Inflammatory T cells rapidly induce differentiation of human bone marrow stromal cells into mature osteoblasts

T cells potentiate PTH-induced cortical bone loss through CD40L signaling

Parathyroid hormone (PTH) promotes bone catabolism by targeting bone marrow (BM) stromal cells (SCs) and their osteoblastic progeny. Here we show that a continuous infusion of PTH that mimics hyperparathyroidism fails to induce osteoclast formation, bone resorption, and cortical bone loss in mice lacking T cells. T cells provide proliferative and survival cues to SCs and sensitize SCs to PTH through CD40 ligand (CD40L), a surface molecule of activated T cells that induces CD40 signaling in SCs. As a result, deletion of T cells or T cell-expressed CD40L blunts the bone catabolic activity of PTH by decreasing bone marrow SC number, the receptor activator of nuclear factor-kappaB ligand (RANKL)/OSTEOPROTEGERN (OPG) ratio, and osteoclastogenic activity. Therefore, T cells play an essential permissive role in hyperparathyroidism as they influence SC proliferation, life span, and function through CD40L. T cell-SC crosstalk pathways may thus provide pharmacological targets for PTH-induced bone disease.

The role of fracture-associated soft tissue injury in the induction of systemic inflammation and remote organ dysfunction after bilateral femur fracture

OBJECTIVES

The storage of preformed cytokines in soft tissue as well as the immunocompetence of adipocytes has been reported. We hypothesized that fracture-associated soft tissue injury plays a pivotal role in the induction of systemic inflammation and remote organ dysfunction after fracture.

MATERIALS AND METHODS

Male C57/BL6 mice sustained either severe soft tissue injury to both thighs (STI), bilateral femur fracture with minimal soft tissue injury (Fx), or the combination of both (Fx+STI) and were euthanized 6 hours after injury. Serum cytokine concentrations were measured using the Luminex multiplexing platform, and serum ALT levels were measured with the Vitros 950 Chemistry System. Hepatic myeloperoxidase activity, a marker for polymorphonuclear cell infiltration, and hepatic IL-6 levels were determined using ELISA kits. Hepatic permeability changes were assessed via measurement of edema formation.

RESULTS

STI as well as Fx both induced significantly elevated serum IL-6 and L-10 levels as compared with Sham animals (P < 0.05). Further, serum IL-6 and IL-10 levels were significantly higher after STI as compared with Fx (P < 0.05). Serum KC and MCP-1 levels were significantly elevated after STI but not after Fx (P < 0.05). STI as well as Fx resulted in significantly increased serum ALT levels and hepatic polymorphonuclear cell infiltration. The combination of both injuries resulted in further increased systemic inflammatory mediators as well as marked liver inflammation and dysfunction represented by significantly elevated serum ALT levels, hepatic polymorphonuclear cell infiltration, hepatic IL-6 concentrations, and hepatic edema formation as compared with Sham animals (P < 0.05).

CONCLUSIONS

Although STI and Fx both induced a systemic inflammatory response, this was more pronounced after STI. However, the combination of both injuries (Fx+STI) was required to induce marked liver dysfunction. Our data indicate that fracture-associated soft tissue injury is a major contributor to the systemic inflammatory response after bilateral femur fracture. This should be recognized especially in multiply injured patients, who are susceptible to systemic inflammation with remote organ dysfunction.

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.

Estrogen deficiency, T cells and bone loss

Estrogen plays a fundamental role in the maintenance of skeletal homeostasis. Although estrogen is established to have direct effects on bone cells, animal studies have identified additional regulatory effects of estrogen centered at the level of the adaptive immune response. Furthermore, a potential role for reactive oxygen species has now been identified in both humans and animals. One of the major challenges has been to integrate a multitude of redundant pathways and cytokines, that all appear capable of playing a relevant role, into a global model of postmenopausal osteoporosis. This review presents our current understanding of the process of estrogen deficiency mediated bone destruction and explores some of the most recent findings and hypotheses to explain estrogen action in bone.

