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

Glucosylated cholesterol accumulates in atherosclerotic lesions and impacts macrophage immune response

Atherosclerosis can be described as a local acquired lysosomal storage disorder (LSD), resulting from the build-up of undegraded material in lysosomes. Atherosclerotic foam cells accumulate cholesterol (Chol) and glycosphingolipids (GSLs) within lysosomes. This constitutes the ideal milieu for the formation of a side product of lysosomal storage: glucosylated cholesterol (GlcChol), previously found in several LSDs. Using LC-MS/MS, we demonstrated that GlcChol is abundant in atherosclerotic lesions. Patients suffering from cardiovascular diseases presented unaltered plasma GlcChol levels but slightly elevated GlcChol/Chol ratios. Furthermore, we mimicked GlcChol formation in vitro by exposing macrophages (Mφ) to a pro-atherogenic oxidized cholesteryl ester, an atherosclerosis foam cell model. Additionally, Mφ exposed to GlcChol exhibited an enlarged and multinucleated phenotype. These Mφ present signs of decreased proliferation and reduced pro-inflammatory capacity. Mechanistically the process seems to be associated with the activation of the AMPK signalling pathway and the cyclin-dependent kinase inhibitor 1 (CDKN1A/p21), in response to DNA damage inflicted by reactive oxygen species (ROS). At the organelle level, exposure to GlcChol impacted the lysosomal compartment, resulting in the activation of the mTOR signalling pathway and lysosomal biogenesis mediated by the transcription factor EB (TFEB). This suggests that high concentrations of GlcChol impact cellular homeostasis. In contrast, under this threshold GlcChol formation most likely represents a relatively innocuous compensatory mechanism to cope with Chol and GSL build-up within lesions. Our findings demonstrate that glycosidase-mediated lipid modifications may play a role in the aetiology of genetic and acquired LSDs, warranting further investigation.

Cell fusion dynamics: mechanisms of multinucleation in osteoclasts and macrophages

Cell-cell fusion is a vital biological process where the membranes of two or more cells merge to form a syncytium. This phenomenon is critical in various physiological and pathological contexts, including embryonic development, tissue repair, immune responses, and the progression of several diseases. Osteoclasts, which are cells from the monocyte/macrophage lineage responsible for bone resorption, have enhanced functionality due to cell fusion. Additionally, other multinucleated giant cells (MGCs) also arise from the fusion of monocytes and macrophages, typically during chronic inflammation and reactions to foreign materials such as prostheses or medical devices. Foreign body giant cells (FBGCs) and Langhans giant cells (LGCs) emerge only under pathological conditions and are involved in phagocytosis, antigen presentation, and the secretion of inflammatory mediators. This review provides a comprehensive overview of the mechanisms underlying the formation of multinucleated cells, with a particular emphasis on macrophages and osteoclasts. Elucidating the intracellular structures, signaling cascades, and fusion-mediating proteins involved in cell-cell fusion enhances our understanding of this fundamental biological process and helps identify potential therapeutic targets for disorders mediated by cell fusion.

Formation and biological activities of foreign body giant cells in response to biomaterials

The integration of biomaterials in medical applications triggers the foreign body response (FBR), a multi-stage immune reaction characterized by the formation of foreign body giant cells (FBGCs). Originating from the fusion of monocyte/macrophage lineage cells, FBGCs are pivotal participants during tissue-material interactions. This review provides an in-depth examination of the molecular processes during FBGC formation, highlighting signaling pathways and fusion mediators in response to both exogenous and endogenous stimuli. Moreover, a wide range of material-specific characteristics, such as surface chemical and physical properties, has been proven to influence the fusion of macrophages into FBGCs. Multifaceted biological activities of FBGCs are also explored, with emphasis on their phagocytic capabilities and extracellular secretory functions, which profoundly affect the vascularization, degradation, and encapsulation of the biomaterials. This review further elucidates the heterogeneity of FBGCs and their diverse roles during FBR, as demonstrated by their distinct behaviors in response to different materials. By presenting a comprehensive understanding of FBGCs, this review intends to provide strategies and insights into optimizing biocompatibility and the therapeutic potential of biomaterials for enhanced stability and efficacy in clinical applications. STATEMENT OF SIGNIFICANCE: As a hallmark of the foreign body response (FBR), foreign body giant cells (FBGCs) significantly impact the success of implantable biomaterials, potentially leading to complications such as chronic inflammation, fibrosis, and device failure. Understanding the role of FBGCs and modulating their responses are vital for successful material applications. This review provides a comprehensive overview of the molecules and signaling pathways guiding macrophage fusion into FBGCs. By elucidating the physical and chemical properties of materials inducing distinct levels of FBGCs, potential strategies of materials in modulating FBGC formation are investigated. Additionally, the biological activities of FBGCs and their heterogeneity in responses to different material categories in vivo are highlighted in this review, offering crucial insights for improving the biocompatibility and efficacy of biomaterials.

The Implant-Induced Foreign Body Response Is Limited by CD13-Dependent Regulation of Ubiquitination of Fusogenic Proteins

Implanted medical devices, from artificial heart valves and arthroscopic joints to implantable sensors, often induce a foreign body response (FBR), a form of chronic inflammation resulting from the inflammatory reaction to a persistent foreign stimulus. The FBR is characterized by a subset of multinucleated giant cells (MGCs) formed by macrophage fusion, the foreign body giant cells (FBGCs), accompanied by inflammatory cytokines, matrix deposition, and eventually deleterious fibrotic implant encapsulation. Despite efforts to improve biocompatibility, implant-induced FBR persists, compromising the utility of devices and making efforts to control the FBR imperative for long-term function. Controlling macrophage fusion in FBGC formation presents a logical target to prevent implant failure, but the actual contribution of FBGCs to FBR-induced damage is controversial. CD13 is a molecular scaffold, and in vitro induction of CD13KO bone marrow progenitors generates many more MGCs than the wild type, suggesting that CD13 regulates macrophage fusion. In the mesh implant model of FBR, CD13KO mice produced significantly more peri-implant FBGCs with enhanced TGF-β expression and increased collagen deposition versus the wild type. Prior to fusion, increased protrusion and microprotrusion formation accompanies hyperfusion in the absence of CD13. Expression of fusogenic proteins driving cell-cell fusion was aberrantly sustained at high levels in CD13KO MGCs, which we show is due to a novel CD13 function, to our knowledge, regulating ubiquitin/proteasomal protein degradation. We propose CD13 as a physiologic brake limiting aberrant macrophage fusion and the FBR, and it may be a novel therapeutic target to improve the success of implanted medical devices. Furthermore, our data directly implicate FBGCs in the detrimental fibrosis that characterizes the FBR.

Multinucleated Giant Cells: Current Insights in Phenotype, Biological Activities, and Mechanism of Formation

Monocytes and macrophages are innate immune cells with diverse functions ranging from phagocytosis of microorganisms to forming a bridge with the adaptive immune system. A lesser-known attribute of macrophages is their ability to fuse with each other to form multinucleated giant cells. Based on their morphology and functional characteristics, there are in general three types of multinucleated giant cells including osteoclasts, foreign body giant cells and Langhans giant cells. Osteoclasts are bone resorbing cells and under physiological conditions they participate in bone remodeling. However, under pathological conditions such as rheumatoid arthritis and osteoporosis, osteoclasts are responsible for bone destruction and bone loss. Foreign body giant cells and Langhans giant cells appear only under pathological conditions. While foreign body giant cells are found in immune reactions against foreign material, including implants, Langhans giant cells are associated with granulomas in infectious and non-infectious diseases. The functionality and fusion mechanism of osteoclasts are being elucidated, however, our knowledge on the functions of foreign body giant cells and Langhans giant cells is limited. In this review, we describe and compare the phenotypic aspects, biological and functional activities of the three types of multinucleated giant cells. Furthermore, we provide an overview of the multinucleation process and highlight key molecules in the different phases of macrophage fusion.

