Live Brucella spp. fail to induce tumor necrosis factor alpha excretion upon infection of U937-derived phagocytes.
Tumor necrosis factor alpha (TNF-alpha) plays a central role in activation of first-line defenses of a host against foreign organisms. To determine whether Brucella infection modulated TNF-alpha production, we measured the biological activity of this cytokine in supernatants of U937 cell-derived macrophages and of fresh human monocytes infected with Brucella spp. Neither the smooth nor rough Brucella strains used induced any measurable TNF-alpha excretion upon infection. On the contrary, as reported before for other gram-negative bacteria, phagocytosis of nonpathogenic Escherichia coli was followed by a rapid and transient induction of TNF-alpha release, suggesting an involvement of this cytokine in some autocrine process. As expected, the Brucella strains tested survived and/or multiplied within U937-derived macrophages, whereas E. coli was rapidly eliminated after phagocytosis. Immunoglobulin G opsonization of E. coli strains enhanced their intracellular killing and strongly potentiated TNF-alpha secretion. Immunoglobulin G opsonization of Brucella strains, in contrast, did not lead to TNF-alpha production, although their rate of intracellular multiplication was reduced. Killed brucellae, however, promoted a significant excretion of TNF-alpha from U937-derived macrophages into cell culture supernatants. We finally demonstrated that pretreatment of U937-derived macrophages with exogenous TNF-alpha significantly inhibited intracellular multiplication of Brucella spp. These results and experiments performed on fresh human monocytes or with isolated lipopolysaccharide (LPS) showed that (i) differences in TNF-alpha production observed during macrophage infection by Brucella spp. and E. coli were not due to differences in LPS structure but resulted from active inhibition of TNF-alpha production by a specific process linked to Brucella spp. and (ii) the capacity of Brucella spp. to use pathways avoiding TNF-alpha production during infection may be considered a major attribute of virulence.
Institut National de la Santé et de la Recherche Médicale U-65, Département Biologie-Santé, Université de Montpellier II, France.
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Caron
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Peyrard
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Köhler
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Cabane
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Liautard
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Dornand
D002002:Brucella / Q000276:immunology* / Q000472:pathogenicity; D002454:Cell Differentiation; D002478:Cells, Cultured; D004926:Escherichia coli / Q000276:immunology; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology; D007963:Leukocytes, Mononuclear / Q000276:immunology / Q000382:microbiology; D008264:Macrophages / Q000276:immunology / Q000382:microbiology; D009895:Opsonin Proteins / Q000276:immunology; D010586:Phagocytes / Q000276:immunology* / Q000382:microbiology; D013045:Species Specificity; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis*
D007074:Immunoglobulin G; D009895:Opsonin Proteins; D014409:Tumor Necrosis Factor-alpha