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Objective This study aimed to determine whether the intra-articular injection of human adipose-derived mesenchymal stem cells (ADSCs) protects against the progression of murine post-traumatic osteoarthritis. Design ADSCs were isolated from human abdomen or buttock adipose tissues. In in vitro study, ADSCs conditioned medium was added to human chondrocytes pre-treated with interleukin-1β (IL-1β), and resultant gene expression of target inflammatory genes was measured by real-time quantitative polymerase chain reaction. A mouse model of knee osteoarthritis was generated by unilaterally transecting the medial meniscus in the right hind limb of 20 female C57BL/6 mice. Mice were randomly assigned to 2 treatment groups that received 6 µl intra-articular injections of either phosphate-buffered saline (control) or 2 × 104 cells/μl of ADSCs 14, 28, and 42 days post-surgery. Mice were euthanized 84 days post-surgery and histological and micro-computed tomography evaluation of knee joints were analyzed. Hind limb weight-bearing distribution was measured pre-surgery and 28 and 84 days post-surgery. Results Conditioned medium from cultured human adipose-derived mesenchymal stem cells suppressed the expression of target inflammatory genes in chondrocytes pre-treated with IL-1β, suggesting anti-inflammatory properties ( P < 0.01). Histological analyses indicated that the progression of destabilization of medial meniscus-induced knee osteoarthritis was suppressed by the administration of ADSCs compared with control group at medial femorotibial joint in vivo. This protective effect was related to a reduction in articular cartilage loss. Conclusion The intra-articular injection of ADSCs suppressed articular cartilage loss in a mouse model of knee osteoarthritis, possible through anti-inflammatory mechanisms.