Impaired skeletal development in interleukin-6-transgenic mice: a model for the impact of chronic inflammation on the growing skeletal system

OBJECTIVE

To identify the mediator responsible for the impact of chronic inflammation on skeletal development in children (bone loss, defective peak bone mass accrual, stunted growth), we evaluated the effects of chronic interleukin-6 (IL-6) overexpression on the skeletons of growing prepubertal mice.

METHODS

We studied IL-6-transgenic mice that had high circulating IL-6 levels since birth. Trabecular and cortical bone structure were analyzed by microcomputed tomography. Epiphyseal ossification, growth plates, and calvariae were studied by histology/histomorphometry. Osteoclastogenesis, osteoblast function/differentiation, and the effects of IL-6 on bone cells were studied in vitro. Osteoblast gene expression was evaluated by reverse transcriptase-polymerase chain reaction. The mineral apposition rate was evaluated dynamically in cortical bone by in vivo double fluorescence labeling.

RESULTS

In prepubertal IL-6-transgenic mice, we observed osteopenia, with severe alterations in cortical and trabecular bone microarchitecture, as well as uncoupling of bone formation from resorption, with decreased osteoblast and increased osteoclast number and activity. Increased osteoclastogenesis and reduced osteoblast activity, secondary to decreased precursor proliferation and osteoblast function, were present. IL-6-transgenic mice also showed impaired development of growth plates and epiphyseal ossification centers. Intramembranous and endochondral ossification and the mineral apposition rate were markedly affected, showing the presence of defective ossification.

CONCLUSION

Chronic overexpression of IL-6 alone induces a skeletal phenotype closely resembling growth and skeletal abnormalities observed in children with chronic inflammatory diseases, pointing to IL-6 as a pivotal mediator of the impact of chronic inflammation on postnatal skeletal development. We hypothesize that IL-6-modifying drugs may reduce skeletal defects and prevent the growth retardation associated with these diseases.

Nonsurgical management of osteolysis: challenges and opportunities

Osteolysis remains a common mode of total hip arthroplasty failure. In vitro and animal models have been used to determine the pathophysiology of osteolysis by carefully dissecting the biochemical pathways leading to particulate wear debris and periprosthetic bone loss. Numerous cytokines and inflammatory mediators, including TNF-alpha and IL-1, are critical participants in this cascade and may represent prime targets for pharmacologic intervention. Osteoclasts, the end effector cells involved in the osteolytic process, also represent potential targets. Cell surface receptors on osteoclast precursors, such as receptor activator of NF-kappaB (RANK) (on osteoclasts) and RANK-ligand (RANKL) (on stromal cells), provide opportunities to arrest osteoclast maturation. Enhancing the naturally occurring osteoprotegerin is another recent attempt at modulating osteoclast behavior and a possible target for pharmacologic therapies. Other nonoperative strategies include intercepting tumor necrosis factor-alpha activity, interfering with the RANK-RANKL interaction necessary for osteoclast development and maturation, bisphosphonate therapy, and using viral vectors to deliver genes. Although each of these approaches has potential benefits, there are substantial challenges to effective implementation. Until there is convincing evidence of efficacy in human clinical trials, we recommend vigilant screening and appropriate surgery with component loosening or substantial likelihood of loosening, periprosthetic fracture, or major bone loss.

Cytokines regulating osteoclast formation and function

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Concise Review: Embryonic Stem Cells: A New Tool to Study Osteoblast and Osteoclast Differentiation

Bone remodeling involves synthesis of organic matrix by osteoblasts and bone resorption by osteoclasts. A tight collaboration between these two cell types is essential to maintain a physiological bone homeostasis. Thus, osteoblasts control bone-resorbing activities and are also involved in osteoclast differentiation. Any disturbance between these effectors leads to the development of skeletal abnormalities and/or bone diseases. In this context, the determination of key genes involved in bone cell differentiation is a new challenge to treat any skeletal disorders. Different models are used to study the differentiation process of these cells, but all of them use pre-engaged progenitor cells, allowing us to study only the latest stages of the differentiation. Embryonic stem (ES) cells come from the inner mass of the blastocyst prior its implantation to the uterine wall. Because of their capacity to differentiate into all germ layers, and so into all tissues of the body, ES cells represent the best model by which to study earliest stages of bone cell differentiation. Osteoblasts are generated by two methods, one including the generation of embryoid body, the other not. Mineralizing cells are obtained after 2 weeks of culture and express all the specific osteoblastic markers (alkaline phosphatase, type I collagen, osteocalcin, and others). Osteoclasts are generated from a single-cell suspension of ES cells seeded on a feeder monolayer, and bone-resorbing cells expressing osteoclastic markers such as tartrate-resistant alkaline phosphatase or receptor activator of nuclear factor kappaB are obtained within 11 days. The aim of this review is to present recent discoveries and advances in the differentiation of both osteoblasts and osteoclasts from ES cells.

