In-vivo studies were carried out to investigate the protective effect of a synthetic viral analogue (
poly I:C) against Ostreid herpes virus (OsHV-1 μvar). Pacific oysters (Crassostrea gigas) were immune-primed by
intramuscular injection of 240 μg of
poly I:C or sterile seawater at 1 day prior to
infection with OsHV-1 μvar.
Poly I:C injection induced an
antiviral state in C. gigas as the percentage of viral-infected oysters at 48 h post
infection was significantly lower in the
poly I:C treatment (11%) compared to seawater controls (100%). In an additional experiment, we demonstrated that the protective role of
poly I:C is reproducible and elicits a specific
antiviral response as immune-priming with heat-killed Vibrio splendidus provided no protection against subsequent
viral infection. In both experiments, genes homologous to a
toll-like receptor (TLR), MyD88,
interferon regulatory factor (IRF) and
protein kinase R (PKR) were up-regulated in oysters immune-primed with
poly I:C compared to seawater controls (p < 0.05). The MyD88, IRF and PKR genes were also significantly up-regulated in response to OsHV-1 μvar
infection (p < 0.05), which is suggestive that they are implicated in the
antiviral response of C. gigas. Our results demonstrate that C. gigas can recognise double-strand
RNA to initiate an innate immune response that inhibits
viral infection. The observed response has striking similarities to the hallmarks of the type-1
interferon response of vertebrates.