Pathological bone resorption in rheumatic diseases

Abstract

Bone resorption in a common feature of rheumatoid and other inflammatory joint conditions. This osteolysis is effected by osteoclasts, highly specialised multinucleated cells which are formed by fusion of mononuclear phagocyte precursors of haematopoietic origin. It has been shown that human osteoclast precursors circulate in the monocyte fraction of peripheral blood and are present in the macrophage population of normal and pathological tissues. The cellular and humoral requirements for human monocyte/macrophage-osteoclast differentiation are the presence of bone-derived stromal cells, which express the membrane-bound protein termed osteoclast differentiation factor or osteoprotegerin ligand (ODF/OPGL), and macrophage-colony stimulating factor (M-CSF). Macrophages form a major component of the inflammatory infiltrate in the rheumatoid synovium and in other conditions such as PVNS and aseptic loosening. Local factors (e.g. cytokines and growth factors) produced in this microenvironment act to stimulate not only osteoclast activity but also osteoclast formation. In the context of RA and other joint conditions in which osteolysis occurs, pathological bone resorption should be regarded as a function not only of osteoclast activity but also of osteoclast formation from mononuclear phagocyte precursors.     

Effect of corticosteroids on human osteoclast formation and activity

Chronic corticosteroid treatment is known to induce bone loss and osteoporosis. Osteoclasts are specialised bone-resorbing cells that are formed from mononuclear phagocyte precursors that circulate in the monocyte fraction. In this study we have examined the effect of the synthetic glucocorticoid, dexamethasone, on human osteoclast formation and bone-resorbing activity. Human monocytes were cultured for up to 21 days on glass coverslips and dentine slices, with soluble receptor activator for nuclear factor kappaB ligand (RANKL; 30 ng/ml) and human macrophage-colony stimulating factor (M-CSF; 25 ng/ml) in the presence and absence of dexamethasone (10(-8) M). The addition of dexamethasone over a period of 7 and 14 days of culture of monocytes (during which cell proliferation and differentiation predominantly occurred) resulted in a marked increase in the formation of tartrate-resistant acid phosphatase-positive multinucleated cells and an increase in lacunar resorption. The addition of dexamethasone to monocyte cultures after 14 days (when resorptive activity of osteoclasts had commenced) reduced the extent of lacunar resorption compared with cultures to which no dexamethasone had been added. The addition of dexamethasone to osteoclasts isolated from giant cell tumours of bone significantly inhibited resorption pit formation. Our findings indicate that dexamethasone has a direct effect on osteoclast formation and activity, stimulating the proliferation and differentiation of human osteoclast precursors and inhibiting the bone-resorbing activity of mature osteoclasts.     

Synovial macrophage-osteoclast differentiation in inflammatory arthritis

BACKGROUND: Pathological bone resorption (marginal erosions and juxta-articular osteoporosis) by osteoclasts commonly occurs in rheumatoid arthritis (RA). OBJECTIVES: To define the nature of the mononuclear precursor cells from which osteoclasts are formed in inflamed synovial tissues and to determine the cellular and humoral factors which influence osteoclast differentiation. METHOD: Macrophage (CD14+), non-macrophage (CD14-), and unsorted (CD14+/CD14-) synovial cell populations from RA and inflammatory/non-inflammatory osteoarthritis (OA) synovium were cultured in the presence of receptor activator for nuclear factor kappaB ligand (RANKL) and monocyte-colony stimulating factor (M-CSF; in the presence/absence of prostaglandin E(2) (PGE(2)), interleukin 1beta (IL1beta), tumour necrosis factor alpha (TNFalpha), and IL6). Osteoclast differentiation was assessed by expression of tartrate resistant acid phosphatase (TRAP), vitronectin receptor (VNR), and lacunar resorption. RESULTS: TRAP+ and VNR+ multinucleated cells capable of lacunar resorption were only formed in cultures of CD14+-containing synovial cell populations (that is, CD14+ and CD14+/CD14- cells). No difference in the extent of osteoclast formation was noted in cultures of CD14+ cells isolated from RA, inflammatory OA, and non-inflammatory OA synovium. However, more TRAP+/VNR+ cells and more lacunar resorption was noted in CD14+/CD14- cells from RA and inflammatory OA synovial tissues. The addition of PGE(2), IL1beta, TNFalpha, and IL6 did not increase RANKL/M-CSF-induced osteoclast formation and lacunar resorption of both CD14+/CD14- and CD14+ synovial cell populations. CONCLUSIONS: Osteoclast precursors in synovial tissues are CD14+ monocyte/macrophages. The increase in osteoclast formation in cultures of CD14+/CD14- compared with CD14+ synovial cells in RA and inflammatory OA points to a role for CD14- cells in promoting osteoclast differentiation and bone resorption in inflamed synovial tissues by a mechanism which does not involve a direct effect of proinflammatory cytokines/prostaglandins on RANKL-induced macrophage-osteoclast differentiation.     