Modulation of osteoclastogenesis induced by nucleoside reverse transcriptase inhibitors

Osteopenia is a common and debilitating side-effect of HAART, yet little is known concerning the effects of HAART on bone metabolism. We reported previously that zidovudine (AZT) stimulates osteoclastogenesis in vitro and causes osteopenia in mice. Here, we confirmed that the AZT-induced osteoclastogenesis is dependent on RANKL in that osteoclastogenesis is blocked by osteoprotegestin. Alendronate, which is used for the treatment of osteopenia and osteoporosis, failed to inhibit AZT-induced osteoclastogenesis in vitro. Osteoclastogenesis in vitro was not affected by tumor necrosis factor-alpha. Two other NRTI drugs, ddl and 3TC, also induced osteoclastogenesis in vitro and induced osteopenia in mice. The osteopenia was associated with an elevation of parameters of osteoclasts, but not with osteoblasts. Combinations of the NRTIs did not result in additive or synergistic effects in vitro or in vivo. Finally, AZT induced osteoclastogenesis of human osteoclast precursors in a RANKL-dependent manner. This in vitro osteoclastogenesis assay using human peripheral blood mononuclear cells could be useful in evaluating bone turnover and the risk of developing osteopenia in AIDS patients on HAART.

MKK3/6-p38 MAPK signaling is required for IL-1beta and TNF-alpha-induced RANKL expression in bone marrow stromal cells

Coupled bone turnover is directed by the expression of receptor-activated NF-kappaB ligand (RANKL) and its decoy receptor, osteoprotegerin (OPG). Proinflammatory cytokines, such as interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) induce RANKL expression in bone marrow stromal cells. Here, we report that IL-1beta and TNF-alpha-induced RANKL requires p38 mitogen-activating protein kinase (MAPK) pathway activation for maximal expression. Real-time PCR was used to assess the p38 contribution toward IL-1beta and TNF-alpha-induced RANKL mRNA expression. Steady-state RANKL RNA levels were increased approximately 17-fold by IL-1beta treatment and subsequently reduced approximately 70%-90% when p38 MAPK was inhibited with SB203580. RANKL mRNA stability data indicated that p38 MAPK did not alter the rate of mRNA decay in IL-1beta-induced cells. Using a RANKL-luciferase cell line receptor containing a 120-kB segment of the 5' flanking region of the RANKL gene, reporter expression was stimulated 4-5-fold by IL-1beta or TNF-alpha treatment. IL-1beta-induced RANKL reporter expression was completely blocked with specific p38 inhibitors as well as dominant negative mutant constructs of MAPK kinase-3 and -6. In addition, blocking p38 signaling in bone marrow stromal cells partially inhibited IL-1beta and TNF-alpha-induced osteoclastogenesis in vitro. Results from these studies indicate that p38 MAPK is a major signaling pathway involved in IL-1beta and TNF-alpha-induced RANKL expression in bone marrow stromal cells.

PLCgamma2 regulates osteoclastogenesis via its interaction with ITAM proteins and GAB2

Excessive bone loss in arthritic diseases is mostly due to abnormal activation of the immune system leading to stimulation of osteoclasts. While phospholipase Cgamma (PLCgamma) isoforms are known modulators of T and B lymphocyte-mediated immune responses, we found that blockade of PLCgamma enzymatic activity also blocks early osteoclast development and function. Importantly, targeted deletion of Plcg2 in mice led to an osteopetrotic phenotype. PLCgamma2, independent of PLCgamma1, was required for receptor activator of NF-kappaB ligand-induced (RANKL-induced) osteoclastogenesis by differentially regulating nuclear factor of activated T cells c1 (NFATc1), activator protein-1 (AP1), and NF-kappaB. Specifically, we show that NFATc1 upregulation is dependent on RANKL-mediated phosphorylation of PLCgamma2 downstream of Dap12/Fc receptor gamma (Dap12/FcRgamma) receptors and is blocked by the PLCgamma inhibitor U73122. In contrast, activation of JNK and NF-kappaB was not affected by U73122 or Dap12/FcRgamma deletion. Interestingly, we found that in osteoclasts, PLCgamma2 formed a complex with the regulatory adapter molecule GAB2, was required for GAB2 phosphorylation, and modulated GAB2 recruitment to RANK. Thus, PLCgamma2 mediates RANKL-induced osteoclastogenesis and is a potential candidate for antiresorptive therapy.

