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Japanese flounder (Paralichthys olivaceus) is one of the most economically important marine fish cultured in north Asia. Vibrio anguillarum is a severe bacterial pathogen to Japanese flounder and many other aquaculture species. In order to understand the immune response of flounder during bacterial infection, we systematically examined the transcriptome profiles of flounder spleen at three time points after V. anguillarum challenge. More than one billion high quality reads were obtained, approximately 80.70% of which were successfully mapped to the reference genome of flounder. A total of 6060, 4688 and 4235 differentially expressed genes (DEGs) were captured at 6, 12 and 24-h post-infection, respectively. The DEGs exhibited dynamic changes in expression and were assigned into four different profiles based on expression trend. GO and KEGG analysis showed that the DEGs were enriched in various immune-related terms, including response to stimulation, immune system and pathways of cytokine-cytokine receptor interaction, Jak-STAT signaling and Toll-like receptor signaling. Furthermore, a network of highly interactive DEGs involved in 11 immune-related pathways was detected by utilizing the weighted co-expressing network analysis (WGCNA). Accordingly, 26 hub genes were discovered that constituted an elaborate immune regulatory network and functioned mainly in pathogen recognition, antigen processing, and molecular signaling. The results of this study provided the first systematical transcriptome profile of flounder in association with V. anguillarum infection and can serve as a valuable resource of target genes for future studies on the molecular mechanisms underlying the immune defense of flounder against bacterial infection. • Vibrio anguillarum -induced transcriptome profiles of flounder were investigated. • A total of 8923 differentially expressed genes (DEGs) were found at 6, 12 and 24 hpi. • Co-expression networks of immune relevant DEGs were constructed. • Twenty-six hub genes with multiple interactions were identified. • The hub genes represented mainly genes involved in immune defense against pathogens. [ABSTRACT FROM AUTHOR]

Interleukin (IL)-34 is a relatively recently discovered cytokine with pleiotropic effects on various cellular activities, including immune response. In fish, the knowledge on the function of IL-34 is limited. In the present work, we investigated the function of Japanese flounder Paralichthys olivaceus IL-34 (PoIL-34) in association with inflammation and immune defense. PoIL-34 possesses the conserved structure of IL-34 superfamily and shares 21.52% sequence identity with murine IL-34. PoIL-34 expression was detected in a wide range of tissues of flounder, in particular intestine, and was regulated to a significant extent by bacterial infection in a time-dependent fashion. In vitro studies showed that recombinant PoIL-34 (rPoIL-34) bound peripheral blood leukocytes (PBLs) and promoted ROS production, acid phosphatase activity, and cellular resistance against bacterial infection. At the molecular level, rPoIL-34 enhanced the expressions of inflammatory cytokines and specific JAK and STAT genes. Similar stimulatory effects of rPoIL-34 were observed in vivo. When PoIL-34 was overexpressed in flounder, the expressions of pro- and anti-inflammatory mediators were significantly affected in a tissue-dependent manner, which correlated with an augmented ability of the fish to eliminate invading pathogens from tissues. Together, these results indicated that PoIL-34 regulated inflammatory response probably via specific JAK/STAT pathways and had a significant influence on the immune defense of flounder against bacterial infection. • PoIL-34 expression occurred in multi-tissues and responded to bacterial infection. • rPoIL-34 bound to and promoted the activation and bacterial resistance of PBLs. • rPoIL-34 regulated the expressions of inflammatory cytokines and specific JAK/STAT. • In vivo PoIL-34 overexpession affected the expressions of inflammatory cytokines. • PoIL-34 overexpession reduced bacterial dissemination in fish tissues. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are involved in many biological activities including immune defense against pathogens. In this study, we applied high-throughput sequencing technology to examine miRNAs in Japanese flounder (Paralichthys olivaceus) infected with Streptococcus iniae at different times. A total of 1038 miRNAs were identified, of which, 249 were novel miRNAs, and 81 showed differential expression (named DEmiRNAs) after S. iniae infection. Of the 81 DEmiRNAs identified, 34 and 58 occurred at 6 h and 24 h post-infection, respectively; most DEmiRNAs were strongly time-specific, and only 13.6% of the DEmiRNAs were shared between the two time points. A total of 9582 target genes were predicted for the 81 DEmiRNAs. The putative target genes were enriched in various GO and KEGG pathways of biological processes and molecular/cellular functions, in particular endocytosis, regulation of transcription, lysososme, and the signaling pathways of MAPK, ErbB, and AMPK. One of the DEmiRNAs, pol-3p-10740_175, was found to target dual specificity phosphatase 6 (Dusp6) and repress the expression of the latter. Transfection of flounder FG cells with pol-3p-10740_175 caused a significant inhibition on S. iniae invasion. The results of this study provided the first S. iniae -induced miRNA profile in Japanese flounder and indicated that flounder miRNAs play an important role in antibacterial immunity. • Streptococcus iniae infection altered the expression of 81 miRNAs in flounder. • There were 34 and 58 differentially expressed miRNAs at 6 and 24 hpi, respectively. • The putative target genes of the miRNAs were enriched in various GO/KEGG pathways. • One miRNA, pol-3p-10740_175, negatively regulated dual specificity phosphatase 6. • Overexpression of pol-3p-10740_175 significantly inhibited S. iniae invasion. [ABSTRACT FROM AUTHOR]