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

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.

Osteoporosis: interferon-gamma-mediated bone remodeling in osteoimmunology

As the world population ages, osteoporosis, the most common disease of bone metabolism, affects more than 200 million people worldwide. The etiology is an imbalance in bone remodeling process resulting in more significant bone resorption than bone remodeling. With the advent of the osteoimmunology field, the immune system's role in skeletal pathologies is gradually being discovered. The cytokine interferon-gamma (IFN-γ), a member of the interferon family, is an important factor in the etiology and treatment of osteoporosis because it mediates bone remodeling. This review starts with bone remodeling process and includes the cellular and key signaling pathways of bone remodeling. The effects of IFN-γ on osteoblasts, osteoclasts, and bone mass are discussed separately, while the overall effects of IFN-γ on primary and secondary osteoporosis are summarized. The net effect of IFN-γ on bone appears to be highly dependent on the environment, dose, concentration, and stage of cellular differentiation. This review focuses on the mechanisms of bone remodeling and bone immunology, with a comprehensive discussion of the relationship between IFN-γ and osteoporosis. Finding the paradoxical balance of IFN-γ in bone immunology and exploring the potential of its clinical application provide new ideas for the clinical treatment of osteoporosis and drug development.

Microenvironment-targeted strategy steers advanced bone regeneration

Treatment of large bone defects represents a great challenge in orthopedic and craniomaxillofacial surgery. Traditional strategies in bone tissue engineering have focused primarily on mimicking the extracellular matrix (ECM) of bone in terms of structure and composition. However, the synergistic effects of other cues from the microenvironment during bone regeneration are often neglected. The bone microenvironment is a sophisticated system that includes physiological (e.g., neighboring cells such as macrophages), chemical (e.g., oxygen, pH), and physical factors (e.g., mechanics, acoustics) that dynamically interact with each other. Microenvironment-targeted strategies are increasingly recognized as crucial for successful bone regeneration and offer promising solutions for advancing bone tissue engineering. This review provides a comprehensive overview of current microenvironment-targeted strategies and challenges for bone regeneration and further outlines prospective directions of the approaches in construction of bone organoids.

Effects of calcium hydroxide intracanal medications on T helper (Th1, Th2, Th9, Th17, and Tfh) and regulatory T (Treg) cell cytokines in apical periodontitis: a CONSORT RCT

INTRODUCTION

This CONSORT RCT investigated T helper (Th1, Th2, Th9, Th17, and Tfh), regulatory T (Treg) cell-type cytokines, and their networks in apical periodontitis (AP). We also assessed the effects of calcium hydroxide [Ca(OH)2] intracanal medications (ICMs) on helper T and Treg cell-type cytokines.

METHODS

Twenty teeth with primary endodontic infection and apical periodontitis (PEIAP) were randomly divided into two groups: Ca(OH)2 + saline solution (SSL; n = 10) and Ca (OH)2 + 2% Chlorhexidine-gel (2% CHX-gel; n = 10). Samples were collected from the periradicular tissue fluid (PTF) before (PTFs1) and after 14 days of ICMs (PTFs2). The Human High Sensitivity T Cell Panel was used to quantify target T-helper (Th)1: IL-2, 12, and IFN-γ; Th2: IL-4, -5, -13; Th9: IL-9; Th17: IL-17; T follicular helper cells (Tfh): IL-21; and Treg-cell-type cytokine: IL-10.

RESULTS

Th1-type cytokines were higher, compared to Th2-type, at PTFs1. Positive (+) associations were found among all Th1-type cytokines and all Th2-type cytokines. There were negative (-) correlations between all Th1- and Th2-type cytokines. Size of radiolucent lesions (SRL) and symptoms (TTP and/or POP) were positively correlated with Th1-type cytokines, IL-17, and IL-21 but negatively correlated with Th2-type cytokines and IL-10 (all, p < 0.001). Both ICMs increased Th2-type cytokines and IL-10 (P < 0.05) and decreased Th1- type cytokines, IL-17, and IL-21 (P < 0.05), with no differences among them (P > 0.05).

CONCLUSIONS

Complex T-cell cytokine networks are involved in AP. Both Ca(OH)2 ICMs effectively increased IL-4, IL-5, IL-10, and IL-13 and lowered IL-2, IL-12, IL-17, IL-21, and IFN-γ.

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.

The Potential Roles of T Cells in Periapical Lesions

INTRODUCTION

Periapical lesions are inflammatory diseases mainly caused by microbial infection from the root canal system, affecting the integrity of alveolar bone, periapical cementum, and periodontal ligament. The invasion of pathogenic microorganisms activates local inflammation and host immune response, especially the recruitment and differentiation of T cells. Many studies have discussed the fundamental roles of T cell-related immunological regulation and the possible clinical significance of cytokine disorders in periapical lesions. However, oral pathogen-mediated T cell immune response is far more clarified. Therefore, the aim of this study was to discuss the research status of T cell-related immunology involved in the progression of periapical lesions and potential future directions.

METHODS

We conducted a literature review focusing on T cell-related immunology in periapical lesions by searching PubMed, Web of Science, Scopus and ScienceDirect online databases.

RESULTS

In total 108 articles were involved in this narrative review. During the development of periapical lesions, the infiltrated number of different types of T cells and the secretion of T cell-related cytokines in root apex region reflected the inflammatory status of periapical lesions. In addition, it was also highly correlated with the periapical bone destruction. Future study could attempt to provide a wider and deeper study on the expression profile and regulatory function of T cells in the development of periapical lesions.

