Published online before print September 12, 2007
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,
,||,1
* Department of Orthopedics,
Center for Vascular and Inflammatory Disease,
Program in Oncology, Marlene and Stewart Greenebaum Cancer Center, and
|| Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, Maryland, USA; and
Department of Hematopoiesis, Jerome H. Holland Laboratory for the Biomedical Sciences, American Red Cross, Rockville, Maryland, USA
1Correspondence: Center for Vascular and Inflammatory Diseases, University of Maryland, Baltimore, 800 W. Baltimore St., Baltimore, MD 21201, USA. E-mail: akeegan{at}som.umaryland.edu
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Key Words: inflammation • phagocytosis • fusion • DC-STAMP • cell differentiation
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MNG are closely related to osteoclasts and share not only a common origin from the hematopoietic-derived monocyte-macrophage lineage but also important morphological and functional characteristics. For example, both develop by the regulated fusion of mononuclear precursors and can be highly multinucleated [16
]. However, their biologic activities are markedly different [8
]. These cells perform highly specific functions: bone resorption by the osteoclast and elimination of extracellular foreign bodies by MNG. We showed previously that IL-4 directly prevents the receptor-activator of NF-
B ligand (RANKL)-induced differentiation of myeloid precursors to osteoclasts and inhibits bone resorption by mature osteoclasts in a STAT6-dependent manner [4
]. During these studies, we observed the formation of tartrate-resistant acid phosphatase (TRAP)-negative MNG in the cultures treated with IL-4. Thus, it would appear that IL-4 is a molecular switch between osteoclasts and MNG.
MNG are the central players in the chronic inflammatory response to foreign materials in vivo [17 , 18 ]. These cells have been observed at the surface of a wide variety of implants (i.e., vascular, cardiovascular, orthopedic, breast prostheses) for extended periods of time [7 , 18 19 20 ] and are thought to be responsible for a variety of biomaterial-mediated, adverse responses, which lead to degradation and implant failure [20 21 22 23 24 25 ]. IL-4 not only promotes the formation of MNG in vitro, but it also participates in the development of the foreign body reaction at the tissue-material interface in vivo [18 ]. However, the mechanism by which IL-4 promotes the formation of MNG is unclear. Therefore, we investigated the mechanism by which IL-4 regulates macrophage fusion during the formation of MNG. In this study, we show that IL-4 promotes the formation of MNG in a STAT6-dependent manner, in part by regulating cell surface expression of E-cadherin, leading to homotypic cell fusion and the incorporation of large foreign bodies.
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-10 [-MEM supplemented with penicillin, streptomycin, and glutamine (all from BioWhittaker, Walkersville, MD, USA) and 10% heat-inactivated FBS (Invitrogen, Frederick, MD, USA)] to deplete adherent stromal cells. The nonadherent mononuclear cells were isolated over Ficoll-Hypaque density gradients as described [26
]. These nonadherent BM mononuclear cells were cultured in the presence of recombinant cytokines to generate MNG, and in other experiments, they were cultured initially for 48 h in the presence of 20 ng/ml recombinant mouse (rm)M-CSF (R&D Systems Inc., Minneapolis, MN, USA) to generate M-CSF-dependent macrophages. In all of the cultures, cells were allowed to adhere for 2 h prior to the addition of cytokines. The cells were cultured for 5 days and then fixed and stained in one step with 1% Crystal violet in 95% ethanol, washed with water, and air-dried before analysis. Cells were plated at 2 x 106 cells/ml, unless otherwise stated.
