Evidence for a CD14-and serum-independent pathway in the induction of endotoxin-tolerance in human monocytes and THP-1 monocytic cells

Lipopolysaccharide (LPS) stimulation of macrophages or monocytes is believed to occur via a serum- and CD14-dependent signaling pathway via toll-like receptor 4 (TLR4). We sought to determine whether serum and/or CD14 are required for LPS to induce the endotoxin-tolerant state in human monocytes. LPS treatments were performed in the presence or absence of an anti-CD14 monoclonal antibody and with or without fetal bovine serum. Endotoxin tolerance was assessed after an 18-h exposure (pretreatment) to 10 ng/mL of LPS. Medium was discarded and cells were challenged with activating (1-1000 ng/mL) doses of LPS. LPS-stimulated tumor necrosis factor (TNF) secretion into culture supernatants was determined after 5 h by ELISA and p44/p42 ERK kinase activation was measured after 30 min by Western blot. Statistical analysis was by ANOVA. LPS induced endotoxin-tolerance with a significant inhibition of LPS-stimulated TNF secretion and less p44/p42 ERK kinase activation. When LPS-stimulation of naive (nontolerant) monocytes was performed in medium with anti-CD14 antibody or without serum, there was marked blunting of TNF release. However, LPS pretreatment in medium without serum or in medium containing anti-CD14 antibody resulted in changes in monocyte activation and function characteristic of endotoxin tolerance. LPS-stimulated p44/p42 ERK kinase activation and TNF release were diminished whether or not anti-CD14 antibody was present during LPS pretreatment. LPS-stimulated TNF secretion and p44/p42 ERK kinase activation require the presence of serum and are inhibited by anti-CD14 antibody. Our findings suggest that LPS induces endotoxin tolerance in human monocytic cells via a pathway that does not require serum or cell surface CD14.