Stimulation of respiratory burst and phagocytic activity in Atlantic salmon (Salmo salar L.) macrophages by lipopolysaccharide

The present paper describes the effect of lipopolysaccharides (LPS) fromAeromonas salmonicida and other Gram-negative bacteria on the respiratory burst, phagocytosis and bactericidal activity of head kidney macrophages from Atlantic salmon (Salmo salar L.) in vitro. Macrophages were first cultured in the presence of various concentrations of LPS from A. salmonicida for 1, 2 and 5 days and then tested for respiratory burst activity (reduction of nitroblue tetrazolium) after exposure to phorbol myristate acetate (PMA). The most marked increase in respiratory burst activity of LPS-treated macrophages was observed after 5 days of incubation with 1, 10 and 100 μg LPS ml −1. The increase appeared to be dose-dependent with a maximal response at 10 μg ml−1. At this LPS-concentration and incubation time the respiratory burst activity was 3·9 times larger in the treated macrophages than in the control macrophages. LPS from three other Gram-negative bacterial salmon pathogens and two non-fish pathogens also enhanced the respiratory burst activity of salmon macrophages. Macrophages incubated with 10 and 50 μg LPS ml −1 also showed a significant increase in PMA-stimulated H2O2-production after 5 days of incubation. LPS also stimulated the phagocytic activity of Atlantic salmon macrophages against opsonized and nonopsonized glucan particles, and glutaraldehyde-fixed sheep red blood cells. LPS-treated macrophages showed an increased ability to kill an avirulent A-layer lacking strain of A. salmonicida, but not a virulent A-layer positive strain.