T cell-mediated increased osteoclast formation from peripheral blood as a mechanism for Crohn's disease-associated bone loss

The immune cells in modulating osteoclast formation and bone metabolism

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

Oxidation of Mesalamine under Phenoloxidase- or Peroxidase-like Enzyme Catalysis

Mesalamine, also called 5-ASA (5-aminosalicylic acid), is a largely used anti-inflammatory agent and is a main choice to treat Ulcerative Colitis. This report is aimed to investigate enzymatic processes involved in the oxidation of mesalamine to better understand some of its side-effects. Oxidation with oxygen (catalyzed by ceruloplasmin) or with hydrogen peroxide (catalyzed by peroxidase or hemoglobin) showed that these oxidases, despite their different mechanisms of oxidation, could recognize mesalamine as a substrate and trigger its oxidation to a corresponding quinone-imine. These enzymes were chosen because they may recognize hydroquinone (a p-diphenol) as substrate and oxidize it to p-benzoquinone and that mesalamine, as a p-aminophenol, presents some similarities with hydroquinone. The UV-Vis kinetics, FTIR and 1H NMR supported the hypothesis of oxidizing mesalamine. Furthermore, mass spectrometry suggested the quinone-imine as reaction product. Without enzymes, the oxidation process was very slow (days and weeks), but it was markedly accelerated with the oxidases, particularly with peroxidase. Cyclic voltammetry supported the hypothesis of the oxidative process and allowed a ranking of susceptibility to oxidizing mesalamine in comparison with other oxidizable drug molecules with related structures. The susceptibility to oxidation was higher for mesalamine, in comparison with Tylenol (acetaminophen) and with aspirin (salicylic acid).

In vitro osteoclastogenesis in autoimmune diseases - Strengths and pitfalls of a tool for studying pathological bone resorption and other disease characteristics

Osteoclasts play a critical role in bone pathology frequently associated with autoimmune diseases. Studying the etiopathogenesis of these diseases and their clinical manifestations can involve in vitro osteoclastogenesis, an experimental technique that utilizes osteoclast precursors that are relatively easily accessible from peripheral blood or synovial fluid. However, the increasing number of methodical options to study osteoclastogenesis in vitro poses challenges in translating findings to clinical research and practice. This review compares and critically evaluates previous research work based on in vitro differentiation of human osteoclast precursors originating from patients, which aimed to explain autoimmune pathology in rheumatic and enteropathic diseases. The discussion focuses primarily on methodical differences between the studies, including the origin of osteoclast precursors, culture conditions, and methods for identifying osteoclasts and assessing their activity. Additionally, the review examines the clinical significance of the three most commonly used in vitro approaches: induced osteoclastogenesis, spontaneous osteoclastogenesis, and cell co-culture. By analyzing and integrating the gathered information, this review proposes general connections between different studies, even in cases where their results are seemingly contradictory. The derived conclusions and future directions aim to enhance our understanding of a potential and limitations of in vitro osteoclastogenesis and provide a foundation for discussing novel methods (such as osteoclastogenesis dynamic) and standardized approaches (such as spontaneous osteoclastogenesis) for future use in autoimmune disease research.

The microbiota-gut-bone axis and bone health

The gastrointestinal tract is colonized by trillions of microorganisms, consisting of bacteria, fungi, and viruses, known as the "second gene pool" of the human body. In recent years, the microbiota-gut-bone axis has attracted increasing attention in the field of skeletal health/disorders. The involvement of gut microbial dysbiosis in multiple bone disorders has been recognized. The gut microbiota regulates skeletal homeostasis through its effects on host metabolism, immune function, and hormonal secretion. Owing to the essential role of the gut microbiota in skeletal homeostasis, novel gut microbiota-targeting therapeutics, such as probiotics and prebiotics, have been proven effective in preventing bone loss. However, more well-controlled clinical trials are still needed to evaluate the long-term efficacy and safety of these ecologic modulators in the treatment of bone disorders.

Absence of αβ T cells accelerates disuse bone loss in male mice after spinal cord injury

Whether T cells promote bone loss following immobilization after spinal cord injury (SCI) remains undetermined. Therefore, wild-type (WT) and T cell-deficient (Tcrb-/- ) male mice underwent sham or contusion SCI to cause hindlimb paralysis. Femurs were isolated and distal and midshaft regions were evaluated by microcomputed tomography scanning. Bone marrow (BM) levels of bone turnover markers, as well as receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG), were measured by ELISA. At 2 weeks post-SCI, immobilization resulted in marked reduction in trabecular fractional bone volume (55%), thickness (40%), connectivity, and cortical thickness only in the Tcrb-/- animals (interaction with P < 0.05). BM analysis revealed lower bone formation (procollagen type 1 intact N-terminal propeptide), higher bone resorption (tartrate-resistant acid phosphatase-5b), and a higher RANKL/OPG ratio in the Tcrb-/- SCI animals. At 5 weeks post-SCI, while both WT and Tcrb-/- paralyzed animals showed deterioration of all indices of bone structure, they were more severe in Tcrb-/- animals. In summary, unlike other skeletal disorders, loss of αβ T cells compromises, rather than preserves, skeletal integrity under conditions of immobilization.

