COX-2 from the injury milieu is critical for the initiation of periosteal progenitor cell mediated bone healing

Although a critical role of COX-2 in bone repair has been established, the mechanism involved remains unclear. During early inflammatory phase of bone healing, COX-2 is produced by the surrounding inflammatory cells as well as bone/cartilage progenitors. Based on the temporal and spatial expression of COX-2 during the early phase of fracture healing, we hypothesize that COX-2 from both sources is critical for progenitor cell activation, proliferation and differentiation. To directly test this we utilized a murine femoral grafting model, in which live segmental grafts from the same strains were transplanted and donor versus host cell involvement in healing was assessed. Specifically, fresh femur cortical bone grafts of 4 mm in length from COX-2−/− (KO) mice were transplanted into wild type (WT) mice with the same sized segmental defect in femurs. Similarly, grafts from WT were transplanted into the defects in KO mice. As controls, transplantations between wild types, and transplantations between KO were also performed. Histologic analyses showed that WT-to-WT transplantation resulted in normal endochondral bone healing as evidenced by markedly induction of neovascularization and periosteal bone formation on donor graft. In contrast, transplantation of KO graft into KO host led to 96% reduction of bone formation and near elimination of donor cell-initiated periosteal bone formation. Similarly, transplantation of WT graft into a KO host resulted in 87% reduction of bone formation (n = 8, p > 0.05), indicating that KO host impaired WT donor progenitor cell expansion and differentiation. When a KO graft was transplanted into WT host, KO donor periosteal cell-initiated endochondral bone formation was restored. Histomorphometric analyses demonstrated 10-fold increase in bone formation and 3-fold increase in cartilage formation compared to KO-to-KO transplantation (n = 8, p < 0.05), suggesting that COX-2 deficient donor cells were capable to differentiate and form bone when placed in a WT host. Taken together, our data strongly suggest that COX-2 is critical for initiation of periosteal cortical bone healing. The early induction of COX-2 constitutes a crucial host-healing environment for activation and differentiation of donor periosteal progenitors. Elimination of COX-2 at the early stage of healing could lead to detrimental effects on periosteal progenitor cell-initiated cortical bone repair.