Interleukin 4 inhibits murine osteoclast formation in vitro

The immune cells in modulating osteoclast formation and bone metabolism

Osteoclasts are pivotal in regulating bone metabolism, with immune cells significantly influencing both physiological and pathological processes by modulating osteoclast functions. This is particularly evident in conditions of inflammatory bone resorption, such as rheumatoid arthritis and periodontitis. This review summarizes and comprehensively analyzes the research progress on the regulation of osteoclast formation by immune cells, aiming to unveil the underlying mechanisms and pathways through which diseases, such as rheumatoid arthritis and periodontitis, impact bone metabolism.

Osteoprotective effect by interleukin-4 (IL-4) on lipoprotein-induced periodontitis

Lipoproteins are immunostimulatory bacterial components suggested to participate in inflammation-induced bone loss in periodontal disease through stimulation of osteoclast differentiation. Toll-like receptor 2 activation by Pam2CSK4 (PAM2), known to mimic bacterial lipoproteins, was previously shown to enhance periodontal bone resorption in mice. The anti-inflammatory cytokine interleukin-4 (IL-4) is a known inhibitor of RANKL-induced bone resorption in vitro. Here, we have investigated whether IL-4 could decrease PAM2-induced periodontal bone loss and osteoclastogenesis in vivo. In a model of periodontitis induced by gingival injections of PAM2 in mice, concomitant injections of IL-4 reduced bone loss. Histologically, IL-4 reduced the recruitment of inflammatory cells and the formation of TRAP+ osteoclasts stimulated by PAM2. Mouse bone marrow macrophages (BMMs) and neonatal calvarial osteoblasts were used to assess the effect of IL-4 on PAM2-induced osteoclastogenesis in vitro. In RANKL-primed BMMs stimulated by PAM2 Nfatc1, Ctsk, and Acp5 gene expression was up-regulated and resulted in robust formation of TRAP+ multinucleated osteoclasts, effects which were impaired by IL-4. These effects were mediated by impairment in PAM2-induced c-fos expression. In primary calvarial osteoblast cultures, IL-4 decreased PAM2-induced Tnfsf11 (encoding RANKL) mRNA and enhanced Tnfrsf11b (encoding OPG) expression. Our data demonstrate that the osteoprotective effect by IL-4 on lipoprotein-induced periodontal disease occurs through the inhibition of osteoclastogenesis by three mechanisms, one by acting directly on osteoclast progenitors, another by acting indirectly through decreasing the expression of osteoclast-regulating cytokines in osteoblasts and a third by decreasing inflammation.

Inflammatory Arthritis and Bone Metabolism Regulated by Type 2 Innate and Adaptive Immunity

While type 2 immunity has traditionally been associated with the control of parasitic infections and allergic reactions, increasing evidence suggests that type 2 immunity exerts regulatory functions on inflammatory diseases such as arthritis, and also on bone homeostasis. This review summarizes the current evidence of the regulatory role of type 2 immunity in arthritis and bone. Key type 2 cytokines, like interleukin (IL)-4 and IL-13, but also others such as IL-5, IL-9, IL-25, and IL-33, exert regulatory properties on arthritis, dampening inflammation and inducing resolution of joint swelling. Furthermore, these cytokines share anti-osteoclastogenic properties and thereby reduce bone resorption and protect bone. Cellular effectors of this action are both T cells (i.e., Th2 and Th9 cells), but also non-T cells, like type 2 innate lymphoid cells (ILC2). Key regulatory actions mediated by type 2 cytokines and immune cells on both inflammation as well as bone homeostasis are discussed.

Current Understanding of Osteoimmunology in Certain Osteoimmune Diseases

The skeletal system and immune system seem to be two independent systems. However, there in fact are extensive and multiple crosstalk between them. The concept of osteoimmunology was created to describe those interdisciplinary events, but it has been constantly updated over time. In this review, we summarize the interactions between the skeletal and immune systems in the co-development of the two systems and the progress of certain typical bone abnormalities and bone regeneration on the cellular and molecular levels according to the mainstream novel study. At the end of the review, we also highlighted the possibility of extending the research scope of osteoimmunology to other systemic diseases. In conclusion, we propose that osteoimmunology is a promising perspective to uncover the mechanism of related diseases; meanwhile, a study from the point of view of osteoimmunology may also provide innovative ideas and resolutions to achieve the balance of internal homeostasis.

The critical role of T cells in glucocorticoid-induced osteoporosis

Glucocorticoids (GC) are widely used clinically, despite the presence of significant side effects, including glucocorticoid-induced osteoporosis (GIOP). While GC are believed to act directly on osteoblasts and osteoclasts to promote osteoporosis, the detailed underlying molecular mechanism of GC-induced osteoporosis is still not fully elucidated. Here, we show that lymphocytes play a pivotal role in regulating GC-induced osteoporosis. We show that GIOP could not be induced in SCID mice that lack T cells, but it could be re-established by adoptive transfer of splenic T cells from wild-type mice. As expected, T cells in the periphery are greatly reduced by GC; instead, they accumulate in the bone marrow where they are protected from GC-induced apoptosis. These bone marrow T cells in GC-treated mice express high steady-state levels of NF-κB receptor activator ligand (RANKL), which promotes the formation and maturation of osteoclasts and induces osteoporosis. Taken together, these findings reveal a critical role for T cells in GIOP.

Sclerostin expression in trabecular bone is downregulated by osteoclasts

Bone tissues have trabecular bone with a high bone turnover and cortical bone with a low turnover. The mechanisms by which the turnover rate of these bone tissues is determined remain unclear. Osteocytes secrete sclerostin, a Wnt/β-catenin signaling antagonist, and inhibit bone formation. We found that sclerostin expression in cortical bone is more marked than in trabecular bone in Sost reporter mice. Leukemia inhibitory factor (LIF) secreted from osteoclasts reportedly suppressed sclerostin expression and promoted bone formation. Here, we report that osteoclasts downregulate sclerostin expression in trabecular bone and promote bone turnover. Treatment of C57BL/6 mice with an anti-RANKL antibody eliminated the number of osteoclasts and LIF-positive cells in trabecular bone. The number of sclerostin-positive cells was increased in trabecular bone, while the number of β-catenin-positive cells and bone formation were decreased in trabecular bone. Besides, Tnfsf11 heterozygous (Rankl^+/−) mice exhibited a decreased number of LIF-positive cells and increased number of sclerostin-positive cells in trabecular bone. Rankl^+/− mice exhibited a decreased number of β-catenin-positive cells and reduced bone formation in trabecular bone. Furthermore, in cultured osteoclasts, RANKL stimulation increased Lif mRNA expression, suggesting that RANKL signal increased LIF expression. In conclusion, osteoclasts downregulate sclerostin expression and promote trabecular bone turnover.

Cytokines and Bone: Osteoimmunology

Cytokines and hematopoietic growth factors have traditionally been thought of as regulators of the development and function of immune and blood cells. However, an ever-expanding number of these factors have been discovered to have major effects on bone cells and the development of the skeleton in health and disease (Table 1). In addition, several cytokines have been directly linked to the development of osteoporosis in both animal models and in patients. In order to understand the mechanisms regulating bone cells and how this may be dysregulated in disease states, it is necessary to appreciate the diverse effects that cytokines and inflammation have on osteoblasts, osteoclasts, and bone mass. This chapter provides a broad overview of this topic with extensive references so that, if desired, readers can access specific references to delve into individual topics in greater detail.

