Cell-based therapy in cartilage repair can benefit from the use of chondroprogenitors; a cell type classified as mesenchymal stem cells, demonstrating reduced hypertrophy. Fibronectin, routinely used to isolate chondroprogenitors, classically binds to α5β1 integrins (CD49e + CD29), of which CD49e is said to be highly expressed in progenitors. The aim of our study was to assess the specificity of CD49e as a distinguishing marker for chondroprogenitors; because studies report low expression in fresh chondrocytes (FCs), but recent conflicting data has exhibited incremental expression of CD49e in cultured chondrocytes.
FCs were isolated from three human osteoarthritic knee joints and CD49e- cells (sorted by flow cytometry) were cultured in adherent and non-adherent conditions and reassessed for CD49e and CD29 at multiple time points. Colony-forming efficiency (CFE) following fibronectin adhesion assay was calculated for FC, CD49e+ and CD49e- cells.
A statistically significant increase in CD49e and CD29 expression was seen in both adherent and non-adherent cultures of CD49e- cells (P < 0.01), as early as 24 h. All groups grew clonally and CFE was similar without any significant difference. CD49e- chondrocytes turned positive when cultured, possibly due to an inherent phenotypic drift, seen after release from cartilage and not because of plastic adherence or chondroprogenitor overgrowth, as non-adherent cultures also showed high expression.
As the specificity of CD49e is questionable, there is a pressing need for a specific differentiating marker, to isolate a pure population of chondroprogenitors, as this cell type shows inherent chondrogenesis and reduced hypertrophy, both requisites for cartilage repair.