A tandem-repeat galectin-9 involved in immune response of yellow catfish, Pelteobagrus fulvidraco, against Aeromonas hydrophila

• PfGAL9 was a tandem-repeat galectin-9 of yellow catfish.
• PfGAL9 was abundant in tested immune tissues.
• PfGAL9 mRNA was sharply increased in liver and head kidney after inactivated Aeromonas hydrophila challenge.
• PfGAL9 agglutinated all checked bacteria including G+ and G−.
• PfGAL9 bond all examined bacteria with specificity.

Galectins exclusively recognize and bind β-galactoside on cell surface by carbohydrate recognition domain (CRD). In spite of extensive study of mammalian galectin importance in immune system, little is known about that of fish. To study the immune response of yellow catfish to pathogens, a tandem-repeat galectin-9 from yellow catfish was identified and named PfGAL9. Its full-length cDNA was 1314 bp, including a 117 bp of 5′ untranslated region (UTR), a 951 bp of open reading frame (ORF), and a 246 bp of 3′ UTR. The ORF encoded 316 amino acids (35.12 KDa), shared the highest 78% identity with the predicted galectin-9 of Ictalurus punctatus. This protein possessed two distinct CRDs with two highly conserved sugar binding motifs. Quantitative PCR showed that PfGAL9 was lowly expressed in skin, gill, fin, muscle, heart, and intestine, highly expressed in tested immune tissues (head kidney, trunk kidney, liver, spleen, and blood) in normal body. After inactivated Aeromonas hydrophila challenge, PfGAL9 was remarkably increased in head kidney and liver in a time-dependent manner. The recombinant protein was expressed in Escherichia coli, which not only agglutinated but also bond all examined bacteria. The binding activities are consistent with the size of aggregates formed by agglutinated bacteria. The agglutination must depend on its direct interaction with bacteria. These results suggested that PfGAL9 was involved in the innate immune response against bacterial infection and clearance of pathogens in yellow catfish.