pOCs were not susceptible to the IFN-β–mediated inhibition of osteoclastogenesis. (A) BMMs were cultured with M-CSF (30 ng/mL) and RANKL (100 ng/mL) for 3 days. IFN-β was added at the start of culture for experiments described in panels B and C. (B) At the end of culture, cells were fixed and stained for TRAP activity. Osteoclasts were identified as TRAP⁺ (purple) multinuclear cells. MR represents M-CSF plus RANKL. Results are representative of at least 3 experiments using 10 U/mL IFN-β. (C) OC numbers were counted after a 3-day culture of BMMs with M-CSF and RANKL in combination with IFN-β. Cells were counted in 3 random fields. Results are presented as means plus or minus SD and are representative of at least 3 experiments with similar results. Asterisks represent statistical difference between the values obtained from cells cultured with or without IFN-β in the presence of M-CSF and RANKL (*P < .05; **P < .01). (D) BMMs were first differentiated into pOCs by incubating cells with M-CSF and RANKL for 2 days, and IFN-β was added at the end of day 2 for experiments in panels E and F. (E) After a 3-day culture, cells were stained for TRAP activity. (F) OC numbers were counted after a 3-day culture of BMMs with M-CSF and RANKL. Varying concentrations of IFN-β were added for the final 24 hours. Data are presented as in panel C.