TARGETED DELETION OF THE LIPOPOLYSACCHARIDE (LPS)-BINDING PROTEIN GENE LEADS TO PROFOUND SUPPRESSION OF LPS RESPONSES EX-VIVO, WHEREAS IN-VIVO RESPONSES REMAIN INTACT

Gram-negative bacterial lipopolysaccharide (LPS) stimulates phagocytic leukocytes by interacting with the cell surface protein CD14. Cellula r responses to LPS are markedly potentiated by the LPS-binding protein (LBP), a lipid-transfer protein that binds LPS aggregates and transfe rs LPS monomers to CD14. LBP also transfers LPS to lipoproteins, there by promoting tile neutralization of LPS. LBP present in normal plasma has been shown to enhance the LPS responsiveness of cells in vitro. Th e role of LBP in promoting LPS responsiveness in vivo was tested in LB P-deficient mice produced by gene targeting in embryonic stem cells. W hole blood from LBP-deficient animals was 1,000-fold less responsive t o LPS as assessed by the release of tumor necrosis factor (TNF)-alpha. Blood from gene-targeted mice was devoid of immunoreactive LBP, essen tially incapable of transferring LPS to CD14 in vitro, and railed to s upport cellular responses to LPS. These activities were restored by th e addition of exogenous recombinant murine LBP to the plasma. Despite these striking in vitro findings, no significant differences in TNF-al pha levels were observed in plasma from wild-type and LBP-deficient mi ce injected with LPS. These data suggest the presence of an LBP-indepe ndent mechanism for responding to LPS. These LBP knockout mice may pro vide a tool for discovering the nature of the presumed second nlechani sm for transferring LPS to responsive cells.