Evidence of a trimolecular complex involving LPS, LPS binding protein and soluble CD14 as an effector of LPS response.

The kinetics of the interaction of lipopolysaccharide (LPS), lipopolysaccharide binding protein (LBP) and CD14 was studied using surface plasmon resonance. The association and dissociation rate constants for the binding of LPS and rsCD14 were 2.9 x 10(4) M(-1) s(-1) and 0.07 s(-1) respectively, yielding a binding constant of 4.2 x 10(5) M(-1). Significantly, the presence of LBP increased not only the association rate but also the association constant for the interaction between LPS and CD14 by three orders of magnitude. Our experimental results suggest that LBP interacts with LPS and CD14 to form a stable trimolecular complex that has significant functional implications as it allows monocytes to detect the presence of LPS at a concentration as low as 10 pg/ml or 2 pM, and to respond by secreting interleukin-6. Thus, LBP is not merely transferring LPS to CD14 but it forms an integral part of the LPS-rLBP-rsCD14 complex.