC were affected by polymer implantation, but not dependent on a specific polymer, IL-6 and TNFalpha were significantly greater in those animals implanted with PEU and SR, materials that do not promote fusion. The results indicate that differential material-dependent cytokine profiles are produced by surface adherent macrophages and foreign body giant cells in vivo.

Alternative activation of macrophages: immune function and cellular biology

Macrophages are the first line of defense of the organism against pathogens and, in response to the microenvironment, become differentially activated. In the presence of IL-4 and IL-13, cytokines that are produced in a Th-2 type response, particularly during allergy and parasitic infections, macrophages become differentially activated. Alternative activated macrophages play an important role in the protection of the host by decreasing inflammation and promoting tissues repair. However, alternative activation of macrophages also downregulates host protection against selected pathogens. This defect is associated with an altered receptor expression pattern and extensive modulation of intracellular membrane trafficking. This review shows how alternative activation of macrophages induces extensive cellular remodelling of phagocytic, endocytic, signaling and secretory pathways which play an important, but unclear role in the pathogenesis of different disease.

The scavenger receptor CD36 plays a role in cytokine-induced macrophage fusion

Multinucleated giant cells, characteristic of granulomatous infections, originate from the fusion of macrophages. Using an antibody screening strategy we found that the scavenger receptor CD36 participates in macrophage fusion induced by the cytokines IL-4 and GM-CSF. Our results demonstrate that exposure of phosphatidylserine on the cell surface and lipid recognition by CD36 are required for cytokine-induced fusion of macrophages. We also show that CD36 acts in a heterotypic manner during giant-cell formation and that the formation of osteoclasts is independent of CD36. The discovery of molecules involved in the formation of multinucleated giant cells will enable us to determine their functional significance. Furthermore, our results suggest that lipid capture by cell surface receptors may be a general feature of cell fusion.

Giant cell formation and function

PURPOSE OF REVIEW

To provide insight into the current state of understanding regarding the molecular and cellular mechanisms underlying the formation and function of various types of multinucleated giant cells.

RECENT FINDINGS

Recent studies involving mainly osteoclasts and foreign body giant cells have revealed a number of common factors, for example, vitronectin, an adhesion protein, dendritic cell-specific transmembrane protein, a fusion factor, and macrophage fusion receptor, that contribute to giant cell formation and function. Insight into common molecules, receptors, and mediators of adhesion and fusion mechanisms of giant cell formation have been complicated by the wide diversity of species, models, and cell types utilized in these studies.

SUMMARY

These recently identified factors together with the well known osteoclast receptor, alphavbeta3, may serve as potential therapeutic targets for the modulation and inhibition of multinucleated giant cell formation and function. Further studies on intracellular and intercellular signaling mechanisms modulating multinucleated giant cell formation and function are necessary for the identification of therapeutic targets as well as a better understanding of giant cell biology.

Macrophage fusion, giant cell formation, and the foreign body response require matrix metalloproteinase 9

Macrophages undergo fusion to form multinucleated giant cells in several pathologic conditions, including the foreign body response (FBR). We detected high levels of matrix metalloproteinase (MMP)-9 during macrophage fusion in vitro and in foreign body giant cells (FBGCs) in vivo. Wild-type (WT) bone marrow-derived macrophages were induced to fuse with IL-4 in the presence of MMP-9 function-blocking antibodies and displayed reduced fusion. A similar defect, characterized by delayed shape change and abnormal morphology, was observed in MMP-9 null macrophages. Analysis of the FBR in MMP-9 null mice was then pursued to evaluate the significance of these findings. Specifically, mixed cellulose ester disks and polyvinyl alcohol sponges were implanted s.c. in MMP-9 null and WT mice and excised 2-4 weeks later. Histochemical and immunohistochemical analyses indicated equal macrophage recruitment between MMP-9 null and WT mice, but FBGC formation was compromised in the former. In addition, MMP-9 null mice displayed abnormalities in extracellular matrix assembly and angiogenesis. Consistent with a requirement for MMP-9 in fusion, we also observed reduced MMP-9 levels in MCP-1 null macrophages, previously shown to be defective in FBGC formation. Collectively, our studies show abnormalities in MMP-9 null mice during the FBR and suggest a role for MMP-9 in macrophage fusion.

Essential role of DAP12 signaling in macrophage programming into a fusion-competent state

Multinucleated giant cells, formed by fusion of macrophages, are a hallmark of granulomatous inflammation. With a genetic approach, we show that signaling through the adaptor protein DAP12 (DNAX activating protein of 12 kD), its associated receptor triggering receptor expressed by myeloid cells 2 (TREM-2), and the downstream protein tyrosine kinase Syk is required for the cytokine-induced formation of giant cells and that overexpression of DAP12 potentiates macrophage fusion. We also present evidence that DAP12 is a general macrophage fusion regulator and is involved in modulating the expression of several macrophage-associated genes, including those encoding known mediators of macrophage fusion, such as DC-STAMP and Cadherin 1. Thus, DAP12 is involved in programming of macrophages through the regulation of gene and protein expression to induce a fusion-competent state.