Mediators of inflammation and bone remodeling in rheumatic disease

Remodeling of bone is a continuous process that occurs throughout life. Under normal physiologic conditions, bone-resorbing osteoclasts and bone-forming osteoblasts are tightly coupled and regulated to ensure proper balance, such that there is no net change in bone mass. However, inflammation perturbs normal bone homeostasis. The impact of inflammation on bone is dependent upon the anatomic site affected, cell types, factors and cytokines present in the local microenvironment, and local mechanical forces. Cytokines are central to the pathogenesis of inflammation-induced bone loss and contribute to the uncoupling of osteoclast-mediated bone resorption and osteoblast-mediated bone formation, thereby disrupting normal remodeling. In this review, we will discuss the effects of cytokines on bone in two settings, rheumatoid arthritis and spondyloarthritis, a disease category that includes ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease, and juvenile onset spondyloarthropathy. The outcome for bone in these disease settings is quite different, and an understanding of the pathogenic mechanisms leading to the net impact on bone has been essential in developing new therapeutic approaches to bone health in these diseases.

Porous bioactive scaffolds: characterization and biological performance in a model of tibial bone defect in rats

The aim of this study was to evaluate the effects of highly porous Biosilicate(®) scaffolds on bone healing in a tibial bone defect model in rats by means of histological evaluation (histopathological and immunohistochemistry analysis) of the bone callus and the systemic inflammatory response (immunoenzymatic assay). Eighty Wistar rats (12 weeks-old, weighing±300 g) were randomly divided into 2 groups (n=10 per experimental group, per time point): control group and Biosilicate® group (BG). Each group was euthanized 3, 7, 14 and 21 days post-surgery. Histological findings revealed a similar inflammatory response in both experimental groups, 3 and 7 days post-surgery. During the experimental periods (3-21 days post-surgery), it was observed that the biomaterial degradation, mainly in the periphery region, provided the development of the newly formed bone into the scaffolds. Immunohistochemistry analysis demonstrated that the Biosilicate® scaffolds stimulated cyclooxygenase-2, vascular endothelial growth factor and runt-related transcription factor 2 expression. Furthermore, in the immunoenzymatic assay, BG presented no difference in the level of tumor necrosis factor alpha in all experimental periods. Still, BG showed a higher level of interleukin 4 after 14 days post-implantation and a lower level of interleukin 10 in 21 days post-surgery. Our results demonstrated that Biosilicate® scaffolds can contribute for bone formation through a suitable architecture and by stimulating the synthesis of markers related to the bone repair.

The immune system, bone and RANKL

Bone and immune systems are tightly linked. In the past years, many molecules originally believed to belong to the immune system were found to function in bone cells. It is now evident that the two systems are coregulated by many shared cytokines and signaling molecules. Here we exemplify the complex interaction between bone metabolism and immune response focusing on the multifaceted role of receptor activator of NF-κB ligand (RANKL). RANKL is expressed by cells of both systems, is an essential regulator of bone degradation and exerts either pro or anti-inflammatory effects on the immune response. In the present review, we summarize the multiple functions of RANKL in bone and in the immune systems, aiming to provide an overview of the field of osteoimmunology.

Activated γδ T cells inhibit osteoclast differentiation and resorptive activity in vitro

Extensive evidence suggests that the immune system exerts powerful effects on bone cells, particularly in chronic disease pathologies such as rheumatoid arthritis (RA). The chronic inflammatory state in RA, particularly the excessive production of T cell-derived proinflammatory cytokines such as tumour necrosis factor (TNF)-α and interleukin (IL)-17, triggers bone erosions through the increased stimulation of osteoclast formation and activity. While evidence supports a role for IL-17 and TNF-α secreted by conventional CD4⁺ T cells in RA, recent evidence in animal models of RA have implicated γδ T cells as a major producer of pathogenic IL-17. However, the capacity of γδ T cells to influence osteoclast formation and activity in humans has not yet been investigated widely. To address this issue we investigated the effects of γδ T cells on osteoclast differentiation and resorptive activity. We have demonstrated that anti-CD3/CD28-stimulated γδ T cells or CD4⁺ T cells inhibit human osteoclast formation and resorptive activity in vitro. Furthermore, we assessed cytokine production by CD3/CD28-stimulated γδ T cells and observed a lack of IL-17 production, with activated γδ T cells producing abundant interferon (IFN)-γ. The neutralization of IFN-γ markedly restored the formation of osteoclasts from precursor cells and the resorptive activity of mature osteoclasts, suggesting that IFN-γ is the major factor responsible for the inhibitory role of activated γδ T cells on osteoclastogenesis and resorptive activity of mature osteoclasts. Our work therefore provides new insights on the interactions between γδ T cells and osteoclasts in humans.

Osteoimmunology and the influence of pro-inflammatory cytokines on osteoclasts

Bone and immune system are functionally interconnected. Immune and bone cells derive from same progenitors in the bone marrow, they share a common microenvironment and are being influenced by similar mediators. The evidence on increased bone resorption associated with inappropriate activation of T cells such as during inflammation, is well established. However, the molecular mechanisms beyond this clinical observation have begun to be intensively studied with the advancement of osteoimmunology. Now days, we have firm evidence on the influence of numerous proinflammatory cytokines on bone cells, with the majority of data focused on osteoclasts, the bone resorbing cells. It has been shown that some proinflammatory cytokines could possess osteoclastogenic and/or anti-osteoclastogenic properties and can target osteoclasts directly or via receptor activator of nuclear factor κB (RANK)/RANK ligand(RANKL)/osteoprotegerin (OPG) system. Several studies have reported opposing data regarding (anti)osteoclastogenic properties of these cytokines.

Therefore, the first part of this review is summarizing current evidence on the influence of pro-inflammatory cytokines on osteoclasts and thus on bone resorption. In the second part, the evidence on the role of pro-inflammatory cytokines in osteoporosis and osteoarthritis is reviewed to show that unravelling the mechanisms beyond such complex bone diseases, is almost impossible without considering skeletal and immune systems as an indivisible integrated system.

IL-4 inhibits TNF-α-mediated osteoclast formation by inhibition of RANKL expression in TNF-α-activated stromal cells and direct inhibition of TNF-α-activated osteoclast precursors via a T-cell-independent mechanism in vivo

It has been reported that osteoclastogenesis is induced by tumor necrosis factor (TNF)-α. Interleukin (IL)-4 is the most important cytokine involved in humoral immunity. However, no studies have investigated the effect of IL-4 on TNF-α-mediated osteoclast formation in vivo. In this study, we investigated the effect of IL-4 on TNF-α-mediated osteoclast formation in vivo. TNF-α was administered with and without IL-4 into the supracalvariae of mice. The number of osteoclasts and the levels of mRNA for cathepsin K and tartrate-resistant acid phosphate, both osteoclast markers, in mice administered TNF-α and IL-4 were lower than those in mice administered TNF-α alone. The level of tartrate-resistant acid phosphatase form 5b (TRACP5b) as a marker of bone resorption in mice administered both TNF-α and IL-4 was also lower. We showed that IL-4 inhibited TNF-α-mediated osteoclast formation in osteoclast precursors in vitro. Expression of receptor activator of NF-κB ligand (RANKL) in TNF-α-activated stromal cells was also inhibited. Furthermore, we investigated whether IL-4 had effects on both stromal cells and osteoclast precursors in TNF-α-mediated osteoclast formation in vivo. Using mice whose stromal cells and osteoclast precursors were chimeric for the presence of TNF receptors, IL-4 inhibited TNF-α-mediated osteoclast formation in the presence of TNF-α-responsive stromal cells, and TNF-α-responsive osteoclast precursors in vivo. IL-4 also inhibited TNF-α-induced RANKL expression in the presence of TNF-α-responsive stromal cells in vivo. This event is dependent on p38 inhibition in vitro. Additionally, IL-4 inhibited TNF-α-mediated osteoclast formation in T cell-depleted mice. In summary, we conclude that IL-4 inhibited TNF-α-mediated osteoclast formation by inhibiting expression of RANKL in TNF-α-activated stromal cells, and directly inhibited TNF-α-activated osteoclast precursors in vivo via a T cell-independent mechanism.

