Histamine participates in the early phase of trabecular bone loss in ovariectomized rats.

We have previously reported that cimetidine, a reference H2 receptor antagonist, attenuates the initial osteoclastic burst and subsequent trabecular bone loss induced by ovariectomy (ovx) in rats. This study was designed to determine whether these effects are specific to H2 antagonism. To this end, we compared the effects of two H2 receptor antagonists, cimetidine and famotidine. In addition, we analyzed the response of histamine-producing cells to these inhibitors. Seventy-two 90-day-old female Sprague-Dawley rats were ovariectomized or sham-operated, and received single daily intramuscular injections of cimetidine (125 mg/kg), famotidine (10 mg/kg), or vehicle. The animals were killed 14 days after surgery and their femurs were processed for histomorphometry. Trabecular bone volume was reduced by 30% in ovx rats and by 15% in cimetidine- and famotidine-treated rats. Architectural parameters were reduced by about 20% in ovx rats. Cimetidine and famotidine attenuated these consequences of ovx by about 50%. Trabecular connectivity was deteriorated by ovx, while cimetidine and famotidine attenuated this effect. Resorption parameters were increased by ovx, while cimetidine and famotidine prevented this increase. Kinetic bone formation parameters were increased by ovx, while cimetidine and famotidine had no influence. Neither cimetidine nor famotidine had any observable effect in sham-treated rats. Mast cell numbers increased by 250% in ovx rats and by only 40% in H2 antagonists-treated ovx rats. A resident histamine-positive, non-mast cell, population found in bone marrow was increased by 25% by ovx. Interestingly, cimetidine and famotidine reduced this population in both sham-operated and ovx rats, famotidine being more potent than cimetidine. These results show that H(2) receptor blockade partially prevents the consequences of castration on cancellous bone resorption in female rats, and strongly suggest that histamine participates in the mediator network regulating estrogen deficiency induced bone resorption. A large population of histamine-producing cells, which differ morphologically from mast cells and belong to an immature marrow population, may be a source of histamine in this model. The H(2) blockers targeted this population, and this effect appeared to explain the anti-resorptive action of the two drugs.

Strontium ranelate inhibits bone resorption while maintaining bone formation in alveolar bone in monkeys (Macaca fascicularis).

Strontium ranelate (S12911) has previously been shown to stimulate bone formation and inhibit bone resorption in rats. To determine whether strontium ranelate affects normal bone remodeling, we studied the effect of strontium ranelate on alveolar bone in monkeys. Strontium ranelate, at dosages of 100, 275, and 750 mg/kg per day, or vehicle, were given by gavage to 31 normal adult monkeys (Macaca fascicularis) (15 males, 16 females), aged 3-4 years. Treatment for 6 months with strontium ranelate resulted in an increase in plasma strontium concentration. Histomorphometric analyses of indices of bone formation and resorption were determined in standardized areas of alveolar bone. Treatment with strontium ranelate decreased the histomorphometric indices of bone resorption (osteoclast surface and number) with a maximal significant effect at the highest dose tested. In contrast to this inhibitory effect on bone resorption, strontium ranelate maintained bone formation. Although the amount of osteoid tended to increase, strontium ranelate, even at the highest dose, had no deleterious effect on bone mineralization, as evaluated by mineral apposition rate and osteoid thickness. These findings show that strontium ranelate decreases indices of bone resorption while maintaining bone formation in the alveolar bone in monkeys.

CD47, a ligand for the macrophage fusion receptor, participates in macrophage multinucleation.

The macrophage fusion receptor (MFR), also called P84/BIT/SIRPalpha/SHPS-1, is a transmembrane glycoprotein that belongs to the superfamily of immunoglobulins. Previously, we showed that MFR expression is highly induced at the onset of fusion in macrophages, and that MFR appears to play a role in macrophage-macrophage adhesion/fusion leading to multinucleation. The recent finding that IAP/CD47 acts as a ligand for MFR led us to hypothesize that it interacts with CD47 at the onset of cell-cell fusion. CD47 is a transmembrane glycoprotein, which, like MFR, belongs to the superfamily of immunoglobulins. We show that macrophages express the hemopoietic form of CD47, the expression of which is induced at the onset of fusion, but to a lower level than MFR. A glutathione S-transferase CD47 fusion protein engineered to contain the extracellular domain of CD47, binds macrophages, associates with MFR, and prevents multinucleation. CD47 and MFR associate via their amino-terminal immunoglobulin variable domain. Of the nine monoclonal antibodies raised against the extracellular domain of CD47, three block fusion, as well as MFR-CD47 interaction, whereas the others have no effect. Together, these data suggest that CD47 is involved in macrophage multinucleation by virtue of interacting with MFR during adhesion/fusion.