Alternative macrophage activation in periprosthetic osteolysis

Several disorders characterized by macrophages accumulating non-disposable (or hard to dispose of) material or formation of multinucleated giant cell containing granulomas have been linked to elicitation of an alternative macrophage activation phenotype. Gene profiling efforts have shown that alternative macrophage activation can exist in numerous forms, each specific for the particular biological niche in which the macrophage finds itself, accentuating the plasticity of this cell type. Periprosthetic osteolysis is characterized by macrophage phagocytosis of particles of wear debris and formation of foreign body granulomas, suggesting the hypothesis that it may represent a new member of this group of diseases characterized by alternative macrophage activation. Gene profiling has provided strong supportive evidence for this hypothesis, revealing that periprosthetic tissues of osteolysis patients show the presence of a pronounced alternative macrophage activation pathway, with the classical pro-inflammatory activation pathway being less evident. These findings have important implications for our understanding of periprosthetic osteolysis and how to approach future investigations into this disease.

Myoblasts and macrophages share molecular components that contribute to cell-cell fusion

Cell–cell fusion is critical to the normal development of certain tissues, yet the nature and degree of conservation of the underlying molecular components remains largely unknown. Here we show that the two guanine-nucleotide exchange factors Brag2 and Dock180 have evolutionarily conserved functions in the fusion of mammalian myoblasts. Their effects on muscle cell formation are distinct and are a result of the activation of the GTPases ARF6 and Rac, respectively. Inhibition of ARF6 activity results in a lack of physical association between paxillin and β₁-integrin, and disruption of paxillin transport to sites of focal adhesion. We show that fusion machinery is conserved among distinct cell types because Dock180 deficiency prevented fusion of macrophages and the formation of multinucleated giant cells. Our results are the first to demonstrate a role for a single protein in the fusion of two different cell types, and provide novel mechanistic insight into the function of GEFs in the morphological maturation of multinucleated cells.

Regulation of alternative macrophage activation by galectin-3

Alternative macrophage activation is implicated in diverse disease pathologies such as asthma, organ fibrosis, and granulomatous diseases, but the mechanisms underlying macrophage programming are not fully understood. Galectin-3 is a carbohydrate-binding lectin present on macrophages. We show that disruption of the galectin-3 gene in 129sv mice specifically restrains IL-4/IL-13-induced alternative macrophage activation in bone marrow-derived macrophages in vitro and in resident lung and recruited peritoneal macrophages in vivo without affecting IFN-gamma/LPS-induced classical activation or IL-10-induced deactivation. IL-4-mediated alternative macrophage activation is inhibited by siRNA-targeted deletion of galectin-3 or its membrane receptor CD98 and by inhibition of PI3K. Increased galectin-3 expression and secretion is a feature of alternative macrophage activation. IL-4 stimulates galectin-3 expression and release in parallel with other phenotypic markers of alternative macrophage activation. By contrast, classical macrophage activation with LPS inhibits galectin-3 expression and release. Galectin-3 binds to CD98, and exogenous galectin-3 or cross-linking CD98 with the mAb 4F2 stimulates PI3K activation and alternative activation. IL-4-induced alternative activation is blocked by bis-(3-deoxy-3-(3-methoxybenzamido)-beta-D-galactopyranosyl) sulfane, a specific inhibitor of extracellular galectin-3 carbohydrate binding. These results demonstrate that a galectin-3 feedback loop drives alternative macrophage activation. Pharmacological modulation of galectin-3 function represents a novel therapeutic strategy in pathologies associated with alternatively activated macrophages.

Modulation of P2X7 receptor expression in macrophages from mineral oil-injected mice

P2X7 receptor activation is involved in a number of pro-inflammatory responses in macrophages and other immune cells. Their expression can be positively modulated with lipopolysaccharide (LPS) and TNFalpha, reinforcing their role during inflammation. We investigated the effect of substances capable of recruiting macrophages into the peritoneal cavity of mice (mineral oil and thioglycolate) on P2X7 receptor expression and function, addressing whether these stimuli can interfere with multinucleated giant cell (MGC) formation, ATP-induced apoptosis, plasma membrane permeabilization and nitric oxide production. It was demonstrated that mineral oil treatment reduces P2X7-dependent MGC formation, whereas thioglycolate treatment does not. Mineral oil treatment reduced P2X7 receptor expression, down-modulating ATP-induced apoptosis, permeabilization and nitric oxide production. In conclusion, mineral oil down modulated P2X7 expression and consequently P2X7-associated phenomena, but thioglycolate did not. These effects might be associated with the unpleasant side effects already described during long-term administration of mineral oil for cosmetic purposes or as a laxative and could be useful in understanding the mechanism of recycling and modulation of P2 receptors present in other situations of immunopathological interest.