Effects of inflammatory and anti-inflammatory cytokines on the bone

BACKGROUND

Inflammatory diseases are linked to enhanced bone loss. The effect of inflammation on bone is mediated by proinflammatory cytokines, which regulate bone formation as well as bone resorption thereby altering bone homeostasis.

MATERIALS AND METHODS

In this article we summarize the key insights in cytokine regulation of bone. We describe the major pro- and anti-inflammatory mediators, which are involved in the regulation of bone and describe the mechanisms by which these cytokines alter bone balance.

RESULTS

We describe the effects of tumor necrosis factor (TNF), interleukin (IL)- 1 family members, IL-6, IL-17 and interferons (IFN) on bone and discuss the mechanisms by which these individual cytokines affect the bone resorbing and the bone forming cells.

CONCLUSIONS

Several proinflammatory cytokines (such as TNFa, IL-1 and IL-17) are major triggers for osteoclast activation explaining the enhanced bone loss during inflammation. Other such as IL-12, IL-18, IL-33 and IFN are strong suppressors of osteoclast differentiation and inhibit bone loss. Thus the cytokine composition of an inflammatory tissue is decisive whether inflammation triggers bone loss or not.

Effects of inflammation on bone: an update

PURPOSE OF REVIEW

To present an updated summary of the relationship between inflammation and localized and generalized bone loss in the rheumatic diseases.

RECENT FINDINGS

In addition to the well established role of inflammatory cytokines in promoting enhanced osteoclast function and bone loss, recent work has discovered the cytokine milieu may also inhibit osteoblast function and bone repair. The WNT and bone morphogenetic protein pathways provide molecular links between inflammation and altered bone homeostasis in chronic inflammatory states. These pathways and others have been the targets of emerging therapies for the management of inflammatory bone loss.

SUMMARY

Inflammation and bone loss are linked through a number of molecular pathways. Both of these processes need to be addressed when designing an effective treatment strategy for the rheumatic diseases.

Interleukin-27 inhibits human osteoclastogenesis by abrogating RANKL-mediated induction of nuclear factor of activated T cells c1 and suppressing proximal RANK signaling

OBJECTIVE

Interleukin-27 (IL-27) has stimulatory and regulatory immune functions and is expressed in rheumatoid arthritis (RA) synovium. This study was undertaken to investigate the effects of IL-27 on human osteoclastogenesis, to determine whether IL-27 can stimulate or attenuate the osteoclast-mediated bone resorption that is a hallmark of RA.

METHODS

Osteoclasts were generated from blood-derived human CD14+ cells. The effects of IL-27 on osteoclast formation were evaluated by counting the number of tartrate-resistant acid phosphatase-positive multinucleated cells and measuring the expression of osteoclast-related genes. The induction of nuclear factor of activated T cells c1 (NFATc1) and the activation of signaling pathways downstream of RANK were measured by immunoblotting. The expression of key molecules implicated in osteoclastogenesis (NFATc1, RANK, costimulatory receptors, and immunoreceptor tyrosine-based activation motif-harboring adaptor proteins) was measured by real-time reverse transcription-polymerase chain reaction. Murine osteoclast precursors obtained from mouse bone marrow and synovial fluid macrophages derived from RA patients were also tested for their responsiveness to IL-27.

RESULTS

IL-27 inhibited human osteoclastogenesis, suppressed the induction of NFATc1, down-regulated the expression of RANK and triggering receptor expressed on myeloid cells 2 (TREM-2), and inhibited RANKL-mediated activation of ERK, p38, and NF-kappaB in osteoclast precursors. Synovial fluid macrophages from RA patients were refractory to the effects of IL-27. In contrast to the findings in humans, IL-27 only moderately suppressed murine osteoclastogenesis, and this was likely attributable to low expression of the IL-27 receptor subunit WSX-1 on murine osteoclast precursors.

CONCLUSION

IL-27 inhibits human osteoclastogenesis by a direct mechanism that suppresses the responses of osteoclast precursors to RANKL. These findings suggest that, in addition to its well-known antiinflammatory effects, IL-27 plays a homeostatic role in restraining bone erosion. This homeostatic function is compromised under conditions of chronic inflammation such as in RA synovitis.

Complex regulation of tartrate-resistant acid phosphatase (TRAP) expression by interleukin 4 (IL-4): IL-4 indirectly suppresses receptor activator of NF-kappaB ligand (RANKL)-mediated TRAP expression but modestly induces its expression directly

Interleukin 4 (IL-4) inhibits receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast formation and functional activity in a STAT6-dependent manner. IL-4 down-regulates expression of tartrate-resistant acid phosphatase (TRAP) in mature osteoclasts. To determine whether IL-4 regulates TRAP promoter activity, RAW264.7 cells were transfected with a TRAP promoter-luciferase reporter. Treatment with IL-4 alone modestly enhanced TRAP luciferase activity. However, IL-4 suppressed the ability of RANKL to up-regulate TRAP-luciferase activity, suggesting that IL-4 has multiple effects on TRAP transcription. IL-4 also reduced the RANKL-induced association of RNA polymerase II with the TRAP gene in osteoclasts. The TRAP promoter contains a STAT6-binding motif, and STAT6 bound to the endogenous TRAP promoter after IL-4 treatment. To determine the impact of STAT6 binding, we transfected cells with STAT6VT, a constitutively active STAT6 mutant. STAT6VT alone up-regulated TRAP-luciferase activity; this effect was abrogated by mutating the STAT6 binding site in the minimal TRAP promoter. STAT6VT did not inhibit the potent up-regulation of TRAP promoter activity caused by overexpression of NFATc1, PU.1, and microphthalmia transcription factor, downstream targets of macrophage colony-stimulating factor and RANKL. IL-4 down-regulated the expression of c-Fos and NFATc1 in mature osteoclasts. Knockdown of NFATc1 by short interfering RNA caused TRAP expression to be down-regulated, and ectopic expression of NFATc1 abrogated the IL-4-induced down-regulation of TRAP. These results suggest that STAT6 plays two distinct roles in TRAP expression. The IL-4-induced activation of STAT6 mediates suppression of the RANKL-induced TRAP promoter activity indirectly by inhibiting NFATc1 expression. However, in the absence of RANKL and osteoclast differentiation, STAT6 binds the TRAP promoter after IL-4 treatment and directly enhances TRAP expression.

The contribution of the synovium, synovial derived inflammatory cytokines and neuropeptides to the pathogenesis of osteoarthritis

Osteoarthritis (OA) is one of the most common and disabling chronic joint disorders affecting horses, dogs and humans. Synovial inflammation or synovitis is a frequently observed phenomenon in osteoarthritic joints and contributes to the pathogenesis of OA through formation of various catabolic and pro-inflammatory mediators altering the balance of cartilage matrix degradation and repair. Catabolic mediators produced by the inflamed synovium include pro-inflammatory cytokines, nitric oxide, prostaglandin E(2) and several neuropeptides, which further contribute to the pathogenesis of OA by increasing cartilage degradation. Recent studies suggest that substance P, corticotropin-releasing factor, urocortin and vasoactive intestinal peptide may also be involved in OA development, but the precise role of these neuropeptides in the pathogenesis of OA is not known. Since increased production of matrix metalloproteinases by the synovium is stimulated by pro-inflammatory cytokines, future anti-inflammatory therapies should focus on the synovium as a means of controlling subsequent inflammatory damage.