CONCLUSION

This review would help us understand the essence of the T cell-related pathology of periapical lesions and raise the potential therapeutic targets for the treatment of apical periodontitis.

Restoring Tissue Homeostasis at Metastatic Sites: A Focus on Extracellular Vesicles in Bone Metastasis

Bone is the most common site of cancer metastasis and the spread of cancer cells to the bone is associated with poor prognosis, pain, increased risk of fractures, and hypercalcemia. The bone marrow microenvironment is an attractive place for tumor dissemination, due to the dynamic network of non-malignant cells. In particular, the alteration of the bone homeostasis favors the tumor homing and the consequent osteolytic or osteoblastic lesions. Extracellular vesicles (EVs) are reported to be involved in the metastatic process, promoting tumor invasion, escape from immune surveillance, extravasation, extracellular matrix remodeling, and metastasis, but the role of EVs in bone metastases is still unclear. Current results suggest the ability of tumor derived EVs in promoting bone localization and metastasis formation, altering the physiological balance between bone destruction and new bone depositions. Moreover, EVs from the bone marrow niche may support the onset of tumor metastasis. This review summarizes recent findings on the role of EVs in the pathological alterations of homeostasis that occur during bone metastasis to show novel potential EV-based therapeutic options to inhibit metastasis formation.

The Role of the Immune Microenvironment in Bone Regeneration

Bone is an active tissue, being constantly renewed in healthy individuals with participation of the immune system to a large extent. Any imbalance between the processes of bone formation and bone resorption is linked to various inflammatory bone diseases. The immune system plays an important role in tissue formation and bone resorption. Recently, many studies have demonstrated complex interactions between the immune and skeletal systems. Both of immune cells and cytokines contribute to the regulation of bone homeostasis, and bone cells, including osteoblasts, osteoclasts, osteocytes, also influence the cellular functions of immune cells. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Therefore, the immune microenvironment is crucial in determining the speed and outcome of bone healing, repair, and regeneration. In this review, we summarise the role of the immune microenvironment in bone regeneration from the aspects of immune cells and immune cytokines. The elucidation of immune mechanisms involved in the process of bone regeneration would provide new therapeutic targets for improving the curative effects of bone injury treatment.

A review of T helper 17 cell-related cytokines in serum and saliva in periodontitis

Periodontitis is a chronic inflammatory disease with a complex underlying immunopathology. Cytokines, as molecular mediators of inflammation, play a role in all stages of disease progression. T helper 17 (Th17) cells are thought to play a role in periodontitis. Th17 cell development and maintenance requires a pro-inflammatory cytokine milieu, with many of the cytokines implicated in the pathogenesis of periodontitis. Serum and saliva are easily accessible biofluids which can represent the systemic and local environment to promote the development of Th17 cells. Here we review human clinical studies that investigate IL-1β, IL-4, IL-6, IL-10, IL-17A, IL-17F, IL-21, IL-22, IL-23, IL-25, IL-31, IL-33, IFN-γ, sCD40L and TNF-α in serum and saliva in periodontitis. We highlight their putative role in the pathogenesis of periodontitis and place them within a wider context of animal and other clinical studies.

Perspective on skeletal health in inflammatory bowel disease

Osteopenia and osteoporosis are common features in inflammatory bowel disease (IBD), comprising both Crohn's disease and ulcerative colitis. Moreover, Crohn's disease is associated with increased fracture risk. The etiology of bone loss in IBD is multifactorial. It includes insufficient intake or absorption of calcium, vitamin D, and potassium; smoking; a low peak bone mass; a low body mass index; and decreased physical activity. In several studies, it has been shown that elevated concentrations of systemic and local pro-inflammatory cytokines, including tumor necrosis factor alpha (TNF-α), interferon-γ (IFNγ), interleukin (IL)-1β, IL-4, IL-5, IL-6, IL-13, and IL-17, present in IBD patients are potentially detrimental for bone metabolism and may be responsible for bone loss and increased fracture risk. This perspective aims to review the current literature on the role of inflammatory factors in the pathophysiology of skeletal problems in IBD and to suggest potential treatment to improve bone health, based on a combination of evidence and clinical and pathophysiological reasoning.

[Inflammation and bone : Osteoimmunological aspects]

Microscopic fractures (so-called microcracks) or traumatic macrofractures require bone, as the basic scaffold of the human body, to have a high regenerative capability. In order to be able to provide this regenerative capability, bone is in a constant process of remodeling. This finely tuned homeostasis of bone formation and degradation can become disrupted, which leads to osteoporosis or other bone disorders. It has been shown that the immune system is substantially involved in the regulation of bone homeostasis and that chronic inflammation in particular can disturb this balance; therefore, this article reviews the osteoimmunological aspects contributing to osteoporosis and other diseases associated with bone degradation.

