Multistep regulation mechanisms for tolerance induction to lipopolysaccharide lethality in the tumor‐necrosis‐factor‐α‐mediated pathway

Lipid A is the active principle of lipopolysaccharide (LPS). Synthetic lipid A analogues with monosaccharide backbones, GLA‐60, GLA‐69 and GLA‐58, which exhibit potent, weak and scarce agonistic activities of LPS, respectively, induced tolerance against LPS lethality in galactosamine‐(GalN)‐sensitized mice while none of them were pyrogenic in rabbits. The tolerance‐inducing mechanisms were investigated focusing on the regulation of tumor‐necrosis‐factor‐α(TNF‐α)‐mediated lethal pathway of LPS. Induction of serum TNF‐α in LPS‐challenged mice was suppressed by prior administration of these analogues as well as LPS. Prior treatment of murine macrophages with the substances suppressed LPS‐stimulated TNF‐α production in the culture supernatant and TNF‐α mRNA expression in the cells as well. Lethal toxicity to TNF‐α in GalN‐sensitized mice was effectively suppressed by prior treatment with LPS, GLA‐60 and GLA‐69 but not by GLA‐58. This protective effect was suggested to be mediated by endogenous TNF‐α, which was induced by prior treatment with the effective substances, because either neutralization of endogenously induced TNF‐α activity with an antibody or deletion of its induction by using LPS‐resistant C3H/HeJ mice reduced the protective effect, and a detectable amount of TNF‐α was produced by stimulating macrophages with the effective substances but not with GLA‐58. These results indicated that multiple regulation steps (one is prior to and the others are following TNF‐α production) are participating in the tolerance induction by LPS and some lipid A analogues and that GLA‐58 is a characteristic compound which induces the tolerance by only blocking the step prior to TNF‐α production.