Human Adipose Tissue-Derived Stromal Cells Suppress Human, but Not Murine Lymphocyte Proliferation, via Indoleamine 2,3-Dioxygenase Activity

Abstract

Over recent years, mesenchymal stromal cells (MSC) have gained immense attraction in immunotherapy, regenerative medicine and tissue engineering. MSC microenvironment modulation occurs through synergy of direct cell–cell contact, and secreted soluble factors and extracellular vesicles (EV). MSC-derived EV have been suggested as cell-free immunomodulatory alternative to MSC; however, previous findings have challenged this. Furthermore, recent data suggest that evaluating the mechanism of action of human MSC (hMSC) in animal models might promote adverse immune reactions or lack of functionality due to xeno-incompatibilities. In this study, we first assessed the immunomodulatory strength of different human MSC sources on in vitro stimulated T cells and compared this to interferon-gamma (IFNγ) primed MSC conditioned medium (CM) and EV. Second, we addressed the main molecular mechanisms, and third, we assessed the MSC in vitro immunosuppressive effect across interspecies barriers. We identified human adipose tissue-derived stromal cells (ASC) with strongest immunomodulatory strength, followed by bone marrow (BM) and cord blood-derived MSC (CB). Whilst CM from primed ASC managed to exert analogous effects as their cellular counterpart, EV derived thereof did not, reproducing previous findings. IFNγ-induced indoleamine 2,3-dioxygenase (IDO) activity was identified as key mechanism to suppress human lymphocyte proliferation, as in the presence of the IDO inhibitor epacadostat (Epac) a stimulation of proliferation was seen. In addition, we revealed MSC immunosuppressive effects to be species-specific, because human cells failed to suppress murine lymphocyte proliferation. In summary, ASC were the strongest immunomodulators with the IDO-kynurenine pathway being key within the human system. Importantly, the in vitro lack of interspecies immunomodulatory strength suggests that preclinical data need to be carefully interpreted especially when considering a possible translation to clinical field.

Human Adipose Tissue-Derived Mesenchymal Stromal Cells Inhibit CD4+ T Cell Proliferation and Induce Regulatory T Cells as Well as CD127 Expression on CD4+CD25+ T Cells

“…The main mechanism involved in human ASC-mediated inhibition of T cell proliferation is the induction of IDO expression, which enzymatically degrades tryptophan, required for T cell proliferation, to kynurenine, a direct inhibitor of T cell proliferation [ 31 ]. To confirm this, IDO was measured in cocultured ASC and kynurenine concentrations in coculture CM.…”

“…IDO is a catabolic enzyme which converts tryptophan to kynurenine [ 40 ]. In a previous study, we showed that increased IDO activity in ASC with subsequent tryptophan reduction and increasing kynurenine levels in their conditioned media was related to suppression of T cell proliferation in vitro [ 31 ]. In line with that, our data show that ASC from stimulated cocultures express higher levels of IDO than the ones from not stimulated cocultures.…”

Therapeutic Properties of Mesenchymal Stromal/Stem Cells: The Need of Cell Priming for Cell-Free Therapies in Regenerative Medicine

“…It has been shown that, using IL-17, MSCs increase the production of IL-6, which suppress T cell proliferation and inhibits Th1 cytokines production [ 119 ]. Instead, when primed with interferon gamma (IFN-γ), MSCs mainly produce indoleamide 2,3-dioxygenase (IDO) (a crucial factor able to suppress lymphocytes proliferation) [ 120 ], and also secrete key regulators of immunomodulation, such as prostaglandin E2 (PGE2), HGF, TGF-β and MCP-1. Furthermore, an increase of MHC I and II expression and of co-stimulatory molecules was also showed [ 121 ].…”

The Unique Immunomodulatory Properties of MSC-Derived Exosomes in Organ Transplantation

“…The inhibition of the proliferation of T and B cells and the maturation of monocytes promotes the production of Tregs and M2 macrophages. Studies have shown that when the paracrine mechanism of MSCs is activated, they can directly secrete anti-inflammatory cytokines, such as indolamine 2,3-dioxygenase (IDO), prostaglandin E2 (PGE2), nitric oxide (NO), HLA-G, transforming growth factor-β (TGF-β), and vascular endothelial growth factor (VEGF) ( 19 , 20 ). These cytokines affect the number of immune cells and achieve the effect of suppressing the immune response.…”