Monocytes/macrophages exposed to LPS show reduced responses to second stimulation with LPS, which is termed LPS tolerance. In this study, we investigated molecular mechanism of LPS tolerance in macrophages. Mouse peritoneal macrophages pre-exposed to LPS exhibited reduced production of inflammatory cytokines in a time- and dose-dependent manner. Activation of neither IL-1 receptor-associated kinase nor NF-κB was observed in macrophages that became tolerant by LPS pretreatment, indicating that the proximal event in Toll-like receptor 4 (TLR4)-MyD88-dependent signaling is affected in tolerant macrophages. Although TLR4 mRNA expression significantly decreased within a few hours of LPS pretreatment and returned to the original level at 24 h, the surface TLR4 expression began to decrease within 1 h, with a gradual decrease after that, and remained suppressed over 24 h. A decrease in inflammatory cytokine production in tolerant macrophages well correlates with down-regulation of the surface TLR4 expression, which may explain one of the mechanisms for LPS tolerance.

CHOP (GADD153) is a member of the C/EBP family and a stress-induced protein. To investigate the role of CHOP in cellular growth, we expressed CHOP conditionally in M1 myeloblastic leukemia cells that do not express p53 protein. More than 60% of M1 cells died through apoptosis 72 h after CHOP induction. Site-directed mutagenesis revealed that this process requires leucine zipper domain but neither intact basic region nor trans-activation domain. CHOP-mediated apoptosis accompanied downregulation of bcl-2 mRNA and overexpression of Bcl-2 delayed the process. Our results indicate that CHOP can induce apoptosis in a p53-independent manner.

IL-18 is an IL-1-related cytokine which shares biological functions with IL-12. These include the activation of NK cells, induction of IFN-gamma production and Th1 cell differentiation. In this study we analyzed the effect of IL-18 deficiency on lipopolysaccharide (LPS)-induced liver injury and endotoxin shock in Propionibacterium acnes-primed mice. P. acnes-primed IL-18-deficient (IL-18KO) mice showed resistance to LPS-induced liver injury. Unexpectedly, P. acnes-primed IL-18KO mice were highly susceptible to LPS-induced endotoxin shock. Serum level of tumor necrosis factor (TNF)-alpha were markedly elevated (approximately 10-fold higher) within 1.5 h after LPS challenge in IL-18KO mice as compared with wild-type mice. Anti-TNF-alpha antibody administration to IL-18KO mice was significantly protective against endotoxin-induced lethality. P. acnes-primed IL-18KO macrophages produced approximately 6-fold more TNF-alpha protein than did P. acnes-primed wild-type control macrophages. Taken together, these findings demonstrate that IL-18 is responsible for the progression of endotoxin-induced liver injury as well as down-regulation of endotoxin-induced TNF-alpha production in P. acnes-primed mice.

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