Toll-like receptors (TLRs) play pivotal roles in inflammation and provide important links between the immune and skeletal systems. Although the activation of TLRs may affect osteoclast differentiation and bone metabolism, whether and how TLRs are required for normal bone remodeling remains to be fully explored. In the current study, we show for the first time that TLR9^−/− mice exhibit a low bone mass and low-grade systemic chronic inflammation, which is characterized by the expansion of CD4⁺ T cells and increased levels of inflammatory cytokines, including TNFα, RANKL, and IL1β. The increased levels of these cytokines significantly promote osteoclastogenesis and induce bone loss. Importantly, TLR9 deletion alters the gut microbiota, and this dysbiosis is the basis of the systemic inflammation and bone loss observed in TLR9^−/− mice. Furthermore, through single-cell RNA sequencing, we identified myeloid-biased hematopoiesis in the bone marrow of TLR9^−/− mice and determined that the increase in myelopoiesis, likely caused by the adaptation of hematopoietic stem cells to systemic inflammation, also contributes to inflammation-induced osteoclastogenesis and subsequent bone loss in TLR9^−/− mice. Thus, our study provides novel evidence that TLR9 signaling connects the gut microbiota, immune system, and bone and is critical in maintaining the homeostasis of inflammation, hematopoiesis, and bone metabolism under normal conditions.

Toll 样受体 (TLR) 在炎症中发挥关键作用，并在免疫系统和骨骼系统之间提供重要联系。尽管 TLRs 的激活可能影响破骨细胞分化和骨代谢，但正常骨重建是否需要 TLRs 以及如何需要 TLRs 仍有待充分探索。在目前的研究中，我们首次表明 TLR9 ^-/-小鼠表现出低骨量和低度全身性慢性炎症，其特征是 CD4 ⁺ T 细胞的扩增和炎性细胞因子水平的升高，包括 TNF α、RANKL 和 IL1 β. 这些细胞因子水平的升高显着促进破骨细胞生成并诱导骨质流失。重要的是，TLR9 缺失会改变肠道微生物群，这种生态失调是在 TLR9 ^−/−小鼠中观察到的全身炎症和骨质流失的基础。^{此外，通过单细胞 RNA 测序，我们在 TLR9 −/−}小鼠的骨髓中发现了骨髓偏向的造血作用，并确定骨髓生成的增加（可能是由造血干细胞适应全身性炎症引起的）也会导致炎症-诱导的破骨细胞生成和随后的 TLR9 骨丢失^-/-老鼠。因此，我们的研究提供了新的证据，证明 TLR9 信号连接肠道微生物群、免疫系统和骨骼，并且在正常条件下对维持炎症、造血和骨代谢的稳态至关重要。