IRF3 in viral infections: more than just triggering the interferon response

Interferon regulatory factor 3 (IRF3) is the first transcription factor activating the expression of type I interferons (IFN-I). It is present in the cytoplasm of most cell types under basal conditions and its activation by phosphorylation allows a rapid triggering of the IFN-I pathway in response to viral infection. This activation of IFN-I is amplified by IRF7, the other major IFN-I transcription factor which expression is induced, in most cell types, by the interferon response. However, recent data have shown that the role of IRF3 in viral infection extends beyond the IFN-I pathway. Here, we review the studies investigating the impact of IRF3 deficiencies in infected cells and in vivo, in mice and in humans. We discuss the discrepancies between and within studies, between isolated cells and whole organisms. While IRF3 is also involved in other pathological processes, we highlight how the newly discovered functions of IRF3 deepen our understanding of its multiple roles in viral infections, which could stimulate the development of pharmacological manipulation of its biological activities.