Unraveling the mystery of Gaucher bone density pathophysiology

Gaucher disease (GD) is caused by pathogenic mutations in GBA1, the gene that encodes the lysosomal enzyme β-glucocerebrosidase. Despite the existence of a variety of specific treatments for GD, they cannot completely reverse bone complications. Many studies have evidenced the impairment in bone tissue of GD, and molecular mechanisms of bone density alterations in GD are being studied during the last years and different reports emphasized its efforts trying to unravel why and how bone tissue is affected. The cause of skeletal density affection in GD is a matter of debates between research groups. and there are two opposing hypotheses trying to explain reduced bone mineral density in GD: increased bone resorption versus impaired bone formation. In this review, we discuss the diverse mechanisms of bone alterations implicated in GD revealed until the present, along with a presentation of normal bone physiology and its regulation. With this information in mind, we discuss effectiveness of specific therapies, introduce possible adjunctive therapies and present a novel model for GD-associated bone density pathogenesis. Under the exposed evidence, we may conclude that both sides of the balance of remodeling process are altered. In GD the observed osteopenia/osteoporosis may be the result of contribution of both reduced bone formation and increased bone resorption.

Chronic Exposure of Gingival Fibroblasts to TLR2 or TLR4 Agonist Inhibits Osteoclastogenesis but Does Not Affect Osteogenesis

Chronic exposure to periodontopathogenic bacteria such as Porphyromonas gingivalis and the products of these bacteria that interact with the cells of the tooth surrounding tissues can ultimately result in periodontitis. This is a disease that is characterized by inflammation-related alveolar bone degradation by the bone-resorbing cells, the osteoclasts. Interactions of bacterial products with Toll-like receptors (TLRs), in particular TLR2 and TLR4, play a significant role in this chronic inflammatory reaction, which possibly affects osteoclastic activity and osteogenic capacity. Little is known about how chronic exposure to specific TLR activators affects these two antagonistic activities. Here, we studied the effect of TLR activation on gingival fibroblasts (GF), cells that are anatomically close to infiltrating bacterial products in the mouth. These were co-cultured with naive osteoclast precursor cells (i.e., monocytes), as part of the peripheral blood mononuclear cells (PBMCs). Activation of GF co-cultures (GF + PBMCs) with TLR2 or TLR4 agonists resulted in a weak reduction of the osteoclastogenic potential of these cultures, predominantly due to TLR2. Interestingly, chronic exposure, especially to TLR2 agonist, resulted in increased release of TNF-α at early time points. This effect, was reversed at later time points, thus suggesting an adaptation to chronic exposure. Monocyte cultures primed with M-CSF + RANKL, led to the formation of bone-resorbing osteoclasts, irrespective of being activated with TLR agonists. Late activation of these co-cultures with TLR2 and with TLR4 agonists led to a slight decrease in bone resorption. Activation of GF with TLR2 and TLR4 agonists did not affect the osteogenic capacity of the GF cells. In conclusion, chronic exposure leads to diverse reactions; inhibitory with naive osteoclast precursors, not effecting already formed (pre-)osteoclasts. We suggest that early encounter of naive monocytes with TLR agonists may result in differentiation toward the macrophage lineage, desirable for clearing bacterial products. Once (pre-)osteoclasts are formed, these cells may be relatively insensitive for direct TLR stimulation. Possibly, TLR activation of periodontal cells indirectly stimulates osteoclasts, by secreting osteoclastogenesis stimulating inflammatory cytokines.

Perspective on skeletal health in inflammatory bowel disease

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

The Interplay between Immune System and Microbiota in Osteoporosis

Osteoporosis is a disease characterized by low bone mass and alterations of bone microarchitecture, with an increased risk of fractures. It is a multifactorial disorder that is more frequent in postmenopausal women but can be associated to other diseases (inflammatory and metabolic diseases). At present, several options are available to treat osteoporosis trying to block bone reabsorption and reduce the risk of fracture. Anyway, these drugs have safety and tolerance problems in long-term treatment. Recently, gut microbiota has been highlighted to have strong influence on bone metabolism, becoming a potential new target to modify bone mineral density. Such evidences are mainly based on mouse models, showing an involvement in modulating the interaction between the immune system and bone cells. Germ-free mice represent a basic model to understand the interaction between microbiota, immune system, and bone cells, even though data are controversial. Anyway, such models have unequivocally demonstrated a connection between such systems, even if the mechanism is unclear. Gut microbiota is a complex system that influences calcium and vitamin D absorption and modulates gut permeability, hormonal secretion, and immune response. A key role is played by the T helper 17 lymphocytes, TNF, interleukin 17, and RANK ligand system. Other important pathways include NOD1, NOD2, and Toll-like receptor 5. Prebiotics and probiotics are a wide range of substances and germs that can influence and modify microbiota. Several studies demonstrated actions by different prebiotics and probiotics in different animals, differing according to sex, age, and hormonal status. Data on the effects on humans are poor and controversial. Gut microbiota manipulation appears a possible strategy to prevent and treat osteopenia and/or osteoporosis as well as other possible bone alterations, even though further clinical studies are necessary to identify correct procedures in humans.

