IFN consensus sequence binding protein potentiates STAT1-dependent activation of IFNγ-responsive promoters in macrophages

IFN gamma, once called the macrophage-activating factor, stimulates many genes in macrophages, ultimately leading to the elicitation of innate immunity. IFN gamma's functions depend on the activation of STAT1, which stimulates transcription of IFN gamma-inducible genes through the GAS element. The IFN consensus sequence binding protein (icsb gamma or IFN regulatory factor 8), encoding a transcription factor of the IFN regulatory factor family, is one of such IFN gamma-inducible genes in macrophages. We found that macrophages from ICSBP-/- mice were defective in inducing some IFN gamma-responsive genes, even though they were capable of activating STAT1 in response to IFN gamma. Accordingly, IFN gamma activation of luciferase reporters fused to the GAS element was severely impaired in ICSBP-/- macrophages, but transfection of ICSBP resulted in marked stimulation of these reporters. Consistent with its role in activating IFN gamma-responsive promoters, ICSBP stimulated reporter activity in a GAS-specific manner, even in the absence of IFN gamma treatment, and in STAT1 negative cells. Indicative of a mechanism for this stimulation, DMA affinity binding assays revealed that endogenous ICSBP was recruited to a multiprotein complex that bound to GAS. These results suggest that ICSBP, when induced by IFN gamma through STAT1, in turn generates a second wave of transcription from GAS-containing promoters, thereby contributing to the elicitation of IFN gamma's unique activities in immune cells.