Human circulating monocytes can express receptor activator of nuclear factor-κB ligand and differentiate into functional osteoclasts without exogenous stimulation

Osteoclast formation from mononuclear precursors is believed to require accessory cells expressing receptor activator of nuclear factor-κB ligand (RANKL). We recently identified a human cell population originated from circulating CD14⁺ monocytes, called monocyte-derived multipotential cells (MOMCs), which can differentiate into several distinct mesenchymal cells, neuron and endothelial cells. This study was undertaken to examine whether MOMCs can differentiate into functional osteoclasts. MOMCs prepared from peripheral blood of healthy volunteers cultured on fibronectin for 7 days at high density (8 × 10⁵ cells cm^-2), but not at regular density (2 × 10⁴ cells cm^-2), resulted in the appearance of tartrate-resistant acid phosphatase-positive giant multi-nucleated cells forming actin ring without exogenous osteoclastogenic factors. A subset of these cells showed bone resorption capacity on dentine slices and expression of genes for cathepsin K and calcitonin receptor, characteristic of functional osteoclasts. Such osteoclast differentiation was not observed in high-density culture of circulating monocytes, macrophages or dendritic cells, or the high-density culture of MOMCs on type I collagen. Among cells of the monocyte lineage, untreated MOMCs exclusively showed gene and protein expression of RANKL. When osteoprotegerin/IgG1 Fc chimera was added to high-density MOMC cultures, osteoclast formation was completely inhibited by neutralizing the endogenous RANKL. These results indicate that human MOMCs derived from circulating monocytes can express RANKL and differentiate into functional osteoclasts without RANKL-expressing accessory cells.