Osteoimmunology: cytokines and the skeletal system

It has become clear that complex interactions underlie the relationship between the skeletal and immune systems. This is particularly true for the development of immune cells in the bone marrow as well as the functions of bone cells in skeletal homeostasis and pathologies. Because these two disciplines developed independently, investigators with an interest in either often do not fully appreciate the influence of the other system on the functions of the tissue that they are studying. With these issues in mind, this review will focus on several key areas that are mediated by crosstalk between the bone and immune systems. A more complete appreciation of the interactions between immune and bone cells should lead to better therapeutic strategies for diseases that affect either or both systems.

Osteoimmunology: interactions of the bone and immune system

Bone and the immune system are both complex tissues that respectively regulate the skeleton and the body's response to invading pathogens. It has now become clear that these organ systems often interact in their function. This is particularly true for the development of immune cells in the bone marrow and for the function of bone cells in health and disease. Because these two disciplines developed independently, investigators in each don't always fully appreciate the significance that the other system has on the function of the tissue they are studying. This review is meant to provide a broad overview of the many ways that bone and immune cells interact so that a better understanding of the role that each plays in the development and function of the other can develop. It is hoped that an appreciation of the interactions of these two organ systems will lead to better therapeutics for diseases that affect either or both.

Macrophage fusion: molecular mechanisms

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

Cell-cell fusion

Cell-cell fusion is a highly regulated and dramatic cellular event that is required for development and homeostasis. Fusion may also play a role in the development of cancer and in tissue repair by stem cells. While virus-cell fusion and the fusion of intracellular membranes have been the subject of intense investigation during the past decade, cell-cell fusion remains poorly understood. Given the importance of this cell-biological phenomenon, a number of investigators have begun analyses of the molecular mechanisms that mediate the specialized fusion events of a variety of cell types and species. We discuss recent genetic and biochemical studies that are beginning to yield exciting insights into the fusion mechanisms of Saccharomyces cerevisiae mating pairs, Caenorhabditis elegans epithelial cells and gametes, Drosophila melanogaster and mammalian myoblasts, and mammalian macrophages.

Cytokines regulating osteoclast formation and function

PURPOSE OF REVIEW

The osteoclast is the principal bone-resorbing cell. Because of its unique ability to efficiently remove both the mineral and the organic matrix of bone, the osteoclast is an important element of the homeostatic mechanisms that maintain skeletal integrity and serum calcium levels. Over the past 30 years, a number of immune cell modulators have been shown to have effects on osteoclast formation and function. This review will briefly summarize the roles that cytokines have in osteoclast regulation.

RECENT FINDINGS

A large number of cytokines have been shown to regulate osteoclast formation and function. In addition, a number of additional cytokines are now known to have a major influence on the ability of osteoclasts to resorb bone. Interactions of the immune system with bone, which has been recently labeled 'osteoimmunology', appear to be mediated mainly by cytokine signals. Cytokines are known to regulate many of the responses of bone to inflammatory conditions; however, they also may regulate physiologic responses of bone.

SUMMARY

In the future it is hoped that therapies that target cytokine actions may be used to reduce the effects of inflammatory diseases on bone, as well as to regulate normal bone physiology.

T lymphocyte-deficient mice lose trabecular bone mass with ovariectomy

We examined OVX-induced bone loss in three TLD mouse models. In TLD mice, OVX caused trabecular bone loss equivalent to that of WT. In contrast, cortical bone loss with OVX was variable. We conclude that T lymphocytes do not influence OVX-induced trabecular bone loss.

INTRODUCTION

We examined ovariectomy (OVX)-induced bone loss in three T lymphocyte-deficient (TLD) mouse models: nude mice, recombination activating gene 2-deficient (RAG2 KO) mice, and T cell receptor alpha chain-deficient (TCRalpha KO) mice.

MATERIALS AND METHODS

Bone mass was examined by DXA, microCT, and histomorphometry. We also examined the effect of OVX on T lymphocytes in the bone marrow and spleens of wildtype (WT) mice and on in vitro osteoclastogenesis and colony forming unit-granulocyte macrophage (CFU-GM) activity in the bone marrow of WT and nude mice.

RESULTS

In WT mice, OVX did not alter T lymphocyte number in the bone marrow but did increase T lymphocytes in the spleen. Comparison of bone mass in nude, RAG2 KO, and TCRalpha KO mice with WT as measured by DXA showed decreased femoral bone mass in nude mice and increased vertebral bone mass in RAG2 KO mice. In TCRalpha KO mice, femoral, tibial, and vertebral bone mass were decreased. In vertebrae and long bones, bone loss with OVX was consistently present in WT mice but variably present in TLD mice as measured by DXA. In contrast, microCT and histomorphometry showed similar trabecular bone loss after OVX in all mice. However, femoral cortical bone loss occurred only in WT and RAG2 KO mice. OVX produced similar trabecular bone loss in WT and TCRalpha KO mice and also induced cortical bone loss in both. Histomorphometry showed that TRACP(+) area in bones was increased by OVX in femurs from both WT and nude mice as was in vitro osteoclast-like cell formation and CFU-GM activity.

CONCLUSIONS

These results show that OVX caused similar trabecular bone loss in both WT and TLD mice. The ability of DXA and measurement of cortical bone loss to show OVX-induced effects on bone mass was variable. It seems that T lymphocytes are not critical for OVX-induced trabecular bone loss in these mouse models.

Tumor necrosis factor-alpha: alternative role as an inhibitor of osteoclast formation in vitro

TNFalpha is known to stimulate the development and activity of osteoclasts and of bone resorption. The cytokine was found to mediate bone loss in conjunction with inflammatory diseases such as rheumatoid arthritis or chronic aseptic inflammation induced by wear particles from implants and was suggested to be a prerequisite for the loss of bone mass under estrogen deficiency. In the present study, the regulation of osteoclastogenesis by TNFalpha was investigated in co-cultures of osteoblasts and bone marrow or spleen cells and in cultures of bone marrow and spleen cells grown with CSF-1 and RANKL. Low concentrations of TNFalpha (1 ng/ml) caused a >90% decrease in the number of osteoclasts in co-cultures, but did not affect the development of osteoclasts from bone marrow cells. In cultures with p55TNFR(-/-) osteoblasts and wt BMC, the inhibitory effect was abrogated and TNFalpha induced an increase in the number of osteoclasts in a dose-dependent manner. Osteoblasts were found to release the inhibitory factor(s) into the culture supernatant after simultaneous treatment with 1,25(OH)(2)D(3) and TNFalpha, this activity, but not its release, being resistant to treatment with anti-TNFalpha antibodies. Dexamethasone blocked the secretion of the TNFalpha-dependent inhibitor by osteoblasts, while stimulating the development of osteoclasts. The data suggest that the effects of TNFalpha on the differentiation of osteoclast lineage cells and on bone metabolism may be more complex than hitherto assumed and that these effects may play a role in vivo during therapies for inflammatory diseases.

