Francisella tularensis is a virulent Gram-negative intracellular pathogen. To address the signaling routes involved in the response of host cells to LPS from F. tularensis live vaccine strain (LVS), experiments were performed in transiently transfected 293 cells. Induction of κB-driven transcriptional activity by 2.5 μg ml-1 F. tularensis LPS isolated by phenol-water and ether-water extraction, was observed in cells transfected with Toll-like receptor (TLR) 4 and MD-2, although CD14 was required for optimal induction. Conversely, TLR2, TLR2/TLR1 or TLR2/TLR6 transfected cells did not show κB-driven transcriptional activity in the presence of F. tularensis LPS. In human monocytic cells, F. tularensis LPS activated extracellular signal-regulated kinases and the production of pro-inflammatory proteins. Concentrations of 5-10 μg ml-1 F. tularensis LPS elicited a similar pattern of mRNA and protein induction than 0.1 μg ml-1 E. coli LPS, including the expression of CXC chemokines (IL-8, Gro and IFN-γ-inducible protein-10); CC chemokines (monocyte chemoattractant protein-1 and -2, macrophage-derived chemoattractant, macrophage inflammatory protein-1α and -1β and RANTES (regulated upon activation, normal T cell expressed and secreted) and pro-inflammatory cytokines (IL-6 and tumor necrosis factor α). Altogether, these data indicate that LPS from F. tularensis LVS signals via TLR4 at higher concentrations than those required for E. coli LPS, which may explain the inflammatory reaction and the low endotoxic response associated to vaccination with LVS in humans. © 2006 Oxford University Press.

Supported by the Intramural Research Program at the National Institutes of Health (NIH), NHLBI, and National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

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