Avian interferon regulatory factor (IRF) family reunion: IRF3 and IRF9 found
Interferon regulatory factors (IRFs) are a family of transcription factors in jawed vertebrates with important functions in immunity and many other key cellular processes. The genomes of most vertebrates encode ten IRF genes (IRF1 to IRF10). IRF3 and IRF9 have key roles in the interferon (IFN) induction and signaling. Most of our knowledge about the IFN pathways originates from the study of the mammalian IFN system, and the description of the corresponding avian components is not as complete. Both IRF3 and IRF9 were considered missing from the chicken genome and also from the genomes of all other avian species. Here we describe multiple avian IRF3 and IRF9 genes, all with difficult GC-rich sequence context which prevented their earlier characterization. IRF3 orthologs are more narrowly distributed and are present in the avian infraclass Palaeognathae residing in a syntenic genomic locus shared with other vertebrates. In contrast, IRF9 orthologs were found in most avian species with the notable exception of the order Galliformes. In about half of the avian orders analyzed, IRF9 was located in noncanonical chromosomal positions indicating past evolutionary translocations. Importantly, phylogenetic analysis confirmed the correct orthology of all newly described avian IRFs. We performed a series of experiments using duck (Anas platyrhynchos) IRF9, confirming its key role in the IFN signaling pathway. Knockout of IRF9 in duck embryonal fibroblasts decreases the induction of IFN-stimulated genes (ISGs). Full induction of ISGs in duck cells requires both intact IRF9 and canonical IFN-stimulated response element (ISRE). Lastly, intact IRF9 is needed for IFN-mediated protection of duck cells against vesicular stomatitis virus (VSV)-induced cytopathic effect. The identification of avian IRFs fills an important gap in our understanding of avian immunology and brings new questions related to the evolution of the IRF family.