Endotoxin and GM-CSF-mediated down-regulation of macrophage apo E secretion is inhibited by a TNF-specific monoclonal antibody.

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.