Leukotriene B4 biosynthetic enzymes and receptors are expressed in both macrophages and osteoclasts. (a) Quantification of leukotriene B4 (LTB4) levels by enzyme-linked immunoassay in the conditioned medium of human peripheral blood mononuclear cells (PBMCs) treated with 25 ng/ml macrophage colony-stimulating factor (M-CSF) and 10 ng/ml interleukin 23 (IL-23) for 3 days. Representative data from three experiments are shown. **P < 0.01. (b) Immunofluorescence of human PBMCs cultured with either M-CSF alone for 8 days or M-CSF + receptor activator of nuclear factor κB ligand (RANKL) for 14 days showing the expression of BLT1, BLT2, 5-lipoxygenase (5-LO) and leukotriene A4 hydrolase (LTA4H) in mononuclear and multinucleated giant cells. Representative images from three experiments are shown. 4′,6-diamidino-2-phenylindole (DAPI) is blue and BLT1, BLT2, 5-LO and LTA4H are in red. The scale bars represent 20 μm in Figure 1b.