Differential macrophage polarization promotes entheseal bone formation in AS

Systemic and focal bone loss is a common feature of many forms of arthritis. However, inappropriate new bone formation also occurs in inflammatory arthritis, including ankylosing spondylitis (AS), osteoarthritis, and rheumatoid arthritis. These features are important signs of systemic inflammation in AS. In general, inflammation is thought to inhibit bone-forming osteoblasts whilst promoting the formation and activation of osteoclasts. More recently, macrophages have been recognized as another component of the bone formation unit. Macrophages (MÏs) are actively involved in tissue remodelling and repair but also in tissue damage and fibrosis. Our research group demonstrated that the interaction between mesenchymal stem cells and monocyte/macrophages leads to an activation of STAT3 that drives osteoblast differentiation. This project objective is to better explain the contribution of monocyte/macrophages to bone formation in a murine model of AS; the 1, 3 beta-glucan (curdlan) challenged SKG mice which develop many features of AS including psoriasis, ileitis, enthesitis and arthritis. In vitro, the role of macrophages as regulators of osteogenic differentiation via direct cell contact with mesenchymal stromal cells (MSC) was investigated. OB differentiation was shown to depend upon contact with pro-inflammatory MÏs rather than anti-inflammatory macrophages. In conclusion, in the in vitro part, we evaluated the capability of GM-CSF MÏs (pro inflammatory) to increase MSCs - OB differentiation using two different genetic strains of mice: SKG and IRF 5 (WT and KO). We assessed that both SKG - GM-CSF MÏs and IRF 5 WT- GM-CSF MÏs could synergize with, but not replace, osteogenic signals in the medium and enhancing MSCs capability towards OB differentiation. However, IRF KO - GM-CSF MÏs showed no effect on MSCs. Moreover, In the in vivo study, we evaluated the occurrence of enthesitis and the role of MÏs in a murine model of AS (SKG). We assessed MÏs activity at the entheseal site by depleting them using clodronate liposomes, which ameliorated the bone phenotype but not the clinical score (all the mice developed severe AS). Previous work in the laboratory showed that MÏs promoted MSC osteogenic differentiation via oncostatin M (OSM). Therefore, OSM was blocked in SKG mice by injecting OSMRâFc directly into the entheseal site. This led to the amelioration of ileitis however there was a dramatic exacerbation of inflammation and bone disease in the paws of the mice indicating that OSM is not a direct mediator of MÏ-induced bone formation at the enthesis in vivo.