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.

The bone wasting induced by estrogen deficiency is, in part, a consequence of increased T cell production of the osteoclastogenic cytokine TNF-␣. This phenomenon is due to an expansion of T cells, but the responsible mechanism is unknown. We now show that ovariectomy (ovx) disregulates T lymphopoiesis and induces bone loss by stimulating, through a rise in IL-7 levels, both thymicdependent differentiation of bone marrow-derived progenitors and thymic-independent, peripheral expansion of mature T cells. Attesting to the relevance of the thymic effects, thymectomy decreases by Ϸ50% the bone loss and the stimulation of T lymphopoiesis induced by ovx. In contrast, in vivo attenuation of the elevated IL-7 completely prevents the stimulation of T lymphopoiesis and the bone loss that follow ovx. Thus, the disruption of both T cell and bone homeostasis induced by ovx is mediated by IL-7 and due to both the thymic and extrathymic mechanisms. We conclude that IL-7 is a pivotal upstream target through which estrogen regulates hematopoietic and immune functions that are critical for bone homeostasis.osteoporosis ͉ ovariectomized ͉ thymus ͉ TNF-␣