Diverse Roles of TRPV4 in Macrophages: A Need for Unbiased Profiling

Transient receptor potential vanilloid 4 (TRPV4) is a non-selective mechanosensitive ion channel expressed by various macrophage populations. Recent reports have characterized the role of TRPV4 in shaping the activity and phenotype of macrophages to influence the innate immune response to pathogen exposure and inflammation. TRPV4 has been studied extensively in the context of inflammation and inflammatory pain. Although TRPV4 activity has been generally described as pro-inflammatory, emerging evidence suggests a more complex role where this channel may also contribute to anti-inflammatory activities. However, detailed understanding of how TRPV4 may influence the initiation, maintenance, and resolution of inflammatory disease remains limited. This review highlights recent insights into the cellular processes through which TRPV4 contributes to pathological conditions and immune processes, with a focus on macrophage biology. The potential use of high-throughput and omics methods as an unbiased approach for studying the functional outcomes of TRPV4 activation is also discussed.

Mechanosensing by TRPV4 mediates stiffness-induced foreign body response and giant cell formation

Implantation of biomaterials or devices into soft tissue often leads to the development of the foreign body response (FBR), an inflammatory condition that can cause implant failure, tissue injury, and death of the patient. Macrophages accumulate and fuse to generate destructive foreign body giant cells (FBGCs) at the tissue-implant interface, leading to the development of fibrous scar tissue around the implant that is generated by myofibroblasts. We previously showed that the FBR in vivo and FBGC formation in vitro require transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive ion channel. Here, we report that TRPV4 was required specifically for the FBR induced by implant stiffness independently of biochemical cues and for intracellular stiffening that promotes FBGC formation in vitro. TRPV4 deficiency reduced collagen deposition and the accumulation of macrophages, FBGCs, and myofibroblasts at stiff, but not soft, implants in vivo and inhibited macrophage-induced differentiation of wild-type fibroblasts into myofibroblasts in vitro. Atomic force microscopy demonstrated that TRPV4 was required for implant-adjacent tissue stiffening in vivo and for cytoskeletal remodeling and intracellular stiffening induced by fusogenic cytokines in vitro. Together, these data suggest a mechanism whereby a reciprocal functional interaction between TRPV4 and substrate stiffness leads to cytoskeletal remodeling and cellular force generation to promote FBGC formation during the FBR.

Fibrin polymer on the surface of biomaterial implants drives the foreign body reaction

Implantation of biomaterials and medical devices in the body triggers the foreign body reaction (FBR) which is characterized by macrophage fusion at the implant surface leading to the formation of foreign body giant cells and the development of the fibrous capsule enveloping the implant. While adhesion of macrophages to the surface is an essential step in macrophage fusion and implanted biomaterials are known to rapidly acquire a layer of host proteins, a biological substrate that is responsible for this process in vivo is unknown. Here we show that mice with genetically imposed fibrinogen deficiency display a dramatic reduction of macrophage fusion on biomaterials implanted intraperitoneally and subcutaneously and are protected from the formation of the fibrin-containing fibrous capsule. Furthermore, macrophage fusion on biomaterials implanted in Fib^AEK mice that express a mutated form of fibrinogen incapable of thrombin-mediated polymerization was strongly reduced. Despite the lack of fibrin, the capsule was formed in Fib^AEK mice, although it had a different composition and distinct mechanical properties than that in wild-type mice. Specifically, while mononuclear α-SMA-expressing macrophages embedded in the capsule of both strains of mice secreted collagen, the amount of collagen and its density in the tissue of Fib^AEK mice was reduced. These data identify fibrin polymer as a key biological substrate driving the development of the FBR.

Mechanotransduction via a TRPV4-Rac1 signaling axis plays a role in multinucleated giant cell formation

Multinucleated giant cells are formed by the fusion of macrophages and are a characteristic feature in numerous pathophysiological conditions including the foreign body response (FBR). Foreign body giant cells (FBGCs) are inflammatory and destructive multinucleated macrophages and may cause damage and/or rejection of implants. However, while these features of FBGCs are well established, the molecular mechanisms underlying their formation remain elusive. Improved understanding of the molecular mechanisms underlying the formation of FBGCs may permit the development of novel implants that eliminate or reduce the FBR. Our previous study showed that transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive ion channel/receptor, is required for FBGC formation and FBR to biomaterials. Here, we have determined that (a) TRPV4 is directly involved in fusogenic cytokine (interleukin-4 plus granulocyte macrophage-colony stimulating factor)-induced activation of Rac1, in bone marrow-derived macrophages; (b) TRPV4 directly interacts with Rac1, and their interaction is further augmented in the presence of fusogenic cytokines; (c) TRPV4-dependent activation of Rac1 is essential for the augmentation of intracellular stiffness and regulation of cytoskeletal remodeling; and (d) TRPV4-Rac1 signaling axis is critical in fusogenic cytokine-induced FBGC formation. Together, these data suggest a novel mechanism whereby a functional interaction between TRPV4 and Rac1 leads to cytoskeletal remodeling and intracellular stiffness generation to modulate FBGC formation.

A versatile and customizable low-cost 3D-printed open standard for microscopic imaging

Modern microscopes used for biological imaging often present themselves as black boxes whose precise operating principle remains unknown, and whose optical resolution and price seem to be in inverse proportion to each other. With UC2 (You. See. Too.) we present a low-cost, 3D-printed, open-source, modular microscopy toolbox and demonstrate its versatility by realizing a complete microscope development cycle from concept to experimental phase. The self-contained incubator-enclosed brightfield microscope monitors monocyte to macrophage cell differentiation for seven days at cellular resolution level (e.g. 2 μm). Furthermore, by including very few additional components, the geometry is transferred into a 400 Euro light sheet fluorescence microscope for volumetric observations of a transgenic Zebrafish expressing green fluorescent protein (GFP). With this, we aim to establish an open standard in optics to facilitate interfacing with various complementary platforms. By making the content and comprehensive documentation publicly available, the systems presented here lend themselves to easy and straightforward replications, modifications, and extensions.

Open standard microscopy is urgently needed to give low-cost solutions to researchers and to overcome the reproducibility crisis in science. Here the authors present a 3D-printed, open-source modular microscopy toolbox UC2 (You. See. Too.) for a few hundred Euros.

Heterogeneity and Actin Cytoskeleton in Osteoclast and Macrophage Multinucleation

Osteoclast signatures are determined by two transcriptional programs, the lineage-determining transcription pathway and the receptor activator of nuclear factor kappa-B ligand (RANKL)-dependent differentiation pathways. During differentiation, mononuclear precursors become multinucleated by cell fusion. Recently, live-cell imaging has revealed a high level of heterogeneity in osteoclast multinucleation. This heterogeneity includes the difference in the differentiation states and the mobility of the fusion precursors, as well as the mode of fusion among the fusion precursors with different numbers of nuclei. In particular, fusion partners often form morphologically distinct actin-based linkages that allow two cells to exchange lipids and proteins before membrane fusion. However, the origin of this heterogeneity remains elusive. On the other hand, osteoclast multinucleation is sensitive to the environmental cues. Such cues promote the reorganization of the actin cytoskeleton, especially the formation and transformation of the podosome, an actin-rich punctate adhesion. This review covers the heterogeneity of osteoclast multinucleation at the pre-fusion stage with reference to the environment-dependent signaling pathway responsible for reorganizing the actin cytoskeleton. Furthermore, we compare osteoclast multinucleation with macrophage fusion, which results in multinucleated giant macrophages.

