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

Xiaokai Hao,Jing Xing,Heng Chi,Xiuzhen Sheng,Xiaoqian Tang,Wenbin Zhan

doi:10.3390/biology10121247

Abstract

Simple SummaryThe study identified a new spliced isoform of anionic β-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 β-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 β-defensins in organisms, which need to be further studied. In this study, a new spliced isoform of anionic β-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.

Highlights

The immune system of vertebrates is divided into two parts: innate immunity and adaptive immunity
FBD was highly similar to defensins from Nile tilapia, grass carp and channel catfish, and it belongs to the group of β-defensins
RfBD significantly increased phagocytic activity of leukocytes from head kidney and spleen. These results indicated that defensins can act as a bridge between innate and adaptive immunity by chemotaxizing leukocytes and modulating phagocytosis

Summary

Introduction

The immune system of vertebrates is divided into two parts: innate immunity and adaptive immunity. As the first line of defense of the immune system, innate immunity fulfills powerful functions and plays an important role. For lower vertebrates such as bony fish, the innate immune system is important because the formation of an adaptive immune response takes a long time and is affected by the ambient temperature [1]. Cysteine-rich cationic host defense peptides, divided into α-, β- and θ-defensins in vertebrates [4]. Θ-defensin, which contains three pairs of disulfide bonds, is a cyclic peptide that has only been identified in non-human primates, such as rhesus monkeys [6]. Zebrafish β-defensin was first identified, and multiple defensin-like genes were subsequently reported in many other species [8,9,10,11,12,13,14]

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