Epigenetic silencing of tumor necrosis factor α during endotoxin tolerance

Sustained silencing of potentially autotoxic acute proinflammatory genes like tumor necrosis factor alpha( TNF alpha) occurs in circulating leukocytes following the early phase of severe systemic inflammation. Aspects of this gene reprogramming suggest the involvement of epigenetic processes. We used THP-1 human promonocytes, which mimic gene silencing when rendered endotoxin-tolerant in vitro, to test whether TNF alpha proximal promoter nucleosomes and transcription factors adapt to an activation-specific profile by developing characteristic chromatin-based silencing marks. We found increased TNF alpha mRNA levels in endotoxin-responsive cells that was preceded by dissociation of heterochromatin-binding protein 1 alpha, demethylation of nucleosomal histone H3 lysine 9 ( H3( Lys(9))), increased phosphorylation of the adjacent serine 10 ( H3( Ser(10))), and recruitment of NF-kappa B RelA/p65 to the TNF alpha promoter. In contrast, endotoxin-tolerant cells repressed production of TNF alpha mRNA, retained binding of heterochromatin-binding protein 1 alpha, sustained methylation of H3( Lys9), reduced phosphorylation of H3( Ser10), and showed diminished binding of NF-kappa B RelA/p65 to the TNF alpha promoter. Similar levels of NF-kappa B p50 occurred at the TNF alpha promoter in the basal state, during active transcription, and in the silenced phenotype. RelB, which acts as a repressor of TNF alpha transcription, remained bound to the promoter during silencing. These results support an immunodeficiency paradigm where epigenetic changes at the promoter of acute proinflammatory genes mediate their repression during the late phase of severe systemic inflammation.