Francisella tularensis inhibits Toll-like receptor-mediated activation of intracellular signalling and secretion of TNF-α and IL-1 from murine macrophages

Microbial ligands, including lipopolysaccharide (LPS) and bacterial lipoproteins, activate Toll-like receptors (TLR) of mononuclear phagocytes, thereby inducing proinflammatory cytokines and antimicrobial activity. We show that Francisella tularensis , an intracellular pathogen, is capable of inhibiting this macrophage response. Infection with the live vaccine strain F. tularensis LVS rendered cells of the murine macrophage-like cell line J774A.1 incapable of secreting TNF-alpha or IL-1beta and mobilizing an antimicrobial activity in response to bacterial lipopeptide or Escherichia coli-derived LPS. Inhibition of TNF-alpha secretion occurred also when J774 cells were infected with F. tularensis LVS in the presence of chloramphenicol, but not when they were infected with a mutant of F. tularensis LVS defective in expression of a 23 kDa protein that is upregulated during intracellular infection. Purified F. tularensis LPS did not show an agonistic or antagonistic effect on the E. coli LPS-induced activation of the J774 cells. Francisella tularensis LVS suppressed the capability of the cells to respond to LPS or bacterial lipopeptide (BLP) with activation of nuclear factor kappa B (NF-kappaB), and degradation of the in-hibitor of NF-kappaB, IkappaB, was blocked during the infection. Also the LPS- or BLP-induced phosphorylation of the mitogen-activated protein kinase p38 and the transcription factor c-Jun was inhibited by F. tularensis LVS but not by the 23 kDa protein mutant. In conclusion, F. tularensis appears capable of abrogating the TNF-alpha and IL-1 responses of macrophages induced by E. coli LPS or BLP via a mechanism that involves suppression of several intracellular pathways and is dependent on expression of a bacterial 23 kDa protein.