Inhibition of NF-kappaB activity and enhancement of apoptosis by the neuropeptide calcitonin gene-related peptide.

Calcitonin gene-related peptide (CGRP) is a neuropeptide produced by the central and peripheral nervous systems and by endocrine cells. CGRP exerts diverse biological effects on the cardiovascular, gastrointestinal, respiratory, central nervous and immune systems. Little is known, however, about the molecular mechanisms that mediate CGRP effects. Using the NFkappaB-luciferase reporter transgenic mice, here we show that CGRP selectively inhibits NF-kappaB-mediated transcription in thymocytes in vitro and in vivo. In contrast, CGRP does not affect transcription mediated by the AP-1 and NFAT transcription factors. CGRP inhibits the accumulation of NF-kappaB complexes in the nucleus by preventing phosphorylation and degradation of the NF-kappaB inhibitor IkappaB. Inhibition of NF-kappaB activity is associated with the induction of apoptosis by CGRP in thymocytes. Together these results demonstrate for the first time the selective implication of the NF-kappaB signaling pathway in the regulatory function of the neuropeptide CGRP. Our study suggests a potential molecular mechanism by which CGRP can induce cell death in thymocytes.

Osteoclasts and giant cells: macrophage-macrophage fusion mechanism.

Membrane fusion is a ubiquitous event that occurs in a wide range of biological processes. While intracellular membrane fusion mediating organelle trafficking is well understood, much less is known about cell-cell fusion mediating sperm cell-oocyte, myoblast-myoblast and macrophage-macrophage fusion. In the case of mononuclear phagocytes, their fusion is not only associated with the differentiation of osteoclasts, cells which play a key role in the pathogenesis of osteoporosis, but also of giant cells that are present in chronic inflammatory reactions and in tumours. Despite the biological and pathophysiological importance of intercellular fusion events, the actual molecular mechanism of macrophage fusion is still unclear. One of the main research themes in my laboratory has been to investigate the molecular mechanism of mononuclear phagocyte fusion. Our hypothesis has been that macrophage-macrophage fusion, similar to virus-cell fusion, is mediated by specific cell surface proteins. But, in contrast with myoblasts and sperm cells, macrophage fusion is a rare event that occurs in specific instances. To test our hypothesis, we established an in vitro cell-cell fusion assay as a model system which uses alveolar macrophages. Upon multinucleation, these macrophages acquire the osteoclast phenotype. This indicates that multinucleation of macrophages leads to a specific and novel functional phenotype in macrophages. To identify the components of the fusion machinery, we generated four monoclonal antibodies (mAbs) which block the fusion of alveolar macrophages and purified the unique antigen recognized by these mAbs. This led us to the cloning of MFR (Macrophage Fusion Receptor). MFR was cloned simultaneously as P84/SHPS-1/SIRPalpha/BIT by other laboratories. We subsequently showed that the recombinant extracellular domain of MFR blocks fusion. Most recently, we identified a lower molecular weight form of MFR that is missing two extracellular immunoglobulin (Ig) C domains. Shortly after we cloned MFR, CD47 was reported to be a ligand for P84/SIRPalpha. We have since generated preliminary results which suggest that CD47 interacts with MFR during adhesion/fusion and is a member of the fusion machinery. We also identified CD44 as a plasma membrane protein which, like MFR, is highly expressed at the onset of fusion. The recombinant soluble extracellular domain of CD44 blocks fusion by interacting with a cell-surface binding site. We now propose a model in which both forms of MFR, CD44, and CD47 mediate macrophage adhesion/fusion and therefore the differentiation of osteoclasts and giant cells.

Targeted expression of calcitonin gene-related peptide to osteoblasts increases bone density in mice.

