had greater heparin binding capacities in vitro and were cleared more rapidly from the plasma of whole animals. Taken together, these data better define how closely related proteins such as BPI and LBP can have opposing effects on the body's response to LPS.
Bacterial endotoxin or lipopolysaccharide (LPS),1 a major component of the outer membrane of Gram-negative bacteria, is a potent mediator of the inflammatory response. Because Gram-negative sepsis remains one of the primary causes of severe systemic inflammation in hospitalized and immunocompromised patients, there is great interest in characterizing proteins involved in the biological response to LPS. In this paper, we focus on two LPS-binding proteins, lipopolysaccharide-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI). LBP and BPI are members of a family of lipid transfer/lipopolysaccharide-binding proteins that also includes cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP). These proteins share significant sequence homology and all bind lipophilic substrates (1).Involved in a complex array of responses to LPS, LBP is a 60-kDA serum glycoprotein that binds the lipid A portion of the LPS molecule to form a high affinity LBP⅐LPS complex (2). This complex potentiates the cellular response to LPS via interaction with the monocytic differentiation antigen CD14 (3, 4). LPS can be transferred from LBP to CD14 (3, 4), present as either a membrane-bound protein on myeloid cells or a soluble serum protein that interacts with endothelial and some epithelial cell lines to elicit an inflammatory response. Recent evidence suggests that LBP may additionally be involved in the neutralization of LPS via interaction with serum lipoproteins (5, 6) or through the internalization of a LBP⅐LPS⅐CD14 complex by neutrophils (7).BPI is a 55-kDa protein found in granules of mature neutrophils and, like LBP, interacts with LPS to form a high affinity complex. BPI, however, binds LPS with higher affinity than does LBP (8 -10), and BPI⅐LPS complexes do not stimulate monocytes or endothelial cells (11-13). The binding of LPS by BPI also prevents binding to LBP, neutralizing the inflammatory activity of LPS (14 -16). BPI and its recombinant N-terminal fragments have been demonstrated to provide protection against challenge with bacteria or purified bacterial endotoxin in several animal models (17-22) and, more recently, in human clinical trials (23). Additionally, Rogy et al. (22) have tested a protein chimera of BPI and LBP, NCY103, in an endotoxin challenge model in baboons.Both BPI and LBP contain 456 amino acids and show an approximate 45% homology at the amino acid level which is distributed over the entire protein sequence. Interestingly, the genes for BPI and LBP lie adjacent to each other in the human genome, suggesting that they might have arisen from a gene duplication event (24). LPS binding is a property of the Nterminal half of both LBP and BPI (9,16,[25][26][27], and a proteolytic N-terminal fragment of BPI (26...
