Clinical trials testing the use of either autologous or allogeneic human bone marrow-derived mesenchymal stromal cells (MSC) as a cell-based pharmaceutical for suppression of autoimmune and alloimmune ailments are underway. Reported results from completed trials vary in effectiveness within and between studies without any clear mechanistic explanation. We propose that these discrepancies may arise from intrinsic variability in the immunosuppressive potential of each MSC donor source. Here, we demonstrate that tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ)-activated MSC derived from normal adult volunteers suppress T cell proliferation in vitro in a variegated manner, an observation linked to IFN-mediated indoleamine 2,3-dioxygenase (IDO) upregulation. We also demonstrate that MSC IDO activity is implicated in the differentiation of monocytes into interleukin (IL)-10-secreting M2 immunosuppressive macrophages (CD14(+)/CD206(+)). Those monocyte-derived M2 are in turn implicated in the suppression of T cell proliferation in an IL-10-independent manner, thus amplifying the immunosuppressive effect generated by MSC. In summary, the immune plasticity of IFN-γ and TNF-α licensed veto function of MSC vary among donors and defines a central role to inducible IDO activity and its bystander effect on lymphomyeloid immune effectors.

Human mesenchymal stromal cells (MSC) can suppress T-cell activation in vitro in an indoleamine 2,3-dioxygenase (IDO)-dependent manner. However, their clinical effects on immune ailments have been inconsistent, with a recent phase III study showing no benefit in acute graft-versus-host disease (GvHD). We here tested the hypothesis that the banked, cryopreserved MSC often used in clinical trials display biologic properties distinct from that of MSC in the log phase of growth typically examined in pre-clinical studies. In freshly thawed cryopreserved MSC derived from normal human volunteers, we observed that MSC up-regulate heat-shock proteins, are refractory to interferon (IFN)-γ-induced up-regulation of IDO, and are compromised in suppressing CD3/CD28-driven T cell proliferation. Immune suppressor activity, IFN-γ responsiveness and induction of IDO were fully restored following 24 h of MSC tissue culture post-thaw. These results highlight a possible cause for the inefficacy of MSC-based immunotherapy reported in clinical trials using cryopreserved MSC thawed immediately prior to infusion.

Bone marrow-derived mesenchymal stromal cells (MSC) possess an immune plasticity manifested by either an immunosuppressive or, when activated with IFN-γ, an APC phenotype. Herein, TLR expression by MSC and their immune regulatory role were investigated. We observed that human MSC and macrophages expressed TLR3 and TLR4 at comparable levels and TLR-mediated activation of MSC resulted in the production of inflammatory mediators such as IL-1β, IL-6, IL-8/CXCL8, and CCL5. IFN-α or IFN-γ priming up-regulated production of these inflammatory mediators and expression of IFNB, inducible NO synthase (iNOS), and TRAIL upon TLR activation in MSC and macrophages, but failed to induce IL-12 and TNF-α production in MSC. Nonetheless, TLR activation in MSC resulted in the formation of an inflammatory site attracting innate immune cells, as evaluated by human neutrophil chemotaxis assays and by the analysis of immune effectors retrieved from Matrigel-embedded MSC injected into mice after in vitro preactivation with cytokines and/or TLR ligands. Hence, TLR-activated MSC are capable of recruiting immune inflammatory cells. In addition, IFN priming combined with TLR activation may increase immune responses induced by Ag-presenting MSC through presentation of Ag in an inflammatory context, a mechanism that could be applied in a cell-based vaccine.