The neuropeptide calcitonin gene-related peptide (CGRP) is concentrated in fine sensory nerve endings innervating all tissues, including bone. CGRP inhibits osteoclasts, stimulates insulin-like growth factor I and inhibits tumor necrosis factor alpha production by osteoblasts in vitro. To investigate the role of CGRP in bone in vivo, mice were engineered to express CGRP in osteoblasts by placing the human CGRP gene under the control of the rat osteocalcin promoter (Ost-CGRP tg+ mice). Calvaria cultures from transgene positive (tg+), but not tg- mice, produced bioactive CGRP. Trabecular bone density and bone volume, determined by peripheral quantitative computed tomography and bone histomorphometry, respectively, were higher in tg+ than tg- littermates. This increase in bone volume was associated with an increased bone formation rate. Trabecular bone density decreased in tg+ mice as a result of ovariectomy, but remained higher than in sham tg- mice. Targeting CGRP to osteoblasts appears to favor the establishment of a higher trabecular bone mass in mice.

CD44 occupancy prevents macrophage multinucleation.

Cells of the mononuclear phagocyte lineage have the capability to adhere to and fuse with each other and to differentiate into osteoclasts and giant cells. To investigate the macrophage adhesion/fusion mechanism, we focused our attention on CD44, a surface glycoprotein known to play a role in hematopoietic cell-cell adhesion. We report that CD44 expression by macrophages is highly and transiently induced by fusogenic conditions both in vitro and in vivo. We show that CD44 ligands, hyaluronic acid, chondroitin sulfates, and osteopontin prevent macrophage multinucleation. In addition, we report that the recombinant extracellular domain of CD44 binds fusing macrophages and prevents multinucleation in vitro. These data suggest that CD44 may control the mononucleated status of macrophages in tissues by virtue of mediating cell-cell interaction.

MFR, a putative receptor mediating the fusion of macrophages.

We had previously identified a macrophage surface protein whose expression is highly induced, transient, and specific, as it is restricted to actively fusing macrophages in vitro and in vivo. This protein is recognized by monoclonal antibodies that block macrophage fusion. We have now purified this protein and cloned its corresponding cDNA. This protein belongs to the superfamily of immunoglobulins and is similar to immune antigen receptors such as the T-cell receptor, B-cell receptor, and viral receptors such as CD4. We have therefore named this protein macrophage fusion receptor (MFR). We show that the extracellular domain of MFR prevents fusion of macrophages in vitro and therefore propose that MFR belongs to the fusion machinery of macrophages. MFR is identical to SHPS-1 and BIT and is a homologue of P84, SIRPalpha, and MyD-1, all of which have been recently cloned and implicated in cell signaling and cell-cell interaction events.

Regulation of interleukin-6 production by prostaglandin E2 in fetal rat osteoblasts: role of protein kinase A signaling pathway.

Prostaglandin E2 (PGE2) is an abundant eicosanoid in bone that has been implicated in a number of pathological states associated with bone loss. Interleukin-6 (IL-6) is a cytokine that plays a critical role in bone remodeling and appears to act as a downstream effector of most bone-resorbing agents. In light of the evidence that PGE2 induces IL-6 in the bone environment, this study was designed to investigate whether PGE2 regulated IL-6 expression by osteoblasts. Here we demonstrate that PGE2 is a potent inducer of IL-6 production by fetal rat osteoblasts and synergizes with lipopolysaccharide to enhance IL-6. We show that PGE2 stimulates the activity of the IL-6 promoter in osteoblasts, suggesting that PGE2 controls IL-6 gene expression at least at the transcriptional level. Moreover, we show that PGE2-mediated IL-6 induction is prevented by the cAMP antagonist, Rp-cAMP, and the protein kinase A (PKA) inhibitors, KT5720 and H89. Thus, our data indicate that PGE2 involves the cAMP-PKA signaling pathway to regulate IL-6 gene expression in osteoblasts.

Effects of calcitonin gene-related peptide on bone turnover in ovariectomized rats.

Calcitonin gene-related peptide (CGRP) is a neuropeptide abundantly concentrated in sensory nerve endings innervating bone metaphysis and periosteum, which indicates that it plays a local role in bone metabolism. CGRP-alpha and -beta share structural and functional homology with calcitonin (CT) and have been shown to inhibit bone resorption in vitro and to induce hypocalcemia in vivo. We recently reported that CGRP stimulates the production of the growth factor insulin-like growth factor-I and inhibits that of the cytokine tumor necrosis factor-alpha by osteoblasts, suggesting that CGRP may control bone cell activity. To investigate this possibility, we used ovariectomized (ovx) rats as a high bone turnover model and compared the effects of CGRP to those of CT. ovx young female rats were injected daily starting the day after surgery with either phosphate-buffered saline, CGRP-alpha (1.15 mg/kg per day), or CT (3 micrograms/kg per day) for 28 days. Ovariectomy induced an increase in bone turnover associated with a 60% loss in trabecular bone volume of the proximal tibia. CGRP inhibited bone resorption but not bone formation, and was nevertheless less efficient than CT in preventing bone loss, since CGRP-treated rats had a loss of 46% of cancellous bone, whereas CT-treated rats had a loss of 21%. This suggests that CGRP is either less potent than CT at inhibiting bone resorption or else very rapidly degraded. These data indicate that CGRP can control bone cells through a mechanism that is in part different from that of CT, and further suggest that CGRP may play a local role in bone metabolism.