A p38alpha selective mitogen-activated protein kinase inhibitor prevents periodontal bone loss

In the oral microbial environment, Gram-negative bacterial derived lipopolysaccharide (LPS) can initiate inflammatory bone loss as seen in periodontal diseases. p38 Mitogen-activated protein kinase (MAPK) signaling is critical to inflammatory cytokine and LPS-induced cytokine expression, which may contribute toward periodontal bone loss. The purpose of this proof-of-principle study was to evaluate the ability of an orally active p38alpha MAPK inhibitor (SD-282) to reduce periopathogenic LPS-induced alveolar bone loss in an experimental rat model. Five groups of Sprague-Dawley rats received one of the following treatments: LPS injected to the palatal gingiva adjacent to the maxillary molars three times per week for 8 weeks, LPS plus two doses of SD-282 (15 or 45 mg/kg) twice daily by oral gavage, or control groups given drug vehicle (1% polyethylene glycol) or SD-282 (45 mg/kg) only. Baseline and 8-week alveolar bone loss was assessed by microcomputed tomography (microCT) and histological examination. LPS induced severe bone loss over this time period, whereas control groups were unchanged from baseline measurements. Both doses of SD-282 showed significant protection from LPS-induced bone loss. Bone area and volumetric analysis of maxillas by microCT indicated significant loss of bone volume with LPS treatment, which was blocked with the p38 inhibitor. Histological examination indicated significantly fewer tartate-resistant acid phosphatase-positive osteoclasts and a significant decrease in interleukin (IL)-6, IL-1beta, and tumor necrosis factor alpha expression in p38 inhibitor-treated groups compared with LPS groups by immunostaining. Results from this in vivo study suggest that orally active p38 MAPK inhibitors can reduce LPS-induced inflammatory cytokine production and osteoclast formation and protect against LPS-stimulated alveolar bone loss.

Soluble RANKL in Crevicular Fluid of Dental Implants: A Pilot Study

BACKGROUND

Receptor activator of NF-kappaB ligand (RANKL), a member of the tumor necrosis factor superfamily, is a key mediator of osteoclast formation, activation, and survival. Thus, it is reasonable to hypothesize that there might be a functional relationship between RANKL expression and peri-implantitis.

PURPOSE

This pilot study was performed to determine the reference levels for soluble RANKL (sRANKL) in peri-implant crevicular fluid and to correlate them with the clinical parameters associated with inflammatory reactions and bone destruction.

MATERIALS AND METHODS

The clinical parameters probing depth (PD), modified bleeding index (MBI), and modified plaque index (MPI) served as indicators for bone resorption and inflammation. Exclusion criteria for calculations were the detection limit of the immunoassay and the minimum acceptable crevicular volume for measurement. From the 84 collected samples of 16 patients, 30-84 years of age, with a total of 19 implants, 29 met these criteria. The absolute amount of sRANKL within crevicular fluid adsorbed to filter strips was a median of 0.18 femtomol (fmol; range, 0.08-0.53) and 0.26 nM (range, 0.09-1.21) when normalized by volume. PD was 4 mm in median and varied within a range between 2 and 12 mm.

RESULTS

Absolute amounts of sRANKL showed no correlation with the adsorbed volume and the clinical parameters PD, MBI, and MPI. When sRANKL was normalized by volume, no correlation with the clinical parameters PD, MBI, and MPI was observed either. The patients' age was not associated with total sRANKL and the concentration of RANKL within crevicular fluid. Absolute levels of sRANKL and sRANKL concentration did not show any differences based on the sampling sites buccal and lingual, or on the patients' gender. A significant difference in sRANKL concentration was detectable when samples from maxillary implants (0.31 nM median; range, 0.12-1.21) were compared with samples from mandibular implants (0.21 nM median; range, 0.09-0.6) (p=.03). Absolute levels of sRANKL were not different between the maxilla and the mandible.

CONCLUSION

Given the limited sample size, our data provide a basis for future prospective longitudinal studies on the possible relevance of sRANKL as a prognostic marker in peri-implantitis, and for an understanding of the pathophysiologic process of the disease as a prerequisite for the design of treatment strategies.

B and T lymphocytes are the primary sources of RANKL in the bone resorptive lesion of periodontal disease

Receptor activator of nuclear factor-kappaB (RANKL)-mediated osteoclastogenesis plays a pivotal role in inflammatory bone resorption. The aim of this study was to identify the cellular source of RANKL in the bone resorptive lesions of periodontal disease. The concentrations of soluble RANKL, but not its decoy receptor osteoprotegerin, measured in diseased tissue homogenates were significantly higher in diseased gingival tissues than in healthy tissues. Double-color confocal microscopic analyses demonstrated less than 20% of both B cells and T cells expressing RANKL in healthy gingival tissues. By contrast, in the abundant mononuclear cells composed of 45% T cells, 50% B cells, and 5% monocytes in diseased gingival tissues, more than 50 and 90% of T cells and B cells, respectively, expressed RANKL. RANKL production by nonlymphoid cells was not distinctly identified. Lymphocytes isolated from gingival tissues of patients induced differentiation of mature osteoclast cells in a RANKL-dependent manner in vitro. However, similarly isolated peripheral blood B and T cells did not induce osteoclast differentiation, unless they were activated in vitro to express RANKL; emphasizing the osteoclastogenic potential of activated RANKL-expressing lymphocytes in periodontal disease tissue. These results suggest that activated T and B cells can be the cellular source of RANKL for bone resorption in periodontal diseased gingival tissue.

