T cells mediate osteoclastogenesis in human MM PBMCs. Osteoclastogenesis occurred in T-cell–depleted MM PBMCs cultured in the absence (A) or in the presence (B) of M-CSF and RANKL. Small-sized OCs developed in the absence of M-CSF and RANKL (A), whereas exogenous cytokines triggered the formation of large-sized OCs (B). Multinucleated and TRAP⁺ cells were identified as OCs. The arrows point to the OCs (magnification × 200).