Analysis of MNG formation
To induce MNG formation, BM cells from BALB/c STAT6+/+ and STAT6–/– mice were plated on glass coverslips (Nalge Nunc International, Naperville, IL, USA) at a density of 2 x 106 cells/ml with 20 ng/ml rmM-CSF and 10 ng/ml rmIL-4 (R&D Systems Inc.). After 5 days in culture, the cells were washed with PBS, fixed with 4% formaldehyde in PBS, and stained with rhodamine phalloidin and Hoechst dye (both from Molecular Probes, Eugene, OR, USA), according to the manufacturer’s instructions. Digital pictures at low power (10x and 20x objectives) were taken with a microscope (ECLIPSE E400, Nikon, Japan) under UV light for the quantification of the number, area, and fusion index of the MNG. Cell area was calculated over the images captured with the 10x objective by delineating all the MNG cells contained on the field. All the pictures were analyzed using image analysis software (Bioquant Image Analysis, Nashville, TN, USA). The fusion index was determined as described previously [7
] by dividing the total number of nuclei within MNG, defined as cells containing greater than or equal to three nuclei/cell, by the number of total nuclei counted and multiplying by 100. Three (10x objective) and five (20x objective) fields per well were counted in duplicate. The images from three independent experiments were evaluated. The unpaired Student’s t-test was used for statistical analysis between wild-type and STAT6–/– groups.
Flow cytometry
M-CSF-dependent macrophages from wild-type STAT6+/+ and STAT6–/– mice were cultured for 48 h in the presence of 20 ng/ml rmM-CSF, in the presence or absence of 10 ng/ml rmIL-4. Cells were exposed to Fluoresbrite AE YGTM microspheres with a diameter of 1 µm for 30 min according to the manufacturer’s instructions (1x108 microspheres/ml, Polysciences Inc., Warrington, PA, USA). The cells were washed extensively with PBS. Bead uptake was quantified using FACScan and analyzed by CellQuest software, Version 3.3 (Becton Dickinson Immunocytometry Systems, Bedford, MA, USA).
Antibodies
mAb against E-cadherin, recognizing the extracellular domain near the transmembrane spanning region (MAB36), was purchased from BD Biosciences (San Diego, CA, USA), the rabbit polyclonal antibody against the C-terminus of E-cadherin and the goat anti-biotin-HRP were purchased from Cell Signaling Technology (Danvers, MA, USA), and the rabbit polyclonal anti-DC-specific transmembrane protein (STAMP) antibody was purchased from Cosmo Bio Co., Ltd. (Tokyo, Japan). The sheep anti-mouse IgG-HRP conjugate was purchased from Amersham (Buckinghamshire, UK), and the goat anti-rabbit IgG-HRP conjugate was obtained from Bio-Rad Laboratories (Hercules, CA, USA). As control for protein loading, rabbit anti-heat shock protein 86 (HSP-86) or mouse mAb against tubulin- (DM1A) was purchased from Lab Vision (Fremont, CA, USA). The mAb against the amino-terminal region of E-cadherin known to block homotypic interaction (ECCD-1) [27
] was purchased from Takara Bio Inc. (Otsu, Japan).
Western blot, cell surface biotinylation, and immunoprecipitation
M-CSF-dependent macrophages from STAT6+/+ and STAT6–/– mice were cultured in the presence of 20 ng/ml rmM-CSF, in the presence or absence of 10 ng/ml rmIL-4 for various times. Subsequently, the cells were lysed in lysis buffer (50 mM Tris, pH 8.0, 150 mM NaCl, 1% Nonidet P-40, 5 mM EDTA, 10 mM Na pyrophosphate, 50 mM NaF, 0.25% Na deoxycholate, 1 mM Na orthovanadate, 1 mM PMSF, pepstatin, leupeptin, and aprotinin) and clarified by centrifugation. Protein concentration was determined in the samples by bicinchoninic acid assay (Pierce, Rockford, IL, USA), and equal amounts of protein were separated on SDS-PAGE before transfer to polyvinylidene difluoride membrane (Immobilon-P, Millipore, Bedford, MA, USA). Membranes were then blocked overnight and probed with antibodies as indicated. The membranes were washed with TBST extensively and developed using Supersignal West Pico chemiluminescent substrate (Pierce). The blots were stripped and reprobed with anti-HSP-86 or anti-tubulin- (DM1A) as control.
