Macrophages that have phagocytosed particles are capable of differentiating into functional osteoclasts.

The aim of the current study was to determine whether human macrophages that have phagocytosed particles are capable of differentiating into osteoclastic bone-resorbing cells. Macrophages isolated from human peripheral blood were cultured with latex particles in the presence of receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) and macrophage colony stimulating factor (M-CSF) on dentine slices and coverslips. After 24 h incubation, particles that had not yet been phagocytosed were removed by washing the slices. Histochemistry and immunohistochemistry was used to determine expression of macrophage and osteoclast markers and lacunae resorption, scanning electron microscopy, and transmission electron microscopy were used to examine cells with phagocytosed particles. Isolated macrophages on dentine slices were noted to contain a large number of particles inside, and no particles were identified outside of culture cells after washing. After 14 days of incubation, numerous tartrate-resistant acid phosphatase-positive multinucleated cells that contained particles in their cytoplasm, capable of extensive lacunae bone resorption, formed in these cultures. Our results clearly indicated that macrophages that have phagocytosed particles were still capable of differentiating into osteoclastic bone-resorbing cells. Macrophages that have phagocytosed wear particles in the pseudomembrane surrounding an implant not only produce cytokines but also may differentiate into functional osteoclasts, and influence bone resorption and loosening of a prosthesis.

Proinflammatory cytokine (TNFalpha/IL-1alpha) induction of human osteoclast formation.

TNFalpha and IL-1alpha are potent stimulators of bone resorption in vivo and in vitro. Recently, it has been demonstrated that these two cytokines directly induce osteoclastogenesis in mouse marrow cultures. This study determined whether TNFalpha (+/- IL-1alpha) is also capable of inducing human osteoclastogenesis. The CD14(+) monocyte fraction of human peripheral mononuclear cells was cultured with TNFalpha +/- IL-1alpha in the presence of M-CSF. TNFalpha induced the formation of multinucleated cells (MNCs) which were positive for TRAP, VNR and cathepsin K and showed evidence of resorption pit formation. IL-1alpha stimulated TNFalpha-induced lacunar resorption two- to four-fold. Osteoprotegerin, the decoy receptor for RANKL, did not inhibit this process. Anti-human IL-1alpha neutralizing antibodies significantly inhibited resorption without inhibiting the formation of TRAP(+)/VNR(+) MNCs. These results suggest that, in the presence of M-CSF, TNFalpha is sufficient for inducing human osteoclast differentiation from circulating precursors by a process which is distinct from the RANK/RANKL signalling pathway.