MLKL-Driven Inflammasome Activation and Caspase-8 Mediate Inflammatory Cell Death in Influenza A Virus Infection

Inflammasome Activation by RNA Respiratory Viruses: Mechanisms, Viral Manipulation, and Therapeutic Insights

Respiratory viruses, including SARS-CoV-2, influenza, parainfluenza, rhinovirus, and respiratory syncytial virus (RSV), are pathogens responsible for lower respiratory tract infections, particularly in vulnerable populations such as children and the elderly. Upon infection, these viruses are recognized by pattern recognition receptors, leading to the activation of inflammasomes, which are essential for mediating inflammatory responses. This review discusses the mechanisms by which these RNA respiratory viruses activate inflammasomes, emphasizing the roles of various signaling pathways and components involved in this process. Additionally, we highlight the specific interactions between viral proteins and inflammasome sensors, elucidating how these viruses manipulate the host immune response to facilitate infection. Understanding the dynamics of inflammasome activation in response to respiratory viruses provides critical insights for developing immunomodulatory therapeutic strategies aimed at mitigating inflammation and improving outcomes in respiratory tract infections.

TRIF-TAK1 signaling suppresses caspase-8/3-mediated GSDMD/E activation and pyroptosis in influenza A virus-infected airway epithelial cells

Pyroptosis plays an important role in attracting innate immune cells to eliminate infected niches. Our study focuses on how influenza A virus (IAV) infection triggers pyroptosis in respiratory epithelial cells. Here, we report that IAV infection induces pyroptosis in a human and murine airway epithelial cell line. Mechanistically, IAV infection activates caspase-8 and caspase-3, which cleave and activate gasdermin (GSDM) D and GSDME, respectively. Z-nucleic acid-binding protein 1 (ZBP1) and receptor-interacting protein kinase (RIPK) 1 activity but not RIPK3 are required for caspase-8/3 and GSDMD/E activation and pyroptosis. GSDMD/E, ZBP1, and RIPK1 knockout all block IAV-induced pyroptosis but enhance virus replication. Transforming growth factor β-activated kinase 1 (TAK1) activation via the adaptor protein TRIF suppresses RIPK1, caspase-8/3, and GSDMD/E activation and pyroptosis. The TAK1 inhibitor 5Z-oxzeneonal (5Z) enhances IAV-induced caspase-8/3 and GSDMD/E cleavage in the lung tissues of IAV-infected mice. Our study unveils a previously unrecognized mechanism of regulation of IAV-induced pyroptosis in respiratory epithelial cells.

ZBP1-driven cell death in severe influenza

Influenza A virus (IAV) infections can cause life-threatening illness in humans. The severity of disease is directly linked to virus replication in the alveoli of the lower respiratory tract. In particular, the lytic death of infected alveolar epithelial cells (AECs) is a major driver of influenza severity. Recent studies have begun to define the molecular mechanisms by which IAV triggers lytic cell death. Z-form nucleic-acid-binding protein 1 (ZBP1) senses replicating IAV and drives programmed cell death (PCD) in infected cells, including apoptosis and necroptosis in AECs and pyroptosis in myeloid cells. Necroptosis and pyroptosis, both lytic forms of death, contribute to pathogenesis during severe infections. Pharmacological blockade of necroptosis shows strong therapeutic potential in mouse models of lethal influenza. We suggest that targeting ZBP1-initiated necroinflammatory cell lysis, either alone or in combination antiviral drugs, will provide clinical benefit in severe influenza.

PANoptosis in intestinal epithelium: its significance in inflammatory bowel disease and a potential novel therapeutic target for natural products

