Background/Purpose: The aim of this study was to examine the importance of TLR5 ligation in the pathogenesis of rheumatoid arthritis (RA) and experimental arthritis.

Methods: The role of TLR5 ligation was determined on RA myeloid cell chemotaxis and osteoclast formation in vitro. Studies were conducted to investigate the mechanism by which TLR5 ligation promotes osteoclast maturation in RA peripheral blood mononuclear cells (PBMC)s. Next, significance of myeloid and T cells were examined in osteoclastogenesis facilitated by TLR5 ligation. Finally, experiments were performed in collagen induced arthritis (CIA) as well as in TLR5 induced arthritis to assess the impact of joint TLR5 ligation on myeloid cell migration and osteoclast differentiation.

Results: We demonstrate that the TLR5 agonist, flagellin, is strongly chemoattractant for monocytes and can further facilitate differentiation of myeloid cells into mature osteoclasts. Consistently, RA synovial fluid induced myeloid cell infiltration and osteoclast differentiation is markedly suppressed by myeloid TLR5 blockade. We show that flagellin ligation of TLR5 promotes RA osteoclast differentiation through induction of receptor activator of nuclear factor kappa-B (RANK) on myeloid cells (by 8 fold) and its corresponding ligand, RANKL, from T cells (by 2 fold). We found that ligation of TLR5 could drive precursor myeloid cells to form fully mature osteoclasts in the absence of T cells when monocytes were cultured in the presence of suboptimal doses of M-CSF and RANKL. These results suggest that monocytes are the effector cells in TLR5 mediated osteoclastogenesis and that flagellin can facilitate osteoclast formation by increasing RANK expression and by allowing the cells to be more responsive to RANKL binding, hence lower levels of RANKL is required for this process. We next asked whether homing and differentiation of myeloid cells to mature osteoclasts are altered in acute and/or chronic arthritis driven by TLR5 ligation. We document that when CIA mice were therapeutically treated with TLR5 agonist, joint swelling was markedly greater in mice that received flagellin treatment compared to the PBS control. We also show that the F480 staining, number of TRAP+ cells and the concentration of bone erosion markers calcitonin receptor, Cathepsin K and RANKL (8-34 folds) were lower in the control ankles compared to the flagellin treated CIA joints. Since CIA ankle swelling and bone erosion were exacerbated by flagellin post onset treatment we next asked whether local injection of flagellin alone could drive joint inflammation and osteoclastogenesis. We demonstrate that ectopic TLR5 ligation elevates ankle circumference from day 0 to 2, subsequently joint inflammation plateaus until day 8, however swelling remains consistently higher than the PBS group. Corroborating with the CIA data, i.a. injection with flagellin resulted in 10 fold greater mature osteoclasts compared to the control group.

Conclusion: These novel results demonstrate for the first time that in RA as well as in acute and chronic experimental arthritis models, flagellin ligation to joint TLR5 contributes to myeloid cell infiltration and osteoclast maturation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/ligation-of-tlr5-promotes-myeloid-cell-infiltration-and-differentiation-into-mature-osteoclasts-in-ra-patients-and-experimental-arthritis/