The multiple roles of osteoclasts in host defense: bone remodeling and hematopoietic stem cell mobilization

Bone remodeling by bone-forming osteoblasts and bone-resorbing osteoclasts dynamically alters the bone inner wall and the endosteum region, which harbors osteoblastic niches for hematopoietic stem cells. Investigators have recently elucidated mechanisms of recruitment and mobilization; these mechanisms consist of stress signals that drive migration of leukocytes and progenitor cells from the bone marrow reservoir to the circulation and drive their homing to injured tissues as part of host defense and repair. The physical bone marrow vasculature barrier that is crossed by mobilized cells actively transmits chemotactic signals between the blood and the bone marrow, facilitating organ communication and cell trafficking. Osteoclasts play a dual role in regulation of bone resorption and homeostatic release or stress-induced mobilization of hematopoietic stem/progenitor cells. In this review, we discuss the orchestrated interplay between bone remodeling, the immune system, and the endosteal stem cell niches in the context of stem cell proliferation and migration during homeostasis, which are accelerated during alarm situations.

Human osteosarcoma cells express functional receptor activator of nuclear factor-kappa B

RANK, RANK ligand (RANKL) and osteoprotegerin (OPG) are the key regulators of bone metabolism, both in normal and pathological conditions. Previous data have demonstrated that human osteosarcoma biopsies express RANKL as well as OPG, and functional RANK is expressed in a murine osteosarcoma cell line. As RANK expression in human osteosarcoma remains controversial, the aim of the present study was to analyse its expression in vitro in human osteosarcoma cell lines, ex vivo using pathological tissues, and then to determine its functionality in terms of signal transduction pathways modulated by RANKL. RT-PCR analysis and immunohistochemistry experiments revealed that RANK is expressed at both transcriptional and protein levels in MNNG/HOS, Saos-2 and MG-63 human osteosarcoma cell lines, in contrast to the U-2 OS osteosarcoma cell line and human osteoblasts, which were negative. RANK was also expressed in 57% of osteosarcoma biopsies. Furthermore, western blot experiments clearly demonstrated the functionality of RANK. Thus, RANKL significantly induced the phosphorylation of ERK1/2, p38 and IkappaB in RANK-positive osteosarcoma cells. This study is the first report of functional RANK expression in human osteosarcoma cells: this strengthens the involvement of the RANK-RANKL-OPG axis in primary bone tumour biology and identifies novel therapeutic approaches targeting RANK-positive osteosarcoma.

B cells and T cells are critical for the preservation of bone homeostasis and attainment of peak bone mass in vivo

Bone homeostasis is regulated by a delicate balance between osteoblastic bone formation and osteoclastic bone resorption. Osteoclastogenesis is controlled by the ratio of receptor activator of NF-kappaB ligand (RANKL) relative to its decoy receptor, osteoprotegerin (OPG). The source of OPG has historically been attributed to osteoblasts (OBs). While activated lymphocytes play established roles in pathological bone destruction, no role for lymphocytes in basal bone homeostasis in vivo has been described. Using immunomagnetic isolation of bone marrow (BM) B cells and B-cell precursor populations and quantitation of their OPG production by enzyme-linked immunosorbent assay (ELISA) and real-time reverse transcriptase-polymerase chain reaction (RT-PCR), cells of the B lineage were found to be responsible for 64% of total BM OPG production, with 45% derived from mature B cells. Consistently B-cell knockout (KO) mice were found to be osteoporotic and deficient in BM OPG, phenomena rescued by B-cell reconstitution. Furthermore, T cells, through CD40 ligand (CD40L) to CD40 costimulation, promote OPG production by B cells in vivo. Consequently, T-cell-deficient nude mice, CD40 KO mice, and CD40L KO mice display osteoporosis and diminished BM OPG production. Our data suggest that lymphocytes are essential stabilizers of basal bone turnover and critical regulators of peak bone mass in vivo.

