Sensing and control of bluetongue virus infection in epithelial cells via RIG-I and MDA5 helicases

Bluetongue virus (BTV), an arthropod-borne member of the Reoviridae family, is a double-stranded RNA virus that causes an economically important livestock disease that has spread across Europe in recent decades. Production of type I interferon (alpha/beta interferon [IFN-α/β]) has been reported in vivo and in vitro upon BTV infection. However, the cellular sensors and signaling pathways involved in this process remain unknown. Here we studied the mechanisms responsible for the production of IFN-β in response to BTV serotype 8. Upon BTV infection of A549 cells, expression of IFN-β and other proinflammatory cytokines was strongly induced at both the protein and mRNA levels. This response appeared to be dependent on virus replication, since exposure to UV-inactivated virus failed to induce IFN-β. We also demonstrated that BTV infection activated the transcription factors IFN regulatory factor 3 and nuclear factor κB. We investigated the role of several pattern recognition receptors in this response and showed that expression of IFN-β was greatly reduced after small-interfering-RNA-mediated knockdown of the RNA helicase encoded by retinoic acid-inducible gene I (RIG-I) or melanoma differentiation-associated gene 5 (MDA5). In contrast, silencing of MyD88, Toll-like receptor 3, or the recently described DexD/H-box helicase DDX1 sensor had no or a weak effect on IFN-β induction, suggesting that the RIG-I-like receptor pathway is specifically engaged for BTV sensing. Moreover, we also showed that overexpression of either RIG-I or MDA5 impaired BTV expression in infected A549 cells. Overall, this indicates that RIG-I and MDA5 can both contribute to the recognition and control of BTV infection.