Tuning the immune reaction to manipulate the cell-mediated degradation of a collagen barrier membrane

In order to elicit a desired barrier function in guided bone regeneration (GBR) or guided tissue regeneration (GTR), a barrier membrane has to maintain its integrity for a certain period of time to guarantee the regeneration of target tissue. Due to the complexity and variety of clinical conditions, the healing time required for tissue regeneration varies from one case to another, which implies the need for tailoring the barrier membranes to diverse conditions via manipulating their degradation property. As a "non-self" biomaterial, a barrier membrane will inevitably trigger host-membrane immune response after implantation, which entails the activation of phagocytic cells. In the degradation process of a barrier membrane, the cell-mediated degradation may play a more vital role than enzymatic and physicochemical dissolution; however, limited studies have been carried out on this topic. In this context, we investigated the cell-mediated degradation and illustrated the possible key cells and mediators for immunomodulation via in vivo and in vitro studies. We discovered that IL-13, a key cytokine mainly released by T helper 2 cells (Th2), induced the formation of foreign body giant cells (FBGCs), thus resulting in membrane degradation. Neutralizing IL-13 could suppress membrane degradation and formation of FBGC. The contributions of this study are (1) unveiling the immune mechanisms underlying the cell-mediated collagen membrane degradation; (2) allowing the formation of an "immunodegradation" strategy to develop an "immune-smart" barrier membrane to manipulate its degradation; (3) providing the key regulatory immune cells and cytokines for the immunomodulation target in collagen membrane degradation. STATEMENT OF SIGNIFICANCE: The significance of this research includes.

Prostaglandin E2 and Its Receptor EP2 Modulate Macrophage Activation and Fusion in Vitro

The foreign body response (FBR) has impaired progress of new implantable medical devices through its hallmark of chronic inflammation and foreign body giant cell (FBGC) formation leading to fibrous encapsulation. Macrophages are known to drive the FBR, but efforts to control macrophage polarization remain challenging. The goal for this study was to investigate whether prostaglandin E2 (PGE2), and specifically its receptors EP2 and/or EP4, attenuate classically activated (i.e., inflammatory) macrophages and macrophage fusion into FBGCs in vitro. Lipopolysaccharide (LPS)-stimulated macrophages exhibited a dose-dependent decrease in gene expression and protein production of tumor necrosis factor alpha (TNF-α) when treated with PGE2. This attenuation was primarily by the EP4 receptor, as the addition of the EP2 antagonist PF 04418948 to PGE2-treated LPS-stimulated cells did not recover TNF-α production while the EP4 antagonist ONO AE3 208 did. However, direct stimulation of EP2 with the agonist butaprost to LPS-stimulated macrophages resulted in a ∼60% decrease in TNF-α secretion after 4 h and corresponded with an increase in gene expression for Cebpb and Il10, suggesting a polarization shift toward alternative activation through EP2 alone. Further, fusion of macrophages into FBGCs induced by interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF) was inhibited by PGE2 via EP2 signaling and by an EP2 agonist, but not an EP4 agonist. The attenuation by PGE2 was confirmed to be primarily by the EP2 receptor. Mrc1, Dcstamp, and Retlna expressions increased upon IL-4/GM-CSF stimulation, but only Retnla expression with the EP2 agonist returned to levels that were not different from controls. This study identified that PGE2 attenuates classically activated macrophages and macrophage fusion through distinct EP receptors, while targeting EP2 is able to attenuate both. In summary, this study identified EP2 as a potential therapeutic target for reducing the FBR to biomaterials.

Inflammation via myeloid differentiation primary response gene 88 signaling mediates the fibrotic response to implantable synthetic poly(ethylene glycol) hydrogels

Synthetic hydrogels, such as poly(ethylene glycol) (PEG), are promising for a range of in vivo applications. However, like all non-biological biomaterials, synthetic hydrogels including PEG elicit a foreign body response (FBR). The FBR is thought to be initiated by adsorbed protein that is recognized by and subsequently activates inflammatory cells, notably macrophages, and culminates with fibrotic encapsulation. However, the molecular mechanisms that drive the FBR are not well understood. Toll-like receptors (TLRs) are key receptors that recognize pathogens, but also recognize altered host proteins that display damage-associated molecular patterns (DAMPs). Thus TLRs may play a role in the FBR. Here, we investigated myeloid differentiation primary response gene 88 (MyD88), a signaling adaptor protein that mediates inflammatory cytokine production induced by most TLRs. An in vitro model was used consisting of macrophages cultured on the surface of synthetic hydrogels, specifically PEG, with pre-adsorbed serum proteins. Our in vitro findings demonstrate that MyD88-dependent signaling is the predominant inflammatory pathway in macrophage activation to synthetic hydrogels. When stimulated with TLR agonists to mimic additional DAMPs present in vivo, MyD88-dependent signaling was also the predominant pathway in macrophage activation. An in vivo model of PEG hydrogels implanted subcutaneously in wild-type and MyD88^-/- mice also demonstrated that MyD88 is the key contributor to the recruitment of inflammatory cells and formation of the fibrous capsule surrounding the implanted hydrogel. Taken together, findings from this study identify MyD88-mediated inflammation as being a critical pathway involved not only in the inflammatory response, but in formation of the fibrous capsule to PEG hydrogels. STATEMENT OF SIGNIFICANCE: Synthetic hydrogels are promising for in vivo applications but, like all non-biological biomaterials, synthetic hydrogels elicit a foreign body response (FBR). The molecular mechanisms that drive the FBR are not well understood. This work identifies the myeloid differentiation primary response gene 88 (MyD88) as a central mediator to macrophage activation in response to a poly(ethylene glycol) hydrogel with pre-adsorbed proteins in vitro. Moreover, MyD88 was also central to the recruitment of inflammatory cells, which included neutrophils, monocytes, and macrophages, to implanted PEG hydrogels and to fibrous encapsulation. These findings demonstrate that MyD88-mediated inflammation is responsible in part for the formation of the fibrous capsule of the FBR.

Factors influencing multinucleated giant cell formation in vitro

Macrophages fuse together to form multinucleated giant cells (MGC) in granulomas associated with various pathological conditions. Improved in vitro methods are required to better enable investigations of MGC biology and potential contribution to disease. There is a need for standardization of MGC quantification, purification of MGC populations, and characterization of how cell culture variables influence MGC formation. This study examined solutions to address these needs while providing context with other current and alternative methods. Primary mouse bone marrow-derived macrophages were treated with interleukin-4, a cytokine known to induce fusion into MGC. This model was used to systematically assess the influence of cell stimulant timing, cell seeding density, colony stimulating factors, and culture vessel type. Results indicated that MGC formation is greatly impacted by alterations in certain culture variables. An assessment of previously published research showed that these culture conditions varied widely between different laboratories, which may explain inconsistencies in the literature. A particularly novel and unexpected observation was that MGC formation appears to be greatly increased by silicone, which is a component of a chamber slide system commonly used for MGC studies. The most successful quantification method was fluorescent staining with semi-automated morphological evaluation. The most successful enrichment method was microfiltration. Overall, this study takes steps toward standardizing in vitro methods, enhancing replicability, and guiding investigators attempting to culture, quantify, and enrich MGC.

An actin-based protrusion originating from a podosome-enriched region initiates macrophage fusion

Macrophage fusion resulting in the formation of multinucleated giant cells occurs in a variety of chronic inflammatory diseases, yet the mechanism responsible for initiating this process is unknown. Here, we used live cell imaging to show that actin-based protrusions at the leading edge initiate macrophage fusion. Phase-contrast video microscopy demonstrated that in the majority of events, short protrusions (∼3 µm) between two closely apposed cells initiated fusion, but occasionally we observed long protrusions (∼12 µm). Using macrophages isolated from LifeAct mice and imaging with lattice light sheet microscopy, we further found that fusion-competent protrusions formed at sites enriched in podosomes. Inducing fusion in mixed populations of GFP- and mRFP-LifeAct macrophages showed rapid spatial overlap between GFP and RFP signal at the site of fusion. Cytochalasin B strongly reduced fusion and when rare fusion events occurred, protrusions were not observed. Fusion of macrophages deficient in Wiskott-Aldrich syndrome protein and Cdc42, key molecules involved in the formation of actin-based protrusions and podosomes, was also impaired both in vitro and in vivo. Finally, inhibiting the activity of the Arp2/3 complex decreased fusion and podosome formation. Together these data suggest that an actin-based protrusion formed at the leading edge initiates macrophage fusion.

