Priming of MSCs with inflammation-relevant signals affects extracellular vesicle biogenesis, surface markers, and modulation of T cell subsets

Although considerable evidence exists supporting the use of mesenchymal stromal cells (MSCs) for treating immune diseases, successful clinical translation has been challenging and has led researchers to investigate cell-free alternatives. MSC-derived extracellular vesicles (MSC-EVs) have been shown to mediate a significant portion of the observed therapeutic effect, including immunosuppression. MSCs have been shown to respond to different aspects of the injury microenvironment such as inflammatory cytokines and hypoxia, although acidosis has not been investigated and different conditions have not been assessed in terms of their effects on MSC-EV function. This study investigated the effects of acidosis, hypoxia, and inflammatory cytokine priming on MSCs and MSC-EVs. We cultured MSCs in the presence of acidosis, hypoxia, or inflammatory cytokines (Interferon-gamma and Tumor Necrosis Factor-alpha) and compared the characteristics of their EVs as well as their uptake by and suppression of different T cell subsets. MSCs showed a greater effect on suppressing activated CD4⁺ and CD8⁺ T cells than MSC-EVs. However, MSC-EVs from MSCs primed with acidosis increased CD4⁺ and CD8⁺ regulatory T cell frequency in vitro. This functional response was reflected by MSC-EV uptake. MSC-EVs from acidosis-primed MSCs were the only EV group to be taken up significantly by CD4⁺ and CD8⁺ regulatory T cells. These data suggest that a simple low-cost alteration in MSC culture conditions, acidosis, can generate extracelluar vesicles that have a desirable influence on anti inflammatory T cell subtypes.