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Journal of Virology, April 2004, p. 4098-4107, Vol. 78, No. 8
0022-538X/04/$08.00+0     DOI: 10.1128/JVI.78.8.4098-4107.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Essential Role of the NV Protein of Novirhabdovirus for Pathogenicity in Rainbow Trout

Unité de Virologie et Immunologie Moléculaires, Institut National de la Recherche Agronomique, 78352 Jouy-en-Josas Cedex, France

Received 25 August 2003/ Accepted 12 December 2003

Novirhabdovirus, infectious hematopoietic necrosis virus (IHNV), and viral hemorrhagic septicemia virus (VHSV) are fish rhabdoviruses that, in comparison to the other rhabdoviruses, contain an additional gene coding for a small nonvirion (NV) protein of unassigned function. A recombinant IHNV with the NV gene deleted but expressing the green fluorescent protein (rIHNV-NV) has previously been shown to be efficiently recovered by reverse genetics (S. Biacchesi et al., J. Virol. 74:11247-11253, 2000). However, preliminary experiments suggested that the growth in cell culture of rIHNV-NV was affected by the NV deletion. In the present study, we show that the growth in cell culture of rIHNV-NV is indeed severely impaired but that a normal growth of rIHNV-NV can be restored when NV is provided in trans by using fish cell clones constitutively expressing the NV protein. These results indicate that NV is a protein that has a crucial biological role for optimal replication of IHNV in cell culture. Although IHNV and VHSV NV proteins do not share any significant identity, we show here that both NV proteins play a similar role since a recombinant IHNV virus, rIHNV-NV_VHSV, in which the IHNV NV open reading frame has been replaced by that of VHSV, was shown to replicate as well as the wild-type (wt) IHNV into fish cells. Finally, data provided by experimental fish infections with the various recombinant viruses strongly suggest an essential role of the NV protein for the pathogenicity of IHNV. Furthermore, we show that juvenile trout immunized with NV-knockout IHNV were protected against challenge with wt IHNV. That opens a new perspective for the development of IHNV attenuated live vaccines.

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