The role of noggin in human mesenchymal stem cell differentiation

Noggin is a secreted protein that inhibits the binding of bone morphogenetic proteins (BMPs) to their cognate receptor. Its role in human mesenchymal stem cell differentiation has not been well studied. Here, we studied the effect of noggin on human mesenchymal stem cell differentiation induced by inflammatory cytokines (activated T-cell conditioned medium (ACTTCM) or the combination of four T-cell cytokines, TNF-alpha, TGF-beta, IFN-gamma, and IL-17 (TTII)), BMPs, or dexamthasone (DEX). HMSC treated with TTII alone rapidly induced alkaline phosphatase (AlkP) activity. Inclusion of noggin resulted in an additive effect. Noggin acted additively with DEX to induce a significantly higher level of AlkP induction than either noggin or DEX alone. Noggin was examined for its ability to inhibit mineralization in long-term cultures of HMSC stimulated with BMP-2, BMP-6, BMP-7, DEX, or TTII. Surprisingly, noggin alone induced mineralization while it did not inhibit mineralization induced by TTII or BMP-2, BMP-6, or BMP-7. Interestingly, when HMSC were treated with both noggin and DEX they acted synergistically to induce mineralization nearly 3-fold over DEX alone and 30-fold over noggin alone. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed that T-cell cytokines induced noggin, Runx2, BMP-2, and osteocalcin gene expression, while noggin alone induced BMP-2 and osteocalcin gene expression, but not Runx2, although it increased the expression of ActRII, a receptor for BMP-2. These results suggest that in HMSC, the anabolic action of inflammation on bone formation occurs through the induction of noggin, which then induces BMP-2 receptor and BMP-2 leading to the activation of the differentiation process.

Osteoimmunology

Osteoimmunology is an interdisciplinary research field combining the exciting fields of osteology and immunology. An observation that contributed enormously to the emergence of osteoimmunology was the accelerated bone loss caused by inflammatory diseases such as rheumatoid arthritis. Receptor activator of nuclear factor kappaB ligand (RANKL), which is the main regulator of osteoclastogenesis, was found to be the primary culprit responsible for the enhanced activation of osteoclasts: activated T cells directly and indirectly increased the expression of RANKL, and thereby promoted osteoclastic activity. Excessive bone loss is not only present in inflammatory diseases but also in autoimmune diseases and cancer. Furthermore, there is accumulating evidence that the very prevalent skeletal disorder osteoporosis is associated with alterations in the immune system. Meanwhile, numerous connections have been discovered in osteoimmunology beyond merely the actions of RANKL. These include the importance of osteoblasts in the maintenance of the hematopoietic stem cell niche and in lymphocyte development as well as the functions of immune cells participating in osteoblast and osteoclast development. Furthermore, research is being done investigating cytokines, chemokines, transcription factors and co-stimulatory molecules which are shared by both systems. Research in osteoimmunology promises the discovery of new strategies and the development of innovative therapeutics to cure or alleviate bone loss in inflammatory and autoimmune diseases as well as in osteoporosis. This review gives an introduction to bone remodeling and the cells governing that process and summarizes the most recent discoveries in the interdisciplinary field of osteoimmunology. Furthermore, an alternative large animal model will be discussed and the pathophysiological alterations of the immune system in osteoporosis will be highlighted.

Estrogen Regulation of Immune Cell Bone Interactions

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

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.