Biotinylation of macrophages was performed with Pierce EZ-Link sulfo-NHS-biotin reagents according to the manufacturer’s instructions. Briefly, M-CSF-dependent macrophages from BALB/c STAT6+/+ mice were cultured for 48 h in the presence of 20 ng/ml rmM-CSF, in the presence or absence of 10 ng/ml rmIL-4. Cells (>95% viable by trypan blue dye exclusion) were washed three times with ice-cold PBS supplemented with 1 mM MgCl2 and 0.1 mM CaCl2. Sulfo-NHS-LC-biotin reagent diluted in PBS (1 mg/mL) was added to cells and incubated for 30 min at 4°C with occasional swirling. The reaction was quenched by treating the cells with PBS containing 100 mM glycine for 15 min at 4°C followed by extensive washing. The quenching reaction was repeated two times. Subsequently, the cells were lysed in lysis buffer and analyzed by immunoprecipitation and Western blotting. For immunoprecipitation, equal amounts of protein were incubated with the primary antibody or with neutravidin-agarose beads. For precipitations with primary antibody, the complexes were collected by immunoprecipitation with protein G or A beads (Invitrogen), according to the manufacturer’s instructions. Immunoprecipitates were separated on SDS-PAGE and analyzed by Western blotting as described above. The membranes were stripped and reprobed with control antibodies as appropriate.
Blocking of MNG formation by anti-E-cadherin
BM cells were seeded at 2 x 106 cells/ml. After 2 h, 20 ng/ml rmM-CSF and 10 ng/ml rmIL-4 to generate MNG or rmM-CSF and RANKL (150 ng/ml) to generate osteoclasts were added in the presence or absence of increasing concentrations of anti-E-cadherin antibody (ECCD-1, Takara Bio Inc.). In other experiments, the blocking antibody was added at different times after the initiation of culture with cytokine as indicated. The media, cytokines, and reagents were added fresh every other day. After 5 days in culture, the cells were fixed and stained in one step with 1% Crystal violet in 95% ethanol for MNG or were stained for TRAP as described [4
] for the osteoclasts, washed with water, and air-dried before analysis.
Confocal microscopy
For the large bead uptake studies, BM cells were isolated from STAT6+/+ and STAT6–/– mice and were cultured on glass coverslips (Nalge Nunc International) for 5 days in the presence of 20 ng/ml rmM-CSF, in the presence or absence of 10 ng/ml rmIL-4. Fluoresbrite AE YGTM microspheres with a diameter of 25 µm (1x103 microspheres/ml, Polysciences Inc.) were added at the beginning of culture. After 5 days, some of the wells were fixed and stained in one step with 1% Crystal violet in 95% ethanol, washed with PBS, and mounted for light and fluorescent microscopy. Others were fixed in 4% formaldehyde in PBS for 10 min at room temperature and stained with rhodamine phalloidin (Molecular Probes), according to the manufacturer’s instructions, and mounted for confocal microscopy analysis. In some cases, the beads were added after 5 days of culture when MNG were already present; these cultures were allowed to proceed for 3 more days after the addition of the beads before analysis. At the end of the 3 days, the cells were labeled with CellTrackerTM CM-DiI C7001 (Molecular Probes), fixed, and permeabilized, according to the manufacturer’s instructions. The nuclei were stained with the nuclear marker TO-PRO3 (Molecular Probes). The images were captured with a confocal microscope Radiance 2000 (Zeiss/BioRad, Thornwood, NY, USA), and the three-dimensional (3D) reconstructions of the confocal images were generated using Volocity software (Improvision, London, UK).
