In vitro activation of rainbow trout macrophages stimulates inhibition of Renibacterium salmoninarum growth concomitant with augmented generation of respiratory burst products

Rainbow trout head kidney macrophages pretreated for 48 h with a supernatant which contained macrophage activating factor (MAF) obtained from Concanavalin A/phorbol myristate acetate stimulated rainbow trout leucocytes express an enhanced ability to inhibit Renibacterium salmoninarum growth in vitro compared with untreated controls. Enhanced 'killing' capacity of these macrophages was detectable after 24 h exposure to R. salmoninarum in vitro. Addition of N ^G -methyl-L-arginine in vitro did not diminish bacterial growth inhibition by MAF-activated macrophages, suggesting that nitric oxide radicals were not involved in this event. However, MAF-treated macrophages expressed enhanced respiratory burst activity compared with control macrophages after a 3 d exposure to R. salmoninarum in vitro. In addition, it was demonstrated that R. salmoninarum was susceptible to hydrogen peroxide (H ₂ O ₂ ) in vitro using a cell-free assay. Introduction of catalase to the in vitro macrophage assay abrogated the 'killing' activity, suggesting that H ₂ O ₂ release from activated macrophages was involved in the inhibition of R. salmoninarum growth. The results are discussed in relation to other intracellular pathogens and the possible importance of cell-mediated immunity for protection against bacterial kidney disease.