Colonic dendritic cells, intestinal inflammation, and T cell-mediated bone destruction are modulated by recombinant osteoprotegerin

Role of RANK ligand and denosumab, a targeted RANK ligand inhibitor, in bone health and osteoporosis: a review of preclinical and clinical data

BACKGROUND

Postmenopausal osteoporosis results from bone loss and decreased bone strength mediated by an increased rate of bone remodeling secondary to reduced estrogen levels. Remodeling cycles are initiated by osteoclasts, the formation, function, and survival of which depend on RANK ligand (RANKL). RANKL inhibition therefore represents a novel strategy for reducing remodeling and its effects on fracture risk.

OBJECTIVES

The goal of this study was to review the preclinical and clinical evidence supporting the value of RANKL inhibition in conditions of bone loss and to provide the rationale for the use of the fully human antibody denosumab, a RANKL inhibitor, in such conditions.

METHODS

We searched PubMed from January 2005 to May 2011 using the following terms: RANK Ligand, RANKL, denosumab, and NOT cancer, metastatic bone, or rheumatoid in the title.

RESULTS

The search method retrieved 111 articles. Preclinical evidence from several bone disease models suggests that RANKL inhibition leads to increased bone volume, density, and strength. Denosumab prevents RANKL from binding to its receptor, decreasing osteoclast activity and thereby reducing bone resorption and increasing cortical and trabecular bone mass and strength. It has consistently been reported to reduce bone turnover, increase bone density, and reduce the risk of fracture in clinical studies of postmenopausal women. Phase III head-to-head trials comparing denosumab with the bisphosphonate alendronate reported that denosumab was associated with significantly greater increases in bone density. Eczema as an adverse event and cellulitis as a serious adverse event were more common with denosumab than with placebo.

CONCLUSIONS

Preclinical studies defined the role of RANKL in bone remodeling and provided evidence for the therapeutic potential of RANKL inhibition in conditions of bone loss. Clinical studies evaluating RANKL inhibition with denosumab in postmenopausal women have reported significant reductions in vertebral, nonvertebral, and hip fractures, providing evidence compatible with the use of denosumab in postmenopausal women with osteoporosis.

Fine-tuning of dendritic cell biology by the TNF superfamily

Members of the tumour necrosis factor (TNF) superfamily have been implicated in a wide range of biological functions, and their expression by cells of the immune system makes them appealing targets for immunomodulation. One common theme for TNF superfamily members is their coordinated expression at the interface between antigen-specific T cells and antigen-presenting dendritic cells and, by virtue of this expression pattern, TNF superfamily members can shape T cell immune responses. Understanding how to manipulate such functions of the TNF superfamily may allow us to tip the balance between immunity and tolerance in the context of human disease.

Extraskeletal benefits and risks of calcium, vitamin D and anti-osteoporosis medications

Drugs used for the prevention and the treatment of osteoporosis exert various favourable and unfavourable extra-skeletal effects whose importance is increasingly recognized notably for treatment selection.

INTRODUCTION

The therapeutic armamentarium for the prevention and the treatment of osteoporosis is increasingly large, and possible extra-skeletal effects of available drugs could influence the choice of a particular compound.

METHODS

The present document is the result of a national consensus, based on a systematic and critical review of the literature.

RESULTS

Observational research has suggested an inverse relationship between calcium intake and cardiovascular diseases, notably through an effect on blood pressure, but recent data suggest a possible deleterious effect of calcium supplements on cardiovascular risk. Many diverse studies have implicated vitamin D in the pathogenesis of clinically important non-skeletal functions or diseases, especially muscle function, cardiovascular disease, autoimmune diseases and common cancers. The possible effects of oral or intravenous bisphosphonates are well-known. They have been associated with an increased risk of oesophageal cancer or atrial fibrillation, but large-scale studies have not found any association with bisphosphonate use. Selective oestrogen receptor modulators have demonstrated favourable or unfavourable extra-skeletal effects that vary between compounds. Strontium ranelate has a limited number of non-skeletal effects. A reported increase in the risk of venous thromboembolism is not found in observational studies, and very rare cases of cutaneous hypersensitivity reactions have been reported. Denosumab has been introduced recently, and its extra-skeletal effects still have to be assessed.

CONCLUSION

Several non-skeletal effects of bone drugs are well demonstrated and influence treatment choices.

Tetracyclines convert the osteoclastic-differentiation pathway of progenitor cells to produce dendritic cell-like cells

Tetracyclines, such as doxycycline and minocycline, are used to suppress the growth of bacteria in patients with inflammatory diseases. Tetracyclines have been shown to prevent bone loss, but the mechanism involved is unknown. Osteoclasts and dendritic cells (DCs) are derived from common progenitors, such as bone marrow-derived macrophages (BMMs). In this article, we show that tetracyclines convert the differentiation pathway, resulting in DC-like cells not osteoclasts. Doxycycline and minocycline inhibited the receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis of BMMs, but they had no effects on cell growth and phagocytic activity. They influenced neither the proliferation nor the differentiation of bone-forming osteoblasts. Surprisingly, doxycycline and minocycline induced the expression of DC markers, CD11c and CD86, in BMMs in the presence of RANKL. STAT5 is involved in DC differentiation induced by GM-CSF. Midostaurin, a STAT5-signaling inhibitor, and an anti-GM-CSF-neutralizing Ab suppressed the differentiation induced by GM-CSF but not by tetracyclines. In vivo, the injection of tetracyclines into RANKL-injected mice and RANKL-transgenic mice suppressed RANKL-induced osteoclastogenesis and promoted the concomitant appearance of CD11c(+) cells. These results suggested that tetracyclines prevent bone loss induced by local inflammation, including rheumatoid arthritis and periodontitis, through osteoclast-DC-like cell conversion.

Animal models of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic intestinal inflammatory condition that is medicated by genetic, immune, and environmental factors. At least 66 different kinds of animal models have been established to study IBD, which are classified primarily into chemically induced, cell-transfer, congenial mutant, and genetically engineered models. These IBD models have provided significant contributions to not only dissect the mechanism but also develop novel therapeutic strategies for IBD. In addition, recent advances on genetically engineered techniques such as cell-specific and inducible knockout as well as knockin mouse systems have brought novel concepts on IBD pathogenesis to the fore. Further, mouse models, which lack some IBD susceptibility genes, have suggested more complicated mechanism of IBD than previously predicted. This chapter summarizes the distinct feature of each murine IBD model and discusses the previous and current lessons from the IBD models.