Direct and indirect estrogen actions on osteoblasts and osteoclasts

Cells of osteoblast and osteoclast lineage are provided with the receptor for sex steroids, but discrepancies concerning mechanism of action still exist. Skeletal estrogen (ER) agonists induce osteoblastic osteoprotegerin (OPG) production through ER receptor-alpha activation in vitro, while immune cells appear to overexpress RANKL in ER deficiency in vivo, not reproduced in in vitro study. It has also been evident that the effects of ER on bone to a large extent are mediated via its action on immune cells. We know now that ER regulates the expression of cytokines that target cell types involved in modulating bone turnover, as IL-1 and IL-6, and the latest findings confirm and expand the concept that T cells are key mediators of bone loss following gonadal failure. Although early work demonstrated that tumor necrosis factor-alpha plays an important role in regulating bone mass, recent studies also implicate the lymphopoietic molecule IL-7: it suppresses the bone-forming osteoblasts, while stimulating formation and function of osteoclasts. More recent in vitro studies, however, indicate a stimulating effect of ER on osteoclastogenesis, which could have a positive effect on maintaining a high level of bone cell activity.

A crucial role for interleukin (IL)-1 in the induction of IL-17-producing T cells that mediate autoimmune encephalomyelitis

It was recently demonstrated that interleukin (IL)-23–driven IL-17–producing (ThIL-17) T cells mediate inflammatory pathology in certain autoimmune diseases. We show that the induction of antigen-specific ThIL-17 cells, but not T helper (Th)1 or Th2 cells, by immunization with antigens and adjuvants is abrogated in IL-1 receptor type I–deficient (IL-1RI^−/−) mice. Furthermore, the incidence of experimental autoimmune encephalomyelitis (EAE) was significantly lower in IL-1RI^−/− compared with wild-type mice, and this correlated with a failure to induce autoantigen-specific ThIL-17 cells, whereas induction of Th1 and Th2 responses was not substantially different. However, EAE was induced in IL-1RI^−/− mice by adoptive transfer of autoantigen-specific cells from wild-type mice with EAE. IL-23 alone did not induce IL-17 production by T cells from IL-1RI^−/− mice, and IL-23–induced IL-17 production was substantially enhanced by IL-1α or IL-1β, even in the absence of T cell receptor stimulation. We demonstrate essential roles for phosphatidylinositol 3-kinase, nuclear factor κB, and novel protein kinase C isoforms in IL-1– and IL-23–mediated IL-17 production. Tumor necrosis factor α also synergized with IL-23 to enhance IL-17 production, and this was IL-1 dependent. Our findings demonstrate that IL-1 functions upstream of IL-17 to promote pathogenic ThIL-17 cells in EAE.

Map kinase c-JUN N-terminal kinase mediates PMMA induction of osteoclasts

Erosive osteolysis induced by implant-derived wear debris is mediated by recruitment and activation of osteoclasts in a pro-inflammatory microenvironment that is enriched with osteoclastogenic and pro-inflammatory cytokines such as RANKL and tumor necrosis factor alpha (TNF-alpha). These cytokines activate the transcription factor NF-kappaB and MAP kinases, including c-Jun, Erks, and p38, all known to be essential for the development of osteoclasts. We have recently documented that TNF and RANKL play a pivotal role in the development of inflammatory osteolysis. We have also found that polymethyl methacrylate (PMMA) particles stimulate osteoclastogenesis, at least in part, by induction of RANKL, TNF, and by activation of NF-kappaB and MAP kinases. More importantly, our data indicate that inhibitors of NF-kappaB and the MAP kinases p38 and ERK abrogate particle-induced osteoclastogenesis. In the current study, we investigated if inhibition of c-Jun N-Terminal kinase (JNK) pathway alters PMMA-induced osteoclastogenesis. Our findings point out that PMMA particles activate the JNK pathway in wild-type and TLR4-null (endotoxin-resistant) osteoclast precursors. This activation was selectively blocked in a dose-dependent fashion by the JNK inhibitor SP600125. Most importantly, we provide evidence that SP600125 inhibits osteoclast formation in a reversible manner. Collectively, our findings demonstrate that activation of the JNK pathway is essential for basal and PMMA-stimulated osteoclastogenesis, and buttress the potential significance of targeting the JNK pathway to inhibit osteolysis.

RANKL-stimulated osteoclast-like cell formation in vitro is partially dependent on endogenous interleukin-1 production

