Interleukin 18 inhibits osteoclast formation via T cell production of granulocyte macrophage colony-stimulating factor

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.

The effect of cytokines on osteoblasts and osteoclasts in bone remodeling in osteoporosis: a review

The complicated connections and cross talk between the skeletal system and the immune system are attracting more attention, which is developing into the field of Osteoimmunology. In this field, cytokines that are among osteoblasts and osteoclasts play a critical role in bone remodeling, which is a pathological process in the pathogenesis and development of osteoporosis. Those cytokines include the tumor necrosis factor (TNF) family, the interleukin (IL) family, interferon (IFN), chemokines, and so on, most of which influence the bone microenvironment, osteoblasts, and osteoclasts. This review summarizes the effect of cytokines on osteoblasts and osteoclasts in bone remodeling in osteoporosis, aiming to providing the latest reference to the role of immunology in osteoporosis.

Effect of T cells on bone

Bone and immune systems mutually influence each other by sharing a variety of regulatory molecules and the tissue microenvironment. The interdisciplinary research field "osteoimmunology" has illuminated the complex and dynamic interactions between the two systems in the maintenance of tissue homeostasis as well as in the development of immune and skeletal disorders. T cells play a central role in the immune response by secreting various immune factors and stimulating other immune cells and structural cells such as fibroblasts and epithelial cells, thereby contributing to pathogen elimination and pathogenesis of immune diseases. The finding on regulation of osteoclastic bone resorption by activated CD4⁺ T cells in rheumatoid arthritis was one of the driving forces for the development of osteoimmunology. With advances in research on helper T cell subsets and rare lymphoid cells such as γδ T cells in the immunology field, it is becoming clear that various types of T cells exert multiple effects on bone metabolism depending on immune context. Understanding the diverse effects of T cells on bone is essential for deciphering the osteoimmune regulatory network in various biological settings.

Associations of IL-18 with Altered Cardiovascular Risk Profile in Psoriatic Arthritis and Ankylosing Spondylitis

Objective: To investigate the associations of IL-18 serum levels with serum lipids, cardiovascular risk, and disease activity in patients with ankylosing spondylitis (AS) and psoriatic arthritis (PsA) with axial (axPsA) and peripheral (perPsA) joint involvement. Methods: 155 adult patients (PsA 61/AS 94) were enrolled in the study. Standard disease activity indices, BASDAI, and ASDAS, were calculated for AS and PsA and DAPSA for PsA. Sera from peripheral blood samples were obtained after night fasting. Serum concentrations of cytokines (IL-18, IL-17) were measured by ELISA, while lipid profile with total cholesterol (TC), triglycerides (TG), low-density cholesterol-(LDL), high-density cholesterol (HDL), and C-reactive protein (CRP) concentrations were determined using routine procedures. The atherogenic index was calculated using the standard formula AI = TC/HDL. Results: Patients with PsA and peripheral joint involvement (perPsA) had significantly higher IL-18 serum levels than axial PsA and AS patients (medians 160 vs. 116 vs. 80 pg/mL). In patients with PsA and in the subgroup with PsA+ ischemic heart disease (IHD), IL-18 positively correlated with atherogenic index (AI) (rho = 0.46 and rho = 0.67, respectively) and TG serum concentrations (rho = 0.4 and rho = 0.675), while negatively with HDL levels (rho = −0.37 and rho = −0.608). In PsA + IHD subgroup IL-18 serum levels correlated positively also with disease activity (DAPSA) (rho = 0.613). Importantly, in patients with perPsA, characterized by the highest IL-18 serum levels, cardiovascular risk, and frequency of both hypertriglyceridemia and IHD, positive correlations between IL-18 and IL-17 (rho = 0.47, p = 0.002), TG (rho = 0.45 p = 0.01) levels and AI (rho = 0.63 p = 0.021) were found. Whereas linear regression models revealed that IL-17, TG concentrations and the tender joint count had an impact on IL-18 Conclusions: We confirmed that patients with perPsA are characterized by a more pronounced proinflammatory and proatherogenic cardiovascular risk profile than patients with axPsA and AS. Importantly our study indicates that in PsA, but not in AS, elevated serum concentration of IL-18 is associated with higher disease activity and proatherogenic lipid profile, leading to a higher cardiovascular risk. Thus, our results point out IL-18 as a critical contributor in these pathological processes and possible therapeutic targets.

Macrophage-Osteoclast Associations: Origin, Polarization, and Subgroups

Cellular associations in the bone microenvironment are involved in modulating the balance between bone remodeling and resorption, which is necessary for maintaining a normal bone morphology. Macrophages and osteoclasts are both vital components of the bone marrow. Macrophages can interact with osteoclasts and regulate bone metabolism by secreting a variety of cytokines, which make a significant contribution to the associations. Although, recent studies have fully explored either macrophages or osteoclasts, indicating the significance of these two types of cells. However, it is of high importance to report the latest discoveries on the relationships between these two myeloid-derived cells in the field of osteoimmunology. Therefore, this paper reviews this topic from three novel aspects of the origin, polarization, and subgroups based on the previous work, to provide a reference for future research and treatment of bone-related diseases.

