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Efficient cellular fusion of mononuclear precursors is the prerequisite for the generation of fully functional multinucleated bone-resorbing osteoclasts. However, the exact molecular factors and mechanisms controlling osteoclast fusion remain incompletely understood. Here we identify RANKL-mediated activation of caspase-8 as early key event during osteoclast fusion. Single cell RNA sequencing-based analyses suggested that activation of parts of the apoptotic machinery accompanied the differentiation of osteoclast precursors into mature multinucleated osteoclasts. A subsequent characterization of osteoclast precursors confirmed that RANKL-mediated activation of caspase-8 promoted the non-apoptotic cleavage and activation of downstream effector caspases that translocated to the plasma membrane where they triggered activation of the phospholipid scramblase Xkr8. Xkr8-mediated exposure of phosphatidylserine, in turn, aided cellular fusion of osteoclast precursors and thereby allowed generation of functional multinucleated osteoclast syncytia and initiation of bone resorption. Pharmacological blockage or genetic deletion of caspase-8 accordingly interfered with fusion of osteoclasts and bone resorption resulting in increased bone mass in mice carrying a conditional deletion of caspase-8 in mononuclear osteoclast precursors. These data identify a novel pathway controlling osteoclast biology and bone turnover with the potential to serve as target for therapeutic intervention during diseases characterized by pathologic osteoclast-mediated bone loss. Proposed model of osteoclast fusion regulated by caspase-8 activation and PS exposure. RANK/RANK-L interaction. Activation of procaspase-8 into caspase-8. Caspase-8 activates caspase-3. Active capase-3 cleaves Xkr8. Local PS exposure is induced. Exposed PS is recognized by the fusion partner. FUSION. PS is re-internalized., (© 2024. The Author(s).)

Chronic Kidney Disease (CKD) is associated with markedly increased cardiovascular (CV) morbidity and mortality. Chronic inflammation, a hallmark of both CKD and CV diseases (CVD), is believed to drive this association. Pro-inflammatory endogenous TLR agonists, Damage-Associated Molecular Patterns (DAMPs), have been found elevated in CKD patients' plasma and suggested to promote CVD, however, confirmation of their involvement, the underlying mechanism(s), the extent to which individual DAMPs contribute to vascular pathology in CKD and the evaluation of potential therapeutic strategies, have remained largely undescribed. A multi-TLR inhibitor, soluble TLR2, abrogated chronic vascular inflammatory responses and the increased aortic atherosclerosis-associated gene expression observed in nephropathic mice, without compromising infection clearance. Mechanistically, we confirmed elevation of 4 TLR DAMPs in CKD patients' plasma, namely Hsp70, Hyaluronic acid, HMGB-1 and Calprotectin, which displayed different abilities to promote key cellular responses associated with vascular inflammation and progression of atherosclerosis in a TLR-dependent manner. These included loss of trans-endothelial resistance, enhanced monocyte migration, increased cytokine production, and foam cell formation by macrophages, the latter via cholesterol efflux inhibition. Calprotectin and Hsp70 most consistently affected these functions. Calprotectin was further elevated in CVD-diagnosed CKD patients and strongly correlated with the predictor of CV events CRP. In nephropathic mice, Calprotectin blockade robustly reduced vascular chronic inflammatory responses and pro-atherosclerotic gene expression in the blood and aorta. Taken together, these findings demonstrated the critical extent to which the DAMP-TLR pathway contributes to vascular inflammatory and atherogenic responses in CKD, revealed the mechanistic contribution of specific DAMPs and described two alternatives therapeutic approaches to reduce chronic vascular inflammation and lower CV pathology in CKD., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Mazzarino, Cetin, Bartosova, Marinovic, Ipseiz, Hughes, Schmitt, Ramji, Labéta and Raby.)

Since the embedding of the principles of the 3Rs (Replacement, Reduction and Refinement) in national and international regulations on the use of animals, scientists have been challenged to find ways to reduce the number of animals in their research. Here, we present a digital platform, called '3R Backboard', linked to a laboratory animal management system, which facilitates sharing of surplus biological materials from animals (e.g. tissues, organs and cells) to other research teams. Based on information provided, such as genotype, age and sex, other animal workers were able to indicate their interest in collecting specific tissues and to communicate with the person providing the animals. A short pilot study of this approach conducted in a limited academic environment presented strong evidence of its effectiveness and resulted in a notable reduction of the number of mice used. In addition, the use of 3R Blackboard led to resource saving, knowledge exchange and even establishment of new collaboration.

