Identification, Expression and Antimicrobial Functional Analysis of Interleukin-8 (IL-8) in Response to Streptococcus iniae and Flavobacterium covae in Asian Seabass (Lates calcarifer Bloch, 1790).

Simple Summary: As a proinflammatory cytokine, interleukin-8 (IL-8) plays a crucial function in inflammatory responses by recruiting and regulating monocytes and lymphocytes during the early stages of inflammation. In this study, a cDNA encoding the Asian seabass (Lates calcarifer) IL-8 gene was cloned and referred to as LcIL-8. Moreover, the expression levels of LcIL-8 in various tissues of normal and diseased fish were analyzed by means of qRT–PCR. Additionally, the recombinant LcIL-8 protein (rLcIL-8) was overexpressed in the Escherichia coli system, and its biological functions under various conditions were investigated. LcIL-8 transcripts were expressed in all the tested tissues of normal Asian seabass. The constitutive mRNA expression of LcIL-8 was quantified in six tissues of fish infected with virulent Streptococcus iniae and Flavobacterium covae at three different concentrations. The minimum inhibitory concentration (MIC) and therapeutic effects of the rLcIL-8 protein against S. iniae were also thoroughly investigated. The obtained findings could be used to further develop prophylactic or therapeutic strategies applicable to the Asian seabass farming industry. In this research, the proinflammatory cytokine interleukin-8 (IL-8) was shown to play a key role in inflammatory responses in fish. This study involved the cloning of the gene that encodes IL-8 in Asian seabass (Lates calcarifer) as well as analyses of its expression and function in this fish. The expression levels of LcIL-8 indicated that it was broadly expressed in most analyzed tissues, with the most predominant expression in the whole blood 6 to 24 h after infection with S. iniae at concentrations of 105 colony-forming units (CFU)/fish (p < 0.05). After fish were immersed in F. covae, the LcIL-8 transcript was upregulated in the gills, liver and intestine, and the highest expression level was observed in the gills. However, LcIL-8 was downregulated in all the tested tissues at 48 and 96 h after infection with the two pathogenic strains, indicating that Lc-IL8 has a short half-life during the early immune responses to pathogens. Moreover, the MIC of the rLcIL-8 protein against S. iniae was 10.42 ± 3.61 µg/mL. Furthermore, functional analyses clearly demonstrated that 10 and 100 µg of the rLcIL-8 protein efficiently enhanced the phagocytic activity of Asian seabass phagocytes in vitro (p < 0.05). Additionally, in vivo injection of S. iniae following the rLcIL-8 protein indicated that 50 and 100 µg of rLc-IL-8 were highly effective in protecting fish from this pathogen (p < 0.001). The obtained results demonstrate that rLcIL-8 possesses a biological function in the defense against bacterial infections in Asian seabass. [ABSTRACT FROM AUTHOR]