Targeting NLRP3 inflammasome as a chief instigator of obesity, contributing to local adipose tissue inflammation and insulin resistance

Inflammasome activity plays a vital role in various non-microbial disease states correlated with prolonged inflammation. NLRP3 inflammasome function and IL-1β formation are augmented in obesity and several obesity-linked metabolic disorders (i.e. diabetes mellitus, hypertension, hepatic steatosis, cancer, arthritis, and sleep apnea). Also, several factors are associated with the progression of diseases viz. increased plasma glucose, fatty acids, and β-amyloid are augmented during obesity and activate NLRP3 inflammasome expression. Prolonged NLRP3 stimulation seems to play significant role in various disorders, though better knowledge of inflammasome regulation and action might result in improved therapeutic tactics. Numerous compounds that mitigate NLRP3 inflammasome expression and suppress its chief effector, IL-1β are presently studied in clinical phases as therapeutics to manage or prevent these common disorders. A deep research on the literature available till date for inflammasome in obesity was conducted using various medical sites like PubMed, HINARI, MEDLINE from the internet, and data was collected simultaneously. The present review aims to examine the prospects of inflammasome as a major progenitor in the progression of obesity via directing their role in regulating appetite.

Inflammasomes in Alveolar Bone Loss

Bone remodeling is tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Fine tuning of the osteoclast-osteoblast balance results in strict synchronization of bone resorption and formation, which maintains structural integrity and bone tissue homeostasis; in contrast, dysregulated bone remodeling may cause pathological osteolysis, in which inflammation plays a vital role in promoting bone destruction. The alveolar bone presents high turnover rate, complex associations with the tooth and periodontium, and susceptibility to oral pathogenic insults and mechanical stress, which enhance its complexity in host defense and bone remodeling. Alveolar bone loss is also involved in systemic bone destruction and is affected by medication or systemic pathological factors. Therefore, it is essential to investigate the osteoimmunological mechanisms involved in the dysregulation of alveolar bone remodeling. The inflammasome is a supramolecular protein complex assembled in response to pattern recognition receptors and damage-associated molecular patterns, leading to the maturation and secretion of pro-inflammatory cytokines and activation of inflammatory responses. Pyroptosis downstream of inflammasome activation also facilitates the clearance of intracellular pathogens and irritants. However, inadequate or excessive activity of the inflammasome may allow for persistent infection and infection spreading or uncontrolled destruction of the alveolar bone, as commonly observed in periodontitis, periapical periodontitis, peri-implantitis, orthodontic tooth movement, medication-related osteonecrosis of the jaw, nonsterile or sterile osteomyelitis of the jaw, and osteoporosis. In this review, we present a framework for understanding the role and mechanism of canonical and noncanonical inflammasomes in the pathogenesis and development of etiologically diverse diseases associated with alveolar bone loss. Inappropriate inflammasome activation may drive alveolar osteolysis by regulating cellular players, including osteoclasts, osteoblasts, osteocytes, periodontal ligament cells, macrophages, monocytes, neutrophils, and adaptive immune cells, such as T helper 17 cells, causing increased osteoclast activity, decreased osteoblast activity, and enhanced periodontium inflammation by creating a pro-inflammatory milieu in a context- and cell type-dependent manner. We also discuss promising therapeutic strategies targeting inappropriate inflammasome activity in the treatment of alveolar bone loss. Novel strategies for inhibiting inflammasome signaling may facilitate the development of versatile drugs that carefully balance the beneficial contributions of inflammasomes to host defense.

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.

Omnipresence of inflammasome activities in inflammatory bone diseases

The inflammasomes are intracellular protein complexes that are assembled in response to a variety of perturbations including infections and injuries. Failure of the inflammasomes to rapidly clear the insults or restore tissue homeostasis can result in chronic inflammation. Recurring inflammation is also provoked by mutations that cause the constitutive assembly of the components of these protein platforms. Evidence suggests that chronic inflammation is a shared mechanism in bone loss associated with aging, dysregulated metabolism, autoinflammatory, and autoimmune diseases. Mechanistically, inflammatory mediators promote bone resorption while suppressing bone formation, an imbalance which over time leads to bone loss and increased fracture risk. Thus, while acute inflammation is important for the maintenance of bone integrity, its chronic state damages this tissue. In this review, we discuss the role of the inflammasomes in inflammation-induced osteolysis.

Zearalenone (ZEA)-induced intestinal inflammation is mediated by the NLRP3 inflammasome

To ascertain whether zearalenone (ZEA) could induce intestinal inflammation and investigate its possible mechanism, we investigated inflammatory cytokine release and the activation of the NLRP3 inflammasome after ZEA treatment both in vitro or in vivo. First, intestinal porcine enterocyte cell line (IPEC-J2) cells and mouse peritoneal macrophages were treated with ZEA to detect NLRP3 inflammasome activation, and the role of reactive oxygen species (ROS) in ZEA-induced inflammation was investigated. Then, Balb/c mice were fed a gavage of ZEA, and the disease activity indices (DAIs) and histological analysis were used to assess intestinal inflammation. Our study showed that the mRNA expression of NLRP3 inflammasome, pro-interleukin-1β (pro-IL-1β), and pro-interleukin-18 (pro-IL-18) was up-regulated 0.5- to 1-fold and that the release of IL-1β and IL-18 increased from 48 pg mL^-1 to 55 pg mL^-1 and 110 pg mL^-1 to 145 pg mL^-1, respectively. However, ROS inhibitor N-acetyl-l-cysteine (NAC) reduced IL-1β and IL-18 release to 45 pg mL^-1 and 108 pg mL^-1. Moreover, the same phenomenon was observed in intestinal tissues of ZEA-treated mice. In addition, clinical parameters of treated mice showed stools became loose and contained mucous. In addition, the presence of gross blood stool was found in the last 2 d. Histological analysis showed obvious inflammatory cell infiltration and tissue damage in the colon. These findings uncovered a possible mechanism of intestinal mucosal innate immunity in response to mycotoxin ZEA that ZEA could activate the ROS-mediated NLRP3 inflammasome and, in turn, contribute to the caspase-1-dependent activation of the inflammatory cytokines IL-1β and IL-18.

