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Regulatory mechanisms of host responsiveness to endotoxin (lipopolysaccharide)
J Mathison1, P Tobias, E Wolfson
1Department of Immunology, Research Institute of Scripps Clinic, La Jolla, Calif.
Abstract:
During Gram-negative endotoxemia, precise regulation of monocyte/macrophage (M phi) responsiveness to lipopolysaccharide (LPS) is critical to preserve host defense while avoiding complications such as organ failure and death. We will discuss regulation of LPS-M phi interactions by LPS-binding plasma proteins and by LPS-induced changes in M phi responsiveness. Upon exposure to plasma, LPS binds to either lipoproteins or LPS-binding protein (LBP; a 60-kilodalton glycoprotein with a high-affinity binding site for the lipid A moiety of rough and smooth LPS). The LPS-LBP complex stimulates the M phi by binding to its cellular receptor, CD14 (a monocyte/M phi-specific, phosphatidylinositol-anchored surface glycoprotein). Pretreatment of whole blood with anti-CD 14 monoclonal antibody reduces the responsiveness of monocytes to LPS [determined by tumor necrosis factor-alpha (TNF-alpha) release]at least 10-fold. Similarly, cellular responsiveness to LPS is diminished at least 100-fold by depletion of plasma LBP with anti-LBP antibody. Compared to LPS-LBP induction of TNF-alpha, LPS-lipoprotein complexes are as much as 10,000-fold less active. Thus, partitioning of LPS between LBP and lipoproteins markedly influences M phi responsiveness to LPS. LPS also directly induces M phi hyporesponsiveness to itself by a process known as adaptation; exposure of M phi to less than or equal to LPS/ml (subthreshold for TNF induction) for 6-9 reduces the sensitivity of the M phi to subsequent challenge up to 1,000-fold, so that 1 microgram/ml rather than 1 ng/ml of LPS is required for maximal induction of TNF-alpha.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Plasma proteins like LPS-binding protein (LBP) and lipoproteins regulate monocyte/macrophage (M phi) responses to lipopolysaccharide (LPS). LPS also induces M phi adaptation, reducing sensitivity to subsequent LPS challenges.
Area of Science:
- Immunology
- Cellular Biology
- Biochemistry
Background:
- Gram-negative endotoxemia requires precise monocyte/macrophage (M phi) regulation in response to lipopolysaccharide (LPS).
- Dysregulation can lead to organ failure and death, highlighting the need to understand LPS-M phi interactions.
- Plasma proteins and cellular adaptation mechanisms significantly influence M phi responsiveness.
Purpose of the Study:
- To elucidate the regulatory mechanisms of M phi responsiveness to LPS during endotoxemia.
- To investigate the roles of LPS-binding plasma proteins and LPS-induced M phi adaptation.
- To quantify the impact of these factors on M phi activation and cytokine release.
Main Methods:
- Investigated LPS binding to plasma lipoproteins and LPS-binding protein (LBP).
- Utilized anti-CD14 and anti-LBP antibodies to assess M phi responsiveness.
- Measured tumor necrosis factor-alpha (TNF-alpha) release as an indicator of M phi activation.
- Studied LPS-induced M phi hyporesponsiveness (adaptation) using varying LPS concentrations and exposure times.
Main Results:
- LPS-LBP complexes are potent stimulators of M phi via CD14, inducing TNF-alpha release.
- Depletion of LBP or blocking CD14 significantly reduced M phi responsiveness to LPS.
- LPS-lipoprotein complexes are substantially less active than LPS-LBP complexes in stimulating M phi.
- M phi adaptation to low LPS levels markedly reduced sensitivity to subsequent LPS challenge (up to 1,000-fold).
Conclusions:
- Plasma protein interactions, particularly with LBP, are critical for M phi activation by LPS.
- LPS-lipoprotein interactions are less effective in M phi stimulation compared to LPS-LBP.
- LPS-induced M phi adaptation is a significant mechanism for limiting excessive inflammatory responses.