Alternatively activated macrophages (AAMs) contribute to the resolution of inflammation and tissue repair. However, molecular pathways that govern their differentiation have remained incompletely understood. Here, we show that uncoupling protein-2-mediated mitochondrial reprogramming and the transcription factor GATA3 specifically controlled the differentiation of pro-resolving AAMs in response to the alarmin IL-33. In macrophages, IL-33 sequentially triggered early expression of pro-inflammatory genes and subsequent differentiation into AAMs. Global analysis of underlying signaling events revealed that IL-33 induced a rapid metabolic rewiring of macrophages that involved uncoupling of the respiratory chain and increased production of the metabolite itaconate, which subsequently triggered a GATA3-mediated AAM polarization. Conditional deletion of GATA3 in mononuclear phagocytes accordingly abrogated IL-33-induced differentiation of AAMs and tissue repair upon muscle injury. Our data thus identify an IL-4-independent and GATA3-dependent pathway in mononuclear phagocytes that results from mitochondrial rewiring and controls macrophage plasticity and the resolution of inflammation., Competing Interests: Declaration of interests The authors declare no competing interests, (Copyright © 2021 Elsevier Inc. All rights reserved.)

The future of regenerative medicine relies on our understanding of the mechanistic processes that underlie tissue regeneration, highlighting the need for suitable animal models. For many years, zebrafish has been exploited as an adequate model in the field due to their very high regenerative capabilities. In this organism, regeneration of several tissues, including the caudal fin, is dependent on a robust epimorphic regenerative process, typified by the formation of a blastema, consisting of highly proliferative cells that can regenerate and completely grow the lost limb within a few days. Recent studies have also emphasized the crucial role of distinct macrophage subpopulations in tissue regeneration, contributing to the early phases of inflammation and promoting tissue repair and regeneration in late stages once inflammation is resolved. However, while most studies were conducted under non-infectious conditions, this situation does not necessarily reflect all the complexities of the interactions associated with injury often involving entry of pathogenic microorganisms. There is emerging evidence that the presence of infectious pathogens can largely influence and modulate the host immune response and the regenerative processes, which is sometimes more representative of the true complexities underlying regenerative mechanics. Herein, we present the current knowledge regarding the paths involved in the repair of non-infected and infected wounds using the zebrafish model., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Bohaud, Johansen, Jorgensen, Ipseiz, Kremer and Djouad.)

Several infectious pathologies in humans, such as tuberculosis or SARS-CoV-2, are responsible for tissue or lung damage, requiring regeneration. The regenerative capacity of adult mammals is limited to few organs. Critical injuries of non-regenerative organs trigger a repair process that leads to a definitive architectural and functional disruption, while superficial wounds result in scar formation. Tissue lesions in mammals, commonly studied under non-infectious conditions, trigger cell death at the site of the injury, as well as the production of danger signals favouring the massive recruitment of immune cells, particularly macrophages. Macrophages are also of paramount importance in infected injuries, characterized by the presence of pathogenic microorganisms, where they must respond to both infection and tissue damage. In this review, we compare the processes implicated in the tissue repair of non-infected versus infected injuries of two organs, the skeletal muscles and the lungs, focusing on the primary role of macrophages. We discuss also the negative impact of infection on the macrophage responses and the possible routes of investigation for new regenerative therapies to improve the recovery state as seen with COVID-19 patients., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Bohaud, Johansen, Jorgensen, Kremer, Ipseiz and Djouad.)

Dysfunction of the immune system underlies a plethora of human diseases, requiring the development of immunomodulatory therapeutic intervention. To date, most strategies employed have been focusing on the modification of T lymphocytes, and although remarkable improvement has been obtained, results often fall short of the intended outcome. Recent cutting-edge technologies have highlighted macrophages as potential targets for disease control. Macrophages play central roles in development, homeostasis and host defence, and their dysfunction and dysregulation have been implicated in the onset and pathogenesis of multiple disorders including cancer, neurodegeneration, autoimmunity and metabolic diseases. Recent advancements have led to a greater understanding of macrophage origin, diversity and function, in both health and disease. Over the last few years, a variety of strategies targeting macrophages have been developed and these open new therapeutic opportunities. Here, we review the progress in macrophage reprogramming in various disorders and discuss the potential implications and challenges for macrophage-targeted approaches in human disease., (© 2021 The Authors. Immunology published by John Wiley & Sons Ltd.)