Elevated levels of Interleukin (IL)-33 induce bone pathology but absence of IL-33 does not negatively impact normal bone homeostasis

Interleukin (IL)-33 is a member of the IL-1 family. IL-33 effects are mediated through its receptor, ST2 and IL-1RAcP, and its signaling induces the production of a number of pro-inflammatory mediators, including TNFα, IL-1β, IL-6, and IFN-γ. There are conflicting reports on the role of IL-33 in bone homeostasis, with some demonstrating a bone protective role for IL-33 whilst others show that IL-33 induces inflammatory arthritis with concurrent bone destruction. To better clarify the role IL-33 plays in bone biology in vivo, we studied IL-33 KO mice as well as mice in which the cytokine form of IL-33 was overexpressed. Mid-femur cortical bone mineral density (BMD) and bone strength were similar in the IL-33 KO mice compared to WT animals during the first 8months of life. However, in the absence of IL-33, we observed higher BMD in lumbar vertebrae and distal femur in female mice. In contrast, overexpression of IL-33 resulted in a marked and rapid reduction of bone volume, mineral density and strength. Moreover, this was associated with a robust increase in inflammatory cytokines (including IL-6 and IFN-γ), suggesting the bone pathology could be a direct effect of IL-33 or an indirect effect due to the induction of other mediators. Furthermore, the detrimental bone effects were accompanied by increases in osteoclast number and the bone resorption marker of C-terminal telopeptide collagen-I (CTX-I). Together, these results demonstrate that absence of IL-33 has no negative consequences in normal bone homeostasis while high levels of circulating IL-33 contributes to pathological bone loss.

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.

Interleukin-17 and interleukin-18 levels in different stages of inflammatory periodontal disease

Context:

Chronic periodontitis is an inflammatory condition of the tooth supporting structures. There is increasing evidence that the cytokines interleukin-17 (IL-17) and interleukin-18 (IL-18) play a role in progression of chronic periodontitis.

Aim:

The objective of this study was to compare the levels of the cytokines IL-17 and IL-18 in gingival tissue extracts from individuals with healthy gingiva, chronic gingivitis, and mild chronic periodontitis.

Settings and Design:

The study was performed in a hospital-based population with an experimental design.

Materials and Methods:

A total of 69 individuals (n = 23 per group) were recruited for the study. Group 1 included 23 individuals with healthy gingiva and Group 2 included 23 chronic gingivitis patients and Group 3 included 23 patients with mild chronic periodontitis. Gingival tissues were collected during surgical procedures and levels of IL-17 and IL-18 were determined using enzyme-linked immunosorbent assay.

Statistical Analysis:

Intergroup comparison was done by posthoc Tukey's test.

Results:

The gingival tissue concentration of IL-17 was found to be highest in Group 2 (415.19 ± 76.84 pg/mg) followed by Group 3 (193.77 ± 37.32 pg/mg) and Group 1 (20.49 ± 6.05 pg/mg). Concentrations of IL-18 were significantly higher (P < 0.01) in Group 2 (1479.42 ± 330.33 pg/mg) when compared with Group 1 (385.18 ± 71.26 pg/mg) and Group 3 (330.24 ± 48.56 pg/mg).

Conclusion:

There appears to be considerable variation of IL-17 and IL-18 levels in gingival tissue during periodontal health and disease.

Novel immunostimulatory effects of osteoclasts and macrophages on human γδ T cells

It has been widely reported that T cells are capable of influencing osteoclast formation and bone remodelling, yet relatively little is known of the reciprocal effects of osteoclasts for affecting T cell function and/or activity. In this study we investigated the effects of human osteoclasts on the function of γδ T cells, a subset of non-CD4(+) T cells implicated in a variety of inflammatory disease states. γδ T cells and CD4(+) T cells were isolated from peripheral blood of healthy volunteers and were co-cultured with autologous mature osteoclasts (generated by treatment with M-CSF and RANKL) before phenotypical and functional changes in the T cell populations were assessed. Macrophages, osteoclasts, and conditioned medium derived from macrophages or osteoclasts induced activation of γδ T cells, as determined by the expression of the early activation marker CD69. TNFα was a major mediator of this stimulatory effect on γδ T cells. Consistent with this stimulatory effect, osteoclasts augmented proliferation of IL-2-stimulated γδ T cells and also supported the survival of unstimulated γδ and CD4(+) T cells, although these effects required co-culture with osteoclasts. Co-culture with osteoclasts also increased the proportion of γδ T cells producing IFNγ, but did not modulate IFNγ or IL-17 production by CD4(+) T cells. We provide new insights into the in vitro interactions between human γδ T cells and osteoclasts/macrophages, and demonstrate that osteoclasts or their precursors are capable of influencing γδ T function both via the release of soluble factors and also through direct cell-cell interactions.

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.

