Extracellular vesicles (EVs) derived from stem and progenitor cells may have therapeutic effects comparable to their parental cells and are considered promising agents for the treatment of a variety of diseases. To this end, strategies must be designed to successfully translate EV research and to develop safe and efficacious therapies, whilst taking into account the applicable regulations. Here, we discuss the requirements for manufacturing, safety, and efficacy testing of EVs along their path from the laboratory to the patient. Development of EV-therapeutics is influenced by the source cell types and the target diseases. In this article, we express our view based on our experience in manufacturing biological therapeutics for routine use or clinical testing, and focus on strategies for advancing mesenchymal stromal cell (MSC)-derived EV-based therapies. We also discuss the rationale for testing MSC-EVs in selected diseases with an unmet clinical need such as critical size bone defects, epidermolysis bullosa and spinal cord injury. While the scientific community, pharmaceutical companies and clinicians are at the point of entering into clinical trials for testing the therapeutic potential of various EV-based products, the identification of the mode of action underlying the suggested potency in each therapeutic approach remains a major challenge to the translational path.

Extracellular vesicles (EVs) released by mesenchymal stromal cells (MSCs) may contribute to biological processes such as tissue regeneration, immunomodulation and neuroprotection. Evaluation of their therapeutic potential and application in future clinical trials demands thorough characterization of EV content and production under defined medium conditions, devoid of xenogenic substances and serum-derived vesicles. Addressing the apparent need for such a growth medium, we have developed a medium formulation based on pooled human platelet lysate (pHPL), free from animal-derived xenogenic additives and depleted of EVs. Depletion of EVs from complete growth medium was achieved by centrifugation at 120 000 g for 3 h, which reduced RNA-containing pHPL EVs to below the detection limit. Bone marrow (BM)-derived MSCs propagated in this medium retained the characteristic surface marker expression, cell morphology, viability and in vitro osteogenic and adipogenic differentiation potential. The proliferation rate was not significantly affected after 48 h but was decreased by 13% after 96 h. EVs collected from BM-MSCs cultured in EV-depleted medium revealed a similar RNA pattern as EVs generated in standard pHPL EV-containing medium but displayed a more clearly defined pattern of proteins characteristic for EVs. Reduction of pHPL content from 10% to 2% or serum-/pHPL-free conditions strongly altered MSC characteristics and RNA content of released EV. The 10% pHPL-based EV-depleted medium is appropriate for purification of exclusively human MSC-derived EVs. With this Good Manufacturing Practice-grade protocol, characterization and establishment of protein and RNA profiles from MSC-derived EVs can now be achieved to identify active components in therapeutic EVs for future clinical application.

The regenerative and immunomodulatory activity of mesenchymal stromal cells (MSCs) is partially mediated by secreted vesicular factors. Extracellular vesicles (EVs) exocytosed by MSCs are gaining increased attention as prospective non-cellular therapeutics for a variety of diseases. However, the lack of suitable in vitro assays to monitor the therapeutic potential of EVs currently restricts their application in clinical studies. We have evaluated a dual in vitro immunomodulation potency assay that reproducibly reports the inhibitory effect of MSCs on induced T-cell proliferation and the alloantigen-driven mixed leukocyte reaction of pooled peripheral blood mononuclear cells in a dose-dependent manner. Phytohemagglutinin-stimulated T-cell proliferation was inhibited by MSC-derived EVs in a dose-dependent manner comparable to MSCs. In contrast, inhibition of alloantigen-driven mixed leukocyte reaction was only observed for MSCs, but not for EVs. Our results support the application of a cell-based in vitro potency assay for reproducibly determining the immunomodulatory potential of EVs. Validation of this assay can help establish reliable release criteria for EVs for future clinical studies.

Extracellular vesicles (EVs) represent a potent intercellular communication system. Within a lipid bilayer such small vesicles transport biomolecules between cells and throughout the body, strongly influencing the fate of recipient cells. Due to their specific biological functions they have been proposed as biomarkers for various diseases and as optimal candidates for therapeutic applications. Despite of their extreme biological relevance, the small size (30 to a few hundred nanometers in diameter) of EVs still poses a great challenge for their isolation, quantification and biophysical/biochemical characterization, therefore the complex network of EVs and cells as well as their interaction remains to be further revealed. Here we propose a multiscale platform based on Atomic Force Microscopy, Small Angle X-ray Scattering, Small Angle Neutron Scattering and Neutron Reflectometry to reveal structure-function correlations of purified EVs through the analysis of their interaction with model membrane systems, in form of both supported lipid bilayers and suspended unilamellar vesicles of variably complex composition. The analysis reveals a strong interaction of EVs with the model membranes and preferentially with liquid ordered raft-like lipid domains, and opens the way to understand uptake mechanisms in different vesicle to cell membrane relative compositions.