Mesenchymal stromal cell exosome-enhanced regulatory T-cell production through an antigen-presenting cell-mediated pathway

Background aims. The immunomodulatory property of mesenchymal stromal cell (MSC) exosomes is well documented. On the basis of our previous report that MSC exosomes increased regulatory T-cell (Treg) production in mice with allogenic skin graft but not in ungrafted mice, we hypothesize that an activated immune system is key to exosome-mediated Treg production. Methods. To test our hypothesis, MSC exosomes were incubated with mouse spleen CD4(+) T cells that were activated with either anti-CD3/CD28 mAbs or allogenic antigen-presenting cell (APC)-enriched spleen CD11c(+) cells to determine whether production of mouse CD4(+) CD25(+) T cells or CD4(+) CD25(+) Foxp3(+) Tregs could be induced. MSC exosomes were also administered to the lethal chimeric human-SCID mouse model of graft-versus-host disease (GVHD) in which human peripheral blood mononuclear cells were infused into irradiated NSG mice to induce GVHD. Results. We report here that MSC exosome-induced production of CD4(+) CD25(+) T cells or CD4(+) CD25(+) Foxp3(+) Tregs from CD4(+) T cells activated by allogeneic APC-enriched CD11C(+) cells but not those activated by anti-CD3/CD28 mAbs. This induction was exosome-and APC dose-dependent. In the mouse GVHD model in which GVHD was induced by transplanted human APC-stimulated human anti-mouse CD4(+) T cell effectors, MSC exosome alleviated GVHD symptoms and increased survival. Surviving exosome-treated mice had a significantly higher level of human CD4(+) CD25(+) CD127(low/-)Tregs than surviving mice treated with Etanercept, a tumor necrosis factor inhibitor. Conclusions. MSC exosome enhanced Treg production in vitro and in vivo through an APC-mediated pathway.