Biophysical regulation of macrophages in health and disease

Macrophages perform critical functions for homeostasis and immune defense in tissues throughout the body. These innate immune cells are capable of recognizing and clearing dead cells and pathogens, and orchestrating inflammatory and healing processes that occur in response to injury. In addition, macrophages are involved in the progression of many inflammatory diseases including cardiovascular disease, fibrosis, and cancer. Although it has long been known that macrophages respond dynamically to biochemical signals in their microenvironment, the role of biophysical cues has only recently emerged. Furthermore, many diseases that involve macrophages are also characterized by changes to the tissue biophysical environment. This review will discuss current knowledge about the effects of biophysical cues including matrix stiffness, material topography, and applied mechanical forces, on macrophage behavior. We will also describe the role of molecules that are known to be important for mechanotransduction, including adhesion molecules, ion channels, as well as nuclear mediators such as transcription factors, scaffolding proteins, and epigenetic regulators. Together, this review will illustrate a developing role of biophysical cues in macrophage biology, and also speculate upon molecular targets that may potentially be exploited therapeutically to treat disease.

An Overview of the Derivation and Function of Multinucleated Giant Cells and Their Role in Pathologic Processes

Monocyte lineage cells play important roles in health and disease. Their differentiation into macrophages is crucial for a broad array of immunologic processes that regulate inflammation, neoplasia, and infection. In certain pathologic conditions, such as foreign body reactions and peripheral inflammatory lesions, monocytes fuse to form large, multinucleated giant cells (MGCs). Currently, our knowledge of the fusion mechanisms of monocytes and the regulation of MGC formation and function in discrete pathologies is limited. Herein, we consider the types and function of MGCs in disease and assess the mechanisms by which monocyte fusion contributes to the formation of MGCs. An improved understanding of the cellular origins and metabolic functions of MGCs will facilitate their identification and ultimately the treatment of diseases and disorders that involve MGCs.

Nanopatterned bulk metallic glass-based biomaterials modulate macrophage polarization

Polarization of macrophages by chemical, topographical and mechanical cues presents a robust strategy for designing immunomodulatory biomaterials. Here, we studied the ability of nanopatterned bulk metallic glasses (BMGs), a new class of metallic biomaterials, to modulate murine macrophage polarization. Cytokine/chemokine analysis of IL-4 or IFNγ/LPS-stimulated macrophages showed that the secretion of TNF-α, IL-1α, IL-12, CCL-2 and CXCL1 was significantly reduced after 24-hour culture on BMGs with 55 nm nanorod arrays (BMG-55). Additionally, under these conditions, macrophages increased phagocytic potential and exhibited decreased cell area with multiple actin protrusions. These in vitro findings suggest that nanopatterning can modulate biochemical cues such as IFNγ/LPS. In vivo evaluation of the subcutaneous host response at 2 weeks demonstrated that the ratio of Arg-1 to iNOS increased in macrophages adjacent to BMG-55 implants, suggesting modulation of polarization. In addition, macrophage fusion and fibrous capsule thickness decreased and the number and size of blood vessels increased, which is consistent with changes in macrophage responses. Our study demonstrates that nanopatterning of BMG implants is a promising technique to selectively polarize macrophages to modulate the immune response, and also presents an effective tool to study mechanisms of macrophage polarization and function.

STATEMENT OF SIGNIFICANCE

Implanted biomaterials elicit a complex series of tissue and cellular responses, termed the foreign body response (FBR), that can be influenced by the polarization state of macrophages. Surface topography can influence polarization, which is broadly characterized as either inflammatory or repair-like. The latter has been linked to improved outcomes of the FBR. However, the impact of topography on macrophage polarization is not fully understood, in part, due to a lack of high moduli biomaterials that can be reproducibly processed at the nanoscale. Here, we studied macrophage interactions with nanopatterned bulk metallic glasses (BMGs), a class of metallic alloys with amorphous microstructure and formability like polymers. We show that nanopatterned BMGs modulate macrophage polarization and transiently induce less fibrotic and more angiogenic responses. Overall, we demonstrate nanopatterning of BMG implants as a technique to polarize macrophages and modulate the FBR.

Zwitterionic PEG-PC Hydrogels Modulate the Foreign Body Response in a Modulus-Dependent Manner

Reducing the foreign body response (FBR) to implanted biomaterials will enhance their performance in tissue engineering. Poly(ethylene glycol) (PEG) hydrogels are increasingly popular for this application due to their low cost, ease of use, and the ability to tune their compliance via molecular weight and cross-linking densities. PEG hydrogels can elicit chronic inflammation in vivo, but recent evidence has suggested that extremely hydrophilic, zwitterionic materials and particles can evade the immune system. To combine the advantages of PEG-based hydrogels with the hydrophilicity of zwitterions, we synthesized hydrogels with comonomers PEG and the zwitterion phosphorylcholine (PC). Recent evidence suggests that stiff hydrogels elicit increased immune cell adhesion to hydrogels, which we attempted to reduce by increasing hydrogel hydrophilicity. Surprisingly, hydrogels with the highest amount of zwitterionic comonomer elicited the highest FBR. Lowering the hydrogel modulus (165 to 3 kPa), or PC content (20 to 0 wt %), mitigated this effect. A high density of macrophages was found at the surface of implants associated with a high FBR, and mass spectrometry analysis of the proteins adsorbed to these gels implicated extracellular matrix, immune response, and cell adhesion protein categories as drivers of macrophage recruitment. Overall, we show that modulus regulates macrophage adhesion to zwitterionic-PEG hydrogels, and demonstrate that chemical modifications to hydrogels should be studied in parallel with their physical properties to optimize implant design.

Common signalling pathways in macrophage and osteoclast multinucleation

Macrophage cell fusion and multinucleation are fundamental processes in the formation of multinucleated giant cells (MGCs) in chronic inflammatory disease and osteoclasts in the regulation of bone mass. However, this basic cell phenomenon is poorly understood despite its pathophysiological relevance. Granulomas containing multinucleated giant cells are seen in a wide variety of complex inflammatory disorders, as well as in infectious diseases. Dysregulation of osteoclastic bone resorption underlies the pathogenesis of osteoporosis and malignant osteolytic bone disease. Recent reports have shown that the formation of multinucleated giant cells and osteoclast fusion display a common molecular signature, suggesting shared genetic determinants. In this Review, we describe the background of cell-cell fusion and the similar origin of macrophages and osteoclasts. We specifically focus on the common pathways involved in osteoclast and MGC fusion. We also highlight potential approaches that could help to unravel the core mechanisms underlying bone and granulomatous disorders in humans.

