In contrast to other teleosts, Atlantic cod (
Gadus morhua
) has an expanded repertoire of MHC-I and TLR components, but lacks the MHC-II, the invariant chain/CD74, and CD4
+
T cell response, essential for production of antibodies and prevention of bacterial infectious diseases. The mechanisms by which
G. morhua
fight bacterial infections are not well understood.
Aeromonas salmonicida
subsp.
salmonicida
is a recurrent pathogen in cultured and wild fish, and has been reported in Atlantic cod. Macrophages are some of the first responders to bacterial infection and the link between innate and adaptive immune response. Here, we evaluated the viability, reactive oxygen species (ROS) production, cell morphology, and gene expression of cod primary macrophages in response to
A. salmonicida
infection. We found that
A. salmonicida
infects cod primary macrophages without killing the cod cells. Likewise, infected Atlantic cod macrophages up-regulated key genes involved in the inflammatory response (e.g.,
IL-1
β and
IL-8
) and bacterial recognition (e.g.,
BPI/LBP
). Nevertheless, our results showed a down-regulation of genes related to antimicrobial peptide and ROS production, suggesting that
A. salmonicida
utilizes its virulence mechanisms to control and prevent macrophage anti-bacterial activity. Our results also indicate that Atlantic cod has a basal ROS production in non-infected cells, and this was not increased after contact with
A. salmonicida
. Transmission electron microscopy results showed that
A. salmonicida
was able to infect the macrophages in a high number, and release outer membrane vesicles (OMV) during intracellular infection. These results suggest that Atlantic cod macrophage innate immunity is able to detect
A. salmonicida
and trigger an anti-inflammatory response, however
A. salmonicida
controls the cell immune response to prevent bacterial clearance, during early infection.
vitamin D 3 , cholecalciferol, induced anti-bacterial innate immunity pathways in Atlantic salmon primary macrophages, suggesting that its utilization as a component of a healthy aquafeed diet in Atlantic salmon could enhance the immune response against A. salmonicida.
Aeromonas salmonicida subsp. salmonicida (hereafter A. salmonicida) is the aetiological agent of furunculosis in marine and freshwater fish. Once A. salmonicida invade the fish host through skin, gut or gills, it spreads and colonizes the head kidney, liver, spleen and brain. A. salmonicida infects leucocytes and exhibits an extracellular phase in the blood of the host; however, it is unknown whether A. salmonicida have an intraerythrocytic phase. Here, we evaluate whether A. salmonicida infects Atlantic salmon (Salmo salar) erythrocytes in vitro and in vivo. A. salmonicida did not kill primary S. salar erythrocytes, even in the presence of high bacterial loads, but A. salmonicida invaded the S. salar erythrocytes in the absence of evident haemolysis. Naïve Atlantic salmon smolts intraperitoneally infected with A. salmonicida showed bacteraemia 5 days post‐infection and the presence of intraerythrocytic A. salmonicida. Our results reveal a novel intraerythrocytic phase during A. salmonicida infection.
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