Autophagic function of the liver of vaccinated rainbow trout (Oncorhynchus mykiss Walbaum) following Yersinia ruckeri infection

The liver plays a critical role in maintaining metabolic homeostasis and immune defence in fish, particularly in response to bacterial infections. Autophagy, a conserved cellular process essential for homeostasis and pathogen clearance, has been implicated in host defence mechanisms. However, the role of autophagy in the liver of vaccinated fish following pathogen exposure remains largely unexplored. Yersinia ruckeri, the causative agent of enteric redmouth disease (ERM), poses a significant threat to rainbow trout (Oncorhynchus mykiss Walbaum) aquaculture, primarily affecting the liver, spleen and kidneys. Vaccination is a widely used preventive strategy, but its effect on autophagic activity during infection is not well understood. The aim of this study was to evaluate the autophagic response in the liver of vaccinated rainbow trout following Y. ruckeri infection by assessing the activity of four lysosomal enzymes: alanyl aminopeptidase (AAP), leucyl aminopeptidase (LAP), acid phosphatase (AcP) and β-N-acetylglucosaminidase (NAG). Rainbow trout were divided into experimental groups: unvaccinated control, vaccinated uninfected, unvaccinated infected and vaccinated infected. The fish were orally immunised with a Y. ruckeri vaccine and challenged with a virulent strain of Y. ruckeri. The results showed significant differences in lysosomal enzyme activity between groups, indicating that vaccination modulated the hepatic autophagic response during bacterial infection. AAP and LAP activity peaked in unvaccinated infected fish, whereas vaccinated fish exhibited a blunted enzymatic response, suggesting that vaccination attenuated excessive autophagic activation. Similarly, AcP and NAG activity patterns indicated an infection-induced autophagic response that was partially attenuated in vaccinated fish. These results suggest that vaccination influences autophagy-related enzymatic activity in the liver of rainbow trout, potentially enhancing pathogen clearance while preventing excessive cellular stress. Understanding the interplay between vaccination, infection and autophagy may provide valuable insights to optimise vaccination strategies and improve disease management in aquaculture.