Gene expression studies of host response to Salmonid alphavirus subtype 3 experimental infections in Atlantic salmon

Abstract

Salmonid alphavirus subtype-3 (SAV-3) infection in Atlantic salmon is exclusively found in Norway. The salmonid alphaviruses have been well characterized at the genome level but there is limited information about the host-pathogen interaction phenomena. This study was undertaken to characterize the replication and spread of SAV-3 in internal organs of experimentally infected Atlantic salmon and the subsequent innate and adaptive immune responses. In addition, suitability of a cohabitation challenge model for this virus was also examined. Groups of fish were infected by intramuscular injection (IM), cohabited (CO) or kept uninfected in a separate tank. Samples of pancreas, kidney, spleen, heart and skeletal muscles were collected at 2, 4 and 8 weeks post infection (wpi). Pathological changes were assessed by histology concurrently with viral loads and mRNA expression of immune genes by real time RT-PCR. Pathological changes were only observed in the pancreas and heart (target organs) of both IM and CO groups, with changes appearing first in the pancreas (2 wpi) in the former. Lesions with increasing severity over time coincided with high viral loads despite significant induction of IFN-α, Mx and ISG15. IFN-γ and MHC-I were expressed in all tissues examined and their induction appeared in parallel with that of IL-10. Inflammatory genes TNF-α, IL-12 and IL-8 were only induced in the heart during pathology while T cell-related genes CD3ε, CD4, CD8, TCR-α and MHC-II were expressed in target organs at 8 wpi. These findings suggest that the onset of innate responses came too late to limit virus replication. Furthermore, SAV-3 infections in Atlantic salmon induce Th1/cytotoxic responses in common with other alphaviruses infecting higher vertebrates. Our findings demonstrate that SAV-3 can be transmitted via the water making it suitable for a cohabitation challenge model.

In vitro adaptation of SAV3 in cell culture correlates with reduced in vivo replication capacity and virulence to Atlantic salmon (Salmo salar L.) parr

“…The viral load measured in heart tissue and the tissue pathology were clearly related in our study, where the pathological changes in pancreas increased with viral load in the heart. The relationship between viral load and tissue pathology for SAV3 has been documented earlier, and a threshold of virus replication has to be reached to trigger pathology (Xu et al, 2012). Difference in tissue tropism between the two passage isolates in our study was less likely since the viral load in head kidney followed the same pattern as heart tissue (results not shown).…”

“…The highest viral load is found in the heart and pancreas (Andersen et al, 2007;Xu et al, 2012). To compare viral infection levels and corresponding pathological changes, 50 Atlantic salmon parr were infected by intramuscular injection with 10 5.17 TCID 50 of H10 P3 and in a parallel tank a similar infection was performed using H10 P14 at the same infection dose.…”

“…Tissue lesions include degeneration and necrosis of cardiomyocytes, pancreatic acinar cell loss and subsequent skeletal muscle degeneration (McLoughlin et al, 2006;Mcvicar, 1987). Virus has been detected in a wide range of tissues (Andersen et al, 2007), and it has been suggested that pancreas, heart, kidney and spleen are infected at about the same time (Xu et al, 2012). However, the virus replicates with highest viral load in the pancreas and heart (Andersen et al, 2007;Xu et al, 2012), and the pancreas is suggested as the preferred site of replication (McLoughlin & Graham, 2007;McLoughlin et al, 1996;Xu et al, 2012).…”

“…Virus has been detected in a wide range of tissues (Andersen et al, 2007), and it has been suggested that pancreas, heart, kidney and spleen are infected at about the same time (Xu et al, 2012). However, the virus replicates with highest viral load in the pancreas and heart (Andersen et al, 2007;Xu et al, 2012), and the pancreas is suggested as the preferred site of replication (McLoughlin & Graham, 2007;McLoughlin et al, 1996;Xu et al, 2012). SAV is currently divided into six different subtypes based on partial sequences of E2 and nsP3 genes (Fringuelli et al, 2008).…”

Natural infection of Atlantic salmon (Salmo salar L.) with salmonid alphavirus 3 generates numerous viral deletion mutants

“…This together with specific environmental conditions and reservoir hosts might lead to restricted geographical distribution (Griffin, 2007). Although SAV3 has been detected by real-time PCR from the salmon louse Lepeophtheirus salmonis collected from diseased fish (Petterson et al, 2009), the role of lice or other invertebrate vectors is yet to be determined as SAV infections can be transmitted without the aid of an arthropod (McLoughlin et al, 1996;Xu et al, 2012).…”