Combining theoretical analysis and experimental data generation reveals IRF9 as a crucial factor for accelerating interferon a-induced early antiviral signalling

Type I interferons (IFN) are important components of the innate antiviral response. A key signalling pathway activated by IFNa is the Janus kinase ⁄ signal transducer and activator of transcription (JAK⁄STAT) pathway. Major components of the pathway have been identified. However, critical kinetic properties that facilitate accelerated initiation of intracellular antiviral signalling and thereby promote virus elimination remain to be determined. By combining mathematical modelling with experimental analysis, we show that control of dynamic behaviour is not distributed among several pathway components but can be primarily attributed to interferon regulatory factor 9 (IRF9), constituting a positive feedback loop. Model simulations revealed that increasing the initial IRF9 concentration reduced the time to peak, increased the amplitude and enhanced termina- tion of pathway activation. These model predictions were experimentally verified by IRF9 over-expression studies. Furthermore, acceleration of signal processing was linked to more rapid and enhanced expression of IFNa target genes. Thus, the amount of cellular IRF9 is a crucial determinant for amplification of early dynamics of IFNa-mediated signal transduction. Huh7.5 wild-type cells or cells over-expressing IRF9 were starved for 3 h and stimulated using IFNa for 0, 1, 2, 3, 4, 8, 12 or 24 h, or left untreated as a control.