A novel role for interferon regulatory factor 1 (IRF1) in regulation of bone metabolism

Increased risk of bone fractures is observed in patients with chronic inflammatory conditions, such as inflammatory bowel disease and rheumatoid arthritis. Members of the Interferon Response Factor family of transcriptional regulators, IRF1 and IRF8, have been identified as genetic risk factors for several chronic inflammatory and autoimmune diseases. We have investigated a potential role for the Irf1 gene in bone metabolism. Here, we report that Irf1(-/-) mutant mice show altered bone morphology in association with altered trabecular bone architecture and increased cortical thickness and cellularity. Ex vivo studies on cells derived from bone marrow stimulated with Rank ligand revealed an increase in size and resorptive activity of tartrate-resistant acid-positive cells from Irf1(-/-) mutant mice compared with wild-type control mice. Irf1 deficiency was also associated with decreased proliferation of bone marrow-derived osteoblast precursors ex vivo, concomitant with increased mineralization activity compared with control cells. We show that Irf1 plays a role in bone metabolism and suggest that Irf1 regulates the maturation and activity of osteoclasts and osteoblasts. The altered bone phenotype of Irf1(-/-) mutants is strikingly similar to that of Stat1(-/-) mice, suggesting that the two interacting proteins play a critical enabling role in the common regulation of these two cell lineages.

IL-18 upregulates the production of key regulators of osteoclastogenesis from fibroblast-like synoviocytes in rheumatoid arthritis

Recent data have demonstrated the importance of IL-18 in the induction and perpetuation of chronic inflammation in experimental arthritis. The aim of the present study was to elucidate whether IL-18 has any indirect effects on osteoclastogenesis by regulating the production of molecules from fibroblast-like synoviocytes (FLS) in rheumatoid arthritis (RA). Human FLS were isolated from RA synovial tissue and cultured in vitro for three to five passages. The expression of IL-18 receptor was determined by RT-PCR. The levels of soluble receptor activator of nuclear factor κB ligand (RANKL), osteoprotegerin (OPG), macrophage colony-stimulating factor (M-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF) in culture supernatants were determined by ELISA. Membrane-bound RANKL expression was analyzed by flow cytometry. Both α and β chains of IL-18 receptor were confirmed in cultured FLS. IL-18 upregulated membrane-bound RANKL expression and soluble RANKL production by FLS in both time- and dose-dependent manners. In addition, IL-18 enhanced production of M-CSF, GM-CSF, and OPG from cultured FLS in a dose-dependent manner. IL-18 also increased the ratio of RANKL/OPG, suggesting that the net effect of IL-18 on FLS favors for the induction of osteoclast formation and bone resorption. In conclusion, IL-18 upregulates the production of key regulators of osteoclastogenesis from FLS in RA.

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.

Inflammatory cell response to calcium phosphate biomaterial particles: an overview

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite (HA) and amorphous calcium phosphate (CaP) crystals deposited in an organic matrix. One objective of bone tissue engineering is to mimic the chemical and structural properties of this complex tissue. CaP ceramics, such as sintered HA and beta-tricalcium phosphate, are widely used as bone substitutes or prosthesis coatings because of their osteoconductive properties. These ceramic interactions with tissues induce a cell response that can be different according to the composition of the material. In this review, we discuss inflammatory cell responses to CaP materials to provide a comprehensive overview of mechanisms governing the integration or loosening of implants, which remains a major concern in tissue engineering. A focus on the effects of the functionalization of CaP biomaterials highlights potential ways to increase tissue integration and limit rejection processes.

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.

Transgenic over-expression of interleukin-33 in osteoblasts results in decreased osteoclastogenesis

Interleukin-33 (IL-33) is the most recently identified member of the IL-1 family of cytokines, which is primarily known for its proinflammatory functions. We have previously reported that IL-33 is expressed by bone-forming osteoblasts, and that administration of recombinant IL-33 to bone marrow cultures inhibits their differentiation into bone-resorbing osteoclasts. Likewise, while the inhibitory effect of IL-33 on osteoclast differentiation was fully abolished in cultures lacking the IL-33 receptor ST2, mice lacking ST2 displayed low bone mass caused by increased osteoclastogenesis. Although these data suggested a physiological role of IL-33 as an inhibitor of bone resorption, direct in vivo evidence supporting such a function was still missing. Here we describe the generation and bone histomorphometric analysis of a transgenic mouse model (Col1a1-Il33) over-expressing IL-33 specifically in osteoblasts. While we did not observe differences in osteoblast number and bone formation between wildtype and Col1a1-Il33 mice, the number of osteoclasts was significantly reduced compared to wildtype littermates in two independent transgenic lines. Since we did not observe quantitative differences in the populations of eosinophils, neutrophils, basophils or M2-macrophages from the bone marrow of wildtype and Col1a1-Il33 mice, our data demonstrate that an inhibition of osteoclastogenesis is one of the major physiological functions of IL-33, at least in mice.

Inflammasome is a central player in the induction of obesity and insulin resistance

Inflammation plays a key role in the pathogenesis of obesity. Chronic overfeeding leads to macrophage infiltration in the adipose tissue, resulting in proinflammatory cytokine production. Both microbial and endogenous danger signals trigger assembly of the intracellular innate immune sensor Nlrp3, resulting in caspase-1 activation and production of proinflammatory cytokines IL-1β and IL-18. Here, we showed that mice deficient in Nlrp3, apoptosis-associated speck-like protein, and caspase-1 were resistant to the development of high-fat diet-induced obesity, which correlated with protection from obesity-induced insulin resistance. Furthermore, hepatic triglyceride content, adipocyte size, and macrophage infiltration in adipose tissue were all reduced in mice deficient in inflammasome components. Monocyte chemoattractant protein (MCP)-1 is a key molecule that mediates macrophage infiltration. Indeed, defective inflammasome activation was associated with reduced MCP-1 production in adipose tissue. Furthermore, plasma leptin and resistin that affect energy use and insulin sensitivity were also changed by inflammasome-deficiency. Detailed metabolic and molecular phenotyping demonstrated that the inflammasome controls energy expenditure and adipogenic gene expression during chronic overfeeding. These findings reveal a critical function of the inflammasome in obesity and insulin resistance, and suggest inhibition of the inflammasome as a potential therapeutic strategy.

