IRF1 cooperates with ISGF3 or GAF to form de novo enhancers in murine macrophages (ChIP-Seq)
Exposure of macrophages to cytokines and pathogen-associated molecular patterns (PAMPs) can alter their subsequent stimulus responses, in part by epigenetic reprogramming. Our previous studies identified dynamic control of NF?B as a specificity mechanism for inducing hundreds of de novo enhancers in murine macrophages. A second set of hundreds of de novo enhancers are associated with the ISRE DNA binding element of the family of interferon regulatory factors (IRFs). In this project, we have dissected the roles of three endotoxin-responsive IRFs in macrophage de novo enhancer formation. While IRF3 was previously hypothesized to be a chromatin remodeling transcription factor, we found that its genetic requirement for enhancer formation (H3K4me1) is indirect, mediated by its role in type I IFN expression which activates the IRF9-containing ISGF3 transcription factor. However, our analysis shows that ISGF3 is unable to trigger enhancer formation on its own – it must cooperate with IRF1. Timecourse analysis of chromatin accessibility by ATAC-seq reveals that IRF1 plays a major role in the early steps of nucleosome eviction, while its partner ISGF3 then reinforces eviction and H3K4me1 deposition on neighboring nucleosomes. Further studies revealed that IRF1 can also cooperate with the type II IFN-induced GAF transcription factor, generating an additional set of IRF1-GAF-specific enhancers. Finally, global gene expression analysis by RNA-seq reveals that these IRF1-dependent enhancers are associated with potentiated expression of nearby genes in response to secondary immune threat exposure. Together our results reveal unexpected combinatorial coordination of IRF transcription factors in epigenomic reprograming to provide innate immune memory in a manner that is restricted to direct pathogen exposure or type II IFN-associated T-cell immunity, but not secondary innate immune stimulation via type I IFN. Overall design: ChIP-seq for H3K4me1 in bone marrow derived macrophages (BMDMs) stimulated with LPS, IFNß, or IFN?.