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Endotoxin and GM-CSF-mediated down-regulation of macrophage apo E secretion is inhibited by a TNF-specific monoclonal
1Lilly Research Labs, Indianapolis, Indiana 46285.
Abstract:
Macrophage activation is associated with increased secretion of monokines, proteases, arachidonic acid metabolites, reactive oxygen, and nitrogen intermediates and yet decreased secretion of apolipoprotein E (apo E). Although the kinetics of apo E down-regulation have been investigated, the mechanism(s) involved remains unknown. In the present study, the question of whether macrophage-activating factors such as lipopolysaccharide (LPS) and granulocyte-macrophage colony-stimulating factor (GM-CSF) directly result in apo E down-regulation or indirectly by inducing the secretion of other inflammatory mediators has been investigated. LPS-stimulated macrophages demonstrated a dose-dependent reduction in apo E secretion with a 70% decrease occurring following a 48-h incubation with 20 ng/ml LPS. Coculture of these cells with a neutralizing concentration of a hamster monoclonal antibody against murine tumor necrosis factor (TNF) inhibited the LPS-mediated reduction in apo E secretion. This inhibitory effect resulting from TNF neutralization was not observed using pooled hamster immunoglobulin G, or hamster monoclonals against murine interleukin-1 alpha (IL-1 alpha), IL-1 beta, or interferon-gamma. Similar results were observed when GM-CSF was used to induce apo E down-regulation. The inhibitory effects of TNF neutralization on endotoxin-induced apo E down-regulation were dependent on LPS concentration and were no longer apparent at concentrations greater than 200 ng/ml. These results suggest that an autocrine, TNF-dependent mechanism may play a role in the down-regulation of apo E secretion during macrophage activation.
Insights
Macrophage activation reduces apolipoprotein E (apo E) secretion. Tumor necrosis factor (TNF) mediates this decrease, suggesting an autocrine mechanism during macrophage activation.
Area of Science:
- Immunology
- Cell Biology
- Molecular Biology
Background:
- Macrophage activation involves complex changes in mediator secretion, including decreased apolipoprotein E (apo E).
- The precise mechanisms driving apo E down-regulation during macrophage activation remain largely unelucidated.
Purpose of the Study:
- To investigate whether macrophage-activating factors like lipopolysaccharide (LPS) and granulocyte-macrophage colony-stimulating factor (GM-CSF) directly or indirectly cause apo E down-regulation.
- To determine the role of specific inflammatory mediators, particularly tumor necrosis factor (TNF), in LPS-induced apo E reduction.
Main Methods:
- Stimulation of macrophages with LPS and GM-CSF.
- Quantification of apo E secretion.
- Inhibition studies using neutralizing antibodies against TNF, IL-1 alpha, IL-1 beta, and interferon-gamma.
Main Results:
- LPS induced a dose-dependent reduction in apo E secretion, with a 70% decrease at 20 ng/ml LPS after 48 hours.
- Neutralization of TNF significantly inhibited LPS-mediated apo E down-regulation, while antibodies against IL-1 or interferon-gamma had no effect.
- GM-CSF also induced apo E down-regulation, which was similarly inhibited by TNF neutralization.
Conclusions:
- Macrophage-activating factors, including LPS and GM-CSF, lead to decreased apo E secretion.
- Tumor necrosis factor (TNF) plays a critical role in mediating the down-regulation of apo E secretion during macrophage activation.
- An autocrine, TNF-dependent pathway is implicated in the reduction of apo E secretion following macrophage activation.