Regulation of pro-inflammatory cytokines by interferon-gamma during endotoxin tolerance (135.38)

Abstract

Abstract Endotoxin tolerance is a phenomenon whereby previous exposure of macrophages to microbial products, such as LPS, induces a transient period of hyporesponsiveness upon subsequent LPS challenge and is characterized by diminished release of pro-inflammatory cytokines. Previous reports have shown that interferon-gamma (IFN-γ) can prevent and reverse endotoxin tolerance, and our study aims to define the mechanisms by which this occurs. We found that exposure of CD14+ human monocytes to 100 pg/mL LPS for 24 h was sufficient to blunt their ability to produce IL-6 and TNFα upon secondary LPS challenge. However, IFN-γ pretreatment abrogated tolerization of IL-6, and to a lesser extent, TNFα, induced by LPS as well as cross-tolerization induced by Pam3Cys and IL-1β. IFN-γ did not reverse the defects in LPS signaling induced by tolerization. Tolerized monocytes exhibited greatly reduced LPS-induced IL-6 gene expression, while IFN-γ-activated monocytes retained the capacity to upregulate IL-6 mRNA despite tolerizing signals. IFN-γ-mediated regulation of tolerance occurred at the level of transcription, as revealed by IL-6 primary transcript measurements and RNA polymerase II binding to the IL-6 promoter. Current efforts are directed at identifying key factors that might mediate the ability of IFN-γ to abrogate endotoxin tolerance. This work is supported by the CRI Predoctoral Fellowship and NIH T32 AIO7621.