Long-term and non-specific immune memory induced by muramyl dipeptide and its protective effect against Vibrio anguillarum in rainbow trout (Oncorhynchus mykiss)

Muramyl dipeptide (MDP), a minimal bioactive peptidoglycan motif derived from the bacterial cell wall, is known to trigger a wide range of signaling cascades related to inflammatory and antibacterial responses, thereby enhancing the antimicrobial function of various innate immune cells including monocytes, macrophages, neutrophils, natural killer (NK) cells, and lymphocytes. To date, several studies have demonstrated that MDP has the capacity to stimulate non-specific immunity in fish. However, the long-term effect of MDP on fish remains unclear. In this study, we aimed to investigate the long-term protective effect of MDP against Vibrio anguillarum in rainbow trout (Oncorhynchus mykiss). MDP was administered to the rainbow trout by intraperitoneal (IP) injection, and the bacterial challenge was conducted at 4 weeks post-administration. The MDP-injected fish exhibited significantly higher survival rates than fish injected with PBS. Following bacterial infection, significantly reduced bacterial loads were shown in the head kidney of MDP-injected fish, accompanied by elevated expression levels of TNF-α, IL-1β, IL-6, IL-12, and HIF-1α. Furthermore, a significant increase of acetylation of histone 3 at lysine 27 (H3K27ac) was evident at the promoter regions of TNF-α and IL-1β in the fish of the MDP group at 4 weeks post-administration. These results suggest that MDP-induced histone acetylation in immune-related genes’ promoters enhanced immune gene expression upon infection, possibly contributing to the observed long-term antibacterial effect and protection against V. anguillarum.