Here we have studied the impact of lice (Lepeophtheirus salmonis) infestation of donor fish on the ability of isolated peripheral blood monocytes (PBMCs) to control the replication of salmonid alphavirus (SAV) ex vivo. PBMCs were collected by Percoll gradients at eight and nine weeks post copepodid infestation of Atlantic salmon post smolt. Uninfested fish were controls. PBMCs were then infected ex vivo with SAV (subtype 3), and samples were collected for analysis at two, four, and six days post virus infection. Virus titer in the supernatant was assayed in CHH-1 cells, and in addition, the relative expression of the virus structural protein E2 and selected host antiviral genes, IRF9, ISG15, Mx, and IFIT5, were assayed using real-time PCR. Significantly higher virus replication was detected in cells collected from lice-infested fish compared to controls. Higher virus titer coincided with an inability to upregulate the expression of different immune genes, IFIT5, IRF9, and Mx. These findings point towards compromised ability of PBMCs from lice-infested fish to control virus replication, and, to our knowledge, is the first report showing the direct effect of lice infestation on the interplay between viruses and immune cells. There is a possible impact on the dynamic spread of viral diseases in the aquatic environment.
Salmon louse, Lepeophtheirus salmonis, represents a major challenge for salmon farming as current treatments impose welfare issues and are costly, and no prophylactic measures are available. Two salmon louse heme peroxidases (LsPxtl-1 and LsPxtl-2) were tested for their importance in parasite development in vitro and as potential vaccine candidates. LsPxtl-1 possesses two heme peroxidase domains and is expressed in ovaries and gut. LsPxtl-1 knockdown in nauplius II stage resulted in poor swimming performance and parasite death when reaching the copepodid stage, indicating LsPxtl-1 being indispensable for parasite development. LsPxtl-2 encodes one peroxidase domain and was predicted to contain an N-terminal signal peptide and an Eph receptor ligand binding domain. LsPxtl-2 knockdown did not impact survival or phenotype. Immunization with DNA plasmids encoding the peroxidases by single or combined injections, gave non-significant reduction in lice numbers post challenge which correlated with low specific antibody levels post vaccination. Combined injections of both plasmids followed by an LsPxtl-2 protein boost induced higher antibody response with reduced lice numbers post challenge but not significantly different from controls. The findings suggest LsPxtl-1 to play a role for parasite development, current formulations and vaccination modalities did not give significant reduction in lice infestation.
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