Targeted deletion of the lipopolysaccharide (LPS)-binding protein gene leads to profound suppression of LPS responses ex vivo, whereas in vivo responses remain intact.
Bottom Line: These activities were restored by the addition of exogenous recombinant murine LBP to the plasma.Despite these striking in vitro findings, no significant differences in TNF-alpha levels were observed in plasma from wild-type and LBP-deficient mice injected with LPS.These data suggest the presence of an LBP-independent mechanism for responding to LPS.
Affiliation: The Rockefeller University, New York 10021, USA.
Gram-negative bacterial lipopolysaccharide (LPS) stimulates phagocytic leukocytes by interacting with the cell surface protein CD14. Cellular responses to LPS are markedly potentiated by the LPS-binding protein (LBP), a lipid-transfer protein that binds LPS aggregates and transfers LPS monomers to CD14. LBP also transfers LPS to lipoproteins, thereby promoting the neutralization of LPS. LBP present in normal plasma has been shown to enhance the LPS responsiveness of cells in vitro. The role of LBP in promoting LPS responsiveness in vivo was tested in LBP-deficient mice produced by gene targeting in embryonic stem cells. Whole blood from LBP-deficient animals was 1,000-fold less responsive to LPS as assessed by the release of tumor necrosis factor (TNF)-alpha. Blood from gene-targeted mice was devoid of immunoreactive LBP, essentially incapable of transferring LPS to CD14 in vitro, and failed to support cellular responses to LPS. These activities were restored by the addition of exogenous recombinant murine LBP to the plasma. Despite these striking in vitro findings, no significant differences in TNF-alpha levels were observed in plasma from wild-type and LBP-deficient mice injected with LPS. These data suggest the presence of an LBP-independent mechanism for responding to LPS. These LBP knockout mice may provide a tool for discovering the nature of the presumed second mechanism for transferring LPS to responsive cells.
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Mentions: To determine if blood contained a functional mutant LBP molecule, we analyzed plasma from KO mice for LBP-like activity. The ability of LBP to bind LPS and to form a stable complex has been well described (9, 16). We incubated plasma from the KO mouse with LPS-coated plastic, washed the wells with buffer, and developed the plate with a polyclonal antiserum against murine LBP. Although hemizygous mice expressed wild-type levels of LBP, their homozygous gene-targeted littermates had no detectable LBP (Fig. 4 a).