Bone, inflammation, and inflammatory bowel disease

Osteoporosis is a leading cause of morbidity in patients with inflammatory bowel disease (IBD). Bone loss is an early systemic process and occurs even before clinical disease manifests. Bone disease is attributed to vitamin D deficiency, steroid use, and/or systemic inflammation. In this review, we discuss the molecular pathways of bone loss mediated by inflammatory cytokines and other mediators. Further research will hopefully clarify the mechanisms of inflammation-induced bone loss in IBD and guide effective treatment modalities.

Fecal osteoprotegerin may guide the introduction of second-line therapy in hospitalized children with ulcerative colitis

BACKGROUND

Osteoprotegerin (OPG) is increased in inflamed colonic mucosa and has a role in immune regulation and apoptosis resistance. Fecal OPG may be useful in predicting corticosteroid resistance in hospitalized children with severe ulcerative colitis (UC). We aimed to determine whether fecal OPG predicts the need for second-line therapies in children hospitalized for UC.

METHODS

We included 83 children with UC admitted for intravenous corticosteroid treatment. Children were classified as responders/nonresponders based on the need for therapy escalation. Fecal OPG results were compared with those of four other fecal markers.

RESULTS

Of the enrolled children, seven had day 1 samples only, 53 children had day 3 samples only, and 23 had both. Twenty-two children failed corticosteroid therapy and required infliximab (n = 20) or colectomy (n = 2). On the third treatment day the median fecal OPG levels were significantly higher in the nonresponders group compared with the responders: 77 pmol/L (interquartile range [IQR] 27-137) versus 13 pmol/L (3-109); P = 0.007. The best day 3 fecal OPG cutoff to predict second-line therapy was >50 pmol/L with a sensitivity of 71% and specificity of 69% (area under the receiver operator curve [ROC] of 0.70%-95% confidence interval [CI] 0.57-0.82). Fecal OPG was superior to day 3 fecal calprotectin, lactoferrin, and S100A12 as a predictor of corticosteroid nonresponse, but equivalent to the less commonly used M2-pyruvate kinase.

CONCLUSIONS

Day 3 fecal OPG may guide the decision to institute second-line therapy in children with severe UC. The role of OPG in the inflammatory response in pediatric UC deserves further study.

Osteoimmunology - Unleashing the concepts

Osteoimmunology is an emerging field of research dedicated to the relationship between the immune processes and the bone metabolism of various inflammatory bone diseases. The regulatory mechanisms governing the osteoclast and osteoblast are critical for understanding the health and disease of the skeletal system. These interactions are either by cell to cell contact or by the secretion of immune regulatory mediators like cytokines and chemokines by immune cells that are governed by the RANKL (TRANCE)-RANK- OPG axis. TRANCE-RANK signaling has served as a cornerstone of osteoimmunology research. There is increased recognition of the importance of the inflammatory and immune responses in the pathogenesis of periodontal disease. Thus, this field has provided a framework for studying the mechanisms underlying periodontal destruction. As bone homeostasis is mainly regulated by both the immune and endocrine systems, there emerged osteoimmunoendocrinology where adipokines take the lead. This review focuses on the underlying concepts of osteoimmunology, its relation to Periodontics.

Osteoporosis and inflammation

Several inflammatory diseases such as rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, celiac disease, cystic fibrosis and chronic obstructive pulmonary disease have been associated to bone resorption. The link between osteoclast, macrophage colony stimulating factor and pro-inflammatory cytokines, especially tumor necrosis factor-alpha and interleukin-1 explain the association between inflammation and osteoporosis. These diseases are related to osteoporosis and high fracture risk independent of other risk factors common to inflammatory diseases such as reduced physical activity, poor nutritional status, hypovitaminosis D, decrease in calcium intake and glucocorticoid treatment. Erythrocyte sedimentation rate and C-reactive protein should always be performed, but the indication about when to perform the densitometry test should be analyzed for each disease. Bisphosphonates are nowadays the best choice of therapy but new medications such as denosumab, IL-1 receptor antagonist, and TNF-alpha antibody have risen as new potential treatments for osteoporosis secondary to inflammation.

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.

Denosumab and bisphosphonates: different mechanisms of action and effects

To treat systemic bone loss as in osteoporosis and/or focal osteolysis as in rheumatoid arthritis or periodontal disease, most approaches target the osteoclasts, the cells that resorb bone. Bisphosphonates are currently the most widely used antiresorptive therapies. They act by binding the mineral component of bone and interfere with the action of osteoclasts. The nitrogen-containing bisphosphonates, such as alendronate, act as inhibitors of farnesyl-pyrophosphate synthase, which leads to inhibition of the prenylation of many intracellular signaling proteins. The discovery of RANKL and the essential role of RANK signaling in osteoclast differentiation, activity and survival have led to the development of denosumab, a fully human monoclonal antibody. Denosumab acts by binding to and inhibiting RANKL, leading to the loss of osteoclasts from bone surfaces. In phase 3 clinical studies, denosumab was shown to significantly reduce vertebral, nonvertebral and hip fractures compared with placebo and increase areal BMD compared with alendronate. In this review, we suggest that the key pharmacological differences between denosumab and the bisphosphonates reside in the distribution of the drugs within bone and their effects on precursors and mature osteoclasts. This may explain differences in the degree and rapidity of reduction of bone resorption, their potential differential effects on trabecular and cortical bone, and the reversibility of their actions.

Do RANKL inhibitors (denosumab) affect inflammation and immunity?

Receptor activator of nuclear factor kappa B ligand (RANKL) and its natural antagonist, osteoprotegerin (OPG), are, respectively, an indispensable factor and a potent inhibitor for osteoclast differentiation, activity, and survival. The development of a human monoclonal antibody to RANKL, denosumab, constitutes a novel approach to prevent fragility fractures in osteoporosis, skeletal complications of malignancy, and potentially bone erosions in rheumatoid arthritis (RA). In addition to being expressed by osteoblasts, RANKL is abundantly produced by activated T cells, and synoviocytes in RA, whereas its receptor, RANK, is also expressed by monocytes/macrophages and dendritic cells. However, in preclinical and clinical studies of RA-including patients with some degree of immunosuppression-RANKL inhibitors did not significantly alter inflammatory processes. RANKL, RANK, and OPG deficiency in murine models highlights the important role of this pathway in the development and maturation of the immune system in rodents, including functions of T and/or B cells, whereas OPG overexpression in mice and rats seems innocuous with regard to immunity. In contrast, loss-of-function mutations in humans have more limited effects on immune cells. In clinical studies, the overall rate of infections, cancer, and death was similar with denosumab and placebo. Nevertheless, the risk of severe infections and cancer in some specific tissues remains to be carefully scrutinized.