For the cell–cell fusion studies, BM-derived macrophages were prepared from STAT6+/+ or STAT6–/– mice as described above. STAT6+/+ macrophages were stained with the CellTrackerTM C7025 (green fluorescence), and STAT6–/– macrophages were stained with CellTrackerTM CM-DiI C7001 (red fluorescence), according to the manufacturer’s instructions (Molecular Probes). The labeled cells were mixed at a 1:1 ratio and cultured at 2 x 106 cells/ml in the presence of 20 ng/ml rmM-CSF and 10 ng/ml rmIL-4 for 96 h. Subsequently, the cells were fixed, and the nuclei were stained with the nuclear marker TO-PRO3 (Molecular Probes). The images were captured with a confocal microscope, Radiance 2000 (Zeiss/BioRad), using a 60x oil objective. Fifteen fields from three independent experiments were analyzed for the total number of cells, total number of MNG (defined as a cell containing greater than or equally to three nuclei), number of green-only MNG, number of red-only MNG, and number of MNG with both colors. The percentage of MNG in each category was calculated by dividing each number by the total number of MNG per field. The unpaired Student’s t-test was used for statistical analysis between groups.
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Figure 1. MNG formation is dependent on IL-4 concentration and cell density. BM mononuclear cells were isolated from BALB/c mice and plated in 48-well plates at increasing cell densities in the presence or absence of various IL-4 concentrations in the continuous presence of 20 ng/ml M-CSF. Media and cytokines were replaced every other day. After 5 days in culture, the plates were fixed and stained with 1% Crystal violet (original magnification, 10x).
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Figure 2. STAT6 requirement for MNG formation. BM cells were isolated from STAT6+/+ and STAT6–/– mice and plated on glass coverslips at 2 x 106 cells/ml for 5 days with 20 ng/ml M-CSF and 10 ng/ml IL-4. Subsequently, they were washed with PBS, fixed with 4% formaldehyde in PBS, and stained with (A) 1% Crystal violet or (B) rhodamine phalloidin and Hoechst’s dye. Digital pictures were taken at low power (10x and 20x objectives). The images from three independent experiments were analyzed for the quantification of the number, area, and fusion index of the MNG using Bioquant software. The area of the cells and the fusion index were determined as described in Materials and Methods. Data represent the average ± SEM. The unpaired Student’s t-test was used for statistical analysis.
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Figure 3. BM precursors in the presence of M-CSF retain their ability to differentiate into MNG cells. BM mononuclear cells from STAT6+/+ animals were seeded on glass coverslips at 2 x 106 cells/ml and remained in the continuous presence of 20 ng/ml M-CSF for the entire length of the experiment. (A) At the indicated times after the beginning of the culture, 10 ng/ml IL-4 was added to the wells. After 120 h total in the presence of IL-4, the cells were fixed and stained with 1% Crystal violet (original magnification, 10x). (B) The cells were exposed to 10 ng/ml IL-4 for the indicated times. Subsequently, the IL-4 was removed by washing the wells three times with complete media and then cultured with M-CSF alone for the remainder of the culture period up to 120 h. Cells were fixed, stained, and photographed as in A. (C) The quantification of the average MNG area in each panel was performed using Bioquant software. The data represent the average ± SEM. These results are representative of three independent experiments.
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Figure 4. Phagocytosis of small particles. BM mononuclear cells were isolated from STAT6+/+ and STAT6–/– mice and cultured in the presence of 20 ng/ml M-CSF for 48 h. After this initial period, the cells were cultured with 20 ng/ml M-CSF in the presence or absence of 10 ng/ml IL-4 for an additional 48 h. At the end of the 48-h period, the cells were cultured in the presence (heavy line) or absence (light line) of Fluoresbrite AE YGTM microspheres with a diameter of 1 µm for 30 min at 37ºC. The cells were subsequently washed extensively with PBS, lifted, and analyzed by flow cytometry for bead content. (A) Representative FACS histograms are shown, and the percentage of cells containing FITC beads is indicated. (B) The effect of IL-4 on the percentage of cells containing beads and on the mean fluorescence intensity (MFI) of cells containing beads was calculated as a percentage of the wild-type (WT), untreated control. The values from three independent experiments are shown in the scatter plots. The average value is indicated by the bar. The unpaired Student’s t-test was used for statistical analysis (P<0.05).