The intestinal epithelium, beyond its role in absorption and digestion, serves as a critical protective mechanical barrier that delineates the luminal contents and the gut microbiota from the lamina propria within resident mucosal immune cells to maintain intestinal homeostasis. The barrier is manifested as a contiguous monolayer of specialized intestinal epithelial cells (IEC), interconnected through tight junctions (TJs). The integrity of this epithelial barrier is of paramount. Consequently, excessive IEC death advances intestinal permeability and as a consequence thereof the translocation of bacteria into the lamina propria, subsequently triggering an inflammatory response, which underpins the clinical disease trajectory of inflammatory bowel disease (IBD). A burgeoning body of evidence illustrates a landscape where IEC undergoes several the model of programmed cell death (PCD) in the pathophysiology and pathogenesis of IBD. Apoptosis, necroptosis, and pyroptosis represent the principal modalities of PCD with intricate specific pathways and molecules. Ample evidence has revealed substantial mechanistic convergence and intricate crosstalk among these three aforementioned forms of cell death, expanding the conceptualization of PANoptosis orchestrated by the PNAoptosome complex. This review provides a concise overview of the molecular mechanisms of apoptosis, necroptosis, and pyroptosis. Furthermore, based on the crosstalk between three cell deaths in IEC, this review details the current knowledge regarding PANoptosis in IEC and its regulation by natural products. Our objective is to broaden the comprehension of innovative molecular mechanisms underlying the pathogenesis of IBD and to furnish a foundation for developing more natural drugs in the treatment of IBD, benefiting both clinical practitioners and research workers.

Pyroptosis the Emerging Link Between Gut Microbiota and Multiple Sclerosis

This review elucidates the pivotal role of pyroptosis, triggered by gut microbiota, in the development of multiple sclerosis (MS), emphasizing its significance within the gut-brain axis. Our comprehensive analysis of recent literature reveals how dysbiosis in the gut microbiota of MS patients-characterized by reduced microbial diversity and shifts in bacterial populations-profoundly impacts immune regulation and the integrity of the central nervous system (CNS). Pyroptosis, an inflammatory form of programmed cell death, significantly exacerbates MS by promoting the release of inflammatory cytokines and causing substantial damage to CNS tissues. The gut microbiota facilitates this detrimental process through metabolites such as short-chain fatty acids and neuroactive compounds, or self-structural products like lipopolysaccharides (LPS), which modulate immune responses and influence neuronal survival. This review highlights the potential of modulating gut microbiota to regulate pyroptosis, thereby suggesting that targeting this pathway could be a promising therapeutic strategy to mitigate inflammatory responses and preserve neuronal integrity in patients with MS.

Advances in research on the impact and mechanisms of pathogenic microorganism infections on pyroptosis

Pyroptosis, also known as inflammatory necrosis, is a form of programmed cell death characterized by the activation of gasdermin proteins, leading to the formation of pores in the cell membrane, continuous cell swelling, and eventual membrane rupture. This process results in the release of intracellular contents, including pro-inflammatory cytokines like IL-1β and IL-18, which subsequently trigger a robust inflammatory response. This process is a crucial component of the body's innate immune response and plays a significant role in combating infections. There are four main pathways through which pathogenic microorganisms induce pyroptosis: the canonical inflammasome pathway, the non-canonical inflammasome pathway, the apoptosis-associated caspase-mediated pathway, and the granzyme-mediated pathway. This article provides a brief overview of the effects and mechanisms of pathogen infections on pyroptosis.

IRE1α silences dsRNA to prevent taxane-induced pyroptosis in triple-negative breast cancer

Chemotherapy is often combined with immune checkpoint inhibitor (ICIs) to enhance immunotherapy responses. Despite the approval of chemo-immunotherapy in multiple human cancers, many immunologically cold tumors remain unresponsive. The mechanisms determining the immunogenicity of chemotherapy are elusive. Here, we identify the ER stress sensor IRE1α as a critical checkpoint that restricts the immunostimulatory effects of taxane chemotherapy and prevents the innate immune recognition of immunologically cold triple-negative breast cancer (TNBC). IRE1α RNase silences taxane-induced double-stranded RNA (dsRNA) through regulated IRE1-dependent decay (RIDD) to prevent NLRP3 inflammasome-dependent pyroptosis. Inhibition of IRE1α in Trp53-/- TNBC allows taxane to induce extensive dsRNAs that are sensed by ZBP1, which in turn activates NLRP3-GSDMD-mediated pyroptosis. Consequently, IRE1α RNase inhibitor plus taxane converts PD-L1-negative, ICI-unresponsive TNBC tumors into PD-L1high immunogenic tumors that are hyper-sensitive to ICI. We reveal IRE1α as a cancer cell defense mechanism that prevents taxane-induced danger signal accumulation and pyroptotic cell death.

