Immune response of the Atlantic halibut (Hippoglossus hippoglossusL.) to infectious pancreatic necrosis virus (IPNV)

Three groups of Atlantic halibut,Hippoglossus hippoglossusL. (mean weight 68 g, age 13–15 months), were bath-challenged with 4×107TCID50ml−1of infectious pancreatic necrosis virus (IPNV), the N1 strain of serotype Sp, for 1 h at 11–12°C. The groups had been previously immunised with either a recombinant IPNV vaccine, a control vaccine or Hankʹs Balanced Salt Solution. The challenge elicited a strong humoral immune response in all groups, but no mortality occurred due to virus exposure. As measured by ELISA, all groups showed a significant increase in IPNV-specific serum antibodies (Abs) starting between 2 and 4 weeks post-challenge (PC) with a peak at 12–14 weeks PC. IPNV titres in pooled kidney and liver samples showed a general decrease during the experiment, and virus-negative individuals appeared from 12 weeks PC. The virus neutralising capacity in serum increased in all groups during the experiment. On an individual level, there was no connection between a high humoral immune response, as measured by ELISA or virus neutralisation, and virus elimination. Western blot analysis of halibut immune sera revealed Abs to VP2 and pVP2 prior to challenge, and an appearance of Abs to VP3 during the experiment. Although all individuals tested before challenge were IPNV negative, high pre-challenge ELISA values together with the pVP2 and VP2 Abs detected pre-challenge suggested prior exposure to the virus. Vaccination did not appear to have any positive effect on the humoral immune parameters measured in this study.