Differential Contribution of Osteoclast‐ and Osteoblast‐Lineage Cells to CpG‐Oligodeoxynucleotide (CpG‐ODN) Modulation of Osteoclastogenesis

CpG‐ODNs modulate osteoclast differentiation through Toll‐like receptor 9 (TLR9). Using TLR9‐deficient mice, we found that activation of TLR9 on both osteoclast precursors and osteoblasts mediate the osteoclastogenic effect of CpG‐ODN. Osteoclastic TLR9 is more important for this activity. Introduction : Bacterial infections cause pathological bone loss by accelerating differentiation and activation of the osteoclast. A variety of bacteria‐derived molecules have been shown to enhance osteoclast differentiation through activation of Toll‐like receptors (TLRs). We have shown that CpG‐oligodeoxynucleotides (CpG‐ODNs), mimicking bacterial DNA and exerting their cellular activities through TLR9, modulate osteoclast differentiation in a complex manner: the ODNs inhibit the activity of the physiological osteoclast differentiation factor RANKL in early osteoclast precursors (OCPs) but markedly stimulate osteoclastogenesis in cells primed by RANKL. Materials and Methods : Osteoclast precursors and osteoblasts from TLR9‐deficient (TLR9 −/− ) and wildtype (TLR9 +/+ ) mice were used for in vitro analyses of osteoclast differentiation and modulation of signal transduction and gene expression. Results : As expected CpG‐ODN did not exert any activity in cells derived from TLR9 −/− mice; these cells, however, responded in a normal manner to other stimuli. Using bone marrow/osteoblasts co‐cultures from all possible combinations of TLR9 −/− and TLR9 +/+ mice‐derived cells, we showed that TLR9 in the two lineages is required for CpG‐ODN induction of osteoclastogenesis. Conclusions : CpG‐ODN modulates osteoclastogenesis in a TLR9‐dependent manner. Activation of TLR9 in bone marrow‐derived osteoclasts precursors is more crucial to induction of osteoclastogenesis than activation of the osteoblastic TLR9.