BACKGROUND: Melioidosis, a lethal tropical infection that is endemic in 
southeast Asia and northern Australia, is caused by the saprophytic 
Gram-negative bacterium Burkholderia pseudomallei. Overall mortality approaches 
40% yet little is known about mechanisms of host defense. Toll-like receptors 
(TLRs) are host transmembrane receptors that recognize conserved pathogen 
molecular patterns and induce an inflammatory response. The lipopolysaccharide 
(LPS) of Gram-negative bacteria is a potent inducer of the host innate immune 
system. TLR4, in association with MD-2, is the archetype receptor for LPS 
although B. pseudomallei LPS has been previously identified as a TLR2 agonist. 
We examined TLR signaling induced by B. pseudomallei, B. pseudomallei LPS, and 
B. pseudomallei lipid A using gain-of-function transfection assays of NF-kappaB 
activation and studies of TLR-deficient macrophages.
RESULTS: In HEK293 cells transfected with murine or human TLRs, CD14, and MD-2, 
heat-killed B. pseudomallei activated TLR2 (in combination with TLR1 or TLR6) 
and TLR4. B. pseudomallei LPS and lipid A activated TLR4 and this TLR4-mediated 
signaling required MD-2. In TLR2-/- macrophages, stimulation with heat-killed B. 
pseudomallei augmented TNF-alpha and MIP-2 production whereas in TLR4-/- cells, 
TNF-alpha, MIP-2, and IL-10 production was reduced. Cytokine production by 
macrophages stimulated with B. pseudomallei LPS or lipid A was entirely 
dependent on TLR4 but was increased in the absence of TLR2. TLR adaptor molecule 
MyD88 strongly regulated TNF-alpha production in response to heat-killed B. 
pseudomallei.
CONCLUSION: B. pseudomallei activates TLR2 and TLR4. In the presence of MD-2, B. 
pseudomallei LPS and lipid A are TLR4 ligands. Although the macrophage cytokine 
response to B. pseudomallei LPS or lipid A is completely dependent on TLR4, in 
TLR2-/- macrophages stimulated with B. pseudomallei, B. pseudomallei LPS or 
lipid A, cytokine production is augmented. Other MyD88-dependent signaling 
pathways may also be important in the host response to B. pseudomallei 
infection. These findings provide new insights into critical mechanisms of host 
defense in melioidosis.