TLR4-dependent upregulation of IL-22-producing T cells in the bone niche links to the imbalance of bone homeostasis.
BACKGROUND Toll-like receptor 4 (TLR4) activation within the bone marrow (BM) microenvironment regulates proinflammatory cytokines and chemokines, which influence immune activity and bone remodeling. However, the immune cell factors mediating TLR4-dependent cross-talk in the bone/BM microenvironment remain insufficiently defined. METHODS We provide a transcriptomic analysis of significant cellular and immune changes in total BM cells of wild-type mice following lipopolysaccharide (LPS)-induced TLR4 activation in vivo compared with TLR4 knockout (KO) mice. To obtain direct evidence that interleukin (IL)-22 contributes to TLR4-driven bone destruction, we analyzed their bone phenotypes following LPS injection into IL-22 KO mice. In addition, co-culture of pre-osteoblasts (OBs) and BM cells was conducted to evaluate the effect of IL-22 in osteoclast (OC) formation in a pre-OB-dependent manner. RESULTS Differentially expressed gene analysis from BM cells of wild-type (WT) mice revealed an increase in IL-22-producing cells, along with increased IL-22 production in a TLR4-dependent manner. Flow cytometry confirmed that LPS enhanced the number of CD4+IL-22+ cells in the BM and decreased in TLR4-deficient BM. LPS also enhanced the differentiation of CD4+IL-22+ T cells and promoted IL-22 induction in T cells. IL-22 KO mice showed minimal LPS-induced bone loss, whereas WT mice exhibited severe bone destruction with increased formation of tartrate-resistant acid phosphatase-positive OCs. Interestingly, while IL-22 did not directly affect OC differentiation, IL-22 enhanced OC differentiation only when BM cells were co-cultured with pre-OBs through the elevation of the receptor activator of nuclear factor-κB ligand (RANKL) in pre-OBs during osteogenesis. Mechanistically, IL-22 induced cyclooxygenase-2 in pre-OBs through extracellular signal-regulated kinase and p38 signaling, driving prostaglandin E2 production and subsequent RANKL induction. CONCLUSIONS TLR4 activation enhanced IL-22 production in T cells, which stimulates RANKL production in pre-OBs to further induce OC formation. These results support that IL-22 acts as a TLR4-driven mediator to shift bone remodeling toward catabolic bone degeneration.