Teneema Kuriakose

ZBP1/DAI is an innate sensor of influenza virus triggering the NLRP3 inflammasome and programmed cell death pathways

ZBP1 activates the NLRP3 inflammasome and programmed cell death. Sensing flu People infected with influenza get sick not only because of the presence of virus but also because of the inflammatory immune response. Now, Kuriakose et al. report that the protein ZBP1/DAI (Z-DNA binding protein 1/DNA-dependent activator of IFN regulatory factors) senses influenza A virus (IAV) and may contribute to this inflammatory pathogenesis. They found that ZBP1/DAI triggered cell death and inflammatory responses after IAV infection, and that ZBP1/DAI deficiency protected mice from IAV-related mortality. These mice had decreased inflammation and less epithelial damage than control animals. If these findings hold true in humans, ZBP1/DAI may be a host-directed target to decrease the severity of IAV pathogenesis. The interferon (IFN)–inducible protein Z-DNA binding protein 1 [ZBP1; also known as DNA-dependent activator of IFN regulatory factors (DAI) and DLM-1] was identified as a double-stranded DNA sensor, which instigates innate immune responses. However, this classification has been disputed, and whether ZBP1 functions as a pathogen sensor during an infection has remained unknown. We demonstrated ZBP1-mediated sensing of the influenza A virus (IAV) proteins NP and PB1, triggering cell death and inflammatory responses via the receptor-interacting protein kinase 1 (RIPK1)–RIPK3–caspase-8 axis. ZBP1 regulates NLRP3 (nucleotide and oligomerization domain, leucine-rich repeat–containing protein family, pyrin domain containing 3) inflammasome activation as well as induction of apoptosis, necroptosis, and pyroptosis in IAV-infected cells. ZBP1 deficiency protected mice from mortality during IAV infection owing to reduced inflammatory responses and epithelial damage. Overall, these findings indicate that ZBP1 is an innate immune sensor of IAV and highlight its importance in the pathogenesis of IAV infection.

Interferon-inducible guanylate-binding proteins at the interface of cell-autonomous immunity and inflammasome activation.

Guanylate‐binding proteins (GBPs) are essential components of cell‐autonomous immunity. In response to IFN signaling, GBPs are expressed in the cytoplasm of immune and nonimmune cells, where they unleash their antimicrobial activity toward intracellular bacteria, viruses, and parasites. Recent studies have revealed that GBPs are essential for mediating activation of the caspase‐1 inflammasome in response to the gram‐negative bacteria Salmonella enterica serovar Typhimurium, Francisella novicida, Chlamydia muridarum, Chlamydia trachomatis, Legionella pneumophila, Vibrio cholerae, Enterobacter cloacae, and Citrobacter koseri. During infection with vacuolar‐restricted gram‐negative bacteria, GBPs disrupt the vacuolar membrane to ensure liberation of LPS for cytoplasmic detection by caspase‐11 and the noncanonical NLRP3 inflammasome. In response to certain cytosolic bacteria, GBPs liberate microbial DNA for activation of the DNA‐sensing AIM2 inflammasome. GBPs also promote the recruitment of antimicrobial proteins, including NADPH oxidase subunits and autophagy‐associated proteins to the Mycobacterium‐containing vacuole to mediate intracellular bacterial killing. Here, we provide an overview on the emerging relationship between GBPs and activation of the inflammasome in innate immunity to microbial pathogens.

Regulation and functions of NLRP3 inflammasome during influenza virus infection.

HighlightsThe NLRP3 inflammasome promotes host protective responses during influenza virus infection.Host factors regulating inflammasome activation include TLR and RIG‐I sensors and type I IFN signaling.The IFN‐inducible protein ZBP1 functions as a common upstream regulator of NLRP3 inflammasome and programmed cell death pathways during influenza virus infection.Virus encoded proteins NS1, M2 and PB1‐F2 facilitate inflammasome activation. Abstract The NLRP3 inflammasome constitutes a major antiviral host defense mechanism during influenza virus infection. Inflammasome assembly in virus‐infected cells facilitates autocatalytic processing of pro‐caspase‐1 and subsequent cleavage and secretion of proinflammatory cytokines IL‐1&bgr; and IL‐18. The NLRP3 inflammasome is critical for induction of both innate and adaptive immune responses during influenza virus infection. Inflammasome‐dependent antiviral responses also regulate immunopathology and tissue repair in the infected lungs. The regulation of NLRP3 inflammasome assembly is an area of active research and recent studies have unraveled multiple cellular and viral factors involved in inflammasome assembly. Emerging studies have also identified the cross talk between inflammasome activation and programmed cell death pathways in influenza virus‐infected cells. Here, we review the current literature regarding regulation and functions of NLRP3 inflammasome during influenza virus infection.

