Interferon Regulatory Factors (IRF1, IRF4, IRF5, IRF7 and IRF9) in Sichuan taimen (Hucho bleekeri): Identification and Functional Characterization

<b>Background/Objectives</b>: Interferon regulatory factors (IRFs) are multifunctional transcription factors that play important roles in the transcriptional regulation of interferons and in the immune response to pathogens. Therefore, studying the interferon system in fish is highly relevant in the prevention and treatment of viral diseases. <b>Methods</b>: In this study, five IRF genes (<i>IRF1</i>, <i>IRF4</i>, <i>IRF5</i>, <i>IRF7</i> and <i>IRF9</i>) were identified and characterized in <i>Hucho bleekeri</i>, and their expression profiles were determined after LPS and Poly(I:C) treatment. <b>Results</b>: These IRFs have typical DNA-binding domains and IRF-association domains. Amino acid sequence comparison revealed high homology between these IRFs and those of other vertebrates, with the highest homology being with other salmonid fish. Phylogenetic analysis revealed that these IRFs are divided into four subfamilies (IRF1, IRF3, IRF4 and IRF5), with both IRF4 and IRF9 belonging to the IRF4 subfamily. IRF genes were widely expressed in all of the tested tissues, with <i>IRF1</i>, <i>IRF4</i> and <i>IRF9</i> being highly expressed in the spleen and kidney and <i>IRF5</i> and <i>IRF7</i> highly expressed in the gonads. <i>IRF1</i>, <i>IRF4</i> and <i>IRF5</i> expression was induced at different time points post-LPS challenge. <i>IRF7</i> and <i>IRF9</i> expression in the spleen and head kidney was not significantly altered by LPS induction. Poly(I:C) treatment altered <i>IRF</i> expression more significantly than LPS treatment. Poly(I:C) significantly altered the spleen and head kidney expression of all five <i>IRFs</i>. <b>Conclusions</b>: These findings reveal the potential role of IRFs in the antiviral response of <i>H. bleekeri</i> and provide a reference for examining signal transduction pathways in the interferon system in fish.