Transcriptional memory is conferred by combined heritable maintenance and local removal of selective chromatin modifications

Interferon-γ (IFNγ) transiently activates genes involved in inflammation and innate immunity. A subset of targets maintain a mitotically heritable memory of prior IFNγ exposure resulting in hyperactivation upon reexposure. Here we discovered that the active chromatin marks H3K4me1, H3K14Ac and H4K16Ac are established during IFNγ priming and selectively maintained on a cluster of GBP genes for at least 7 days in dividing cells in the absence of transcription. The histone acetyltransferase KAT7 is required for the accelerated GBP reactivation upon reexposure to IFNγ. In naïve cells, we find the GBP cluster is maintained in low-level repressive chromatin marked by H3K27me3 limiting priming in a PRC2-dependent manner. Unexpectedly, IFNγ results in transient accumulation of this repressive mark but is then selectively depleted from primed GBP genes during the memory phase facilitating hyperactivation of primed cells. Furthermore, we identified a cis-regulatory element that makes transient, long-range contacts across the GBP cluster and acts as a repressor, primarily to curb the hyperactivation of previously IFNγ-primed cells. Combined our results identify the putative chromatin basis for long-term transcriptional memory of interferon signalling that may contribute to enhanced innate immunity.