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.
Related in: MedlinePlus
Mentions: We predicted that the LBP KO mouse would be markedly hyporesponsive to the systemic administration of LPS. KO mice, representing F2 through F4 generations, were matched for age and sex and injected intravenously with graded concentrations of LPS. Surprisingly, the KO mice responded nearly identically to the hemizygous littermates over a wide range of doses (Fig. 6). Although at the lowest dose of LPS, a trend towards less response may have been seen in the KO animals, these doses were far lower than the dose of LPS needed to kill an animal (or even produce visible signs of morbidity). This was true regardless of the route of administration, as intraperitoneal injections of endotoxin produced similar results (data not shown). These results contrast dramatically with the differences observed ex vivo in which blood from the mutant LBP−/− mice was ∼1,000-fold less responsive than blood from hemizygous LBP+/− littermates.