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Figure 5. STAT6+/+-derived MNG can incorporate large foreign bodies. BM mononuclear cells, isolated from STAT6+/+ and STAT6–/– mice, were cultured on glass coverslips at 2 x 106/ml for 5 days in the presence of 20 ng/ml M-CSF, in the presence or absence of 10 ng/ml IL-4. Fluoresbrite microspheres with a diameter of 25 µm were added at the beginning of culture. After 5 days, some of the wells were stained with 1% Crystal violet (A–E) and pictured with light (A and C) or fluorescent microscopy (B, D, and E). Others were fixed in 4% formaldehyde and stained with rhodamine phalloidin for confocal microscopy analysis (F and G). After 5 days, when mature MNG cells were present, Fluoresbrite microspheres with a diameter of 25 µm were added to the culture. After 3 days, the membrane of the cells was labeled with CellTrackerTM CM-DiI C7001 (red fluorescence). The cells were fixed with 4% formaldehyde and permeabilized with acetone, and the nuclei were stained with TO-PRO3. The slides were analyzed by confocal microscopy. (H) The FITC bead and membrane labeling, (I) the nuclei stain with TO-PRO3, and (J) all three stains in a 3D reconstruction of the shaded area.
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Our results, indicating the dependency of MNG formation on cell density, suggest the importance of cell–cell contact in the fusion process. Thus, we examined a potential role for cell surface molecules. Osteoclasts and MNG are derived from a common mononuclear monocyte-macrophage progenitor and form as the result of cell–cell fusion [16 ]. E-cadherin has been implicated extensively in cell–cell contact [27 ] and also in osteoclast progenitor fusion, as antibodies to E-cadherin block osteoclast development and activity [31 ]. Therefore, we tested the role of E-cadherin on the IL-4-induced MNG formation. We analyzed the levels of E-cadherin during MNG formation on STAT6+/+ macrophages by Western blotting with two different antibodies (Fig. 6A ). We observed a clear induction of E-cadherin protein starting at 8 h after the addition of IL-4, reaching maximum levels after 16 h. The level of E-cadherin remained high after 48 h in the presence of IL-4 (Fig. 6A) . To verify that the increase in E-cadherin protein observed by Western blot translated into higher levels of E-cadherin protein on the cell surface, we performed cell surface biotinylation (Fig. 6B) . Macrophages from STAT6+/+ mice were treated in the presence or absence of IL-4 before the cell surface proteins were labeled with biotin. Precipitation of cell lysates with neutravidin beads pulled down E-cadherin only from cells treated with IL-4. In the reverse experiment, precipitation of cell lysates with anti-E-cadherin pulled down biotinylated E-cadherin only from cells treated with IL-4. These results indicate that IL-4 induced expression of E-cadherin and that it was expressed on the cell surface. In the next experiment, we analyzed the induction of E-cadherin on STAT6–/–-derived macrophages (Fig. 6C) . IL-4 failed to up-regulate E-cadherin expression in the absence of STAT6. In addition, we analyzed the expression of another protein, DC-STAMP, recently shown to be essential for the formation of osteoclasts induced by M-CSF plus RANKL treatment and MNG induced by M-CSF plus IL-3 and IL-4 treatment [32 ]. Similar to E-cadherin, we found that expression of DC-STAMP protein was induced by IL-4 in macrophages derived from STAT6+/+ mice, but not in macrophages derived from STAT6–/– mice (Fig. 6C) . Taken together, these results suggest that IL-4 acts to modulate E-cadherin and DC-STAMP expression in a STAT6-dependent manner and thus, facilitates cell–cell fusion.