Suppression of ZBP1-mediated NLRP3 inflammasome by the tegument protein VP22 facilitates pseudorabies virus infection

The interaction between Z-DNA binding protein 1 (ZBP1) and the NLR family pyrin domain-containing 3 (NLRP3) inflammasome has been uncovered in several viral infections. However, the role of this molecular pathway during infection with the alpha-herpesvirus pseudorabies virus (PRV) remains largely elusive. Here, we report that during PRV infection, ZBP1-mediated NLRP3 inflammasome activation is inhibited by the viral tegument protein VP22, thereby facilitating viral infection. Through a combination of RNA sequencing and genetic studies, we demonstrate that PRV VP22 functions as a virus-encoded virulence factor by evading the inhibitory effects of ZBP1 on virus infection. Importantly, the replication and pathogenicity of a recombinant PRV lacking VP22 are significantly increased in ZBP1-deficient cells and mice. Mechanistically, PRV VP22 interacts with ZBP1, impeding the recruitment of receptor-interacting protein kinase 3 and Caspase-8, thereby inhibiting NLRP3 activation. Furthermore, we show that the N-terminal 1-50 amino acid domain of VP22 dominantly destabilizes ZBP1-mediated function. Taken together, these findings identify a functional link between PRV infection and ZBP1-mediated NLRP3 inflammatory response, providing novel insights into the pathogenesis of PRV and other herpesviruses.

IMPORTANCE

Z-DNA binding protein 1 (ZBP1) functions as a pivotal innate immune sensor that regulates inflammatory cell death during viral infections. However, its role in pseudorabies virus (PRV) infection remains unknown. Here, we demonstrate that ZBP1 serves as a restrictive factor by triggering the activation of the NLR family pyrin domain-containing 3 inflammasome, a process counteracted by PRV-encoded protein VP22. Furthermore, VP22 interferes with the interaction between ZBP1 and receptor-interacting protein kinase 3/Caspase-8, particularly through its N-terminal 1-50 amino acids. Importantly, deficiency in ZBP1 enhances the replication and virulence of recombinant viruses lacking VP22 or its N-terminal 1-50 amino acids. These findings reveal how PRV escapes ZBP1-mediated inflammatory responses during infection, potentially informing the rational design of therapeutic interventions.

Porcine parvovirus infection induces necroptosis of porcine placental trophoblast cells via a ZBP1-mediated pathway

Porcine parvovirus (PPV) infection induces germ cell death, leading to reproductive disorders in first-pregnant sows. Porcine placental trophoblast cells (PTCs) are the major target of PPV, and we have previously found that PPV infection leads to the death of PTCs by a non-apoptotic process, which may be related to PPV pathogenicity. The Z-nucleic acid-binding protein 1 (ZBP1) is often activated after virus invasion and mediates subsequent cell death. Here, we found that PPV infection induced ZBP1-mediated necroptosis of porcine PTCs in the presence of the apoptosis inhibitor, AC-DEVD-CHO. ZBP1 expression was upregulated during PPV infection, and ZBP1 knockout or RNA interference significantly reduced its expression along with the PPV-induced necroptosis. We discovered that RIPK3 and MLKL, but not Caspase-8, were involved in downstream signaling of ZBP1 during PPV infection; the phosphorylation levels of RIPK3 and MLKL were enhanced, but Caspase-8 was not significantly cleaved. The knockout of RIPK3 and MLKL significantly reduced the PPV infection-induced necroptosis of porcine PTCs. RIPK3 knockout did not affect the PPV infection-induced upregulation of ZBP1 expression, but blocked the activation of MLKL. MLKL knockout did not affect the upregulation of ZBP1 expression and RIPK3 phosphorylation during PPV infection. UV-inactivated PPV induced significantly less necroptosis of porcine PTCs than non-irradiated PPV. A PPV strain with a mutation in the translation initiation codon was still able to induce necroptosis of PTCs through the ZBP1/RIPK3/MLKL pathway. These results provide new insights into the pathogenic mechanism of PPV infection and suggest that PPV infection of porcine PTCs induces necroptosis through the viral DNA-dependent ZBP1/RIPK3/MLKL pathway.

