Mesenchymal stromal cells (MSCs) are under investigation for treating a wide range of immune disorders due to their immunomodulatory capacity. Although intracellular factors like indoleamine-2,3-dioxygenase have been implicated as important, secreted factors, including exosomes, may also play a major role. Curiously, while MSCs are not very immunosuppressive at baseline, as they have not been “primed” towards a suppressive phenotype, exosomes from the supernatant of these naïve MSCs still show therapeutic efficacy. Continuing our previous work on how MSC priming influences their therapeutic efficacy, we sought to extend this exploration to their exosomes. We used a polymer method to isolate exosomes from the supernatant of unprimed MSCs or those primed with 48-hours of combined hypoxia/IFN-γ stimulation. There were consistently 2-4x more exosomes secreted from primed MSCs. We labeled these exosomes and dosed them into mixed lymphocyte reactions (MLRs) at concentrations of 10, 50, 125 and 250 μg/mL. This showed dose-dependent uptake of the exosomes into mononuclear cells (PBMCs), but the uptake was less profound when exosomes came from primed MSCs. While no division occurred when the exosomes were added to only responder PBMCs, in a full MLR, the exosomes attenuated T-cell division in a dose-dependent manner. Doses > 50 μg/mL exacerbated T-cell division from both exosome sources, but exosomes from primed MSCs were able to inhibit T-cell division once the concentration was lowered to 50 μg/mL. We are still investigating the mechanisms of this dose and source-dependent attenuation, but it is clear that monocytes (not T-cells) predominantly take up the exosomes.
Mikati Foundation Professor of Biomedical Engineering and Medical Sciences