MD-2, a novel accessory molecule, is involved in species-specific actions of Salmonella lipid A.

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 (mCD14), 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 (compound 516). 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 (compound 506) were not affected. These results indicate that both TLR4 and MD-2 participate in the species-specific action of Salmonella lipid A.