Biodefense function of fish lectin family which bind to pathogenic bacteria and microspodian parasites

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L-Rhamnose-binding lectins (RBLs), named CSL1〜3, SML, and SFL, were isolated and characterized from the eggs of chum salmon, Spanish mackerel, and ayu, respectively. They were composed of two or three tandemly repeated domains, which consisted of about 100 amino acid residues with 30〜55% sequence identity. SML was the first RBL, which was a glycoprotein. The structure of the sugar chain and the assignment of 8 disulfide bonds of SML were determined. On the basis of the sequences, the molecular evolution of the carbohydrate recognition domains (CRD) was discussed phylogenetically in term of their structure-function relationship. RBLs agglutinated Gram-negative and Gram positive bacteria by recognizing the structures of lipopolysaccharide and lipoteichoic acid on their surfaces, respectively. RBLs tightly bound to the microspodian, Glugea plecoglossi, via glycoproteins and glycolipids on the surface in a concentration-dependent manner as shown by fluorescence spectrometer and confocal laser scanning microscope. The binding could not be inhibited by L-rhmnose, indicating the presence of a specific binding site besides CRD. The treatment of the spores with SFL could not prevent the transmission to ayu. RBLs also bound to rainbow trout gonal cultured cells, RTG-2, and stimulated the interleukin-8 gene expression, but not IL-1β gene in the cells. The tissue distribution of RBLs demonstrated that the lectins expressed in the liver was transported to the immune system and oocytes via blood stream. These results suggest that RBLs function as non-self recognition molecules in the innate immunity not only in the eggs but also in the adult fishes.