Requirement of IFN-γ–Mediated Indoleamine 2,3-Dioxygenase Expression in the Modulation of Lymphocyte Proliferation by Human Adipose–Derived Stem Cells

Abstract

Human adipose–derived mesenchymal stem cells (hASCs) are mesenchymal stem cells (MSCs) with reduced immunogenicity and capability to modulate immune responses. Whereas the immunosuppressive activity of bone marrow–MSCs has received considerable attention during the last few years, the specific mechanisms underlying hASC-mediated immunosuppression have been poorly studied. Recent studies comparing both cell types have reported differences at transcriptional and proteomic levels, suggesting that hASCs and bone marrow–MSCs, while having similarities, are quite different. This suggests that different mechanisms of immunosuppression may apply. Here, we report that hASCs inhibit peripheral blood mononuclear cells (PBMCs), and CD4⁺ and CD8⁺ T cell proliferation in both cell–cell contact and transwell conditions, which is accompanied by a reduction of proinflammatory cytokines. We demonstrate that hASCs do not constitutively express immunomodulatory factors. Conditioned supernatants from hASCs stimulated by IFN-γ, PBMCs, or activated PBMCs highly inhibited PBMC proliferation, indicating that inhibitory factors are released upon hASC activation. Many factors have been involved in MSC-mediated immunosuppression, including IFN-γ, IL-10, hepatocyte growth factor, prostaglandin E2, transforming growth factor-β1, indoleamine 2,3-dioxygenase (IDO), nitric oxide, and IL-10. Using pharmacological inhibitors, neutralizing antibodies, and genetically modified hASCs that constitutively express or silence IDO enzyme, we demonstrate that, in the case of hASCs, the IFN-γ/IDO axis is essential. Taken together, our data support the key role of IDO in the therapeutic use of hASC on immunomediated diseases.