Anti-Donor Immune Responses Elicited by Allogeneic Mesenchymal Stem Cells and Their Extracellular Vesicles: Are We Still Learning?

Cell-based therapy, e.g., multipotent mesenchymal stromal cell (MSC) treatment, shows promise for the treatment of various diseases. The strong paracrine capacity of these cells and not their differentiation capacity, is the principal mechanism of therapeutic action. MSCs robustly release exosomes, membrane vesicles (~30–100 nm) originally derived in endosomes as intraluminal vesicles, which contain various molecular constituents including proteins and RNAs from maternal cells. Contained among these constituents, are small non-coding RNA molecules, microRNAs (miRNAs), which play a key role in mediating biological function due to their prominent role in gene regulation. The release as well as the content of the MSC generated exosomes are modified by environmental conditions. Via exosomes, MSCs transfer their therapeutic factors, especially miRNAs, to recipient cells, and therein alter gene expression and thereby promote therapeutic response. The present review focuses on the paracrine mechanism of MSC exosomes, and the regulation and transfer of exosome content, especially the packaging and transfer of miRNAs which enhance tissue repair and functional recovery. Perspectives on the developing role of MSC mediated transfer of exosomes as a therapeutic approach will also be discussed.

Human bone marrow-derived mesenchymal stromal cells (MSCs) inhibit proliferation of activated T cells, and IFN-γ plays an important role in this process. This IFN-γ-licensed veto property is IDO-dependent. To further decipher the mechanistic underpinnings of MSC veto function on T cells, we investigated the effect of MSCs and IFN-γ-licensed MSCs on T cell effector function as assayed by cytokine secretion of T cells. Although MSCs and IFN-γ-licensed MSCs inhibit T cell proliferation, only IFN-γ-licensed MSCs significantly inhibit Th1 cytokine (IFN-γ, TNF-α, and IL-2) production by T cells. Additionally, IFN-γ-licensed MSCs inhibit T cell degranulation as well as single, double, and triple cytokine-producing T cells. Although IFN-γ-licensed MSCs upregulate their IDO activity, we found that MSC IDO catalytic function is dispensable with regard to MSC-driven inhibition of T cell effector function. Novel flow cytometry based functional screening of MSC-expressed, IFN-γ-licensed inhibitory molecules identified B7H1 and B7DC/PD1 pathways as essential effectors in blocking T cell function. Small interfering RNA-mediated blocking of B7H1 and B7DC reverses the inhibitory potential of IFN-γ-licensed MSCs on T cell effector function. Mechanistic analysis revealed that clustering of MHC and coinhibitory molecules are indispensable for the inhibitory effect of IFN-γ MSCs. Although exogenous IL-2 reverses B7H1-Ig-mediated inhibition of T cell proliferation, it does not affect the veto function of IFN-γ MSCs on both T cell proliferation and effector function. Our results reveal a new immunosuppressive property of IFN-γ-licensed MSCs that inhibits T cell effector function independent of IDO but through the ligands for PD1.

Cardiac cell therapy for older patients who experience a myocardial infarction may require highly regenerative cells from young, healthy (allogeneic) donors. Bone marrow mesenchymal stem cells (MSCs) are currently under clinical investigation because they can induce cardiac repair and may also be immunoprivileged (suitable for allogeneic applications). However, it is unclear whether allogeneic MSCs retain their immunoprivilege or functional efficacy late after myocardial implantation. We evaluated the effects of MSC differentiation on the immune characteristics of cells in vitro and in vivo and monitored cardiac function for 6 months after post-myocardial infarction MSC therapy. In the in vitro experiments, inducing MSCs to acquire myogenic, endothelial, or smooth muscle characteristics (via 5-azacytidine or cytokine treatment) increased major histocompatibility complex-Ia and -II (immunogenic) expression and reduced major histocompatibility complex-Ib (immunosuppressive) expression, in association with increased cytotoxicity in coculture with allogeneic leukocytes. In the in vivo experiments, we implanted allogeneic or syngeneic MSCs into infarcted rat myocardia. We measured cell differentiation and survival (immunohistochemistry, real-time polymerase chain reaction) and cardiac function (echocardiography, pressure-volume catheter) for 6 months. MSCs (versus media) significantly improved ventricular function for at least 3 months after implantation. Allogeneic (but not syngeneic) cells were eliminated from the heart by 5 weeks after implantation, and their functional benefits were lost within 5 months. The long-term ability of allogeneic MSCs to preserve function in the infarcted heart is limited by a biphasic immune response whereby they transition from an immunoprivileged to an immunogenic state after differentiation, which is associated with an alteration in major histocompatibility complex-immune antigen profile.