IFN c Inhibits the Cytosolic Replication of Shigella flexneri via the Cytoplasmic RNA Sensor RIGI

The activation of host cells by interferon gamma (IFNc) is essential for inhibiting the intracellular replication of most microbial pathogens. Although significant advances have been made in identifying IFNc-dependent host factors that suppress intracellular bacteria, little is known about how IFNc enables cells to recognize, or restrict, the growth of pathogens that replicate in the host cytoplasm. The replication of the cytosolic bacterial pathogen Shigella flexneri is significantly inhibited in IFNc-stimulated cells, however the specific mechanisms that mediate this inhibition have remained elusive. We found that S. flexneri efficiently invades IFNc-activated mouse embryonic fibroblasts (MEFs) and escapes from the vacuole, suggesting that IFNc acts by blocking S. flexneri replication in the cytosol. This restriction on cytosolic growth was dependent on interferon regulatory factor 1 (IRF1), an IFNc-inducible transcription factor capable of inducing IFNcmediated cell-autonomous immunity. To identify host factors that restrict S. flexneri growth, we used whole genome microarrays to identify mammalian genes whose expression in S. flexneri-infected cells is controlled by IFNc and IRF1. Among the genes we identified was the pattern recognition receptor (PRR) retanoic acid-inducible gene I (RIG-I), a cytoplasmic sensor of foreign RNA that had not been previously known to play a role in S. flexneri infection. We found that RIG-I and its downstream signaling adaptor mitochondrial antiviral signaling protein (MAVS)—but not cytosolic Nod-like receptors (NLRs)—are critically important for IFNc-mediated S. flexneri growth restriction. The recently described RNA polymerase III pathway, which transcribes foreign cytosolic DNA into the RIG-I ligand 59-triphosphate RNA, appeared to be involved in this restriction. The finding that RIG-I responds to S. flexneri infection during the IFNc response extends the range of PRRs that are capable of recognizing this bacterium. Additionally, these findings expand our understanding of how IFNc recognizes, and ultimately restricts, bacterial pathogens within host cells. Citation: Jehl SP, Nogueira CV, Zhang X, Starnbach MN (2012) IFNc Inhibits the Cytosolic Replication of Shigella flexneri via the Cytoplasmic RNA Sensor RIGI. PLoS Pathog 8(8): e1002809. doi:10.1371/journal.ppat. 1002809 Editor: Mary X. D. O’Riordan, University of Michigan Medical School, United States of America Received June 27, 2011; Accepted June 5, 2012; Published August 9, 2012 Copyright: 2012 Jehl et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by National Institutes of Health Grant AI055962 to M.N. S. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: starnbach@hms.harvard. edu