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Figure 6. IL-4 induced E-cadherin expression in a STAT6-dependent manner. (A) STAT6+/+-derived macrophages were cultured with 20 ng/ml M-CSF in the presence or absence of 10 ng/ml IL-4 for different times as indicated. After treatment, the cells were lysed, and the lysates were analyzed for E-cadherin expression by SDS-PAGE and Western blotting as described in Materials and Methods with a mAb (MAB36) or a rabbit polyclonal antibody against E-cadherin. After stripping, the blots were reprobed with anti-HSP-86. (B) STAT6+/+-derived macrophages were cultured with 20 ng/ml M-CSF in the presence or absence of 10 ng/ml IL-4 for 48 h. After this, the cells were subjected to cell surface biotinylation and lysed. The lysates were subjected to immunoprecipitation (IP) with Neutravidin beads or anti-E-cadherin antibodies and analyzed by SDS-PAGE and Western blotting (WB) with anti-biotin or anti-E-cadherin antibodies as indicated. (C) STAT6+/+- and STAT6–/–-derived M-CSF-dependent macrophages were cultured with 20 ng/ml M-CSF in the presence or absence of 10 ng/ml IL-4 for the indicated times. At the end of the treatment, the cells were lysed, and the lysates were analyzed for E-cadherin expression by SDS-PAGE and Western blotting with a mAb (MAB36) against E-cadherin or a polyclonal antibody against DC-STAMP. After stripping, the blots were reprobed with anti- tubulin or anti-HSP-86.
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3 µg/ml dramatically reduced the area of the MNG or osteoclast and the number of nuclei contained in each cell. We noticed that anti-E-cadherin did not suppress expression of TRAP in these cultures, in contrast to IL-4 treatment [4
]. These results suggest an important role for E-cadherin in the IL-4-induced formation of MNG and the RANKL-induced formation of osteoclasts from mononuclear precursor cells.
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Figure 7. Blocking MNG formation by anti-E-cadherin. BM mononuclear cells from STAT6+/+ animals were cultured in 48-well plates at 2 x 106 cells/ml in the continuous presence of 20 ng/ml M-CSF and 10 ng/ml IL-4 or M-CSF (20 ng/ml) and RANKL (150 ng/ml) as indicated. Increasing concentrations of a blocking mAb against murine E-cadherin (ECCD1) were added to the cultures. Media, cytokines, and blocking antibody were exchanged every other day. (A) After 5 days, the cultures were fixed and stained with 1% Crystal violet for MNG or for TRAP in the case of the osteoclasts (original magnification, 4x). (B) The quantification of the average number, average area, and average number of nuclei per MNG or osteoclasts (OC) in each panel was performed using Bioquant software. The average ± SEM is shown. These results are representative of three independent experiments.
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Figure 8. Time-dependent effect of E-cadherin blockade on MNG formation. BM mononuclear cells from STAT6+/+ animals were cultured in 48-well plates at 2 x 106 cells/ml in the continuous presence of 20 ng/ml M-CSF and 10 ng/ml IL-4. (A) At the indicated times after the beginning of the culture, 5 µg/ml blocking mAb against murine E-cadherin (ECCD1) was added to the wells. After 120 h total in the presence of IL-4, the cells were fixed and stained with 1% Crystal violet (original magnification, 4x). (B) The quantification of the average number and MNG area was performed using Bioquant software. The average ± SEM is shown. These results are representative of two independent experiments.
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25%) of green MNG cells containing some red label (example marked with arrows, showing as yellow on the merged images; Fig. 9
, A and B). The green STAT6+/+ cells and the red STAT6–/– cells remained healthy and viable during the course of the experiment (Fig. 9
, and data not shown). It is interesting that we noticed green MNG in close proximity to single red STAT6–/– cells (Fig. 9C)
. In addition, we frequently observed single green STAT6+/+ cells in close interaction with single red STAT6–/– cells. In many cases, the STAT6+/+ cells with a single nucleus contained red and green dye. These results suggest that homotypic STAT6+/+ to STAT6+/+ interactions are necessary for the efficient formation of MNG in response to IL-4 but that heterotypic interactions may proceed less efficiently. It is also possible that once formed, the STAT6+/+ MNG can phagocytose a labeled STAT6–/– macrophage (a process termed cellocytosis [30
]) or that they can take up released red dye.
