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-KB 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.