The gills are an important site of iNOS expression in rainbow trout Oncorhynchus mykiss after challenge with the Gram-positive pathogen Renibacteriumsalmoninarum

Following injection challenge of rainbow trout with the Gram-positive patho gen Renibacterium salmoninarum?, serum nitrate levels increased indicative of NO production. The timing and amount of nitrate produced varied with the virulence of the bacterial strain used, with the highest levels seen in fi sh challenged with the most virulent (autoaggregating) strain. Immunization with a killed R. salmoninarum preparation in Freund's incomplete adjuvant significantly increased nitrate levels after challenge. Inducible nitric ox ide synthase (iNOS) transcript expression was detectable in rainbow trout t issues after injection challenge with R salmoninarum, and its induction in the gills was both quick (between 3 and 6 hr) and relatively prolonged (las ting several days). iNOS expression in the kidney was also seen at a later stage (24 hr) but appeared to switch off relatively rapidly. Bath challenge with R salmoninarum also induced iNOS expression in gill, and a variable e xpression in the gut and kidney also occurred. These results highlight the importance of the gills, not only as a point of entry of pathogens but also as a tissue capable of mounting an immune response.