Co-ordinated Role of TLR3, RIG-I and MDA5 in the Innate Response to Rhinovirus in Bronchial Epithelium

Abstract

The relative roles of the endosomal TLR3/7/8 versus the intracellular RNA helicases RIG-I and MDA5 in viral infection is much debated. We investigated the roles of each pattern recognition receptor in rhinovirus infection using primary bronchial epithelial cells. TLR3 was constitutively expressed; however, RIG-I and MDA5 were inducible by 8–12 h following rhinovirus infection. Bronchial epithelial tissue from normal volunteers challenged with rhinovirus in vivo exhibited low levels of RIG-I and MDA5 that were increased at day 4 post infection. Inhibition of TLR3, RIG-I and MDA5 by siRNA reduced innate cytokine mRNA, and increased rhinovirus replication. Inhibition of TLR3 and TRIF using siRNA reduced rhinovirus induced RNA helicases. Furthermore, IFNAR1 deficient mice exhibited RIG-I and MDA5 induction early during RV1B infection in an interferon independent manner. Hence anti-viral defense within bronchial epithelium requires co-ordinated recognition of rhinovirus infection, initially via TLR3/TRIF and later via inducible RNA helicases.

Author Summary

Host-pathogen interactions are mediated by pattern recognition receptors that identify conserved structures of micro-organisms that are distinct from self. During a viral infection, important pattern recognition receptors include the endosomal Toll-like receptors (TLRs), and a second set of cytoplasmic pattern recognition receptors known as the RNA helicases. Many studies have highlighted the importance of TLR3, TLR7/8 and the RNA helicases in providing robust anti-viral immunity via interferon induction and inflammation. Both endosomal TLR and cytoplasmic RNA helicase mediated pathways are believed to exist as separate yet non-redundant entities; however, little thought is given to why both systems exist, and few studies also consider how both pathways together contribute to anti-viral immunity. Using models of rhinovirus infection in primary bronchial epithelial cell culture in vitro and experimental infection in mouse and human models in vivo, we show that the RNA helicases are preferentially induced early in the infection cycle via TLR3 mediated signaling events, and work in a co-ordinated, systematic manner. The results help understand the complex events that determine effective innate immunity to rhinovirus infection and how these processes contribute to virus induced exacerbations of asthma and chronic obstructive pulmonary disease.