IRF-mediated regulation of IFN antiviral response in fish

Members of the interferon regulatory factor (IRF) family are essential transcription factors that regulate the expression of IFN genes individually and collectively in viral-infected cells. IRF family proteins are structurally conserved. The N-terminal DBD domain endows IRF proteins with the binding affinity to IFN gene promoter DNAs, while the C-terminal IAD domains mediate protein-protein interaction, e. g. interaction with other IRFs or transcription factors from other families, leading to functional specificity and diversity of IRF proteins. That is, when a given cell type is infected with a given type of virus, different signaling pathways will be triggered, where different IRF members are activated to turn on the transcription of different IFN genes. Mammalian IRF family includes 9 members, IRF1-9, and a total of 11 members are identified in fish IRF family. In addition to IRF1~9, fish genomes possess teleost-specific IRF11, and IRF10, which also exists in bird genomes. As a transcription factor, IRF proteins have to enter cellular nucleus first, followed by initiation of IFN gene transcription. In mammals, accumulated evidence has shown that IRF1/3/5/7/9, although localized to cytoplasm or nucleus in resting cells, positively regulate the transcription of IFN genes upon virus infection, but IRF2 exerts a negative role. In the past two decades, significant progresses have been made on molecular cloning, expression characterization and function analyses of fish IRF family genes, although these conclusions are often drawn from different fish species or different experiment systems. It is noted that only in vitro data have been acquired, and future work needs to focus on in vivo studies regarding the physiological function of fish IRF members. This review summarizes the recent mechanism research progresses on expression characterization, subcellular localization and molecular regulation of fish IRF family genes during IFN antiviral immune response.