Lipopolysaccharides (LPS, endotoxin) stimulate mononuclear cells to release cytokines which initiate endotoxic effects. Interaction of LPS at low concentrations with target cells is CD14-independent whereas at high LPS concentrations it is CD14-independent. Here, we demonstrate by resonance energy transfer (RET) technique that nonspecific, CD14-independent intercalation of LPS into membrane systems can be mediated by lipopolysaccharide-binding protein (LBP). It is proposed that in this pathway, LBP breaks down LPS aggregates, transports the smaller units to and inserts them into the phospholipid cell matrix. We furthermore show that LBP also mediates the intercalation of other negatively charged amphiphilic molecules. We propose a model explaining CD14-independent cell activation at high endotoxin concentrations.
