| Research Abstract |
Salmonella lipid A is inactive in human macrophages despite being potently active in murine macrophages. We investigated the molecular basis for this species-specific action of Salmonella lipid A. When murine CD14(mCD 14), mTLR4,and mMD-2 were all expressed in human monocytic THP-1 cells, these cells were capable of responding to Salmonella lipid A. Expressing each of these proteins separately did not impart such responsiveness. Expression of mTLR4 plus mMD-2,but not mCD14 plus mTLR4 or mCD14 plus mMD-2,conferred this responsiveness. In THP-1 cells expressing mCD14,mTLR4,and mMD-2,replacing mCD14 with human CD14 had no effect on responsiveness to Salmonella lipid A or synthetic Salmonella-type lipid A. When mTLR4 was replaced with human TLR4,the responses to these lipid A preparations were decreased to half, and the replacement of mMD-2 decreased responsiveness to one-third, although the responses to Escherichia. coli lipid A or synthetic E. coli-type lipid A were not affected. These results indicate that both TLR4 and MD-2 participate in the species-specific action of Salmonella lipid A. Furthermore, we found that Salmonella lipid A is inactive in human macrophages, despite its LPS being highly active, and proved that the polysaccharide portion covalently bound to lipid A plays the principal role in Salmonella LPS-induced activation of NF-κB through human CD14/TLR4/MD-2. In addition CD14 is also found to contribute to the species specificity of LPS action. The C-terminal part of mouse CD14,which is not required for the LPS response in human cells, is required for ThR4-mediated activation of NF-κB, showing that it involves in species-specific action of Salmonella lipid A. Taken together, all these factors such as CD14,TLR4 and MD-2 play a role for the recognition of endotoxin molecule including polysaccharide portion.
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