Immunoregulatory properties of bone marrow mesenchymal stromal cell-derived extracellular vesicles

Mesenchymal stromal cells (MSCs) are adult stem cells of mesodermal origin that can be isolated from various tissues, including bone marrow (BM), adipose tissue and amniotic fluid. MSCs express mesenchymal markers, i.e. CD73, CD90, and CD105, and lack expression of hematopoietic markers, such as CD45, CD34, CD11b and CD14. In addition to their tri-lineage differentiation towards adipocytes, chondrocytes and osteoblasts, MSCs modulate the immune response. In fact, MSCs can regulate the proliferation and activation of different immune effector cells (IECs), including T, B and NK cells. The biological effects of MSCs are not exclusively related to their close interaction with target cells by cell-to-cell contact, but can be mediated by molecule release. For instance, MSC immunomodulation may occur through paracrine mechanisms, including indolamine 2,3 dioxygenase, prostaglandin E2, heme-oxygenese-1, and TGF-β. In the last decade, a key mechanism of cell-to-cell communication of MSCs through extracellular vesicles (EVs) has been clarified. The potential therapeutic role of MSC-derived EVs has been described in different diseases, including cardiovascular disease, acute kidney injury, and lung injury. EVs are molecular shuttles consisting of a phospholipid bilayer containing different molecules, including proteins and different types of RNAs (mRNA and miRNA). EVs are a family of different shedding vesicles, including exosomes (EXs, 50-100 nm), microvesicles (MVs, 100-1000 nm), and apoptotic bodies (ABs, 50-500 nm). EXs originate by multivesicular body and express specific markers, such as CD63, CD9 and Alix. MVs result from the plasmatic membrane and express specific proteins of the cells of origin. To understand whether the MSC immunomodulatory effects are mediated by EV release, we characterized the protein content and immunomodulatory functions towards different immune effector cells of EVs derived from BM-MSCs. In addition, we evaluated the capability of unfractionated PBMCs to internalize MSC-derived EVs. We observed that the rate of EV internalization was higher in B cells and correlated with their capability to reduce B cell proliferation. By using a reproducible and standardized method we showed a new mechanism of MSC-mediated immunosuppression, thus characterizing better the biological function of MSC-derived EVs and paving the way to a possible clinical application of EVs as alternative cell-free therapy.