Human MM cell–derived IL-34 accelerates osteoclast formation. (A) ELISA measurement of IL-34 in the supernatants of IM-9, KMS-11, OPC, OPM-2, and U226B1 human MM cell lines stimulated with IL-1β, IL-6, TGFβ, or TNFα (100 ng/mL) for 7 days. (B) Intracellular staining of IL-34 in CD19⁻CD138⁺ populations in BM aspirates collected from MM patients (Pt.). Gray histogram, isotype; red line, IL-34. (C) Fold induction of DC-STAMP (top) or OC-STAMP (bottom) expression in human CD14⁺ monocytes cocultured with MM cells or stimulated with BMF from IL-34^High or IL-34^Absent patients in the presence or absence of RANKL (referred to as R). A neutralizing antibody against IL-34 (αIL-34) or an isotype control antibody (immunoglobulin G [IgG]) was added to the culture. Unstimulated monocytes were considered as 1. Data are mean ± SEM; *P < .05.