Regulation of osteoclastogenesis by integrated signals from toll-like receptors

A variety of pathogen-derived molecules have been shown to cause bone loss by enhancing osteoclast differentiation through activation of toll-like receptors (TLRs). The pathogen-derived molecules (TLR-ligands) modulate osteoclastogenesis in a complex manner: inhibition of the osteoclast differentiation factor RANKL in early precursors and osteoclastogenesis stimulation in RANKL-primed cells. Since organisms may be challenged by several TLR ligands at a time, we investigated osteoclastogenesis modulation by simultaneous challenge with different TLR ligands. As an example we used ligands for TLR3 (Synthetic double stranded RNA [dsRNA], polyinosinic-polycytidylic acid [poly(I:C)] mimicking viral dsRNA), TLR4 (lipopolysaccharide [LPS], found in the outer membrane of Gram-negative bacteria) and TLR9 (Synthetic oligodeoxynucleotide mimicking bacterial DNA [CpG-ODN]). In osteoclastogenesis-inhibition, synergy between LPS and CpG-ODN or LPS and poly(I:C) while in stimulation, synergy between LPS and CpG-ODN or CpG-ODN and poly(I:C) were observed. Modulation of molecules involved in osteoclastogenesis (c-Fos, M-CSF receptors [M-CSFR], TNF-α, IL-6, and IL-12 and the three TLRs tested) was examined. The results indicate that M-CSFR plays a role only in the inhibitory effect while c-Fos plays a role in the two effects. TLR3 and TLR9 levels were increased by the TLRs ligands, suggesting that this may be part of the mechanism leading to the synergy. While TLRs activation in RANKL-primed cells, increasing osteoclastogenesis, explains pathogen-induced bone loss, activation of TLRs in early cells inhibiting osteoclastogenesis could attenuate excessive resorption, and promote differentiation of common precursor cells into inflammatory cells. The synergism between TLR ligands enables the individual to initiate response at a lower level of pathogen.