Activation of Toll-like receptor 3 amplifies mesenchymal stem cell trophic factors and enhances therapeutic potency

Clinical trials of bone marrow mesenchymal stem cell (MSC) therapy have thus far demonstrated moderate and inconsistent benefits, indicating an urgent need to improve therapeutic efficacy. Although administration of sufficient cells is necessary to achieve maximal therapeutic benefits, documented MSC clinical trials have largely relied on injections of ∼1 × 106 cells/kg, which appears too low to elicit a robust therapeutic response according to published preclinical studies. However, repeated cell passaging necessary for large-scale expansion of MSC causes cellular senescence and reduces stem cell potency. Using the RNA mimetic polyinosinic-polycytidylic acid [poly(I:C)] to engage MSC Toll-like receptor 3 (TLR3), we found that poly(I:C), signaling through multiple mitogen-activated protein kinase pathways, induced therapeutically relevant trophic factors such as interleukin-6-type cytokines, stromal-derived factor 1, hepatocyte growth factor, and vascular endothelial growth factor while slightly inhibiting the proliferation and migration potentials of MSC. At the suboptimal injection dose of 1 × 106 cells/kg, poly(I:C)-treated MSC, but not untreated MSC, effectively stimulated regeneration of the failing hamster heart 1 mo after cell administration. The regenerating heart exhibited increased CD34+/Ki67+ and CD34+/GATA4+ progenitor cells in the presence of decreased inflammatory cells and cytokines. Cardiac functional improvement was associated with a ∼50% reduction in fibrosis, a ∼40% reduction in apoptosis, and a ∼55% increase in angiogenesis, culminating in prominent cardiomyogenesis evidenced by abundant distribution of small myocytes and a ∼90% increase in wall thickening. These functional, histological, and molecular characterizations thus establish the utility of TLR3 engagement for enabling the low-dose MSC therapy that may be translated to more efficacious clinical applications.