Antiviral immunity in Drosophila involves RNA interference and poorly characterized inducible responses. Here, we showed that two components of the IMD pathway, the kinase dIKKβ and the transcription factor Relish, were required to control infection by two picorna-like viruses. We identified a set of genes induced by viral infection and regulated by dIKKβ and Relish, which included an ortholog of STING. We showed that dSTING participated in the control of infection by picorna-like viruses, acting upstream of dIKKβ to regulate expression of Nazo, an antiviral factor. Our data reveal an antiviral function for STING in an animal model devoid of interferons and suggest an evolutionarily ancient role for this molecule in antiviral immunity.

Cyclic GMP–AMP synthase (cGAS) is a cytosolic DNA sensor that produces the second messenger cG[2′–5′]pA[3′–5′]p (2′3′-cGAMP) and controls activation of innate immunity in mammalian cells1–5. Animal genomes typically encode multiple proteins with predicted homology to cGAS6–10, but the function of these uncharacterized enzymes is unknown. Here we show that cGAS-like receptors (cGLRs) are innate immune sensors that are capable of recognizing divergent molecular patterns and catalysing synthesis of distinct nucleotide second messenger signals. Crystal structures of human and insect cGLRs reveal a nucleotidyltransferase signalling core shared with cGAS and a diversified primary ligand-binding surface modified with notable insertions and deletions. We demonstrate that surface remodelling of cGLRs enables altered ligand specificity and used a forward biochemical screen to identify cGLR1 as a double-stranded RNA sensor in the model organism Drosophila melanogaster. We show that RNA recognition activates Drosophila cGLR1 to synthesize the novel product cG[3′–5′]pA[2′–5′]p (3′2′-cGAMP). A crystal structure of Drosophila stimulator of interferon genes (dSTING) in complex with 3′2′-cGAMP explains selective isomer recognition, and 3′2′-cGAMP induces an enhanced antiviral state in vivo that protects from viral infection. Similar to radiation of Toll-like receptors in pathogen immunity, our results establish cGLRs as a diverse family of metazoan pattern recognition receptors.

No abstract

We previously reported that an ortholog of STING regulates infection by picorna-like viruses in Drosophila. In mammals, STING is activated by the cyclic dinucleotide 2′3′-cGAMP produced by cGAS, which acts as a receptor for cytosolic DNA. Here, we showed that injection of flies with 2′3′-cGAMP induced the expression of dSTING-regulated genes. Coinjection of 2′3′-cGAMP with a panel of RNA or DNA viruses resulted in substantially reduced viral replication. This 2′3′-cGAMP–mediated protection was still observed in flies with mutations in Atg7 and AGO2, genes that encode key components of the autophagy and small interfering RNA pathways, respectively. By contrast, this protection was abrogated in flies with mutations in the gene encoding the NF-κB transcription factor Relish. Transcriptomic analysis of 2′3′-cGAMP–injected flies revealed a complex response pattern in which genes were rapidly induced, induced after a delay, or induced in a sustained manner. Our results reveal that dSTING regulates an NF-κB–dependent antiviral program that predates the emergence of interferons in vertebrates.