Aspirin Blocks Orthodontic Relapse via Inhibition of CD4+ T Lymphocytes

Immunologic response plays an important role in orthodontic tooth movement (OTM) and relapse. Nonsteroidal anti-inflammatory drugs, such as aspirin, affect immune cells and clinical orthodontic treatment. However, the mechanisms by which nonsteroidal anti-inflammatory drugs regulate immune cells to affect orthodontic relapse are unclear. In this study, male Sprague-Dawley rats were grouped as relapse and relapse + aspirin for 10 d after 14 d of OTM. Silicone impressions of the rats’ maxillary dentitions were obtained to record the distance of OTM at the indicated time point. CD4+ T lymphocytes in spleen were examined by flow cytometry. Serum levels of type 1 T-helper (Th1) cell–associated cytokines tumor necrosis factor α (TNF-α), and interferon γ (IFN-γ) were determined through enzyme-linked immunosorbent assay. The effects of aspirin on CD4+ T and Th1 cells were also analyzed in vitro. Aspirin treatment significantly reduced the relapse rate. More interestingly, injection of CD25 neutralizing antibody basiliximab or TNF-α inhibitor etanercept can significantly reduce the relapse rate as well. Correspondingly, aspirin treatment significantly accelerated the decrease of orthodontic force–induced secretion of TNF-α and IFN-γ in serum and the expression of TNF-α and IFN-γ in periodontal ligament during relapse. Furthermore, aspirin treatment in vitro significantly repressed the differentiation of CD4+ T and Th1 cells. Overall, results indicated that aspirin treatment can block orthodontic relapse by regulating Th1 cells.

T Cells Are Required for Orthodontic Tooth Movement

The immune system plays a pivotal role during bone remodeling process. Orthodontic tooth movement (OTM) induces local inflammation in periodontium, but whether systemic immune response is involved in OTM remains unknown. In this study, we show that tooth movement distance was significantly reduced in T-cell-deficient immunocompromised mice compared with wild-type (WT) mice. Intravenous infusion of allogeneic T cells to the immunocompromised mice rescued the OTM distance. Correspondingly, increased numbers of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts were detected around the alveolar bone after OTM in WT mice but were barely detected in immunocompromised mice. Moreover, intravenous infusion of T cells rescued the number of TRAP-positive osteoclasts in the OTM area of the immunocompromised mice, thus suggesting T cells are required for OTM. We then reveal that OTM induced a significant elevation of type 1 T helper cell (Th1) cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) around periodontal tissue in WT but not in immunocompromised mice. Infusion of T cells could increase the levels of TNF-α and IFN-γ in periodontal tissues of immunocompromised mice. More interestingly, intraperitoneal injection of TNF-α inhibitor etanercept significantly reduced the distance of OTM in T-cell-infused immunocompromised mice. In summary, this study demonstrates a previously unrecognized mechanism that T cells are required for OTM depending on Th1-associated cytokines.

Osteoclast formation elicited by interleukin-33 stimulation is dependent upon the type of osteoclast progenitor

Osteoclasts are bone resorbing multinucleated cells (MNCs) derived from macrophage progenitors. IL-33 has been reported to drive osteoclastogenesis independently of receptor activator of NFκB ligand (RANKL) but this remains controversial as later studies did not confirm this. We found IL-33 clearly elicited functional dentine-resorbing osteoclast formation from human adult monocytes. However, monocytes from only 3 of 12 donors responded this way, while all responded to RANKL. Human cord blood-derived progenitors and murine bone marrow macrophages lacked an osteoclastogenic response to IL-33. In RAW264.7 cells, IL-33 elicited NFκB and p38 responses but not NFATc1 signals (suggesting poor osteoclastogenic responses) and formed only mononuclear tartrate-resistant acid phosphatase positive (TRAP(+)) cells. Since TGFβ boosts osteoclastogenesis in RAW264.7 cells we employed an IL-33/TGFβ co-treatment, which resulted in small numbers of MNCs expressing key osteoclast markers TRAP and calcitonin receptors. Thus, IL-33 possesses weak osteoclastogenic activity suggesting pathological significance and, perhaps, explaining previous conflicting reports.

Osteoimmunology in orthodontic tooth movement

The skeletal and immune systems share a multitude of regulatory molecules, including cytokines, receptors, signaling molecules, and signaling transducers, thereby mutually influencing each other. In recent years, several novel insights have been attained that have enhanced our current understanding of the detailed mechanisms of osteoimmunology. In orthodontic tooth movement, immune responses mediated by periodontal tissue under mechanical force induce the generation of inflammatory responses with consequent alveolar bone resorption, and many regulators are involved in this process. In this review, we take a closer look at the cellular/molecular mechanisms and signaling involved in osteoimmunology and at relevant research progress in the context of the field of orthodontic tooth movement.

Coupling the activities of bone formation and resorption: a multitude of signals within the basic multicellular unit

Coupling between bone formation and bone resorption refers to the process within basic multicellular units in which resorption by osteoclasts is met by the generation of osteoblasts from precursors, and their bone-forming activity, which needs to be sufficient to replace the bone lost. There are many sources of activities that contribute to coupling at remodeling sites, including growth factors released from the matrix, soluble and membrane products of osteoclasts and their precursors, signals from osteocytes and from immune cells and signaling taking place within the osteoblast lineage. Coupling is therefore a process that involves the interaction of a wide range of cell types and control mechanisms. As bone remodeling occurs at many sites asynchronously throughout the skeleton, locally generated activities comprise very important control mechanisms. In this review, we explore the potential roles of a number of these factors, including sphingosine-1-phosphate, semaphorins, ephrins, interleukin-6 (IL-6) family cytokines and marrow-derived factors. Their interactions achieve the essential tight control of coupling within individual remodeling units that is required for control of skeletal mass.

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.