The role of T lymphocytes in bone metabolism

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

T-cell cytokine induction of BMP-2 regulates human mesenchymal stromal cell differentiation and mineralization

How T-cells, attracted to local sites of inflammation in arthritides, affect heterotopic ossification is presently unknown. Here, we tested the hypothesis that T-cell cytokines play a role in the differentiation of human mesenchymal stromal cells (HMSC) into the osteoblast phenotype by inducing autologous BMP-2, providing a possible mechanism for heterotopic ossification. HMSC from multiple donor bones were treated with either activated T-cell conditioned medium (ACTTCM) or physiological concentrations of the major inflammatory cytokines, TNF-alpha, TGF-beta, IFN-gamma, and IL-17 (TTII), individually or in combinations. ACTTCM induced BMP-2 protein in a time-dependent manner over a 48 h period and alkaline phosphatase (AlkP) within 7 days. In combination, TTII, like ACTTCM, induced AlkP and synergistically induced BMP-2 protein. Either individually, or in combinations of up to three, the T-cell cytokines failed to induce BMP-2 above control levels while a combination of all four cytokines synergistically induced BMP-2 10-fold as assessed by ELISA. TTII induced mineralized matrix as effectively as dexamethasone. Inhibition of p38 MAPK completely inhibited TTII-induced BMP-2 production and matrix mineralization. Real time RT-PCR analysis demonstrated a striking early (within 4 h) increase in BMP-2 gene expression by TTII, which was suppressed by p38 MAP kinase inhibition. In localized chronic inflammatory diseases, T-cell cytokines released at localized sites of inflammation may be the driving force for differentiation of local mesenchymal stromal cells into the osteoblast phenotype thereby playing a significant role in the heterotopic ossification observed in these diseases.

Immunocompetent properties of human osteoblasts: interactions with T lymphocytes

We sought to determine whether osteoblasts (OBs) can serve as accessory cells (ACs) for T-cell activation and whether T cells directly activate OB production of IL-6, using primary human OBs (NHOst), the transformed fetal osteoblast line hFOB1.19, and an osteosarcoma line SaOS-2. Robust, bidirectional activating interactions were shown using each of these three human ostoblast lines.

INTRODUCTION

Osteoblasts (OBs) could come into contact with lymphocytes during inflammatory joint destruction and fracture repair.

MATERIALS AND METHODS

We used several in vitro assays to assess the ability of T cells and OBs to interact in the generation of immune and inflammatory responses.

RESULTS

By flow cytometry, three OB cell lines all were found to express ligands for T-cell co-stimulation. The integrin ligand CD54/ICAM-1 was constitutively expressed by hFOB1.19 and NHOst and was upregulated on SaOS-2 by IFN-gamma. MHC Class II was upregulated on all three lines by IFN-gamma. CD166/ALCAM, a ligand of the T-cell molecule CD6, was constitutively expressed on all three lines. A second putative CD6 ligand designated 3A11 was expressed on hFOB1.19 and NHOst, but not consistently on SaOS-2. The ectoenzyme CD26 (dipeptidyl peptidase IV) was expressed on hFOB1.19 and NHOst, but not on SaOS-2. All three cell lines presented superantigen to T cells, especially after treatment with IFN-gamma. Superantigen presentation was inhibited by antibodies to the leukocyte integrin CD11a/CD18 (LFA-1), MHC Class II, and CD54/ICAM-1. T cells, particularly when cytokine activated for 7 days before co-culture, stimulated all three osteoblast lines to produce interleukin (IL)-6, and this effect was boosted when IL-17 was added to the co-cultures with either resting T cells or cytokine-activated T cells.

CONCLUSIONS

Bidirectional activating interactions are readily shown between human T cells and several types of human OBs. The expression by OBs of ligands for the T cell-specific molecule CD6, as well as other molecules involved in immune interactions, strongly suggests that such in vitro interactions are representative of physiologic or pathologic events that occur in vivo.

Involvement of subchondral bone marrow in rheumatoid arthritis: lymphoid neogenesis and in situ relationship to subchondral bone marrow osteoclast recruitment

OBJECTIVE

To evaluate the presence and immunohistochemical characteristics of subchondral bone marrow inflammatory infiltrate in rheumatoid arthritis (RA) and to determine the in situ relationship between marrow inflammation and osteoclast recruitment.

