High dose IFN-β activates GAF to enhance expression of ISGF3 target genes in epithelial cells

Interferon β (IFN- β ) signaling activates the transcription factor complex ISGF3 to induce gene expression programs critical for antiviral defense and host immune responses. It has also been observed that IFN- β activates a second transcription factor, γ-activated factor (GAF), but the significance of this coordinated activation is unclear. We report that in respiratory epithelial cells high doses of IFN- β indeed activate both ISGF3 and GAF, which bind to distinct genomic locations defined by their respective DNA sequence motifs. In contrast, low doses of IFN- β preferentially activate ISGF3 but not GAF. Surprisingly, in epithelial cells GAF binding does not induce nearby gene expression even when strongly bound to the promoter. Yet expression of interferon stimulated genes is enhanced when GAF and ISGF3 are both active compared to ISGF3 alone. Our data suggest that GAF enhances ISGF3 target gene expression by co-localizing with ISGF3 at some promoters and facilitating chromosome looping between distal enhancers and other promoters. We propose that GAF may function as a dose-sensitive amplifier of ISG expression to enhance antiviral immunity and establish pro-inflammatory states in respiratory epithelial cells. One sentence summary GAF is transcriptionally inactive in epithelial cells but enhances expression of ISGF3 target genes, thus functioning as a dose-sensitive amplifier of the IFN- β response. ### Competing Interest Statement The authors have declared no competing interest.