Modulation of osteoclast function in bone by the immune system

Osteoclast differentiation and function is regulated by cellular signals and cytokines that also play significant roles in the immune system. There is much scope, therefore, for immune cell influence on osteoclasts and bone metabolism. Many examples of this have been identified and T cells in particular are a source of factors affecting osteoclast formation and activity, a number which have either pro-osteolytic or anti-osteolytic actions depending on the cellular and microenvironmental context. For example, IL-12 and IL-18 participate in inflammatory processes that can lead to highly destructive osteolysis, yet these cytokines potently block osteoclast formation through mediation of T cells. IL-23 participates in chronic inflammatory processes, but lack of this cytokine results in reduced bone mass in mice, pointing to an influence on physiological regulation of bone mass. Such insights suggest that therapies that target immune responses may significantly influence osteolysis. Investigations into links between the immune system and bone metabolism are thus uncovering important information about the functioning of both systems.

IL-12 stimulates the osteoclast inhibitory peptide-1 (OIP-1/hSca) gene expression in CD4+ T cells

Immune cell products such as interferon (IFN)-gamma and interleukin (IL)-12 are potent inhibitors of osteoclast formation. We previously characterized the human osteoclast inhibitory peptide-1 (OIP-1/hSca), a Ly-6 gene family member and showed IFN-gamma modulation of OIP-1 expression in bone marrow cells. Whether, IL-12 regulates OIP-1 expression in the bone microenvironment is unclear. Real-time PCR analysis revealed that IL-12 treatment significantly enhanced OIP-1 mRNA expression in human bone marrow mononuclear cells. Because IL-12 induces IFN-gamma production by T cells, we tested whether IFN-gamma participates in IL-12 stimulation of OIP-1 gene expression in these cells. IL-12 treatment in the presence of IFN-gamma neutralizing antibody significantly increased OIP-1 mRNA expression, suggesting that IL-12 directly regulates OIP-1 gene expression. Interestingly, real-time PCR analysis demonstrated that IL-12 induces OIP-1 expression (3.2-fold) in CD4+ T cells; however, there was no significant change in CD8+ T cells. Also, IL-12 (10 ng/ml) treatment of Jurkat cells transfected with OIP-1 gene (-1 to -1,988 bp) promoter-luciferase reporter plasmid demonstrated a 5-fold and 2.7-fold increase in OIP-1 gene promoter activity in the presence and absence of antibody against IFN-gamma, respectively. We showed that STAT-1,3 inhibitors treatment significantly decreased IL-12 stimulated OIP-1 promoter activity. Chromatin immunoprecipitation (ChIP) assay confirmed STAT-3, but not STAT-1 binding to the OIP-1 gene promoter in response to IL-12 stimulation. These results suggest that IL-12 stimulates the OIP-1 gene expression through STAT-3 activation in CD4+ T cells.

Locally administered T cells from mice immunized with lipopolysaccharide (LPS) accelerate LPS-induced bone resorption

T cells play important roles in bone destruction and osteoclastogenesis and are found in chronic destructive bone lesions. Lipopolysaccharide (LPS) is one of several pathological factors involved in inflammatory bone destruction. We previously described the importance of T cells in the inflammatory bone resorption that occurs after repeated LPS administration. However, whether local or systemic T cells are important for inflammatory bone resorption and whether immunization of host animals influences bone resorption remain unclear. The present study examines the effects of local extant T cells from LPS-immunized mice on LPS-induced bone resorption. T cells from LPS-immunized or non-immunized mice were injected together with LPS into the gingival tissues of mice with severe combined immunodeficiency disease that lack both T and B cells. We histomorphometrically evaluated bone resorption at sites of T cell injections and examined the influence of T cells from LPS-immunized mice on osteoclastogenesis in vitro. We found that locally administered T cells from LPS-immunized but not non-immunized mice accelerated LPS-induced bone resorption in vivo. Moreover, T cells from LPS-immunized mice increased osteoclastogenesis in vitro induced by receptor activator of NF-kappa B ligand and LPS and anti-tumor necrosis factor (TNF)-alpha antibody inhibited this increase. These results demonstrated that local extant T cells accelerate inflammatory bone resorption. Furthermore, T cells from LPS-immunized mice appear to elevate LPS-induced bone resorption using TNF-alpha.

IL-23 inhibits osteoclastogenesis indirectly through lymphocytes and is required for the maintenance of bone mass in mice

IL-23 stimulates the differentiation and function of the Th17 subset of CD4(+) T cells and plays a critical role in chronic inflammation. The IL-23 receptor-encoding gene is also an inflammatory disease susceptibility gene. IL-23 shares a common subunit with IL-12, a T cell-dependent osteoclast formation inhibitor, and we found that IL-23 also dose-dependently inhibited osteoclastogenesis in a CD4(+) T lymphocyte-dependent manner. When sufficiently enriched, gammadelta T cells also mediated IL-23 inhibition. Like IL-12, IL-23 acted synergistically with IL-18 to block osteoclastogenesis but, unlike IL-12, IL-23 action depended on T cell GM-CSF production. IL-23 did not mediate IL-12 action although IL-12 induced its expression. Male mice lacking IL-23 (IL-23p19(-/-)) had approximately 30% lower bone mineral density and tibial trabecular bone mass (bone volume (BV)/total volume (TV)) than wild-type littermates at 12 wk and 40% lower BV/TV at 26 wk of age; male heterozygotes also had lower bone mass. Female IL-23p19(-/-) mice also had reduced BV/TV. IL-23p19(-/-) mice had no detectable osteoclast defect in trabecular bone but IL-23p19(-/-) had thinner growth plate hypertrophic and primary spongiosa zones (and, in females, less cartilage remnants) compared with wild type. This suggests increased osteoclast action at and below the growth plate, leading to reduced amounts of mature trabecular bone. Thus, IL-23 inhibits osteoclast formation indirectly via T cells in vitro. Under nonpathological conditions (unlike inflammatory conditions), IL-23 favors higher bone mass in long bones by limiting resorption of immature bone forming below the growth plate.