The in vitro effects of macrophages on the osteogenic capabilities of MC3T3-E1 cells encapsulated in a biomimetic poly(ethylene glycol) hydrogel

Poly(ethylene glycol) PEG-based hydrogels are promising for cell encapsulation and tissue engineering, but are known to elicit a foreign body response (FBR) in vivo. The goal of this study was to investigate the impact of the FBR, and specifically the presence of inflammatory macrophages, on encapsulated cells and their ability to synthesize new extracellular matrix. This study employed an in vitro co-culture system with murine macrophages and MC3T3-E1 pre-osteoblasts encapsulated in a bone-mimetic hydrogel, which were cultured in transwell inserts, and exposed to an inflammatory stimulant, lipopolysaccharide (LPS). The co-culture was compared to mono-cultures of the cell-laden hydrogels alone and with LPS over 28 days. Two macrophage cell sources, RAW 264.7 and primary derived, were investigated. The presence of LPS-stimulated primary macrophages led to significant changes in the cell-laden hydrogel by a 5.3-fold increase in percent apoptotic osteoblasts at day 28, 4.2-fold decrease in alkaline phosphatase activity at day 10, and 7-fold decrease in collagen deposition. The presence of LPS-stimulated RAW macrophages led to significant changes in the cell-laden hydrogel by 5-fold decrease in alkaline phosphatase activity at day 10 and 4-fold decrease in collagen deposition. Mineralization, as measured by von Kossa stain or quantified by calcium content, was not sensitive to macrophages or LPS. Elevated interleukin-6 and tumor necrosis factor-α secretion were detected in mono-cultures with LPS and co-cultures. Overall, primary macrophages had a more severe inhibitory effect on osteoblast differentiation than the macrophage cell line, with greater apoptosis and collagen I reduction. In summary, this study highlights the detrimental effects of macrophages on encapsulated cells for bone tissue engineering.

STATEMENT OF SIGNIFICANCE

Poly(ethylene glycol) (PEG)-based hydrogels are promising for cell encapsulation and tissue engineering, but are known to elicit a foreign body response (FBR) in vivo. The impact of the FBR on encapsulated cells and their ability to synthesize tissue has not been well studied. This study utilizes thiol-ene click chemistry to create a biomimetic, enzymatically degradable hydrogel system with which to encapsulate MC3T3-E1 pre-osteoblasts. The osteogenic capabilities and differentiation of these cellswerestudied in co-culture with macrophages, known drivers of the FBR.This study demonstrates that macrophages reduce osteogenic capabilities of encapsulated cellsin vitroand suggestthat the FBR should be considered for in vivo tissue engineering.

Multicompartment Drug Release System for Dynamic Modulation of Tissue Responses

Pharmacological modulation of responses to injury is complicated by the need to deliver multiple drugs with spatiotemporal resolution. Here, a novel controlled delivery system containing three separate compartments with each releasing its contents over different timescales is fabricated. Core-shell electrospun fibers create two of the compartments in the system, while electrosprayed spheres create the third. Utility is demonstrated by targeting the foreign body response to implants because it is a dynamic process resulting in implant failure. Sequential delivery of a drug targeting nuclear factor-κB (NF-κB) and an antifibrotic is characterized in in vitro experiments. Specifically, macrophage fusion and p65 nuclear translocation in the presence of releasate or with macrophages cultured on the surfaces of the constructs are evaluated. In addition, releasate from pirfenidone scaffolds is shown to reduce transforming growth factor-β (TGF-β)-induced pSMAD3 nuclear localization in fibroblasts. In vivo, drug eluting constructs successfully mitigate macrophage fusion at one week and fibrotic encapsulation in a dose-dependent manner at four weeks, demonstrating effective release of both drugs over different timescales. Future studies can employ this system to improve and prolong implant lifetimes, or load it with other drugs to modulate other dynamic processes.

Morphologic, phenotypic, and transcriptomic characterization of classically and alternatively activated canine blood-derived macrophages in vitro

Macrophages are a heterogeneous cell population playing a pivotal role in tissue homeostasis and inflammation, and their phenotype strongly depends on the micromilieu. Despite its increasing importance as a translational animal model for human diseases, there is a considerable gap of knowledge with respect to macrophage polarization in dogs. The present study comprehensively investigated the morphologic, phenotypic, and transcriptomic characteristics of unstimulated (M0), M1- (GM-CSF, LPS, IFNγ-stimulated) and M2- (M-CSF, IL-4-stimulated)-polarized canine blood-derived macrophages in vitro. Scanning electron microscopy revealed distinct morphologies of polarized macrophages with formation of multinucleated cells in M2-macrophages, while immunofluorescence employing literature-based prototype-antibodies against CD16, CD32, iNOS, MHC class II (M1-markers), CD163, CD206, and arginase-1 (M2-markers) demonstrated that only CD206 was able to discriminate M2-macrophages from both other phenotypes, highlighting this molecule as a promising marker for canine M2-macrophages. Global microarray analysis revealed profound changes in the transcriptome of polarized canine macrophages. Functional analysis pointed out that M1-polarization was associated with biological processes such as "respiratory burst", whereas M2-polarization was associated with processes such as "mitosis". Literature-based marker gene selection revealed only minor overlaps in the gene sets of the dog compared to prototype markers of murine and human macrophages. Biomarker selection using supervised clustering suggested latexin (LXN) and membrane-spanning 4-domains, subfamily A, member 2 (MS4A2) to be the most powerful predicting biomarkers for canine M1- and M2-macrophages, respectively. Immunofluorescence for both markers demonstrated expression of both proteins by macrophages in vitro but failed to reveal differences between canine M1 and M2-macrophages. The present study provides a solid basis for future studies upon the role of macrophage polarization in spontaneous diseases of the dog, a species that has emerging importance for translational research.

The host response to naturally-derived extracellular matrix biomaterials

Biomaterials based on natural materials including decellularized tissues and tissue-derived hydrogels are becoming more widely used for clinical applications. Because of their native composition and structure, these biomaterials induce a distinct form of the foreign body response that differs from that of non-native biomaterials. Differences include direct interactions with cells via preserved moieties as well as the ability to undergo remodeling. Moreover, these biomaterials could elicit adaptive immune responses due to the presence of modified native molecules. Therefore, these biomaterials present unique challenges in terms of understanding the progression of the foreign body response. This review covers this response to natural materials including natural polymers, decellularized tissues, cell-derived matrix, tissue derived hydrogels, and biohybrid materials. With the expansion of the fields of regenerative medicine and tissue engineering, the current repertoire of biomaterials has also expanded and requires continuous investigation of the responses they elicit.

Gpr132 sensing of lactate mediates tumor-macrophage interplay to promote breast cancer metastasis

Macrophages are prominent immune cells in the tumor microenvironment that exert potent effects on cancer metastasis. However, the signals and receivers for the tumor-macrophage communication remain enigmatic. Here, we show that G protein-coupled receptor 132 (Gpr132) functions as a key macrophage sensor of the rising lactate in the acidic tumor milieu to mediate the reciprocal interaction between cancer cells and macrophages during breast cancer metastasis. Lactate activates macrophage Gpr132 to promote the alternatively activated macrophage (M2)-like phenotype, which, in turn, facilitates cancer cell adhesion, migration, and invasion. Consequently, Gpr132 deletion reduces M2 macrophages and impedes breast cancer lung metastasis in mice. Clinically, Gpr132 expression positively correlates with M2 macrophages, metastasis, and poor prognosis in patients with breast cancer. These findings uncover the lactate-Gpr132 axis as a driver of breast cancer metastasis by stimulating tumor-macrophage interplay, and reveal potential new therapeutic targets for breast cancer treatment.

Regulation of cell-cell fusion by nanotopography

Cell-cell fusion is fundamental to a multitude of biological processes ranging from cell differentiation and embryogenesis to cancer metastasis and biomaterial-tissue interactions. Fusogenic cells are exposed to biochemical and biophysical factors, which could potentially alter cell behavior. While biochemical inducers of fusion such as cytokines and kinases have been identified, little is known about the biophysical regulation of cell-cell fusion. Here, we designed experiments to examine cell-cell fusion using bulk metallic glass (BMG) nanorod arrays with varying biophysical cues, i.e. nanotopography and stiffness. Through independent variation of stiffness and topography, we found that nanotopography constitutes the primary biophysical cue that can override biochemical signals to attenuate fusion. Specifically, nanotopography restricts cytoskeletal remodeling-associated signaling, which leads to reduced fusion. This finding expands our fundamental understanding of the nanoscale biophysical regulation of cell fusion and can be exploited in biomaterials design to induce desirable biomaterial-tissue interactions.