Effect of osteoprotegerin and osteoprotegerin ligand on osteoclast formation by arthroplasty membrane derived macrophages

OBJECTIVE: Osteoprotegerin ligand (OPGL) is a newly discovered molecule, which is expressed by osteoblasts/bone stromal cells. This ligand and M-CSF are now known to be essential for osteoclast differentiation from marrow and circulating precursors. This study examined whether OPGL and its soluble receptor osteoprotegerin (OPG), influenced osteoclast formation from human arthroplasty derived macrophages, to determine if the effects of OPGL and OPG on these cells could contribute to the osteolysis of aseptic loosening. METHODS: OPGL (+/- dexamethasone/M-CSF) was added to cultures of macrophages isolated from the pseudomembrane of loosened hip arthroplasties incubated on glass coverslips and dentine slices. OPG was added to cocultures of arthroplasty derived macrophages and UMR106 osteoblast-like cells. Osteoclast differentiation in long term cultures was assessed by expression of macrophage (CD14) and osteoclast markers (tartrate resistant acid phosphatase (TRAP), vitronectin receptor (VNR) and lacunar resorption). RESULTS: In the absence of osteoblastic cells, the addition of OPGL alone was sufficient to induce differentiation of macrophages (CD14(+), TRAP(-), VNR(-)) into TRAP(+) and VNR(+) multinucleated cells, capable of extensive lacunar resorption. OPG was found to inhibit osteoclast formation by arthroplasty macrophages in a dose dependent manner. OPG (100 ng/ml) more than halved the formation of TRAP(+) and VNR(+) cells and the extent of lacunar resorption in co-cultures of UMR106 cells and arthroplasty macrophages. CONCLUSIONS: This study has shown that macrophages, isolated from the pseudomembrane surrounding loose arthroplasty components, are capable of differentiating into osteoclastic bone resorbing cells and that OPGL is required for this to occur. OPG inhibits this process, most probably by interrupting the cell-cell interaction between osteoblasts and mononuclear phagocyte osteoclast precursors present in the pseudomembrane.     

Joint erosion in rheumatoid arthritis: interactions between tumour necrosis factor alpha, interleukin 1, and receptor activator of nuclear factor kappaB ligand (RANKL) regulate osteoclasts

BACKGROUND: Osteoclasts, specialised bone resorbing cells regulated by RANKL and M-CSF, are implicated in rheumatoid joint erosion. Lymphocyte-monocyte interactions activate bone resorption, this being attributed to tumour necrosis factor alpha (TNFalpha) and interleukin 1 beta (IL1beta) enhanced osteoblast expression of RANKL. In animal studies, TNF potently increases osteoclast formation in the presence of RANKL. RANKL-independent osteoclastogenesis also occurs, though IL1 is required for resorptive function in most studies. These inflammatory cytokines have a pivotal role in rheumatoid arthritis, OBJECTIVE: To study the interactions of TNFalpha and IL1beta with RANKL, particularly the time course of the interactions and the role of lymphocytes. METHOD: Cultures of lymphocytes and monocytes (osteoclast precursors) or of purified CD14(+) cells alone (osteoclast precursors) were exposed to various combinations of TNFalpha, RANKL, and IL1beta or the inhibitors osteoprotegerin, IL1 receptor antagonist, or neutralising antibodies to RANKL or to IL1. Osteoclastogenesis and resorptive activity were assessed on microscopy of dentine slices. RESULTS: TNFalpha potently increased osteoclast proliferation/differentiation in the presence of RANKL. This effect was greatest when RANKL was present before but not after exposure of osteoclast precursor cells to TNFalpha. The resorptive activity of osteoclasts generated by TNFalpha in the absence of RANKL was critically dependent upon IL1, which was expressed by lymphocyte-monocyte interaction. CONCLUSION: TNFalpha potently enhances RANKL mediated osteoclast activity. Interactions between TNFalpha and IL1 also result in osteoclastic activity independently of RANKL. These findings will inform therapeutic approaches to the prevention of joint erosion in rheumatoid arthritis.     

