Elsevier

Fish & Shellfish Immunology

Volume 57, October 2016, Pages 35-40
Fish & Shellfish Immunology

Full length article
Protection of Atlantic salmon against salmonid alphavirus infection by type I interferons IFNa, IFNb and IFNc

https://doi.org/10.1016/j.fsi.2016.08.020Get rights and content

Highlights

  • IFNa, IFNb and IFNc all induced antiviral activity against SAV3 in cultured cells.

  • IFNb and IFNc plasmids, but not IFNa plasmid induced expression of the antiviral protein Mx in pancreas.

  • IFNc plasmid gave highest protection against SAV3 infection 10 weeks after injection.

  • IFNb plasmid induced lower protection against SAV3 infection than IFNc plasmid while IFNa plasmid showed no protection.

Abstract

Salmonid alphavirus 3 (SAV3) causes pancreas disease (PD), which is a major problem in Norwegian aquaculture of Atlantic salmon. In this work we studied antiviral activities of salmon type I interferons IFNa, IFNb and IFNc against SAV3 infection in cell culture and in live fish to increase the understanding of the innate immunity of salmon against this virus. Recombinant IFNa, IFNb and IFNc all induced antiviral activity against SAV3 in ASK cells. For in vivo studies, we injected salmon presmolts intramuscularly with plasmids encoding salmon IFNa, IFNb and IFNc or a control plasmid and measured expression of the antiviral protein Mx in pancreas after 2 and 10 weeks and protection against SAV3 infection after 10 weeks. IFNb and IFNc plasmids, but not IFNa plasmid induced Mx expression in pancreas as shown by RT-qPCR and immunohistochemistry. A high level of protection against SAV3 infection by IFNc plasmid was observed by a strong reduction of virus load in serum and by a marked reduction in pathology of pancreas and heart compared to control fish. Lesser but significant protection was observed with IFNb plasmid while no protection was observed after treatment with IFNa plasmid. Taken together, this work suggests that IFNa provides protection of salmon against SAV3 locally in an infected area while IFNb and IFNc provides systemic protection against the virus.

Introduction

Salmonid alphavirus (SAV) causes pancreas disease (PD) of farmed Atlantic salmon in Norway, Ireland and Scotland [1]. Histopathological characteristics of salmon with PD include severe degeneration, necrosis and inflammation in pancreas, heart and skeletal muscle [2], [3], [4], [5]. Six subtypes of SAV (SAV1-SAV6) have so far been identified [6], [7]. Different geographic locations of the PD epidemics are associated with different SAV strains. SAV1 is the main problem in Ireland and Scotland while SAV3 is the cause of the PD epidemic in western Norway and a marine SAV2 is causing the epidemic in mid-Norway [8].

More knowledge about how SAV interacts with the immune system of salmon is needed since the PD problem in Atlantic salmon farming persists despite the use of vaccines. In this work we have studied antiviral activities of salmon type I interferons (IFN-I) against SAV3 infection in cell culture and in live fish. IFN-I play important roles in both innate and adaptive immunity against viruses. IFN-I are induced upon host cell recognition of viral nucleic acids and protect other cells against infection by inducing numerous antiviral proteins such as Mx, ISG15 and viperin [9], [10], [11]. Four IFN-I subtypes, named IFNa, IFNb, IFNc and IFNd, have so far been characterized in Atlantic salmon [12], [13]. The four IFNs show major differences in sequence, cellular expression and antiviral activities [12], [14]. IFNa and IFNc showed similar strong antiviral activity against infectious pancreatic necrosis virus (IPNV) in salmon TO cells [14]. IFNb was less active and IFNd showed no antiviral activity. IFNa, IFNb and IFNc provided only transient inhibition of infectious salmon anemia virus (ISAV) replication in TO cells [15]. On the other hand, intramuscular (i.m.) injection of an IFNc expression plasmid (pIFNc) into Atlantic salmon provided protection against ISAV infection for at least 8 weeks apparently due to systemic induction of antiviral genes in the fish [16]. In contrast, injection of IFNa plasmid (pIFNa) did not cause systemic induction of antiviral genes and did not protect salmon against ISAV. IFNb plasmid (pIFNb) caused systemic up-regulation of antiviral genes, but did not protect salmon against ISAV infection apparently because IFNb induced antiviral genes in fewer cell types than IFNc.

Previously, IFNa has been shown to have antiviral activity against SAV3 in cell culture [17], [18], but the activities of IFNb and IFNc against SAV are unknown. Moreover, the antiviral activity of the salmon IFN-I against SAV in vivo has not been studied. In this work we therefore studied antiviral activity of recombinant IFNb and IFNc against SAV3 infection in cell culture and whether i.m. injection of IFN plasmids into Atlantic salmon could induce antiviral genes in pancreas and protection against SAV3 infection.

Section snippets

Fish

Atlantic salmon (Salmo salar L.) presmolts (35–45 g) of the strain Aquagen standard (Aquagen, Kyrksæterøra, Norway) were kept at Tromsø Aquaculture Research Station, Norway in 300 l tanks supplied with fresh water at 10 °C and were fed commercial dry food. Prior to treatments, the fish were anesthetized with 0.005% benzocaine (ACD Pharmaceuticals, Norway). Fish groups were labeled by tattooing (2% alcian blue, Panjet inoculator). The fish were killed by an overdose of 0.01% benzocaine prior to

Recombinant IFNa, IFNb and IFNc inhibit SAV3 replication in ASK cells

Antiviral activity of recombinant IFNa, IFNb and IFNc against SAV3 was measured by treating ASK cells with recombinant IFNs (1000 U/ml) for 24 h, infecting the cells with SAV3 and measuring E2 transcripts in the cells at 4, 24, 72 and 120 h post infection (hpi) (Fig. 1). SAV3 replication was significantly inhibited by all three IFNs at 24 hpi (p < 0.05), 72 hpi (p < 0.005) and 120 hpi (p < 0.05). At the peak of virus replication, 72 hpi, the fold reduction in virus E2 copy numbers were 783, 120

Discussion

The present work showed that recombinant IFNa, IFNb and IFNc all induced strong antiviral activity against SAV3 infection in cell culture while only IFNb and IFNc displayed protective effects against SAV3 infection in live Atlantic salmon. The in vivo studies were performed by injecting salmon with IFN-expressing plasmids, control plasmid or PBS followed by measurement of Mx expression in pancreas and protection against SAV3 infection. Pancreas was chosen for Mx-expression studies because this

Acknowledgements

We want to thank researcher Jinni Gu from Norwegian Veterinary Institute in Trondheim for helping us establishing the method for scoring of histopathological changes in SAV3 infected fish. This work was in part funded by the Marine Biotechnology Programme in Tromsø (MABIT) (grant AF0070).

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