P133 RIG-I mediates recognition of Salmonella RNA in non-phagocytic cells and contributes to early bacterial replication in vivo

Abstract

Introduction Cytosolic pattern recognition receptors (PRR) have been shown to detect nucleic acids from RNA and DNA viruses in order to launch a type I interferon mediated antiviral state [1] . Here we show that RNA of the facultative intracellular bacterium Salmonella typhimurium is a ligand for RIG-I, triggers production of type-I interferons and contributes to pathogenicity in vivo . Methods We use RIG-I−/− MEFs [2] or human epithelial cells depleted of RIG-I by lentiviral shRNA transduction for transfection and infection experiments. IFNbeta production was measured by 293T IFNbeta- reporter cells or qPCR. Intracellular bacterial titers were determined by gentamycin protection assay. In vivo infection experiments were performed as described [3] using SL1344 or SL1344 aroA deficient Salmonella strains in RIG-I+/+ and RIG-I−/− mice [4] . Results Infection of fibroblasts and epithelial cells with S. typhimurium, but not with non-invasive E. coli, triggers IFNbeta transcription, suggesting that intracellular replication is required. RIG-I dependent recognition of bacterial RNA is dominant in non-phagocytic cells, as shown in RIG-I knockout and knockdown model cells. Despite presence of other pathogen-associated patterns (PAMP), like LPS or flagellin, no IFNbeta was produced in fibroblasts and epithelial cells lacking RIG-I upon S. typhimurium infection. In contrast, in macrophages TLR dependent recognition of bacterial PAMPs through TRIF/Myd88 overcomes RIG-I deficiency and leads to robust induction of type I interferon. We observed higher bacterial titers at early time points after infection in the cecum of S. typhimurium infected RIG-I−/− mice, when compared to wild type animals. However, at later stages of infection, S. typhimurium overcomes the innate response leading to similar weight loss and mortality in wildtype and RIG-I−/− mice. Conclusion In summary, our data implicate a role of RIG-I mediated innate immune recognition of bacterial RNA in early control of bacterial replication, most likely mediated by non-phagocytic intestinal epithelial cells targeted by S. typhimurium.