Interferon-γ plays a role in bone formation in vivo and rescues osteoporosis in ovariectomized mice

Interferon γ (IFN-γ) is a cytokine produced locally in the bone microenvironment by cells of immune origin as well as mesenchymal stem cells. However, its role in normal bone remodeling is still poorly understood. In this study we first examined the consequences of IFN-γ ablation in vivo in C57BL/6 mice expressing the IFN-γ receptor knockout phenotype (IFNγR1(-/-)). Compared with their wild-type littermates (IFNγR1(+/+)), IFNγR1(-/-) mice exhibit a reduction in bone volume associated with significant changes in cortical and trabecular structural parameters characteristic of an osteoporotic phenotype. Bone histomorphometry of IFNγR1(-/-) mice showed a low-bone-turnover pattern with a decrease in bone formation, a significant reduction in osteoblast and osteoclast numbers, and a reduction in circulating levels of bone-formation and bone-resorption markers. Furthermore, administration of IFN-γ (2000 and 10,000 units) to wild-type C57BL/6 sham-operated (SHAM) and ovariectomized (OVX) female mice significantly improved bone mass and microarchitecture, mechanical properties of bone, and the ratio between bone formation and bone resorption in SHAM mice and rescued osteoporosis in OVX mice. These data therefore support an important physiologic role for IFN-γ signaling as a potential new anabolic therapeutic target for osteoporosis.

Interleukin-33, a target of parathyroid hormone and oncostatin m, increases osteoblastic matrix mineral deposition and inhibits osteoclast formation in vitro

IL-33 is an important inflammatory mediator in allergy, asthma, and joint inflammation, acting via its receptor, ST2L, to elicit Th₂ cell cytokine secretion. IL-33 is related to IL-1 and IL-18, which both influence bone metabolism, IL-18 in particular inhibiting osteoclast formation and contributing to PTH bone anabolic actions. We found IL-33 immunostaining in osteoblasts in mouse bone and IL-33 mRNA expression in cultured calvarial osteoblasts, which was elevated by treatment with the bone anabolic factors oncostatin M and PTH. IL-33 treatment strongly inhibited osteoclast formation in bone marrow and spleen cell cultures but had no effect on osteoclast formation in receptor activator of nuclear factor-κB ligand/macrophage colony-stimulating factor-treated bone marrow macrophage (BMM) or RAW264.7 cultures, suggesting a lack of direct action on immature osteoclast progenitors. However, osteoclast formation from BMM was inhibited by IL-33 in the presence of osteoblasts, T cells, or mature macrophages, suggesting these cell types may mediate some actions of IL-33. In bone marrow cultures, IL-33 induced mRNA expression of granulocyte macrophage colony-stimulating factor, IL-4, IL-13, and IL-10; osteoclast inhibitory actions of IL-33 were rescued only by combined antibody ablation of these factors. In contrast to osteoclasts, IL-33 promoted matrix mineral deposition by long-term ascorbate treated primary osteoblasts and reduced sclerostin mRNA levels in such cultures after 6 and 24 h of treatment; sclerostin mRNA was also suppressed in IL-33-treated calvarial organ cultures. In summary, IL-33 stimulates osteoblastic function in vitro but inhibits osteoclast formation through at least three separate mechanisms. Autocrine and paracrine actions of osteoblast IL-33 may thus influence bone metabolism.

IL-33 shifts the balance from osteoclast to alternatively activated macrophage differentiation and protects from TNF-alpha-mediated bone loss

IL-33 is a new member of the IL-1 family, which plays a crucial role in inflammatory response, enhancing the differentiation of dendritic cells and alternatively activated macrophages (AAM). Based on the evidence of IL-33 expression in bone, we hypothesized that IL-33 may shift the balance from osteoclast to AAM differentiation and protect from inflammatory bone loss. Using transgenic mice overexpressing human TNF, which develop spontaneous joint inflammation and cartilage destruction, we show that administration of IL-33 or an IL-33R (ST2L) agonistic Ab inhibited cartilage destruction, systemic bone loss, and osteoclast differentiation. Reconstitution of irradiated hTNFtg mice with ST2(-/-) bone marrow led to more bone loss compared with the chimeras with ST2(+/+) bone marrow, demonstrating an important endogenous role of the IL-33/ST2L pathway in bone turnover. The protective effect of IL-33 on bone was accompanied by a significant increase of antiosteoclastogenic cytokines (GM-CSF, IL-4, and IFN-γ) in the serum. In vitro IL-33 directly inhibits mouse and human M-CSF/receptor activator for NF-κB ligand-driven osteoclast differentiation. IL-33 acts directly on murine osteoclast precursors, shifting their differentiation toward CD206(+) AAMs via GM-CSF in an autocrine fashion. Thus, we show in this study that IL-33 is an important bone-protecting cytokine and may be of therapeutic benefit in treating bone resorption.