Inhibition of Hormone and Cytokine-stimulated Osteoclastogenesis and Bone Resorption by Interleukin-4 and Interleukin-13 Is Associated with Increased Osteoprotegerin and Decreased RANKL and RANK in a STAT6-dependent Pathway

Interleukin (IL)-4 and IL-13 are cytokines that inhibit bone resorption. Data showing an inhibitory effect of IL-4 and IL-13 on RANK mRNA in mouse calvariae were first reported at the 22nd American Society for Bone and Mineral Research Meeting (Lerner, U.H., and Conaway, H. H. 2000) J. Bone Min. Res. 15, Suppl. 1, Abstr. SU 230). In the present study, release of 45Ca from cultured mouse calvarial bones stimulated by different cytokines, peptides, and steroid hormones was inhibited by IL-4 and IL-13. IL-4 and IL-13 decreased receptor activator of nuclear factor-kappaB ligand (RANKL) and RANK mRNA and increased osteoprotegerin (OPG) mRNA in calvariae. Additionally, the cytokines decreased RANKL protein and increased OPG protein in calvarial bones. In osteoblasts isolated from calvariae, both an increase in RANKL mRNA and a decrease in OPG mRNA and protein elicited by vitamin D3 were reversed by IL-4 and IL-13. IL-4 and IL-13 decreased the number of tartrate-resistant acid phosphatase positive multinucleated cells and the mRNA expression of calcitonin receptor, tartrate-resistant acid phosphatase, and cathepsin K in mouse spleen cells and bone marrow macrophages (BMM) treated with macrophage colony-stimulating factor and RANKL. Inhibition of mRNA for RANK and the transcription factor NFAT2 was also noted in spleen cell and BMM cultures treated with IL-4 and IL-13. In addition, RANK mRNA and RANK protein were decreased by IL-4 and IL-13 in RAW 264.7 cells. Osteoblasts, spleen cells, and BMM expressed mRNA for the four proteins making up the IL-4 and IL-13 receptors. No effects by IL-4 on bone resorption and osteoclast formation or on RANKL and RANK mRNA expression were seen in Stat6-/- mice. The data indicate that IL-4 and IL-13, via a STAT6-dependent pathway, inhibit osteoclast differentiation and bone resorption by activating receptors on osteoblasts and osteoclasts that affect the RANKL/RANK/OPG system.

Interleukin-4 directly inhibits tumor necrosis factor-alpha-mediated osteoclast formation in mouse bone marrow macrophages

Recently it has been found that osteoclast differentiation is induced by tumor necrosis factor (TNF)-alpha. Interleukin (IL)-4 was reported to suppress osteoclast differentiation and bone resorption. However, no study has investigated the effect of IL-4 on TNF-alpha-induced osteoclast formation. In this study, we investigated whether IL-4 inhibits TNF-alpha-mediated osteoclast formation in mouse bone marrow derived macrophages (BMM). First, IL-4 suppresses RANKL-induced osteoclast formation and bone resorption. Next, when BMM were cultured with TNF-alpha, osteoclast-like cells were formed. When they were cultured with both TNF-alpha and IL-4, osteoclast formation and bone resorption was suppressed by IL-4 in a dose-dependent manner. It has been recently found that TNF-alpha and RANKL synergistically promote osteoclastogenesis. Finally, we investigated whether IL-4 had the ability to inhibit synergistic TNF-alpha and RANKL-induced osteoclastogenesis, with the result that it effectively inhibited the synergistic osteoclast formation in a dose-dependent manner. We conclude that IL-4 can strongly inhibit osteoclast formation that is related to both physiological bone resorption induced by RANKL and pathological bone resorption induced by TNF-alpha.

IL-4 suppresses osteoclast development and mature osteoclast function by a STAT6-dependent mechanism: irreversible inhibition of the differentiation program activated by RANKL

Numerous reports have described the effects of interleukin-4 (IL-4) on bone biology. Previous studies, performed using complex coculture systems, demonstrated the effects of IL-4 on osteoblasts and osteoclasts. To directly test the effect of IL-4 on osteoclasts, we took advantage of a simplified system using recombinant receptor activator of nuclear factor kappaB ligand (RANKL) as the osteoclast differentiation factor. We analyzed the ability of IL-4 to directly regulate osteoclast differentiation and mature osteoclast function. We found that IL-4 inhibited the differentiation of osteoclasts from bone marrow precursors in an irreversible manner and also inhibited the resorptive capacity of mature osteoclasts. In the presence of IL-4, we detected the appearance of tartrate-resistant acid phosphatase (TRAP)-negative multinucleated giant (MNG) cells. Both IL-4 effects were dependent on signal transducer and activator of transcription 6 (STAT6). We found that IL-4 suppresses RANK mRNA expression in the developing precursor cells. When RANK was ectopically expressed under the cytomegalovirus (CMV) promoter in RAW264.7 macrophages, IL-4 treatment did not inhibit osteoclast development. Furthermore, when osteoclastogenesis was induced independently of RANKL by using tumor necrosis factor-alpha (TNF-alpha), IL-4 inhibited osteoclast differentiation through a STAT6-dependent mechanism. These results suggest that IL-4 regulates osteoclast development by regulating gene expression, including RANK. We propose that IL-4 irreversibly regulates the lineage commitment of precursor cells by regulating gene expression, resulting in the suppression of osteoclast development and the generation of MNG cells as an alternative pathway of differentiation.

T-cells mediate an inhibitory effect of interleukin-4 on osteoclastogenesis

IL-4 is an important cytokine that can influence bone. We identified two distinct actions of IL-4 to inhibit osteoclast formation: one direct on osteoclast progenitors and the second through the production of a novel T-cell surface-associated molecule(s). These data show a new link between the immune system and bone. The Th2 cytokine interleukin (IL)-4 inhibits osteoclast formation in vitro but also acts on other cell types found in bone, including T-cells and macrophages. Because some osteoclastogenesis inhibitors (e.g., IL-12) act indirectly through T-cells, we investigated IL-4 action on osteoclastogenesis in the presence of T-cells. Osteoclast formation from murine spleen cells treated with RANKL and macrophage colony-stimulating factor (M-CSF) was blocked by IL-4 even when spleen cells were depleted of T-cells (Thy 1.2+) and/or B-cells (B220+). Also, IL-4 inhibited osteoclastogenesis in RANKL/M-CSF-stimulated adherent spleen cells, Rag1 -/- (lymphocyte-deficient) spleen cells, and bone marrow macrophages, indicating an action on myelomonocytic cells to block osteoclastogenesis. In contrast, IL-4 did not inhibit osteoclastogenesis in cells from IL-4 receptor null mice (IL-4R -/-). However, when wildtype T-cells were added to IL-4R -/- spleen cell cultures, IL-4 inhibited osteoclast formation, indicating a T-cell-dependent action. Osteoclast formation in RANKL-stimulated RAW 264.7 cells was not inhibited by IL-4 unless T-cells were added to the culture. Separation of RAW 264.7 cells and T-cells by semipermeable membrane ablated this action of IL-4, suggesting the induction of a membrane-associated osteoclastogenesis inhibitor. However, membrane-bound inhibitors thymic shared antigen-1 (TSA-1) and osteoclast inhibitory lectin (OCIL) were not regulated by IL-4. In summary, at least two mechanisms of IL-4 -mediated osteoclastogenesis inhibition exist, including a direct action on myelomonocytic progenitors (from which osteoclasts derive) and an indirect action through T-cells that may involve novel anti-osteoclastic factors.

Modulation of growth factor/cytokine synthesis and signaling by 1alpha,25-dihydroxyvitamin D(3): implications in cell growth and differentiation

Distinct from its classic functions in the regulation of calcium and phosphorus metabolism as a systemic hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is involved in the local control and regulation of cellular growth and differentiation in various tissues, including epidermis (keratinocytes) and bone (osteoblasts and osteoclasts). In this review, the impact of 1alpha,25(OH)(2)D(3) on growth factor/cytokine synthesis and signaling is discussed, particularly as it pertains to bone cells and keratinocytes. 1alpha,25(OH)(2)D(3) not only regulates growth factor/cytokine synthesis but may also alter growth factor signaling. Recently discovered examples for such interactions are the interactions between the vitamin D receptor and the mothers against decapentaplegic-related proteins that function downstream of TGFbeta receptors. Inhibitory effects of 1alpha,25(OH)(2)D(3) on keratinocytes through TGFbeta activation and IL-1alpha, IL-6, and IL-8 suppression may provide a rationale for its beneficial effects in the treatment of hyperproliferative skin disorders, whereas stimulatory effects through the epidermal growth factor-related family members and platelet-derived growth factor may be operative in its beneficial effects in skin atrophy and wound healing. Modulation of cytokines and growth factors by 1alpha,25(OH)(2)D(3) during bone remodeling plays an important role in the coupling of osteoblastic bone formation with osteoclastic resorption to maintain bone mass.

