Mitigating the challenges in the Atlantic salmon aquaculture caused by salmonid alphavirus by unveiling the underlying immune mechanisms

Pancreas disease (PD), caused by Salmonid alphavirus (SAV), is a major problem in salmon aquaculture. However, knowledge about how viral virulence, and the physiological and immunological state of the salmon influences susceptibility to SAV is limited. Atlantic salmon is very sensitive to handling in the phase of transition to seawater. A major goal of this project was to characterize host-pathogen interactions during SAV infection. A bath challenge model for SAV at the smolt- and post-smolts phase was established and used in several studies. The strength of this model is to give a specific time of exposure to pathogen giving much better control of time during studies of immune responses. Based on the results from susceptibility, innate and adaptive immune responses and immune cells in target organs it could be established that salmon adapted to seawater for at least 9 weeks before they meet pathogen were much more robust than when they might be exposed to pathogens at earlier time. The method for detection of SAV in seawater was optimized to be much more sensitive which will facilitate future detection of SAV during outbreaks in the field. This will further help in the management strategies by calculating the risk of potential transmission to neighbouring farms. Considering the potential use of triploid salmon in aquaculture to reduce the risk of genetic impact on wild fish, the susceptibility of triploid and diploid salmon to SAV was assessed. Triploid salmon showed to be more resilient where SAV infection was accumulated much slower than in diploid salmon. Microbiome analysis of salmon post-smolts revealed that SAV infection causes dysbiosis in skin and might render the fish more susceptible to other pathogens. Using assays established in this project for multi-panel flowcytometry and immunohistochemistry to study the role of various subsets of immune cells during SAV infection both systematically and locally in target organs for SAV, in addition to study specific immune cells infiltrating the peritoneum upon vaccination will facilitate development of better vaccines. Looking into the role of nutritional status on the immune response to SAV infection and disease progression of PD showed that the dietary lipids influence inflammation, immune response and tissue pathology, and that saturated lipids in connection with a low omega-6/omega-3 ratio may be beneficial for the outcome. To sum up, throughout the project, a holistic approach was taken to understand the impact of several parameters such as infection dynamics, pathogen, physiological status of salmon, immune responses, environmental parameters, microbiota, nutrition, isogenic salmon lines, and ploidy, thus contributing to an in-depth understanding in SAV-salmon interaction. Such knowledge is necessary for improved strategies to combat SAV infections, including better vaccination approaches and clarification of how processes such as smoltification influence SAV susceptibility.

1.Establishing SAV challenge model- Bath challenge model was established and used in several studies 2.Immune gene regulation-Immune gene regulation in addition to role of specific immune cells was studied in salmon post-smolts to understand role of the different immune cells, molecules and markers in response to SAV infection 3. Several antibodies were used to establish and optimize assays for immunohistochemistry and flowcytometry 4. Comparison of susceptibility of diploid and triploid salmon using bath challenge model was tested, combined with stress by increasing temperature sub-optimal for triploids. Samples from this experiment were analysed by RNA seq analysis 5.Microbiome analysis of SAV infected salmon-Microbiome analysis of salmon post-smolts revealed that SAV infection causes dysbiosis in skin and might render the fish more susceptible to other pathogens. Further, gills from diploids and triploids subjected to bath challenge with SAV were analysed

Pancreas disease (PD), caused by Salmonid alphavirus (SAV), is a major problem in salmon aquaculture. However, knowledge about how viral virulence, and the physiological and immunological state of the salmon influences susceptibility to SAV is limited. A major goal of this project is to characterise host-pathogen interactions during SAV infection. Such knowledge will suggest improved strategies to combat SAV infections, including better vaccination approaches and clarification of how processes such as smo ltification influence SAV susceptibility. Our approach is to study the mechanisms at whole animal, cellular and molecular level. We aim to establish SAV challenge models at the smolt- and post-smolts phase. Access to a panel of antibodies will allow us to study the roles of various subsets of immune cells during SAV infection. Complimentary studies using microarrray will also be used to study gene expression levels during infection. To understand the route of entry of SAV, salmon will be infected with a r ecombinant chemiluminescent and/or fluorescent SAV to localise: 1. the site(s) of virus entry; 2. the route of virus dissemination to target organs; and 3. the site(s) of viral replication. As there is increasing focus on the future use of triploid salmon to reduce the risk of genetic impact on wild fish, we will also assess the susceptibility of triploid salmon to SAV. This project will work closely with other ongoing projects that study smolt quality, stress, and charac-terization of the Atlantic salmon immune system, through active collaboration with other national and international research groups. This will be a concerted action to develop new tools to study immune function in Atlantic salmon and to use these tools to gain increased knowledge of the salmon-SAV host-pathogen relationship. This knowledge will contribute to an improved, science-based approach, to combat the economic and welfare challenges posed by SAV in salmon aquaculture.