Binding of Legionella pneumophila to macrophages increases cellular cytokine mRNA

Abstract

Infection of macrophages with Legionella pneumophila induces formation of interleukin 1β (IL-1β), but the molecular basis of this is not understood. Binding of bacteria to macrophage surfaces is the first step in an infection process. Therefore, we examined whether this step was sufficient to increase the cellular level of mRNAs for IL-1β and other cytokines. To assess the effect of binding of L. pneumophila on the steady-state levels of cytokine mRNAs, cultures of thioglycolate-elicited macrophages from L. pneumophila- susceptible A/J mice were treated with cytochalasin D and infected with L. pneumophila and the total RNA was extracted for analysis by reverse transcription-PCR with primers for IL-1α, IL-1β, IL-6, tumor necrosis factor alpha, granulocyte macrophage colony-stimulating factor, and beta interferon (IFN-β). L. pneumophila treatment increased the cellular steady- state mRNA levels of all cytokines except IFN-β. To determine the specificity of this effect, macrophage cultures were treated with cytochalasin D and either bacterial lipopolysaccharide, bovine serum albumin- sensitized latex, Salmonella typhimurium, or Escherichia coli. Lipopolysaccharide treatment increased all mRNAs, bovine serum albumin- sensitized latex had no significant effect, and treatment with S. typhimurium or E. coli increased all mRNAs except that of IFN-β. These results suggested that the binding of gram-negative bacteria to the macrophage surface was sufficient to induce a unique pattern of cytokine mRNAs. Additional studies that examined the characteristics of the bacterial ligands involved indicated involvement of both heat-labile and heat-stable surface ligands.