Mesenchymal stromal cells : mode of action and clinical translation

Abstract

Mesenchymal Stromal Cells (MSCs) are an extensively used cell type in clinical trials for the treatment of various diseases. In order to obtain clinically relevant numbers, MSCs need to be expanded in vitro, usually relying on the use of foetal calf serum (FCS), which is now not recommended by the regulatory authorities. In addition, the precise mechanism of action of MSCs remains unclear. The initial rationale for the therapeutic potential of MSCs was based on their engraftment and differentiation ability. It gets increasingly clear that MSCs exert their effects in a paracrine manner, by the release of soluble factors and extracellular vesicles (EVs). The aims of this study are: 1) to investigate the feasibility of replacing FCS with human platelet lysate (PLT) for the expansion of MSCs and assess its effect on MSC general characteristics and immunosuppressive potential; 2) to assess the immunoregulatory function of EVs derived from PLT-expanded MSCs (MSC-PLT). Data presented in this thesis demonstrated that MSC-PLT maintained their general characteristics and immunosuppressive potential, while exhibiting enhanced proliferative properties compared to FCS-expanded MSCs (MSC-FCS). As the first comparative study on the global surface protein profile of MSC-FCS and MSC-PLT, we showed that PLT induced little changes on the expression of markers involved in the enhancement of proliferation and differentiation properties of MSCs. Furthermore, this study has demonstrated that MSC-derived EVs retained the immunosuppressive function of the parent cells, although with a lower potency. Here we also report for the first time, that MSC-EVs skewed DC maturation into a tolerogenic phenotype and impaired their phagocytic ability and migratory potential. We also found that MSC-derived EVs are rich in a wide variety of microRNAs that target a plethora of genes involved in various pathways related to development, trafficking and modulation of immune responses, including dendritic cell maturation and function. The findings in this study support the notion that PLT is a suitable supplement for in vitro MSC expansion and that MSC-PLT produce EVs with the ability to modulate immune responses.