Receptor activator of NF-kappaB ligand (RANKL) and interleukin-1 (IL-1) individually plays a critical role in the differentiation and activation of osteoclasts in bone. In addition, both RANKL and IL-1 activate similar signal transduction pathways including p38 MAP kinase and c-Jun NH(2) terminal kinase (JNK). We examined if endogenously produced IL-1 influenced osteoclast-like cell (OCL) formation in murine bone marrow and bone marrow monocyte (BMM) cultures that were stimulated with M-CSF and RANKL. RANKL stimulated OCL formation in a dose-dependent manner in bone marrow cultures, and this response was significantly inhibited by IL-1 RA (100 ng/ml), a specific IL-1 antagonist. Interleukin-1 further increased OCL formation in BMM cultures that were treated with M-CSF (30 ng/ml) and RANKL (1, 3, 10 and 30 ng/ml). In addition, BMM cultures from IL-1 type I receptor-deficient mice, which do not respond to IL-1, demonstrated significantly less OCL formation compared to wild-type BMM cultures. We examined the time course and dose response of IL-1alpha protein expression by ELISA in BMM cultures that were treated with or without M-CSF and RANKL. RANKL dose dependently stimulated IL-1alpha protein significantly (up to 46%) in 6-day cultures. The interaction of RANKL and IL-1 on osteoclastogenesis did not appear significantly dependent on prostaglandin synthesis since PGE(2) expression in the conditioned medium of BMM cultures was nearly undetectable and the PGHS-2 specific inhibitor, NS-398, was without effect. We also investigated the effect of IL-1 on p38 MAP kinase and JNK in BMM cultures. The combination of RANKL and IL-1 had additive effects on JNK but not p38 MAP kinase compared to results in cultures treated with RANKL or IL-1 alone. In addition, SP600125, a specific JNK inhibitor, markedly reduced OCL formation in BMM cultures that were treated with RANKL or the combination of RANKL and IL-1. These findings demonstrate that endogenously produced IL-1 augments the response of bone marrow cells to RANKL, and this effect appears mediated by mechanisms that are associated with enhancement of JNK activity.

Estrogen Regulation of Immune Cell Bone Interactions

Estrogen deficiency is one of the most frequent causes of osteoporosis in women and a possible cause of bone loss and insufficient skeletal development in men. Estrogen deficiency results from menopause but also by a number of conditions, such as stress, excessive physical activity, and low body weight. The mechanism by which estrogen deficiency causes bone loss remains largely unknown. Estrogen deficiency leads to an increase in the immune function, which culminates in an increased production of TNF by activated T cells. TNF increases osteoclast formation and bone resorption both directly and by augmenting the sensitivity of maturing osteoclasts to the essential osteoclastogenic factor RANKL. Increased T cell production of TNF is induced by estrogen deficiency via a complex mechanism mediated by antigen-presenting cells and involving the cytokines IFN-gamma, IL-7, and TGF-beta. Herein we review the experimental evidence that suggests that estrogen prevents bone loss by regulating T cell function and immune cell bone interactions.

Pathogenesis of osteopenia/osteoporosis induced by highly active anti-retroviral therapy for AIDS

The advent of highly active anti-retroviral therapy (HAART) has dramatically decreased the rate of AIDS-related mortality and significantly extended the life span of patients with AIDS. A variety of metabolic side effects are associated with these therapies, one of which is metabolic bone disease. A higher prevalence of osteopenia and osteoporosis in HIV-infected patients receiving anti-retroviral therapy than in patients not on therapy has now been reported in several studies. Several factors have been demonstrated to influence HIV-associated decreases in bone mineral density (BMD), including administration of nucleoside reverse transcriptase inhibitors (NRTIs). In this article, discussion will focus on the molecular pathogenesis and treatment of HAART-associated osteopenia and osteoporosis.

Cathepsin K levels in the crevicular fluid of dental implants: a pilot study

OBJECTIVE

To determine the concentration of cathepsin K secreted into the crevicular fluid around dental implants and its correlation with clinical parameters of healthy implants and implants showing clinical signs of peri-implantitis.

MATERIAL AND METHODS

Nineteen patients with 40 implants with and without peri-implantitis were enrolled in the study. Peri-implantitis was diagnosed by the pocket probing depth (PD), the modified bleeding index (MBI), the modified plaque index (MPI) and by radiographic signs of bone loss. Gingival crevicular fluid collected from the buccal and lingual sites was adsorbed to filter strips. Cathepsin K levels and total protein within the crevicular fluid were determined by immunoassay and the bicinchoninic method, respectively.

RESULTS

Cathepsin K per filter strip normalized to the time of collection was 10.1 (0-33.5) pmol/sample around control implants and 22.4 (3.7-56.3) pmol/sample in the peri-implantitis group. The difference between the medians was significant (p < 0.01). Absolute cathepsin K levels in the crevicular fluid of all implants investigated showed a positive correlation with PD (R = 0.25; p = 0.03), MPI (R = 0.28; p = 0.01) and MBI (R = 0.32; p < 0.01). Absolute cathepsin K levels in the crevicular fluid also correlated with the adsorbed volume of gingival crevicular fluid (R = 0.51; p < 0.01). When normalized to the adsorbed volume of gingival crevicular fluid, the concentration of cathepsin K was 2.2 (0.01-6.4) nM around control implants and 1.7 (0.4-4.6) nM in the peri-implantitis group (p = 0.33). Patients' age correlated with sample volume and with cathepsin K normalized to the adsorbed volume of gingival crevicular fluid (R = 0.39; p < 0.01). Moreover, significant differences between male and female (p < 0.01, p < 0.01), and between mandible and maxilla (p < 0.05, p < 0.01), but not between buccal and lingual sites (p = 0.99, p= 0.93), were observed when analysed for the parameters adsorbed volume and absolute cathepsin K levels.

