RNA-sensing pattern-recognition receptors synergize with TLR2 or dectin-1 to trigger high production of IL-12p70 by human dendritic cells

IL-12p70 is crucial for T helper 1 polarization and the generation of type 1 immunity that is required to fight cancer and intracellular pathogens. Therefore, strategies to optimize the production of IL-12p70 by dendritic cells (DCs) may significantly improve the efficacy of vaccines and immunotherapies for cancer. However, the rules governing the production of IL-12p70 remain obscure. Here, we stimulated pattern recognition receptors (PRRs) representing all five families of PRRs, to evaluate their ability to elicit high production of IL-12p70 by human DCs. We used ten well-characterized agonists and stimulated human monocyte-derived DCs in vitro with either single agonists or 26 different combinations. We found that poly(I:C), which engages the RNA-sensing PRRs TLR3 and/or MDA5, was the only agonist that could elicit IL-12p70 production when used alone. Combinations of agonists of cell surface and intracellular PRRs were found to synergize to induce high IL-12p70 production, given that the combination included poly(I:C) or resiquimod, which both are agonists of intracellular, RNA sensing PRRs (TLR3/MDA5 and TLR7/8, respectively). Our data show that production of high IL-12p70 is strictly controlled, which is different from what we observed for IFNβ, whose production could be elicited by several intracellular PRRs. In conclusion, we identified six different combinations of PRR ligands able to induce high IL-12p70 production by human DCs. The identified synergistic PRR ligand combinations may represent strong adjuvant candidates in particular for therapeutic cancer vaccines.