Dissociation of lipopolysaccharide (LPS)-inducible gene expression in murine macrophages pretreated with smooth LPS versus monophosphoryl lipid A.

Lipopolysaccharide (LPS) and the nontoxic derivative of lipid A, monophosphoryl lipid A (MPL), were employed to assess the relationship between expression of LPS-inducible inflammatory genes and the induction of tolerance to LPS in murine macrophages. Both LPS and MPL induced expression (as assessed by increased steady-state mRNA levels) of a panel of seven "early" inflammatory genes including the tumor necrosis factor alpha (TNF-alpha), interleukin-1 beta, type 2 TNF receptor (TNFR-2), IP-10, D3, D8, and D2 genes (the last four represent LPS-inducible early genes whose functions remain unknown). In addition, LPS and MPL were both capable of inducing tolerance to LPS. The two stimuli differed in the relative concentration required to induce various outcome measures, with LPS being 100- to 1,000-fold more potent on a mass concentration basis. Characterization of the tolerant state identified three distinct categories of responsiveness. Two genes (IP-10 and D8) exhibited strong desensitization in macrophages pretreated with tolerance-inducing concentrations of either LPS or MPL. In macrophages rendered tolerant by pretreatment with LPS or MPL, a second group of inducible mRNAs (TNF-alpha, interleukin-1 beta, and D3) showed moderate suppression of response to secondary stimulation by LPS. The third category of inducible genes (TNFR-2 and D2) showed increased expression in macrophages pretreated with tolerance-inducing concentrations of either LPS or MPL. All of the LPS-inducible genes examined exhibited modest superinduction with less than tolerance-inducing concentrations of either stimulus, suggesting a priming effect of these adjuvants at low concentration. The differential behavior of the members of this panel of endotoxin-responsive genes thus offers insight into molecular events associated with acquisition of transient tolerance to LPS.