Mechanisms Underlying Bone Loss Associated with Gut Inflammation

Patients with gastrointestinal diseases frequently suffer from skeletal abnormality, characterized by reduced bone mineral density, increased fracture risk, and/or joint inflammation. This pathological process is characterized by altered immune cell activity and elevated inflammatory cytokines in the bone marrow microenvironment due to disrupted gut immune response. Gastrointestinal disease is recognized as an immune malfunction driven by multiple factors, including cytokines and signaling molecules. However, the mechanism by which intestinal inflammation magnified by gut-residing actors stimulates bone loss remains to be elucidated. In this article, we discuss the main risk factors potentially contributing to intestinal disease-associated bone loss, and summarize current animal models, illustrating gut-bone axis to bridge the gap between intestinal inflammation and skeletal disease.

Immune Function and Diversity of Osteoclasts in Normal and Pathological Conditions

Osteoclasts (OCLs) are key players in controlling bone remodeling. Modifications in their differentiation or bone resorbing activity are associated with a number of pathologies ranging from osteopetrosis to osteoporosis, chronic inflammation and cancer, that are all characterized by immunological alterations. Therefore, the 2000s were marked by the emergence of osteoimmunology and by a growing number of studies focused on the control of OCL differentiation and function by the immune system. At the same time, it was discovered that OCLs are much more than bone resorbing cells. As monocytic lineage-derived cells, they belong to a family of cells that displays a wide heterogeneity and plasticity and that is involved in phagocytosis and innate immune responses. However, while OCLs have been extensively studied for their bone resorption capacity, their implication as immune cells was neglected for a long time. In recent years, new evidence pointed out that OCLs play important roles in the modulation of immune responses toward immune suppression or inflammation. They unlocked their capacity to modulate T cell activation, to efficiently process and present antigens as well as their ability to activate T cell responses in an antigen-dependent manner. Moreover, similar to other monocytic lineage cells such as macrophages, monocytes and dendritic cells, OCLs display a phenotypic and functional plasticity participating to their anti-inflammatory or pro-inflammatory effect depending on their cell origin and environment. This review will address this novel vision of the OCL, not only as a phagocyte specialized in bone resorption, but also as innate immune cell participating in the control of immune responses.

What Are the Peripheral Blood Determinants for Increased Osteoclast Formation in the Various Inflammatory Diseases Associated With Bone Loss?

Local priming of osteoclast precursors (OCp) has long been considered the main and obvious pathway that takes place in the human body, where local bone lining cells and RANKL-expressing osteocytes may facilitate the differentiation of OCp. However, priming of OCp away from bone, such as in inflammatory tissues, as revealed in peripheral blood, may represent a second pathway, particularly relevant in individuals who suffer from systemic bone loss such as prevalent in inflammatory diseases. In this review, we used a systematic approach to review the literature on osteoclast formation in peripheral blood in patients with inflammatory diseases associated with bone loss. Only studies that compared inflammatory (bone) disease with healthy controls in the same study were included. Using this core collection, it becomes clear that experimental osteoclastogenesis using peripheral blood from patients with bone loss diseases in prevalent diseases such as rheumatoid arthritis, osteoporosis, periodontitis, and cancer-related osteopenia unequivocally point toward an intrinsically increased osteoclast formation and activation. In particular, such increased osteoclastogenesis already takes place without the addition of the classical osteoclastogenesis cytokines M-CSF and RANKL in vitro. We show that T-cells and monocytes as OCp are the minimal demands for such unstimulated osteoclast formation. In search for common and disease-specific denominators of the diseases with inflammation-driven bone loss, we demonstrate that altered T-cell activity and a different composition—such as the CD14+CD16+ vs. CD14+CD16– monocytes—and priming of OCp with increased M-CSF, RANKL, and TNF- α levels in peripheral blood play a role in increased osteoclast formation and activity. Future research will likely uncover the barcodes of the OCp in the various inflammatory diseases associated with bone loss.

Fracture Risk in Patients With Inflammatory Bowel Disease: A Nationwide Population-Based Cohort Study From 1964 to 2014

INTRODUCTION

Most studies on fractures in inflammatory bowel disease (IBD) are based on patients from tertiary centers or patients followed up before the introduction of immunomodulators or biologics. In addition, the role of corticosteroids in fracture risk has rarely been examined.

METHODS

We conducted a nationwide population-based cohort study of 83,435 patients with incident IBD (ulcerative colitis [UC]: n = 50,162, Crohn's disease [CD]: n = 26,763, and IBD unclassified: 6,510) and 825,817 reference individuals from 1964 to 2014. Using multivariable Cox regression, we estimated hazard ratios (HRs) for hip fracture and any fracture and the association with cumulative corticosteroid exposure.

RESULTS

During 1,225,415 person-years of follow-up in patients with IBD, there were 2,491 first-time hip fractures (203/100,000 person-years) compared with 20,583 hip fractures during 12,405,642 person-years in reference individuals (159/100,000 person-years). This corresponded to an HR of 1.42 (95% confidence interval [CI] = 1.36-1.48). The risk for hip fracture was higher in CD compared with UC (P < 0.001). Inflammatory bowel disease was also associated with any fracture (IBD: HR = 1.18; 95% CI = 1.15-1.20). Hazard ratios for hip fracture had not changed since the introduction of immunomodulators or biologics. Increasing exposure to corticosteroids was associated with hip fracture in both IBD and non-IBD individuals (P < 0.001), but only in elderly (>60 years) patients with IBD. The association between IBD and hip fracture was nonsignificant among individuals without corticosteroids (HR = 1.11; 95% CI = 0.86-1.44).