Targeted expression of the neuropeptide calcitonin gene-related peptide to beta cells prevents diabetes in NOD mice.

To investigate whether the immunosuppressive neuropeptide calcitonin gene-related peptide (CGRP) was a potential candidate for tissue-specific gene therapy, we engineered nonobese diabetic (NOD) mice to produce CGRP in beta cells by placing the modified calcitonin gene under the control of the rat insulin promoter. CGRP inhibits CD4 T cell production of the cytokines that have been implicated in the pathogeny of type I diabetes. Three transgene-positive mouse lines were obtained, two of which expressed immunoreactive CGRP in beta cells (NOD-CGRP mice). Isolated islets from one of these two transgene-positive founders produced active CGRP, whereas islets from transgene-negative littermates did not. The production of CGRP by beta cells prevented insulin-dependent diabetes mellitus in male and reduced its incidence by 63% in female mice. This prevention was due to a local immunosuppressive effect of CGRP as no difference was detected between NOD-CGRP and NOD littermate lymph node, spleen, and thymus cells by either FACS analysis or proliferative response to stimulation by Ag, alloantigen or anti-CD3. These data suggest that CGRP is a potential therapeutic molecule to prevent or treat diabetes and possibly other diseases and conditions in which immune cells are involved. These data also suggest that endogenous CGRP concentrated in sensory nerve endings may regulate locally the immune response, further strengthening the importance of the functional neuroimmune link.

The neuropeptide calcitonin gene-related peptide inhibits TNF-alpha but poorly induces IL-6 production by fetal rat osteoblasts.

Tumour necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) are potent inflammatory cytokines produced by osteoblasts and whose contribution to bone loss occurring in oestrogen deficiency is well documented. Calcitonin gene-related peptide (CGRP) is a neuropeptide abundantly concentrated in sensory nerve endings innervating bone metaphyses and periosteum suggesting that it controls bone homeostasis locally. Since CGRP was shown to inhibit TNF-alpha production by T cells and stimulate IL-6 expression by fibroblasts, this study was designed to investigate whether CGRP regulated TNF-alpha and IL-6 production by osteoblasts. We show that CGRP inhibits the production of TNF-alpha by both lipopolysaccharide (LPS)- and IL-1-stimulated fetal rat osteoblasts. Like CGRP, the cAMP agonists prostaglandin E2 (PGE2), dibutyryl cAMP (Bt2cAMP) and forskolin inhibit TNF-alpha production by osteoblasts. Exposure of osteoblasts to a high dose of phorbol myristoyl acetate (PMA) to deplete PKC activity abolished CGRP-mediated TNF-alpha suppression. In contrast with its potent inhibition of TNF-alpha production, we show that CGRP is a weak inducer of IL-6 when compared to PGE2, Bt2cAMP and forskolin. However, in presence of isobutylmethylxanthine (IBMX) CGRP stimulates the production of IL-6. Collectively, these data suggest that the inhibition of TNF-alpha CGRP is cAMP dependent and PMA sensitive and that the concentration of intracellular cAMP may be a regulatory mechanism for IL-6 expression in osteoblasts.

The neuropeptide calcitonin gene-related peptide stimulates insulin-like growth factor I production by primary fetal rat osteoblasts.

Calcitonin gene-related peptide (CGRP)-immunoreactive sensory nerve terminals infiltrate all tissues including bone, in which CGRP may play a local regulatory role. To initiate studies on the role of this neuropeptide in bone, osteoblasts were isolated from fetal rat calvariae, treated with CGRP, and analyzed for cAMP and insulin-like growth factor I (IGF-I) production. CGRP alpha and -beta induced a cAMP accumulation in osteoblastic cells, suggesting that they express functional receptors for CGRP. CGRP induced an increase in both IGF-I transcripts and immunoreactive polypeptide. In contrast to prostaglandin E2 (PGE2) treatment, this increase was not accompanied by an augmentation in IGF binding proteins. Although PGE2 induced a more significant increase in IGF-I transcripts than did CGRP, the concentration of IGF-I polypeptide produced by osteoblasts was similar in response to both treatments. It is concluded from this study that CGRP has potent anabolic effects on osteoblasts, an observation which opens possibilities to study the potential therapeutic role of CGRP in osteoporosis.

