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

Abstract

The present paper describes the effect of lipopolysaccharides (LPS) from Aeromonas 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 H 2O 2-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.