Osteoclast formation from circulating precursors in osteoporosis

OBJECTIVE: An imbalance between bone formation and bone resorption is thought to underlie the pathogenesis of reduced bone mass in osteoporosis. Bone resorption is carried out by osteoclasts. which are formed from marrow-derived cells that circulate in the monocyte fraction. The aim of this study was to determine the role of osteoclast formation in the pathogenesis of bone loss in osteoporosis. METHODS: The proportion of circulating osteoclast precursors and their relative sensitivity to the osteoclastogenic effects of M-CSF. 1,25(OH)2D3 and RANKL were assessed in primary osteoporosis patients and normal controls. RESULTS: Although there was no difference in the number of circulating osteoclast precursors in osteoporosis patients and normal controls. osteoclasts formed from osteoporosis patients exhibited substantially increased resorptive activity relative to normal controls. Although no increased sensitivity to the osteoclastogenic effects of 1,25(OH)D3 or M-CSF was noted, increased bone resorption was found in osteoporosis peripheral blood mononuclear cell (PBMC) cultures to which these factors were added. CONCLUSION: Our findings suggest that osteoclast functional activity rather than formation is increased in primary involutional osteoporosis and that dexamethasone acts to increase osteoclast formation.     

LIGHT (TNFSF14), a novel mediator of bone resorption, is elevated in rheumatoid arthritis

OBJECTIVE: Human osteoclast formation from mononuclear phagocyte precursors involves interactions between tumor necrosis factor (TNF) ligand superfamily members and their receptors. LIGHT is a transmembrane protein expressed and shed from the surface of activated T cells. Since activated T cells have been implicated in osteoclastogenesis in rheumatoid arthritis (RA), this study sought to determine whether LIGHT can regulate RANKL/cytokine-induced osteoclast formation, to identify the mechanism by which LIGHT influences osteoclastogenesis, and to investigate the presence of LIGHT in the serum of RA patients. METHODS: The effect of LIGHT on human and murine osteoclast formation was assessed in the presence and absence of neutralizing reagents to known osteoclastogenic factors. Serum levels of LIGHT in RA patients were measured by enzyme-linked immunosorbent assay. RESULTS: In the presence and absence of RANKL, LIGHT induced osteoclast formation from both human peripheral blood mononuclear cells and murine macrophage precursors, in a dose-dependent manner, whereas no inhibition was observed by adding osteoprotegerin, RANK:Fc, TNFalpha, or interleukin-8 or by blocking the LIGHT receptors herpesvirus entry mediator or lymphotoxin beta receptor. However, formation of osteoclasts was significantly decreased by the soluble decoy receptor for LIGHT, DcR3, and by blocking antibodies to the p75 component of the TNF receptor. A significant increase in LIGHT levels in the serum of RA patients compared with normal controls was also noted. CONCLUSION: Our results indicate that LIGHT promotes RANKL-mediated osteoclastogenesis and that it can induce osteoclast formation by a mechanism independent of RANKL. The increased concentration of LIGHT in patients with RA raises the possibility that LIGHT may play a role in immunopathogenic conditions that are associated with localized or systemic bone loss.     

Osteoclast formation and activity in the pathogenesis of osteoporosis in rheumatoid arthritis

OBJECTIVE: Rheumatoid arthritis (RA) is often complicated by generalized osteopenia due to increased bone resorption by osteoclasts. We analysed a number of cellular and humoral factors that influence osteoclast formation from circulating precursors in RA patients. METHODS: Monocytes isolated from RA patients and normal controls were cultured with macrophage colony-stimulating factor (M-CSF) and nuclear factor-kappaB ligand (RANKL), or with RANKL-expressing UMR106 cells and 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. Osteoclast differentiation was assessed by expression of tartrate-resistant acid phosphatase (TRAP) and vitronectin receptors (VNR) and lacunar resorption. RESULTS: Osteoclasts formed from RA patients exhibited increased resorptive activity but there was no difference in the relative proportion of circulating osteoclast precursors between RA patients and normal controls. Osteoclast precursors in RA patients were not more sensitive to the osteoclastogenic effects of 1,25(OH)(2)D(3), M-CSF or RANKL. Dexamethasone, but not interleukin (IL) 1beta, tumour necrosis factor alpha and IL-6, increased osteoclast formation and lacunar resorption. CONCLUSION: There is an increase in the extent of lacunar resorption carried out by osteoclasts formed from circulating precursors in RA patients. This is not due to an increase in the number of circulating precursors or increased sensitivity to the osteoclastogenic effects of 1,25(OH)(2)D(3), M-CSF, RANKL or inflammatory cytokines. Our findings suggest that increased osteoclast functional activity rather than osteoclast formation is more likely to play a role in the generalized bone loss that occurs in RA, and that corticosteroids stimulate osteoclast formation and resorption.     