The alarm cytokine interleukin-1β (IL-1β) is a potent activator of the inflammatory cascade following pathogen recognition. IL-1β production typically requires two signals: first, priming by recognition of pathogen-associated molecular patterns leads to the production of immature pro-IL-1β; subsequently, inflammasome activation by a secondary signal allows cleavage and maturation of IL-1β from its pro-form. However, despite the important role of IL-1β in controlling local and systemic inflammation, its overall regulation is still not fully understood. Here we demonstrate that peritoneal tissue-resident macrophages use an active inhibitory pathway, to suppress IL-1β processing, which can otherwise occur in the absence of a second signal. Programming by the transcription factor Gata6 controls the expression of prostacyclin synthase, which is required for prostacyclin production after lipopolysaccharide stimulation and optimal induction of IL-10. In the absence of secondary signal, IL-10 potently inhibits IL-1β processing, providing a previously unrecognized control of IL-1β in tissue-resident macrophages., (© 2020 The Authors. Published under the terms of the CC BY 4.0 license.)

Bone turnover, which is determined by osteoclast-mediated bone resorption and osteoblast-mediated bone formation, represents a highly energy consuming process. The metabolic requirements of osteoblast differentiation and mineralization, both essential for regular bone formation, however, remain incompletely understood. Here we identify the nuclear receptor peroxisome proliferator-activated receptor (PPAR) δ as key regulator of osteoblast metabolism. Induction of PPARδ was essential for the metabolic adaption and increased rate in mitochondrial respiration necessary for the differentiation and mineralization of osteoblasts. Osteoblast-specific deletion of PPARδ in mice, in turn, resulted in an altered energy homeostasis of osteoblasts, impaired mineralization and reduced bone mass. These data show that PPARδ acts as key regulator of osteoblast metabolism and highlight the relevance of cellular metabolic rewiring during osteoblast-mediated bone formation and bone-turnover.

Tissue-resident macrophages exhibit specialized phenotypes dependent on their in vivo physiological niche. Investigation of their function often relies upon complex whole mouse transgenic studies. While some appropriate lineage-associated promoters exist, there are no options for tissue-specific targeting of macrophages. We have developed full protocols for in vivo productive infection (defined by stable transgene expression) of tissue-resident macrophages with lentiviral vectors, enabling RNA and protein overexpression, including expression of small RNA species such as shRNA, to knock down and modulate gene expression. These approaches allow robust infection of peritoneal tissue-resident macrophages without significant infection of other cell populations. They permit rapid functional study of macrophages in homeostatic and inflammatory settings, such as thioglycolate-induced peritonitis, while maintaining the cells in their physiological context. Here we provide detailed protocols for the whole workflow: viral production, purification, and quality control; safety considerations for administration of the virus to mice; and assessment of in vivo transduction efficiency and the low background levels of inflammation induced by the virus. In summary, we present a quick and accessible protocol for the rapid assessment of gene function in peritoneal tissue-resident macrophages in vivo ., (© 2019 The Authors.)

NR4A1 (Nur77 or NGFI-B), an orphan member of the nuclear receptor superfamily, has been identified as a key regulator of the differentiation and function of myeloid, lymphoid, and mesenchymal cells. The detailed role of NR4A1 in bone biology is incompletely understood. Here, we report a role for NR4A1 as novel factor controlling the migration and recruitment of osteoclast precursors during bone remodeling. Myeloid-specific but not osteoblast-specific deletion of NR4A1 resulted in osteopenia due to an increase in the number of bone-lining osteoclasts. Although NR4A1-deficient osteoclast precursors displayed a regular differentiation into mature osteoclasts, they showed a hyper-motile phenotype that was largely dependent on increased osteopontin expression, suggesting that expression of NR4A1 negatively controlled osteopontin-mediated recruitment of osteoclast precursors to the trabecular bone. Pharmacological activation of NR4A1, in turn, inhibited osteopontin expression and osteopontin-dependent migration of osteoclast precursors resulted in reduced abundance of bone-resorbing osteoclasts in vivo as well as in an ameliorated bone loss after ovariectomy in mice. This study identifies NR4A1 as a crucial player in the regulation of osteoclast biology and bone remodeling and highlights this nuclear receptor as a promising target for therapeutic intervention during the treatment of osteoporosis. © 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc., (© 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.)

