A tandem-repeat galectin-4 from Nile tilapia (Oreochromis niloticus) is involved in immune response to bacterial infection via mediating pathogen recognition and opsonization

Highlights

• A tandem-repeat galectin-4 (OnGal-4) was identified and characterized from Nile tilapia, Oreochromis niloticus.

• OnGal-4 was widely distributed in various tissues and was induced by bacterial infection in vivo or in vitro.

• rOnGal-4 had bacterial binding and agglutination activities, as well as enhanced the bactericidal activity and cytokines expressions of Mo/MΦ.

• OnGal-4 overexpression in vivo could increase tilapia survival rate by reducing tissue damage and inflammation.

Abstract

Galectins are the family of carbohydrate-binding proteins that participate in host-pathogen interaction. In this study, a galectin-4 homolog (OnGal-4) from Nile tilapia (Oreochromis niloticus) was characterized. The open reading frame of OnGal-4 was 1194 bp, encoding a peptide of 397 amino including two CRD regions and two carbohydrate recognition sites. OnGal-4 mRNA was expressed in all examined tissues with the highest level in spleen. After Streptococcus agalactiae (S.agalactiae) challenge, the OnGal-4 expression was up-regulated in the spleen, head kidney, brain, and monocytes/macrophages (Mo/MΦ). The in vitro experiments showed that recombinant OnGal-4 (rOnGal-4) protein could bind and agglutinate S.agalactiae and A.hydrophila. Also, rOnGal-4 could induce cytokines expressions and increased bactericidal activity of Mo/MΦ. Further in vivo analysis indicated that OnGal-4 overexpression could protect O.niloticus from S.agalactiae infection through modulating inflammation response. Our study suggested that OnGal-4 could improve immune response against bacterial infection by mediating pathogen recognition and opsonization.

Introduction

Galectins are kinds of proteins that bind specifically to beta-galactoside sugars, which have a variety of functions including modulation of cell-cell interactions, cell-matrix adhesion and signal transduction(Niu et al., 2019c). Galectins are widely expressed in different tissues and most immune cells including dendritic cells, macrophages, mast cells, natural killer cells, and activated B and T cells(Stowell et al., 2008; Vasta, 2009). The secreted galectins from immune cells can oligomerize to form lattice that trigger immune responses via binding to surface polysaccharides of pathogen(Rabinovich et al., 2002, 2007b; Vasta, 2012). To date, fifteen galectins have been discovered in mammals and are subdivided into three groups based on their conserved carbohydrate recognition domain (CRD): “prototype”, “chimera” and “tandem repeat” (Bai et al., 2017; Niu et al., 2019a).

As a tandem-repeat galectin, galectin-4 (Gal-4) possess two CRDs with distinct specificity, which make Gal-4 can serve as a crosslinker and vital regulator in many biological processes(Cao and Guo, 2016; Oda et al., 1993). In mammals, Gal-4 is mainly expressed in intestine and the intestinal Gal-4 is considered as the largest reservoir of macrophages (Platt and Mowat, 2008) that can induces the productions of inflammation factors (Hokama et al., 2004; Hong et al., 2019). Besides, Gal-4 can bind to blood group B-associated antigens expressed on E. coli and lead to its death(Stowell et al., 2010). In experimental colitis models, Gal-4 improves mucosal inflammation and reduces the secretion of pro-inflammatory cytokines (Paclik et al., 2008; Satelli et al., 2011). However, the information of Gal-4 in lower vertebrate is quite limited.

Nile tilapia (Oreochromis niloticus) is one of the most important economically farmed fish species in the world (Sifa et al., 2002). Recently, the outbreak of bacterial disease has resulted in huge loss in tilapia culture(Suanyuk et al., 2005). While the roles of tilapia Gal-4 during bacterial infection remains unclear. In this study, a tandem-repeat galectin-4 (OnGal-4) was obtained from O. niloticus. The bacterial binding and agglutination activity, the regulatory roles on monocytes/macrophages (Mo/MΦ) activity and protective effect against bacterial infection of OnGal-4 were investigated. Our data will expand the knowledge about the mechanism of fish galectins against bacterial infection.