A FADD-dependent innate immune mechanism in mammalian cells

Vertebrate innate immunity provides a first line of defence against pathogens such as viruses and bacteria. Viral infection activates a potent innate immune response, which can be triggered by double-stranded (ds) RNA produced during viral replication(1-3). Here, we report that mammalian cells lacking the death-domain-containing protein FADD(4,5) are defective in intracellular dsRNA-activated gene expression, including production of type I (a/b) interferons, and are thus very susceptible to viral infection. The signalling pathway incorporating FADD is largely independent of Toll-like receptor 3 and the dsRNA-dependent kinase PKR, but seems to require receptor interacting protein 1 as well as Tank-binding kinase 1-mediated activation of the transcription factor IRF-3. The requirement for FADD in mammalian host defence is evocative of innate immune signalling in Drosophila, in which a FADD-dependent pathway responds to bacterial infection by activating the transcription of antimicrobial genes(6). These data therefore suggest the existence of a conserved pathogen recognition pathway in mammalian cells that is essential for the optimal induction of type I interferons and other genes important for host defence.