Inflammatory responses require mobilization of innate immune cells from the bone marrow. The functionality of this process depends on the state of the bone marrow microenvironment. We therefore hypothesized that molecular changes in osteoblasts, which are essential stromal cells of the bone marrow microenvironment, influence the inflammatory response. Here, we show that osteoblast-specific expression of the AP-1 transcription factor Fra-2 (Fra-2 Ob-tet ) induced a systemic inflammatory state with infiltration of neutrophils and proinflammatory macrophages into the spleen and liver as well as increased levels of proinflammatory cytokines, such as interleukin-1β (IL-1β), IL-6, and granulocyte-macrophage colony-stimulating factor (GM-CSF). By in vivo inhibition of osteopontin (OPN) in Fra-2 Ob-tet mice, we demonstrated that this process was dependent on OPN expression, which mediates alterations of the bone marrow niche. OPN expression was transcriptionally enhanced by Fra-2 and stimulated mesenchymal stem cell (MSC) expansion. Furthermore, in a murine lung injury model, Fra-2 Ob-tet mice showed increased inflammatory responses and more severe disease features via an enhanced and sustained inflammatory response to lipopolysaccharide (LPS). Our findings demonstrate for the first time that molecular changes in osteoblasts influence the susceptibility to inflammation by altering evasion of innate immune cells from the bone marrow space., (Copyright © 2018 American Society for Microbiology.)

Eosinophils were reported to serve as an essential component of the plasma cell niche within the bone marrow. As the potential contribution of eosinophils to humoral immunity has remained incompletely understood, we aimed to further characterize their role during antibody responses and to additionally investigate their role in autoimmune disease. Contrary to our expectations and the currently prevailing paradigm, we found that eosinophils are fully dispensable for the survival of murine bone marrow plasma cells and accordingly do not contribute to antibody production and autoantibody-mediated disease. Littermate wild type and eosinophil-deficient ΔdblGATA-1 animals showed similar numbers and frequencies of plasma cells and did not differ in steady state levels of immunoglobulins or their ability to raise antigen-specific antibody responses. Eosinophils were likewise dispensable for autoantibody production or autoantibody-induced disease in a mouse model of systemic lupus erythematosus. Our findings thus argue against a role of eosinophils during the maintenance of the plasma cell pool and challenge the hitherto postulated concept of an eosinophil-sustained bone marrow niche., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mesenchymal stem cells (MSC) are highly immunosuppressive cells able to reduce chronic inflammation through the active release of mediators. Recently, we showed that glucocorticoid-induced leucine zipper (Gilz) expression by MSC is involved in their therapeutic effect by promoting the generation of regulatory T cells. However, the mechanisms underlying this pivotal role of Gilz remain elusive. Methods and Results In this study, we have uncovered evidence that Gilz modulates the phenotype and function of Th1 and Th17 cells likely by upregulating the level of Activin A and NO 2 secreted by MSC. Adoptive transfer experiments sustained this Gilz-dependent suppressive effect of MSC on Th1 and Th17 cell functions. In immunoregulatory MSC, obtained by priming with IFN-γ and TNF-α, Gilz was translocated to the nucleus and bound to the promoters of inos and Activin βA to induce their expression. The increased expression of Activin A directly impacted on Th17 cells fate by repressing their differentiation program through the activation of Smad3/2 and enhancing IL-10 production. Conclusion Our results reveal how Gilz controls inos and Activin βA gene expression to ultimately assign immunoregulatory status to MSC able to repress the pathogenic Th17 cell differentiation program and uncover Activin A as a novel mediator of MSC in this process., Competing Interests: Competing Interests: The authors have declared that no competing interest exists.

