Quantitative expression (Walbaum) of immunological factors in rainbow trout, Oncorhynchus mykiss (Walbaum), after infection with either Flavobacterium psychrophilum, Aeromonas salmonicida, or infectious haematopoietic necrosis virus

To further enhance our understanding of immunological gene expression in rainbow trout, Oncorhynchus mykiss, after infection with naturally occurring pathogens, a series of probes and primers were developed for the quantification of immune factors. Separate groups of specific-pathogen-free rainbow trout were infected with either Flavobacterium psychrophilum, Aeromonas salmonicida or infectious haematopoietic necrosis virus (IHNV). Three different concentrations of each pathogen were used and samples from infected and mock-infected fish were taken at either 1 or 5 days after infection. Ten fish were sampled at each time point for individual sections of liver, spleen and head kidney. Organ specimens from five of the fish were used to re-isolate and quantify the pathogen at the time the samples were taken. Total RNA was extracted from the organs of the remaining five animals. Using real-time polymerase chain reaction with fluorescent-labelled probes, the RNA from these organs was examined for the level of expression of the following immunological factors; an interferon related protein (MX-1), interleukin-8 (IL-8), the cytotoxic T-cell marker CD-8 and complement factor C3 (C3). They were also measured for the level of beta-actin, which was used as a standardization control for cellular RNA expression. Infection with IHNV produced the greatest change in expression level for all the immunological related factors examined in this study. IHNV elicited the best dose response profile, which was typically seen at 5-days post-infection for MX-1, C3, IL-8 and CD-8. Infection with A. salmonicida and F. psychrophilum showed elevated, but variable expression levels for several of the genes tested.