Advances in the understanding of mineral and bone metabolism in inflammatory bowel diseases

Chronic inflammatory disorders such as inflammatory bowel diseases (IBDs) affect bone metabolism and are frequently associated with the presence of osteopenia, osteoporosis, and increased risk of fractures. Although several mechanisms may contribute to skeletal abnormalities in IBD patients, inflammation and inflammatory mediators such as TNF, IL-1β, and IL-6 may be the most critical. It is not clear whether the changes in bone metabolism leading to decreased mineral density are the result of decreased bone formation, increased bone resorption, or both, with varying results reported in experimental models of IBD and in pediatric and adult IBD patients. New data, including our own, challenge the conventional views, and contributes to the unraveling of an increasingly complex network of interactions leading to the inflammation-associated bone loss. Since nutritional interventions (dietary calcium and vitamin D supplementation) are of limited efficacy in IBD patients, understanding the pathophysiology of osteopenia and osteoporosis in Crohn's disease and ulcerative colitis is critical for the correct choice of available treatments or the development of new targeted therapies. In this review, we discuss current concepts explaining the effects of inflammation, inflammatory mediators and their signaling effectors on calcium and phosphate homeostasis, osteoblast and osteoclast function, and the potential limitations of vitamin D used as an immunomodulator and anabolic hormone in IBD.

Pre-B cell colony enhancing factor/NAMPT/visfatin and its role in inflammation-related bone disease

Chronic inflammation affects bone metabolism and is commonly associated with the presence of osteoporosis. Bone loss is directed by various immune mediators such as the pro-inflammatory cytokines tumour necrosis factor-alpha, interleukin 1-beta or interferon-gamma. Pre-B cell colony enhancing factor (PBEF)/nicotinamide phosphoribosyl transferase (NAMPT)/visfatin is a pleiotropic mediator acting as growth factor, cytokine and enzyme involved in energy and nicotinamide adenine dinucleotide (NAD) metabolism. PBEF/NAMPT/visfatin has been recently demonstrated to exert several pro-inflammatory functions. We studied serum levels of PBEF/NAMPT/visfatin in patients with inflammatory bowel diseases (IBD) and their relation with bone mineral density (BMD). Furthermore, we were interested whether PBEF/NAMPT/visfatin affects osteoclastogenesis and involved mediators. PBEF/NAMPT/visfatin serum levels were increased in patients with IBD, correlated positively with disease activity and negatively with BMD, especially in the lumbar spine. Osteoclast precursor cells were generated from peripheral blood mononuclear cells after stimulation with various growth factors such as macrophage colony-stimulating factor (M-CSF) and soluble ligand of receptor activator of nuclear factor kappa B (RANK). In these in vitro studies, PBEF/NAMPT/visfatin suppressed osteoclastogenesis and inhibited the differentiation of osteoclast precursors into tartrate-resistant acid phosphatase positive multinucleated cells. These effects were paralleled by the suppression of the osteoclast typical markers RANK, nuclear factor of activated T-cells c1 (NFATc1) and cathepsin-K. This is the first report demonstrating a potential role for this important cytokine/enzyme in inflammation-related bone disease.

Exaggerated inflammatory response of primary human myeloid dendritic cells to lipopolysaccharide in patients with inflammatory bowel disease

Inflammatory bowel disease (IBD) results from a breakdown of tolerance towards the indigenous flora in genetically susceptible hosts. Failure of dendritic cells (DC) to interpret molecular microbial patterns appropriately when directing innate and adaptive immune responses is conceivable. Primary (conventional, non-monocyte generated) CD1c(+)CD11c(+)CD14(-)CD16(-)CD19(-) myeloid blood or mucosal dendritic cells (mDC) from 76 patients with Crohn's disease (CD) or ulcerative colitis (UC) in remission, during flare-ups (FU) and 76 healthy or non-IBD controls were analysed by fluorescence activated cell sorter (FACS) flow cytometry and real-time polymerase chain reaction. Cytokine secretion of freshly isolated, cultured and lipopolysaccharide (LPS)-stimulated highly purified mDC (purity >95%) was assessed using cytometric bead arrays (CBA). More cultured and stimulated circulating mDC express CD40 in IBD patients. Stimulated circulating mDC from IBD patients secrete significantly more tumour necrosis factor (TNF)-alpha and interleukin (IL)-8. Toll-like receptor (TLR)-4 expression by mDC was higher in remission and increased significantly in flaring UC and CD patients compared with remission (P < 0.05) and controls (P < 0.001). Fluorochrome-labelled LPS uptake by mDC was evaluated at different time-points over 24 h by measuring mean fluorescence intensity (MFI). Circulating mDC from IBD patients take up more LPS and the uptake begins earlier compared with controls (P < 0.05 in CD-FU and UC-FU at 24 h). The frequency of mucosal mDC (P < 0.05) and the number of CD40 expressing mucosal mDC is significantly greater in UC and CD compared with non-IBD controls (P < 0.001 versus P < 0.01, respectively). Our data suggest an aberrant LPS response of mDC in IBD patients, resulting in an inflammatory phenotype and possibly intestinal homing in acute flares.

Regulation of dendritic cell survival and cytokine production by osteoprotegerin

The TNF family ligand, RANKL, and its two TNFR family receptors, RANK and OPG, enable coordinated regulation between the skeletal and immune systems. Relatively little is known about how OPG influences RANKL-RANK interactions for the regulation of DCs. Here, we show that OPG KO bone marrow-derived DCs survive better and produce more TNF-alpha, IL-12p40, and IL-23 in response to Escherichia coli LPS than WT DCs. RANKL is induced on DCs within 24 h after LPS stimulation. OPG limits RANKL-RANK interactions between DCs, which can promote DC survival and elevated expression of proinflammatory cytokines. Survival of and cytokine production by OPG KO DCs are inhibited by soluble OPG; conversely, anti-OPG enhances survival and cytokine production by WT DCs. Bim KO DCs, like OPG KO, also survive longer and produce more TNF-alpha than WT DCs; however, unlike OPG KO, Bim KO DCs do not produce more IL-23. In addition, after inoculation with LPS, OPG KO mice produce more TNF-alpha and IL-12p40 than WT mice but not more IL-6. Thus, OPG regulates not only DC survival but also the nature of DC-dependent inflammatory responses.

