Molecular immune responses of lumpfish (Cyclopterus lumpus) to aeromonas salmonicida

Chakraborty, Setu (2023) Molecular immune responses of lumpfish (Cyclopterus lumpus) to aeromonas salmonicida. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Lumpfish (Cyclopterus lumpus) have been utilized to biocontrol sea-lice (e.g., Lepeophtheirus salmonis) infestations in Atlantic salmon (Salmo salar) farms in the North Atlantic region. Aeromonas salmonicida subsp. salmonicida is a Gram-negative facultative intracellular pathogen infecting several fish species, including lumpfish. Here, I established an A. salmonicida systemic infection model in lumpfish. Juvenile lumpfish were intraperitoneally (ip) injected with different doses of A. salmonicida strain J223. A. salmonicida median lethal dose (LD50) was calculated at 102 colony-forming units (CFU)/dose. Samples from blood, head kidney, spleen, and liver were collected at different time points to determine the infection kinetics. Triplicated RNA samples from the head kidney, spleen, and liver of non-infected and infected (104 CFU/dose) lumpfish at 3 and 10 days post-infection (dpi) were utilized for RNA sequencing (RNA-Seq). Analysis of RNA-Seq data suggested that A. salmonicida could induce lethal infection in lumpfish by septic-like shock, suppression of the adaptive immune system, impairment of the DNA repair system, and disruption of cytoskeleton structure. Thus, an effective vaccine for lumpfish against A. salmonicida is highly needed. Here, I evaluated the immune protective effect of A. salmonicida bacterins and outer membrane proteins (OMPs) expressing iron-regulated outer membrane proteins (IROMPs) in lumpfish. However, my results demonstrate that vaccines do not confer protection to lumpfish against A. salmonicida J223. Additionally, I observed that formalin-killed A. salmonicida J223 bacterins do not increase IgM titers in lumpfish serum. Instead, the bacterins downregulate genes encoding IgM, MHC-II, and CD4, which indicates immune suppression and vaccine’s inability to trigger humoral and cell-mediated immune responses. Furthermore, different post-transcriptional factors (e.g., microRNAs (miRNAs)) that significantly determine an organism’s disease state are not explored yet in lumpfish. Therefore, I characterized miRNA encoding genes in lumpfish from three developmental stages (adult, embryos, and larvae) in this study, providing the reference miRNAome for future functional studies. Altogether, 443 unique mature miRNAs were identified in lumpfish. Transcriptomics analysis suggested organ-specific and age-specific expression of miRNAs. This thesis contributes significantly to understanding lumpfish- A. salmonicida interactions and provides guidelines for future host-pathogen interaction and vaccine studies.

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