Cytokine-controlled RANKL and osteoprotegerin expression by human and mouse synovial fibroblasts: fibroblast-mediated pathologic bone resorption

OBJECTIVE

To determine whether proinflammatory cytokine treatment or the complete absence of select cytokines modulates the expression of RANKL and osteoprotegerin (OPG) in synovial fibroblasts.

METHODS

Fibroblasts were isolated from normal and rheumatoid human synovium and from normal or arthritic joints of wild-type and cytokine gene-deficient (interleukin-4-knockout [IL-4 (-/-)] and interferon-gamma-knockout [IFNgamma (-/-)]) mice. Fibroblasts were stimulated with proinflammatory cytokines (tumor necrosis factor alpha [TNFalpha], IL-1beta, and IL-17) or antiosteoclastogenic cytokines (IL-4 and IFNgamma), alone or in combination, and the expression of RANKL and OPG was measured.

RESULTS

Proinflammatory cytokine-stimulated fibroblasts from rheumatoid and arthritic mouse joints expressed higher levels of RANKL and OPG than those from normal joints. IL-4 suppressed RANKL expression and increased OPG expression, IFNgamma reduced the production of both RANKL and OPG, and IL-17 had only a modest effect on the expression of RANKL or OPG. Additive effects of combination treatment (TNFalpha/IL-17 or IL-1beta/IL-17) were observed only in the human system. Extensive destruction was observed in the arthritic joints of IL-4 (-/-) mice, with a corresponding upward shift of the RANKL:OPG ratios. However, an IL-17 deficiency did not attenuate arthritis or reduce bone resorption.

CONCLUSION

Proinflammatory cytokines induce the expression of RANKL and OPG in both human and murine synovial fibroblasts. The RANKL:OPG ratios are shifted in favor of bone protection by IL-4 treatment, and, to a lesser extent, by IFNgamma treatment. Unexpectedly, an IL-17 deficiency alone does not induce reduced inflammatory bone destruction. Our results suggest that synovial fibroblasts may significantly contribute to bone resorption through modulation of RANKL and OPG production in a cytokine-rich milieu of inflamed joints.

Lymphocyte-derived cytokines in inflammatory arthritis

Inflammatory bone loss is observed in a number of disorders including rheumatoid arthritis (RA), osteoporosis and periodontal disease. Lymphocytes are key components in the onset and exacerbation of autoimmune diseases and the cytokines produced by these cells have a powerful impact on disease progression. Many cytokines implicated in inflammation impact upon osteoclast (OCL) differentiation and function either directly or indirectly by modulating the relative expression of RANKL and OPG. This review highlights the contribution of lymphocyte-derived cytokines to the bone loss observed in RA and other autoimmune disorders. A greater understanding of the cytokines involved in these disorders will ultimately lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases.

Normal human primary CD4+ T lymphocytes synthesize and release functional osteoprotegerin in vitro

Osteoprotegerin (OPG) acts as a decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL) and TNF-related apoptosis-inducing ligand (TRAIL). OPG regulates bone remodeling and the immune response. The primary objective was to decipher, among human peripheral blood mononuclear leukocytes (PBML) that produce OPG, the subset(s) responsible for this synthesis and its regulation. To this end, normal human PBML and CD4-, 8-, 19-, 14-enriched subpopulations were studied in vitro for OPG synthesis. PBML were subjected to adherence and immunomagnetic separation, and OPG expression was analyzed by PCR, northern and western blotting, and ELISA. The antiapoptotic effects of OPG were studied on TRAIL-stimulated RPMI 8226 myeloma cells. OPG was time-dependently produced by primary CD4+ T lymphocytes exclusively. OPG secretion was upregulated by anti-CD3 antibody stimulation or incubation with interleukin (IL)-4, IL-1beta, TNF-alpha, GM-CSF, and vitamin D(3). In contrast, IL-10 inhibited the basal and IL-4-induced production of OPG by T cells. Conditioned media from activated T lymphocytes decreased TRAIL-induced apoptosis of RPMI 8226 cells. This effect was reversed by addition of RANKL to the T-cell conditioned media. As human immunodeficiency virus-1 (HIV-1) targets CD4+ T cells, we evaluated the effects of recombinant HIV-1 gp120 proteins on OPG synthesis. The gp120 from three different HIV-1 strains significantly reduced the basal output of OPG from T cells. Furthermore, all four protease inhibitors (PIs) used in highly active antiretroviral therapy decreased OPG synthesis by human blood T cells, nelfinavir being the most efficient PI. The simultaneous presence of an HIV-1 gp120 and a PI abrogated the basal output of OPG. In conclusion, these results highlight a new role for T lymphocytes involved in pathologies. Activated CD4+ T cells could, through OPG release, have a paracrine effect on adjacent cells and contribute to reduce the local process of bone remodeling and cellular apoptosis.

