Small self-RNA generated by RNase L amplifies antiviral innate immunity

Antiviral innate immunity is initiated in response to RNA molecules that are produced in virus- infected cells(1). These RNAs activate signalling cascades that activate the genes that encode alpha- and beta- interferon ( IFN). Signalling occurs through the interaction of the RNAs with either of two pathogen recognition receptors, retinoic acid- inducible gene- I ( RIG- I, also known as DDX58) and melanoma differentiation associated gene- 5 ( MDA5, also known as IFIH1), which contain amino- terminal caspase activation and recruitment domains ( CARD) and carboxy- terminal DExD/ H Box RNA helicase motifs(2-5). RIG- I and MDA5 interact with another CARD protein, interferon-beta promotor stimulator protein- 1 ( IPS- 1, also known as MAVS, VISA and Cardif), in the mitochondrial membrane, which relays the signal through the transcription factors interferon regulatory factor 3 ( IRF- 3) and nuclear factor ( NF)- kappa B to the IFN-beta gene(6-10). Although the signalling pathway is well understood, the origin of the RNA molecules that initiate these processes is not. Here we show that activation of the antiviral endoribonuclease, RNase L-11, by 2',5'- linked oligoadenylate ( 2-5A)(12) produces small RNA cleavage products from self- RNA that initiate IFN production. Accordingly, mouse embryonic fibroblasts lacking RNase L were resistant to the induction of IFN-beta expression in response to 2- 5A, dsRNA or viral infection. Single-stranded regions of RNA are cleaved 39 of UpUp and UpAp sequences by RNase L during viral infections, resulting in small, often duplex, RNAs13,14. We show that small self- RNAs produced by the action of RNase L on cellular RNA induce IFN-beta expression and that the signalling involves RIG- I, MDA5 and IPS- 1. Mice lacking RNase L produce significantly less IFN-beta during viral infections than infected wild- type mice. Furthermore, activation of RNase L with 2- 5A in vivo induced the expression of IFN-beta in wild- type but not RNase L- deficient mice. Our results indicate that RNase L has an essential role in the innate antiviral immune response that relieves the requirement for direct sensing of non- self RNA.