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Figure 9. Analysis of cell–cell fusion. BM-derived macrophages were prepared from STAT6+/+ or STAT6–/– mice and cultured in the presence of 20 ng/ml rmM-CSF for 48 h. STAT6+/+ macrophages were stained with the probe CellTrackerTM C7025 (green fluorescence), and STAT6–/– macrophages were stained with CellTrackerTM CM-DiI C7001 (red fluorescence). The labeled cells were mixed at a 1:1 ratio and cultured at 2 x 106 cells/ml in the presence of 20 ng/ml rmM-CSF and 10 ng/ml rmIL-4 for 96 h. Subsequently, the cells were fixed, and the nuclei were stained with the nuclear marker TO-PRO3 (Molecular Probes). The images were captured with a confocal microscope, Radiance 2000 (Zeiss/BioRad), using a 60x oil objective. (A) Representative panels illustrating typical staining for nuclei (blue, TO-PRO3), STAT6+/+ cells (green, C7025), and STAT6–/– cells (red, C7001) and all three images merged are shown. Optical-section thickness was 0.8 µm for green and red and 1.5 µm for blue. MNG are highlighted with a dotted line. An example of a MNG containing red and green dye is highlighted with arrows. (B) Fifteen fields originating from three independent experiments were analyzed for the total number of cells, total number of MNG (defined as a cell containing greater than or equal to three nuclei), number of green-only MNG, number of red-only MNG, and number of MNG with both colors (showing as yellow staining). The percentage of MNG in each category was calculated by dividing each number by the total number of MNG per field. The unpaired Student’s t-test was used for statistical analysis between the green and yellow groups (**, P<0.001). (C) Representative images of MNG and mononuclear cells.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A recent report by McNally and Anderson [28 ] showed that the process of macrophage fusion exhibits several features of phagocytosis, as it required the participation of microtubules, V-type ATPase, and independent phospholipase A2. The authors demonstrated that components of the endoplasmic reticulum (ER) were present at fusion interfaces and on macrophage surfaces during MNG formation. Furthermore, it has been shown that many cytoskeletal components known to participate in adhesion are enriched in the phagocytic cup [36 37 38 ]. These results indicate that the cell uses similar proteins and mechanisms for phagocytosis and cell–cell fusion, which may be limited in quantity. In keeping with this idea, we observed that IL-4 treatment suppressed the number of beads phagocytized per cell by STAT6+/+ macrophages over a period of 30 min. These cells are programmed to fuse and differentiate into MNG. Perhaps the machinery available for rapid and efficient phagocytic uptake of beads is diverted to the fusion process in STAT6+/+ cells responding to IL-4. An in-depth analysis of the contribution of the phagocytic machinery to IL-4-induced, STAT6-dependent fusion mechanisms will require further study.
When we challenged the macrophage cultures with large particles (25 µm diameter, larger than a single macrophage), we found that STAT6+/+ MNG were able to internalize numerous particles per cell, indicating these cells were functionally active. However, in contrast to small beads (1 µm), STAT6–/– cells were not able to incorporate any large particles. Nonetheless, STAT6–/– cells were detected, attached to the surface of the beads, suggesting a frustrated attempt to internalize the particles. In agreement with this data, others have reported that the size of the cell correlates with the size of the internalized particle [39 40 41 ]. Von Knoch et al. [42 ] have shown that the mean intracellular polyethylene particle size is significantly greater in FBGC than in macrophages. Thus, the lack of IL-4-induced fusion in the absence of STAT6 likely prevents the incorporation of large particles, although the phagocytic capacity of the STAT6–/–-derived cells remained intact.
