Objective: We investigated the effects of single or repetitive intra-articular injections of synovial mesenchymal stem cells (MSCs) on a rat osteoarthritis (OA) model, and elucidated the behaviors and underlying mechanisms of the stem cells after the injection. Design: One week after the transection of the anterior cruciate ligament (ACL) of wild type Lewis rats, one million synovial MSCs were injected into the knee joint every week. Cartilage degeneration was evaluated with safranin-o staining after the first injection. To analyze cell kinetics or MSC properties, luciferase, LacZ, and GFP expressing synovial MSCs were used. To confirm the role of MSCs, species-specific microarray and PCR analyses were performed using human synovial MSCs. Results: Histological analysis for femoral and tibial cartilage showed that a single injection was ineffective but weekly injections had significant chondroprotective effects for 12 weeks. Histological and flow-cytometric analyses of LacZ and GFP expressing synovial MSCs revealed that injected MSCs migrated mainly into the synovium and most of them retained their undifferentiated MSC properties though the migrated cells rapidly decreased. In vivo imaging analysis revealed that MSCs maintained in knees while weekly injection. Species-specific microarray and PCR analyses showed that the human mRNAs on day 1 for 21 genes increased over 50-fold, and increased the expressions of PRG-4, BMP-2, and BMP-6 genes encoding chondroprotective proteins, and TSG-6 encoding an anti-inflammatory one. Conclusion: Not single but periodic injections of synovial MSCs maintained viable cells without losing their MSC properties in knees and inhibited osteoarthritis (OA) progression by secretion of trophic factors.
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