Spontaneous formation of multinucleated giant cells with bone resorbing activity by long culture of human peripheral blood CD14-positive monocytes in vitro.

Several reports indicate that multinucleated giant cells that derived from human peripheral blood CD14-positive monocytes have osteoclastic properties, and although the mechanism is not completely understood, the authors have previously demonstrated that spontaneous osteoclastogenesis from monocytes can occur. Here, the authors investigated the effect of detachment and long-term cultures in this process. When monocytes were incubated for 2 weeks, spontaneous formation of polykaryocytes was rarely observed. In addition, when monocytes precultured for 2 weeks were detached by a cell scraper and further subcultured, almost all cells died. Surprisingly, when monocytes were incubated for 8 weeks without any pro-osteoclastogenic factors and without detachment, the authors observed the spontaneous formation of tartrate-resistant acid phosphatase (TRAP)-positive polykaryocytes that were able of lacunae resorption. These findings indicate that cell adhesion is a prerequisite for differentiation and survival of CD14-positive monocytes, and that a long incubation period spontaneously induces multinucleation and bone-resorbing activity of monocytes, even in the absence of osteoclastogenesis-stimulating factors.