Expression of the Infectious Salmon Anemia Virus Receptor on Atlantic Salmon Endothelial Cells Correlates with the Cell Tropism of the Virus

Abstract

Infectious salmon anemia (ISA) is a World Organization for Animal Health (OIE)-listed disease of farmed Atlantic salmon, characterized by slowly developing anemia and circulatory disturbances. The disease is caused by ISA virus (ISAV) in the Orthomyxoviridae family; hence, it is related to influenza. Here we explore the pathogenesis of ISA by focusing on virus tropism, receptor tissue distribution, and pathological changes in experimentally and naturally infected Atlantic salmon. Using immunohistochemistry on ISAV-infected Atlantic salmon tissues with antibody to viral nucleoprotein, endotheliotropism was demonstrated. Endothelial cells lining the circulatory system were found to be infected, seemingly noncytolytic, and without vasculitis. No virus could be found in necrotic parenchymal cells. From endothelium, the virus budded apically and adsorbed to red blood cells (RBCs). No infection or replication within RBCs was detected, but hemophagocytosis was observed, possibly contributing to the severe anemia in fish with this disease. Similarly to what has been done in studies of influenza, we examined the pattern of virus attachment by using ISAV as a probe. Here we detected the preferred receptor of ISAV, 4- O -acetylated sialic acid (Neu4,5Ac 2 ). To our knowledge, this is the first report demonstrating the in situ distribution of this sialic acid derivate. The pattern of virus attachment mirrored closely the distribution of infection, showing that the virus receptor is important for cell tropism, as well as for adsorption to RBCs.

First field evidence of the evolution from a non-virulent HPR0 to a virulent HPR-deleted infectious salmon anaemia virus

“…the functional shift of cell and organ tropism from an epithelial to an endothelial infection, including generalized dissemination. This is in agreement with our previous findings that non-virulent ISAV-HPR0 causes only a localized epithelial infection of mucosal surfaces of the gills and skin [19,49], whereas virulent ISAV-HPR-deleted shows a generalized endothelial infection of the circulatory system [50]. Furthermore, recent functional analysis demonstrated that deletions in the HE-HPR promote ISAV receptor fusion of viral and cellular membranes by which the virus delivers its genetic material into the host cell for subsequent replication [23].…”

Transcriptional Characterization of the T Cell Population within the Salmonid Interbranchial Lymphoid Tissue

“…This finding demonstrated that the ILT responded to systemic infection in a distinct manner compared with the other tissues examined, especially noteworthy the gills, in which the ILT is embedded. Previous investigations have shown the presence of the ISAV cellular receptor in the ILT by IHC (28), whereas the absence of replicating ISAV has been shown by a RT-qPCR assay (4). These findings indicate that the changes observed in the ILT most likely are consequences of a systemic and not a local response.…”

Infectious salmon anaemia virus infection of Atlantic salmon gill epithelial cells

“…Previous studies have demonstrated ISAV receptor expression on gill mucosal epithelial cells [33] and high prevalences of ISAV positive gills by RT-PCR [31,34], suggesting ISAV infection of gill cells. However, such infections have not been confirmed at the cellular level.…”

“…As they filter large amounts of water they offer a suitable target for invading infectious agents. While RT-PCR detection of ISAV on whole gill preparations has suggested the gills as an entry-site for ISAV infection [31], in vivo studies have only reported ISAV infection within endothelial cells and leucocytes [11,32,33]. We showed previously that gill and hind-gut mucosal epithelial cells express the ISAV 4- O- acetylated sialic acid receptor on the cell surface [33], indicating the potential for ISAV-infection of epithelial cells.…”

“…While RT-PCR detection of ISAV on whole gill preparations has suggested the gills as an entry-site for ISAV infection [31], in vivo studies have only reported ISAV infection within endothelial cells and leucocytes [11,32,33]. We showed previously that gill and hind-gut mucosal epithelial cells express the ISAV 4- O- acetylated sialic acid receptor on the cell surface [33], indicating the potential for ISAV-infection of epithelial cells. High prevalences of ISAV positive gills by RT-PCR, have been documented, both for low pathogenic ISAV-HPR0 cases in the Faroe Islands [34], and in high pathogenic ISA outbreaks in Norway [35].…”