Articular Cartilage Repair with Mesenchymal Stem Cells After Chondrogenic Priming: A Pilot Study

Bone marrow-derived mesenchymal stromal stem cells (BMSCs) are a promising cell source for treating articular cartilage defects. The objective of this study was to assess a protocol that involved autologous transplantation of BMSCs into full-thickness cartilage defects in sheep following isolation, expansion, and a short period (4 days) of chondrogenic priming. The impact of oxygen tension during preimplantation culture was investigated. It was hypothesized that chondrogenically primed BMSCs would produce superior cartilaginous repair tissue relative to control defects, and that culture under hypoxia would yield improved repair tissue in comparison to normoxia. Ovine BMSCs were isolated, expanded to passage 2, seeded within a hyaluronic acid (HYAFF) scaffold, and primed ex vivo in chondrogenic medium for 4 days under normoxia (21% oxygen) or hypoxia (3% oxygen). Full-thickness, 7-mm-diameter articular cartilage defects were created in the femoral condyles of five sheep. Twenty defects were treated with normoxia-cultured, autologous BMSC-seeded scaffolds (eight); hypoxia-cultured, autologous BMSC-seeded scaffolds (eight); cell-free scaffolds (two); or no implants (two). Preimplantation priming was evaluated through gene expression analysis using reverse transcription quantitative polymerase chain reaction. After 6 months, histological assessment was performed on repair tissues with a modified O'Driscoll scoring system and tissue dimension analysis. Priming of preimplantation BMSC-seeded scaffolds in chondrogenic medium for 4 days resulted in significantly increased gene expression of hyaline cartilage-related collagen II and aggrecan relative to unprimed BMSCs (p < 0.05). Defects implanted with chondrogenically primed BMSC-seeded scaffolds developed cartilaginous repair tissues that contained safranin O-positive proteoglycans, and had significantly larger repair tissue areas, higher percentages of defect fill, and improved histological scores than cell-free controls (p < 0.05). Although hypoxic culture improved the preimplantation gene expression profile, a consistent difference in histological scores was not found between normoxia- and hypoxia-seeded BMSC-seeded scaffolds after 6 months (p = 0.90). This study demonstrates in a sheep model that (1) chondrogenic priming ex vivo improves the gene expression profile of BMSCs; (2) chondrogenically primed BMSCs are associated with the development of superior cartilaginous tissue to cell-free controls within cartilage defects; and (3) oxygen tension during preimplantation ex vivo culture does not consistently modulate cartilaginous repair tissue formation following BMSC transplantation into cartilage defects.