Osteoimmunology: crosstalk between the immune and bone systems

INTRODUCTION

The interaction between the immune and skeletal systems has long been acknowledged, but investigation into rheumatoid arthritis (RA) as well as the various bone phenotypes found in immunocompromised gene-deficient mice has highlighted the importance of the dynamic interplay between the two systems. This has led to the recent emergence and subsequent rapid evolution of the field of osteoimmunology. BONE DESTRUCTION WITH ARTHRITIS AS A RANKL DISEASE: In the bone destruction associated with RA, IL-17-producing helper T cells (T(H)17) play a major role by inducing receptor activator of nuclear factor-kappaB ligand (RANKL). RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity.

NEW PLAYERS IN OSTEOIMMUNOLOGY

In addition to cellular interactions via cytokines, the immune and skeletal systems share various molecules, including transcription factors, signaling molecules, and membrane receptors.

CONCLUSION

The scope of osteoimmunology has grown to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases of both the immune and skeletal systems.

RANK-RANKL signaling pathway is critically involved in the function of CD4+CD25+ regulatory T cells in chronic colitis

It is now clear that functional CD4(+)CD25(+) regulatory T (T(R)) cells exist as part of the normal immune population and prevent the development of intestinal inflammation. We have recently shown that CD4(+)CD25(+) T(R) cells reside in the intestine and control intestinal homeostasis in humans and mice. In this study, we demonstrate that the TNF family molecule RANKL and its receptor RANK are critically involved in controlling the function of CD4(+)CD25(+) T(R) cells in the intestine. We first found that RANKL was preferentially expressed on both CD4(+)CD25(+) T(R) cells and colitogenic CD4(+) T cells, whereas RANK was expressed on dendritic cells. Although neutralizing anti-RANKL mAb did not affect T(R) activity of CD4(+)CD25(+) T(R) cells to suppress the proliferation of CD4(+) responder cells in vitro, in vivo administration of anti-RANKL mAb abrogated CD4(+)CD25(+) T(R) cell-mediated suppression of colitis induced by adoptive transfer of CD4(+)CD45RB(high) T cells into SCID mice. Interestingly, an adoptive transfer experiment using Ly5.1(+)CD4(+)CD45RB(high) cells and Ly5.2(+)CD4(+)CD25(+) T(R) cells revealed that the ratio of CD4(+)CD25(+) T(R) cells in total CD4(+) T cells in inflamed mucosa was significantly decreased by anti-RANKL mAb treatment. Consistent with this, the expression of RANK on lamina propria CD11c(+) cells from colitic mice was significantly increased as compared with that from normal mice, and in vitro treatment with anti-RANKL mAb suppressed the expansion of CD4(+)Foxp3(+) T(R) cells in culture with colitic lamina propria CD11c(+) cells. Together, these results suggest that the RANK-RANKL signaling pathway is critically involved in regulating the function of CD4(+)CD25(+) T(R) cells in colitis.

Inflammatory bowel disease causes reversible suppression of osteoblast and chondrocyte function in mice

Decreased bone density and stature can occur in pediatric patients with inflammatory bowel disease (IBD). Little is known about how IBD broadly impacts the skeleton. To evaluate the influence of an acute episode of IBD on growing bone, 4-wk-old mice were administered 5% dextran sodium sulfate (DSS) for 5 days to induce colitis and their recovery was monitored. During active disease and early recovery, trabecular bone mineral density, bone volume, and thickness were decreased. Cortical bone thickness, outer perimeter, and density were also decreased, whereas inner perimeter and marrow area were increased. These changes appear to maintain bone strength since measures of moments of inertia were similar between DSS-treated and control mice. Histological (static and dynamic), serum, and RNA analyses indicate that a decrease in osteoblast maturation and function account for changes in bone density. Unlike some conditions of bone loss, marrow adiposity did not increase. Similar to reports in humans, bone length decreased and correlated with decreases in growth plate thickness and chondrocyte marker expression. During disease recovery, mice experienced a growth spurt that led to their achieving final body weights and bone length, density, and gene expression similar to healthy controls. Increased TNF-alpha and decreased IGF-I serum levels were observed with active disease and returned to normal with recovery. Changes in serum TNF-alpha (increased) and IGF-I (decreased) paralleled changes in bone parameters and returned to normal values with recovery, suggesting a potential role in the skeletal response.

Rapid dendritic cell mobilization to the large intestinal epithelium is associated with resistance to Trichuris muris infection

The large intestine is a major site of infection and disease, yet little is known about how immunity is initiated within this site and the role of dendritic cells (DCs) in this process. We used the well-established model of Trichuris muris infection to investigate the innate response of colonic DCs in mice that are inherently resistant or susceptible to infection. One day postinfection, there was a significant increase in the number of immature colonic DCs in resistant but not susceptible mice. This increase was sustained at day 7 postinfection in resistant mice when the majority of the DCs were mature. There was no increase in DC numbers in susceptible mice until day 13 postinfection. In resistant mice, most colonic DCs were located in or adjacent to the epithelium postinfection. There were also marked differences in the expression of colonic epithelial chemokines in resistant mice and susceptible mice. Resistant mice had significantly increased levels of epithelium-derived CCL2, CCL3, CCL5, and CCL20 compared with susceptible mice. Furthermore, administering neutralizing CCL5 and CCL20 Abs to resistant mice prevented DC recruitment. This study provides clear evidence of differences in the kinetics of DC responses in hosts inherently resistant and susceptible to infection. DC responses in the colon correlate with resistance to infection. Differences in the production of DC chemotactic chemokines by colonic epithelial cells in response to infection in resistant vs susceptible mice may explain the different kinetics of the DC response.

Involvement of dendritic cells in the pathogenesis of inflammatory bowel disease

In conclusion, during inflammation, DCs are likely activated by inflammatory signals and induced to migrate to T cell zones of organized lymphoid tissues where the cells induce T cell responses. In addition to their established role in T cell priming and the induction of tolerance, DCs may act to enhance (or possibly suppress) T cell responses at sites of mucosal inflammation. Determining the importance of DCs in this regard, as well as establishing a potential role for DCs in continuous activation of naive or central memory cells in lymph nodes draining inflammatory sites, will elucidate the role of DCs as a potential therapeutic target for chronic inflammatory diseases, like IBD. Resident intestinal macrophages are noninflammatory and do not efficiently present antigens to intestinal T cells, yet are avidly phagocytic and able to kill internalized organisms. During intestinal inflammation, monocytes are recruited from the blood, become inflammatory macrophages in the inflamed tissue, and are major contributors to tissue destruction and perpetuation of inflammation via their production of chemokines and pro-inflammatory cytokines. Macrophages may also contribute directly to DC activation and maturation, which would drive DCs to present antigens from the bacterial flora to T cells locally within tissue or to more efficiently traffic to T cell zones of lymphoid tissue. Thus, DCs and macrophages have evolved functional niches that promote cooperation in the prevention of untoward intestinal inflammation in the steady state and in the eradication of invasive microorganisms during infection. The balance between suppressing inflammation and promoting host defense is altered in humans with IBD allowing a persistent inflammatory response to commensal bacteria. Based on studies from animal models, the pathogenesis of IBD likely involves either the lack of appropriate regulation from T cells, or an over-production of effector T cells. The end result of these potential mechanisms is the abnormal induction and/or survival of effector T cells and the production of factors such as cytokines by inflammatory macrophages and neutrophils that result in tissue destruction. The destructive process likely involves normally tolerizing DCs, which in the microenvironment of the inflamed mucosa activate T cell responses to normal flora in both draining lymphoid tissues and at sites of inflammation, with macrophages and neutrophils contributing the bulk of inflammatory and destructive cytokines.