Mechanistic Insights into Influenza A Virus-Induced Cell Death and Emerging Treatment Strategies

Influenza A virus (IAV) infection initiates a complex interplay of cell death modalities, including apoptosis, necroptosis, pyroptosis, and their integration, known as PANoptosis, which significantly impacts host immune responses and tissue integrity. These pathways are intricately regulated by viral proteins and host factors, contributing to both viral clearance and pathogenesis-related tissue damage. This review comprehensively explores the molecular mechanisms underlying these cell death processes in influenza infection. We highlight the roles of key regulatory proteins, such as ZBP1 (Z-DNA binding protein 1) and RIPK3 (receptor-interacting protein kinase 3), in orchestrating these responses, emphasizing the dual roles of cell death in both antiviral defense and tissue injury. Furthermore, we discuss emerging therapeutic strategies targeting these pathways, aiming to enhance antiviral efficacy while minimizing collateral tissue damage. Future research should focus on targeted approaches to modulate cell death mechanisms, aiming to reduce tissue damage and improve clinical outcomes for patients with severe influenza.

Influenza A virus infection activates caspase-8 to enhance innate antiviral immunity by cleaving CYLD and blocking TAK1 and RIG-I deubiquitination

Caspase-8, an aspartate-specific cysteine protease that primarily functions as an initiator caspase to induce apoptosis, can downregulate innate immunity in part by cleaving RIPK1 and IRF3. However, patients with caspase-8 mutations or deficiency develop immunodeficiency and are prone to viral infections. The molecular mechanism underlying this controversy remains unknown. Whether caspase-8 enhances or suppresses antiviral responses against influenza A virus (IAV) infection remains to be determined. Here, we report that caspase-8 is readily activated in A549 and NL20 cells infected with the H5N1, H5N6, and H1N1 subtypes of IAV. Surprisingly, caspase-8 deficiency and two caspase-8 inhibitors, Z-VAD and Z-IETD, do not enhance but rather downregulate antiviral innate immunity, as evidenced by decreased TBK1, IRF3, IκBα, and p65 phosphorylation, decreased IL-6, IFN-β, MX1, and ISG15 gene expression; and decreased IFN-β production but increased virus replication. Mechanistically, caspase-8 cleaves and inactivates CYLD, a tumor suppressor that functions as a deubiquitinase. Caspase-8 inhibition suppresses CYLD cleavage, RIG-I and TAK1 ubiquitination, and innate immune signaling. In contrast, CYLD deficiency enhances IAV-induced RIG-I and TAK1 ubiquitination and innate antiviral immunity. Neither caspase-3 deficiency nor treatment with its inhibitor Z-DEVD affects CYLD cleavage or antiviral innate immunity. Our study provides evidence that caspase-8 activation in two human airway epithelial cell lines does not silence but rather enhances innate immunity by inactivating CYLD.

Necroptosis in immunity, tissue homeostasis, and cancer

Immune and tissue homeostasis is achieved through balancing signals that regulate cell survival, proliferation, and cell death. Recent studies indicate that certain cell death programs can stimulate inflammation and are often referred as 'immunogenic cell death' (ICD). ICD is a double-edged sword that can confer protection against pathogen infection but also cause tissue damage. Necroptosis is a key ICD module that has been shown to participate in host defense against pathogen infection, tissue homeostasis, and cancer response to immunotherapy. Here, we will review recent findings on the regulation of necroptosis signaling and its role in pathogen infection, tissue homeostasis, and cancer.

