The effect of viral hemorrhagic septicemia virus (VHSV) was studied on the established rainbow trout (Oncorhynchus mykiss) monocyte/macrophage-like cell line RTS11. The virus was not able to complete its replication cycle as infectious viral particles were not released from the cells. However, in RTS11, the virus was capable of producing
mRNA from at least its N and G genes. At the
protein level, only N
protein was detected 2 days post-
infection, whereas a faint band corresponding to the
G protein was also observed after 5 days post-
infection. These results suggest an interruption of
viral protein translation at some point. The expression of N
mRNA was significantly inhibited in cells pre-treated with
Poly I:C, but not affected by
2-aminopurine (2-AP), an inhibitor of the dsRNA-dependent
protein kinase (PKR), thus indicating that PKR has no effect on
mRNA expression directly. However, when cells were preincubated with
Poly I:C in the presence of 2-AP, the levels of N
mRNA were restored suggesting that
Poly I:C can limit viral transcription through an
antiviral mechanism dependent of PKR. The effect of VHSV on the expression of transcripts for different immune genes was determined, but significant induction was found only for genes related to the
type I interferon (IFN) response, such as IFN-1 and -2 and the three Mx
isoforms. Heat-inactivated virus failed to induce IFN-1 and -2, suggesting that early events in the VHSV life cycle were necessary for the type I IFN response.
Poly I:C alone also induced transcripts for the
antiviral Mx
proteins. Prior exposure of RTS11 to VHSV did not prevent
Poly I:C from inducing transcripts for Mx1, Mx2 and Mx3. Perhaps the failure of VHSV to disable
antiviral mechanisms in RTS11 accounts for the aborted
infections.