Human multipotent mesenchymal stromal cells inhibit proliferation of PBMCs independently of IFNgammaR1 signaling and IDO expression

Multipotent mesenchymal stromal cells (MSCs) inhibit proliferation, helper, and effector functions in most if not all peripheral blood mononuclear cell (PBMC) subpopulations in vitro. The molecular mechanism is widely thought to imply tryptophan degradation by the interferon-γ (IFNγ)-induced expression of indoleamine 2,3-dioxygenase (IDO). However, IDO inhibitors were not able to restore proliferation of PBMCs in each case. Moreover, human MSCs with an IFNγ receptor 1 (R1) defect inhibited proliferation of HLA-mismatched PBMCs to a similar extent as control MSCs. In contrast to healthy MSCs, IFNγR1-deficient MSCs showed no detectable mRNA for IDO - neither in the absence nor in the presence of recombinant human IFNγ, nor in coculture with HLA-mismatched PBMCs. Based on gene expression profiling, we were able to show that insulinlike growth factor (IGF)-binding proteins contribute to the inhibitory mechanism of MSCs. Taken together, human MSCs exert important immunomodulatory functions in the absence of IFNγR1 signaling and IDO, partially accounted for by IGF-binding proteins.