Transferrin from obscure puffer Takifugu obscurus promotes antimicrobial immune response

Transferrin (TF) is an iron-binding glycoprotein that plays a crucial role in host defense by restricting iron availability to pathogens and activating antimicrobial immune responses. However, its antimicrobial role in fish remains largely unexplored. In this study, we identified and characterized transferrin from obscure puffer Takifugu obscurus, named ToTF, and investigated its functions in bacterial infections. The coding region of the ToTF gene consists of 2295 bp, encoding a polypeptide of 764 amino acids. Tissue distribution analysis revealed that ToTF is primarily expressed in the liver. The mRNA expression of ToTF was significantly upregulated in both the liver and kidney following challenge with Staphylococcus aureus and Vibrio harveyi. Moreover, recombinant ToTF (rToTF) markedly inhibits iron-dependent bacteria growth, including S. aureus and V. harveyi. The iron-binding capacity of rToTF peaked at pH 8 and decreased significantly under acidic conditions. Furthermore, the antibacterial activity of rToTF peaked at near-neutral pH but decreased significantly at pH below 5. This dual mechanism deprives bacteria of essential iron through high-affinity binding and directly inhibits their growth under optimal conditions. These findings highlight the dual role of ToTF in the antibacterial immunity of T. obscurus, mediated by iron sequestration and direct bacteriostatic effects. This study supports the development of TF-based antibacterial agents and enhances strategies for aquatic disease prevention and fish disease resistance.