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Abstract

Mesenchymal stem/stromal cells (MSC) have been known to possess strong therapeutic potential for many diseases. MSC are capable of inhibiting apoptosis, promoting migration and angiogenesis. One of MSC key modes of action is via immunomodulation, being known to suppress T, B and NK cells, to polarize monocytes/macrophages and to induce regulatory T cells. MSC T cell proliferation inhibition has been closely related to an increased indoleamine 2,3-dioxygenase (IDO) secretion, breaking down the essential amino acid tryptophan into kynurenine.

Nevertheless, there is a need to understand whether direct cell to cell contact is needed to exert MSC immune responses or whether MSC derived conditioned media (CM) and extracellular vesicles (EV) possess similar immunomodulatory potential. We speculate that EV are directly involved in MSC mediated suppression, thus being a potential cell-free therapeutic agent.

Our main objectives were to investigate whether MSC isolated from different human tissue sources would differ in their immunomodulatory functions. We aimed to evaluate MSC-derived CM and EV immune strength, identify related molecular mechanisms and compare it to the effects of their cellular counterpart. Furthermore, we focused on defining different MSC modulatory properties and mechanisms.

We compared adipose (ASC), bone marrow (BM) and cord blood (CB) derived MSC, hypothesizing that MSC from different sources possess differing modulatory capacities. To assess their immunosuppression capacities, we set up direct cocultures with human peripheral blood mononuclear cells (PBMC) and purified CD4+ T cells and assessed suppression of PHA-induced proliferation.

In addition, we compared direct coculture setups with parallel transwell and EV cocultures, in order to determine whether MSC exert their modulatory capacities in a paracrine manner through secreted factors or even through ultracentrifuge-isolated EV. We investigated IDO as a candidate involved in the inhibition of PBMC proliferation, as well as the kynurenine pathway of tryptophan metabolism.