Difference in Legionella pneumophila growth permissiveness between J774.1 murine macrophage-like JA-4 cells and lipopolysaccharide (LPS)-resistant mutant cells, LPS1916, after stimulation with LPS.

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To elucidate the role of the oxidative burst in macrophage resistance to Legionella infection, we examined a murine macrophage-like cell line, J774.1, for permissiveness to Legionella growth, using a mutant that has a selective defect in the oxidative burst after lipopolysaccharide (LPS) stimulation. Legionella pneumophila serogroup 1 was infected into J774.1 monolayers, and then the extent of bacterial growth was estimated by a CFU assay. Both the parental cell line, JA-4, and the LPS-resistant mutant, LPS1916, were permissive for Legionella growth but became nonpermissive after pretreatment with gamma interferon. However, pretreatment of LPS1916 cells with LPS failed to inhibit bacterial growth, although LPS-treated JA-4 cells exhibited inhibited multiplication of the bacteria. The bacterial growth inhibition in JA-4 and mutant LPS1916 cells was correlated with the extent of the oxidative burst in the cells, as judged by cytochrome c reduction but not nitrite production. Neither transferrin receptor expression nor the iron content in JA-4 and LPS1916 cells, with or without LPS treatment, was correlated with suppression of Legionella growth. These results suggest that the restriction of Legionella growth in J774.1 cells is due to a bactericidal effect of the oxidative burst rather than reduction of the iron supply to the intracellular bacteria and that the effectors are reactive oxygen intermediates and not reactive nitrogen intermediates.

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