Interleukin-1ß Induces mtDNA Release to Activate Innate Immune Signaling via cGAS-STING.

Interleukin-1 beta (IL-1ß) is a pleiotropic mediator of inflammation and is produced in response to a wide range of stimuli. During infection, IL-1ß production occurs in parallel with the onset of innate antimicrobial defenses, but the contribution of IL-1ß signaling to cell-intrinsic immunity is not defined. Here, we report that exogenous IL-1ß induces interferon regulatory factor 3 (IRF3) activation in human myeloid, fibroblast, and epithelial cells. IRF3 activation by IL-1ß is dependent upon the DNA-sensing pathway adaptor, stimulator of interferon genes (STING), through the recognition of cytosolic mtDNA by cyclic guanosine monophosphate (GMP)-AMP synthase (cGAS). IL-1ß treatment results in interferon (IFN) production and activation of IFN signaling to direct a potent innate immune response that restricts dengue virus infection. This study identifies a new function for IL-1ß in the onset or enhancement of cell-intrinsic immunity, with important implications for cGAS-STING in integrating inflammatory and microbial cues for host defense.

Comparative Analysis of African and Asian Lineage-Derived Zika Virus Strains Reveals Differences in Activation of and Sensitivity to Antiviral Innate Immunity.

In recent years, Asian lineage Zika virus (ZIKV) strains emerged to cause pandemic outbreaks associated with a high rate of congenital ZIKV syndrome (CZVS). The reasons for the enhanced spread and severe disease caused by newly emerging strains are not fully understood. Here we compared viral sequences, viral replication, and innate immune signaling induction of three different ZIKV strains derived from African and Asian lineages and West Nile virus, another flavivirus. We found pronounced differences in activation of innate immune signaling and inhibition of viral replication across ZIKV strains. The newly emerged Asian ZIKV strain Brazil Fortaleza 2015, which is associated with a higher rate of neurodevelopmental disorders like microcephaly, induced much weaker and delayed innate immune signaling in infected cells. However, superinfection studies to assess control of innate immune signaling induced by Sendai virus argue against an active block of IRF3 activation by the Brazilian strain of ZIKV and rather suggest an evasion of detection by host cell pattern recognition receptors. Compared to the Asian strain FSS13025 isolated in Cambodia, both ZIKV Uganda MR766 and ZIKV Brazil Fortaleza appear less sensitive to the interferon-induced antiviral response. ZIKV infection studies of cells lacking the different RIG-I-like receptors identified RIG-I as the major cytosolic pattern recognition receptor for detection of ZIKV.IMPORTANCE Zika Virus (ZIKV), discovered in 1947, is divided into African and Asian lineages. Pandemic outbreaks caused by currently emerging Asian lineage strains are accompanied by high rates of neurological disorders and exemplify the global health burden associated with this virus. Here we compared virological and innate immunological aspects of two ZIKV strains from the Asian lineage, an emerging Brazilian strain and a less-pathogenic Cambodian strain, and the prototypic African lineage ZIKV strain from Uganda. Compared to the replication of other ZIKV strains, the replication of ZIKV Brazil was less sensitive to the antiviral actions of interferon (IFN), while infection with this strain induced weaker and delayed innate immune responses in vitro Our data suggest that ZIKV Brazil directs a passive strategy of innate immune evasion that is reminiscent of a stealth virus. Such strain-specific properties likely contribute to differential pathogenesis and should be taken into consideration when choosing virus strains for future molecular studies.

Interleukin-1ß Signaling in Dendritic Cells Induces Antiviral Interferon Responses.

Induction of interferon beta (IFN-ß), IFN-stimulated genes (ISGs), and inflammatory responses is critical for control of viral infection. We recently identified an essential linkage of stimulation of the inflammatory cytokine interleukin-1ß (IL-1ß) and induction of ISGs that function as host restriction pathways against the emerging flavivirus West Nile virus (WNV) in vivo Here we utilized ex vivo global transcriptome analysis of primary dendritic cells, known targets of WNV replication, to define gene signatures required for this IL-1ß-driven antiviral response. Dendritic cells that were deficient in IL-1 receptor signaling showed dysregulation of cell-intrinsic defense genes and loss of viral control during WNV infection. Surprisingly, we found that in wild-type cells, IL-1ß treatment, in the absence of infection, drove the transcription of IFN-ß and ISGs at late times following treatment. Expression of these antiviral innate immune genes was dependent on the transcription factor IFN regulatory factor 3 (IRF3) and appears to reflect a general shift in IL-1ß signaling from an early inflammatory response to a late IFN-mediated response. These data demonstrate that inflammatory and antiviral signals integrate to control viral infection in myeloid cells through a process of IL-1ß-to-IRF3 signaling crosstalk. Strategies to exploit these cytokines in the activation of host defense programs should be investigated as novel therapeutic approaches against individual pathogens.IMPORTANCE West Nile virus is an emerging mosquito-borne flavivirus that can result in serious illness, neuropathology, and death in infected individuals. Currently, there are no vaccines or therapies for human use against West Nile virus. Immune control of West Nile virus infection requires inflammatory and antiviral responses, though the effect that each arm of this response has on the other is unclear. The significance of our research is in defining how virus-induced inflammatory responses regulate critical antiviral immune programs for effective control of West Nile virus infection. These data identify essential mechanisms of immune control that can inform therapeutic efforts against West Nile virus, with potential efficacy against other neuroinvasive viruses.

The Activation-Induced Assembly of an RNA/Protein Interactome Centered on the Splicing Factor U2AF2 Regulates Gene Expression in Human CD4 T Cells.

Activation of CD4 T cells is a reaction to challenges such as microbial pathogens, cancer and toxins that defines adaptive immune responses. The roles of T cell receptor crosslinking, intracellular signaling, and transcription factor activation are well described, but the importance of post-transcriptional regulation by RNA-binding proteins (RBPs) has not been considered in depth. We describe a new model expanding and activating primary human CD4 T cells and applied this to characterizing activation-induced assembly of splicing factors centered on U2AF2. We immunoprecipitated U2AF2 to identify what mRNA transcripts were bound as a function of activation by TCR crosslinking and costimulation. In parallel, mass spectrometry revealed the proteins incorporated into the U2AF2-centered RNA/protein interactome. Molecules that retained interaction with the U2AF2 complex after RNAse treatment were designated as "central" interactome members (CIMs). Mass spectrometry also identified a second class of activation-induced proteins, "peripheral" interactome members (PIMs), that bound to the same transcripts but were not in physical association with U2AF2 or its partners. siRNA knockdown of two CIMs and two PIMs caused changes in activation marker expression, cytokine secretion, and gene expression that were unique to each protein and mapped to pathways associated with key aspects of T cell activation. While knocking down the PIM, SYNCRIP, impacts a limited but immunologically important set of U2AF2-bound transcripts, knockdown of U2AF1 significantly impairs assembly of the majority of protein and mRNA components in the activation-induced interactome. These results demonstrated that CIMs and PIMs, either directly or indirectly through RNA, assembled into activation-induced U2AF2 complexes and play roles in post-transcriptional regulation of genes related to cytokine secretion. These data suggest an additional layer of regulation mediated by the activation-induced assembly of RNA splicing interactomes that is important for understanding T cell activation.