A single-nucleotide polymorphism in the gene encoding osteoprotegerin, an anti-inflammatory protein produced in response to infection with diarrheagenic Escherichia coli, is associated with an increased risk of nonsecretory bacterial diarrhea in North American travelers to Mexico

BACKGROUND

Osteoprotegerin (OPG), an immunoregulatory member of the TNF receptor superfamily, is expressed in inflamed intestinal mucosa. We investigated whether OPG is produced by intestinal epithelial cells and tested the hypothesis that single-nucleotide polymorphisms (SNPs) in the gene encoding OPG (TNFRSF11B) are associated with traveler's diarrhea (TD) among North American travelers to Mexico.

METHODS

OPG concentration was measured in the supernatants of T84 cells infected with various diarrheagenic Escherichia coli pathotypes. Genotyping was performed for 4 SNPs in the OPG gene for 968 North American travelers with or without TD. Stool samples from travelers with TD were evaluated for the presence of enteric pathogens.

RESULTS

T84 cells produced higher OPG levels in response to infection with various diarrheagenic E. coli pathotypes than with E. coli controls (P<.05). A SNP in the exon 1 region of the OPG gene (OPG+1181G>C) was associated with TD in white travelers who stayed in Mexico for >1 week during the summer (P=.009) and for TD due to nonsecretory pathogens (P=.001).

CONCLUSIONS

Our study suggests that OPG is secreted by intestinal epithelial cells in response to enteropathogens and that a polymorphism in the OPG gene is associated with an increased susceptibility to TD.

New insights into the roles of dendritic cells in intestinal immunity and tolerance

Dendritic cells (DCs) play a critical key role in the initiation of immune responses to pathogens. Paradoxically, they also prevent potentially damaging immune responses being directed against the multitude of harmless antigens, to which the body is exposed daily. These roles are particularly important in the intestine, where only a single layer of epithelial cells provides a barrier against billions of commensal microorganisms, pathogens, and food antigens, over a huge surface area. In the intestine, therefore, DCs are required to perform their dual roles very efficiently to protect the body from the dual threats of invading pathogens and unwanted inflammatory reactions. In this review, we first describe the biology of DCs and their interactions with other cells types, paying particular attention to intestinal DCs. We, then, examine the ways in which this biology may become misdirected, resulting in inflammatory bowel disease. Finally, we discuss how DCs potentiate immune responses against viral, bacterial, parasitic infections, and their importance in the pathogenesis of prion diseases. We, therefore, provide an overview of the complex cellular interactions that affect intestinal DCs and control the balance between immunity and tolerance.

Osteoclasts and the immune system

Investigation into arthritis as well as the numerous bone phenotypes found in mice lacking immune-related genes has highlighted the importance of the dynamic interplay between the bone and immune systems. It has recently led to both the emergence and subsequent rapid evolution of the field of osteoimmunology. Receptor activator of nuclear factor-kappaB ligand (RANKL) stimulates osteoclastogenesis through the nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity. Studies on RANKL signaling revealed various immune-related genes which are involved in the regulation of osteoclastogenesis. Bone destruction in rheumatoid arthritis is caused by the enhanced activity of osteoclasts resulting from the activation of T cells. Here we describe our efforts to address the challenging question as to how abnormal T-cell activation mechanistically induces bone destruction. The scope of osteoimmunology has been extended to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases related to both the bone and immune systems.

Bone marrow microenvironment controls the in vivo differentiation of murine dendritic cells into osteoclasts

Finding that activated T cells control osteoclast (OCL) differentiation has revealed the importance of the interactions between immune and bone cells. Dendritic cells (DCs) are responsible for T-cell activation and share common precursors with OCLs. Here we show that DCs participate in bone resorption more directly than simply through T-cell activation. We show that, among the splenic DC subsets, the conventional DCs have the higher osteoclastogenic potential in vitro. We demonstrate that conventional DCs differentiate into functional OCLs in vivo when injected into osteopetrotic oc/oc mice defective in OCL resorptive function. Moreover, this differentiation involves the presence of activated CD4+ T cells controlling a high RANK-L expression by bone marrow stromal cells. Our results open new insights in the differentiation of OCLs and DCs and offer new basis for analyzing the relations between bone and immune systems.

Dextran sodium sulfate-induced colitis causes rapid bone loss in mice

INTRODUCTION

Osteopenia is a common complication of human inflammatory bowel disease (IBD). We evaluated the contribution of colonic inflammation to osteopenia and its mechanism in a murine colitis model.

METHODS

Colitis was induced by adding dextran sodium sulfate (DSS) to the drinking water for 2 weeks to nine-week-old Balb/C male mice. 5% DSS was added on the first week and was reduced to 2.5% on the second week. Age- and sex-matched Balb/C mice served as the control group. Indices of femoral bone mass and architecture were determined by micro computed tomography (muCT). Bone formation parameters and osteoclast number were determined by dynamic histomorphometry. The degree of colonic inflammation was assessed by a clinical disease activity index, and colonic mucosal myeloperoxidase activity.

RESULTS

DSS-treated mice exhibited a significantly lower bone mass compared to controls as indicated by decreased trabecular bone volume (BV/TV) of 32%. This reduction was accompanied by decreased trabecular number (23%) and connectivity density (37%) compared to the controls. No changes were observed in cortical bone indices. Osteopenia resulted from suppressed bone formation, as indicated by decreased trabecular double-labeled surface (dL%) of 90%, mineralizing surface (MS) of 62%, and bone formation rate (BFR) of 67%, and increased bone resorption as indicated by a 34% increase in osteoclast number in DSS-treated mice compared to the controls. Myeloperoxidase activity inversely correlated with trabecular BV/TV (r=-0.67, p=0.02), trabecular number (r=-0.86, p=0.0008) and connectivity density (r=-0.63, p=0.03). Myeloperoxidase activity inversely correlated with the bone formation indices: dL%, MS, and BFR (r=-0.79, p=0.007, r=-0.84, p=0.002, r=-0.83, p=0.003, respectively).

