Epiphyseal Chondroprogenitors Provide a Stable Cell Source for Cartilage Cell Therapy

Articular cartilage regeneration poses particularly tough challenges for implementing cell-based therapies. Many cell types have been investigated looking for a balanced combination of responsiveness and stability, yet techniques are still far from defining a gold standard. The work presented focuses on the reliable expansion and characterization of a clinical grade human epiphyseal chondroprogenitor (ECP) cell bank from a single tissue donation. A parental human ECP cell bank was established, which provides the seed material for master and working cell banks. ECPs were investigated at both low and high cumulative population doublings looking at morphology, monolayer expansion kinetics, resistance to cryogenic shock, colony-forming efficiency, and cell surface markers. Three-dimensional micropellet assays were used to determine spontaneous extracellular matrix deposition at varying population doublings and monolayer 2D differentiation studies were undertaken to assess the propensity for commitment into other lineages and their stability. ECPs exhibited remarkable homogeneity in expansion with a steady proliferative potential averaging three population doublings over 8 days. Surface marker analysis revealed no detectable contaminating subpopulations or population enrichment during prolonged culture periods. Despite a slight reduction in Sox9 expression levels at higher population doublings in monolayer, nuclear localization was equivalent both in monolayer and in micropellet format. Equally, ECPs were capable of depositing glycosaminoglycans and producing aggrecan, collagen I, and collagen II in 3D pellets both at low and high population doublings indicating a stable spontaneous chondrogenic potential. Osteogenic induction was differentially restricted in low and high population doublings as observed by Von Kossa staining of calcified matrix, with a notable collagen X, MMP13, and ADAMTS5 downregulation. Rare adipogenic induction was seen as evidenced by cytoplasmic lipid accumulation detectable by Oil Red O staining. These findings highlight the reliability, stability, and responsiveness of ECPs over prolonged culture, making them ideal candidates in defining novel strategies for cartilage regeneration.