c-Fms inhibition blocks macrophage differentiation and joint infiltration. (a) Representative immunohistochemistry images of sections of ankle joint tissue from DBA/1 mice treated with vehicle, GW2580, or imatinib in a collagen-induced arthritis prevention study. Joint sections were stained with antibodies against total c-Fms, the macrophage marker F4/80, or antibody isotype controls. Images are shown at × 400 magnification and are representative of at least three independent experiments. (b, c) Differentiation to macrophages. Bone marrow cells from naïve BALB/c mice were treated with macrophage colony-stimulating factor (M-CSF) alone for 5 days to promote monocyte maturation and then incubated with (+) or without (-) M-CSF for an additional 48 hours in the presence of GW2580 or imatinib, as indicated. (b) Representative inverted microscopic images of untreated monocytes (left panel) and M-CSF-treated monocytes ± GW2580 or imatinib. (c) The percentage of macrophages in untreated or M-CSF-treated cultures in the presence of 0 to 10 μM GW2580 or imatinib was determined with an assay that detects α-naphtyl acetate esterase activity, coupled with fluoride inhibition. **P < 0.01.