OCs generated from human MM PBMCs. (A) OCs were obtained from unfractionated PBMCs of patients with MM with or without osteolysis (ii,iii,v,vi) and controls (i,iv). Numerous and large-sized OCs (arrows) developed in the unstimulated cultures from patients with MM with osteolysis (ii), whereas rare and small-sized OCs were observed in the cultures from patients with MM without osteolysis (iii) and controls (i). No significant increase in OC formation was observed in MM PBMCs from the patients with osteolysis by exogenous M-CSF and RANKL (v), whereas these cytokines were essential to triggering the OC formation in patients with MM without osteolysis (vi) and controls (iv). Multinucleated (> 3 nuclei per cell) and TRAP⁺ cells were identified as OCs. The arrows point to the OCs (magnification, × 200). (B) The inhibition of RANKL by RANK-Fc prevented in a dose-dependent manner the OC formation in unstimulated and unfractionated PBMC cultures from patients with MM. ^*P < .001. (C) Photomicrographs of the pits formed on Millennium Osteologic slides by the OCs. Numerous and large resorption areas (arrows) were observed in the MM bone disease samples (ii) with respect to the few and small pits detected in the controls (i). The percentage of mineral surface resorbed by the OCs was quantified with an image analyzer. The data reported in the graph correspond to the mean ± SE (iii). See “Cell cultures” for figure acquisition info.