Systemic immune responses do not affect significant immune responses in the skin.

Fish skin plays an important role in defending against pathogens in water, primarily through the secretion of skin mucus containing various immune-related factors. Local immune responses in the skin activate systemic immune responses by inflammatory cytokines. However, it remains unclear whether immune responses in the skin occur after systemic immune responses caused by pathogen invasion into the fish body. This study aimed to clarify the relationship between systemic immune responses and skin responses after intraperitoneal injection of formalin-killed cells (FKC) of Vibrio anguillarum. Although systemic inflammatory responses were observed in the spleen after injection, expression changes in the skin did not show significant differences. In contrast, expression of hemoglobin subunit genes significantly increased in the skin after FKC injection, suggesting that erythrocytes infiltrate extravascularly.

A novel liver-specific immunoglobulin heavy chain-like gene in a cartilaginous fish.

We identified a novel immunoglobulin (Ig) heavy chain-like gene (tsIgH) expressed in the liver of the banded houndshark Triakis scyllium by preliminary transcriptomic analysis. The tsIgH gene showed less than 30% of amino acid identities to Ig genes of the shark. The gene encodes one variable domain (VH) and three conserved domains (CH1-CH3) with a predicted signal peptide. Interestingly, this protein has only one cysteine residue in a linker region between VH and CH1 other than those required for the formation of the immunoglobulin domain. Genome sequencing revealed that each of the domains was encoded by a corresponding single exon, and the exon-intron structures of the homologues are conserved in the other cartilaginous fishes. By RT-qPCR analysis, the transcript of the tsIgH gene was observed only in the liver, while that of the IgM was mainly detected in the epigonal organ, liver, and spleen. The novel Ig-heavy chain-like gene in cartilaginous fish may provide new clues to the evolution of immunoglobulin genes.

Chicken-type lysozyme is a major bacteriolytic enzyme in the blood of the banded houndshark Triakis scyllium.

We examined lysozyme activities in the serum and the leukocyte extracts of the banded houndshark Triakis scyllium. The serum exhibited lytic activity, but not the leukocyte extracts. The lytic substance in the serum was of approximately 14 kDa and the N-terminal amino acid sequence was YVYSK. cDNA cloning identified a C-type lysozyme (TsLysC) gene and two G-type lysozyme (TsLysG) cDNA clones of different lengths. The TsLysC gene encodes 149 amino acids residues, and the sequence derived from the N-terminal amino acid sequencing was displayed at position 17-21. TsLysG, on the other hand, contains two ORFs that are homologous to the N- and C-terminal regions of G-type lysozyme of other fish species. TsLysC mRNA levels were high in the liver. TsLysG mRNA level was significantly lower than TsLysC mRNA in the liver.

Preliminary characterization of pathogen-detection activities of serum antibodies from the banded houndshark Triakis scyllium.

Antibodies of cartilaginous fish are of scientific interest due to their phylogenetic position. In the present study, we developed antiserum against IgM of the banded houndshark, Triakis scyllium, and characterized binding activity of the IgM against fish pathogenic bacteria. Pentameric and monomeric IgM antibodies were separated by gel filtration chromatography using high performance liquid chromatography and SDS-PAGE. Antisera were developed by immunizing rabbits with unfractionated IgM antibodies separated by SDS-PAGE electrophoresis. Shark serum antibodies were found to have binding affinity for Aeromonas hydrophila, Vibrio anguillarum, Edwardsiella tarda, and Pseudomonas plecoglossicida antigens but not Lactococcus garvieae by enzyme-linked immunosorbent assay. We speculate the binding activities of shark antibodies may confer protection against certain bacterial pathogens.