Mesenbio's aim is to accelerate the clinical development of extracellular vesicle (EV) biotherapeutics. We have engineered a human mesenchymal stem cell line to produce a limitless, consistent supply EVs with intrinsic anti-inflammatory and tissue regenerating properties. Our intention is to develop these EVs as a disruptive new therapy for inflammatory arthritis, with refractory juvenile idiopathic arthritis (JIA) as our initial target indication. EVs are nano-sized, membrane-bound structures containing biological material, including nucleic acids and proteins. EVs are released by cells to deliver their bioactive cargoes and elicit functional change in target cells (e.g. growth, differentiation, immune suppression). Therefore EVs are increasingly viewed as an exciting new biotherapeutic paradigm, uncomplicated by the problems associated with cellular therapies, such as immune-compatibility, survival, storage, transport and costs. The key challenges for bringing EV therapies to market are consistency, efficacy and scale-up, which all link back to the source and supply of the EV-producing cells, which cannot rely on the constant sampling of uncharacterised donated human cells of variable quality and quantity. In 2010, we initiated a programme of work to identify authentic mesenchymal stem cells (MSCs) within a mixed population of human bone marrow stromal cells. Heterogeneous stromal cells were immortalised and expanded as single cell-derived colonies. The immortalised status enabled in-depth characterisation of many clones over several years and we succeeded in identifying clone Y201, which has the marker profile, morphological features and all the differentiation and immunosuppressive characteristics of true MSCs (James S. et al, Stem Cell Reports, 2015; Kay AG et al, Front. Immunol., 06 June 2022). We have demonstrated that Y201 MSCs produce a continuous, consistent supply of EVs that have both immunosuppressive and tissue-forming capabilities. In 2021 we founded the company, Mesenbio, to accelerate the development of Y201 EVs as a production platform to treat inflammatory joint disease, with refractory JIA as our first clinical target. The aim of this proposal is to advance the preclinical development of Y201 EVs to support regulatory approval for first-in-human clinical trials. We will focus here on upstream processing, characterisation and safety and efficacy of the EV product with the aim of attracting further funding to progress the therapy to clinical trial application.