IL-4 inhibits the bone resorption activity of mature osteoclasts. (A) BMMs were isolated from wild-type mice and differentiated to osteoclasts by culturing with rmM-CSF (20 ng/mL) and RANKL (150 ng/mL), as in the previous experiments. Cells were plated on a layer of type 1 collagen. Six days later, when mature multinucleated osteoclasts were abundant, the plates were treated with collagenase to release the osteoclasts. These osteoclasts were then cultured on dentin slices with or without 10 ng/mL rmIL-4 for 24 hours. Resorption pits were visualized by staining with toluidine blue; original magnification, × 10 for both panels. (B) Cells from wild-type and STAT6^-/- mice were isolated and cultured as described in panel A and were plated on hydroxyapatite-coated discs with or without 10 ng/mL rmIL-4. Areas of resorbed substrate were quantified by image analysis. Data represent the mean ± SD of 3 independent experiments.