COMPETITION BETWEEN RBPI(23), A RECOMBINANT FRAGMENT OF BACTERICIDAL/PERMEABILITY-INCREASING PROTEIN, AND LIPOPOLYSACCHARIDE (LPS)-BINDING PROTEIN FOR BINDING TO LPS AND GRAM-NEGATIVE BACTERIA

Lipopolysaccharide (LPS)-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI) are two structurally related lipid A-binding proteins with divergent functional activities. LBP mediates activation of macrophage and other proinflammatory cells. In contrast, BPI has potent bactericidal and LPS-neutralizing activities. A recombinant fragment of BPI (rBPI(23)) retains the potent biological activities of the holo protein and may represent a novel therapeutic agent for the treatment of gram-negative infections, sepsis, and endotoxemia. For therapeutic effectiveness in many clinical situations, rBPI(23) will have to successfully compete with high serum levels of LBP for binding to endotoxin and gram-negative bacteria. The relative binding affinities of rBPI(23) and human recombinant LBP (rLBP) for lipid A and gram-negative bacteria were evaluated. The binding of both proteins to lipid A was specific and saturable with apparent K(d)s of 2.6 nM for rBPI(23) and 58 nM for rLBP. rBPI(23) was approximately 75-foId more potent than rLBP in inhibiting the binding of I-125-rLBP to lipid A. The binding affinity of rBPI(23)(K-d = 70 nM) for Escherichia coli J5 bacteria was also significantly higher than that of rLBP (K-d = 1,050 nM). In addition, rBPI(23) at 0.2 mu g/ml was able to inhibit LPS-induced tumor necrosis factor release from monocytes in the presence of 20 mu g of rLBP per mi; These results demonstrate that rBPI,, binds more avidly to endotoxin than does rLBP and that, even in the presence of a 100-fold weight excess of rLBP, rBPI(23) effectively blocks the proinflammatory response of peripheral blood mononuclear cells to endotoxin.