The virus-induced cyclic dinucleotide 2'3'-c-di-GMP mediates STING-dependent antiviral immunity in Drosophila.

In mammals, the enzyme cGAS senses the presence of cytosolic DNA and synthesizes the cyclic dinucleotide (CDN) 2'3'-cGAMP, which triggers STING-dependent immunity. In Drosophila melanogaster, two cGAS-like receptors (cGLRs) produce 3'2'-cGAMP and 2'3'-cGAMP to activate STING. We explored CDN-mediated immunity in 14 Drosophila species covering 50 million years of evolution and found that 2'3'-cGAMP and 3'2'-cGAMP failed to control infection by Drosophila C virus in D. serrata and two other species. We discovered diverse CDNs produced in a cGLR-dependent manner in response to viral infection in D. melanogaster, including 2'3'-c-di-GMP. This CDN was a more potent STING agonist than cGAMP in D. melanogaster and it also activated a strong antiviral transcriptional response in D. serrata. Our results shed light on the evolution of cGLRs in flies and provide a basis for understanding the function and regulation of this emerging family of pattern recognition receptors in animal innate immunity.