Interleukin-33 is expressed in differentiated osteoblasts and blocks osteoclast formation from bone marrow precursor cells

Since the hematopoetic system is located within the bone marrow, it is not surprising that recent evidence has demonstrated the existence of molecular interactions between bone and immune cells. While interleukin 1 (IL-1) and IL-18, two cytokines of the IL-1 family, have been shown to regulate differentiation and activity of bone cells, the role of IL-33, another IL-1 family member, has not been addressed yet. Since we observed that the expression of IL-33 increases during osteoblast differentiation, we analyzed its possible influence on bone formation and observed that IL-33 did not affect matrix mineralization but enhanced the expression of Tnfsf11, the gene encoding RANKL. This finding led us to analyze the skeletal phenotype of Il1rl1-deficient mice, which lack the IL-33 receptor ST2. Unexpectedly, these mice displayed normal bone formation but increased bone resorption, thereby resulting in low trabecular bone mass. Since this finding suggested a negative influence of IL-33 on osteoclastogenesis, we next analyzed osteoclast differentiation from bone marrow precursor cells and observed that IL-33 completely abolished the generation of TRACP(+) multinucleated osteoclasts, even in the presence of RANKL and macrophage colony-stimulating factor (M-CSF). Although our molecular studies revealed that IL-33 treatment of bone marrow cells caused a shift toward other hematopoetic lineages, we further observed a direct negative influence of IL-33 on the osteoclastogenic differentiation of RAW264.7 macrophages, where IL-33 repressed the expression of Nfatc1, which encodes one of the key transciption factors of osteoclast differentiation. Taken together, these findings have uncovered a previously unknown function of IL-33 as an inhibitor of bone resorption.

IL-18 production downstream of the Nlrp3 inflammasome confers protection against colorectal tumor formation

Colorectal cancer is a leading cause of cancer-related deaths worldwide. Chronic inflammation is recognized as a predisposing factor for the development of colon cancer, but the molecular mechanisms linking inflammation and tumorigenesis have remained elusive. Recent studies revealed a crucial role for the NOD-like receptor protein Nlrp3 in regulating inflammation through the assembly of proinflammatory protein complexes termed inflammasomes. However, its role in colorectal tumor formation remains unclear. In this study, we showed that mice deficient for Nlrp3 or the inflammasome effector caspase-1 were highly susceptible to azoxymethane/dextran sodium sulfate-induced inflammation and suffered from dramatically increased tumor burdens in the colon. This was a consequence of markedly reduced IL-18 levels in mice lacking components of the Nlrp3 inflammasome, which led to impaired production and activation of the tumor suppressors IFN-γ and STAT1, respectively. Thus, IL-18 production downstream of the Nlrp3 inflammasome is critically involved in protection against colorectal tumorigenesis.

Central roles of NLRs and inflammasomes in viral infection

The immune response to viral infections is determined by a complex interplay between the pathogen and the host. Innate immune cells express a set of cytosolic sensors to detect viral infection. Recognition by these sensors induces the production of type I interferons and the assembly of inflammasome complexes that activate caspase-1, leading to production of interleukin-1β (IL-1β) and IL-18. Here, I discuss recent progress in our understanding of the central roles of NOD-like receptors (NLRs) and inflammasomes in the immune response during viral infections. This information will improve our understanding of host defence mechanisms against viruses and provide new avenues for interfering in the pathogenesis of infectious diseases.

The NLRP3 inflammasome protects against loss of epithelial integrity and mortality during experimental colitis

Decreased expression of the Nlrp3 protein is associated with susceptibility to Crohn's disease. However, the role of Nlrp3 in colitis has not been characterized. Nlrp3 interacts with the adaptor protein ASC to activate caspase-1 in inflammasomes, which are protein complexes responsible for the maturation and secretion of interleukin-1beta (IL-1beta) and IL-18. Here, we showed that mice deficient for Nlrp3 or ASC and caspase-1 were highly susceptible to dextran sodium sulfate (DSS)-induced colitis. Defective inflammasome activation led to loss of epithelial integrity, resulting in systemic dispersion of commensal bacteria, massive leukocyte infiltration, and increased chemokine production in the colon. This process was a consequence of a decrease in IL-18 in mice lacking components of the Nlrp3 inflammasome, resulting in higher mortality rates. Thus, the Nlrp3 inflammasome is critically involved in the maintenance of intestinal homeostasis and protection against colitis.

IL-18 inhibits TNF-alpha-induced osteoclastogenesis possibly via a T cell-independent mechanism in synergy with IL-12 in vivo

It has recently been reported that tumor necrosis factor (TNF)-alpha has the ability to accelerate osteoclastogenesis. We previously reported that the proinflammatory cytokine interleukin (IL)-18 inhibits TNF-alpha-mediated osteoclastogenesis in mouse bone marrow cultures. In the present study, the effect of IL-18 on TNF-alpha-mediated osteoclastogenesis was investigated in vivo. We administered TNF-alpha with or without IL-18 into the supracalvaria of mice. The number of osteoclasts in the suture of the calvaria was increased in mice administered TNF-alpha. The number of osteoclasts in mice administered both TNF-alpha and IL-18 was lower than that in mice administered TNF-alpha alone. We previously showed that IL-12 and IL-18 synergistically inhibit TNF-alpha-mediated osteoclastogenesis in vitro. To assess the ability of these two cytokines to synergistically inhibit TNF-alpha-induced osteoclastogenesis in vivo, mice were administered the two cytokines at doses that did not inhibit osteoclast formation. The combination of IL-12 and IL-18 markedly inhibited TNF-alpha-induced osteoclastogenesis in vivo. To evaluate how IL-12 and IL-18 synergistically affect TNF-alpha-induced osteoclastogenesis, the IL-18 receptor (IL-18R) and IL-12R expression levels were analyzed by RT-PCR in bone marrow cells cultured with IL-12 or IL-18. IL-18R mRNA was increased in cells cultured with IL-12, while IL-12R mRNA was increased in cells cultured with IL-18. In addition, IL-18 inhibited TNF-alpha-induced osteoclastogenesis in mice with T-cell depletion caused by anti-CD4 and anti-CD8 antibodies. The present results suggest that IL-18 may inhibit TNF-alpha-mediated osteoclastogenesis in vivo via a T cell-independent mechanism.

