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The fish gills represent a crucial organ for the communication with the aquatic environment. Transcriptional changes in gills of two hatchery rainbow trout strains in response to injection with the potent pathogen Aeromonas salmonicida were detected by global gene expression profiling using a 4×44K oligonucleotide microarray. Emphasis was placed on 'day 3 postinfection' representing a decisive time point for the resolution of inflammation. The comparison of features and pathways differentially regulated in branchial tissues revealed that the local breeding strain BORN and imported American rainbow trout apply common and specific immune strategies. In gills of infected BORN trout, we observed a dynamic regulation of genes controlling NF-κB pathways and the induction of factors promoting the development of myeloid cells, whereas an increased expression of lysozyme and immunoglobulin genes was obvious in gills of infected import trout. In order to prove the relevance of the array-predicted candidates as well as well-known immune genes for gill immunity, a subsequent in vitro experiment was conducted. Altogether, we uncovered dynamic but moderate changes in the expression of a broad range of immune-relevant features implying the gill's involvement in pathogen defense strategies. [ABSTRACT FROM AUTHOR]