Grass carp catalase inhibits GCRV replication through ROS scavenging–mediated immune suppression and autophagy

Catalase (CAT) is a key antioxidant enzyme that eliminates reactive oxygen species (ROS), thereby protecting organisms from oxidative stress and maintaining cellular homeostasis. In this study, we cloned and characterized a CAT gene (CiCAT) from grass carp (Ctenopharyngodon idellus), an important aquaculture species in China. The full-length cDNA of CiCAT is 2263 bp, with a 1575 bp open reading frame (ORF), and the encoded protein is highly conserved among fish species. CiCAT was constitutively expressed across all examined tissues, and its expression levels were significantly altered following infection with grass carp reovirus (GCRV) or stimulation with pathogen-associated molecular patterns (PAMPs). Subcellular localization analysis revealed that CiCAT was widely distributed in cells but co-localized with peroxisomes under H₂O₂ treatment. Functionally, CiCAT enhanced Escherichia coli resistance to H₂O₂ and heavy metal stress, and it inhibited GCRV replication in fish cells. Mechanistic investigations demonstrated that CiCAT suppresses GCRV replication not through activation of the host antiviral immune response, but by scavenging ROS, which attenuates host immune signaling. This reduced immune activity in turn promotes autophagy via the mTORC1/ULK1 pathway, thereby restricting GCRV replication. Collectively, our findings provide novel insights into the antiviral role of CAT in teleost fish.