The effect of selective cyclooxygenase-2 inhibitor on human osteoclast precursors to influence osteoclastogenesis in vitro

Abstract

The inducible prostaglandin synthesis enzyme, cyclooxygenase-2 (COX-2), is involved in bone resorption and osteoclastogenesis, and acts indirectly through prostaglandin E2 (PG E2) produced by osteoblastic cells. This study was undertaken to investigate whether celecoxib (a selective COX-2 inhibitor) has a direct effect on human osteoclast precursors to influence osteoclastogenesis in vitro. Human peripheral blood mononuclear cells (PBMCs) were cultured on glass coverslips and dentine slices with soluble receptor activator of NF-kB ligand (sRANKL) and macrophage colony stimulating factor (M-CSF). COX inhibitors including celecoxib were added to the cultures. Osteoclast formation was assessed as the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNCs), and the functional evidence of lacunar resorption pits on dentine slices was assessed. Celecoxib and indomethacin inhibited osteoclast formation and the extent of lacunar resorption in a dose-dependent manner, but the effect of indomethacin was less than that of celecoxib. Mofezolac affected neither the number of TRAP-positive MNCs nor the extent of lacunar resorption pits. These results indicate that celecoxib influences not only osteoclast formation through osteoblastic cells but also acts directly on circulating osteoclast precursors to influence human osteoclast differentiation. The effect of celecoxib on osteoclast precursors may be related to the COX-2 signal pathway.     

The effect of selective cyclooxygenase-2 inhibitor on human osteoclast precursors to influence osteoclastogenesis in vitro

The inducible prostaglandin synthesis enzyme, cyclooxygenase-2 (COX-2), is involved in bone resorption and osteoclastogenesis, and acts indirectly through prostaglandin E2 (PG E2) produced by osteoblastic cells. This study was undertaken to investigate whether celecoxib (a selective COX-2 inhibitor) has a direct effect on human osteoclast precursors to influence osteoclastogenesis in vitro. Human peripheral blood mononuclear cells (PBMCs) were cultured on glass coverslips and dentine slices with soluble receptor activator of NF-kB ligand (sRANKL) and macrophage colony stimulating factor (M-CSF). COX inhibitors including celecoxib were added to the cultures. Osteoclast formation was assessed as the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNCs), and the functional evidence of lacunar resorption pits on dentine slices was assessed. Celecoxib and indomethacin inhibited osteoclast formation and the extent of lacunar resorption in a dose-dependent manner, but the effect of indomethacin was less than that of celecoxib. Mofezolac affected neither the number of TRAP-positive MNCs nor the extent of lacunar resorption pits. These results indicate that celecoxib influences not only osteoclast formation through osteoblastic cells but also acts directly on circulating osteoclast precursors to influence human osteoclast differentiation. The effect of celecoxib on osteoclast precursors may be related to the COX-2 signal pathway.     

Regulation of macrophage function by adenosine

Following its release into the extracellular space in response to metabolic disturbances, the endogenous nucleoside adenosine exerts a range of immunomodulatory effects and cells of the mononuclear phagocyte system are among its major targets. Adenosine governs mononuclear phagocyte functions via 4 G-protein-coupled cell membrane receptors, which are denoted A(1), A(2A), A(2B), and A(3) receptors. Adenosine promotes osteoclast differentiation via A(1) receptors and alters monocyte to dendritic cell differentiation through A(2B) receptors. Adenosine downregulates classical macrophage activation mainly through A(2A) receptors. In contrast A(2B) receptor activation upregulates alternative macrophage activation. Adenosine promotes angiogenesis, which is mediated by inducing the production of vascular endothelial growth factor by mononuclear phagocytes through A(2A), A(2B), and A(3) receptors. By regulating mononuclear phagocyte function adenosine dictates the course of inflammatory and vascular diseases and cancer.     

