Lumpfish (Cyclopterus lumpus), a native fish of the North Atlantic Ocean, has been utilized as cleaner fish to biocontrol sea-lice (Lepeophtheirus salmonis ) infestations in Atlantic salmon (Salmo salar ) farms in the North Atlantic region. Aeromonas salmonicida subspecies salmonicida is a Gram-negative facultative intracellular pathogen that causes lethal furunculosis in several fish species, including lumpfish. The molecular immune response of lumpfish to A. salmonicida infection is unknown. In this study, we established an A. salmonicida systemic infection model in lumpfish, and examined the transcriptome profile of central and peripherical lymphoid tissues after infection. Groups of lumpfish (50±7 g; n =60 per group) were intraperitoneally (ip) injected with different doses of A. salmonicida to calculate the median lethal dose (LD50). Samples of blood, head kidney, spleen, and liver were collected at different time points to determine the infection kinetics. We determined that A. salmonicida lethal dose 50 (LD50) is 102 bacterial cells per dose. The infection kinetic analysis indicated that the head-kidney is the primary target organ for A. salmonicida infection. Triplicate biological samples were collected from infected head kidney, spleen, and liver at 3- and 10-days post infection (dpi) and compared to non-infected organs. Transcriptome sequencing was performed in Nova-Seq illumina platform, generating total 1,034 million pair-end reads . Gene enrichment analyses showed that A. salmonicida
induces lethal infection in lumpfish by uncontrolled and detrimental blood coagulation, complement activation, and inflammation. Such responses lead to hypoxia, internal organs hemorrhages, suppression of adaptive immune system, and impairment of DNA repair system, which results in cell cycle arrest, and death. These results concordance with the fish behavior before disease onset and mortality and coincident with A. salmonicida described virulence factors.
This study has provided a valuable resource and novel insight into the interaction of fish-pathogens.