The 4–1BB ligand and 4–1BB expressed on osteoclast precursors enhance RANKL‐induced osteoclastogenesis via bi‐directional signaling

Abstract The 4–1BB is a costimulatory molecule similar to the receptor activator of NF‐κB ligand (RANKL), both of which are key factors for the differentiation of osteoclasts and are expressed mainly by activated T cells. The 4–1BB shares common signaling pathways with RANK, suggesting a potential role in osteoclastogenesis. In this study, the role of 4–1BB and 4–1BB ligand (4–1BBL) in osteoclastogenesis was investigated using 4–1BB –/– and 4–1BB +/+ mice. Osteoclast precursors normally express 4–1BB and 4–1BBL after exposure to RANKL, which was confirmed by semi‐quantitative RT‐PCR and flow cytometry. The 4–1BB –/– mice had a slightly increased bone mass accompanied by a reduced osteoclastogenic ability of 4–1BB –/– bone marrow‐derived macrophages (BMM) ex vivo . In addition, 4–1BB –/– BMM demonstrated hypophosphorylation of JNK and p38 and decreased induction of c‐Fos in response to RANKL stimulation. Retroviral transduction of wild‐type as well as partial‐length 4–1BB, which lacks TNF receptor‐associated factor 2‐binding sites for signaling, restored the osteoclastogenic ability of 4–1BB –/– BMM. Furthermore, both recombinant 4–1BB and 4–1BBL enhanced RANKL‐induced osteoclastogenesis by 4–1BB +/+ BMM and the induction of c‐Fos and NFATc1.Together, these results indicate that 4–1BBL and 4–1BB expressed on osteoclast precursors enhance RANKL‐induced osteoclastogenesis via bi–directional signaling, findings that may delineate the complex nature of the 4–1BBL and 4–1BB interaction. Supporting Information for this article is available at www.wiley‐vch.de/contents/jc_2040/2008/37650_s.pdf