Characterization of the bone-resorptive effect of interleukin-11 in cultured mouse calvarial bones

Interleukin-11 (IL-11) is a stromal cell-derived cytokine that can enhance osteoclast formation and stimulate bone resorption. In the present study, the characteristics of the resorptive effect of IL-11 in mouse calvarial bones were investigated. Both recombinant mouse IL-11 and human IL-11 caused concentration- and time-dependent stimulations of (45)Ca release from prelabeled mouse calvariae. Half-maximal responses were obtained at 0.7 ng/mL (approximately 40 pmol/L). Mouse and human IL-11 also stimulated release of (3)H from [(3)H]-proline-labeled bones. The magnitude of the (45)Ca and (3)H release (1.4-1.6-fold) caused by a maximally effective concentration of IL-11 was less than the stimulation (2.5-4.0-fold) elicited by a maximum concentration of parathyroid hormone (PTH). Release of (45)Ca by IL-11 was unaffected by the mitotic inhibitors, hydroxyurea and aphidicolin. In addition to resorption of bone, IL-11 caused a small (1.5-2.0-fold) enhancement of prostaglandin E(2) (PGE(2)) biosynthesis in calvariae, but had no effect on the mRNA expression of cyclooxygenase-1 and -2, or cytosolic phospholipase A(2). Indomethacin and flurbiprofen abolished the formation of PGE(2) and partially reduced (45)Ca release stimulated by IL-11. When either mouse interleukin-4 (IL-4) or interleukin-13 (IL-13) was added to calvariae treated with IL-11, (45)Ca release was inhibited. Resorption caused by IL-11 was also inhibited by both anti-mouse glycoprotein 130 (gp130) and an antibody neutralizing IL-11, but these agents had no effect on (45)Ca release caused by PTH or 1,25(OH)(2)vitamin D(3) (D(3)). Real-time, quantitative polymerase chain reaction (PCR) analysis (TaqMan PCR) and semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) demonstrated that IL-11 caused concentration-dependent enhancements of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) mRNA, without affecting the mRNA expression of RANK. Mouse RANKL stimulated (45)Ca release in the calvarial bones. The stimulatory effects of RANKL and IL-11 were inhibited by mouse OPG. These data demonstrate that IL-11 stimulates osteoclastic resorption in mouse calvariae by mechanisms that are independent of cell proliferation; partially dependent on prostaglandin biosynthesis; sensitive to inhibition by IL-4, IL-13, and OPG; and associated with enhanced expression of RANKL and OPG. In addition, IL-11 was not found to play an essential role in resorption stimulated by other calciotropic agents in calvariae.

Interleukin-4 reversibly inhibits osteoclastogenesis via inhibition of NF-kappa B and mitogen-activated protein kinase signaling

To define the molecular mechanism(s) by which interleukin (IL)-4 reversibly inhibits formation of osteoclasts (OCs) from bone marrow macrophages (BMMs), we examined the capacity of this T cell-derived cytokine to impact signals known to modulate osteoclastogenesis, which include those initiated by macrophage colony-stimulating factor (M-CSF), receptor for activation of NF-kappa B ligand (RANKL), tumor necrosis factor (TNF), and IL-1. We find that although pretreatment of BMMs with IL-4 does not alter M-CSF signaling, it reversibly blocks RANKL-dependent activation of the NF-kappa B, JNK, p38, and ERK signals. IL-4 also selectively inhibits TNF signaling, while enhancing that of IL-1. Contrary to previous reports, we find that MEK inhibitors dose-dependently inhibit OC differentiation. To identify more proximal signals mediating inhibition of OC formation by IL-4, we used mice lacking STAT6 or SHIP1, two adapter proteins that bind the IL-4 receptor. IL-4 fails to inhibit RANKL/M-CSF-induced osteoclastogenesis by BMMs derived from STAT6-, but not SHIP1-, knockout mice. Consistent with this observation, the inhibitory effects of IL-4 on RANKL-induced NF-kappa B and mitogen-activated protein kinase activation are STAT6-dependent. We conclude that IL-4 reversibly arrests osteoclastogenesis in a STAT6-dependent manner by 1) preventing I kappa B phosphorylation and thus NF-kappa B activation, and 2) blockade of the JNK, p38, and ERK mitogen-activated protein kinase pathways.

Potential biologic agents for treating rheumatoid arthritis

The encouraging clinical results observed in trials using anti-TNF therapy clearly warrant further studies to determine whether TNF inhibitors are capable of modifying the destructive component of this disease in long-term follow-up studies as well as to assess the safety of long-term use (see the article by Keystone in this issue). It is also reasonable to propose that interfering with the cytokine cascade earlier in the course of disease may be of even greater therapeutic benefit. As the pathogenetic mechanisms in RA are more clearly defined, especially in early disease and in those individuals destined to develop severe disease, the potential of other biologic agents to specifically inhibit these critical pathways may provide better treatments for our patients. Many potential targets in the immune-mediated process of RA are currently being rigorously evaluated in clinical trials. Use of combinations of biologic therapies, perhaps in human patients with RA, should be of considerable interest in future trials.

IL-10, but not IL-4, suppresses infection-stimulated bone resorption in vivo

Periapical bone resorption occurs following infection of the dental pulp and is mediated mainly by IL-1alpha in the murine model. The production and activity of IL-1alpha is modulated by a network of regulatory cytokines, including those produced by Th1 (pro-inflammatory) and Th2 (anti-inflammatory) subset T cells. This study was designed to assess the functional role of the Th2-type cytokines IL-4 and IL-10 in infection-stimulated bone resorption in vivo. The dental pulps of the first molars were exposed and infected with a mixture of four common endodontic pathogens, and bone destruction was determined by micro-computed tomography at sacrifice on day 21. The results demonstrate that IL-10(-/-) mice had significantly greater infection-stimulated bone resorption in vivo compared with wild-type mice (p < 0.001), whereas IL-4(-/-) exhibited no increased resorption. IL-10(-/-) had markedly elevated IL-1alpha production within periapical inflammatory tissues (>10-fold) compared with wild type (p < 0.01), whereas IL-4(-/-) exhibited decreased IL-1alpha production (p < 0.05). IL-10 also suppressed IL-1alpha production by macrophages in a dose-dependent fashion in vitro, whereas IL-4 had weak and variable effects. We conclude that IL-10, but not IL-4, is an important endogenous suppressor of infection-stimulated bone resorption in vivo, likely acting via inhibition of IL-1alpha expression.

Cytokine receptor profile of arthroplasty macrophages, foreign body giant cells and mature osteoclasts

In the arthroplasty pseudomembrane surrounding a loose prosthesis there is a marked macrophage and foreign body giant cell (FBGC) response to implant-derived wear particles. These cells contribute to the osteolysis of loosening by releasing cytokines and growth factors which influence the formation and activity of osteoclasts. Using a panel of monoclonal antibodies directed against known cytokine/growth factor receptors, we have determined by immunohistochemistry whether arthroplasty macrophages, FB-GCs and osteoclasts express receptors for cytokines and growth factors that are known to modulate osteolysis. All these cell types reacted with antibodies directed against the following cytokine/growth factor receptors: gp130, IL-1R type 1, IL-2R, IL-4R, IL-6R, TNFR, M-CSFR, GM-CSFR and SCFR but not with antibodies directed against IL-3R and IL-8R. Arthroplasty macrophages, FBGCs and osteoclasts thus show a similar pattern of cytokine/growth factor receptor expression. This reflects the fact that arthroplasty macrophages are capable of osteoclast differentiation and that these cell types form part of the mononuclear phagocyte system. As regards the osteolysis of aseptic loosening, it also indicates that these cells are targets for numerous cytokines and growth factors which influence osteoclast formation and bone resorption.