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

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

Rheumatoid synovial endothelial cells produce macrophage colony-stimulating factor leading to osteoclastogenesis in rheumatoid arthritis

OBJECTIVES: Periarticular osteoporosis and joint destruction are major complications in rheumatoid arthritis (RA), caused by osteoclast-mediated bone resorption. However, the mechanisms of monocyte/osteoclast maturation and role of RA endothelial cells (RAECs) in the control of osteoclastogenesis remain unclear. The present study was designed to determine the most important factors that influence monocyte accumulation and osteoclast formation among the many factors produced by RAEC. METHODS: We analysed the expression profiles of various genes in human endothelial cells from various organs (RA synovium, umbilical vein, skin, liver sinusoid, renal glomerulus and brain) using oligonucleotide microarrays. Specifically, up-regulated gene in RAECs was assessed by real-time quantitative polymerase chain reaction, enzyme-linked immunosorbent assay and immunostaining of RA synovia. Migration of monocytes was assessed by the chemotactic chamber EZ-TAXIScan. Tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cell (MNC) formation was observed by microscopy. RESULTS: Among many epithelial-expressed factors, macrophage colony-stimulating factor (M-CSF) gene was abundantly expressed specifically in RAECs. Genes of fibroblast growth factor-2, interleukin-6 and osteoprotegerin were also overexpressed in RAECs. Migration of monocytes and osteoclast formation in co-cultures promoted by culture supernatants of RAECs were inhibited by M-CSF neutralizing antibody. CONCLUSIONS: M-CSF produced by RAECs is involved in osteoclastogenesis from monocytes, migration and TRAP-positive MNC formation.     

Current insights into the role of transforming growth factor-beta in bone resorption

Transforming growth factor-beta (TGF-beta) elicits a variety of effects on cellular proliferation and differentiation. The major repository for TGF-beta is bone, where it possesses separate facilitative and suppressive actions on osteoclast differentiation and bone resorption. Without a direct enabling stimulus from TGF-beta monocytes cannot form osteoclasts but instead follow macrophage differentiation pathways. This facilitative action depends on an ability to promote a state in which precursors are resistant to anti-osteoclastic inflammatory signals. Following the initiation of resorption TGF-beta is released from bone matrix. This acts on osteoblasts to reduce the availability of the osteoclast differentiation factor, RANKL (receptor activator of NFkappaB ligand) and thereby indirectly limits further osteoclast formation. Thus TGF-beta has a fundamental role in the control of bone resorption having actions that first allow monocytes to develop into osteoclasts then subsequently limiting the extent and duration of resorption after its release from the bone matrix.     

The effect of long-term treatment with steroid hormones or tamoxifen on oestrogen receptors (alpha and beta) in the endometrium of ovariectomized cynomolgus macaques

A number of bone diseases characterised by excessive osteolysis (e.g. osteoporosis and Paget's disease) exhibit a marked gender difference in prevalence and are more common in the elderly population. Bone resorption is carried out by osteoclasts, which are formed by fusion of circulating mononuclear precursor cells of haematopoietic origin. In this study, we have determined whether there are gender- and age-related differences in osteoclast formation from circulating precursors. Peripheral blood mononuclear cells (PBMCs) were co-cultured with UMR106 osteoblast-like cells in the presence of macrophage-colony stimulating factor (M-CSF) and 1,25 dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) or cultured alone in the presence of sRANKL (soluble receptor activator of nuclear factor kappa B ligand) and M-CSF. As assessed by the formation of tartrate resistant acid phosphatase (TRAP)-positive (TRAP(+)) and vitronectin receptor-positive (VNR(+)) multinucleated cells (MNCs), there was no difference in the number of circulating osteoclast precursors in males and females. Lacunar resorption carried out by osteoclasts formed from these precursors was generally increased in males compared with females (P=0.03). An increase in the number of TRAP(+) and VNR(+) MNCs formed from male PBMCs was noted in response to 1,25(OH)(2)D(3) (P<0.005). An increase in lacunar resorption in cultures of PBMCs (10(5) per well) from males was also noted in response to 10(-9) M 1,25(OH)(2)D(3) (P<0.05) and sRANKL (P=0.05), but not M-CSF. The addition of dexamethasone resulted in a marked increase in osteoclast formation and lacunar resorption in both males and females. Post-menopausal females and males of comparable age showed similar levels of osteoclastogenesis. Pre-menopausal women showed similar levels of osteoclastogenesis but less resorption (P=0.01) compared with males of comparable age. These results show that there are specific gender/age-related differences in osteoclast formation and bone resorption and have implications for evaluating osteoclastogenesis in skeletal diseases such as primary osteoporosis and Paget's disease.     