ZBP1/DAI ubiquitination and sensing of influenza vRNPs activate programmed cell death

Innate sensing of influenza virus infection induces activation of programmed cell death pathways. We have recently identified Z-DNA–binding protein 1 (ZBP1) as an innate sensor of influenza A virus (IAV). ZBP1-mediated IAV sensing is critical for triggering programmed cell death in the infected lungs. Surprisingly, little is known about the mechanisms regulating ZBP1 activation to induce programmed cell death. Here, we report that the sensing of IAV RNA by retinoic acid inducible gene I (RIG-I) initiates ZBP1-mediated cell death via the RIG-I–MAVS–IFN-&bgr; signaling axis. IAV infection induces ubiquitination of ZBP1, suggesting potential regulation of ZBP1 function through posttranslational modifications. We further demonstrate that ZBP1 senses viral ribonucleoprotein (vRNP) complexes of IAV to trigger cell death. These findings collectively indicate that ZBP1 activation requires RIG-I signaling, ubiquitination, and vRNP sensing to trigger activation of programmed cell death pathways during IAV infection. The mechanism of ZBP1 activation described here may have broader implications in the context of virus-induced cell death.

ZBP1: Innate Sensor Regulating Cell Death and Inflammation

Z-DNA-binding protein 1 (ZBP1), initially reported as an interferon (IFN)-inducible tumor-associated protein, harbors nucleic acid-binding domains for left-handed helix (Z-form) and receptor-interacting protein homotypic interaction motif (RHIM) domains for protein homotypic interactions. Recent studies have identified ZBP1 as an innate sensor of viral infections and a target of viral evasion strategies, regulating cell death, inflammasome activation, and proinflammatory responses. ZBP1 also functions during development and can trigger perinatal lethality when its RHIM-dependent interactions are not restricted. Here we review the history and emergence of ZBP1 as a pathogen sensor and a central regulator of cell death and inflammatory responses. We also discuss the gaps in our knowledge regarding the regulation and functions of ZBP1 and highlight potential avenues for future research.

IRF1 Is a Transcriptional Regulator of ZBP1 Promoting NLRP3 Inflammasome Activation and Cell Death during Influenza Virus Infection

Innate immune sensing of influenza A virus (IAV) induces activation of various immune effector mechanisms, including the nucleotide and oligomerization domain, leucine-rich repeat–containing protein family, pyrin domain containing 3 (NLRP3) inflammasome and programmed cell death pathways. Although type I IFNs are identified as key mediators of inflammatory and cell death responses during IAV infection, the involvement of various IFN-regulated effectors in facilitating these responses are less studied. In this study, we demonstrate the role of IFN regulatory factor (IRF)1 in promoting NLRP3 inflammasome activation and cell death during IAV infection. Both inflammasome-dependent responses and induction of apoptosis and necroptosis are reduced in cells lacking IRF1 infected with IAV. The observed reduction in inflammasome activation and cell death in IRF1-deficient cells during IAV infection correlates with reduced levels of Z-DNA binding protein 1 (ZBP1), a key molecule mediating IAV-induced inflammatory and cell death responses. We further demonstrate IRF1 as a transcriptional regulator of ZBP1. Overall, our study identified IRF1 as an upstream regulator of NLRP3 inflammasome and cell death during IAV infection and further highlights the complex and multilayered regulation of key molecules controlling inflammatory response and cell fate decisions during infections.

Deficiency of the NOD-Like Receptor NLRC5 Results in Decreased CD8+ T Cell Function and Impaired Viral Clearance

ABSTRACT Pathogen recognition receptors are vital components of the immune system. Engagement of these receptors is important not only for instigation of innate immune responses to invading pathogens but also for initiating the adaptive immune response. Members of the NOD-like receptor (NLR) family of pathogen recognition receptors have important roles in orchestrating this response. The NLR family member NLRC5 regulates major histocompatibility complex class I (MHC-I) expression during various types of infections, but its role in immunity to influenza A virus (IAV) is not well studied. Here we show that Nlrc5 −/− mice exhibit an altered CD8+ T cell response during IAV infection compared to that of wild-type (WT) mice. Nlrc5 −/− mice have decreased MHC-I expression on hematopoietic cells and fewer CD8+ T cells prior to infection. NLRC5 deficiency does not affect the generation of antigen-specific CD8+ T cells following IAV infection; however, a change in epitope dominance is observed in Nlrc5 −/− mice. Moreover, IAV-specific CD8+ T cells from Nlrc5 −/− mice have impaired effector functions. This change in the adaptive immune response is associated with impaired viral clearance in Nlrc5 −/− mice. Collectively, our results demonstrate an important role for NLRC5 in regulation of antiviral immune responses and viral clearance during IAV infection. IMPORTANCE The NOD-like receptor family member NLRC5 is known to regulate expression of MHC-I as well as other genes required for antigen processing. In addition, NLRC5 also regulates various immune signaling pathways. In this study, we investigated the role of NLRC5 during influenza virus infection and found a major role for NLRC5 in restricting virus replication and promoting viral clearance. The observed increases in viral titers in NLRC5-deficient mice correlated with impaired effector CD8+ T cell responses. Although NLRC5-deficient mice were defective at clearing the virus, they did not show an increase in morbidity or mortality following influenza virus infection because of other compensatory immune mechanisms. Therefore, our study highlights how NLRC5 regulates multiple immune effector mechanisms to promote the host defense during influenza virus infection.

cGAMP: A tale of two signals

In this issue of JEM, Swanson et al. report an unanticipated role for cGAMP in priming and activation of inflammasomes in addition to its well-characterized function as an endogenous second messenger inducing type I interferons in the cytosolic DNA-sensing pathway.