Selection of Molecular Structure and Delivery of RNA Oligonucleotides to Activate TLR7 versus TLR8 and to Induce High Amounts of IL-12p70 in Primary Human Monocytes

Detection of non-self RNA by TLRs within endosomes and by retinoic acid-inducible gene I (RIG-I)-like helicases in the cytosol is central to mammalian antiviral immunity. In this study, we used pathway-specific agonists and targeted delivery to address RNA immunorecognition in primary human immune cells. Within PBMC, plasmacytoid dendritic cells (pDC) and monocytes were found to be responsible for IFN-alpha production upon immunorecognition of RNA. The mechanisms of RNA recognition in pDC and monocytes were distinct. In pDC, recognition of ssRNA and dsRNA oligonucleotides was TLR7-dependent, whereas a 5' triphosphate moiety (RIG-I ligand activity) had no major contribution to IFN-alpha production. In monocytes, the response to RNA oligonucleotides was mediated by either TLR8 or RIG-I. TLR8 was responsible for IL-12 induction upon endosomal delivery of ssRNA oligonucleotides and RIG-I was responsible for IFN-alpha production upon delivery of 5' triphosphate RNA into the cytosol. In conclusion, the dissection of these pathways by selecting the appropriate structure and delivery of RNA reveals pDC as major producer of IFN-alpha upon TLR-mediated stimulation and monocytes as major producer of IFN-alpha upon RIG-I-mediated stimulation. Furthermore, our results uncover the potential of monocytes to function as major producers of IL-12p70, a key Th1 cytokine classically ascribed to myeloid dendritic cells that cannot be induced by CpG oligonucleotides in the human system.