CONCLUSION

Clinical parameters of peri-implantitis are associated with a higher amount of cathepsin K and a higher volume adsorbed to filters strips. To establish cathepsin K as a biochemical parameter to monitor peri-implant tissue health, age, sex and collection site should be considered to avoid interfering influences because of sample inhomogenity. Also a prospective study over time including more patients would be necessary.

-511 C/T IL1B gene polymorphism is associated to resistance to bisphosphonates treatment in Paget disease of bone

INTRODUCTION

Osteoclasts are the most important cells involved in the pathogenesis of Paget disease of bone (PDB). Cytokines stimulate osteoclast differentiation and activation, with some of them over-expressed in pagetic osteoclasts. We have assessed whether genetic variability in genes coding of proteins from the IL1 pathway clustered in chromosome 2 is associated with clinical characteristics and the therapeutic response of patients with PDB.

METHODS

We have studied -511 C/T and +3953 T/C polymorphisms of the IL1B gene, a HinfI polymorphism in the 5'UTR of the IL1R1 gene, and a variable number of tandem repeats (VNTR) in the intron 2 of the IL1RN gene, in 165 patients diagnosed as suffering from PDB and in 122 healthy controls. Distribution of genotypes and alleles was studied for association with clinical and laboratory data and response to bisphosphonate (BSP) treatment.

RESULTS

No differences were observed in the distribution of genotypes or alleles between PDB patients and control subjects. We also failed to detect differences concerning epidemiological, clinical and laboratory data in the series of PDB patients. However, the -511 CC genotype of the IL1B gene was associated with a higher percentage of resistance to BSP (49% vs. 20%; P = 0.00 for all BSP, 60% vs. 39%, P = 0.17 for etidronate, 50% vs. 37% P = 0.53 for clodronate, 48 vs. 34% P = 0.05 for tiludronate and 50% vs. 4% P = 0.01 for risedronate).

CONCLUSIONS

Our results suggest that the -511 CC genotype of the IL1B gene could be related to resistance to bisphosphonates in patients with PDB.

Bone loss in inflammatory arthritis: mechanisms and therapeutic approaches with bisphosphonates

The inflammatory process in rheumatoid arthritis provokes intense bone resorption, evidenced as bone erosions, juxta-articular osteopenia and generalized osteoporosis. These types of bone loss share a common pathogenesis, and the role of osteoclasts in focal bone erosion was verified in elegant animal studies. The tumour necrosis factor (TNF) family of cytokines and receptors--specifically TNF-alpha, RANKL, RANK and OPG--are dominant regulators of osteoclastic bone resorption in rheumatoid arthritis. The confirmation of the osteoclast mechanism provides new insight into the structural joint protection afforded by disease-modifying drugs and suggests innovative approaches to limit bone destruction. Emerging treatment strategies for bone disease in rheumatoid arthritis are the use of monoclonal antibodies to neutralize RANKL, and powerful bisphosphonates that target pathogenic osteoclasts.

Peripheral cannabinoid receptor, CB2, regulates bone mass

The endogenous cannabinoids bind to and activate two G protein-coupled receptors, the predominantly central cannabinoid receptor type 1 (CB1) and peripheral cannabinoid receptor type 2 (CB2). Whereas CB1 mediates the cannabinoid psychotropic, analgesic, and orectic effects, CB2 has been implicated recently in the regulation of liver fibrosis and atherosclerosis. Here we show that CB2-deficient mice have a markedly accelerated age-related trabecular bone loss and cortical expansion, although cortical thickness remains unaltered. These changes are reminiscent of human osteoporosis and may result from differential regulation of trabecular and cortical bone remodeling. The CB2(-/-) phenotype is also characterized by increased activity of trabecular osteoblasts (bone-forming cells), increased osteoclast (the bone-resorbing cell) number, and a markedly decreased number of diaphyseal osteoblast precursors. CB2 is expressed in osteoblasts, osteocytes, and osteoclasts. A CB2-specific agonist that does not have any psychotropic effects enhances endocortical osteoblast number and activity and restrains trabecular osteoclastogenesis, apparently by inhibiting proliferation of osteoclast precursors and receptor activator of NF-kappaB ligand expression in bone marrow-derived osteoblasts/stromal cells. The same agonist attenuates ovariectomy-induced bone loss and markedly stimulates cortical thickness through the respective suppression of osteoclast number and stimulation of endocortical bone formation. These results demonstrate that the endocannabinoid system is essential for the maintenance of normal bone mass by osteoblastic and osteoclastic CB2 signaling. Hence, CB2 offers a molecular target for the diagnosis and treatment of osteoporosis, the most prevalent degenerative disease in developed countries.