Macrophage fusion: molecular mechanisms

Macrophages are the most versatile, plastic, and mobile cells in the animal kingdom. They are present in all tissues and might even define a true " body-wide" network that maintains health and ensures the repair of tissues and organs. In specific and rare instances, macrophages fuse to form multinucleate osteoclasts and giant cells in bone and in chronic inflammatory reactions, respectively. While macrophages lose most of their plasticity and mobility after they become multinucleate, at the same time they acquire the capacity to resorb calcified tissues, such as bone, and foreign bodies, such as pathogens and implants, and they mediate the replacement of the resorbed tissue by new tissue. There is evidence to suggest that macrophages might also fuse with somatic cells to repair tissues and with tumor cells to trigger the metastatic process. The molecular machinery of macrophage fusion remains poorly characterized, but it is likely to be shared by all fusing macrophages.

Methods to fuse macrophages in vitro

Macrophages are mononucleate cells that fuse in rare and specific instances to form osteoclasts in bone or giant cells in chronic inflammatory conditions. Because of the central role these cells play in bone metabolism and in inflammation, respectively, methods to study their formation in vitro are described.

Foreign body reaction to biomaterials

The foreign body reaction composed of macrophages and foreign body giant cells is the end-stage response of the inflammatory and wound healing responses following implantation of a medical device, prosthesis, or biomaterial. A brief, focused overview of events leading to the foreign body reaction is presented. The major focus of this review is on factors that modulate the interaction of macrophages and foreign body giant cells on synthetic surfaces where the chemical, physical, and morphological characteristics of the synthetic surface are considered to play a role in modulating cellular events. These events in the foreign body reaction include protein adsorption, monocyte/macrophage adhesion, macrophage fusion to form foreign body giant cells, consequences of the foreign body response on biomaterials, and cross-talk between macrophages/foreign body giant cells and inflammatory/wound healing cells. Biomaterial surface properties play an important role in modulating the foreign body reaction in the first two to four weeks following implantation of a medical device, even though the foreign body reaction at the tissue/material interface is present for the in vivo lifetime of the medical device. An understanding of the foreign body reaction is important as the foreign body reaction may impact the biocompatibility (safety) of the medical device, prosthesis, or implanted biomaterial and may significantly impact short- and long-term tissue responses with tissue-engineered constructs containing proteins, cells, and other biological components for use in tissue engineering and regenerative medicine. Our perspective has been on the inflammatory and wound healing response to implanted materials, devices, and tissue-engineered constructs. The incorporation of biological components of allogeneic or xenogeneic origin as well as stem cells into tissue-engineered or regenerative approaches opens up a myriad of other challenges. An in depth understanding of how the immune system interacts with these cells and how biomaterials or tissue-engineered constructs influence these interactions may prove pivotal to the safety, biocompatibility, and function of the device or system under consideration.

The molecular basis of macrophage fusion

Multinucleated giant cells (MGCs), characteristic of granulomatous infections as well as multinucleated osteoclasts originate from fusion of macrophages. While intracellular and viral membrane fusion have been studied in detail, much less is known about the machinery which mediates cell-to-cell fusion, in particular macrophage polykaryon formation. Several molecules have been implicated in this process which may involve the action of multiple glycoproteins mediating membrane attachment and fusion. Macrophage fusion can be induced by soluble mediators such as cytokines and growth factors, even though several other stimuli may be involved, especially for the induction of granuloma-associated giant cells. The function of MGCs during granulomatous diseases is currently unknown. However, a better understanding of the mechanistic basis of macrophage fusion may lead to a better understanding of the function of MGCs found in granulomas.

Foreign body giant cell formation is preceded by lamellipodia formation and can be attenuated by inhibition of Rac1 activation

Macrophages that are recruited to the site of implanted biomaterials undergo fusion to form surface-damaging foreign body giant cells. Exposure of peripheral blood monocytes to interleukin-4 can recapitulate the fusion process in vitro. In this study, we used interleukin-4 to induce multinucleation of murine bone marrow-derived macrophages and observed changes in cell shape, including elongation and lamellipodia formation, before fusion. Because cytoskeletal rearrangements are regulated by small GTPases, we examined the effects of inhibitors of Rho kinase (Y-32885) and Rac activation (NSC23766) on fusion. Y-32885 did not prevent cytoskeletal changes or fusion but limited the extent of multinucleation. NSC23766, on the other hand, inhibited lamellipodia formation and fusion in a dose-dependent manner. In addition, we found that in control cells, these changes were preceded by Rac1 activation. However, NSC23766 did not block the uptake of polystyrene microspheres. Likewise, short interfering RNA knockdown of Rac1 limited fusion without limiting phagocytosis. Thus, phagocytosis and fusion can be partially decoupled based on their susceptibility to NSC23766. Furthermore, poly(ethylene-co-vinyl acetate) scaffolds containing NSC23766 attenuated foreign body giant cell formation in vivo. These observations suggest that targeting Rac1 activation could protect biomaterials without compromising the ability of macrophages to perform beneficial phagocytic functions at implantation sites.

New vistas on macrophage differentiation and activation

Plasticity and heterogeneity are hallmarks of myelomonocytic differentiation and polarized activation. Evidence published in this issue of the European Journal of Immunology, together with other recent data, add new elements and perspectives to the current understanding of mononuclear phagocyte differentiation and activation and are discussed in this Commentary.