Atlantic salmon immune responses after infection with intracellular pathogens

Abstract

We focused on the responses of two arms of the Atlantic salmon immune system against intracellular pathogens: the antibody (Ab) production by B cells after challenge with the facultative intracellular bacterium <i>Pisciricketsia salmonis</i> and the interferon (IFN) responses in a salmonid cell line against salmon alphavirus (SAV), infectious necrotic pancreas virus (IPNV), and <i>P. salmonis</i>. We developed tools and generated knowledge on these immune responses to contribute to the development of effective vaccines against intracellular pathogens, focusing on <i>P. salmonis</i>. We induced CRISPR-Cas knock outs (KOs) in the salmonid cell line CHSE-214 for IRF3, IRF7, and MAVS. The induction of IFN responses was disrupted in the IRF3 and MAVS KOs, while we did not observe effects of similar magnitude in the IRF7 KO. Although replication of SAV was positively affected in the KOs with disrupted IFN induction, IPNV replication and <i>P. salmonis</i> growth were negatively affected by these KOs. To investigate Ab production after <i>P. salmonis</i> infection of Atlantic salmon, we developed two intraperitoneal challenge models. In the first study, we observed a significant increase in anti-<i>P. salmonis</i> serum Abs at 14 weeks post challenge (wpc), but not at 18 wpc. The fish were protected against a secondary challenge at 14 wpc, while the protection might have been reduced at 18 wpc. In the second in vivo study, we investigated the origin and specificity of early Ab responses using ELISpot and ELISA assays. The intraperitoneal challenge resulted in a major increase of leukocytes, total IgM Ab secreting cells (ASC), and anti-<i>P. salmonis</i> ASC in the peritoneal cavity (PerC), compared to the head kidney and spleen. Furthermore, we observed an early increase of non-specific Ab production, while specific Abs dominated the later time point. We discuss how our findings fit together in a model of specific and non-specific activation of B cells through B cell receptors and pattern recognition receptors, respectively, which could explain the early presence of non-specific Abs. Our model addresses the location of early Ab production, the transition to a more specific Ab response, and the possible functions of the PerC and its adipose tissue in this. Finally, we address the duration of Ab responses against <i>P. salmonis</i>, how our findings can contribute to the development of vaccine strategies with long term protection, and possible avenues for future research.