Atlantic halibut experimentally infected with nodavirus shows increased levels of T-cell marker and IFNγ transcripts

The transcript levels of viral RNAs, selected T-cell marker and cytokine genes, toll like receptor (TLR) 7, and two interferon stimulated genes (ISG) were analysed in sexually immature adult Atlantic halibut (Hippoglossus hippoglossus L.) experimentally infected with nodavirus. The expression of the T-cell markers, TLR7 and the cytokine genes was further explored in in vitro stimulated anterior kidney leucocytes (AK leucocytes) isolated from the experiment fish and from additional untreated non-injected fish. The levels of viral RNA1 and RNA2 were increasing in brain and eye at around 4 and 8weeks post injection (wpi), respectively, and still increasing at the end of the experiment, especially in eye. Immuno-positive cells and signs of vacuolisation in both brain and eye were seen at 14wpi. Increased transcript levels of TCRβ, CD4-2, CD4, CD8α, and Lck in brain and eye of the experimentally infected halibut suggested an involvement of halibut T-cells in the immune response against nodavirus. Interestingly, a similar expression pattern of TCRβ, CD4 and Lck was seen in both brain and eye. However, compared to brain that showed elevated transcript levels of TCRβ, CD4 and Lck mainly at 10 and 14wpi, the increase appeared earlier between 3 and 4wpi in the eye. Yet, an increase in the transcript level of IFNγ was seen at 10 and 14wpi in both organs. Moreover, elevated levels of TLR7, IL-1β, IL-6, ISG15 and Mx were detected in vivo. The in vitro experiments, stimulating AK leucocytes with ConA-PMA, imiquimod or nodavirus, further supported an involvement of IL-6 and IFNγ in the immune response against nodavirus and the involvement of CD8β(+) cells. Results from the present study thus indicate an importance of T-cells, IFNγ and the analysed ISGs in the immune response against nodavirus in Atlantic halibut, and would be of great help in future vaccination trials giving the possibility to monitor the immune response rather than mortality during post-vaccination challenge experiments.