Background: Listeria monocytogenes tends to survive in phagocytes. Granulocyte macrophage colony-stimulating factor (GM-CSF) protects mice against L. monocytogenes infection, and mice knocked out for the GM-CSF gene are more susceptible to these infections.
Methods: THP-1 cells were used to characterize the GM-CSF receptor (binding isotherms; STAT5 phosphorylation), measure the intracellular growth of L. monocytogenes (5 h after phagocytosis), examine the influence of a 24-h incubation with GM-CSF before infection, measure the production of tumor necrosis factor (TNF)-alpha and the expression of nitric oxide synthase (iNOS), and evaluate the influence of anti-GM-CSF receptor (GM-CSFR alpha ) and anti-TNF-alpha antibodies and the addition of N(omega)-nitro-L-arginine methyl ester (L-NAME) and catalase.
Results: THP-1 cells display functional GM-CSFR alpha. GM-CSF impairs the intracellular growth of L. monocytogenes to approximately 65% of its value in unstimulated cells. This effect is abolished by anti-GM-CSFR alpha, anti-TNF-alpha antibodies, and catalase (and, to a lesser extent, by L-NAME). GM-CSF stimulates the release of TNF-alpha and the expression of iNOS. TNF-alpha added to unstimulated cells (even in large amounts) does not fully reproduce the impairment in the growth of L. monocytogenes caused by GM-CSF.
Conclusions: GM-CSF impairs the intracellular growth of L. monocytogenes by a synergistic action of the GM-CSF-triggered release of autocrine TNF-alpha and hydrogen peroxide and the production of NO (associated with the stimulation of the expression of iNOS).