Identification of an inducible surface molecule specific to fusing macrophages.

Multinucleated giant cells and osteoclasts arise through the fusion of mononuclear phagocyte precursors. To elucidate the mechanism by which cells of monocytic lineage fuse and differentiate into giant cells and osteoclasts, we hypothesized that, as with other cell fusion events, specific surface molecules mediate the adhesion/fusion process. It has been observed that macrophages can be induced to fuse with one another in response to specific stimuli or when placed in a specific microenvironment. The formation of giant cells is primarily associated with chronic inflammatory reactions and tumors, while osteoclasts differentiate on bone which they resorb. The fact that, under normal conditions, macrophages and monocytes fail to fuse in regions and tissues where they are present in large numbers suggests the regulated and transient expression of potential fusion molecules. To identify such a fusion-associated molecule, we established a macrophage fusion assay and generated monoclonal antibodies (mAbs) that alter the fusion of macrophages in vitro. We selected four mAbs that each had the ability to block the fusion but not the aggregation of macrophages in vitro. All four antibodies recognize surface proteins of 150 kDa. The expression of the antigens recognized by all four mAbs is restricted to macrophages that have been induced to fuse in vitro and in vivo and is inducible, transient, and regulated, as neither nonfusing macrophages nor macrophages fused in vitro express these antigens. These results support the hypothesis that macrophage fusion is mediated by specific fusion/adhesion molecules and also provide a means to study the molecular mechanisms of macrophage fusion.

Bone cell defects in osteogenesis imperfecta.

Iliac crest bone biopsies from nine children (6-15 years old) with osteogenesis imperfecta tarda (O.I.) have been studied by bone histomorphometry after double fluorescent labeling with tetracycline and compared to five unlabeled biopsies from normal children in the same age group. The results indicate that children with O.I. have a low trabecular bone volume associated with an increased bone turnover rate. Bone formation is increased at the tissue level despite a decrease in the activity of individual osteoblasts. The original defects in O.I. seem to be due to the altered rate of matrix synthesis by osteoblasts. It is, however, compensated by an increase in the number of these cells. These results suggest that these children were not losing bone at the time of the biopsy, which fits with the clinical stability of O.I. with age. Our study therefore suggests that the osteopenia observed in O.I. is most likely due to an inability to accumulate bone during growth, as normal children do, rather than to a progressive net loss of bone.

Calcitonin gene-related peptide inhibits interleukin 2 production by murine T lymphocytes.

Evidence from the literature suggests that the nervous and the immune systems closely interact via neuromediators, which affect the immune system, and cytokines, which control nerve cell growth and activity. Calcitonin gene-related peptide (CGRP) is a neuropeptide that has been identified in numerous tissues including immune organs and inhibits the proliferation of spleen cells. We investigated whether CGRP altered the function of T lymphocytes. We present evidence that CGRP induces a dose-dependent cAMP accumulation in interleukin 2-producing TH1 cells and inhibits their production of interleukin 2. These effects are prevented by CGRP8-37, a CGRP antagonist that is missing the first 7 amino acids. This CGRP-mediated inhibition of interleukin 2 production is accompanied by a decrease in interleukin 2 mRNA accumulation. CGRP also inhibits the accumulation of mRNA coding for tumor necrosis factor-alpha and -beta and interferon-gamma. Thus, we have identified one mechanism by which CGRP inhibits the proliferation of spleen cells.

Multinucleated rat alveolar macrophages express functional receptors for calcitonin.

