Expansion of human mesenchymal stem/stromal cells (hMSCs) in bioreactors using microcarriers: lessons learnt and what the future holds.

Human mesenchymal stem/stromal cells (hMSCs) present a key therapeutic cellular intervention for use in cell and gene therapy (CGT) applications due to their immunomodulatory properties and multi-differentiation capability. Some of the indications where hMSCs have demonstrated pre-clinical or clinical efficacy to improve outcomes are cartilage repair, acute myocardial infarction, graft versus host disease, Crohn's disease and arthritis. The current engineering challenge is to produce hMSCs at an affordable price and at a commercially-relevant scale whilst minimising process variability and manual, human operations. By employing bioreactors and microcarriers (due to the adherent nature of hMSCs), it is expected that production costs would decrease due to improved process monitoring and control leading to better consistency and process efficiency, and enabling economies of scale. This approach will result in off the shelf (allogeneic) hMSC-based products becoming more accessible and affordable. Importantly, cell quality, including potency, must be maintained during the bioreactor manufacturing process. This review aims to examine the various factors to be considered when developing a hMSC manufacturing process using microcarriers and bioreactors and their potential impact on the final product. As concluding remarks, gaps in the current literature and potential future areas of research are also discussed.