Interleukin-4 and interferon-gamma inhibit prostaglandin production by interleukin-1beta-stimulated human periodontal ligament fibroblasts

The purpose of the present study was to investigate the involvement of cyclooxygease-1 (COX-1) and cyclooxygenase-2 (COX-2) in prostaglandin (PG) production by human periodontal ligament (PDL) fibroblasts stimulated with a proinflammatory cytokine, inerleukin-1beta (IL-1beta), and to examine the effect of interleukin-4 (IL-4), a Th2 cytokine, and interferon-gamma (IFN-gamma), a Th1 cytokine, on PG production by the cells. IL-1beta-stimulated PDL fibroblasts produced prostaglandin E2 (PGE2) in a time-dependent manner. Indomethacin, a non-selective COX-1/COX-2 inhibitor, and NS-398, a selective COX-2 inhibitor, completely inhibited PGE2 production by IL-1beta-stimulated cells. Northern blot analysis showed that COX-2 mRNA was detected in IL-1beta-stimulated PDL cells, although not detected in unstimulated cells, while expression of COX-1 mRNA was in the same extent in both the cells. Dexamethasone inhibited COX-2 mRNA expression, COX activity and PGE2 production in IL-1beta-stimulated cells. IL-4 and IFN-gamma suppressed PGE2 production by IL-1beta-stimulated PDL fibroblasts, but COX activity enhanced by IL-1beta treatment was significantly inhibited by IL-4, not by IFN-gamma. Northern blot analysis showed that IL-4 depressed COX-2 mRNA expression with no effect on COX-1 mRNA expression. On the other hand, IFN-gamma had no effect on expression of COX-1 and -2 mRNA. These data suggest that COX-2 is primarily responsible for PGE2 production by IL-1beta-stimulated human PDL fibroblasts and that IL-4 inhibited PGE2 production by IL-1beta-stimulated PDL fibroblasts through down-regulation of COX-2 expression, while IFN-gamma suppressed the PGE2 production with no effect on COX-2 expression.

Osteochondrodysplasia occurring in transgenic mice expressing interferon-gamma

In addition to various biological activities, interferon-gamma (IFN-gamma) inhibits bone resorption and collagen synthesis. We produced a transgenic mouse line expressing the murine IFN-gamma gene and protein in the bone marrow and thymus. Forty-five transgenic FVB/NCr mice, 23 days-9 months of age, were studied for anomalies in the skeletal system. The transgenic mice had short, wide, and deformed long bones. Young transgenic mice had epiphyseal plates severely thickened with zones of hypertrophy and degeneration with irregular metaphyseal borders. Cartilagenous masses were also observed in the metadiaphyseal marrow cavities. These lesions were primarily seen in long bones and ribs. Adult transgenic mice had residues of degenerated cartilagenous masses in the diaphyses. Many osteoclasts with well-developed ruffled borders were present on the metaphyseal cartilagenous masses in young transgenic mice. Adult transgenic mice had less prominent primary spongiosa with fewer osteoclasts at the metaphysis as compared with nontransgenic controls. The cortical bones of the transgenic mice were thinner and more immature compared with controls. Transgenic mice also had fractures, disruption of the epiphyseal plate, and degeneration of articular cartilage. Thus, the IFN-gamma transgenic mice developed a complex chondro-osseous lesion that was diagnosed as osteochondrodysplasia. The lesions may originate from primarily decreased matrix synthesis in bone and cartilage and also possible osteoclast-related changes caused by IFN-gamma overexpression in the bone marrow. Our IFN-gamma transgenic mouse will be a useful model to investigate the role of IFN-gamma in bone metabolism.

Interleukin-18 (interferon-gamma-inducing factor) is produced by osteoblasts and acts via granulocyte/macrophage colony-stimulating factor and not via interferon-gamma to inhibit osteoclast formation

We have established by differential display polymerase chain reaction of mRNA that interleukin (IL)-18 is expressed by osteoblastic stromal cells. The stromal cell populations used for comparison differed in their ability to promote osteoclast-like multinucleated cell (OCL) formation. mRNA for IL-18 was found to be expressed in greater abundance in lines that were unable to support OCL formation than in supportive cells. Recombinant IL-18 was found to inhibit OCL formation in cocultures of osteoblasts and hemopoietic cells of spleen or bone marrow origin. IL-18 inhibited OCL formation in the presence of osteoclastogenic agents including 1alpha,25-dihydroxyvitamin D3, prostaglandin E2, parathyroid hormone, IL-1, and IL-11. The inhibitory effect of IL-18 was limited to the early phase of the cocultures, which coincides with proliferation of hemopoietic precursors. IL-18 has been reported to induce interferon-gamma (IFN-gamma) and granulocyte/macrophage colony-stimulating factor (GM-CSF) production in T cells, and both agents also inhibit OCL formation in vitro. Neutralizing antibodies to GM-CSF were able to rescue IL-18 inhibition of OCL formation, whereas neutralizing antibodies to IFN-gamma did not. In cocultures with osteoblasts and spleen cells from IFN-gamma receptor type II-deficient mice, IL-18 was found to inhibit OCL formation, indicating that IL-18 acted independently of IFN-gamma production: IFN-gamma had no effect in these cocultures. Additionally, in cocultures in which spleen cells were derived from receptor-deficient mice and osteoblasts were from wild-type mice and vice versa, we identified that the target cells for IFN-gamma inhibition of OCL formation were the hemopoietic cells. The work provides evidence that IL-18 is expressed by osteoblasts and inhibits OCL formation via GM-CSF production and not via IFN-gamma production.

Increased osteoclastic differentiation by PMMA particle-associated macrophages. Inhibitory effect by interleukin 4 and leukemia inhibitory factor

To determine the influence of polymethylmethacrylate (PMMA) wear particles on macrophage-osteoclast differentiation, PMMA particles were added to mouse monocytes which were cocultured with UMR 106 osteoblast-like cells in the presence of 1,25 dihydroxy vitamin D3[1,25(OH)2D3] for up to 7 days on glass coverslips and for up to 14 days on human cortical bone slices. An increase in osteoclast differentiation, as evidenced by the expression of the osteoclast-associated enzyme tartrate-resistant acid phosphatase (TRAP) and the extent of lacunar bone resorption, was observed in monocyte cultures to which PMMA had been added. Interleukin 4 (IL-4) and Leukemia Inhibitory Factor (LIF) added to these cocultures caused considerably less expression of TRAP and significant inhibition of lacunar bone resorption. This inhibitory effect was reversed by the addition of specific neutralizing antibodies to LIF and IL-4. These findings show that PMMA-wear particle-associated macrophages exhibit an enhanced capacity for differentiation to osteoclastic bone-resorbing cells.

