Transcriptional repressor Capicua is a gatekeeper of cell-intrinsic interferon responses.

Early detection of viral infection and rapid activation of host antiviral defenses through transcriptional upregulation of interferons (IFNs) and IFN-stimulated genes (ISGs) are critical for controlling infection. However, aberrant production of IFN in the absence of viral infection leads to auto-inflammation and can be detrimental to the host. Here, we show that the DNA-binding transcriptional repressor complex composed of Capicua (CIC) and Ataxin-1 like (ATXN1L) binds to an 8-nucleotide motif near IFN and ISG promoters and prevents erroneous expression of inflammatory genes under homeostasis in humans and mice. By contrast, during respiratory viral infection, activation of the mitogen-activated protein kinase (MAPK) pathway results in rapid degradation of the CIC-ATXN1L complex, thereby relieving repression and allowing for robust induction of IFN and ISGs. Together, our studies define a new paradigm for host regulation of IFN and ISGs through the evolutionarily conserved CIC-ATXN1L transcriptional repressor complex during homeostasis and viral infection.