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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]

Bmal1 is the key component of the central molecular clock that mediates the circadian control of various biological activities including immune reactions. In fish, the role of Bmal1 in immunity is unclear. In this study, we examined the expressional regulation and potential role of Japanese flounder Bmal1 (named PoBmal1) in pathogen associated immune response. PoBmal1 shares 85% identity with its human counterpart and possesses the conserved helix-loop-helix domain and PAS domain. PoBmal1 transcript was distributed extensively in nine tissues and altered to significant extents during the infection of two bacterial pathogens. PoBmal1 , as well as IL-1β, IL-6, IL-8, and TNF-α, exhibited oscillating expression through the light-dark cycle in a tissue-dependent manner, and the expression patterns of the cytokines were largely similar to that of PoBmal1 , both being higher during the photophase. Knockdown of PoBmal1 reduced the expressions of two clock genes (Cry1 and Cry2) and IL-1β, IL-6, IL-8, and TNF-α in a time-dependent manner. During bacterial infection, fish with PoBmal1 knockdown exhibited significantly increased expression of inflammatory cytokines in a tissue- and time-dependent fashion, which largely correlated with significantly decreased bacterial dissemination in tissues. Taken together, our study revealed that flounder Bmal1 exerted a regulatory effect on inflammation response in a manner that was situation-dependent, and that Bmal1 had a significant impact on the outcome of bacterial infection. These results provided new insights into the immune function of fish Bmal1. • PoBmal1 expression occurred in 9 tissues and was regulated by bacterial infection. • PoBmal1 and inflammatory cytokine expressions oscillated similarly with light input. • PoBmal1 knockdown impaired the expressions of Cry and inflammatory cytokines. • PoBmal1 knockdown enhanced inflammatory cytokine expression during bacterial infection. • PoBmal1 knockdown reduced bacterial dissemination in fish tissues. [ABSTRACT FROM AUTHOR]