Editors' ChoiceImmunology

Choosing between interferon and death

Sci. Signal.  24 May 2016:
Vol. 9, Issue 429, pp. ec121
DOI: 10.1126/scisignal.aag1864

Many RNA viruses and some DNA viruses activate the cytosolic RIG-I–like receptors (RLRs), leading to phosphorylation of the transcription factor interferon regulatory factor 3 (IRF3). Phosphorylated IRF3 dimerizes, translocates to the nucleus, and induces the expression of genes encoding type I interferons (IFNs). If the cell remains infected, the RLR-induced, IRF3-mediated pathway of apoptosis (RIPA) induces cell death through the interaction of IRF3 with the apoptotic protein Bax at mitochondria. Chattopadhyay et al. found that overexpression of ubiquitin promoted RIPA in a human fibrosarcoma cell line. Experiments with IRF3-deficient mouse embryonic fibroblasts (MEFs) expressing mutant IRF3 proteins identified specific lysine residues that were required for the apoptotic, but not transcriptional, activity of IRF3. RLR activation resulted in the linear ubiquitylation of IRF3 by the LUBAC complex, which coimmunoprecipitated with IRF3 from RLR-stimulated cells. Mutation of two serine residues in IRF3 (to generate the S1 IRF3 mutant) blocked its transcriptional activity but had no effect on its ability to induce RIPA in IRF3-deficient MEFs. Cell fractionation experiments showed that wild-type IRF3 translocated to both mitochondria and the nucleus in response to RLR stimulation, whereas the S1 IRF3 mutant translocated only to mitochondria. In culture, cells expressing either the wild-type or the S1 mutant IRF3 protein similarly inhibited the replication of Sendai virus. Cells from mice engineered to express the S1 mutant instead of wild-type IRF3 exhibited RIPA activation, but not gene expression in response to RLR stimulation. However, the S1 IRF3 mutant mice were as effective as wild-type mice in inhibiting Sendai virus replication after intranasal infection. Only the wild-type mice exhibited type I IFN production in the lung, whereas only the S1 IRF3 mutant mice showed apoptotic cell death in the lung. Together, these data suggest that the ubiquitylation state of IRF3 determines whether viral infection induces a nonlethal, transcriptional response or a lethal apoptotic response and indicate that either pathway inhibits viral replication.

S. Chattopadhyay, T. Kuzmanovic, Y. Zhang, J. L. Wetzel, G. C. Sen, Ubiquitination of the transcription factor IRF-3 activates RIPA, the apoptotic pathway that protects mice from viral pathogenesis. Immunity 44, 1151–1161 (2016). [PubMed]