Ferritin heavy chain mediates antibacterial immunity via TGF-β1 signaling in Oreochromis niloticus

Ferritin serves as both a regulator of iron homeostasis and an active participant in the innate immune response, playing multiple functions in immune defense, inflammation regulation, antioxidation, and immune protection. In this study, ferritin heavy chain (Fth) and medium chain (Ftm) were identified from Nile tilapia ( Oreochromis niloticus ) and characterized at both the expression and antibacterial functional levels. Both OnFth and OnFtm were widely distributed, occurring in all examined tissues. Notably, the recombinant OnFth protein significantly reduced the proliferation of Streptococcus agalactiae and Aeromonas hydrophila , whereas OnFtm exhibited no comparable antibacterial activity. Further analysis revealed that OnFth expression was significantly increased in the head kidney, spleen, liver, and intestine following bacterial challenge. Additionally, OnFth dampened inflammation, reduced pathological tissue damage, and improved the survival rate of tilapia upon bacterial challenge. Following OnFth knockdown, both tissue iron homeostasis and intestinal microecological stability were severely compromised in tilapia, accompanied by reduced host resistance against pathogenic bacterial infection. Mechanistic investigations revealed that OnFth knockdown inhibits the TGF-β1/Smad2/3 signaling pathway, which in turn reduces host resistance to bacterial infection. Moreover, zebrafish ( Danio rerio ) DrFth, an ortholog of OnFth also confers immune protection. Collectively, these results suggest that ferritin not only contributes to maintaining host iron homeostasis but also executes functions analogous to those of antimicrobial peptides, while remaining highly conserved during evolution. These findings also expand our understanding of the regulatory mechanisms underlying ferritin function in fishes and provide valuable insights into the evolution of innate immunity.