The fusion of mononuclear phagocytes occurs spontaneously in vivo and leads to the differentiation of either multinucleated giant cells or osteoclasts in chronic inflammatory sites or in bone, respectively. Although osteoclasts are responsible for resorbing bone, the functional role of giant cells in chronic inflammatory reactions and tumors remains poorly understood. We recently reported that the plasma membrane of multinucleated macrophages is, like that of osteoclasts, enriched in Na-K-adenosinetriphosphatases (ATPases). We also observed that the localization of their Na-K-ATPases is restricted to the nonadherent domain of the plasma membrane of cells both in vivo and in vitro, thus imposing a functional polarity on their organization. By following this observation, we wished to investigate whether these cells also expressed, like osteoclasts, functional receptors for calcitonin (CT). To this end, alveolar macrophages were fused in vitro, and both their structural and functional association with CT was analyzed and compared with those of mononucleated peritoneal and alveolar macrophages. Evidence is presented that multinucleated alveolar macrophages express a high copy number of functional receptors for CT. Our results also indicate that alveolar macrophages, much like peritoneal, express functional receptors for calcitonin gene-related peptide. It is suggested that multinucleated rat alveolar macrophages offer a novel model system to study CT receptors and that calcitonin may control local immune reactions where giant cells differentiate.

Macrophages express functional receptors for calcitonin-gene-related peptide.

The present study was designed to investigate whether non-activated macrophages express calcitonin (CT) or calcitonin-gene-related peptide (CGRP) receptors. To this end, we first analyzed whether CT and CGRP induce a cAMP accumulation in macrophages. Macrophages were treated for 2 min with increasing concentrations of either CT or CGRP in the presence or absence of IBMX. A dose-dependent cAMP accumulation was measured in response to CGRP with a half-maximal effect attained with 1 nM CGRP. CT failed at all doses to induce an accumulation of cAMP. The effects of CT and CGRP on the activation of the Na-H exchanger were next assessed by spectrofluorometry by using the pH-sensitive dye 2,7 biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). Steady-state pHi of macrophages in a 7.4, HCO3-free solution (HEPES-buffered) was 7.04 +/- 0.08 (n = 22). pHi recovery following an NH4+/NH3 acid load was inhibited by the removal of Na+ or by the addition of the amiloride analog EIPA; therefore recovery is dependent on Na-H exchange activity. CT had no effect on steady-state pHi but CGRP increased pHi in a dose-dependent fashion (10(-12) to 10(-6) M). The pHi change induced by CGRP was due to the stimulation of the Na-H exchanger as CGRP enhanced the rate of recovery (dpHi/dt) from an acid load from 45.3 to 77.2 microMs-1 (n = 8, P less than 0.002) and was completely blocked by EIPA. These data indicate that CGRP both enhances the activity of the Na-H exchanger and increases intracellular cAMP, thus demonstrating that macrophages express functional CGRP receptors.

Detection of the Na(+)-K(+)-ATPase alpha 3-isoform in multinucleated macrophages.

We previously reported that multinucleated macrophages express a high concentration of Na(+)-K(+)-ATPases that are concentrated on the nonadherent domain of their plasma membrane (A. Vignery, T. Niven-Fairchild, D. H. Ingbar, and M. Caplan. J. Histochem. Cytochem. 37: 1265-1271, 1989). We also showed that an increase in newly synthesized alpha-subunit occurred during cell culture and multinucleation. We now present evidence that macrophage multinucleation in vitro is accompanied by an increased accumulation of Na(+)-K(+)-ATPase alpha-subunit mRNA. Most interesting is the detection of significant amount of both alpha 1- and alpha 3-isoform mRNA and peptide in these cells by in situ hybridization, Northern and Western blot analyses. These qualitative and quantitative variations in Na(+)-K(+)-ATPase expression suggest that macrophage multinucleation is accompanied by a coordinated regulation of gene expression and that multinucleation confers a specific function to macrophages. Multinucleated macrophages offer a novel model system to investigate not only the specific function(s) of the alpha 3-isoform but also the role of the Na(+)-K(+)-ATPase in giant cells and osteoclasts.

Recombinant murine interferon-gamma inhibits the fusion of mouse alveolar macrophages in vitro but stimulates the formation of osteoclastlike cells on implanted syngeneic bone particles in mice in vivo.

Osteoclasts are multinucleated cells that originate from the fusion of mononuclear precursors and are responsible for bone resorption. Indirect evidence from in vitro studies suggests that IFN-gamma and TNF-alpha inhibit and stimulate bone resorption, respectively, but contradictory results have emerged from the literature regarding the effects of IFN-gamma on macrophage multinucleation. Using highly sensitive model systems, the present work demonstrates that, in mice, rMuIFN-gamma inhibits the fusion of alveolar macrophages in vitro but augments the number of osteoclastlike cells on implanted syngeneic bone particles in vivo. Although rMuTNF-alpha fails to stimulate macrophage multinucleation in either system, treatment of implanted animals with rMuIFN-gamma appears to limit the inflammatory reaction and favor tissue repair.