The In Vitro and In Vivo Response to MMP-Sensitive Poly(Ethylene Glycol) Hydrogels

Enzyme-sensitive hydrogels are a promising class of materials for cell encapsulation and tissue engineering because their ability to be degraded by cell-secreted factors. However, it is well known that nearly all synthetic biomaterials elicit a foreign body response (FBR) upon implantation. Therefore, this study aimed to evaluate the in vitro and in vivo response to an enzyme-sensitive hydrogel. Hydrogels were formed from poly(ethylene glycol) with the peptide crosslinker, C-VPLS↓LYSG-C, which is susceptible to matrix metalloproteinases 2 and 9. We evaluated the hydrogel by exogenously delivered enzymes, encapsulated mesenchymal stem cells as a tissue engineering relevant cell type, and by macrophage-secreted factors in vitro and for the FBR through macrophage attachment in vitro and in a subcutaneous mouse model. These hydrogels rapidly degraded upon exposure to exogenous MMP-2 and to lesser degree with MMP-9. Encapsulated mesenchymal stem cells were capable of degrading the hydrogels via matrix metalloproteinases. Inflammatory macrophages were confirmed to attach to the hydrogels, but were not capable of rapidly degrading the hydrogels. In vivo, these hydrogels remained intact after 4 weeks and exhibited a classic FBR with inflammatory cells at the hydrogel surface and a fibrous capsule. In summary, these findings suggest that while this MMP-2/9 sensitive hydrogel is readily degraded in vitro, it does not undergo rapid degradation by the FBR. Thus, the long term stability of these hydrogels in vivo coupled with the ability for encapsulated cells to degrade the hydrogel makes them promising materials for tissue engineering.

Nanoparticle delivery of miR-223 to attenuate macrophage fusion

The foreign body response (FBR) begins with injury acquired during implantation of a biomaterial (BM) and is detrimental due to the eventual encapsulation of the implant. Fusion of macrophages to form foreign body giant cells (FBGC), a hallmark of the FBR, is the consequence of a multistep mechanism induced by interleukin (IL)-4 that includes the acquisition of a fusion competent state and subsequent cytoskeletal rearrangements. However, the precise mechanism, regulation, and interplay among molecular mediators to generate FBGCs are insufficiently understood. Seeking novel mediators of fusion that might be regulated at the post-transcriptional level, we examined the role of microRNAs (miRs) in this process. A miR microarray was screened and identified miR-223 as a negative regulator of macrophage fusion. In addition, transfection of primary macrophages with a mir-223 mimic attenuated IL-4-induced fusion. Furthermore, miR-223 KO mice and mir-223 deficient cells displayed increased fusion in vivo and in vitro, respectively. Finally, we developed a method for in vivo delivery of miR-223 mimic utilizing PLGA nanoparticles, which inhibited FBGC formation in a biomaterial implant model. Our results identify miR-223 as a negative regulator of fusion and demonstrate miR-223 mimic-loaded nanoparticles as a therapeutic inhibitor of macrophage fusion.

Biomimetic collagenous scaffold to tune inflammation by targeting macrophages

The inflammatory response following implantation of a biomaterial is one of the major regulatory aspects of the overall regenerative process. The progress of inflammation determines whether functional tissue is restored or if nonfunctional fibrotic tissue is formed. This delicate balance is directed by the activity of different cells. Among these, macrophages and their different phenotypes, the inflammatory M1 to anti-inflammatory M2, are considered key players in the process. Recent approaches exploit macrophage’s regenerative potential in tissue engineering. Here, we propose a collagen scaffold functionalized with chondroitin sulfate (CSCL), a glycosaminoglycan known to be able to tune inflammation. We studied CSCL effects on bone-marrow-derived macrophages in physiological, and lipopolysaccharides-inflamed, conditions in vitro. Our data demonstrate that CSCL is able to modulate macrophage phenotype by inhibiting the LPS/CD44/NF-kB cascade. As a consequence, an upregulation of anti-inflammatory markers (TGF-β, Arg, MRC1, and IL-10) was found concomitantly with a decrease in the expression of pro-inflammatory markers (iNOS, TNF-α, IL-1β, IL-12β). We then implanted CSCL subcutaneously in a rat model to test whether the same molecular mechanism could be maintained in an in vivo environment. In vivo data confirmed the in vitro studies. A significant reduction in the number of infiltrating cells around and within the implants was observed at 72 h, with a significant downregulation of pro-inflammatory genes expression. The present work provides indications regarding the immunomodulatory potential of molecules used for the development of biomimetic materials and suggests their use to direct macrophage immune modulation for tissue repair.

Multinucleated Giant Cells Are Specialized for Complement-Mediated Phagocytosis and Large Target Destruction

Multinucleated giant cells (MGCs) form by fusion of macrophages and are presumed to contribute to the removal of debris from tissues. In a systematic in vitro analysis, we show that IL-4-induced MGCs phagocytosed large and complement-opsonized materials more effectively than their unfused M2 macrophage precursors. MGC expression of complement receptor 4 (CR4) was increased, but it functioned primarily as an adhesion integrin. In contrast, although expression of CR3 was not increased, it became functionally activated during fusion and was located on the extensive membrane ruffles created by excess plasma membrane arising from macrophage fusion. The combination of increased membrane area and activated CR3 specifically equips MGCs to engulf large complement-coated targets. Moreover, we demonstrate these features in vivo in the recently described complement-dependent therapeutic elimination of systemic amyloid deposits by MGCs. MGCs are evidently more than the sum of their macrophage parts.

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MGCs are specialized for phagocytosis of large and complement-opsonized particles

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MGCs show extensive membrane ruffles containing pre-activated complement receptor 3

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Membrane ruffles provide excess membrane for ingestion of large materials

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MGCs eliminate systemic amyloid deposits after immunotherapeutic targeting

MicroRNA-mediated immune modulation as a therapeutic strategy in host-implant integration

The concept of implanting an artificial device into the human body was once the preserve of science fiction, yet this approach is now often used to replace lost or damaged biological structures in human patients. However, assimilation of medical devices into host tissues is a complex process, and successful implant integration into patients is far from certain. The body's immediate response to a foreign object is immune-mediated reaction, hence there has been extensive research into biomaterials that can reduce or even ablate anti-implant immune responses. There have also been attempts to embed or coat anti-inflammatory drugs and pro-regulatory molecules onto medical devices with the aim of preventing implant rejection by the host. In this review, we summarize the key immune mediators of medical implant reaction, and we evaluate the potential of microRNAs to regulate these processes to promote wound healing, and prolong host-implant integration.

Th2-M2 immunity in lesions of muscular sarcoidosis and macrophagic myofasciitis

OBJECTIVE

To analyse the paradox of a lack of giant cell formation and fibrosis in chronic lesions of macrophagic myofasciitis (MMF) in comparison with muscular sarcoidosis (MuS).

METHODS

Inflammatory lesions and contiguous muscle regions from biopsy samples of 10 patients with MuS and 10 patients with MMF were cut out by laser microdissection. Mediators of the T helper cell (Th)1 inducing classical macrophage activation (e.g. STAT1, IFNγ and CXCR3), and Th2 inducing alternative activation of macrophages (e.g. CD206/MRC1, STAT6, SOCS1), molecules involved in development of fibrosis (e.g. TGFβ) and giant cells (e.g. TYROBP), were assessed by immunohistochemistry and real-time polymerase chain reaction (PCR).

RESULTS

STAT6-induced Th2 immunity was associated with up-regulated gene expression of MRC1, SOCS1 and TGFB in inflammatory foci, in comparison with adjacent tissue. TYROBP and TREM2, genes regulating giant cell formation, were more strongly expressed in lesions of MuS patients than in those of MMF. TGFβ co-localized with CD206(+) macrophages in MuS but not in MMF. Conversely, Th1 immunity was illustrated by STAT1 staining both in macrophages and myofibres in MuS, but not in MMF. Also, STAT1-induced IFNG and CXCR3 expression in lesions and the surrounding tissue was elevated compared with normal controls, but without statistically significant differences.

