Late pre-osteoclasts are resistant to IFN-γ-induced surface expression of CD11b and receptor-activator of NFκB (RANK). (a) Cells were treated with the indicated cytokines for 4 days, then analyzed for CD11b expression by FACS. Late pre-osteoclasts (RAW cells pretreated with 200 ng/ml receptor-activator of NFκB ligand [RANKL] for 2 days prior to IFN-γ treatment) expressed CD11b at levels nearly identical to RANKL-only treated cells, suggesting that late pre-osteoclasts are resistant to IFN-γ-induced CD11b upregulation. (b) Identically treated cells were analyzed for RANK expression by FACS. The proportion of RANKhi cells was nearly identical in RANKL-only treated cells and late pre-osteoclasts, suggesting that late pre-osteoclasts are resistant to IFN-γ-induced upregulation of RANK. MFI, mean fluorescence intensity.