Cytokine Modulation of TLR Expression and Activation in Mesenchymal Stromal Cells Leads to a Proinflammatory Phenotype

Bone marrow-derived mesenchymal stromal cells (MSC) possess an immune plasticity manifested by either an immunosuppressive or, when activated with IFN-gamma, 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-1beta, IL-6, IL-8/CXCL8, and CCL5. IFN-alpha or IFN-gamma 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-alpha 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.