CD14 is well known as a receptor for LPS. However, the mechanism of signal transduction which LPS initiates upon binding to CD14 is not known. One theory is that CD14 presents LPS to another molecule which in turn initiates cellular activation [Ulevitch RJ, Tobias PS. Receptor-dependent mechanisms of cell stimulation by bacterial endotoxin. Annu Rev Immunol 1995; 13: 437]. In this context one may ask whether the postulated presentation function of CD14 is limited to simple concentration of LPS at the cell surface or whether CD14 performs some other more specific function. LPS binding protein (LBP) is able to bind and transfer LPS to a variety of acceptors [Wurfel MM, Wright SD. Lipopolysaccharide-binding protein and soluble CD14 transfer lipopolysaccharide to phospholipid bilayers: preferential interaction with particular classes of lipid. J Immunol 1997; 158: 3925] and, as such, is a candidate as an alternative to CD14 if the function of CD14 is simply to concentrate LPS at the cell surface. To address this, CHO and U373 cells were engineered to express a membrane anchored form of LBP (mLBP) for comparison with cells expressing membrane anchored CD14 (mCD14) as well as empty vector transfected cells. Activation of the cells by LPS was assessed using activation of NFκB (CHO and U373 cells), and surface expression of ICAM and secretion of IL-6 (U373 cells). Despite the observation that LBP did concentrate LPS at the cell surface, as assessed by binding of either fluorescein or tritium labeled LPS, the activation of LBP expressing cells was several orders of magnitude less efficient than activation of the mCD14 expressing cells and only minimally more efficient than activation of empty vector transfected cells. This slight effect is likely to be due to the presence of a small amount of soluble CD14 (sCD14) and the ability of mLBP to foster formation of LPS-sCD14 complexes which are agonistic for these cells. Thus LBP clearly cannot act as an efficient surrogate for CD14, and the function of CD14 is more complicated than the simple concentration of LPS at the cell surface.