Osteoimmunological insight into bone damage in rheumatoid arthritis

Research into the bone destruction associated with rheumatoid arthritis has highlighted the importance of the interplay of the immune and skeletal systems. Arthritic bone destruction is attributable to the defective control of osteoclastogenesis by T cells. We revealed that excessive expression of receptor activator of NF-kappaB ligand (RANKL) and a paucity of interferon-gamma underlie the enhanced osteoclastogenesis in arthritis. The interdisciplinary research field called osteoimmunology has attracted further attention after identification of a number of unexpected bone phenotypes in mice lacking immunomodulatory molecules. Accumulating evidence suggests that the immune and skeletal systems share not only cytokines but also various signaling molecules, transcription factors, and membrane receptors. Thus, bone turns out to be a dynamic tissue that is constantly renewed, where the immune system participates to a hitherto unexpected extent. This emerging field will be of great importance for a better understanding and treatment of rheumatic diseases.

The multiple roles of osteoclasts in host defense: bone remodeling and hematopoietic stem cell mobilization

Bone remodeling by bone-forming osteoblasts and bone-resorbing osteoclasts dynamically alters the bone inner wall and the endosteum region, which harbors osteoblastic niches for hematopoietic stem cells. Investigators have recently elucidated mechanisms of recruitment and mobilization; these mechanisms consist of stress signals that drive migration of leukocytes and progenitor cells from the bone marrow reservoir to the circulation and drive their homing to injured tissues as part of host defense and repair. The physical bone marrow vasculature barrier that is crossed by mobilized cells actively transmits chemotactic signals between the blood and the bone marrow, facilitating organ communication and cell trafficking. Osteoclasts play a dual role in regulation of bone resorption and homeostatic release or stress-induced mobilization of hematopoietic stem/progenitor cells. In this review, we discuss the orchestrated interplay between bone remodeling, the immune system, and the endosteal stem cell niches in the context of stem cell proliferation and migration during homeostasis, which are accelerated during alarm situations.

Alveolar bone loss in T helper 1/T helper 2 cytokine-deficient mice

BACKGROUND AND OBJECTIVES

The role of cytokines in bone loss is important in the context of periodontitis, where inflammation-induced bone destruction is a major manifestation. Numerous cytokines have been implicated as mediators of bone resorption. The purpose of this study was to observe the impact of targeted gene deletion of T helper 1 (Th1) and T helper 2 (Th2) cytokines on naturally occurring alveolar bone loss in genetically modified mice.

MATERIAL AND METHODS

Alveolar bone loss was measured histomorphometrically in interleukin-4, interleukin-10, interleukin-12p40, interferon-gamma (IFN-gamma) and tumor necrosis factor (TNF) knockout mice at 6, 16 and 30 wk of age.

RESULTS

Both Th1 (interleukin-12p40, IFN-gamma, TNF) and Th2 (interleukin-10, interleukin-4) knockout mice exhibited significantly more alveolar bone loss than their respective wild-type control mice (p<0.001). Interleukin-10-/- and interleukin-12p40-/- mice exhibited a three-fold increase in alveolar bone loss at 30 wk of age, whereas bone loss in IFN-gamma-/-, TNF-/- and interleukin-4-/- mice was 1.5- to two-fold higher compared with wild-type control mice.

CONCLUSION

The results of the present study indicate that both Th1 and Th2 cytokines play an important role in maintaining alveolar bone homeostasis. The kinetics of alveolar bone loss seen in cytokine gene knockout mice indicates that bone loss is age dependent and late in onset.

Osteoimmunology

Osteoimmunology is an interdisciplinary research field combining the exciting fields of osteology and immunology. An observation that contributed enormously to the emergence of osteoimmunology was the accelerated bone loss caused by inflammatory diseases such as rheumatoid arthritis. Receptor activator of nuclear factor kappaB ligand (RANKL), which is the main regulator of osteoclastogenesis, was found to be the primary culprit responsible for the enhanced activation of osteoclasts: activated T cells directly and indirectly increased the expression of RANKL, and thereby promoted osteoclastic activity. Excessive bone loss is not only present in inflammatory diseases but also in autoimmune diseases and cancer. Furthermore, there is accumulating evidence that the very prevalent skeletal disorder osteoporosis is associated with alterations in the immune system. Meanwhile, numerous connections have been discovered in osteoimmunology beyond merely the actions of RANKL. These include the importance of osteoblasts in the maintenance of the hematopoietic stem cell niche and in lymphocyte development as well as the functions of immune cells participating in osteoblast and osteoclast development. Furthermore, research is being done investigating cytokines, chemokines, transcription factors and co-stimulatory molecules which are shared by both systems. Research in osteoimmunology promises the discovery of new strategies and the development of innovative therapeutics to cure or alleviate bone loss in inflammatory and autoimmune diseases as well as in osteoporosis. This review gives an introduction to bone remodeling and the cells governing that process and summarizes the most recent discoveries in the interdisciplinary field of osteoimmunology. Furthermore, an alternative large animal model will be discussed and the pathophysiological alterations of the immune system in osteoporosis will be highlighted.

