Kathryn M. Monroe

STING is a direct innate immune sensor of cyclic di-GMP

The innate immune system detects infection by using germline-encoded receptors that are specific for conserved microbial molecules. The recognition of microbial ligands leads to the production of cytokines, such as type I interferons (IFNs), that are essential for successful pathogen elimination. Cytosolic detection of pathogen-derived DNA is one major mechanism of inducing IFN production, and this process requires signalling through TANK binding kinase 1 (TBK1) and its downstream transcription factor, IFN-regulatory factor 3 (IRF3). In addition, a transmembrane protein called STING (stimulator of IFN genes; also known as MITA, ERIS, MPYS and TMEM173) functions as an essential signalling adaptor, linking the cytosolic detection of DNA to the TBK1–IRF3 signalling axis. Recently, unique nucleic acids called cyclic dinucleotides, which function as conserved signalling molecules in bacteria, have also been shown to induce a STING-dependent type I IFN response. However, a mammalian sensor of cyclic dinucleotides has not been identified. Here we report evidence that STING itself is an innate immune sensor of cyclic dinucleotides. We demonstrate that STING binds directly to radiolabelled cyclic diguanylate monophosphate (c-di-GMP), and we show that unlabelled cyclic dinucleotides, but not other nucleotides or nucleic acids, compete with c-di-GMP for binding to STING. Furthermore, we identify mutations in STING that selectively affect the response to cyclic dinucleotides without affecting the response to DNA. Thus, STING seems to function as a direct sensor of cyclic dinucleotides, in addition to its established role as a signalling adaptor in the IFN response to cytosolic DNA. Cyclic dinucleotides have shown promise as novel vaccine adjuvants and immunotherapeutics, and our results provide insight into the mechanism by which cyclic dinucleotides are sensed by the innate immune system.

Cell death by pyroptosis drives CD4 T-cell depletion in HIV-1 infection

The pathway causing CD4 T-cell death in HIV-infected hosts remains poorly understood although apoptosis has been proposed as a key mechanism. We now show that caspase-3-mediated apoptosis accounts for the death of only a small fraction of CD4 T cells corresponding to those that are both activated and productively infected. The remaining over 95% of quiescent lymphoid CD4 T cells die by caspase-1-mediated pyroptosis triggered by abortive viral infection. Pyroptosis corresponds to an intensely inflammatory form of programmed cell death in which cytoplasmic contents and pro-inflammatory cytokines, including IL-1β, are released. This death pathway thus links the two signature events in HIV infection—CD4 T-cell depletion and chronic inflammation—and creates a pathogenic vicious cycle in which dying CD4 T cells release inflammatory signals that attract more cells to die. This cycle can be broken by caspase 1 inhibitors shown to be safe in humans, raising the possibility of a new class of ‘anti-AIDS’ therapeutics targeting the host rather than the virus.

The N-Ethyl-N-Nitrosourea-Induced Goldenticket Mouse Mutant Reveals an Essential Function of Sting in the In Vivo Interferon Response to Listeria monocytogenes and Cyclic Dinucleotides

ABSTRACT Type I interferons (IFNs) are central regulators of the innate and adaptive immune responses to viral and bacterial infections. Type I IFNs are induced upon cytosolic detection of microbial nucleic acids, including DNA, RNA, and the bacterial second messenger cyclic-di-GMP (c-di-GMP). In addition, a recent study demonstrated that the intracellular bacterial pathogen Listeria monocytogenes stimulates a type I IFN response due to cytosolic detection of bacterially secreted c-di-AMP. The transmembrane signaling adaptor Sting (Tmem173, Mita, Mpys, Eris) has recently been implicated in the induction of type I IFNs in response to cytosolic DNA and/or RNA. However, the role of Sting in response to purified cyclic dinucleotides or during in vivo L. monocytogenes infection has not been addressed. In order to identify genes important in the innate immune response, we have been conducting a forward genetic mutagenesis screen in C57BL/6 mice using the mutagen N-ethyl-N-nitrosourea (ENU). Here we describe a novel mutant mouse strain, Goldenticket (Gt), that fails to produce type I IFNs upon L. monocytogenes infection. By genetic mapping and complementation experiments, we found that Gt mice harbor a single nucleotide variant (T596A) of Sting that functions as a null allele and fails to produce detectable protein. Analysis of macrophages isolated from Gt mice revealed that Sting is absolutely required for the type I interferon response to both c-di-GMP and c-di-AMP. Additionally, Sting is required for the response to c-di-GMP and L. monocytogenes in vivo. Our results provide new functions for Sting in the innate interferon response to pathogens.