TSG‐6 inhibits osteoclast activity via an autocrine mechanism and is functionally synergistic with osteoprotegerin

Objective

TSG‐6 (the product of tumor necrosis factor [TNF]–stimulated gene 6) has a potent inhibitory effect on RANKL‐mediated bone erosion. The aim of this study was to compare the activity of TSG‐6 with that of osteoprotegerin (OPG) and to investigate its role as an autocrine modulator of cytokine‐mediated osteoclast formation/activation. We also determined TSG‐6 expression in inflammatory joint disease.

Methods

The effects of TSG‐6, OPG, and the inflammation mediators TNFα, interleukin‐1 (IL‐1), and IL‐6 on the formation of osteoclasts from peripheral blood mononuclear cells and synovial fluid (SF) macrophages were determined by tartrate‐resistant acid phosphatase staining. Lacunar resorption and filamentous actin ring formation were measured as indicators of osteoclast activity. The amount of TSG‐6 in culture media or SF was quantified by enzyme‐linked immunosorbent assay, and expression of TSG‐6 in synovial tissue was assessed by immunohistochemistry.

Results

TSG‐6 acted in synergy with OPG to inhibit RANKL‐mediated bone resorption and was produced by osteoclast precursors and mature osteoclasts in response to TNFα, IL‐1, and IL‐6. Expression of TSG‐6 correlated with inhibition of lacunar resorption; this effect was ameliorated by an anti–TSG‐6 antibody. The level of TSG‐6 protein was determined in SF from patients with various arthritides; it was highest in patients with inflammatory conditions such as rheumatoid arthritis, in which it correlated with the amount of TSG‐6 immunostaining in the synovium. TSG‐6 inhibited the activation but not the formation of osteoclasts from SF macrophages.

Conclusion

In the presence of inflammatory cytokines, osteoclasts produced TSG‐6 at concentrations that are sufficient to inhibit lacunar resorption. This may represent an autocrine mechanism to limit the degree of bone erosion during joint inflammation.

     

Infliximab acts directly on human osteoclast precursors and enhances osteoclast formation induced by receptor activator of nuclear factor κB ligand in vitro

Infliximab is known to protect against the development of joint destruction. In the present study, we sought to determine whether Infliximab acts directly on human osteoclast precursors and influences monocyte-osteoclast differentiation induced by receptor activator of nuclear factor B ligand (RANKL) in vitro. Peripheral blood mononuclear cells (PBMCs) isolated from rheumatoid arthritis (RA) patients and normal controls were cultured in the presence of RANKL and macrophage colony stimulating factor. Infliximab, antihuman tumor necrosis factor (TNF), antihuman TNF soluble receptor p55 (TNFR p55), and antihuman TNF soluble receptor p75 (TNFR p75) antibodies were added. Osteoclast formation was determined by assessing the number of tartrate-resistant acid phosphatase (TRAP) staining cells and the extent of lacunar resorption. Addition of Infliximab resulted in a marked increase in the number of TRAP-positive multinucleated cells (TRAP⁺ MNCs) and in the extent of lacunar resorption compared with the control cultures. Antihuman TNF antibody showed the same effect; however, the addition of neither TNFR p55 nor TNFR p75 antibody affected the extent of TRAP⁺ MNCs and lacunar resorption. Our results suggest that infliximab acts directly on early osteoclast precursors, and stimulates osteoclast formation and lacunar resorption induced by RANKL in vitro.     

Cell biology of the osteoclast.

The osteoclast is a hematopoietic cell derived from CFU-GM and branches from the monocyte-macrophage lineage early during the differentiation process. The marrow microenvironment appears critical for osteoclast formation due to production of RANK ligand, a recently described osteoclast differentiation factor, by marrow stromal cells in response to a variety of osteotropic factors. In addition, factors such as osteoprotegerin, a newly described inhibitor of osteoclast formation, as well as secretory products produced by the osteoclast itself and other cells in the marrow enhance or inhibit osteoclast formation. The identification of the role of oncogenes such as c-fos and pp60 c-src in osteoclast differentiation and bone resorption have provided important insights in the regulation of normal osteoclast activity. Current research is beginning to delineate the signaling pathways involved in osteoclastic bone resorption and osteoclast formation in response to cytokines and hormones. The recent development of osteoclast cell lines may make it possible for major advances to our understanding of the biology of the osteoclast to be realized in the near future.