The Clathrin-Mediated Endocytic Pathway Participates in dsRNA-Induced IFN-β Production

TLR3 and cytoplasmic RIG-I-like receptor (RLR) recognize virus-derived dsRNA and induce type I IFN production in a distinct manner. Human TLR3 localizes to the endosomal compartments in myeloid dendritic cells (mDCs), while it localizes to both the cell surface and interior in fibroblasts and epithelial cells. TLR3 signaling arises in the intracellular compartment in both cell types and requires endosomal maturation. The mechanisms by which extracellular dsRNA is delivered to the TLR3-containing organelle remain largely unknown. Among various synthetic dsRNAs, poly(I:C) is preferentially internalized and activates TLR3 in mDCs. In vitro transcribed dsRNAs hardly induce IFN-beta production in mDCs. In this study, we demonstrate that the clathrin-dependent endocytic pathway mediates cell entry of poly(I:C) to induce IFN-beta gene transcription. Furthermore, poly(I:C)-induced IFN-beta production is inhibited by pretreatment of cells with B- and C-type oligodeoxynucleotides (ODNs) but not with TLR7/8 ligands. The binding and internalization of B-type ODNs by mDCs was reduced in the presence of poly(I:C), suggesting that poly(I:C) shares the uptake receptor with B- and C-type ODNs. Hence, foreign dsRNA is recognized by differently categorized receptors, cytoplasmic RIG-I-like receptor, membrane-bound TLR3 and cell-surface RNA capture. The endocytic pathway is critical for dsRNA-induced TLR3-mediated cell activation.