Differential Immune Gene Expression in Rainbow Trout, Oncorhynchus Mykiss (walbaum), Exposed to Five Pathogens: Aeromonas Salmonicida, Flavobacterium Psychrophilum, Vibrio Anguillarum, Yersinia Ruckeri and Ichthyophthirius Multifiliis Sincerely, Per Kania Associate Professor PH.D., M.Sc. University of Copenhagen Laboratory of Aquatic Pathobiology DIR +45 35 33 27 40 MOB +45 93565317

Rainbow trout, Oncorhynchus mykiss, originating from the same outbred population and reared under the same conditions, were bath exposed to five different pathogens (Aeromonas salmonicida, Flavobacterium psychrophilum, Vibrio anguillarum, Yersinia ruckeri, Ichthyophthirius multifiliis). The exposures were conducted as independent experiments with non-exposed fish as time-point controls. At peak of infections, fish showing significant clinical signs, fish without clinical signs, and time-point control fish were euthanized, and gills, spleen, and liver were sampled for gene expression analysis. At the end of the infection course, i.e. a period without any disease signs, surviving fish were sampled as well. Genes encoding 14 cytokines, 5 immune cell markers, 5 immune effector molecules and 3 reference/housekeeping genes were analyzed. The host fish exhibited different reaction patterns dependent on the pathogen. Especially trout infected with I. multifiliis showed a characteristic Th2 expression signature in gills, whereas only innate responses occurred in internal organs. Rainbow trout exposed to F. psychrophilum displayed a relatively lower expression of immune genes contrasting the host responses to A. salmonicida, V. anguillarum and Y. ruckeri. The bacterial pathogens induced a relatively similar expression profile in internal organs of trout with a few exceptions. Thus, F. psychrophilum induced upregulation of genes earlier in gills than internal organs, and A. salmonicida infected fish showed a notable downregulation of immune genes in gills. The differential expression patterns may reflect the different invasion mechanisms and target tissues associated with the five pathogens.