Lumpfish (Cyclopterus lumpus) Is Susceptible to Renibacterium salmoninarum Infection and Induces Cell-Mediated Immunity in the Chronic Stage

Abstract

<i>Renibacterium salmoninarum</i> is a Gram-positive, intracellular pathogen that causes Bacterial Kidney Disease (BKD) in several fish species in freshwater and seawater. Lumpfish (<i>Cyclopterus lumpus</i>) is utilized as a cleaner fish to biocontrol sea lice infestation in Atlantic salmon (<i>Salmo salar</i>) farms. Atlantic salmon is susceptible to <i>R. salmoninarum</i>, and it can transfer the infection to other fish species. Although BKD outbreaks have not been reported in lumpfish, its susceptibility and immune response to <i>R. salmoninarum</i> is unknown. In this study, we evaluated the susceptibility and immune response of lumpfish to <i>R. salmoninarum</i> infection. Groups of lumpfish were intraperitoneally (i.p.) injected with either <i>R. salmoninarum</i> (1×10⁷, 1×10⁸, or 1×10⁹ cells dose^-1) or PBS (control). <i>R. salmoninarum</i> infection kinetics and mortality were followed for 98 days post-infection (dpi). Transcript expression levels of 33 immune-relevant genes were measured in head kidney (<i>n</i> = 6) of fish infected with 1×10⁹ cells/dose and compared to the control at 28 and 98 dpi. Infected lumpfish displayed characteristic clinical signs of BKD. Lumpfish infected with high, medium, and low doses had a survival rate of 65%, 93%, and 95%, respectively. Mortality in the high-dose infected group stabilized after 50 dpi, but <i>R. salmoninarum</i> persisted in the fish tissues until 98 dpi. Cytokines (<i>il1β</i>, <i>il8a</i>, <i>il8b</i>), pattern recognition receptors (<i>tlr5a</i>), interferon-induced effectors (<i>rsad2, mxa</i>, <i>mxb</i>, <i>mxc</i>), and iron regulation (<i>hamp</i>) and acute phase reactant (<i>saa5</i>) related genes were up-regulated at 28 dpi. In contrast, cell-mediated adaptive immunity-related genes (<i>cd4a</i>, <i>cd4b</i>, <i>ly6g6f</i>, <i>cd8a</i>, <i>cd74</i>) were down-regulated at 28 dpi, revealing the immune suppressive nature of <i>R. salmoninarum</i>. However, significant upregulation of <i>cd74</i> at 98 dpi suggests induction of cell-mediated immune response. This study showed that <i>R. salmoninarum</i> infected lumpfish in a similar fashion to salmonid fish species and caused a chronic infection, enhancing cell-mediated adaptive immune response.