The Functions of β-Defensin in Flounder (Paralichthys olivaceus): Antibiosis, Chemotaxis and Modulation of Phagocytosis

Simple Summary: The study identified a new spliced isoform of anionic beta-defensin from flounder (Paralichthys olivaceus, fBD) and examined its antibiosis, chemotaxis and modulation of phagocytosis. It also analyzed the contributions of fBD to the antimicrobial activity of extracellular traps (ETs). The analyses found that an anionic beta-defensin in fish possesses strong bacteriostatic ability in line with that of cationic defensins and also plays an important role in immune response. This study provides new insights into the biological function of anionic defensins, which can serve as one of the important effectors in extracellular traps and contribute to the immune response. Most defensins are cationic antimicrobial peptides with broad-spectrum killing activity against bacteria, fungi and enveloped viruses. However, it should be recognized that there are some non-cationic beta-defensins in organisms, which need to be further studied. In this study, a new spliced isoform of anionic beta-defensin from flounder (Paralichthys olivaceus, fBD) was identified, and its antibiosis, chemotaxis and modulation of phagocytosis were examined. In addition, the contributions of fBD to the antimicrobial activity of extracellular traps (ETs) were also analyzed. The recombinant fBD (rfBD) could effectively inhibit the growth of Gram-positive bacteria (S. aureus, Micrococcus luteus) and Gram-negative bacteria (E. coli, V. alginolyticus, V. anguillarum). An indirect immunofluorescence assay showed that the fBD was co-localized in the extracellular traps released by the leukocytes. When the ETs were blocked with antibodies against rfBD, the proliferation of S. aureus and E. coli incubated with ETs tended to increase compared with that in the control group. In addition, the results obtained by flow cytometry showed that the rfBD could significantly chemoattract leukocytes and increase phagocytic activity in vitro. In conclusion, this study provides new insights into the biological function of anionic defensins, which can serve as one of the important effectors in extracellular traps and as a bridge between innate and adaptive immunity in teleosts.

Automated summary

The study identified a new spliced isoform of anionic beta-defensin from flounder and examined its antibiosis, chemotaxis, and modulation of phagocytosis. It also analyzed the contributions of this defensin to the antimicrobial activity of extracellular traps. The findings suggest that anionic beta-defensins play a crucial role in immunity and can be important effectors in extracellular traps.