PCR phenotyping of cytokines, growth factors and their receptors and bone matrix proteins in human osteoblast-like cell lines

The expression of a total of 58 cytokines, growth factors, and their corresponding receptors and bone matrix proteins was assessed using reverse transcription-linked polymerase chain reaction (RT-PCR) analysis to determine the similarity in the expression profile between clonal osteosarcoma-derived human osteoblast-like cell lines and primary human osteoblast-like cell cultures derived from human trabecular bone explants. The spectrum of cytokines, growth factors, and bone-related proteins expressed by three human osteosarcoma-derived cell lines, TE-85, MG-63, SaOS-2, and primary human osteoblast-like cells was found to be highly comparable and for the first time the expression of EGF, ECGF, FGF beta, oncostatin M, TNF beta, and SCF by human osteoblast-like cells was detected. Also the expression of several receptor types including IL-4R, IL-7R, IFN alpha/beta R, and SCFR was detected that has not been previously described for human osteoblast-like cells. For the factors examined, no qualitative variations in the expression profile were observed in the six primary human osteoblast-like cell cultures used in this study. Of the 58 factors examined, only 13 showed some degree of nonuniformity of expression between all of the three cell lines and primary cell cultures. These differences were seen especially in the expression of cytokine receptor mRNA and to a lesser extent with some cytokines. Differences in receptor expression would suggest that the possible spectrum of response to exogenously added factors, or even autocrine/ paracrine networks would be determined by the repertoire of receptors expressed by each cell type. Whether the differences are related to the status of cell maturation within the osteoblast development lineage or to their abberant regulation of expression cannot be concluded at this stage. However, this PCR-phenotyping approach rapidly provides a resource of information, which can be subsequently used for further in depth studies to facilitate the analysis of the molecular mechanisms, whereby the target gene of interest is modulated in a model cell line. In addition, this study indicates that at least based on the transcript expression profile of the factors analyzed, human osteosarcoma-derived osteoblast-like cells are useful as models for their nontransformed counterparts.

Osteoclast inhibition by factors from cells associated with regenerative tissue

Guided tissue regeneration (GTR) uses expanded polytetrafluoroethylene (ePTFE) membranes to favor the repopulation of the healing wound with cells with bone regenerative potential. As bone remodeling is a tightly coupled process, inhibition of osteoclast-mediated bone resorption may be critical to regeneration. Thus, this study was undertaken to determine whether cells associated with regenerative tissue can inhibit osteoclast differentiation and activity and to begin characterizing and identifying the factor(s) mediating the observed effects. Conditioned media were harvested from human periodontal cell lines established in culture: cells adherent to ePTFE membranes, recovered from patients after GTR; cells adherent to ePTFE augmentation membranes, recovered from edentulous ridge augmentation procedures; and periodontal ligament cells from periodontally healthy bicuspids. Conditioned medium from each of these regenerative cell lines reduced the number of tartrate-resistant acid phosphatase-positive osteoclast-like cells formed from hemopoietic precursors in mouse bone marrow cultures. Also, both the total resorbed surface area and number of resorption pits formed by these cells on calcium phosphate ceramic films were reduced. The factor in the conditioned medium which inhibited osteoclast differentiation was soluble, heat labile, and resided in the lower molecular weight (< 30 kDa) fraction, the same fraction which would contain cytokines. Western blot analysis of the conditioned medium detected a band at the molecular weight of interferon gamma (IFN-gamma), using a polyclonal rabbit anti-human IFN-gamma. Thus, the factor in the conditioned medium with inhibitory activity may have identity with IFN-gamma or one of the other anti-inflammatory cytokines.

Interleukin-4 inhibits prostaglandin G/H synthase-2 and cytosolic phospholipase A2 induction in neonatal mouse parietal bone cultures

We have shown previously that prostaglandin (PG) production in 7-day-old neonatal mouse calvarial cultures is regulated largely by changes in prostaglandin G/H synthase-2 (PGHS-2) expression and to a lesser extent by changes in arachidonic acid (AA) release. In this study, we examined the effects of interleukin-4 (IL-4), and its interactions with other cytokines and with parathyroid hormone (PTH), on mRNA levels of PGHS-2, PGHS-1, and cytosolic phospholipase A2 (cPLA2) and on medium protaglandin E2 (PGE2) levels in calvarial cultures. IL-1 and tumor necrosis factor-alpha (TNF-alpha), both at 1-100 ng/ml, and PTH at 0.1-10 nM increased PGHS-2 and cPLA2 mRNA and medium PGE2 levels dose-dependently after 4 h of treatment. IL-6 and IL-11 at 1-100 ng/ml did not affect mRNA or PGE2 levels. IL-4 at 1-100 ng/ml decreased PGHS-2 and cPLA2 mRNA and PGE2 levels in control as well as IL-1, TNF-alpha, and PTH-stimulated cultures. The inhibition of PGHS-2 and cPLA2 mRNA expression by IL-4 (10 ng/ml) was present at 1 h, reached a maximum at 4 h, and persisted for 24 h. The effects were maintained in the presence of cycloheximide. IL-4 also decreased PGHS-2 protein levels in control and IL-1-stimulated cultures. PGHS-1 mRNA levels were not stimulated by any of the factors studied nor inhibited by IL-4. IL-4 partially inhibited control and PTH-stimulated 45Ca release from prelabeled mouse calvariae at 4 days. However, neither the inhibition of resorption by IL-4 nor the stimulation by IL-1 and PTH were altered by indomethacin (1 microM). We conclude that (1) IL-1, TNF-alpha, and PTH, but not IL-6 nor IL-11, can increase the expression of PGHS-2, cPLA2, and PGE2 production in cultured mouse calvariae; (2) IL-4 inhibits PGE2 production in both control and stimulated calvarial cultures by inhibiting PGHS-2 and cPLA2; and (3) IL-4 has an inhibitory effect on bone resorption which is independent of PG production.

Interleukin-4 as a bone regulatory factor: effects on murine osteoblast-like cells

Bone remodelling is regulated at the local level by an incompletely elucidated cytokine network. In the present study we have determined the effect of interleukin-4 (IL-4), a cytokine produced by T lymphocytes and other cells, on the activity of murine osteoblasts. IL-4 (0.1-10 ng/ml) did not influence the proliferation rate of the osteoblast-like cell line MC3T3, but inhibited the expression of alkaline phosphatase. In long-term cultures supplemented with ascorbic acid and glycerophosphate such an effect was accompanied by a retardation of matrix mineralization. IL-4 also stimulated M-CSF expression by MC3T3 cells, both at the RNA and bioactivity levels. However, no stimulation of IL-1, IL-6, GM-CSF or PGE2 production was observed. An IL-4-induced inhibition of alkaline phosphatase expression and retardation of mineralization was also found in cultures of primary osteoblast-like cells isolated from neonatal mice calvariae. These results suggest that IL-4, probably released by cells within the bone marrow, may locally influence the activity of bone-forming cells.

Interleukin-4 induces expression of the integrin alpha v beta 3 via transactivation of the beta 3 gene

Osteoclastic bone resorption is dependent upon cell-matrix recognition. This process is mediated by the integrin alpha v beta 3 whose expression is enhanced, in avian osteoclast precursors, by bone-seeking steroids. The purpose of this study was to determine if bone-modulating cytokines impact on alpha v beta 3 expression by mouse marrow macrophages (BMMs), known to differentiate into osteoclasts. Of the cytokines tested. Interleukin-4 (IL-4) is most effective in increasing beta 3 mRNA levels by a mechanism involving transactivation of the beta 3 gene. Moreover, IL-4 augmented beta 3 mRNA is mirrored by plasma membrane appearance of alpha v beta 3. As IL-4 induces beta 3 and not alpha v mRNA, the beta 3 chain appears to regulate surface expression of the heterodimer. The functional significance of IL-4-induced alpha v beta 3 is underscored by the fact that, while attachment to fibronectin is unaltered, treatment of BMMs with the cytokine enhances alpha v beta 3-mediated binding to vitronectin 5-fold. Expression of this heterodimer by BMMs driven along a non-osteoclastic lineage suggests alpha v beta 3 may play a role in the inflammatory response of macrophages.