CONCLUSIONS

DSS-induced colitis is associated with reduced femoral bone mass and altered micro architecture, which results from suppressed bone formation and increased bone resorption. The decrease in indices of bone mass, structure and formation are directly linked to the degree of colonic mucosal inflammation. DSS-induced colitis can be used to study pharmacological interventions for bone loss in colitis.

Pathophysiological roles of osteoprotegerin (OPG)

Osteoprotegerin (OPG) is a secreted glycoprotein central to bone turnover via its role as a decoy receptor for the receptor activator of nuclear factor kappaB ligand (RANKL) and has traditionally been linked to a number of bone-related diseases. However, there is additional evidence that OPG can promote cell survival by inhibiting TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. As a result, a number of in vitro, in vivo and clinical studies have been performed assessing the role of OPG in tumourigenesis. Similar studies have been performed regarding vascular pathologies, resulting from observations of expression and regulation of OPG in the vasculature. This review aims to provide an update on this area and assess the potential protective or detrimental role of OPG in both vascular pathologies and tumourigenesis.

Cryptosporidium infection of human intestinal epithelial cells increases expression of osteoprotegerin: a novel mechanism for evasion of host defenses

Cryptosporidium parasites are pathogens of human intestinal epithelial cells. To determine which genes are regulated during early infection, human ileal mucosa cultured as explants was infected with C. parvum or C. hominis, and gene expression was analyzed by microarray. The gene for osteoprotegerin (OPG) was up-regulated by both parasites. OPG mRNA was also significantly increased in biopsy specimens obtained from a volunteer experimentally infected with C. meleagridis, compared with levels in a prechallenge biopsy specimen. After in vitro infection of HCT-8 cells, there was an early peak in production of OPG mRNA protein. Treatment of infected cells with the OPG ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induced epithelial cell apoptosis and reduced parasite numbers, and recombinant OPG blocked these effects. These results suggest a novel TRAIL-mediated pathway for elimination of Cryptosporidium infection and a role for OPG in modulating this host response.

Chronic pediatric inflammatory diseases: effects on bone

In children, chronic inflammatory diseases present a significant challenge to long-term skeletal health. These conditions are often associated with poor appetite and suboptimal overall nutrition, altered nutrient utilization, delayed puberty, inactivity, and reduced muscle mass, all of which can alter bone metabolism. In addition, bone cell activity is susceptible to the effects of the immune response that characterizes these diseases. Moreover, drugs used to treat these maladies, notably glucocorticoids, may have negative effects on bone formation and on linear growth in developing children. As a result, predicted peak bone mass may not be achieved, and fracture risk may be increased in the short term or in the future. Studies using primarily dual energy X-ray absorptiometry have documented that deficits in bone mass are common in these diseases. However, there are wide variations in the prevalence of low bone mass, largely due to differences in the characteristics of each study population. Recent studies provide insight into the pathogenesis of decreased bone mass in these conditions. In this paper we will provide an overview of the effects of chronic inflammatory conditions on bone mass in children. We will also present relevant data from adult patients, when pediatric data are scant or not available.

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.

The dynamic interplay between osteoclasts and the immune system

Investigation into arthritis, as well as numerous bone phenotypes found in mice lacking immune-related genes, has highlighted the importance of the interplay between the bone and immune systems, which has led to the emergence and evolution of the field of osteoimmunology. RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells (NFAT) c1, which is also a crucial regulator of immunity. In rheumatoid arthritis, bone destruction is caused by the enhanced activity of osteoclasts, which is mainly dependent on interleukin-17-producing helper T cells (T(H)17). The scope of osteoimmunology has been extended to encompass a wide range of molecular and cellular interactions. The framework of osteoimmunology will provide a scientific basis for future therapeutic approaches to diseases related to both of these systems.

Continuous RANKL inhibition in osteoprotegerin transgenic mice and rats suppresses bone resorption without impairing lymphorganogenesis or functional immune responses

Receptor activator of NF-kappaB ligand (RANKL) is an essential mediator of osteoclast formation, function, and survival. The effects of RANKL are inhibited by a soluble decoy receptor called osteoprotegerin (OPG). Total ablation of RANKL in knockout mice leads to high bone mass, lymph node agenesis, and altered lymphocyte differentiation. In contrast, RANKL inhibition via OPG suppresses bone resorption but not inflammation in animal models of inflammatory bone loss. This suggests that the immune phenotype of RANKL knockout mice is related to total RANKL ablation. We hypothesized that prenatal RANKL inhibition via OPG overexpression would suppress bone resorption without influencing lymph node formation or subsequent immune responses. Transgenic rats were created, wherein soluble OPG was overexpressed by 100-fold vs wild type (WT) controls, by gestational day 11 (i.e., before lymph node formation). The structure of lymph nodes, spleen, and thymus of OPG-transgenic (OPG-Tg) animals were comparable to those of age-matched WT rats at gestational day 19 and in adulthood. The OPG-Tg neonates had elevated bone mass, confirming the prenatal inhibition of RANKL. Adult OPG-Tg rats and OPG-Tg mice exhibited no significant functional alterations relative to WT controls when subjected to immune challenges to test for altered innate and humoral responses (e.g., contact hypersensitivity to oxazolone, IgM response to Pneumovax, IgG response to keyhole limpet hemocyanin, or cytokine response to LPS). In summary, prenatal RANKL inhibition did not impair lymph node development, nor did continuous life-long RANKL inhibition cause obvious changes in innate or humoral immune responses in mice or rats.

