Retinoic acid-inducible gene I mediates early antiviral response and Toll-like receptor 3 expression in respiratory syncytial virus-infected airway epithelial cells.

Respiratory syncytial virus (RSV) is one of the most common viral pathogens causing severe lower respiratory tract infections in infants and young children. Infected host cells detect and respond to RNA viruses using different mechanisms in a cell-type-specific manner, including retinoic acid-inducible gene I (RIG-I)-dependent and Toll-like receptor (TLR)-dependent pathways. Because the relative contributions of these two pathways in the recognition of RSV infection are unknown, we examined their roles in this study. We found that RIG-I helicase binds RSV transcripts within 12 h of infection. Short interfering RNA (siRNA)-mediated RIG-I "knockdown" significantly inhibited early nuclear factor-kappaB (NF-kappaB) and interferon response factor 3 (IRF3) activation 9 h postinfection (p.i.). Consistent with this finding, RSV-induced beta interferon (IFN-beta), interferon-inducible protein 10 (IP-10), chemokine ligand 5 (CCL-5), and IFN-stimulated gene 15 (ISG15) expression levels were decreased in RIG-I-silenced cells during the early phase of infection but not at later times (18 h p.i.). In contrast, siRNA-mediated TLR3 knockdown did not affect RSV-induced NF-kappaB binding but did inhibit IFN-beta, IP-10, CCL-5, and ISG15 expression at late times of infection. Further studies revealed that TLR3 knockdown significantly reduced NF-kappaB/RelA transcription by its ability to block the activating phosphorylation of NF-kappaB/RelA at serine residue 276. We further found that TLR3 induction following RSV infection was regulated by RIG-I-dependent IFN-beta secreted from infected airway epithelial cells and was mediated by both IFN response-stimulated element (ISRE) and signal transducer and activator of transcription (STAT) sites in its proximal promoter. Together these findings indicate distinct temporal roles of RIG-I and TLR3 in mediating RSV-induced innate immune responses, which are coupled to distinct pathways controlling NF-kappaB activation.