CONCLUSIONS

Inflammatory bowel disease (CD and UC) is associated with an increased risk of hip fracture and any fracture, but not in individuals without a history of corticosteroid treatment. The association between corticosteroids and hip fracture was restricted to elderly patients with IBD.

Overexpression of miR‑146a blocks the effect of LPS on RANKL‑induced osteoclast differentiation

The concept that inflammation serves a leading role in osteoclast-induced bone loss under pathological circumstances is now widely accepted. In the present study, it was observed that lipopolysaccharides (LPSs) demonstrated a synergic effect on receptor activator of nuclear factor κ-B ligand (RANKL)-induced osteoclast differentiation in Raw264.7 cells, with increasing levels of multiple pro-inflammatory cytokines including interleukin (IL)-6, tumor necrosis factor-α and IL-1β. Furthermore, microRNA (miR)-146a was highly induced by LPS and RANKL co-stimulation during the process of osteoclast differentiation. Overexpression of miR-146a inhibited osteoclast transformation by targeting the key regulators of nuclear factor (NF)-κβ signaling, TNF receptor-associated factor 6 and interleukin-1 receptor-associated kinase 1. The downstream activation of NF-κβ signaling was also inhibited by transfection with a miR-146a mimic. Altogether, the results of the present study demonstrated that miR-146a prevents osteoclast differentiation induced by LPS and RANKL co-stimulation, suggesting that miR-146a may be a promising therapeutic target for treatment of inflammation mediated bone loss.

Survival, Retention, and Selective Proliferation of Lymphocytes Is Mediated by Gingival Fibroblasts

Periodontitis, a chronic inflammatory disease of the periodontium, is characterized by osteoclast-mediated alveolar bone destruction. Gingival fibroblasts (GFs) present in the bone-lining mucosa have the capacity to activate the formation of osteoclasts, but little is known about which local immune cells (co-)mediate this process. The aim of this study was to investigate the cellular interactions of GFs with immune cells, including the contribution of GFs to osteoclast formation and their possible role in the proliferation of these immune cells. In addition, we investigated the expression of adhesion molecules and the inflammatory cytokines that are evoked by this interaction. GFs were cocultured with peripheral blood mononuclear cells (PBMCs), CD14+ monocytes or peripheral blood lymphocytes (PBLs) for 7, 14, and 21 days. After 21 days, comparable numbers of multinucleated cells (osteoclasts) were found in gingival fibroblast (GF)-PBMC and GF-monocyte cocultures. No osteoclasts were formed in GF-PBL cocultures, indicating that the PBLs present in GF-PBMC cocultures do not contribute to osteoclastogenesis. Persisting mononuclear cells were interacting with osteoclasts in GF-PBMC cocultures. Remarkably, a predominance of CD3+ T cells was immunohistochemically detected in GF cocultures with PBLs and PBMCs for 21 days that frequently interacted with osteoclasts. Significantly more T, B (CD19+), and NK (CD56+CD3-) cells were identified with multicolor flow cytometry in both GF-PBMC and GF-PBL cocultures compared to monocultures without GFs at all time points. GFs retained PBLs independently of the presence of monocytes or osteoclasts over time, showing a stable population of T, B, and NK cells between 7 and 21 days. T helper and cytotoxic T cell subsets remained stable over time in GF cocultures, while the number of Th17 cells fluctuated. Lymphocyte retention is likely mediated by lymphocyte-function-associated antigen-1 (LFA-1) expression, which was significantly higher in GF-PBL cultures compared to GF-monocyte cultures. When assessing inflammatory cytokine expression, high tumor necrosis alpha expression was only observed in the GF-PBMC cultures, indicating that this tripartite presence of GFs, monocytes, and lymphocytes is required for such an induction. Carboxyfluorescein succinimidyl ester-labeling showed that only the CD3+ cells proliferated in presence of GFs. This study demonstrates a novel role for GFs in the survival, retention, and selective proliferation of lymphocytes.

Gut microbiome and bone

The gut microbiome is now viewed as a tissue that interacts bidirectionally with the gastrointestinal, immune, endocrine and nervous systems, affecting the cellular responses in numerous organs. Evidence is accumulating of gut microbiome involvement in a growing number of pathophysiological processes, many of which are linked to inflammatory responses. More specifically, data acquired over the last decade point to effects of the gut microbiome on bone mass regulation and on the development of bone diseases (such as osteoporosis) and of inflammatory joint diseases characterized by bone loss. Mice lacking a gut microbiome have bone mass alteration that can be reversed by gut recolonization. Changes in the gut microbiome composition have been reported in mice with estrogen-deficiency osteoporosis and have also been found in a few studies in humans. Probiotic therapy decreases bone loss in estrogen-deficient animals. The effect of the gut microbiome on bone tissue involves complex mechanisms including modulation of CD4⁺T cell activation, control of osteoclastogenic cytokine production and modifications in hormone levels. This complexity may contribute to explain the discrepancies observed betwwen some studies whose results vary depending on the age, gender, genetic background and treatment duration. Further elucidation of the mechanisms involved is needed. However, the available data hold promise that gut microbiome manipulation may prove of interest in the management of bone diseases.

