Trophic stimulation of articular chondrocytes by late-passage mesenchymal stem cells in coculture

Coculture of mesenchymal stem cells (MSCs) with articular chondrocytes (ACs) increases glycosaminoglycan (GAG) accumulation compared to monoculture. MSCs might (1) differentiate into ACs (progenitor role) and/or (2) stimulate AC matrix metabolism (trophic role). MSCs lose the ability to undergo chondrogenesis after extended passaging. We hypothesized that MSCs act predominantly as progenitors, and that late-passage MSCs without chondrogenic potential would be unable to increase GAG in coculture. Early- and late-passage human MSCs (hMSCs) were grown in pellet monoculture under chondrogenic conditions and in pellet coculture with bovine ACs. Chondrogenesis was assessed with GAG quantification, safranin-O staining, and quantitative PCR (qPCR). Contributions of human and bovine cells were assessed with species-specific qPCR and human-specific immunostaining. Late-passage hMSCs did not undergo chondrogenesis in monoculture with chondrogenic stimuli or in coculture with ACs. Early-passage hMSCs underwent chondrogenesis only in response to chondrogenic stimuli. Coculture pellets in both cases accumulated as much GAG/DNA as monoculture AC pellets. Aggrecan transcription was not increased in coculture. Late-passage hMSCs that do not undergo chondrogenesis are capable of stimulating GAG accumulation in coculture with ACs. This supports a trophic effect of hMSCs on ACs. hMSCs may have therapeutic utility even after prolonged passaging.