Transcriptomic responses in rainbow trout gills upon infection with viral hemorrhagic septicemia virus (VHSV)

It has been previously demonstrated that even though the fin bases constitute the main portal of entry of viral hemorrhagic septicemia virus (VHSV) in rainbow trout (Oncorhynchus mykiss), an important number of chemokine genes are up-regulated in the gills upon bath exposure to the virus. Because chemokines mediate the recruitment of leukocytes through the action of specific chemokine receptors, in the current study, we have studied the transcription of several immune genes in response to a VHSV bath infection in the gills, focusing both on chemokine receptor genes and on genes characteristic of distinct leukocyte populations such as IgM, IgD, IgT, CD4, CD8, perforin and MHC-II. We have studied the response to the virus in naïve fish as well as in fish that had been previously intramuscularly (i.m.) injected with a VHSV DNA vaccine. Additionally, we have sorted both IgM+ and CD8+ cells from the gills of naïve and infected animals to study some of these up-regulated genes in specific leukocyte populations. Our results indicate that despite the low replication level, VHSV provokes an up-regulation of IgM, IgT, CD3 and perforin transcription together with the up-regulation of CCR7, CCR9, CXCR3B and CXCR4 mRNA levels. Interestingly, MHC-II mRNA was up-regulated and CCR7 was down-modulated in IgM+ cells from infected gills, whereas perforin, CCR7 and CXCR4 mRNA levels were higher in sorted CD8+ cells from infected animals. Surprisingly, when fish had been previously injected with either the empty plasmid or the VHSV DNA vaccine, these up-regulations in immune gene transcription were no longer observed. Our results point to the gills as an important site for innate and acquired viral defense.