Distinct sequential death complexes regulate pyroptosis and IL-1β release in response to Yersinia blockade of immune signaling

Pathogen infection of host cells triggers an inflammatory cell death termed pyroptosis via activation of inflammatory caspases. However, blockade of immune signaling kinases by the Yersinia virulence factor YopJ triggers cell death involving both apoptotic caspase-8 and pyroptotic caspase-1. While caspase-1 is normally activated within inflammasomes, Yersinia-induced caspase-1 activation is independent of known inflammasome components. We report that caspase-8 is an essential initiator, while caspase-1 is an essential amplifier of its own activation through two feed-forward loops involving caspase-1 auto-processing and caspase-1-dependent activation of gasdermin D and NLPR3. Notably, while Yersinia-induced caspase-1 activation and cell death are inflammasome-independent, IL-1β release requires NLPR3 inflammasome activation. Mechanistically, caspase-8 is rapidly activated within multiple foci throughout the cell, followed by assembly of a canonical inflammasome speck, indicating that caspase-8 and canonical inflammasome complex assemblies are kinetically and spatially distinct. Our findings reveal that functionally interconnected but distinct death complexes mediate pyroptosis and IL-1β release in response to pathogen blockade of immune signaling.

Initiator cell death event induced by SARS-CoV-2 in the human airway epithelium

Virus-induced cell death is a key contributor to COVID-19 pathology. Cell death induced by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is well studied in myeloid cells but less in its primary host cell type, angiotensin-converting enzyme 2 (ACE2)-expressing human airway epithelia (HAE). SARS-CoV-2 induces apoptosis, necroptosis, and pyroptosis in HAE organotypic cultures. Single-cell and limiting-dilution analysis revealed that necroptosis is the primary cell death event in infected cells, whereas uninfected bystanders undergo apoptosis, and pyroptosis occurs later during infection. Mechanistically, necroptosis is induced by viral Z-RNA binding to Z-DNA-binding protein 1 (ZBP1) in HAE and lung tissues from patients with COVID-19. The Delta (B.1.617.2) variant, which causes more severe disease than Omicron (B1.1.529) in humans, is associated with orders of magnitude-greater Z-RNA/ZBP1 interactions, necroptosis, and disease severity in animal models. Thus, Delta induces robust ZBP1-mediated necroptosis and more disease severity.

miR-126a-5p inhibits H1N1-induced inflammation and matrix protease secretion in lung fibroblasts by targeting ADAMTS-4

Upregulation of ADAMTS-4 has been reported to have an important role in lung injury, and ADAMTS-4 expression is regulated by miR-126a-5p in abdominal aortic aneurysms. The aim of this study was to investigate whether miR-126a-5p/ADAMTS-4 plays a role in influenza-virus-induced lung injury. Lung fibroblasts were infected with H1N1 influenza virus to detect changes in miR-126a-5p and ADAMTS-4 expression, and cell viability was measured by CCK-8 assay. Inflammatory factors and matrix protease levels were examined using ELISA kits, and cell apoptosis was assessed by measuring the levels of apoptosis-related proteins. A dual luciferase assay was used to verify the regulatory relationship between miR-126a-5p and ADAMTS-4. H1N1 influenza virus reduced fibroblast viability, inhibited miR-126a-5p expression, and promoted ADAMTS-4 expression. Overexpression of miR-126a-5p attenuated the cellular inflammatory response, apoptosis, matrix protease secretion, and virus replication. Luciferase reporter assays revealed that miR-126a-5p inhibited ADAMTS-4 expression by targeting ADAMTS-4 mRNA. Further experiments showed that overexpression of ADAMTS-4 significantly reversed the inhibitory effects of miR-126a-5p on fibroblast inflammation, apoptosis, matrix protease secretion, and virus replication. Upregulation of miR-126a-5p inhibits H1N1-induced apoptosis, inflammatory factors, and matrix protease secretion, as well as virus replication in lung fibroblasts.

Host Innate Antiviral Response to Influenza A Virus Infection: From Viral Sensing to Antagonism and Escape

Influenza virus possesses an RNA genome of single-stranded, negative-sensed, and segmented configuration. Influenza virus causes an acute respiratory disease, commonly known as the "flu" in humans. In some individuals, flu can lead to pneumonia and acute respiratory distress syndrome. Influenza A virus (IAV) is the most significant because it causes recurring seasonal epidemics, occasional pandemics, and zoonotic outbreaks in human populations, globally. The host innate immune response to IAV infection plays a critical role in sensing, preventing, and clearing the infection as well as in flu disease pathology. Host cells sense IAV infection through multiple receptors and mechanisms, which culminate in the induction of a concerted innate antiviral response and the creation of an antiviral state, which inhibits and clears the infection from host cells. However, IAV antagonizes and escapes many steps of the innate antiviral response by different mechanisms. Herein, we review those host and viral mechanisms. This review covers most aspects of the host innate immune response, i.e., (1) the sensing of incoming virus particles, (2) the activation of downstream innate antiviral signaling pathways, (3) the expression of interferon-stimulated genes, (4) and viral antagonism and escape.

