Date Published: April 22, 2019
Publisher: Public Library of Science
Author(s): R. Paul Wilson, Sarah A. Tursi, Glenn J. Rapsinski, Nicole J. Medeiros, Long S. Le, Kevin P. Kotredes, Sajan Patel, Elisabetta Liverani, Shuang Sun, Wenhan Zhu, Laurie Kilpatrick, Sebastian E. Winter, Ana M. Gamero, Çagla Tükel, Renée M. Tsolis.
The mechanisms by which the gut luminal environment is disturbed by the immune system to foster pathogenic bacterial growth and survival remain incompletely understood. Here, we show that STAT2 dependent type I IFN signaling contributes to the inflammatory environment by disrupting hypoxia enabling the pathogenic S. Typhimurium to outgrow the microbiota. Stat2-/- mice infected with S. Typhimurium exhibited impaired type I IFN induced transcriptional responses in cecal tissue and reduced bacterial burden in the intestinal lumen compared to infected wild-type mice. Although inflammatory pathology was similar between wild-type and Stat2-/- mice, we observed decreased hypoxia in the gut tissue of Stat2-/- mice. Neutrophil numbers were similar in wild-type and Stat2-/- mice, yet Stat2-/- mice showed reduced levels of myeloperoxidase activity. In vitro, the neutrophils from Stat2-/- mice produced lower levels of superoxide anion upon stimulation with the bacterial ligand N-formylmethionyl-leucyl-phenylalanine (fMLP) in the presence of IFNα compared to neutrophils from wild-type mice, indicating that the neutrophils were less functional in Stat2-/- mice. Cytochrome bd-II oxidase-mediated respiration enhances S. Typhimurium fitness in wild-type mice, while in Stat2-/- deficiency, this respiratory pathway did not provide a fitness advantage. Furthermore, luminal expansion of S. Typhimurium in wild-type mice was blunted in Stat2-/- mice. Compared to wild-type mice which exhibited a significant perturbation in Bacteroidetes abundance, Stat2-/- mice exhibited significantly less perturbation and higher levels of Bacteroidetes upon S. Typhimurium infection. Our results highlight STAT2 dependent type I IFN mediated inflammation in the gut as a novel mechanism promoting luminal expansion of S. Typhimurium.
A healthy gastrointestinal microbiota is characterized by the dominance of obligate anaerobic members of the phyla Bacteroidetes and Firmicutes. The expansion of facultative anaerobic Enterobacteriaceae (phylum Proteobacteria) is considered a microbial signature for gut inflammation and dysbiosis [1, 2]. This signature is observed in severe human intestinal diseases including inflammatory bowel disease (IBD), [3–5] colorectal cancer  and necrotizing enterocolitis . Several mechanisms by which the enteric pathogen, Salmonella enterica serovar Typhimurium, capitalizes on multiple processes induced by inflammation and outcompete the commensal have been described. Infection with S. Typhimurium starts with the invasion of intestinal epithelial cells using its type III secretion system (T3SS-1) . After crossing the intestinal barrier, the bacterium is rapidly recognized by Pattern Recognition Receptors (PRRs), such as Toll-like receptors (TLRs) and Nod-like receptors (NLRs), and is internalized by macrophages or dendritic cells. In macrophages, S. Typhimurium survives using its T3SS-2 . Epithelial invasion, recognition of Pathogen-Associated Molecular Patterns (PAMPs) and macrophage survival leads to the production of chemokines and cytokines triggering an inflammatory environment and acute colitis [10–12]. In the lumen, S. Typhimurium employs mechanisms to utilize unique respiratory electron acceptors (e.g. tetrathionate and nitrate) which are generated as byproducts of the inflammatory burst. Most commensal members of the microbiota are unable to metabolize nitrate and tetrathionate [13, 14]. As a result, S. Typhimurium outcompetes the healthy microbiota enabling its luminal expansion and eventually facilitating the transmission to subsequent hosts [13–16].
The immune system deploys multiple mechanisms to eradicate invading microbes and infections. Induction of type I IFNs is a critical mechanism that the immune system exploits to fight viral infections. Type I IFNs (IFNs α and β) induce antiviral responses by binding to their cognate receptor IFNAR ubiquitously expressed on many cell types. The transcription factor STAT2 takes center stage in the type I IFN response as it is essential to mediate an antiviral state that helps the host clear a viral infection . Research over the past few years has suggested that type I IFNs are intricate players during bacterial infections. Although type I IFN responses mounted against viral infections provide a common anti-viral state among a broad range of viral pathogens, the type I IFN response generated against bacteria varies based on the specific bacterial pathogen. Recently, it was reported that Ifnβ−/− mice exhibit greater resistance to oral S. Typhimurium infection and a slower spread of S. Typhimurium to distal sterile sites . These results are consistent with our findings using Stat2-/- mice (Fig 2). Nevertheless, the previous study did not use streptomycin pre-treatment to induce colitis during infection, which models more accurately the course of S. Typhimurium infection. Hence the role of type I IFNs during gut inflammation and dysbiosis has remained unclear. Several studies have emerged showing that not all type I IFN responses involve the classical ISGF3 complex. STAT2 homodimers have been shown to bind IRF9 and activate ISG expression of antiviral genes in the absence of STAT1 [37, 48]. The expression of a subset of ISGs stimulated by STAT2 homodimers/IRF9 exhibits a delayed kinetics compared to the classical ISGF3, however, this is sufficiently robust to evoke an innate response . These observations together with our own findings indicate that S. Typhimurium exploits the type I IFN pathway by relying on STAT2 and potentially in the absence of STAT1.