IL-12 inhibits TNF-alpha induced osteoclastogenesis via a T cell-independent mechanism in vivo

It has been reported that TNF-alpha plays an important role in bone resorption in pathological conditions. IL-12, which is a T cell mediator, is also an important inflammatory cytokine. We previously reported that IL-12 induces apoptosis in bone marrow cells treated with TNF-alpha in vitro via an interaction between TNF-alpha-induced Fas and IL-12-induced Fas ligand (FasL), and that, as a result, osteoclastogenesis is inhibited. The purpose of this study was to investigate the effects of IL-12 on TNF-alpha-mediated osteoclastogenesis in vivo. We administered TNF-alpha with and without IL-12 into the supracalvaria in mice. The numbers of osteoclasts in the sutures in the calvaria were higher in mice administered TNF-alpha than in control mice not administered TNF-alpha. The numbers of osteoclasts in mice administered both TNF-alpha and IL-12 were lower than those in mice administered only TNF-alpha. Next, we determined the levels of mRNAs for cathepsin K and tartrate-resistant acid phosphatase (TRAP). mRNA levels were increased in mice administered TNF-alpha compared with control mice, but not in mice administered both TNF-alpha and IL-12. We also evaluated the amounts of tartrate-resistant acid phosphatase 5b (TRACP 5b) in mouse sera. The levels of TRACP 5b in mice administered TNF-alpha were higher than those in control mice. On the other hand, in mice administered both TNF-alpha and IL-12, the levels were lower than those in mice administered TNF-alpha alone. Fas and FasL expression levels were analyzed by real-time RT-PCR. The levels of Fas mRNA were increased in the calvaria of mice administered TNF-alpha compared with control mice, while those of FasL mRNAs were increased in the calvaria of mice administered IL-12. In TdT-mediated dUTP-biotin nick end-labeling (TUNEL) assays, many apoptotic cells were found in the sutures in the calvaria of mice administered both TNF-alpha and IL-12. IL-12 also inhibited TNF-alpha-induced osteoclastogenesis in mice whose T cells were blocked by anti-CD4 and anti-CD8 antibodies. These results suggest that IL-12 inhibits TNF-alpha-mediated osteoclastogenesis and induces apoptotic changes through an interaction between TNF-alpha-induced Fas and IL-12-induced FasL, in vivo, via a T cell-independent mechanism.

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.

Parathyroid hormone signaling in bone and kidney

PURPOSE OF REVIEW

Parathyroid hormone (PTH) maintains a physiological balance of calcium and phosphate concentrations by binding to its receptor on the plasma membrane of cells in bone and kidney. It signals through multiple pathways, including protein kinase A and protein kinase C, although a preference for certain pathways is apparent in each organ and function. Here, we will review the recent advancements regarding PTH signaling in bone and kidney.

RECENT FINDINGS

Wnt proteins have been reported as important regulators of bone metabolism in both PTH-dependent and independent pathways. Recent studies emphasize its role as a mediator of PTH signaling, as PTH treatment increased the expression of wnt4 and sfrp4 and decreased the expression of Wnt inhibitors such as Sost and sclerostin, leading to an increase in Wnt signaling. In kidney, sodium-hydrogen exchanger regulatory factor 1, originally known for its role in the retention of NaPi-IIa at the apical membrane, was shown to have multiple roles in PTH signaling, both as a mediator and regulator.

SUMMARY

PTH activates a number of different signaling pathways by binding to a single receptor in bone and kidney. Recent studies demonstrate the involvement of novel factors as well as additional roles for previously identified downstream factors of PTH.

Inflammasomes: guardians of cytosolic sanctity

The innate immune system is critical in recognizing bacterial and viral infections to evoke a proper immune response. Certain members of the intracellular nucleotide-binding and oligomerization domain (NOD)-like receptor (NLR) family detect microbial components in the cytosol and trigger the assembly of large caspase-1-activating complexes termed inflammasomes. Autoproteolytic maturation of caspase-1 zymogens within these inflammasomes leads to maturation and secretion of the pro-inflammatory cytokines interleukin-1 beta (IL-1 beta) and IL-18. The NLR proteins ICE protease-activating factor (IPAF), NALP1b (NACHT domain-, leucine-rich repeat-, and PYD-containing protein 1b), and cryopyrin/NALP3 assemble caspase-1-activating inflammasomes in a stimulus-dependent manner. Bacterial flagellin is sensed by IPAF, whereas mouse NALP1b detects anthrax lethal toxin. Cryopyrin/NALP3 mediates caspase-1 activation in response to a wide variety of microbial components and in response to crystalline substances such as the endogenous danger signal uric acid. Genetic variations in Nalp1 and cryopyrin/Nalp3 are associated with autoinflammatory disorders and increased susceptibility to microbial infection. Further understanding of inflammasomes and their role in innate immunity should provide new insights into the mechanisms of host defense and the pathogenesis of autoimmune diseases.

