Competition between Bactericidal/Permeability-Increasing Protein and Lipopolysaccharide-Binding Protein for Lipopolysaccharide Binding to Monocytes

Abstract

The bactericidal/permeability-increasing protein (BPI) inhibits the lipopolysaccharide (LPS)-mediated activation of monocytes. Due to its inhibitory activity for various LPS, BPI has therapeutic potential in endotoxic shock. To be efficient in vivo, BPI should overcome the action of LPS-binding protein (LBP), a serum molecule that increases the expression of LPS-inducible genes via CD 14 of monocytes. rBPI 23 , a recombinant fragment of BPI, prevented in a dose-dependent manner the binding and the internalization of LPS mediated by LBP. Consequently, rBPI 23 also inhibited LPS-induced tumor necrosis factor (TN Fa) synthesis from monocytes. LPS-and LBP-mediated activation of monocytes was totally inhibited when LPS was preincubated with rBPI 23 • Adding rBPI 23 at the same time as LBP resulted in an important but partial inhibition of TNFa release, but this inhibition vanished with delaying the time of addition of rBPI 23 • These studies suggest that the inhibitory activity of BPI is related to its ability to compete with LBP for LPS.Living organisms have developed several protective mechanisms against toxic lipopolysaccharide (LPS) of gram-negative bacteria. Very recently, two members ofa family of proteins have been recognized that possess LPS-binding sites. Indeed, the LPS-binding protein (LBP) and the bactericidalf permeability-increasing protein (BPI) share the ability to bind to smooth and rough LPS as well as to lipid A [1, 2]. There is a striking homology in the DNA sequence of the two human, rabbit, or bovine proteins [3]. However, despite their structural similarities, the functions of LBP and BPI appear an tagonistic.Most circulating LPS binds to plasma LBP, and LPS linked to LBP is transferred to CD 14 on the surface of cells of the monocytic lineage and neutrophils, resulting in increased expression of LPS-inducible genes [3,4]. LBP-mediated processes include increased tumor necrosis factor-a (TNFa) production by monocytes [3][4][5], accelerated priming of neutrophils for the oxidative response to FMLP [6], and increased adhesive capacity of complement receptor CR3 of neutrophils [7]. LBP was originally described as an acutephase reactant in the rabbit [8] . Soluble BPI, recombinant BPI, or its 25-kDa N-terminal fragment have been shown to inhibit endotoxin activity by neutralizing LPS, as detected by the inhibition of the proteolytic cascade in the limulus amebocyte lysate assay, by the inhibition of the priming of neutrophils by LPS, and by the inhibition of LPS-mediated production of TNFa by monocytes [11][12][13]. Because of the apparent antagonistic properties of these two proteins that bind LPS, it is important to better define the respective roles of LBP and BPI in the LPS-induced activation process, inasmuch as BPI has therapeutic potential in endotoxic shock. We characterized in vitro the direct competition between LBP and BPI for binding ofLPS to cells of the monocytic lineage and investigated the competition between these two molecules in the activation process of monocytes resulting in TN...

The bactericidal/permeability-increasing protein (BPI) in infection and inflammatory disease

“…In contrast to the effect of binding of LBP to endotoxin, binding of BPI does not lead to extraction and transfer of endotoxin to CD14 and thus can preclude MD-2/TLR4-dependent cell activation by endotoxin [51,66]. This difference reflects differences in the structural and functional properties of the C-terminal domain of LBP and of BPI [48,51,81,107]. Additional differences in the structural and functional properties of the N-terminal domain of BPI and of LBP account for the higher affinity of BPI (vs. LBP) for purified or outer membrane-inserted endotoxin [9,52,53].…”

“…Tissue-derived BPI has been found on apical and basolateral epithelial surfaces [38,40,42] and on dermal fibroblasts [39] derived from subcutaneous connective tissue, however, its quantitative contribution to the initial host interactions with GNB in tissue are as yet unknown. The presence of BPI at these sites could enhance the ability of local defenses to eliminate small amounts of invading bacteria and also diminish the availability of endotoxin for stimulation of sub-epithelial host cells via the LBP/CD14/MD-2/TLR4 pathway [80][81][82]. Under these circumstances, incidental incursions of GNB may be resolved locally without a significant inflammatory response and little or no influx of neutrophils.…”

“…Additional differences in the structural and functional properties of the N-terminal domain of BPI and of LBP account for the higher affinity of BPI (vs. LBP) for purified or outer membrane-inserted endotoxin [9,52,53]. Thus, BPI can effectively compete with LBP for binding to endotoxin and thus inhibit LBP-dependent (and -independent) extraction and transfer of endotoxin to CD14/ MD-2/TLR4 [81,82].…”

“…Single endotoxin molecules from OM blebs or other endotoxin containing remnants of GNB can be extracted and processed by the LBP/ CD14/MD-2/TLR-4 pathway resulting an activation of CD-14 positive monocytes. In contrast, BPI bound endotoxin is not accessible for LBP/CD14 mediated LOS/LPS extraction and does not cause an inflammatory response in monocytes [81,82,107]. Shown is the prevalence of BPI-ANCA in selected studies of healthy persons and patients with infectious diseases, rheumatic diseases, inflammatory bowel diseases and immunodeficiency [17,18,115,118,119,126].…”

An opsonic function of the neutrophil bactericidal/permeability-increasing protein depends on both its N- and C-terminal domains

“…In the dose range tested, LBP͞BPI was also unable to promote uptake of K1͞r, suggesting that both the N-and C-terminal domains derived from BPI were needed for opsonic activity toward K1͞r. This finding may reflect the lower affinity of the N-terminal region of LBP for LPS (18,19), producing insufficient bacterial binding for the BPI-derived C-terminal region to mediate an opsonic effect.…”