Abstract
The immune response and morphological changes in the gills of rainbow trout fry after immersion in hydrogen peroxide ( H 2 O 2 ), F lavobacterium psychrophilum or combined exposure were examined. The gills were sampled 4, 48, 125 and 192 h after exposure, and the regulation of expression of the following genes was investigated using q PCR : I g T , I g M , CD 8, CD 4, MHC I, MHC II, IL ‐4/13A, T c R ‐β, IL ‐10, IL ‐1β, IL ‐17, SAA and F ox P 3. Bacteria were not observed in haematoxylin‐and‐eosin‐stained gill tissue, but the presence of F . psychrophilum 16 S r RNA was detected using q PCR . The 16 S r RNA levels were correlated with gene expression. Although pretreatment with H 2 O 2 before immersion in F . psychrophilum did not significantly alter the amount of bacteria found in the gill, the immune response was influenced: exposure to F . psychrophilum resulted in a negative correlation with expression of IL ‐17c1, MHC I and MHC II, while pretreatment with H 2 O 2 resulted in a positive correlation with IL ‐4/13 A and I g M . Exposure to either H 2 O 2 or F . psychrophilum influenced the regulation of gene expression and damaged tissue. Exposure to both combined altered the immune response to infection and postponed healing of gill tissue.