Periodontal ligament fibroblasts as a cell model to study osteogenesis and osteoclastogenesis in fibrodysplasia ossificans progressiva

Fibrodysplasia Ossificans Progressiva (FOP) is a progressive disease characterized by periods of heterotopic ossification of soft connective tissues, including ligaments. Though progress has been made in recent years in unraveling the underlying mechanism, patient-derived cell models are necessary to test potential treatment options. Periodontal ligament fibroblasts (PLF) from extracted teeth can be used to study deviant bone modeling processes in vitro since these cells are derived from genuine ligaments. They further provide a tool to study the hitherto unknown role of the bone morphogenesis protein receptor type 1 (BMPR-1) Activin A type 1 receptor ACVR1-R206H mutation in osteoclastogenesis. To further validate this potential model, osteogenesis and osteoclastogenesis was studied in the presence of TGF-β/activin receptor inhibitor GW788388. Control and FOP fibroblasts (n=6 of each) were used in osteogenesis and osteoclastogenesis assays in the absence or presence of TGF-β/activin receptor inhibitor GW788388. For osteogenesis, alkaline phosphatase (ALP) activity, alizarin red staining for mineralization and qPCR for expression of osteogenic markers was assessed. TRACP staining, multinuclearity and expression of osteoclastogenesis markers were used as a measure of osteoclast formation. FOP fibroblasts cultured in osteogenic medium displayed a trend of higher ALP activity at 7days. Gene expression of ALP from FOP fibroblasts was significantly higher at 3days. Mineralization was similar at 21days for both groups. GW788388 did not influence mineral deposition in both groups. Osteoclast formation was inhibited by GW788388 on plastic for both controls and FOP. On cortical bone slices, however, osteoclast formation was significantly lowered by GW788388, only in FOP cultures. qPCR revealed strong expression of RANKL at 7days and a significant decline at 14 and 21days in both FOP and control cultures. In contrast to the osteoclastogenesis results, the RANKL/OPG ratio was higher in the presence of GW788388, only in FOP cultures. TGF-β expression was significantly higher at 14 and 21days compared to 7days, possibly signifying a role in later stages of osteoclast formation. Addition of GW788388 strongly decreased TGF-β expression. Our study shows that periodontal ligament fibroblasts from FOP patients displayed at most slightly enhanced in vitro osteogenesis and osteoclastogenesis. This model could be useful to elucidate molecular mechanisms leading to heterotopic ossification in FOP such as in the presence of specific ACVR1-R206H activators as Activin A.

Integrin αMβ2 is differently expressed by subsets of human osteoclast precursors and mediates adhesion of classical monocytes to bone

Bone-degrading osteoclasts are formed through fusion of their monocytic precursors. In the population of human peripheral blood monocytes, three distinct subsets have been identified: classical, intermediate and non-classical monocytes. We have previously shown that when the monocyte subsets are cultured on bone, significantly more osteoclasts are formed from classical monocytes than from intermediate or non-classical monocytes. Considering that this difference does not exist when monocyte subsets are cultured on plastic, we hypothesized that classical monocytes adhere better to the bone surface compared to intermediate and non-classical monocytes. To investigate this, the different monocyte subsets were isolated from human peripheral blood and cultured on slices of human bone in the presence of the cytokine M-CSF. We found that classical monocytes adhere better to bone due to a higher expression of the integrin αMβ2 and that their ability to attach to bone is significantly decreased when the integrin is blocked. This suggests that integrin αMβ2 mediates attachment of osteoclast precursors to bone and thereby enables the formation of osteoclasts.

Dendritic-cell-derived osteoclasts: a new game changer in bone-resorption-associated diseases

Bone-resorbing cells, osteoclasts (OCs), and antigen-presenting cells, dendritic cells (DCs), share several features. They are derived from a common hematopoietic precursor, exhibit phagocytic activities and their functions are dependent upon receptor activator of nuclear factor κB ligand (RANKL). Upon inflammatory conditions, DCs can transdifferentiate toward functional OCs in the presence of RANKL. It has then been assumed that the increase in proinflammatory cytokines could provide a supportive environment for this transdifferentiation. In this review, we emphasize the molecular mechanisms underlying the potential for DCs to give rise to resorbing OCs in the context of bone-destruction-associated diseases upon inflammatory conditions. Whether these mechanisms reveal new strategies for the discovery of therapeutic targets and drugs is discussed extensively.

Osteoimmune Interactions in Inflammatory Bowel Disease: Central Role of Bone Marrow Th17 TNFα Cells in Osteoclastogenesis

Osteoimmunology is an interdisciplinary research field dedicated to the study of the crosstalk between the immune and bone systems. CD4⁺ T cells are central players in this crosstalk. There is an emerging understanding that CD4⁺ T cells play an important role in the bone marrow (BM) under physiological and pathological conditions and modulate the differentiation of bone-resorbing osteoclasts. However, identification of the mechanisms that maintain CD4⁺ T cells in the BM is still a matter of investigation. This article describes the CD4⁺ T cell populations of the BM and reviews their role as osteoclastogenic population in inflammatory bowel disease.