Cloning and analysis of rat osteoclast inhibitory lectin gene promoter

Osteoclast inhibitory lectin (OCIL) is a novel regulator of bone remodeling, however, little is known concerning how OCIL is regulated to date. In this study, approximately 4.4 kb of the 5'-flanking sequence of rat OCIL gene was cloned into the promoter-less reporter vector pGL3-basic and transiently transfected into three different cell lines. The differences in the levels of luciferase activity paralleled well with the levels of OCIL mRNA expression in these cells, suggesting that the regulation of rat OCIL gene expression occurs mainly at the transcriptional level. Additional luciferase assays using a series of constructs containing unidirectionally deleted fragments showed that the construct-1819/pGL3 (-1819 to +118) exhibited the highest luciferase activity, suggesting the presence of functional promoter in this region. The region from -4370 to -2805 might contain negative regulatory elements, while the region from -1819 to -1336 might have important positive regulatory elements that enhance OCIL transcription. Sequence analysis of the promoter revealed the absence of both TATA and CAAT boxes. However, in the proximal promoter region (-81 to +118), several potential transcription factor binding sites that may be responsible for the basal transcriptional activity of rat OCIL promoter were observed. The promoter contains several potential Sp1 binding sites, and cotransfection of a shRNA expression plasmid that knockdowns Sp1 significantly reduced OCIL promoter activity and endogenous gene expression and moreover, overexpressing Sp7, a Sp1 family member that also binds to Sp1 binding sequence, increased OCIL promoter activity and gene expression, suggesting a role of Sp1 family proteins in regulation of OCIL transcription.

The chemokine Cxcl1 is a novel target gene of parathyroid hormone (PTH)/PTH-related protein in committed osteoblasts

The PTH receptor (PTHR1) is expressed on osteoblasts and responds to PTH or PTHrP in an endocrine or autocrine/paracrine manner, respectively. A microarray study carried out on PTHR1-positive osteoblasts (Kusa 4b10 cells) identified the cysteine-X-cysteine (CXC) family chemokine ligand 1 (Cxcl1) as a novel immediate PTH/PTHrP-responsive gene. Cxcl1 is a potent neutrophil chemoattractant with recognized roles in angiogenesis and inflammation, but a role in bone biology has not been described. Cxcl1 mRNA levels were up-regulated 1 h after either PTH or PTHrP treatment of differentiated Kusa 4b10 osteoblasts (15-fold) and mouse calvarial osteoblasts (160-fold) and in rat metaphyseal bone (5-fold) 1 h after a single sc injection of PTH. Furthermore, PTH treatment stimulated a 10-fold increase in secreted Cxcl1 protein by both Kusa 4b10 cells and calvarial osteoblasts. Immunohistochemistry and PCR demonstrated that CXCR2, the receptor for Cxcl1, is highly expressed in osteoclast precursors (hemopoietic cells) but is predominantly undetectable in the osteoblast lineage, suggesting that osteoblast-derived Cxcl1 may act as a chemoattractant for osteoclast precursors. Confirming this hypothesis, recombinant Cxcl1 dose-dependently stimulated migration of osteoclast precursors in cell culture studies, as did conditioned media from Kusa 4b10 cells treated with PTH. These data indicate that local action through the PTHR1 could stimulate cells of the osteoblast lineage to release a chemokine capable of attracting osteoclast precursors to the bone environment.

Increased interleukin-18 expression in nonrheumatic aortic valve stenosis

BACKGROUND

It is unknown whether interleukin-18 (IL-18) participates in the pathophysiology of nonrheumatic aortic stenosis (NR-AS).

METHODS

We examined IL-18 expression in human NR-AS valves by immunohistochemistry and Western blot analysis.

RESULTS

Immunohistochemistry revealed that NR-AS valves showed increased IL-18 expression compared with controls. IL-18 receptor was also expressed in NR-AS valves. Western blot analysis showed that IL-18 was expressed as an active form in NR-AS valves. Furthermore, increased IL-18 expression was correlated with the advanced clinical severity of NR-AS.

CONCLUSIONS

IL-18 was expressed in the aortic valves and up-regulated in NR-AS valves. IL-18 may contribute to the pathophysiology of NR-AS.

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.

Osteoclast inhibitory lectin, an immune cell product that is required for normal bone physiology in vivo

Osteoclast inhibitory lectin (OCIL or clrb) is a member of the natural killer cell C-type lectins that have a described role mostly in autoimmune cell function. OCIL was originally identified as an osteoblast-derived inhibitor of osteoclast formation in vitro. To determine the physiological function(s) of OCIL, we generated ocil(-/-) mice. These mice appeared healthy and were fertile, with no apparent immune function defect, and phenotypic abnormalities were limited to bone. Histomorphometric analysis revealed a significantly lower tibial trabecular bone volume and trabecular number in the 10- and 16-week-old male ocil(-/-) mice compared with wild type mice. Furthermore, ocil(-/-) mice showed reduced bone formation rate in the 10-week-old females and 16-week-old males while Static markers of bone formation showed no significant changes in male or female ocil(-/-) mice. Examination of bone resorption markers in the long bones of ocil(-/-) mice indicated a transient increase in osteoclast number per unit bone perimeter. Enhanced osteoclast formation was also observed when either bone marrow or splenic cultures were generated in vitro from ocil(-/-) mice relative to wild type control cultures. Loss of ocil therefore resulted in osteopenia in adult mice primarily as a result of increased osteoclast formation and/or decreased bone formation. The enhanced osteoclastic activity led to elevated serum calcium levels, which resulted in the suppression of circulating parathyroid hormone in 10-week-old ocil(-/-) mice compared with wild type control mice. Collectively, our data suggest that OCIL is a physiological negative regulator of bone.

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.