Transcriptomic and functional evidence reveals dual-module immune response of human-nasal staphylococcus aureus in Koi Carp (Cyprinus carpio).

Staphylococcus aureus is a significant pathogen that poses a threat to both human and animal health. Its pathogenicity in humans has been extensively studied, however, the signaling pathways and key genes in Koi Carp responding to S. aureus from human rhinitis remain unclear. In this study, we established an intraperitoneal infection model in koi carp (Cyprinus carpio) using an S. aureus isolate from patients with rhinitis and integrated RNA-seq, qPCR, and ELISA to dissect the host response. Our findings reveal a dual-module immune evasion strategy employed by S. aureus in koi carp. Module I: The pathogen down-regulated the entire complement coagulation cascade (C3, C9, CFH, F7/9/10) and apolipoprotein-mediated opsonins (APOA1, APOB, APOC1/2), thereby crippling innate clearance. Module II: The host mounted a restricted but potent counter-response, characterized by type I IFN signalling (gvin1, MHC-I), NK/T-cell co-stimulation (CD244, SLAMF5), and the selective induction of IL-8 and IL-1β, while IL-6, IL-10, and TNF-α remained unchanged. Functionally, serum superoxide dismutase (SOD), catalase (CAT), and lysozyme (LZM) activities surged, confirming an oxidative burst, whereas splenic CD22R protein decreased, indicating B-cell disinhibition. These results establish a molecular basis for understanding the interaction between human-derived S. aureus and the immune system of aquatic organisms.