TRAP-positive osteoclast precursors mediate ROS/NO-dependent bactericidal activity via TLR4

Osteoclastogenesis was induced by RANKL stimulation in mouse monocytes to examine the possible bactericidal function of osteoclast precursors (OCp) and mature osteoclasts (OCm) relative to their production of NO and ROS. Tartrate-resistant acid phosphatase (TRAP)-positive OCp, but few or no OCm, phagocytized and killed Escherichia coli in association with the production of reactive oxygen species (ROS) and nitric oxide (NO). Phagocytosis of E. coli and production of ROS and NO were significantly lower in TRAP+ OCp derived from Toll-like receptor (TLR)-4 KO mice than that derived from wild-type (WT) or TLR2-KO mice. Interestingly, after phagocytosis, TRAP+ OCp derived from wild-type and TLR2-KO mice did not differentiate into OCm, even with continuous exposure to RANKL. In contrast, E. coli-phagocytized TRAP+ OCp from TLR4-KO mice could differentiate into OCm. Importantly, neither NO nor ROS produced by TRAP+ OCp appeared to be engaged in phagocytosis-induced suppression of osteoclastogenesis. These results suggested that TLR4 signaling not only induces ROS and NO production to kill phagocytized bacteria, but also interrupts OCm differentiation. Thus, it can be concluded that TRAP+ OCp, but not OCm, can mediate bactericidal activity via phagocytosis accompanied by the production of ROS and NO via TLR4-associated reprograming toward phagocytic cell type. (Working Hypothesis) M-CSF combined with RANKL can induce osteoclast precursors (OCp) from monocytes, while M-CSF alone induces macrophages. TRAP-positive OCp, but not mature multinuclear osteoclasts (OCm), phagocytize bacteria through TLR4 activation. Subsequently, phagocytized bacteria in TRAP-positive OCp are killed, in part, by the production of ROS and NO promoted by TLR4. After bacterial phagocytosis, regardless of continuous exposure to M-CSF/RANKL, TRAP-positive OCp lost the ability to differentiate into mature osteoclasts in a manner independent of ROS and NO. ROS and NO are positively engaged in the RANKL-mediated differentiation of bacteria-unexposed TRAP-positive OCp into mature multinucleated osteoclasts. [Display omitted] •TRAP+ mononuclear osteoclast precursors phagocytize bacteria via TLR4 activation.•Phagocytized bacteria are killed by the production of ROS and NO.•Bacteria phagocytosis inhibits RANKL-dependent osteoclastogenesis.•Phagocytosis-mediated inhibition of osteoclastogenesis is independent of ROS/NO.