Robert S. Munford

Although animals mobilize their innate defenses against gram-negative bacteria when they sense the lipid A moiety of bacterial lipopolysaccharide (LPS), excessive responses to this conserved bacterial molecule can be harmful. Of the known ways for decreasing the stimulatory potency of LPS in blood, the binding and neutralization of LPS by plasma lipoproteins is most prominent. The mechanisms by which host lipoproteins take up the native LPS that is found in bacterial membranes are poorly understood, however, since almost all studies of host-LPS interactions have used purified LPS aggregates. Using nativeSalmonella entericaserovar Typhimurium outer membrane fragments (blebs) that contained³H-labeled lipopolysaccharide (LPS) and³⁵S-labeled protein, we found that two human plasma proteins, LPS-binding protein (LBP) and phospholipid transfer protein (PLTP), can extract [³H]LPS from bacterial membranes and transfer it to human high-density lipoproteins (HDL). Soluble CD14 (sCD14) did not release LPS from blebs yet could facilitate LBP-mediated LPS transfer to HDL. LBP, but not PLTP, also promoted the activation of human monocytes by bleb-derived LPS. Whereas depleting or neutralizing LBP significantly reduced LPS transfer from blebs to lipoproteins in normal human serum, neutralizing serum PLTP had no demonstrable effect. Of the known lipid transfer proteins, LBP is thus most able to transfer LPS from bacterial membranes to the lipoproteins in normal human serum.

Human acyloxyacyl hydrolase is an unusual lipase, found in phagocytic cells, that removes acyl chains from bacterial lipopolysaccharides (LPS) and glycerolipids. It is a heterodimer in which two glycosylated peptides are linked by disulfide bonding. The large subunit contains the active site serine, while the smaller subunit has striking sequence similarity to the saposins, peptide cofactors for several sphingolipid hydrolases. Since rabbits and mice are widely used for studies of LPS—animal interactions, we asked if murine and lapine AOAHs resemble the human enzyme. We report here that murine and lapine AOAHs share the distinctive features of the human AOAH primary sequence and have similar affinity for LPS. The structure of this unusual lipase appears to have been highly conserved.