TGF-β1 and IFN-γ Direct Macrophage Activation by TNF-α to Osteoclastic or Cytocidal Phenotype

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Abstract
TNF-related activation-induced cytokine (TRANCE; also called receptor activator of NF-κB ligand (RANKL), osteoclast differentiation factor (ODF), osteoprotegerin ligand (OPGL), and TNFSF11) induces the differentiation of progenitors of the mononuclear phagocyte lineage into osteoclasts in the presence of M-CSF. Surprisingly, in view of its potent ability to induce inflammation and activate macrophage cytocidal function, TNF-α has also been found to induce osteoclast-like cells in vitro under similar conditions. This raises questions concerning both the nature of osteoclasts and the mechanism of lineage choice in mononuclear phagocytes. We found that, as with TRANCE, the macrophage deactivator TGF-β1 strongly promoted TNF-α-induced osteoclast-like cell formation from immature bone marrow macrophages. This was abolished by IFN-γ. However, TRANCE did not share the ability of TNF-α to activate NO production or heighten respiratory burst potential by macrophages, or induce inflammation on s.c. injection into mice. This suggests that TGF-β1 promotes osteoclast formation not only by inhibiting cytocidal behavior, but also by actively directing TNF-α activation of precursors toward osteoclasts. The osteoclast appears to be an equivalent, alternative destiny for precursors to that of cytocidal macrophage, and may represent an activated variant of scavenger macrophage.