Direct inhibition of human RANK ⁺ osteoclast precursors identifies a homeostatic function of IL-1β

IL-1β is a key mediator of bone resorption in inflammatory settings, such as rheumatoid arthritis (RA). IL-1β promotes osteoclastogenesis by inducing RANKL expression on stromal cells and synergizing with RANKL to promote later stages of osteoclast differentiation. Because IL-1Rs share a cytosolic Toll-IL-1R domain and common intracellular signaling molecules with TLRs that can directly inhibit early steps of human osteoclast differentiation, we tested whether IL-1β also has suppressive properties on osteoclastogenesis in primary human peripheral blood monocytes and RA synovial macrophages. Early addition of IL-1β, prior to or together with RANKL, strongly inhibited human osteoclastogenesis as assessed by generation of TRAP ⁺ multinucleated cells. IL-1β acted directly on human osteoclast precursors (OCPs) to strongly suppress expression of RANK, of the costimulatory triggering receptor expressed on myeloid cells 2 receptor, and of the B cell linker adaptor important for transmitting RANK-induced signals. Thus, IL-1β rendered early-stage human OCPs refractory to RANK stimulation. Similar inhibitory effects of IL-1β were observed using RA synovial macrophages. One mechanism of RANK inhibition was IL-1β-induced proteolytic shedding of the M-CSF receptor c-Fms that is required for RANK expression. These results identify a homeostatic function of IL-1β in suppressing early OCPs that contrasts with its well-established role in promoting later stages of osteoclast differentiation. Thus, the rate of IL-1-driven bone destruction in inflammatory diseases, such as RA, can be restrained by its direct inhibitory effects on early OCPs to limit the extent of inflammatory osteolysis.