Identification, Expression and Antimicrobial Functional Analysis of Interleukin-8 (IL-8) in Response to <em>Streptococcus iniae</em> and Flavobacterium Covae in Asian Seabass (Lates calcarifer Bloch, 1790)

Abstract

Using qRT-PCR analysis, the LcIL-8 expression level indicated broad expression in most detected tissues, with the most predominant expression in whole blood at 6 to 24 h after infection with S. iniae at concentrations of 105 colony-forming unit (CFU)/fish (p &amp;amp;lt; 0.05). After immersion with F. covae, the LcIL-8 transcript was upregulated in the gills, liver and intestine, with the highest expression level found in the gills. However, LcIL-8 was downregulated in all 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. MIC analysis of rLcIL-8 protein against S. iniae was 10.42 ± 3.61 µg/mL. Furthermore, the functional analyses clearly demonstrated that 10 and 100 µg of rLcIL-8 protein efficiently enhanced the phagocytic activity of Asian seabass phagocytes in vitro (p &amp;amp;lt; 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 the tested pathogen (p &amp;amp;lt; 0.001). These results indicate that rLcIL-8 plays a crucial role in the defense mechanisms against bacterial infections in the target fish.