Need for standardization of Influenza A virus-induced cell death in vivo to improve consistency of inter-laboratory research findings

The involvement of necroptosis in the control of influenza A virus (IAV) infection has been reported in multiple studies. Downstream of the nucleic acid sensor ZBP1, RIPK3 kinase activity is critically involved in the induction of necroptotic cell death by phosphorylating MLKL, while RIPK3 as a scaffold can induce apoptosis. Paradoxically, RIPK3-deficiency of mice may result in increased or decreased susceptibility to IAV infection. Here, we critically review the published reports on the involvement of RIPK3 in IAV infection susceptibility and try to identify differences in experimental settings that could explain seemingly conflicting outcomes. Analysis of the experimental reports revealed differences in the IAV challenge dose, the IAV inoculum preparation, IAV titer assessment, as well as the route of inoculation between studies. Furthermore, differences were noticed in the inclusion of littermate controls, which show high variance in viral sensitivity. Our evaluation argues for a standardized setup for IAV infection experiments including the preparation of the IAV virus, the use of different IAV infectious doses description and the proper experimental genetic controls of the mouse strains to increase inter-laboratory consistency in this field. Workflow for IAV infection studies in vivo: Viral preparation and titer assessment should be as standardized as possible with the use of a universal repository (such as BEI resources). Infection studies in genetically modified mice and littermate controls should include dose-response experimentation, following a defined infection route and inoculation volume. Data are generated by consistent analysis methods.

Novel insights into double-stranded RNA-mediated immunopathology

Recent progress in human and mouse genetics has transformed our understanding of the molecular mechanisms by which recognition of self double-stranded RNA (self-dsRNA) causes immunopathology. Novel mouse models recapitulate loss-of-function mutations in the RNA editing enzyme ADAR1 that are found in patients with Aicardi-Goutières syndrome (AGS) - a monogenic inflammatory disease associated with increased levels of type I interferon. Extensive analyses of the genotype-phenotype relationships in these mice have now firmly established a causal relationship between increased intracellular concentrations of endogenous immunostimulatory dsRNA and type I interferon-driven immunopathology. Activation of the dsRNA-specific immune sensor MDA5 perpetuates the overproduction of type I interferons, and chronic engagement of the interferon-inducible innate immune receptors PKR and ZBP1 by dsRNA drives immunopathology by activating an integrated stress response or by inducing excessive cell death. Biochemical and genetic data support a role for the p150 isoform of ADAR1 in the cytosol in suppressing the spontaneous, pathological response to self-dsRNA.

Apoptosis dysfunction: unravelling the interplay between ZBP1 activation and viral invasion in innate immune responses

Apoptosis plays a pivotal role in pathogen elimination and maintaining homeostasis. However, viruses have evolved strategies to evade apoptosis, enabling their persistence within the host. Z-DNA binding protein 1 (ZBP1) is a potent innate immune sensor that detects cytoplasmic nucleic acids and activates the innate immune response to clear pathogens. When apoptosis is inhibited by viral invasion, ZBP1 can be activated to compensate for the effect of apoptosis by triggering an innate immune response. This review examined the mechanisms of apoptosis inhibition and ZBP1 activation during viral invasion. The authors outlined the mechanisms of ZBP1-induced type I interferon, pyroptosis and necroptosis, as well as the crosstalk between ZBP1 and the cGAS-STING signalling pathway. Furthermore, ZBP1 can reverse the suppression of apoptotic signals induced by viruses. Intriguingly, a positive feedback loop exists in the ZBP1 signalling pathway, which intensifies the innate immune response while triggering a cytokine storm, leading to tissue and organ damage. The prudent use of ZBP1, which is a double-edged sword, has significant clinical implications for treating infections and inflammation.