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

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

Basic and clinical aspects of osteoporosis in inflammatory bowel disease

Low bone mineral density and the increased risk of fracture in gastrointestinal diseases have a multifactorial pathogenesis. Inflammatory bowel disease (IBD) has been associated with an increased risk of osteoporosis and osteopenia and epidemiologic studies have reported an increased prevalence of low bone mass in patients with IBD. Certainly, genetics play an important role, along with other factors such as systemic inflammation, malnutrition, hypogonadism, glucocorticoid therapy in IBD and other lifestyle factors. At a molecular level the proinflammatory cytokines that contribute to the intestinal immune response in IBD are known to enhance bone resorption. There are genes influencing osteoblast function and it is likely that LRP5 may be involved in the skeletal development. Also the identification of vitamin D receptors (VDRs) and some of its polymorphisms have led to consider the possible relationships between them and some autoimmune diseases and may be involved in the pathogenesis through the exertion of its immunomodulatory effects during inflammation. Trying to explain the physiopathology we have found that there is increasing evidence for the integration between systemic inflammation and bone loss likely mediated via receptor for activated nuclear factor kappa-B (RANK), RANK-ligand, and osteoprotegerin, proteins that can affect both osteoclastogenesis and T-cell activation. Although glucocorticoids can reduce mucosal and systemic inflammation, they have intrinsic qualities that negatively impact on bone mass. It is still controversial if all IBD patients should be screened, especially in patients with preexisting risk factors for bone disease. Available methods to measure BMD include single energy x-ray absorptiometry, DXA, quantitative computed tomography (QCT), radiographic absorptiometry, and ultrasound. DXA is the establish method to determine BMD, and routinely is measured in the hip and the lumbar spine. There are several treatments options that have proven their effectiveness, while new emergent therapies such as calcitonin and teriparatide among others remain to be assessed.

Osteoimmunology: shared mechanisms and crosstalk between the immune and bone systems

Osteoimmunology is an interdisciplinary research field focused on the molecular understanding of the interplay between the immune and skeletal systems. Although osteoimmunology started with the study of the immune regulation of osteoclasts, its scope has been extended to encompass a wide range of molecular and cellular interactions, including those between osteoblasts and osteoclasts, lymphocytes and osteoclasts, and osteoblasts and haematopoietic cells. Therefore, the two systems should be understood to be integrated and operating in the context of the 'osteoimmune' system, a heuristic concept that provides not only a framework for obtaining new insights by basic research, but also a scientific basis for the discovery of novel treatments for diseases related to both systems.

Inflammatory bowel disease: cause and immunobiology

Crohn's disease and ulcerative colitis are idiopathic inflammatory bowel disorders. In this paper, we discuss how environmental factors (eg, geography, cigarette smoking, sanitation and hygiene), infectious microbes, ethnic origin, genetic susceptibility, and a dysregulated immune system can result in mucosal inflammation. After describing the symbiotic interaction of the commensal microbiota with the host, oral tolerance, epithelial barrier function, antigen recognition, and immunoregulation by the innate and adaptive immune system, we examine the initiating and perpetuating events of mucosal inflammation. We pay special attention to pattern-recognition receptors, such as toll-like receptors and nucleotide-binding-oligomerisation-domains (NOD), NOD-like receptors and their mutual interaction on epithelial cells and antigen-presenting cells. We also discuss the important role of dendritic cells in directing tolerance and immunity by modulation of subpopulations of effector T cells, regulatory T cells, Th17 cells, natural killer T cells, natural killer cells, and monocyte-macrophages in mucosal inflammation. Implications for novel therapies, which are discussed in detail in the second paper in this Series, are covered briefly.

Osteoprotegerin expression in dendritic cells increases with maturation and is NF-κB-dependent

Dendritic cells (DC) comprise a unique leukocyte population which controls primary immune responses. Recent studies indicate that DC express osteoprotegerin (OPG), a secreted tumor necrosis factor receptor homolog, which regulates DC survival, monocyte chemotaxis, and B cell development and function by ligating TNF family member receptor activator of NF-kappaB ligand (RANKL). The precise regulators of OPG expression in DC have not been investigated. In this study, we assessed OPG mRNA steady state levels by Northern blot analysis and OPG protein secretion by an immunoassay in monocyte-derived DC of different maturation, and the effect of different cytokines and hormones on OPG expression. OPG was upregulated with maturation of DC, whereas pretreatment of DC with 1alpha,25(OH)(2) vitamin D(3), tamoxifen, or dexamethasone, agents that inhibit differentiation of DC, decreased OPG expression. In vivo, OPG was found to be colocalized with mature CD83(+) DC in human tonsils by immunofluorescence confocal microscopy analysis. Furthermore, OPG was upregulated by TNF superfamily members TNF-alpha, anti-CD40, and RANKL, and by ligands of the Toll-like/IL-1 receptor family including IL-1beta, double-stranded RNA (poly I:C), or lipopolysaccharide (LPS), all of which induce maturation of DC. Gene silencing by small interfering RNA (siRNA) directed against transcription factor NF-kappaB abrogated the expression of OPG as demonstrated by real-time PCR. In summary, we describe that the expression of OPG by DC increases with maturation and is NF-kappaB-dependent, possibly regulating immune responses in lymphoid tissues.

Epidermal RANKL controls regulatory T-cell numbers via activation of dendritic cells

Regulatory CD4(+)CD25(+) T cells are important in suppressing immune responses. The requirements for the maintenance of peripheral CD4(+)CD25(+) T cells remain incompletely understood. Receptor activator of NF-kappaB (RANK) and its ligand (RANKL; also known as CD254, OPGL and TRANCE) are key regulators of bone remodeling, mammary gland formation, lymph node development and T-cell/dendritic cell communication. Here we report that RANKL is expressed in keratinocytes of the inflamed skin. RANKL overexpression in keratinocytes resulted in functional alterations of epidermal dendritic cells and systemic increases of regulatory CD4(+)CD25(+) T cells. Thus, epidermal RANKL expression can change dendritic cell functions to maintain the number of peripheral CD4(+)CD25(+) regulatory T cells. Epidermal RANKL mediated ultraviolet-induced immunosuppression and overexpression of epidermal RANKL suppressed allergic contact hypersensitivity responses and the development of systemic autoimmunity. Therefore, environmental stimuli at the skin can rewire the local and systemic immune system by means of RANKL.

CD8+ cytotoxic T-APC stimulate central memory CD8+ T cell responses via acquired peptide-MHC class I complexes and CD80 costimulation, and IL-2 secretion

We previously showed that naive CD4+ Th cells acquire peptide-MHC class I (pMHC I) and costimulatory molecules from OVA-pulsed dendritic cells (DC(OVA)), and act as Th-APCs in stimulation of CD8+ CTL responses. In this study, we further demonstrated that naive CD8+ cytotoxic T (Tc) cells also acquire pMHC I and costimulatory CD54 and CD80 molecules by DC(OVA) stimulation, and act as Tc-APC. These Tc-APC can play both negative and positive modulations in antitumor immune responses by eliminating DC(OVA) and neighboring Tc-APC, and stimulating OVA-specific CD8+ central memory T responses and antitumor immunity. Interestingly, the stimulatory effect of Tc-APC is mediated via its IL-2 secretion and acquired CD80 costimulation, and is specifically targeted to OVA-specific CD8+ T cells in vivo via its acquired pMHC I complexes. These principles could be applied to not only antitumor immunity, but also other immune disorders (e.g., autoimmunity).