As IL-4 inhibited the formation of multinucleated osteoclasts but promoted the formation of MNG, we suspected originally that the fusion mechanisms used by osteoclasts and MNG would be different. However, recent studies suggest that there is a common fusion mechanism used by these cell types [29 , 32 ]. We found evidence for an important role for E-cadherin in the formation of MNG and osteoclasts from mononuclear precursor cells. Furthermore, we found that IL-4 clearly induced the membrane expression of E-cadherin in a STAT6-dependent manner on BM macrophages. It was shown recently that expression of the ER and cell surface protein, DC-STAMP, is critical for the formation of MNG and osteoclasts [32 ]. We also observed a STAT6-dependent induction of DC-STAMP protein in response to IL-4, suggesting the participation of E-cadherin and DC-STAMP in the IL-4-induced fusion.
The antibody-blocking experiments revealed a critical role for E-cadherin in the fusion process for MNG and osteoclasts. The addition of the blocking antibody at later time-points showed less-efficient inhibition of MNG formation. This may be a result of a loss of antibody epitope caused by homotypic interaction of E-cadherins on neighboring cells or by lateral interaction of E-cadherin molecules on the same cell [43 ]. The analysis of the number of MNG generated in the antibody-blocking experiments using the classic definition of MNG (containing at least three nuclei per cell) did not reveal a clear impact of E-cadherin on the fusion process (Figs. 7 and 8) . This is in contrast with the profound effect of STAT6 deficiency on the numbers of MNG in IL-4-treated cultures (Fig. 2) . However, analysis of cell area and nuclei content per cell clearly indicated a suppressive effect of anti-E-cadherin antibodies on MNG formation. The difference between STAT6 deficiency and E-cadherin blockade on MNG number may be a result of the contribution of other STAT6-regulated molecules, such as DC-STAMP, in the formation of small (approximately three nuclei) MNG.
The STAT6 dependency of MNG formation does not appear to be mediated by production of soluble factors. We found that cell-free supernatants from IL-4-treated STAT6+/+ cells did not induce MNG formation from STAT6–/– precursors (data not shown). Furthermore, the addition of the chemokine CCL2, shown to be required for MNG formation [44 , 45 ], was not able to stimulate MNG formation in the STAT6–/– cultures (data not shown). Using cell-mixing experiments, we found that STAT6+/+ cells would fuse efficiently with STAT6+/+ cells in response to IL-4 and form MNG but that STAT6+/+ cells would rarely form MNG containing STAT6–/– cells (Fig. 9) . These results indicate a requirement for cell intrinsic homotypic STAT6+/+ to STAT6+/+ interaction for efficient formation of MNG. However, Yagi et al. [32 ] found that expression of DC-STAMP was only required on one of the cell partners to drive efficient fusion induced by treatment with IL-3 plus IL-4; DC-STAMP+/+ cells fused efficiently with DC-STAMP–/– cells. Thus, the mechanism by which IL-4 promotes homotypic fusion in the presence of M-CSF likely involves the STAT6-dependent regulation of several cell surface molecules, including E-cadherin and DC-STAMP. Furthermore, several other candidate molecules known to play a role in the fusion of alveolar macrophages may participate in the IL-4-induced fusion [29 , 46 ]. A complete understanding of the detailed molecular interactions activated by IL-4 to promote macrophage fusion and MNG formation will require additional investigation.
Received January 23, 2007; revised August 10, 2007; accepted August 13, 2007.
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B J. Clin. Invest. 107,1375-1385[Medline] B and mitogen-activated protein kinase signaling J. Biol. Chem. 277,6622-6630
, and prostaglandin E impact the osteoclastic cell-forming potential of murine bone marrow macrophages Endocrinology 136,2367-2376[Abstract] differentially regulate macrophage tumoricidal activation Immunology 80,617-624[Medline] isozyme of protein kinase C and myosin I in zymosan phagocytosis by macrophages J. Exp. Med. 182,829-840This article has been cited by other articles:
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