Figure 5

Model of known components of the IFN signalling pathway and explanation of observed results. Transfection of siRNA using Lipofectamine2000 in mouse BMDMs inductes a type I IFN response. This probably occurrs through the activation of pattern recognition receptors (PRRs) by dsRNA and/or liposome complexes. Downstream of PRR activation interferon regulatory factor 3 (Irf3) is phospohorylated and translocates to the nucleus where it which binds to the IFNβ promotor to induce expression of the IFNβ transcript (Ifnb1). It has been proposed that interferon regulatory factor 5 (Irf5) contributes in a similar manner to induce type I IFN during the antiviral response, and components of the NF-kB pathway are also known to contribute at this level. Once induced, the IFNβ acts in an autocrine manner to activate JAK-STAT signalling and subsequent formation of the interferon-stimulated gene factor 3 (ISGF3), a transcription factor complex composed of Stat1, Stat2 and Irf9 proteins. ISGF3 is responsible for driving the expression of type I interferon-stimulated genes via the ISRE (interferon stimulate response element) found within the promoter sequence of many ISGs affected in this study. Our data suggest that siRNAs targeted to the Ifnb1, Irf3, Irf5, Stat1, Stat2 and Nfkb2 transcripts all disrupt the type I IFN response at a similar level, and therefore alter the downstream expression of several hundred IFN-inducible genes in a similar way. Our data also suggest that type I IFN signalling strongly down-regulates cell cycle genes and influences the activity of the NF-κB signalling and many of the genes associated with a type II IFNγ response. Subsequent activation by IFNγ influences the expression of a different but significantly overlapping set of genes and the magnitude of this response is clearly influenced by the prior activation of the cells by type I signalling.