Microglia cells fulfill key homeostatic functions and essentially contribute to host defense within the CNS. Altered activation of microglia, in turn, has been implicated in neuroinflammatory and neurodegenerative diseases. In this study, we identify the nuclear receptor (NR) Nr4a1 as key rheostat controlling the activation threshold and polarization of microglia. In steady-state microglia, ubiquitous neuronal-derived stress signals such as ATP induced expression of this NR, which contributed to the maintenance of a resting and noninflammatory microglia phenotype. Global and microglia-specific deletion of Nr4a1 triggered the spontaneous and overwhelming activation of microglia and resulted in increased cytokine and NO production as well as in an accelerated and exacerbated form of experimental autoimmune encephalomyelitis. Ligand-induced activation of Nr4a1 accordingly ameliorated the course of this disease. Our current data thus identify Nr4a1 as regulator of microglia activation and potentially new target for the treatment of inflammatory CNS diseases such as multiple sclerosis., (Copyright © 2017 by The American Association of Immunologists, Inc.)

Peroxisome proliferator-activated receptors (PPARs) have emerged as key regulators of physiological and immunological processes. Recently, one of their members PPARβ/δ has been identified as major player in the maintenance of bone homeostasis, by promoting Wnt signalling activity in osteoblast and mesenchymal stem cells (MSC). PPARβ/δ not only controls the fate of MSC but also regulates their immunosuppressive properties by directly modulating their NF-κB activity. In this review, we discuss how the regulation of PPARβ/δ provides an innovative strategy for an optimisation of MSC-based therapy., (Copyright © 2016 Elsevier B.V. and Société Française de Biochimie et Biologie Moléculaire (SFBBM). All rights reserved.)

The checkpoints and mechanisms that contribute to autoantibody-driven disease are as yet incompletely understood. Here we identified the axis of interleukin 23 (IL-23) and the T H 17 subset of helper T cells as a decisive factor that controlled the intrinsic inflammatory activity of autoantibodies and triggered the clinical onset of autoimmune arthritis. By instructing B cells in an IL-22- and IL-21-dependent manner, T H 17 cells regulated the expression of β-galactoside α2,6-sialyltransferase 1 in newly differentiating antibody-producing cells and determined the glycosylation profile and activity of immunoglobulin G (IgG) produced by the plasma cells that subsequently emerged. Asymptomatic humans with rheumatoid arthritis (RA)-specific autoantibodies showed identical changes in the activity and glycosylation of autoreactive IgG antibodies before shifting to the inflammatory phase of RA; thus, our results identify an IL-23-T H 17 cell-dependent pathway that controls autoantibody activity and unmasks a preexisting breach in immunotolerance., Competing Interests: The authors declare no competing financial interests.

The effect of metabolic stress on the bone marrow microenvironment is poorly defined. We show that high-fat diet (HFD) decreased long-term Lin(-)Sca-1(+)c-Kit(+) (LSK) stem cells and shifted lymphoid to myeloid cell differentiation. Bone marrow niche function was impaired after HFD as shown by poor reconstitution of hematopoietic stem cells. HFD led to robust activation of PPARγ2, which impaired osteoblastogenesis while enhancing bone marrow adipogenesis. At the same time, expression of genes such as Jag-1, SDF-1, and IL-7 forming the bone marrow niche was highly suppressed after HFD. Moreover, structural changes of microbiota were associated to HFD-induced bone marrow changes. Antibiotic treatment partially rescued HFD-mediated effects on the bone marrow niche, while transplantation of stools from HFD mice could transfer the effect to normal mice. These findings show that metabolic stress affects the bone marrow niche by alterations of gut microbiota and osteoblast-adipocyte homeostasis., (Copyright © 2015 Elsevier Inc. All rights reserved.)

DCs are able to undergo rapid maturation, which subsequently allows them to initiate and orchestrate T cell-driven immune responses. DC maturation must be tightly controlled in order to avoid random T cell activation and development of autoimmunity. Here, we determined that 12/15-lipoxygenase-meditated (12/15-LO-mediated) enzymatic lipid oxidation regulates DC activation and fine-tunes consecutive T cell responses. Specifically, 12/15-LO activity determined the DC activation threshold via generation of phospholipid oxidation products that induced an antioxidative response dependent on the transcription factor NRF2. Deletion of the 12/15-LO-encoding gene or pharmacologic inhibition of 12/15-LO in murine or human DCs accelerated maturation and shifted the cytokine profile, thereby favoring the differentiation of Th17 cells. Exposure of 12/15-LO-deficient DCs to 12/15-LO-derived oxidized phospholipids attenuated both DC activation and the development of Th17 cells. Analysis of lymphatic tissues from 12/15-LO-deficient mice confirmed enhanced maturation of DCs as well as an increased differentiation of Th17 cells. Moreover, experimental autoimmune encephalomyelitis in mice lacking 12/15-LO resulted in an exacerbated Th17-driven autoimmune disease. Together, our data reveal that 12/15-LO controls maturation of DCs and implicate enzymatic lipid oxidation in shaping the adaptive immune response.