The role of T lymphocytes in bone metabolism

Recent findings from animal models suggest that the bone loss induced by estrogen deficiency may stem in large measure from a pathological upregulation of the adaptive immune response. While the role of activated T cells in the osteoporosis driven by inflammatory conditions and infection has been well documented, only recently has the role of T cells in the bone destruction associated with estrogen deficiency begun to be appreciated. In vivo and in vitro models of postmenopausal osteoporosis demonstrate that estrogen deficiency leads to an increase in the adaptive immune function that culminates in an increased production of tumor necrosis factor alpha (TNF) by activated T cells. TNF increases osteoclast (OC) formation and bone resorption both directly and by augmenting the sensitivity of maturing OCs to the essential osteoclastogenic factor receptor activator of nuclear factor kappaB ligand. The activation and expansion of TNF-producing T cells are key steps in estrogen deficiency-driven bone loss and are regulated by multiple interacting cytokines including transforming growth factor-beta, interleukin-7, and interferon-gamma, as well as by the process of antigen presentation. Herein, we review the experimental evidence that suggests estrogen prevents bone loss by regulating T-cell function and immune cell bone interactions.

Rheumatic diseases: the effects of inflammation on bone

Rheumatoid arthritis, juvenile idiopathic arthritis, the seronegative spondyloarthropathies including psoriatic arthritis, and systemic lupus erythematosus are all examples of rheumatic diseases in which inflammation is associated with skeletal pathology. Although some of the mechanisms of skeletal remodeling are shared among these diseases, each disease has a unique impact on articular bone or on the axial or appendicular skeleton. Studies in human disease and in animal models of arthritis have identified the osteoclast as the predominant cell type mediating bone loss in arthritis. Many of the cytokines and growth factors implicated in the inflammatory processes in rheumatic diseases have also been demonstrated to impact osteoclast differentiation and function either directly, by acting on cells of the osteoclast-lineage, or indirectly, by acting on other cell types to modulate expression of the key osteoclastogenic factor receptor activator of nuclear factor (NF) kappaB ligand (RANKL) and/or its inhibitor osteoprotegerin (OPG). Further elucidation of the mechanisms responsible for inflammation-induced bone loss will potentially lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases. In this review, we provide an overview of the cell types, inflammatory mediators, and mechanisms that are implicated in bone loss and new bone formation in inflammatory joint diseases.

The role of the immune system in the pathophysiology of osteoporosis

The role of the immune system in the development of senile osteoporosis, which arises primarily through the effects of estrogen deficiency and secondary hyperparathyroidism, is slowly being unraveled. This review focuses on our current understanding of how the components of this complex-interlinked system are regulated and how these fit with previous models of senile and postmenopausal osteoporosis. There is certainly substantial evidence that bone remodeling is a tightly regulated, finely balanced process influenced by subtle changes in proinflammatory and inhibitory cytokines as well as hormones and cellular components that act primarily but not exclusively through the receptor activator of nuclear factor-kappaB (RANK)/RANK ligand/osteoprotegerin system. In addition, an acute or chronic imbalance in the system due to infection or inflammation could contribute to systemic (or local) bone loss and increase the risk of fracture. Although significant progress has been made, there remains much to be done in unraveling this complex interaction between the immune system and bone.

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.

Bone health in pediatric rheumatic disease

PURPOSE OF REVIEW

Children with chronic rheumatic disease have decreased bone mass. In adults, lowered bone mineral density is associated with increased fracture risk. This morbidity is undetermined in pediatric rheumatic disease as osteoporosis has not been well-defined in children. This review compares methods for determining bone mass in children, examines insights into molecular mechanisms of bone metabolism, and discusses the prevention and treatment of decreased bone mass in children.

RECENT FINDINGS

Peak bone mass, attained during adolescence and early adulthood, is critical in determining fracture risk. Studies of children with chronic rheumatic disease demonstrate decreased bone mineral density, and potentially lowered peak bone mass. Dual energy x-ray absorptiometry is the most commonly used technique for monitoring bone mineral density and should be interpreted utilizing age-appropriate Z-scores. Recent studies suggest quantitative ultrasound may be as reliable as dual energy x-ray absorptiometry and lacks radiation exposure. The molecular mechanisms by which inflammation alters bone mineral density involve receptor activator of nuclear factor kappaB ligand (RANKL)/osteoprotegerin, tumor necrosis factor-alpha, and interleukin-1. Limited data on the use of bisphosphonates and calcitonin in children suggest they are safe and effective, but should be used cautiously.

SUMMARY

Children with chronic rheumatic disease should have bone mass monitored by dual energy x-ray absorptiometry Z-scores. Targeting the RANKL/osteoprotegerin pathway may lead to therapies that improve bone health in this population. More studies on the role of bisphosphonates and calcitonin must be pursued to establish guidelines for use in pediatric patients with chronic rheumatic disease. For now, supplemental calcium and vitamin D should be implemented in these children.

Bone disease and HIV infection

The high prevalence of bone demineralization among human immunodeficiency virus (HIV)-infected patients in the current therapeutic era has been described in multiple studies, sounding the alarm that we may expect an epidemic of fragility fractures in the future. However, despite noting high overall prevalences of osteopenia and osteoporosis, recent longitudinal studies that we review here have generally not observed accelerated bone loss during antiretroviral therapy beyond the initial period after treatment initiation. We discuss the continued progress toward understanding the mechanisms of HIV-associated bone loss, particularly the effects of HIV infection, antiretroviral therapy, and host immune factors on bone turnover. We summarize results of clinical trials published in the past year that studied the safety and efficacy of treatment of bone loss in HIV-infected patients and provide provisional opinions about who should be considered for bone disease screening and treatment.

