Interleukin (IL)-33 inhibited receptor activator of nuclear factor kappa-B ligand (RANKL)- and tumor necrosis factor-α (TNF-α)-induced osteoclast formation in vitro. (a) Microscopic images and the number of TRAP-positive cells of osteoclast precursors cultured with macrophage colony stimulating factor (M-CSF), M-CSF + RANKL, M-CSF + RANKL + IL-33, or IL-33. (b) Microscopic images and the number of TRAP-positive cells of osteoclast precursors cultured with M-CSF, M-CSF + TNF-α, M-CSF + TNF-α + IL-33, or IL-33. TRAP-positive cells with ≥3 nuclei were considered to be osteoclasts. (c) Microscopic images and the percentage of resorption pits. (d) Tartrate-resistant acid phosphatase (TRAP) mRNA levels of osteoclasts detected using real-time RT-PCR. Results are expressed as means ± SD. The statistical significance of differences was determined by Scheffe’s test (n = 4; * p < 0.05, ** p < 0.01). Scale bars = 200 µm.