Practical considerations in transforming MSC therapy for neurological diseases from cell to EV.

Cell-based therapy using Mesenchymal Stromal Cell (MSC) has generally been efficacious in treating a myriad of diseases in animal models and clinical trials. The rationale for MSC therapy was predicated on the potential of MSC to differentiate and form new replacement cells in the diseased tissue. However, pre-clinical animal and clinical data were more consistent with a secretion- and not a differentiation-based rationale. Analysis of MSC secretion led to the identification of small extracellular vesicles (sEVs) as therapeutically active, secretory agents. MSC-sEVs are defined as bi-lipid membrane vesicles of 50-200 nm in diameter that are secreted by MSCs. They reportedly exert similar therapeutic efficacy as MSCs in many diseases including neurological diseases. MSC-sEVs being small and non-living are intrinsically safer than living MSCs. Manufacturing of MSC-sEVs may also be less complex. Nevertheless, realising the therapeutic potential of MSC-sEVs will require exacting scientific rigor and robustness, as well as compliance to regulatory oversight. This review summarises the scientific rationale for the transition of MSC therapy from a cell- to an EV-based therapy and discusses critical scientific issues in the development of MSC-sEVs therapy.