METHODS

Bone samples and paired synovia from 8 RA patients undergoing joint surgery were analyzed by immunohistochemistry and in situ hybridization for specific lymphoid neogenetic features, such as T and B cell composition, follicular dendritic cell (FDC) networks, peripheral lymph node addressin (PNAd)-positive high endothelial venules, and lymphoid chemokine expression. Osteoclasts were identified as multinucleated tartrate-resistant acid phosphatase (TRAP)-positive and cathepsin K-positive cells adherent to the bone surface.

RESULTS

An inflammatory infiltrate with perivascular aggregates of variable size was detected in 7 (87.5%) of 8 synovial samples and in paired bone samples. Lymphoid neogenetic features typical of rheumatoid synovium were also recognized in the bone marrow. PNAd+ blood vessels were found in 4 of 8 patients, CD21+ FDC networks in 2 patients, CXCL13+ cells in 5 patients, and CCL21+ cells in 6 patients. TRAP-positive and cathepsin K-positive osteoclasts were identified on both the synovial and marrow sides of the bone surface. Bone marrow samples showing a higher degree of inflammation were characterized by a significantly increased number of osteoclasts adherent to the subchondral bone.

CONCLUSION

Our data demonstrate that lymphoid aggregates with lymphoid neogenetic features are detectable on the subchondral side of the joint in established RA. Moreover, the local inflammation/aggregation process appears to be related to osteoclast differentiation on the marrow side of subchondral bone, supporting a functional role of the bone compartment in local damage.

Bone morphogenetic proteins in vascular calcification

Vascular calcification is a common problem among the elderly and those with chronic kidney disease (CKD) and diabetes. The process of tunica media vascular calcification in CKD appears to involve a phenotypic change in the vascular smooth muscle cell (VSMC) resulting in cell-mediated mineralization of the extracellular matrix. The bone morphogenetic proteins (BMPs) are important regulators in orthotopic bone formation, and their localization at sites of vascular calcification raises the question of their role. In this review, we will discuss the actions of the BMPs in vascular calcification. Although the role of BMP-2 in vascular calcification is not proven, it has been the most studied member of the BMP family in this disease process. The role of BMP-2 may be through inducing osteoblastic differentiation of VSMCs through induction of MSX-2, or by inducing apoptosis of VSMCs, a process thought critical in the initiation of vascular calcification. Additionally, BMP-2 may be related to loss of regulation of the matrix Gla protein. A second BMP, BMP-7, less studied than BMP-2 may have opposing actions in vascular calcification. In postnatal life, BMP-7 is expressed primarily in the kidney, and expression is diminished by renal injury. BMP-7 is an important regulator of skeletal remodeling and the VSMC phenotype. BMP-7 restores skeletal anabolic balance in animal models of CKD with disordered skeletal modeling, also reducing serum phosphate in the process. BMP-7 also reverses vascular calcification in CKD, and reduction in vascular calcification is due, in part, to reduced serum phosphate, an important inducer of VSMC-mediated vascular mineralization and in part to direct actions on the VSMC.

Arthritis induces lymphocytic bone marrow inflammation and endosteal bone formation

Arthritis can destroy the cortical bone barrier and expose bone marrow to synovial tissue. This study examines bone marrow changes in arthritis and its effects on cortical bone remodeling. Bone marrow next to arthritic lesions exhibits B-lymphocyte-rich infiltrates, which express BMPs and stimulate endosteal bone formation. Thus, bone marrow actively participates in the arthritic process.

INTRODUCTION

Imaging studies have shown that bone marrow changes occur in patients with rheumatoid arthritis (RA). To examine whether bone marrow is affected during arthritis, human TNF transgenic (hTNFtg) mice, which constitute an established animal model of human RA, were examined for bone marrow changes.

MATERIALS AND METHODS

The hind paws (tarsal area) of 22 untreated hTNFtg mice, 5 hTNFtg mice treated with anti-TNF (infliximab), and 5 wildtype (WT) mice were examined histologically, immunohistochemically, and by means of mRNA in situ hybridization.

