CaIRF1 promotes antibacterial immune defense and acts as a vaccine adjuvant in Chromileptes altivelis.

Interferon regulatory factor 1 (IRF1), a transcription factor featuring a unique helix-turn-helix DNA-binding domain, plays an essential role in the innate immune response of vertebrates by regulating the type I interferon signaling pathway and contributing to defense against viral and bacterial pathogens. In this study, we identified and characterized the IRF1 homolog from Chromileptes altivelis (CaIRF1). Structural analysis revealed that CaIRF1 possesses a conserved DNA-binding domain and an IRF association domain with high similarity to mammalian IRF1. CaIRF1 was constitutively expressed in multiple tissues, with highest expression in immune-related organs such as gill, spleen, and head kidney. Following infection with Vibrio harveyi, CaIRF1 expression was significantly upregulated in liver, spleen, and head kidney. Subcellular localization studies indicated that CaIRF1 is distributed in both the nucleus and cytoplasm, with a predominant nuclear presence. Functional analyses demonstrated that overexpression of CaIRF1 reduced bacterial loads in vivo and enhanced cell survival post-infection in vitro, whereas knockdown of CaIRF1 compromised antibacterial responses. Moreover, co-immunization with CaIRF1 improved immune parameters and increased survival rates after bacterial challenge, suggesting its potential utility as an immune adjuvant. Collectively, this study identifies CaIRF1 as a key regulator of antibacterial immunity in C. altivelis and underscores its promising application in the development of vaccine adjuvants for aquaculture.