Prolonged LPS treatment modulates crosstalk between monocytes and osteoclast progenitors via Hyaluronan

Objectives: The classical pathological feature of chronic periodontal disease is irreversible alveolar bone loss. In the periodontal tissues, monocytes and macrophages are continuously exposed to LPS in the accumulated subgingival plaque. Constant exposure to LPS results in down-regulation of osteoclastogenic factors (endotoxin tolerance) by monocytes / macrophages, yet bone resorption still occurs in the periodontal lesion. Therefore, other simultaneous mechanisms promoting osteoclastogenesis must be present. Hyaluronan (HA) was identified as a potential activator of osteoclastogenic factors in monocytes, via TLR4 / CD44 signaling pathways. The objective of this study was to assess LPS induction of HA synthesis genes in LPS challenged monocytes and the effects of HA protein on osteoclast differentiation.
Methods: THP-1 cells were treated continuously with LPS over 1-9 days and expression of Hyaluronan Synthase genes were measured by real time RT-PCR. Osteoclasts were generated by treatment of human primary bone marrow cells (BMCs) with RANKL and M-CSF for 9 days. To assess the effects of HA protein on the differentiation of BMCs into osteoclasts, BMCs were treated with HA (+/- RANKL & M-CSF) for 9 days. Osteoclast differentiation was determined by TRAP and immunofluorescent staining

Results: LPS modulated the expression of HAS genes in THP-1 monocytes which could further affect osteoclast differentiation. High molecular weight HA protein increased osteoclast differentiation from BMCs and low molecular weight HA opposed the effects.
Conclusions: Prolonged exposure to LPS in the periodontal lesion could lead to increased production of HA, which in turn increases the differentiation of infiltrating monocytes into osteoclasts, leading to increased bone resorption. These data demonstrate a novel mechanism by which LPS can induce osteoclast differentiation, even in typically “tolerogenic” conditions