The role of "bone immunological niche" for a new pathogenetic paradigm of osteoporosis

Osteoporosis is characterized by low bone mass and microarchitectural deterioration of bone tissue. The etiology and pathogenetic mechanisms of osteoporosis have not been clearly elucidated. Osteoporosis is linked to bone resorption by the activation of the osteoclastogenic process. The breakdown of homeostasis among pro- and antiosteoclastogenic cells causes unbalanced bone remodeling. The complex interactions among these cells in the bone microenvironment involve several mediators and proinflammatory pathways. Thus, we may consider the bone microenvironment as a complex system in which local and systemic immunity are regulated and we propose to consider it as an "immunological niche." The study of the "bone immunological niche" will permit a better understanding of the complex cell trafficking which regulates bone resorption and disease. The goal of a perfect therapy for osteoporosis would be to potentiate good cells and block the bad ones. In this scenario, additional factors may take part in helping or hindering the proosteoblastogenic factors. Several proosteoblastogenic and antiosteoclastogenic agents have already been identified and some have been developed and commercialized as biological therapies for osteoporosis. Targeting the cellular network of the "bone immunological niche" may represent a successful strategy to better understand and treat osteoporosis and its complications.

Bone marrow Th17 TNFα cells induce osteoclast differentiation, and link bone destruction to IBD

OBJECTIVE

Under both physiological and pathological conditions, bone volume is determined by the rate of bone formation by osteoblasts and bone resorption by osteoclasts. Excessive bone loss is a common complication of human IBD whose mechanisms are not yet completely understood. Despite the role of activated CD4(+) T cells in inflammatory bone loss, the nature of the T cell subsets involved in this process in vivo remains unknown. The aim of the present study was to identify the CD4(+) T cell subsets involved in the process of osteoclastogenesis in vivo, as well as their mechanism of action.

DESIGN

CD4(+) T cells were studied in IL10-/- mice and Rag1-/- mice adoptively transferred with naive CD4(+)CD45RB(high) T cells, representing two well-characterised animal models of IBD and in patients with Crohn's disease. They were phenotypically and functionally characterised by flow cytometric and gene expression analysis, as well as in in vitro cocultures with osteoclast precursors.

RESULTS

In mice, we identified bone marrow (BM) CD4(+) T cells producing interleukin (IL)-17 and tumour necrosis factor (TNF)-α as an osteoclastogenic T cell subset referred to as Th17 TNF-α(+) cells. During chronic inflammation, these cells migrate to the BM where they survive in an IL-7-dependent manner and where they promote the recruitment of inflammatory monocytes, the main osteoclast progenitors. A population equivalent to the Th17 TNF-α(+) cells was also detected in patients with Crohn's disease.

CONCLUSIONS

Our results highlight the osteoclastogenic function of the Th17 TNF-α(+) cells that contribute to bone loss in vivo in IBD.

Tumor necrosis factor-α antagonist infliximab inhibits osteoclast formation of peripheral blood mononuclear cells but does not affect periodontal ligament fibroblast-mediated osteoclast formation

BACKGROUND AND OBJECTIVE

The inflammatory cytokine tumor necrosis factor-alpha (TNF-α) is elevated in inflamed periodontal tissues and may contribute to periodontitis progression. TNF-α stimulates formation and activity of osteoclasts, the cells that are recruited in periodontitis, that cause alveolar bone degradation and subsequent tooth loss. We previously showed that TNF-α is elevated in co-cultures of periodontal ligament fibroblast (PDLF) and peripheral blood mononuclear cells (PBMC). Hence, TNF-α could be a determining factor in osteoclast formation in these cultures, as osteoclasts are formed despite the fact that prototypical osteoclast differentiation factor receptor activator of nuclear factor kappa-B ligand is outnumbered at least 100-fold by its inhibitor osteoprotegerin in these cultures.

MATERIAL AND METHODS

To assess the role of TNF-α in periodontitis-associated osteoclast formation in vitro, osteoclast formation was analyzed in the presence of the anti-TNF-α therapeutic agent infliximab in two culture systems: (i) PBMC in co-culture with PDLFs from controls and patients with periodontitis, or (ii) with PBMC only. PDLFs from control and patients with periodontitis were exposed to infliximab, PBMCs were added and the formation of osteoclast-like cells was assessed.

RESULTS

TNF-α was highest levels in supernatants at 7 d in co-cultures and declined at 14 and 21 d. TNF-α was undetectable in cultures that received infliximab. The formation and activity of osteoclasts in co-cultures was not affected by infliximab. In contrast, infliximab in cultures of only PBMC significantly reduced the formation of osteoclasts. This reduction was accompanied by a decreased number and size of cell clusters, a step that precedes the formation of osteoclasts. TNF-α was again undetectable in the supernatant of infliximab-treated cultures, but was detectable at similar levels in cell lysates of control and infliximab-treated PBMC cultures.

CONCLUSION

Our study shows that the contribution of TNF-α to osteoclast formation is cell system dependent. It contributes to PBMC-induced osteoclast formation, possibly by establishing stronger cell-cell interactions that precede osteoclast formation.

Bone-immune cell crosstalk: bone diseases

Bone diseases are associated with great morbidity; thus, the understanding of the mechanisms leading to their development represents a great challenge to improve bone health. Recent reports suggest that a large number of molecules produced by immune cells affect bone cell activity. However, the mechanisms are incompletely understood. This review aims to shed new lights into the mechanisms of bone diseases involving immune cells. In particular, we focused our attention on the major pathogenic mechanism underlying periodontal disease, psoriatic arthritis, postmenopausal osteoporosis, glucocorticoid-induced osteoporosis, metastatic solid tumors, and multiple myeloma.