Osteoporosis, inflammation and ageing

Osteoporosis is a condition characterized by low bone mass and increased bone fragility, putting patients at risk of fractures, which are major causes of morbidity substantially in older people. Osteoporosis is currently attributed to various endocrine, metabolic and mechanical factors. However, emerging clinical and molecular evidence suggests that inflammation also exerts significant influence on bone turnover, inducing osteoporosis. Numerous proinflammatory cytokines have been implicated in the regulation of osteoblasts and osteoclasts, and a shift towards an activated immune profile has been hypothesized as important risk factor. Chronic inflammation and the immune system remodelling characteristic of ageing, as well as of other pathological conditions commonly associated with osteoporosis, may be determinant pathogenetic factors. The present article will review the current perspectives on the interaction between bone and immune system in the elderly, providing an interpretation of osteoporosis in the light of inflamm-ageing.

Mechanisms of Disease: the link between RANKL and arthritic bone disease

Chronic inflammation and bone loss are closely linked pathophysiologic events. The most typical example of inflammatory bone loss is seen in patients with rheumatoid arthritis who develop systemic osteopenia as well as local breakdown of bone in the direct vicinity of inflamed joints. Understanding the mechanisms of arthritic bone degradation is crucial for designing therapies that can specifically protect joints from structural damage. Since osteoclast differentiation and activity are key events in arthritic bone damage, the signals that trigger osteoclastogenesis are potential therapeutic targets. Receptor activator of nuclear factor-kappaB (RANK) is activated by its ligand, RANKL, an essential molecule for osteoclast development: in the absence of RANKL or RANK, osteoclast differentiation from monocyte precursors does not occur. RANKL is expressed on T cells and fibroblasts within the synovial inflammatory tissue of patients with RA and its expression is regulated by proinflammatory cytokines. In animal models of arthritis, blockade of RANKL-RANK interactions, or a genetic absence of RANKL or RANK, protects against joint damage despite the presence of joint inflammation. Therefore, inhibition of RANKL is regarded as a promising future strategy for inhibiting inflammatory bone loss in patients with chronic inflammatory arthritis.

Role of the immune system in postmenopausal bone loss

Postmenopausal osteoporosis stems from estrogen deficiency. The mechanisms by which estrogen deficiency drives bone destruction are complex and poorly understood. Recent findings from animal models suggest that postmenopausal bone loss may stem in large measure from a pathologic upregulation of the adaptive immune response. While the role of activated T cells in the bone loss driven by inflammatory conditions such as rheumatoid arthritis has been well documented, only recently has the role of T cells in the bone destruction associated with estrogen deficiency begun to be appreciated. In vivo and in vitro models of postmenopausal osteoporosis demonstrate that the activation and expansion of tumor necrosis factor-a producing T cells is a key step in estrogen deficiency driven bone loss and is regulated by multiple interacting cytokines including transforming growth factor-b, interleukin-7, and interferon-g, as well as by the process of antigen presentation. This paper presents recent findings pertaining to this new view of postmenopausal osteoporosis.

Central IL-1 receptor signaling regulates bone growth and mass

The proinflammatory cytokine IL-1, acting via the hypothalamic IL-1 receptor type 1 (IL-1RI), activates pathways known to suppress bone formation such as the hypothalamo pituitary-adrenocortical axis and the sympathetic nervous system. In addition, peripheral IL-1 has been implicated as a mediator of the bone loss induced by sex hormone depletion and TNF. Here, we report an unexpected low bone mass (LBM) phenotype, including impairment of bone growth, in IL-1RI-deficient mice (IL-1rKO mice). Targeted overexpression of human IL-1 receptor antagonist to the central nervous system using the murine glial fibrillary acidic protein promoter (IL-1raTG mice) resulted in a similar phenotype, implying that central IL-1RI silencing is the causative process in the LBM induction. Analysis of bone remodeling indicates that the process leading to the LBM in both IL-1rKO and IL-1raTG is characterized mainly by doubling the osteoclast number. Either genetic modification does not decrease testosterone or increase corticosterone serum levels, suggesting that systems other than the gonads and hypothalamo pituitary-adrenocortical axis mediate the central IL-1RI effect on bone. We further demonstrate that WT mice express mouse IL-1ra in bone but not in the hypothalamus. Because low levels of IL-1 are present in both tissues, it is suggested that skeletal IL-1 activity is normally suppressed, whereas central IL-1 produces a constant physiologic stimulation of IL-1RI signaling. Although the pathway connecting the central IL-1RI signaling to bone remodeling remains unknown, the outburst of osteoclastogenesis in its absence suggests that normally it controls bone growth and mass by tonically restraining bone resorption.