CONCLUSION

Giant cell and typical granuloma formations, including fibrogenesis, is dependent on two main mechanisms, both involving specific macrophage activation: a strong Th2-M2 polarization and a significant expression of TYROBP and TGFβ in macrophages. The low-grade alternative activation of macrophages in MMF lesions and poor TYROBP and TGFβco-expression are obviously insufficient to produce giant cells.

Regulation of macrophage motility by the water channel aquaporin-1: crucial role of M0/M2 phenotype switch

The water channel aquaporin-1 (AQP1) promotes migration of many cell types. Although AQP1 is expressed in macrophages, its potential role in macrophage motility, particularly in relation with phenotype polarization, remains unknown. We here addressed these issues in peritoneal macrophages isolated from AQP1-deficient mice, either undifferentiated (M0) or stimulated with LPS to orientate towards pro-inflammatory phenotype (classical macrophage activation; M1). In non-stimulated macrophages, ablation of AQP1 (like inhibition by HgCl2) increased by 2-3 fold spontaneous migration in a Src/PI3K/Rac-dependent manner. This correlated with cell elongation and formation of lamellipodia/ruffles, resulting in membrane lipid and F4/80 recruitment to the leading edge. This indicated that AQP1 normally suppresses migration of resting macrophages, as opposed to other cell types. Resting Aqp1-/- macrophages exhibited CD206 redistribution into ruffles and increased arginase activity like IL4/IL13 (alternative macrophage activation; M2), indicating a M0-M2 shift. In contrast, upon M1 orientation by LPS in vitro or peritoneal inflammation in vivo, migration of Aqp1-/- macrophages was reduced. Taken together, these data indicate that AQP1 oppositely regulates macrophage migration, depending on stimulation or not by LPS, and that macrophage phenotypic and migratory changes may be regulated independently of external cues.

Linking the foreign body response and protein adsorption to PEG-based hydrogels using proteomics

Poly(ethylene glycol) (PEG) hydrogels with their highly tunable properties are promising implantable materials, but as with all non-biological materials, they elicit a foreign body response (FBR). Recent studies, however, have shown that incorporating the oligopeptide RGD into PEG hydrogels reduces the FBR. To better understand the mechanisms involved and the role of RGD in mediating the FBR, PEG, PEG-RGD and PEG-RDG hydrogels were investigated. After a 28-day subcutaneous implantation in mice, a thinner and less dense fibrous capsule formed around PEG-RGD hydrogels, while PEG and PEG-RDG hydrogels exhibited stronger, but similar FBRs. Protein adsorption to the hydrogels, which is considered the first step in the FBR, was also characterized. In vitro experiments confirmed that serum proteins adsorbed to PEG-based hydrogels and were necessary to promote macrophage adhesion to PEG and PEG-RDG, but not PEG-RGD hydrogels. Proteins adsorbed to the hydrogels in vivo were identified using liquid chromatography-tandem mass spectrometry. The majority (245) of the total proteins (≥300) that were identified was present on all hydrogels with many proteins being associated with wounding and acute inflammation. These findings suggest that the FBR to PEG hydrogels may be mediated by the presence of inflammatory-related proteins adsorbed to the surface, but that macrophages appear to sense the underlying chemistry, which for RGD improves the FBR.

Loss of monocyte chemoattractant protein-1 alters macrophage polarization and reduces NFκB activation in the foreign body response

Implantation of biomaterials elicits a foreign body response characterized by fusion of macrophages to form foreign body giant cells and fibrotic encapsulation. Studies of the macrophage polarization involved in this response have suggested that alternative (M2) activation is associated with more favorable outcomes. Here we investigated this process in vivo by implanting mixed cellulose ester filters or polydimethylsiloxane disks in the peritoneal cavity of wild-type (WT) and monocyte chemoattractant protein-1 (MCP-1) knockout mice. We analyzed classical (M1) and alternative (M2) gene expression via quantitative polymerase chain reaction, immunohistochemistry and enzyme-linked immunosorbent assay in both non-adherent cells isolated by lavage and implant-adherent cells. Our results show that macrophages undergo unique activation that displays features of both M1 and M2 polarization including induction of tumor necrosis factor α (TNF), which induces the expression and nuclear translocation of p50 and RelA determined by immunofluorescence and Western blot. Both processes were compromised in fusion-deficient MCP-1 KO macrophages in vitro and in vivo. Furthermore, inclusion of BAY 11-7028, an inhibitor of NFκB activation, reduced nuclear translocation of RelA and fusion in WT macrophages. Our studies suggest that peritoneal implants elicit a unique macrophage polarization phenotype leading to induction of TNF and activation of the NFκB pathway.

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.

Immunomodulation by mesenchymal stem cells combats the foreign body response to cell-laden synthetic hydrogels

The implantation of non-biological materials, including scaffolds for tissue engineering, ubiquitously leads to a foreign body response (FBR). We recently reported that this response negatively impacts fibroblasts encapsulated within a synthetic hydrogel and in turn leads to a more severe FBR, suggesting a cross-talk between encapsulated cells and inflammatory cells. Given the promise of mesenchymal stem cells (MSCs) in tissue engineering and recent evidence of their immunomodulatory properties, we hypothesized that MSCs encapsulated within poly(ethylene glycol) (PEG) hydrogels will attenuate the FBR. In vitro, murine MSCs encapsulated within PEG hydrogels attenuated classically activated primary murine macrophages by reducing gene expression and protein secretion of pro-inflammatory cytokines, most notably tumor necrosis factor-α. Using a COX2 inhibitor, prostaglandin E2 (PGE2) was identified as a mediator of MSC immunomodulation of macrophages. In vivo, hydrogels laden with MSCs, osteogenically differentiating MSCs, or no cells were implanted subcutaneously into C57BL/6 mice for 28 days to assess the impact of MSCs on the fibrotic response of the FBR. The presence of encapsulated MSCs reduced fibrous capsule thickness compared to acellular hydrogels, but this effect diminished with osteogenic differentiation. The use of MSCs prior to differentiation in tissue engineering may therefore serve as a dynamic approach, through continuous cross-talk between MSCs and the inflammatory cells, to modulate macrophage activation and attenuate the FBR to implanted synthetic scaffolds thus improving the long-term tissue engineering outcome.

Engineering cellular response using nanopatterned bulk metallic glass

Nanopatterning of biomaterials is rapidly emerging as a tool to engineer cell function. Bulk metallic glasses (BMGs), a class of biocompatible materials, are uniquely suited to study nanopattern-cell interactions as they allow for versatile fabrication of nanopatterns through thermoplastic forming. Work presented here employs nanopatterned BMG substrates to explore detection of nanopattern feature sizes by various cell types, including cells that are associated with foreign body response, pathology, and tissue repair. Fibroblasts decreased in cell area as the nanopattern feature size increased, and fibroblasts could detect nanopatterns as small as 55 nm in size. Macrophages failed to detect nanopatterns of 150 nm or smaller in size, but responded to a feature size of 200 nm, resulting in larger and more elongated cell morphology. Endothelial cells responded to nanopatterns of 100 nm or larger in size by a significant decrease in cell size and elongation. On the basis of these observations, nondimensional analysis was employed to correlate cellular morphology and substrate nanotopography. Analysis of the molecular pathways that induce cytoskeletal remodeling, in conjunction with quantifying cell traction forces with nanoscale precision using a unique FIB-SEM technique, enabled the characterization of underlying biomechanical cues. Nanopatterns altered serum protein adsorption and effective substrate stiffness, leading to changes in focal adhesion density and compromised activation of Rho-A GTPase in fibroblasts. As a consequence, cells displayed restricted cell spreading and decreased collagen production. These observations suggest that topography on the nanoscale can be designed to engineer cellular responses to biomaterials.