Uptake of apoptotic cells (ACs) by macrophages ensures the nonimmunogenic clearance of dying cells, as well as the maintenance of self-tolerance to AC-derived autoantigens. Upon ingestion, ACs exert an inhibitory influence on the inflammatory signaling within the phagocyte. However, the molecular signals that mediate these immune-modulatory properties of ACs are incompletely understood. In this article, we show that the phagocytosis of apoptotic thymocytes was enhanced in tissue-resident macrophages where this process resulted in the inhibition of NF-κB signaling and repression of inflammatory cytokines, such as IL-12. In parallel, ACs induced a robust expression of a panel of immediate early genes, which included the Nr4a subfamily of nuclear receptors. Notably, deletion of Nr4a1 interfered with the anti-inflammatory effects of ACs in macrophages and restored both NF-κB signaling and IL-12 expression. Accordingly, Nr4a1 mediated the anti-inflammatory properties of ACs in vivo and was required for maintenance of self-tolerance in the murine model of pristane-induced lupus. Thus, our data point toward a key role for Nr4a1 as regulator of the immune response to ACs and of the maintenance of tolerance to "dying self."

Peroxisome proliferator-activated receptors (PPARs) act as metabolic sensors and central regulators of fat and glucose homeostasis. Furthermore, PPARγ has been implicated as major catabolic regulator of bone mass in mice and humans. However, a potential involvement of other PPAR subtypes in the regulation of bone homeostasis has remained elusive. Here we report a previously unrecognized role of PPARβ/δ as a key regulator of bone turnover and the crosstalk between osteoblasts and osteoclasts. In contrast to activation of PPARγ, activation of PPARβ/δ amplified Wnt-dependent and β-catenin-dependent signaling and gene expression in osteoblasts, resulting in increased expression of osteoprotegerin (OPG) and attenuation of osteoblast-mediated osteoclastogenesis. Accordingly, PPARβ/δ-deficient mice had lower Wnt signaling activity, lower serum concentrations of OPG, higher numbers of osteoclasts and osteopenia. Pharmacological activation of PPARβ/δ in a mouse model of postmenopausal osteoporosis led to normalization of the altered ratio of tumor necrosis factor superfamily, member 11 (RANKL, also called TNFSF11) to OPG, a rebalancing of bone turnover and the restoration of normal bone density. Our findings identify PPARβ/δ as a promising target for an alternative approach in the treatment of osteoporosis and related diseases.

Rapid clearance of apoptotic cells, frequently referred to as efferocytosis, is crucial for the maintenance of tissue homeostasis and the prevention of autoimmunity. The common model of apoptotic cell clearance involves a system of released "Find me" and exposed "Eat me" signals on apoptotic cells, detected and recognized by matching receptors on macrophages or dendritic cells (DC), referred to as the phagocytic synapse. Osteoclasts share the monocyte lineage with these professional mononuclear phagocytes, thus raising the question if, in addition to bone resorption, osteoclasts can act as scavengers for apoptotic cells. Our qPCR data clearly show that osteoclasts express most of the genes required for dying cell clearance at mRNA levels similar to or even higher than those observed in M1-macrophages, M2-macrophages or DC. Our microscopical analyses reveal that osteoclasts in fact can bind and/or engulf apoptotic cells in an essentially serum-independent fashion. Together with our data on the abundance of the respective mRNAs, these results identify the vitronectin receptor (integrin α(ν)β(3))/milk fat globule-EGF factor 8 protein (MFG-E8) axis, the scavenger receptors (CD36, CD68 and class A macrophage scavenger receptor (SR-A)), the complement/complement receptor axis, the Mer/Tyro3/Protein S axis, and the phosphatidylserine (PS) receptor brain-specific angiogenesis inhibitor 1 (BAI1) as the most promising candidates to be involved in osteoclast-mediated efferocytosis.