RESULTS AND CONCLUSIONS

All untreated hTNFtg mice with moderate (n = 10) and severe (n = 7) disease developed inflammatory bone marrow lesions during the course of disease, whereas no such lesions appeared in hTNFtg mice with mild disease (n = 5) and WT mice. Bone marrow infiltrates were almost exclusively composed of lymphocytes, and the overwhelming proportion (>80%) was B-cells. Presence and extent of bone marrow infiltrates were closely linked to severity of arthritis. In addition, blockade of TNF effectively reduced bone marrow inflammation. Interestingly, osteoblast numbers were increased at the endosteal surface in the vicinity of these lesions. Moreover, osteoid deposition; expression of bone matrix proteins, such as osteocalcin and osteopontin; and mineralization were enhanced, suggesting that inflammatory bone marrow infiltrates induce bone formation. Indeed, B-lymphocytes of these lesions expressed bone morphogenetic protein (BMP)-6 and -7, which are important stimulators of new bone formation. Thus, we conclude that bone marrow actively participates in destructive arthritis by generating B-lymphocyte-rich bone marrow lesions and inducing endosteal bone formation.

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

Autoimmune associated bone disease and intestinal inflammation are closely linked with deregulation and hyperactivation of autoreactive CD4 T cells. How these T cells are activated and mediate disease is not clear. Here we show that in the Interleukin 2-deficient mouse model of autoimmunity spontaneous osteopenia and colitis are caused by increased production of the ligand for receptor activator of NFkappaB (RANKL). RANKL acting via its receptor, receptor activator of NFkappaB (RANK), increases bone turnover and promotes intestinal dendritic cell (DC) survival in vivo. Modulation of RANKL-RANK interactions with exogenous recombinant osteoprotegerin (Fc-OPG) reverses skeletal abnormalities and reduces colitis by decreasing colonic DC numbers. This study identifies a common causal link between bone disease and intestinal inflammation and establishes the importance of DC in mediating colonic inflammation in vivo.

IL-13 regulates vascular cell adhesion molecule-1 expression in human osteoblasts

Activated T cells (Act T) produce multiple cytokines that affect osteoblast function as well as osteoclastogenesis. One of these cytokines, IL-13, is a multifunctional cytokine elaborated by Act T that regulates vascular cellular adhesion molecule (VCAM)-1 expression in endothelial cells. VCAM-1 has also been implicated in osteoclast formation by myeloma cells. We therefore studied whether IL-13 regulates VCAM-1 in human osteoblastic cells since these cells express RANKL, the major osteoclastogenic factor and osteoclast precursors are found adjacent to osteoblasts. Human T cells were activated in the absence or presence of Cyclosporin A (CsA), an inhibitor of the production of most activated T cell cytokines. Conditioned media were assayed for IL-13 by ELISA. Act T produced IL-13 and, unlike other T cell cytokines, this was elevated 3-fold by CsA. Exposure of human osteoblasts (hOB) to doses of recombinant human IL-13 (rhIL-13, 0-10 ng/ml) resulted in an increase of VCAM-1 mRNA (up to 5-fold) within 4 h with a maximum stimulation at 1 ng/ml. CsA had no effect on basal hOB VCAM-1 mRNA expression. Examination of VCAM-1 on the cell surface of hOB, by immunocytochemistry, revealed increasing levels of surface expression of the protein within 16 h after stimulation with doses of rhIL-13 (0.1-10 ng/ml) which were reflective of the mRNAs. IL-6 production was also stimulated in a dose dependent manner with a maximum of 2.5-fold with 1 ng/ml rhIL-13 within 16 h. Since both VCAM-1 and IL-6 showed similar responses to IL-13, IL-6 was examined for its ability to induce VCAM-1. Immunocytochemistry demonstrated no effect of IL-6 on VCAM-1 expression. These data demonstrate that during pathological processes associated with T cell activation, such as rheumatoid arthritis or possibly post-menopausal osteoporosis, T cells may play a pivotal role in osteoclast precursor adhesion to osteoblasts as a first step prior to RANKL signaling.