Autocrine amplification of type I interferon gene expression mediated by interferon stimulated gene factor 3 (ISGF3).

Interferon regulatory factor (IRF)-1 and IRF-2 have been implicated for the virus-induced expression of the interferon-alpha and beta (type I IFN) genes. However, recent gene disruption studies in mice suggested the presence of other factor(s) interacting with overlapping promoter elements. In the present paper, we describe the characterization of a DNA binding factor which is strongly induced after virus infection and recognizes these promoter elements. After extensive purification, the factor was revealed to be identical to IFN-stimulated gene factor 3 (ISGF3), a transcription factor complex activated by IFN treatment. ISGF3 binds to the promoter element of IFN-beta, positive regulatory domain I (PRDI), with significantly higher affinity than IRF-1, 2, and mutational analysis of PRDI showed that the gene expression and binding of ISGF3, but not of IRF-1, 2, are highly correlated. Furthermore, our functional analysis involving a dominant negative inhibitor for ISGF3 activation and an anti-IFN neutralizing antibody clearly demonstrated the presence of a positive feedback path way for type I IFN genes mediated by ISGF3.