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The F-lectin, a fucose-binding protein found from invertebrates to ectothermic vertebrates, is the last lectin family to be discovered. Here, we describe effects of two different types of stressors, bacterial infection and confinement stress, on the modulation of European sea bass Dicentrarchus labrax (L.) F-lectin (DlFBL), a well-characterized serum opsonin, using a specific antibody. The infection of the Vibrio alginolyticus bacterial strain increased the total haemagglutinating activity during the 16-day testing period. The DlFBL value showed an upward regulation on the first, second and last days and underwent a slight downward regulation 4 days post-challenge. In contrast, the effect of confinement and density stress showed a decrease in the plasma concentration of lectin, ranging from 50% to 60% compared with the control. The modulation of DlFBL is in line with the hypothesis that humoral lectins could be involved and recruited in the initial recognition step of the inflammation, which leads to agglutination, and the activation of mechanisms responsible for killing of the pathogens., (© 2014 John Wiley & Sons Ltd.)

The discovery of rhamnose-binding lectins (RBLs) in teleost fish eggs led to the identification of a novel lectin family characterized by a unique sequence motif and a structural fold, and initially proposed to modulate fertilization. Further studies of the RBL tissue localization and gene organization were also suggestive of role(s) in innate immunity. Here we describe the purification, and biochemical and functional characterization of a novel RBL (DlRBL) from sea bass (Dicentrarchus labrax) serum. The purified DlRBL had electrophoretic mobilities corresponding to 24 kDa and 100 kDa under reducing and non-reducing conditions, respectively, suggesting that in plasma the DlRBL is present as a physiological homotetramer. DlRBL subunit transcripts revealed an open reading frame encoding 212 amino acid residues that included two tandemly-arrayed carbohydrate-recognition domains, and an 18-residue signal sequence at the N-terminus. The deduced size of 24.1 kDa for the mature protein was in good agreement with the subunit size of the isolated lectin. Binding activity of DlRBL for rabbit erythrocytes could be inhibited in the presence of rhamnose or galactose, did not require calcium, and was optimal at around 20°C and within the pH 6.5-8.0 range. DlRBL agglutinated Gram positive and Gram negative bacteria, and exposure of formalin-killed Escherichia coli to DlRBL enhanced their phagocytosis by D. labrax peritoneal macrophages relative to the unexposed controls. Taken together, the results suggest that plasma DlRBL may play a role in immune recognition of microbial pathogens and facilitate their clearance by phagocytosis., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

The purification, cloning, sequencing, molecular properties and expression of a fucose-binding lectin from the serum of Dicentrarchus labrax (DlFBL) have been previously reported. We now describe the distribution and expression of DlFBL during fish ontogeny. Immunohistochemistry and in situ hybridization assays were carried out at various developmental stages (from 10 days post-hatching larvae to juveniles). Another fucose-binding lectin, similar to DlFBL in biochemical, immunochemical and agglutinating properties, was extracted and purified from eggs and appeared to be localized in the embryo yolk sack residual. DlFBL was found in columnar and goblet cells of the intestinal epithelium of larvae (from 20 days post-hatching) and juveniles and in parenchymal tissue of juveniles. DlFBL mRNA and protein were detected in the intestinal epithelium and in hepatocytes. An amplification product from degenerate primers indicates that lectin isotypes with DlFBL epitopes are expressed in eggs and embryos. Whether the lectin fraction isolated from eggs and embryos includes DlFBL of maternal origin remains unclear.

Recently described biochemical and structural aspects of fucose-binding lectins from the European eel (Anguilla anguilla) and striped bass (Morone saxatilis) led to the identification of a novel lectin family ("F-type" lectins) characterized by a unique sequence motif and a characteristic structural fold. The F-type fold is shared not only with other members of this lectin family, but also with apparently unrelated proteins ranging from prokaryotes to vertebrates. Here we describe the purification, biochemical and molecular properties, and the opsonic activity of an F-type lectin (DlFBL) isolated from sea bass (Dicentrarchus labrax) serum. DlFBL exhibits two tandemly arranged carbohydrate-recognition domains that display the F-type sequence motif. In situ hybridization and immunohistochemical analysis revealed that DlFBL is specifically expressed and localized in hepatocytes and intestinal cells. Exposure of formalin-killed Escherichia coli to DlFBL enhanced their phagocytosis by D. labrax peritoneal macrophages relative to the unexposed controls, suggesting that DlFBL may function as an opsonin in plasma and intestinal mucus.