Identification of galectin-9 and its antibacterial function in Yellow River carp ().

Introduction: Galectin-9 is a β-galactoside-binding lectin that functions as a critical pattern recognition receptor (PRR) in the host immune system, initiating immune defense responses by recognizing and binding to pathogen-associated molecular patterns (PAMPs) on the surface of microorganisms. In this study, we identified and characterized a novel galectin-9 cDNA, designated CcGal-9, from Yellow River carp ().

Methods: The full-length CcGal-9 cDNA was cloned and sequenced, and its structural features were analyzed. Tissue distribution of CcGal-9 mRNA was examined by quantitative real-time PCR. Expression changes following and infections were evaluated. Recombinant CcGal-9 (rCcGal-9) was expressed in BL21 (DE3), purified, and assessed for binding to various PAMPs and microorganisms. Agglutination assays and survival experiments were conducted to determine functional roles in immune defense.

Results: The CcGal-9 cDNA is 963 bp in length and encodes a 320-amino acid protein with two distinct carbohydrate recognition domains (CRDs), characteristic of tandem-repeat type galectins. C mRNA was predominantly expressed in the spleen, testicle, and head kidney, with lower levels in the liver and intestine. Upon bacterial infection, CcGal-9 expression was significantly upregulated in multiple immune-related tissues. Purified rCcGal-9 bound LPS, PGN, mannan, and both Gram-positive and Gram-negative bacteria, and exhibited broad-spectrum agglutination activity. Administration of rCcGal-9 markedly improved the survival rate of carp challenged with A. .

Discussion: These findings indicate that CcGal-9 is an important PRR in C. , contributing to immune defense against pathogenic microorganisms through PAMP recognition and microbial agglutination. This study enhances our understanding of galectin-mediated immunity in teleost fish.