Megakaryocyte-mediated inhibition of osteoclast development

A growing body of evidence indicates that megakaryocytes (MK) or their growth factors play a role in skeletal homeostasis. We previously identified a novel regulatory pathway that controls bone formation, which is mediated by MK. In vivo megakaryocytosis resulted in massive bone formation. The co-culture of MK with osteoblasts (OB) resulted in increased OB proliferation in vitro, by a mechanism that required direct cell-to-cell contact. Here, we examined a second MK-mediated pathway that regulates osteoclast (OC) development. We have begun examining the unique inhibitory effect of MK on OC development. Spleen or bone marrow (BM) cells from C57BL/6 mice, as a source of OC precursors, were cultured with M-CSF and RANKL to induce OC development. MK were prepared by culturing fetal liver cells with thrombopoietin and separating cells into MK and non-MK populations. MK were titrated into spleen cell cultures and OC were identified as tartrate-resistant acid phosphatase-positive giant cells with >3 nuclei. There was a significant, P < 0.001, up to 10-fold reduction in OC formed when MK were added to the spleen cell cultures. We determined that 30% (vol:vol) MK conditioned media (CM) were able to completely block OC development from precursors, whereas 3% MK CM resulted in up to a 10-fold reduction in OC development, P < 0.001. These data indicate that a soluble factor(s) was responsible, at least in part, for the inhibition. We examined MK CM for known inhibitors of OC formation, using ELISAs. IL-4 was undetectable in MK CM, whereas IL-10 and IFN-gamma levels were similar in MK and non-MK CM. TGFbeta-1 levels were increased 2-fold in MK CM compared to control CM but were not responsible for the inhibition in OC development. Although, we found a significant increase in the levels of osteoprotegerin (OPG) in MK CM, antibody neutralization studies, MK derived from OPG-deficient mice, and tandem mass spectrophotometry, all confirm that OPG was not responsible for the MK-mediated inhibition of OC development. Overall, these data suggest that an unidentified factor(s) is present in MK CM that inhibits OC development. These studies indicate that MK can play a dual role in skeletal homeostasis by stimulating OB proliferation and simultaneously inhibiting OC development.

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.

Extending missing-self? Functional interactions between lectin-like NKrp1 receptors on NK cells with lectin-like ligands

The functions of natural killer (NK) cells are clearly regulated by major histocompatibility complex (MHC) class I molecules on their cellular targets. In mice, this is due to the action of MHC-specific inhibitory receptors belonging to the Ly49 family oflectin-like molecules. The Ly49 receptors are encoded in the NK gene complex (NKC) that contains clusters of genes for other lectin-like receptors on NK cells and other hematopoietic cells. Interestingly, recent studies have shown that some of these lectin-like receptors, belonging to the Nkrpl family, can recognize other lectin-like molecules, termed Clr, also encoded in the NKC. These genetically linked loci for receptor-ligand pairs suggest a genetic strategy to preserve this interaction and show several other contrasts with Ly49-MHC interactions. In this review, we discuss these issues and summarize recent developments concerning this non-MHC-dependent regulation of NK cell function.

Peripheral blood T lymphocytes from patients with early rheumatoid arthritis express RANKL and interleukin-15 on the cell surface and promote osteoclastogenesis in autologous monocytes

OBJECTIVE

To investigate the osteoclastogenic potential of T cells from the peripheral blood (PB) and synovial fluid (SF) of patients with rheumatoid arthritis (RA) on autologous monocytes, and to study the cytokines implicated in this process.

METHODS

T cells and monocytes were isolated from the PB of 20 healthy subjects and 20 patients with early RA, and from the SF of 20 patients with established RA. Autologous T cell/monocyte cocultures were established in the absence of exogenous cytokines or growth factors in order to examine spontaneous ex vivo osteoclast differentiation by tartrate-resistant acid phosphatase staining and calcified matrix resorption activity.

RESULTS

Surface RANKL was expressed on freshly isolated T cells from the PB of patients with early RA and the SF of patients with established RA. In addition, surface interleukin-15 (IL-15) was detected on freshly isolated T cells and monocytes from the PB of patients with early RA and the SF of patients with established RA. Autologous T cell/monocyte cocultures derived from the SF of patients with established RA and from the PB of patients with early RA, but not from the PB of healthy controls, resulted in osteoclast differentiation that was significantly inhibited by osteoprotegerin (OPG) and by neutralizing monoclonal antibodies to IL-15, IL-17, tumor necrosis factor alpha (TNFalpha), and IL-1beta. OPG, anti-TNFalpha, and anti-IL-1beta demonstrated a cooperative inhibitory effect. At 1-year followup, surface RANKL and IL-15 and ex vivo osteoclastogenesis were no longer observed on PB T cells or monocytes from patients with early RA in whom clinical remission had been achieved with treatment.

CONCLUSION

T cells are important contributors to the pathogenesis of bone erosions in RA through interaction with osteoclast precursors of the monocyte/macrophage lineage.

Inflammatory bone destruction and osteoimmunology

OBJECTIVES

The metabolism of hard tissue is influenced by the immune system. Research into the bone destruction associated with inflammatory diseases such as periodontal disease and rheumatoid arthritis has highlighted the importance of the interplay of the immune and skeletal systems. This interdisciplinary research field, called 'osteoimmunology', has become increasingly important for each system by itself as well as the biology linking them. The history and recent progress of this field are reviewed.

MATERIAL AND METHODS

'Osteoimmunology' was coined to describe the pioneering work on the T-cell regulation of osteoclastogenesis by the receptor activator of nuclear factor-kappaB ligand (RANKL) and interferon (IFN)-gamma. Accumulating evidence suggests that the immune and skeletal systems share not only cytokines but also various signaling molecules, transcription factors and membrane receptors. The contribution of T cells to the pathogenesis of inflammatory bone destruction is discussed, and our recent findings are summarized to illustrate how the osteoimmunological network functions.

RESULTS

RANKL is an osteoclastogenic cytokine that links bone and the immune system. Immunomodulatory cytokines such as IFNs also participate in the regulation of RANKL signaling and inflammatory bone loss. The transcription factor nuclear factor of activated T cells c1 (NFATc1) has been identified as a master switch regulator of osteoclastogenesis. In addition, immunoglobulin-like receptors are critically involved in bone homeostasis.

