Homeostatic and Interferon-induced gene expression represent different states of promoter-associated transcription factor ISGF3

Host defense by the innate immune system requires the establishment of antimicrobial states allowing cells to cope with microorganisms before the onset of the adaptive immune response. Interferons (IFN) are of vital importance in the establishment of cell-autonomous antimicrobial immunity. Speed is therefore an important attribute of the cellular response to IFN. With much of the antimicrobial response being installed de novo, this pertains foremost to gene expression, the rapid switch between resting-state and active-state transcription of host defense genes. Mechanisms to meet this demand on the relevant molecular machinery include remodeling of chromatin but also changes in transcription factor interaction prior and during the IFN response. Our results show how distinct transcription factor complexes, determine the responsiveness of Interferon stimulated genes to different IRF9-containing complexes. Raw 264.7 macrophages expressing a doxycycline-inducible, myc-tagged versions of each IRF9-BirA*, STAT2-BirA* and STAT1-BirA* fusion genes were used to study complex formation in vivo. Furthermore, we extended the BioID proximity labeling by coupling it to parallel reaction monitoring to determine the degree and quantity of association between IRF9 and STATs in resting and interferon-treated macrophages.