BACKGROUND:Bone marrow mesenchymal stem cel s are considered to have good proliferation and differentiation potentials. Sox9 is a transcription factor that is essential for chondrogenesis and has been termed as a“master regulator”of the chondrocyte phenotype. OBJECTIVE:To study the therapeutic effects of Sox9-transfected bone marrow mesenchymal stem cel s on knee osteoarthritis. METHODS:The bone marrow mesenchymal stem cel s were transfected with Lenti-Sox9-EGFP in vitro. The model of murine knee osteoarthritis was established by cutting off the anterior cruciate ligament. Thirty model mice were randomly divided into three groups, as normal saline group, bone mesenchymal stem cel group and Sox9-transfected bone mesenchymal stem cel group. 0.1 mL of normal saline, 0.1 mL of normal saline containing non-transfected bone marrow mesenchymal cel s (non-transfected group), or 0.1 mL of normal saline containing Sox9-transfected bone marrow mesenchymal cel s (Sox9-transfected group) was injected into the knee joint cavity of mice in the corresponding group, respectively. After 4, 8, 12 weeks, the repair of articular cartilage lesions was evaluated by toluidine blue and immunohistochemical staining. RESULTS AND CONCLUSION:The lesions of articular cartilage were more serious in the normal saline group, compared with the other two groups, and the difference became more obvious over time. Damaged articular cartilage was improved in the non-transfected group, but the improvement was less than that in the Sox9-transfected group. Immunohistochemistry staining revealed that in the Sox9-transfected group, the positive type II col agen expression was stronger than that in the other two groups, but this positive expression was decreased over time in al the three groups. These results suggest that Sox9-transfected bone marrow mesenchymal stem cel s promote the repair of damaged cartilage in mice with knee osteoarthritis.