CONCLUSION

Bone turns out to be a dynamic tissue that is constantly renewed, where the immune system participates to a hitherto unexpected extent. This emerging field will be of great importance to a better understanding and treatment of diseases of the skeletal and immune systems, as well as to the fundamental biology underpinning both.

IL-4 Inhibits Bone-Resorbing Activity of Mature Osteoclasts by Affecting NF-κB and Ca2+ Signaling

IL-4 is an important immune cytokine that regulates bone homeostasis. We investigated the molecular mechanism of IL-4 action on bone-resorbing mature osteoclasts. Using a highly purified population of mature osteoclasts, we show that IL-4 dose-dependently inhibits receptor activator of NF-kappaB ligand (RANKL)-induced bone resorption by mature osteoclasts. We detected the existence of IL-4R mRNA in mature osteoclasts. IL-4 decreases TRAP expression without affecting multinuclearity of osteoclasts, and inhibits actin ring formation and migration of osteoclasts. Interestingly, IL-4 inhibition of bone resorption occurs through prevention of RANKL-induced nuclear translocation of p65 NF-kappaB subunit, and intracellular Ca(2+) changes. Moreover, IL-4 rapidly decreases RANKL-stimulated ionized Ca(2+) levels in the blood, and mature osteoclasts in IL-4 knockout mice are sensitive to RANKL action to induce bone resorption and hypercalcemia. Furthermore, IL-4 inhibits bone resorption and actin ring formation by human mature osteoclasts. Thus, we reveal that IL-4 acts directly on mature osteoclasts and inhibits bone resorption by inhibiting NF-kappaB and Ca(2+) signaling.

The Heat Shock Protein 90 Inhibitor, 17-Allylamino-17-demethoxygeldanamycin, Enhances Osteoclast Formation and Potentiates Bone Metastasis of a Human Breast Cancer Cell Line

Breast cancer metastasis to the bone occurs frequently, causing numerous complications including severe pain, fracture, hypercalcemia, and paralysis. Despite its prevalence and severity, few effective therapies exist. To address this, we examined whether the heat shock protein 90 (Hsp90) inhibitor, 17-allylamino-17-demethoxygeldanamycin (17-AAG), would be efficacious in inhibiting breast cancer metastasis to bone. Utilizing the human breast cancer subline, MDA-MB-231SA, previously in vivo selected for its enhanced ability to generate osteolytic bone lesions, we determined that 17-AAG potently inhibited its in vitro proliferation and migration. Moreover, 17-AAG significantly reduced MDA-MB-231SA tumor growth in the mammary-fat pad of nude mice. Despite these findings, 17-AAG enhanced the incidence of bone metastasis and osteolytic lesions following intracardiac inoculation in the nude mouse. Consistent with these findings, 17-AAG enhanced osteoclast formation 2- to 4-fold in mouse bone marrow/osteoblast cocultures, receptor activator of nuclear factor kappaB ligand (RANKL)-stimulated bone marrow, and RAW264.7 cell models of in vitro osteoclastogenesis. Moreover, the drug enhanced osteoclastogenesis in human cord blood progenitor cells, demonstrating that its effects were not limited to mouse models. In addition to 17-AAG, other Hsp90 inhibitors, such as radicicol and herbimycin A, also enhanced osteoclastogenesis. A pro-osteolytic action of 17-AAG independent of tumor presence was also determined in vivo, in which 17-AAG-treated tumor-naive mice had reduced trabecular bone volume with an associated increase in osteoclast number. Thus, HSP90 inhibitors can stimulate osteoclast formation, which may underlie the increased incidence of osteolysis and skeletal tumor incidence caused by 17-AAG in vivo. These data suggest an important contraindication to the Hsp90 targeted cancer therapy currently undergoing clinical trial.

Modulation of osteoclast formation

Osteoclasts are derived following the fusion of precursors of hematopoietic and myelomonocytic origin after appropriate stimulus, such as that afforded by RANKL and M-CSF. Thus the osteoclast can be considered as a specialized type of macrophage, and several of the factors that affect osteoclast formation also have affects upon macrophage differentiation. Inhibitors of osteoclast formation may perturb RANKL or M-CSF signalling or affect other signalling pathways. Several of these inhibitors are discussed with the view of their capacity to influence osteoclast differentiation, but not necessarily their activity.

Interleukin-4 and interleukin-13 potentiate interleukin-1 induced secretion of interleukin-6 in human osteoblast-like cells

Formation of interleukin-6 (IL-6) in osteoblasts and bone marrow stromal cells is believed to regulate osteoclast recruitment. The anti-inflammatory cytokines interleukin-4 and -13 (IL-4 and IL-13) stimulate IL-6 production in human osteoblasts. We investigated the relative potencies, and synergistic effects, between IL-4, IL-13 and interleukin-1 (IL-1) on IL-6 formation in human osteoblast-like cells. Isolated human osteoblast-like cells were incubated for 72 h in the presence of various concentrations of IL-4, IL-13 and IL-1, and IL-6 secretion was measured by ELISA. All cytokines stimulated the secretion of IL-6. The rank order of potency was IL-1>>IL-4>IL-13. There were no additive or synergistic effects between IL-4 and IL-13. However, co-stimulation with IL-1 and IL-4 resulted in a marked synergistic effect on IL-6 secretion. Co- stimulation with IL-1 and IL-13 gave a minor synergistic effect. In conclusion, IL-4/13 synergistically potentiates IL-1 induced secretion of IL-6 in human osteoblast-like cells.