Epigenetic regulation by CpG methylation splits strong from retarded IFNγ-induced IL-18BP in epithelial versus monocytic cells

Interferon (IFN)-γ-inducing interleukin (IL)-18 is a crucial inflammatory cytokine systemically provided by monocytes. It is counteracted by IL-18 binding protein (IL-18BP), a decoy receptor that displays IFNγ-inducibility thus curbing inflammation by negative feedback. Since IL18BP inducibility is pronounced in human epithelial cells but diminished in monocytes, differential IL18BP regulation was investigated herein in both types of cells. Interestingly, DNA-demethylating 5-aza-2'-deoxycytidine enhanced IFNγ-induced IL-18BP only in monocytic but not in epithelial cells. Subsequent promoter analysis brought into focus a specific CpG (coined CpG2) neighboring a γ-activated site responsible for IL18BP induction. CpG2 was consistently methylated in monocytic but unmethylated in epithelial cells. Notably, demethylation by 5-aza-2'-deoxycytidine treatment of monocytic cells impeded methyl-CpG-binding protein-2 (MeCP2) interaction with CpG2, increased adjacent histone H3K9-acetylation, and enhanced RNA-polymerase-II recruitment to the nearby IL18BP transcriptional start. Both latter observations are indicative of a gene locus displaying augmented transcriptional activity. Data suggest that epigenetic silencing by single CpG methylation determines differential IL18BP inducibility in monocytic versus epithelial cells. This regulatory principle should serve and control pivotal IL-18-related cell type-specific (patho)-physiological functions. Whereas epithelial IL-18BP evidently counteracts pathological inflammation at biological barriers, retarded IL18BP inducibility in monocytes may be key to combat blood-borne infections in IL-18-dependent manner.