Enhanced lysozyme production in Atlantic salmon (Salmo salar L.) macrophages treated with yeast β-glucan and bacterial lipopolysaccharide

Atlantic salmon head kidney macrophages grown in the presence of particulate yeast β-glucan and bacterial lipopolysaccharide (LPS) showed increased production of lysozyme in the culture supernatants compared to non-treated controls. The increased lysozyme production started at day 3 and was five- to six-fold higher compared to controls at day 6 in culture. β-Glucan showed an approximate linear dose–response curve between 1 and 250 μg ml−1whereas LPS showed a dose–response curve with a well-defined optimum concentration (10 μg ml−1). The increase in lysozyme activity was accompanied by an accumulation of lysozyme gene transcript in the stimulated cells. Recombinant human tumor necrosis factor alpha, known for its ability to stimulate lysozyme in human macrophages and to elevate respiratory burst activity of rainbow trout macrophages, failed to stimulate lysozyme production of Atlantic salmon macrophages. Macrophages isolated from fish suffering from a non-lethal Ichthyobodo necator infection displayed a highly increased ability to produce lysozyme in response to both β-glucan and LPS. As in higher vertebrates, lysozyme production may reflect the differentiation stage of the Atlantic salmon macrophages as well as a direct activation of lysozyme gene transcription by biological response modifiers. The rather late increase in lysozyme production induced by β-glucan and LPS may thus be explained by stimulation of differentiation of the macrophages in culture eventually combined with direct activation of transcription of the lysozyme gene.