Effect of early infectious salmon anaemia virus (ISAV) infection on expression of MHC pathway genes and type I and II interferon in Atlantic salmon (Salmo salar L.) tissues.

A number of viral diseases affecting teleost fish are characterized but few studies have addressed the effects of viral infection on gene expression in vivo. In this study, we investigated the effect of the early stages of infectious salmon anaemia virus (ISAV) infection on important components of the innate and adaptive immune response by monitoring expression of five genes in the MHC class I pathway, MHC class IIbeta, type I IFN-alpha, Mx, and type II IFN-gamma from cohabitant-infected Atlantic salmon tissues using quantitative real-time PCR. There was an increased expression of type I IFN-alpha in all tissues analyzed in response to infection that was proportional to viral load (relative to virus RNA levels) in gills and head kidney. Basal expression of IFN-gamma was modest or absent in all tissues, but expression was strongly induced and proportional to ISAV RNA levels in heart, spleen and head kidney. A 10-fold or higher level of virally induced IFN-alpha, in addition to significantly elevated levels of IFN-gamma, enhanced transcription of MHC class I pathway genes in heart, spleen and head kidney. In gills, the main entry site for ISAV, there was no induction of MHC class I pathway genes. MHC IIbeta and PSMB9 were not significantly induced in any tissue. Thus, by analysing various immune genes in a range of tissues from early cohabitant ISAV-infected salmon, we demonstrate that ISAV infection induced a rapid type I and II IFN response in the major infected lymphoid tissues, which was concurrent with induced expression of MHC class I pathway genes but not MHC IIbeta. This may suggest that CD8(+) T cell responses are more important than CD4(+) T cell responses during early ISAV viraemia.