Raftlin Is Involved in the Nucleocapture Complex to Induce Poly(I:C)-mediated TLR3 Activation

The double-stranded RNA analog, poly(I: C), extracellularly activates both the endosomal Toll-like receptor (TLR) 3 and the cytoplasmic RNA helicase, melanoma differentiation-associated gene 5, leading to the production of type I interferons (IFNs) and inflammatory cytokines. The mechanism by which extracellular poly(I: C) is delivered to TLR3-positive organelles and the cytoplasm remains to be elucidated. Here, we show that the cytoplasmic lipid raft protein, Raftlin, is essential for poly(I: C) cellular uptake in human myeloid dendritic cells and epithelial cells. When Raftlin was silenced, poly(I: C) failed to enter cells and induction of IFN-beta production was inhibited. In addition, cellular uptake of B-type oligodeoxynucleotide that shares its uptake receptor with poly(I: C) was suppressed in Raftlin knockdown cells. Upon poly(I: C) stimulation, Raftlin was translocated from the cytoplasm to the plasma membrane where it colocalized with poly(I: C), and thereafter moved to TLR3-positive endosomes. Thus, Raftlin cooperates with the uptake receptor to mediate cell entry of poly(I: C), which is critical for activation of TLR3.