Mesenchymal stem cells (MSCs) can be widely isolated from various tissues including bone marrow, umbilical cord, and adipose tissue, with the potential for self-renewal and multipotent differentiation. There is compelling evidence that the therapeutic effect of MSCs mainly depends on their paracrine action. Extracellular vesicles (EVs) are fundamental paracrine effectors of MSCs and play a crucial role in intercellular communication, existing in various body fluids and cell supernatants. Since MSC-derived EVs retain the function of protocells and have lower immunogenicity, they have a wide range of prospective therapeutic applications with advantages over cell therapy. We describe some characteristics of MSC-EVs, and discuss their role in immune regulation and regeneration, with emphasis on the molecular mechanism and application of MSC-EVs in the treatment of fibrosis and support tissue repair. We also highlight current challenges in the clinical application of MSC-EVs and potential ways to overcome the problem of quality heterogeneity.

BackgroundMesenchymal stromal cells (MSCs) are a promising therapy for preventing chronic Graft-Versus-Host Disease (cGVHD) due to their potent immunomodulatory properties. However, the safety concerns regarding the use of MSCs remain unsolved, and conflicting effects are observed due to the heterogeneity of MSCs. Recently, exosomes were shown to mediate the paracrine effects of MSCs, making it a potential candidate for cell-free therapies. The aim of this study is to investigate the efficacy and safety of MSCs-derived exosomes (MSCs-exo) in an established cGVHD mouse model.MethodsBone marrow (BM)-derived MSCs were cultured, and the supernatants of these cultures were collected to prepare exosomes using ultracentrifugation. Exosomes from human dermal fibroblasts (Fib-exo) were used as a negative control. The cGVHD model was established, and tail vein injections of MSCs-exo or Fib-exo were administered once per week for 6 weeks. The symptoms and signs of cGVHD were monitored, and histopathological changes were detected by hematoxylin and eosin and Masson staining. The effects of MSCs-exo on Th17, Th1, and Treg were evaluated by flow cytometry, qPCR, and Luminex. In addition, human peripheral blood mononuclear cells (PBMCs) were stimulated and treated with MSCs-exo in vitro. IL-17-expressing Th17 and IL-10-expressing Treg were evaluated by flow cytometry, qPCR, and ELISA.ResultsWe found that MSCs-exo effectively prolonged the survival of cGVHD mice and diminished the clinical and pathological scores of cGVHD. Fibrosis in the skin, lung, and liver was significantly ameliorated by MSCs-exo application. In MSCs-exo treated mice, activation of CD4+ T cells and their infiltration into the lung were reduced. Of note, MSCs-exo exhibited potent immunomodulatory effects via the inhibition of IL-17-expressing pathogenic T cells and induction of IL-10-expressing regulatory cells during cGVHD. The expressions of Th17 cell-relevant transcription factors and pro-inflammatory cytokines was markedly reduced after MSCs-exo treatment. In vitro, MSCs-exo blocked Th17 differentiation and improved the Treg phenotype in PBMCs obtained from healthy donors and patients with active cGVHD, further indicating the regulatory effect of MSCs-exo on GVHD effector T cells.ConclusionsOur data suggested that MSCs-exo could improve the survival and ameliorate the pathologic damage of cGVHD by suppressing Th17 cells and inducing Treg. This finding provides a novel alternative approach for the treatment of cGVHD.Electronic supplementary materialThe online version of this article (10.1186/s13045-018-0680-7) contains supplementary material, which is available to authorized users.

Mesenchymal stromal cells (MSC) are a heterogeneous cell population with self-renewal and the ability to differentiate into different lineages. The novel regulatory role of MSC in both adaptive and innate immune responses got extensive investigation and MSC have been widely used in clinical trials as immunosuppressive agents for autoimmune and inflammatory diseases, including graft-versus-host disease (GVHD), multiple sclerosis (MS), systemic lupus erythematosus (SLE), chronic kidney disease, etc. Recent studies have found that MSC exerted their immunomodulation function through secreting extracellular vesicles (EVs), which delivered parent cell cargo to recipient cells without oncogenicity or variability. Since MSC-EVs exhibit most of the properties of MSC and take advantage of their cellular immunomodulatory fuction, MSC-EVs appear to a promising none-cell therapy in various human diseases. In this review, we summarize the pivotal roles of MSC-EVs as agents for immunotherapy in diseases.