Cytokine-mediated bone destruction in rheumatoid arthritis

Bone homeostasis, which involves formation and resorption, is an important process for maintaining adequate bone mass in humans. Rheumatoid arthritis (RA) is an autoimmune disease characterized by inflammation and bone loss, leading to joint destruction and deformity, and is a representative disease of disrupted bone homeostasis. The bone loss and joint destruction are mediated by immunological insults by proinflammatory cytokines and various immune cells. The connection between bone and immunity has been intensely studied and comprises the emerging field of osteoimmunology. Osteoimmunology is an interdisciplinary science investigating the interplay between the skeletal and the immune systems. The main contributors in osteoimmunology are the bone effector cells, such as osteoclasts or osteoblasts, and the immune cells, particularly lymphocytes and monocytes. Physiologically, osteoclasts originate from immune cells, and immune cells regulate osteoblasts and vice versa. Pathological conditions such as RA might affect these interactions, thereby altering bone homeostasis, resulting in the unfavorable outcome of bone destruction. In this review, we describe the osteoclastogenic roles of the proinflammatory cytokines and immune cells that are important in the pathophysiology of RA.

Treatment of bone loss in osteopenic patients with Crohn's disease: a double-blind, randomised trial of oral risedronate 35 mg once weekly or placebo, concomitant with calcium and vitamin D supplementation

OBJECTIVE

Osteoporosis and fractures are frequently encountered in patients with Crohn's disease. In order to prevent fractures, treatment with bone protecting drugs appears warranted early in the course of bone disease when bone loss is not yet prominent. We therefore aimed to demonstrate a beneficial effect on bone density of the bisphosphonate risedronate in osteopenic Crohn's disease patients.

METHODS

This double-blind, placebo-controlled randomised trial of risedronate with calcium and vitamin D supplementation was performed in osteopenic Crohn's disease patients. Patients were treated for 2 years with follow-up after 3 and after every 6 months. Disease characteristics and activity and bone turnover markers were assessed at all visits; dual x-ray absorptiometry was performed at baseline, 12 and 24 months; radiographs of the spine at baseline and 24 months.

RESULTS

Of 132 consenting patients, 131 were randomised (67 placebo and 64 risedronate). Patient characteristics were similar in both groups, although the risedronate group was slightly heavier (body mass index 24.3 vs 23.0 kg/m(2)). Bone mineral density at lumbar spine increased 0.04 g/cm(2) on average in the risedronate group versus 0.01 g/cm(2) in the placebo group (p=0.007). The mean increase in total hip bone mineral density was 0.03 versus 0.01 g/cm(2), respectively (p=0.071). Fracture prevalence and incidence were similar. Change of T-scores and concentrations of bone turnover markers were consistent with a beneficial effect of risedronate when compared with placebo. The effect of risedronate was primarily demonstrated in the first 12 months of treatment. No serious unexpected suspected adverse events were observed.

CONCLUSIONS

A 24-month treatment course with risedronate 35 mg once weekly, concomitant with calcium and vitamin D supplementation, in osteopenic Crohn's disease patients improved bone density at lumbar spine. NTR 163 Dutch Trial Register.

Progressive Improvement of T-Scores in Men with Osteoporosis and Subnormal Serum Testosterone Levels upon Treatment with Testosterone over Six Years

Testosterone deficiency leads to bone loss and testosterone treatment has a beneficial effect. This study investigated the effects of normalizing serum testosterone on bone mineral density in 45 men with osteoporosis, diagnosed with testosterone deficiency (serum testosterone levels <12.1 nmol/L, T-scores: (mean ± SD) -3.12 ± 0.45, minimum: -4.10, and maximum: -2.60). In a cumulative, prospective, registry study of hypogonadal men (mean age: 53 ± 7 years) they received parenteral testosterone undecanoate of 1000 mg/12 weeks for up to six years. After one year 44 men were included in the registry, after two years 36 men, after three years 32 men, after four years 25 men, after five years 10 men and after six years 4 men. The declining numbers do not reflect drop-out rates but are a result of the registry design. Over the 6 year period there was a significant and progressive improvement of the T-scores in these men. Normalizing of serum testosterone leads to an improvement of bone mineral density and this improvement was progressive with the time period of testosterone administration. In this study of 6-years many men with testosterone deficiency suffered from classical diagnoses (Klinefelter's syndrome and testicular pathology) hitherto undiagnosed.

The role of T helper (TH)17 cells as a double-edged sword in the interplay of infection and autoimmunity with a focus on xenobiotic-induced immunomodulation

Extensive research in recent years suggests that exposure to xenobiotic stimuli plays a critical role in autoimmunity induction and severity and that the resulting response would be exacerbated in individuals with an infection-aroused immune system. In this context, heavy metals constitute a prominent category of xenobiotic substances, known to alter divergent immune cell responses in accidentally and occupationally exposed individuals, thereby increasing the susceptibility to autoimmunity and cancer, especially when accompanied by inflammation-triggered persistent sensitization. This perception is learned from experimental models of infection and epidemiologic studies and clearly underscores the interplay of exposure to such immunomodulatory elements with pre- or postexposure infectious events. Further, the T_H17 cell subset, known to be associated with a growing list of autoimmune manifestations, may be the “superstar” at the interface of xenobiotic exposure and autoimmunity. In this review, the most recently established links to this nomination are short-listed to create a framework to better understand new insights into T_H17's contributions to autoimmunity.

