Palmatine as a potent immunomodulator: Enhancing resistance to Micropterus salmoides rhabdovirus in largemouth bass through innate immune activation and viral suppression.

Micropterus salmoides rhabdovirus (MSRV) poses a significant threat to aquaculture, causing substantial economic losses. In this study, we evaluated the antiviral efficacy and immunomodulatory potential of palmatine, a plant-derived monomer, against MSRV infection in largemouth bass. Our results demonstrated that palmatine significantly inhibited MSRV replication, with a reduction in viral nucleoprotein expression by 85 % at a safe concentration. Additionally, palmatine pre-treatment of EPC cells enhanced their antiviral capacity, with a maximum inhibition rate of 82 % following 24 h pre-incubation. Palmatine also effectively reduced MSRV-induced cytopathic effects, protecting cellular integrity and maintaining mitochondrial membrane potential. In vivo studies revealed that palmatine immersion at 80 mg/L was non-toxic and significantly suppressed MSRV replication in largemouth bass, increasing survival rates by 53 % over 15 d. Furthermore, palmatine pre-treatment enhanced the fish's resistance to MSRV, with a 78 % inhibition rate of viral replication and a 46 % increase in survival rate. Mechanistically, palmatine activated key immune genes, including IRF3, IRF7, and IFN, indicating its role in boosting innate immune responses. The compound also reduced horizontal transmission of MSRV in a cohabitation model, decreasing viral spread by up to 78 % over nine days. These findings highlight palmatine's potential as a small-molecule immunomodulator in aquaculture, offering a sustainable approach to disease management and enhancing fish health and welfare. Integrating palmatine into fish diets as an immunostimulant could provide a continuous, proactive defense against viral outbreaks, promoting more resilient and sustainable aquaculture practices.