A high-content imaging assay for the quantification of the Burkholderia pseudomallei induced multinucleated giant cell (MNGC) phenotype in murine macrophages

BACKGROUND

Burkholderia pseudomallei (Bp), a Gram-negative, motile, facultative intracellular bacterium is the causative agent of melioidosis in humans and animals. The Bp genome encodes a repertoire of virulence factors, including the cluster 3 type III secretion system (T3SS-3), the cluster 1 type VI secretion system (T6SS-1), and the intracellular motility protein BimA, that enable the pathogen to invade both phagocytic and non-phagocytic cells. A unique hallmark of Bp infection both in vitro and in vivo is its ability to induce cell-to-cell fusion of macrophages to form multinucleated giant cells (MNGCs), which to date are semi-quantitatively reported following visual inspection.

RESULTS

In this study we report the development of an automated high-content image acquisition and analysis assay to quantitate the Bp induced MNGC phenotype. Validation of the assay was performed using T6SS-1 (∆hcp1) and T3SS-3 (∆bsaZ) mutants of Bp that have been previously reported to exhibit defects in their ability to induce MNGCs. Finally, screening of a focused small molecule library identified several Histone Deacetylase (HDAC) inhibitors that inhibited Bp-induced MNGC formation of macrophages.

CONCLUSIONS

We have successfully developed an automated HCI assay to quantitate MNGCs induced by Bp in macrophages. This assay was then used to characterize the phenotype of the Bp mutants for their ability to induce MNGC formation and identify small molecules that interfere with this process. Successful application of chemical genetics and functional reverse genetics siRNA approaches in the MNGC assay will help gain a better understanding of the molecular targets and cellular mechanisms responsible for the MNGC phenotype induced by Bp, by other bacteria such as Mycobacterium tuberculosis, or by exogenously added cytokines.

The responses of endothelial cells to Zr₆ ₁Ti ₂Cu₂₅Al₁₂ metallic glass in vitro and in vivo

The goal of this study was to evaluate the biocompatibility of newly developed Zr61Ti2Cu25Al12 metallic glass (denoted ZT1) and its parallel material, commercially pure titanium (CP-Ti), as dental implants. To this end, we evaluated their cytotoxicity in vitro using human umbilical vein endothelial cells (HUVECs) and in vivo by performing an oral mucosa irritation test in hamsters. Specifically, cytotoxicity was determined in HUVECs by evaluating their cell morphology using scanning electron microscopy (SEM) and their cell viability using CCK-8 assays. Moreover, we examined the mRNA levels of vascular endothelial growth factor and von Willebrand factor by quantitative PCR (qPCR). In the early stages of proliferation and differentiation, no differences were observed between HUVECs inoculated on ZT1 compared to those on CP-Ti. However, in the later stages of proliferation and differentiation, the HUVECs inoculated on ZT1 were significantly better than the cells cultured on CP-Ti. In the oral mucosa irritation test, we sutured sample discs into the cheek pouch of hamsters. After 2, 3, and 4weeks, we harvested the corresponding mucosal tissues, stained them with hematoxylin and eosin, obtained blood samples for biochemical analysis, and finally, observed the topography of the sample discs by SEM. Immunohistochemistry and hematology analyses showed no differences in the biocompatibility of ZT1 and CP-Ti, and neither of these compounds caused irritation of the mucosa. In addition, SEM images showed that no pitting occurred on the sample discs. Together, these data indicate that ZT1 may be a good candidate for dental implants and should be further studied.

Lipopolysaccharide-induced multinuclear cells: increased internalization of polystyrene beads and possible signals for cell fusion

A murine macrophage-derived line, RAW264.7, becomes multinuclear on stimulation with lipopolysaccharide (LPS), an outer membrane component of Gram-negative bacteria. These multinuclear cells internalized more polystyrene beads than mononuclear cells or osteoclasts (Nakanishi-Matsui, M., Yano, S., Matsumoto, N., and Futai, M., 2012). In this study, we analyzed the time courses of cell fusion in the presence of large beads. They were internalized into cells actively fusing to become multinuclear. However, the multinuclear cells once formed showed only low phagocytosis activity. These results suggest that formation of the multinuclear cells and bead internalization took place simultaneously. The formation of multinuclear cells was blocked by inhibitors for phosphoinositide 3-kinase, phospholipase C, calcineurin, and c-Jun N-terminal kinase. In addition, interleukin 6 and 10 also exhibited inhibitory effects. These signaling molecules and cytokines may play a crucial role in the LPS-induced multinuclear cell formation.

Modulation of macrophage phenotype by cell shape

Phenotypic polarization of macrophages is regulated by a milieu of cues in the local tissue microenvironment. Although much is known about how soluble factors influence macrophage polarization, relatively little is known about how physical cues present in the extracellular environment might modulate proinflammatory (M1) vs. prohealing (M2) activation. Specifically, the role of cell shape has not been explored, even though it has been observed that macrophages adopt different geometries in vivo. We and others observed that macrophages polarized toward different phenotypes in vitro exhibit dramatic changes in cell shape: M2 cells exhibit an elongated shape compared with M1 cells. Using a micropatterning approach to control macrophage cell shape directly, we demonstrate here that elongation itself, without exogenous cytokines, leads to the expression of M2 phenotype markers and reduces the secretion of inflammatory cytokines. Moreover, elongation enhances the effects of M2-inducing cytokines IL-4 and IL-13 and protects cells from M1-inducing stimuli LPS and IFN-γ. In addition shape- but not cytokine-induced polarization is abrogated when actin and actin/myosin contractility are inhibited by pharmacological agents, suggesting a role for the cytoskeleton in the control of macrophage polarization by cell geometry. Our studies demonstrate that alterations in cell shape associated with changes in ECM architecture may provide integral cues to modulate macrophage phenotype polarization.

STATs and macrophage fusion

Macrophages play a pivotal role in host defense against multiple foreign materials such as bacteria, parasites and artificial devices. Some macrophage lineage cells, namely osteoclasts and foreign body giant cells (FBGCs), form multi-nuclear giant cells by the cell-cell fusion of mono-nuclear cells. Osteoclasts are bone-resorbing cells, and are formed in the presence of RANKL on the surface of bones, while FBGCs are formed in the presence of IL-4 or IL-13 on foreign materials such as artificial joints, catheters and parasites. Recently, fusiogenic mechanisms and the molecules required for the cell-cell fusion of these macrophage lineage cells were, at least in part, clarified. Dendritic cell specific transmembrane protein (DC-STAMP) and osteoclast stimulatory transmembrane protein (OC-STAMP), both of which comprise seven transmembrane domains, are required for both osteoclast and FBGC cell-cell fusion. STAT6 was demonstrated to be required for the cell-cell fusion of FBGCs but not osteoclasts. In this review, advances in macrophage cell-cell fusion are discussed.

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.

RNA therapeutics targeting osteoclast-mediated excessive bone resorption

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing technique developed with dramatically increasing utility for both scientific and therapeutic purposes. Short interfering RNA (siRNA) is currently exploited to regulate protein expression relevant to many therapeutic applications, and commonly used as a tool for elucidating disease-associated genes. Osteoporosis and their associated osteoporotic fragility fractures in both men and women are rapidly becoming a global healthcare crisis as average life expectancy increases worldwide. New therapeutics are needed for this increasing patient population. This review describes the diversity of molecular targets suitable for RNAi-based gene knock down in osteoclasts to control osteoclast-mediated excessive bone resorption. We identify strategies for developing targeted siRNA delivery and efficient gene silencing, and describe opportunities and challenges of introducing siRNA as a therapeutic approach to hard and connective tissue disorders.