Bone effects of biologic drugs in rheumatoid arthritis

Biologic agents used in the treatment of rheumatoid arthritis (RA) are able to reduce both disease activity and radiographic progression of joint disease. These drugs are directed against several proinflammatory cytokines (TNF α , IL-6, and IL-1) which are involved both in the pathogenesis of chronic inflammation and progression of joint structural damage and in systemic and local bone loss typically observed in RA. However, the role of biologic drugs in preventing bone loss in clinical practice has not yet clearly assessed. Many clinical studies showed a trend to a positive effect of biologic agents in preventing systemic bone loss observed in RA. Although the suppression of inflammation is the main goal in the treatment of RA and the anti-inflammatory effects of biologic drugs exert a positive effect on bone metabolism, the exact relationship between the prevention of bone loss and control of inflammation has not been clearly established, and if the available biologic drugs against TNF α , IL-1, and IL-6 can exert their effect on systemic and local bone loss also through a direct mechanism on bone cell metabolism is still to be clearly defined.

Estrogen deficiency induces the differentiation of IL-17 secreting Th17 cells: a new candidate in the pathogenesis of osteoporosis

Th17 cells produce IL-17, and the latter promotes bone loss in collagen-induced arthritis in mice. Blocking IL-17 action in mouse model of rheumatoid arthritis reduces disease symptoms. These observations suggest that Th17 cells may be involved in the pathogenesis of bone loss. However, the role of Th17 cell in estrogen (E2) deficiency-induced bone loss is still not very clear. We investigated the effect of E2 on Th17 differentiation in vivo and IL-17 mediated regulation of osteoclast and osteoblast differentiation. Additionally, effect of IL-17 functional block under E2 deficiency-induced bone loss was studied. In murine bone marrow cells, E2 suppressed IL-17 mediated osteoclast differentiation. IL-17 inhibited formation of mineralized nodules in osteoblasts and this effect was suppressed by E2. E2 treatment to mouse calvarial osteoblasts inhibited the IL-17-induced production of osteoclastogenic cytokines and NF-kB translocation. In ovariectomized mice, there was increase in the number of Th17 cells, transcription factors promoting Th17 cell differentiation and circulating IL-17 levels. These effects were reversed by E2 supplementation. Treatment of neutralizing IL-17 monoclonal antibody to Ovx mice mitigated the E2 deficiency-induced trabecular bone loss and reversed the decreased osteoprotegerin-to-receptor activator of nuclear factor kappa B ligand (RANKL) transcript levels in long bones, increased osteoclast differentiation from the bone marrow precursor cells and decreased osteoblast differentiation from the bone marrow stromal cells. Our findings indicate that E2 deficiency leads to increased differentiation of Th17 cells with attendant up regulation of STAT3, ROR-γt and ROR-α and downregulation of Foxp3 which antagonizes Th17 cell differentiation. Increased IL-17 production in turn induces bone loss by increasing pro-osteoclastogenic cytokines including TNF-α, IL-6 and RANKL from osteoblasts and functional block of IL-17 prevents bone loss. IL-17 thus plays a critical causal role in Ovx-induced bone loss and may be considered a potential therapeutic target in pathogenesis of post menopausal osteoporosis.

Pathways for bone loss in inflammatory disease

Chronic inflammation including autoimmune disease is an important risk factor for the development of osteoporosis. Receptor activator of nuclear factor-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) play a central role in osteoclast differentiation and function, and the molecular pathways by which M-CSF and RANKL induce osteoclast differentiation have been analyzed in detail. Proinflammatory cytokines directly or indirectly regulate osteoclastogenesis and bone resorption providing a link between inflammation and osteoporosis. Tumor necrosis factor-α, interleukin (IL)-1, IL-6, and IL-17 are the most important proinflammatory cytokines triggering inflammatory bone loss. Inhibition of these cytokines has provided potent therapeutic effects in the treatment of diseases such as rheumatoid arthritis. Further investigation is needed to understand the pathophysiology and to develop new strategies to treat inflammatory bone loss. This review summarizes new data on inflammatory bone loss obtained in 2011.

Intercellular cross-talk among bone cells: new factors and pathways

Intercellular communication within the bone microenvironment is critical for the maintenance of normal bone structure. Osteoblast-lineage cells at all stages of differentiation, from pluripotent precursors to matrix-embedded osteocytes, produce regulatory factors that modulate the differentiation and activity of both bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoclasts can also release factors that feed back to regulate osteoblast activity. Intercellular cross-talk within the bone microenvironment is not restricted only to these bone cells. Other cells within the bone marrow microenvironment, including adipocytes, T cells, and macrophages, play key roles that influence the processes of bone formation and resorption. This review discusses recent work that provides new insights into some of these communication networks and the factors involved, including osteocytic production of receptor activator of nuclear factor-κB ligand (RANKL) and sclerostin, osteoblastic production of interleukin-33, osteoclast-derived Semaphorin 4D, ephrin signaling, and signals from T helper cells and resident osteal macrophages (osteomacs).