Induction of endotoxin tolerance in vivo inhibits LPS-inducible activation of IRAK4, p38 and NF-κB, decreases expression of pro-inflammatory cytokines and induces negative regulators of TLR4 signaling (116.14)

Abstract

Abstract Toll-like receptors (TLRs) are conserved sensors of microbial pathogens and environmental or host-derived “danger” molecules. Development of TLR tolerance in septic patients re-programs monocyte responses to LPS, decreasing expression of pro-inflammatory cytokines without inhibiting anti-inflammatory and antimicrobial mediators, and increases a risk of secondary infections. To address the impact of endotoxin tolerization on activation of proximal TLR4 signaling pathways and expression of regulators of TLR4 signaling, we used an in vivo model of endotoxin tolerance. C57BL/6 mice were i.p. injected with LPS or PBC (used as a control), followed by in vitro challenge of isolated macrophages with medium or LPS and analyses of TLR4 signaling outputs and negative regulatory molecules. Compared to the control group, administration of LPS led to decreased TLR4-driven induction of IRAK4 kinase activity, degradation of IκB-α and phosphorylation of p38, and reduced expression of TNF-α, IL-6, and KC mRNA in peritoneal and splenic macrophages upon in vitro LPS challenge. In contrast, real-time PCR analyses revealed increased expression of IRAK-M, Tollip, SHIP, SARM, and SIKE in macrophages obtained from LPS-injected mice. We conclude that induction of endotoxin tolerance in vivo inhibits expression of pro-inflammatory cytokines and chemokines via impaired activation of IRAK4, p38 and NF-κB and up-regulation of negative regulators of MyD88- and TRIF-dependent pathways.