Analysis of immunoglobulin and T cell receptor gene expression in ballan wrasse (Labrus bergylta) revealed an extraordinarily high IgM expression in the gut

Sumaira Bilal,Kai Kristoffer Lie,Alf Seljenes Dalum,Odd Andrè Karlsen,Ivar Hordvik

doi:10.1016/j.fsi.2019.02.007

Abstract

The serum IgM concentration of ballan wrasse is relatively high, estimated to approximately 13 mg/ml in adult wild fish of 800 g. The present study revealed an unusual high abundance of IgM mRNA in the gut of ballan wrasse. Initially, transcripts encoding IgM, IgT, IgD, TCRα, TCRδ and CD3ε were quantified by RT-qPCR in several tissues of wild caught fish (approx. 800 g), indicating an elevated immune activity in hindgut and an extraordinarily high expression of IgM. Subsequently, a new RT-qPCR analysis was performed on the entire intestine, cut into four different segments, of reared fish (32–100 g). The analysis indicated immune activity along the entire intestine, but not as strong as in the hindgut. Furthermore, similar to the larger fish, the relative abundance of IgM transcripts was higher in the hindgut than in kidney and spleen, although the absolute level of IgM was in general higher in the larger fish. The secreted form of IgM was completely dominant in comparison to the membrane bound form of IgM and the other analysed genes. IgM was purified from gut mucus and external mucosal surfaces by magnetic beads coated with protein A. Mucus IgM reacted with rabbit antisera raised against serum IgM and contained subunits of the same size. Regarding the elevated immune activity in the intestine it is tempting to speculate on a possible compensatory strategy in this lineage of stomach-less fish, and that natural antibodies have an important role in the first line defence.

Highlights

Most teleosts have three immunoglobulin (Ig) classes, IgM, IgD and IgT/IgZ, defined by the heavy chains of the molecules μ, δ and τ, respectively
In catfish (Ictalurus punctatus), IgD was proposed to act as a pattern recognition molecule [3] and in the gills of rainbow trout (Oncorhynchus mykiss) it functions in maintaining microbiotal homeostasis [4]
IgT and IgD sequences from Atlantic salmon were used as queries to search for counterparts in an intestinal transcriptome database of ballan wrasse

Summary

Introduction

Most teleosts have three immunoglobulin (Ig) classes, IgM, IgD and IgT/IgZ, defined by the heavy chains of the molecules μ, δ and τ, respectively. The IgT molecules were found as monomers in serum of rainbow trout whereas in gut mucus they were tetramers [2]. In another study of rainbow trout, the IgD response against viral infection was found to be weak [5]. The concentration of total IgM in serum vary to a large extent among teleost species, and according to several other parameters, like age/size, gender, season, environment and vaccination/infection status (Hordvik, 2015). Protein A affinity to IgM varies among teleosts, from almost no binding in for example salmonids to high binding in other species [9,10,11,12]

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Conclusion