Innate immunity induced by composition-dependent RIG-I recognition of hepatitis C virus RNA

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Abstract
Innate immunity is an important defence against infection by viruses, triggered by host recognition of 'PAMPS', or pathogen-associated molecular patterns. Saito et al. have now identified a conserved poly-uridine motif in the 3´ non-transcribed region of the hepatitis C virus genome as the relevant PAMP for detection by the RNA helicase RIG-I, a protein previously shown to have an essential function in double-stranded RNA-induced innate antiviral responses. Innate immune defences are essential for the control of virus infection and are triggered through host recognition of viral macromolecular motifs known as pathogen-associated molecular patterns (PAMPs)1. Hepatitis C virus (HCV) is an RNA virus that replicates in the liver, and infects 200 million people worldwide2. Infection is regulated by hepatic immune defences triggered by the cellular RIG-I helicase. RIG-I binds PAMP RNA and signals interferon regulatory factor 3 activation to induce the expression of interferon-α/β and antiviral/interferon-stimulated genes (ISGs) that limit infection3,4,5,6,7,8,9,10. Here we identify the polyuridine motif of the HCV genome 3′ non-translated region and its replication intermediate as the PAMP substrate of RIG-I, and show that this and similar homopolyuridine or homopolyriboadenine motifs present in the genomes of RNA viruses are the chief feature of RIG-I recognition and immune triggering in human and murine cells8. 5′ terminal triphosphate on the PAMP RNA was necessary but not sufficient for RIG-I binding, which was primarily dependent on homopolymeric ribonucleotide composition, linear structure and length. The HCV PAMP RNA stimulated RIG-I-dependent signalling to induce a hepatic innate immune response in vivo, and triggered interferon and ISG expression to suppress HCV infection in vitro. These results provide a conceptual advance by defining specific homopolymeric RNA motifs within the genome of HCV and other RNA viruses as the PAMP substrate of RIG-I, and demonstrate immunogenic features of the PAMP–RIG-I interaction that could be used as an immune adjuvant for vaccine and immunotherapy approaches.