Inflammatory bowel diseases as secondary causes of osteoporosis

Inflammatory bowel disease (IBD), comprising Crohn's disease and ulcerative colitis, is associated with an increased risk of osteoporosis and bone fractures. Initial studies suggested very high rates of osteoporosis in IBD, but more recent studies have suggested that bone mineral density (BMD) is often normal in patients with IBD and typically changes little over time. Nonetheless, IBD is associated with an increased risk of fractures. Doctors managing patients with IBD must consider a variety of risk factors, not just BMD measurements, in assessing fracture risk. Advances have been made in exploring the pathogenesis of osteoporosis in IBD. The evolution of knowledge regarding receptor for activated factor of nuclear factor kappaB (RANK), its ligand RANKL, and osteoprotegerin (OPG), which serves as a decoy receptor, has enhanced the understanding of both osteoporosis and T-cell immunobiology. Recent clinical studies in patients with IBD have revealed that serum OPG levels may be elevated and inflamed intestinal tissue secretes increased amounts of OPG. It is suspected that OPG levels are elevated as a counterregulatory response to low BMD, as serum OPG levels in IBD have been found to be inversely associated with BMD. Finally, in animal models of IBD, exogenous OPG has reversed both the osteopenia and the enterocolitis, suggesting that it may have a therapeutic role in human IBD.

Prevention and treatment of osteoporosis in inflammatory bowel disease

The following are guidelines for evaluation and consideration for treatment of patients with inflammatory bone disease (IBD) after bone mineral density (BMD) measurements. The Crohn's & Colitis Foundation of America (CCFA) has indicated that its recommendations are intended to serve as reference points for clinical decision-making, not as rigid standards, limits, or rules. They should not be interpreted as quality standards.

OSTEOIMMUNOLOGY: Interplay Between the Immune System and Bone Metabolism

Studies of bone and the immune system have converged in recent years under the banner of osteoimmunology. The immune system is spawned in the bone marrow reservoir, and investigators now recognize that important niches also exist there for memory lymphocytes. At the same time, various factors produced during immune responses are capable of profoundly affecting regulation of bone. Mechanisms have evolved to prevent excessive interference by the immune system with bone homeostasis, yet pathologic bone loss is a common sequela associated with autoimmunity and cancer. There are also developmental links, or parallels, between bone and the immune system. Cells that regulate bone turnover share a common precursor with inflammatory immune cells and may restrict themselves anatomically, in part by utilizing a signaling network analogous to lymphocyte costimulation. Efforts are currently under way to further characterize how these two organ systems overlap and to develop therapeutic strategies that benefit from this understanding.

Elevated serum levels of the decoy receptor osteoprotegerin in children with Langerhans cell histiocytosis

Langerhans cell histiocytosis (LCH) is characterized by an accumulation of dendritic Langerhans cells in granulomatous lesions in the bone, skin, and other sites in the body. The pathogenesis of the disease remains unknown. We measured serum levels of the decoy receptor osteoprotegerin (OPG), an important regulator of bone metabolism as well as immune responses, in 18 children with single system (SS) or multi-system (MS) forms of LCH and 20 pediatric controls. OPG levels were significantly increased in LCH patients at diagnosis when compared with controls, and pretreatment levels of OPG were elevated in MS compared with SS patients. Moreover, OPG levels in LCH patients were elevated in patients with involvement of risk organs (liver, lungs, hematopoietic system, or spleen) when compared with patients without risk organ involvement, indicative of an association between OPG values and disease severity. We also observed a positive correlation between serum levels of OPG and tumor necrosis factor (TNF)-alpha, a pro-inflammatory cytokine, at the time of onset of disease. These findings show, for the first time, that serum OPG levels are elevated in LCH patients, and suggest that OPG may play a role in the pathogenesis of this enigmatic disease.

Osteoclast precursors, RANKL/RANK, and immunology

Rapid progress has been made in recent years in our understanding of the mechanisms regulating the formation, activation, and survival of osteoclasts, which are derived from precursor cells in the myeloid lineage. In contrast, study of the regulation of osteoclast precursors (OCPs) has been relatively slow, in part because it has been hard to accurately identify them. However, following the discovery of cell-surface markers that facilitated purification of OCPs, recent studies have demonstrated that peripheral blood OCP numbers are increased in tumor necrosis factor (TNF)-mediated arthritis, both in animals and humans, and these numbers correlate with serum TNF levels. The increase can be reversed by anti-TNF therapy. Furthermore, the precursor cells that give rise to osteoclasts can also differentiate into other cell types, including dendritic cells. Receptor activator nuclear factor-kappaB ligand (RANKL) stimulates OCPs to produce pro-inflammatory cytokines and chemokines, and RANKL blockade prevents joint inflammation in a murine model of inflammatory arthritis. These findings suggest that OCPs may serve as a source for both osteoclasts and other effector cells and participate actively in the pathogenesis of diseases. Here, we review our current understanding of the regulation of OCP formation and differentiation and provide a model of a vicious cycle in which pro-inflammatory cytokines produced in inflamed joints feedback on the bone marrow to promote the generation and release of OCPs. The OCPs then home to the inflamed joints to differentiate into mature osteoclasts or to produce more inflammatory factors in the presence of RANKL. Disruption of this cycle could provide a new strategy for the development of drugs to treat inflammatory arthritis and other disorders associated with elevated OCP/myeloid progenitors.

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.

Perforin-dependent elimination of dendritic cells regulates the expansion of antigen-specific CD8+ T cells in vivo

The lifespan and survival of dendritic cells (DC) in vivo are potentially critical to the expansion of T cell immune responses. We have previously reported that DC loaded with specific antigen are rapidly eliminated by cytotoxic T lymphocytes (CTL) in vivo, but the site, mechanism, and consequences of DC elimination were not defined. In this article we show that DC elimination in vivo occurs in a perforin-dependent manner and does not require IFN-gamma or the presence of CD4(+)CD25(+) regulatory T cells. Most importantly, failure to eliminate DC had profound consequences on the CTL immune response. Perforin-deficient mice showed a progressive increase in the numbers of antigen-specific CD8(+) T cells after repeated immunizations with DC. In contrast, in control mice the number of antigen-specific CD8(+) T cells did not notably increase with repeated immunizations. Lastly, we also show that CTL-mediated elimination of DC occurs in peripheral tissues but not in the lymph node. Our data suggest that CTL act as "gatekeepers" that control access of antigen-loaded DC into the lymph node, thereby preventing continued expansion of antigen-specific T cells.