T cell-produced cytokines play a pivotal role in the bone loss caused by inflammation, infection, and estrogen deficiency. IFN-γ is a major product of activated T helper cells that can function as a pro-or antiresorptive cytokine, but the reason why IFN-γ has variable effects in bone is unknown. Here we show that IFN-γ blunts osteoclast formation through direct targeting of osteoclast precursors but indirectly stimulates osteoclast formation and promotes bone resorption by stimulating antigen-dependent T cell activation and T cell secretion of the osteoclastogenic factors RANKL and TNF-α. Analysis of the in vivo effects of IFN-γ in 3 mouse models of bone loss -ovariectomy, LPS injection, and inflammation via silencing of TGF-β signaling in T cells -reveals that the net effect of IFN-γ in these conditions is that of stimulating bone resorption and bone loss. In summary, IFN-γ has both direct anti-osteoclastogenic and indirect pro-osteoclastogenic properties in vivo. Under conditions of estrogen deficiency, infection, and inflammation, the net balance of these 2 opposing forces is biased toward bone resorption. Inhibition of IFN-γ signaling may thus represent a novel strategy to simultaneously reduce inflammation and bone loss in common forms of osteoporosis.

Summary Intermittent administration of parathyroid hormone (iPTH) is used to treat osteoporosis as it improves bone architecture and strength, but the underlying cellular and molecular mechanisms are unclear. Here we show that iPTH increases the production of Wnt10b by bone marrow CD8+ T cells, and induces these lymphocytes to activate canonical Wnt-signaling in pre-osteoblasts. Accordingly, in responses to iPTH, T cell null mice display diminished Wnt signaling in pre-osteoblasts and blunted osteoblastic commitment, proliferation, differentiation and lifespan which result in decreased trabecular bone anabolism and no increase in strength. Demonstrating the specific role of lymphocytic Wnt10b, iPTH has no anabolic activity in mice lacking T cell produced Wnt10b. Therefore, T cell mediated activation of Wnt signaling in osteoblastic cells plays a key permissive role in the mechanism by which iPTH increases bone strength, suggesting that T cell osteoblast cross-talk pathways may provide pharmacological targets for bone anabolism.

PTH promotes bone catabolism by targeting bone marrow 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, RANKL/OPG production and osteoclastogenic activity. Therefore, T cells play an essential permissive role in hyperparathyroidism as they influence SC proliferation, lifespan and function through CD40L. T cell-SC cross-talk pathways may thus provide pharmacological targets for PTH induced bone disease.

Increased production of tumor necrosis factor ␣ (TNF) in the bone marrow (BM) in response to both oxidative stress and T cell activation contributes to the bone loss induced by estrogen deficiency, but it is presently unknown whether oxidative stress causes bone loss through T cells. Here we show that ovariectomy causes an accumulation in the BM of reactive oxygen species, which leads to increased production of TNF by activated T cells through upregulation of the costimulatory molecule CD80 on dendritic cells. Accordingly, bone loss is prevented by treatment of ovariectomized mice with either antioxidants or CTLA4-Ig, an inhibitor of the CD80/CD28 pathway. In summary, reactive oxygen species accumulation in the BM is an upstream consequence of ovariectomy that leads to bone loss by activating T cells through enhanced activity of BM dendritic cells, and these findings suggest that the CD80/CD28 pathway may represent a therapeutic target for postmenopausal bone loss.CTLA-4Ig ͉ reactive oxygen species ͉ T cells ͉ osteoporosis ͉ tumor necrosis factor P ostmenopausal bone loss is induced by ovarian involution and the resulting increase in follicle-stimulating hormone production through multiple effects on bone marrow (BM) and bone cells (1-4), but the most relevant consequence of natural menopause and ovariectomy (ovx) is a cytokine-driven increase in osteoclast (OC) formation (2, 4). OC formation occurs when bone marrow macrophages (BMMs) are costimulated by the essential osteoclastogenic factors receptor activator of nuclear factor-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) (2), but after menopause TNF is produced in greater amounts in the BM and up-regulates OC formation (4, 5). TNF stimulates the production of RANKL and M-CSF by bone and BM cells (6, 7), augments the responsiveness of OC precursors to RANKL (8, 9), and induces additional osteoclastogenic cytokines such as IL-1, IL-6, and IL-7 (4). Studies with genetic and pharmacological models have demonstrated the pivotal role of TNF in ovx-induced bone loss in rodents and humans (10-13) and have provided insights on the estrogen (E)-regulated sources of TNF. Ovx upregulates the production of TNF by BM T lymphocytes (8, 12) by increasing thymic T cell output (14) and by stimulating the antigen (Ag)-dependent activation and expansion of T cells in the BM (15). Attesting to the relevance of T cell-produced TNF, T cell-deficient nude mice are protected against the loss of cortical and trabecular bone induced by ovx (8,12,16). Furthermore, the capacity of ovx to induce bone loss is restored by adoptive transfer into nude mice of T cells from wild-type (WT) but not of those from TNF Ϫ/Ϫ mice (12).T cell activation is caused by the presentation to T cells of Ag fragments bound to MHC molecules expressed on Ag-presenting cells (APCs) (17). Ovx up-regulates the capacity of BMMs to present Ag to CD4ϩ T cells (4, 15), but no information is available on how ovx activates CD8ϩ T cells and on whether ovx has additional stimulatory effects on BM DCs, t...

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