IL‐17 induces osteoclastogenesis from human monocytes alone in the absence of osteoblasts, which is potently inhibited by anti‐TNF‐α antibody: A novel mechanism of osteoclastogenesis by IL‐17

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IL-17 is a proinflammatory cytokine crucial for osteoclastic bone resorption in the presence of osteoblasts or synoviocytes in rheumatoid arthritis. However, the role of IL-17 in osteoclastogenesis from human monocytes alone remains unclear. Here, we investigated the role of IL-17 in osteoclastogenesis from human monocytes alone and the direct effect of infliximab on the osteoclastogenesis induced by IL-17. Human peripheral blood mononuclear cells (PBMC) were cultured for 3 days with M-CSF. After non-adherent cells were removed, IL-17 was added with either infliximab or osteoprotegerin (OPG). Seven days later, adherent cells were stained for vitronectin receptor. On the other hand, CD11b-positive monocytes purified from PBMC were also cultured and stained as described above. CD11b-positive cells were cultured with TNF-alpha and receptor activator of NF-kappaB ligand (RANKL). In the cultures of both adherent cells and CD11b-positive cells, IL-17 dose-dependently induced osteoclastogenesis in the absence of soluble-RANKL. OPG or infliximab inhibited IL-17-induced osteoclastogenesis. Interestingly, in the culture of CD11b-positive cells, the osteoclastogenesis was more potently inhibited by infliximab than by OPG. TNF-alpha and RANKL synergistically induced osteoclastogenesis. The present study clearly demonstrated the novel mechanism by which IL-17 directly induces osteoclastogenesis from human monocytes alone. In addition, infliximab potently inhibits the osteoclastogenesis directly induced by IL-17.