IFI16 DNA Sensor Is Required for Death of Lymphoid CD4 T-cells Abortively Infected with HIV

The progressive depletion of quiescent “bystander” CD4 T cells, which are nonpermissive to HIV infection, is a principal driver of the acquired immunodeficiency syndrome (AIDS). These cells undergo abortive infection characterized by the cytosolic accumulation of incomplete HIV reverse transcripts. These viral DNAs are sensed by an unidentified host sensor that triggers an innate immune response, leading to caspase-1 activation and pyroptosis. Using unbiased proteomic and targeted biochemical approaches, as well as two independent methods of lentiviral short hairpin RNA–mediated gene knockdown in primary CD4 T cells, we identify interferon-γ–inducible protein 16 (IFI16) as a host DNA sensor required for CD4 T cell death due to abortive HIV infection. These findings provide insights into a key host pathway that plays a central role in CD4 T cell depletion during disease progression to AIDS. The intracellular sensor that triggers the death of human lymphoid CD4 T cells abortively infected with HIV is identified. [Also see Perspective by Gaiha and Brass] Sensing HIV The depletion of quiescent CD4+ T cells from lymphoid organs is a major event contributing to the development of AIDS. The accumulation of incomplete HIV DNA transcripts in the cytoplasm of these cells, which do not themselves become productively infected, is somehow sensed, which triggers cell death. Monroe et al. (p. 428, published online 19 December; see the Perspective by Gaiha and Brass) now identify the host DNA sensor as interferon-γ–inducible protein 16, which senses viral DNA and activates pyroptosis, an inflammatory cell death pathway.

A host type I interferon response is induced by cytosolic sensing of the bacterial second messenger cyclic-di-GMP

The innate immune system responds to unique molecular signatures that are widely conserved among microbes but that are not normally present in host cells. Compounds that stimulate innate immune pathways may be valuable in the design of novel adjuvants, vaccines, and other immunotherapeutics. The cyclic dinucleotide cyclic-di–guanosine monophosphate (c-di-GMP) is a recently appreciated second messenger that plays critical regulatory roles in many species of bacteria but is not produced by eukaryotic cells. In vivo and in vitro studies have previously suggested that c-di-GMP is a potent immunostimulatory compound recognized by mouse and human cells. We provide evidence that c-di-GMP is sensed in the cytosol of mammalian cells via a novel immunosurveillance pathway. The potency of cytosolic signaling induced by c-di-GMP is comparable to that induced by cytosolic delivery of DNA, and both nucleic acids induce a similar transcriptional profile, including triggering of type I interferons and coregulated genes via induction of TBK1, IRF3, nuclear factor κB, and MAP kinases. However, the cytosolic pathway that senses c-di-GMP appears to be distinct from all known nucleic acid–sensing pathways. Our results suggest a novel mechanism by which host cells can induce an inflammatory response to a widely produced bacterial ligand.

Induction of type I interferons by bacteria.

Type I interferons (IFNs) are secreted cytokines that orchestrate diverse immune responses to infection. Although typically considered to be most important in the response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. Although diverse mechanisms have been described, bacterial induction of type I IFNs occurs upon stimulation of two main pathways: (i) Toll‐like receptor (TLR) recognition of bacterial molecules such as lipopolysaccharide (LPS); (ii) TLR‐independent recognition of molecules delivered to the host cell cytosol. Cytosolic responses can be activated by two general mechanisms. First, viable bacteria can secrete stimulatory ligands into the cytosol via specialized bacterial secretion systems. Second, ligands can be released from bacteria that lyse or are degraded. The bacterial ligands that induce the cytosolic pathways remain uncertain in many cases, but appear to include various nucleic acids. In this review, we discuss recent advances in our understanding of how bacteria induce type I interferons and the roles type I IFNs play in host immunity.

Cell-to-Cell Transmission of HIV-1 Is Required to Trigger Pyroptotic Death of Lymphoid-Tissue-Derived CD4 T Cells.

The progressive depletion of CD4 T cells underlies clinical progression to AIDS in untreated HIV-infected subjects. Most dying CD4 T cells correspond to resting nonpermissive cells residing in lymphoid tissues. Death is due to an innate immune response against the incomplete cytosolic viral DNA intermediates accumulating in these cells. The viral DNA is detected by the IFI16 sensor, leading to inflammasome assembly, caspase-1 activation, and the induction of pyroptosis, a highly inflammatory form of programmed cell death. We now show that cell-to-cell transmission of HIV is obligatorily required for activation of this death pathway. Cell-free HIV-1 virions, even when added in large quantities, fail to activate pyroptosis. These findings underscore the infected CD4 T cells as the major killing units promoting progression to AIDS and highlight a previously unappreciated role for the virological synapse in HIV pathogenesis.

Corrigendum: Cell death by pyroptosis drives CD4 T-cell depletion in HIV-1 infection

This corrects the article DOI: 10.1038/nature12940