Comparison of gene expression in post‐smolt Atlantic salmon challenged by LF‐89‐like and EM‐90‐like Piscirickettsia salmonis isolates reveals differences in the immune response associated with pathogenicity

Abstract

Piscirickettsiosis is the main bacterial disease affecting the Chilean salmon farming industry and is responsible for high economic losses. The aim of this study was to describe and comparatively quantify the immune response of post-smolt Atlantic salmon infected by cohabitation with fish bearing LF-89-like and EM-90-like Piscirickettsia salmonis. The expression of 17 genes related to the immune response was studied in head kidney from cohabitant fish by RT-qPCR. Our results at the transcriptomic level suggest that P. salmonis is able to manipulate the kinetics of cytokine production in a way that might constitute a virulence mechanism that promotes intracellular bacterial replication in cells of Atlantic salmon. This strategy involves the creation of an ideal environment for the microorganism based on induction of the inflammatory and IFN-mediated response, modulation of Th1 polarization, reduced antigen processing and presentation, modulation of the evasion of the immune response mediated by CD8 T cells and promotion of the CD4 T-cell response during the late stage of infection as a mechanism to escape host defences. This response was significantly exacerbated in fish infected by PS-EM-90 compared with fish infected by PS-LF-89, a finding that is probably associated with the higher pathogenicity of PS-EM-90.

Comparative evaluation of experimental challenge by intraperitoneal injection and cohabitation of Atlantic salmon (Salmo salar L) after vaccination against Piscirickettsia salmonis (EM90‐like)

“…In the present study, the CD8α expression was downregulated and CD4-1 expression was upregulated in the spleen after challenge by both routes. This is in agreement with the results obtained in the study performed by Rozas-Serri, Peña, Arriagada, Enríquez, and Maldonado (2018). were found in highest frequency in fish vaccinated with the highest antigen dose (group A) at 2 wpc (i.p.…”

“…It has been described that P . salmonis is able to modulate cell‐mediated immunity and thus promoting intracellular replication of the bacteria and its survival inside the host cells (Rozas‐Serri et al, ). Thus, the acquired immune response developed in fish after vaccination against P .…”

“…There has been described that increased cell density in vaccines could improve their efficacy (Makrinos & Bowden, ). Therefore, future studies should include an increased concentration of antigen in the vaccine formulation, for example from 5× to 10×, with the objective to stimulate a protective immune response, especially the antigen‐specific CD8 T‐cell response, described as essential for the immune protection against intracellular pathogen (Rozas‐Serri et al, ). Other alternatives to improve the stimulation of a specific immune response could be by live‐attenuated vaccines, which have been described to stimulate a stronger cellular immune response to intracellular pathogens (Evensen, ; Tandberg et al, ).…”

“…In addition, a dose-response relationship between CD8α expression and vaccines A, B and C was observed after challenge by injection, with the highest CD8α expression identified in the spleen of fish vaccinated with vaccine A. It has been described that P. salmonis is able to modulate cell-mediated immunity and thus promoting intracellular replication of the bacteria and its survival inside the host cells (Rozas-Serri et al, 2018). Thus, the acquired immune response developed in fish after vaccination against P. salmonis could avoid downregulation of CD8α.…”

Biofilm Produced In Vitro by Piscirickettsia salmonis Generates Differential Cytotoxicity Levels and Expression Patterns of Immune Genes in the Atlantic Salmon Cell Line SHK-1

“…In this sense, the expression of immunity genes evaluated indicated increments on il-1β , il-8 , and iκb-α genes during the 24 hpi, which later resulted in an increased cytotoxicity produced by the bacteria. Additionally, Rozas et al [ 77 ] reported an increase in the expression of il-1β , il-8 , tnf-α , and ifn-ɣ genes in the cohabiting fish kidneys at day 28 after being exposed to Trojan fish. It is important to mention that this finding was described in fish infected with both LF-89 and EM-90 strains, and the mortality in both groups began after the increment in the immune gene expression, between days 45 and 42 post challenge, respectively.…”