The role of caspase-8 in inflammatory signalling and pyroptotic cell death

The programmed cell death machinery exhibits surprising flexibility, capable of crosstalk and non-apoptotic roles. Much of this complexity arises from the diverse functions of caspase-8, a cysteine-aspartic acid protease typically associated with activating caspase-3 and - 7 to induce apoptosis. However, recent research has revealed that caspase-8 also plays a role in regulating the lytic gasdermin cell death machinery, contributing to pyroptosis and immune responses in contexts such as infection, autoinflammation, and T-cell signalling. In mice, loss of caspase-8 results in embryonic lethality from unrestrained necroptotic killing, while in humans caspase-8 deficiency can lead to an autoimmune lymphoproliferative syndrome, immunodeficiency, inflammatory bowel disease or, when it can't cleave its substrate RIPK1, early onset periodic fevers. This review focuses on non-canonical caspase-8 signalling that drives immune responses, including its regulation of inflammatory gene transcription, activation within inflammasome complexes, and roles in pyroptotic cell death. Ultimately, a deeper understanding of caspase-8 function will aid in determining whether, and when, targeting caspase-8 pathways could be therapeutically beneficial in human diseases.

ZBP1 Drives IAV-Induced NLRP3 Inflammasome Activation and Lytic Cell Death, PANoptosis, Independent of the Necroptosis Executioner MLKL

Influenza A virus (IAV) continues to pose a significant global health threat, causing severe respiratory infections that result in substantial annual morbidity and mortality. Recent research highlights the pivotal role of innate immunity, cell death, and inflammation in exacerbating the severity of respiratory viral diseases. One key molecule in this process is ZBP1, a well-recognized innate immune sensor for IAV infection. Upon activation, ZBP1 triggers the formation of a PANoptosome complex containing ASC, caspase-8, and RIPK3, among other molecules, leading to inflammatory cell death, PANoptosis, and NLRP3 inflammasome activation for the maturation of IL-1β and IL-18. However, the role for other molecules in this process requires further evaluation. In this study, we investigated the role of MLKL in regulating IAV-induced cell death and NLRP3 inflammasome activation. Our data indicate IAV induced inflammatory cell death through the ZBP1-PANoptosome, where caspases and RIPKs serve as core components. However, IAV-induced lytic cell death was only partially dependent on RIPK3 at later timepoints and was fully independent of MLKL throughout all timepoints tested. Additionally, NLRP3 inflammasome activation was unaffected in MLKL-deficient cells, establishing that MLKL and MLKL-dependent necroptosis do not act upstream of NLRP3 inflammasome activation, IL-1β maturation, and lytic cell death during IAV infection.

Sequentially activated death complexes regulate pyroptosis and IL-1β release in response to Yersinia blockade of immune signaling

The Yersinia virulence factor YopJ potently inhibits immune signaling in macrophages by blocking activation of the signaling kinases TAK1 and IKK. In response, macrophages trigger a backup pathway of host defense that mediates cell death via the apoptotic enzyme caspase-8 and pyroptotic enzyme caspase-1. While caspase-1 is normally activated within multiprotein inflammasome complexes that contain the adaptor ASC and NLRs, which act as sensors of pathogen virulence, caspase-1 activation following Yersinia blockade of TAK1/IKK surprisingly requires caspase-8 and is independent of all known inflammasome components. Here, we report that caspase-1 activation by caspase-8 requires both caspase-8 catalytic and auto-processing activity. Intriguingly, while caspase-8 serves as an essential initiator of caspase-1 activation, caspase-1 amplifies its own activation through a feed-forward loop involving auto-processing, caspase-1-dependent cleavage of the pore-forming protein GSDMD, and subsequent activation of the canonical NLRP3 inflammasome. Notably, while caspase-1 activation and cell death are independent of inflammasomes during Yersinia infection, IL-1β release requires the canonical NLPR3 inflammasome. Critically, activation of caspase-8 and activation of the canonical inflammasome are kinetically and spatially separable events, as rapid capase-8 activation occurs within multiple foci throughout the cell, followed by delayed subsequent assembly of a single canonical inflammasome. Importantly, caspase-8 auto-processing normally serves to prevent RIPK3/MLKL-mediated necroptosis, and in caspase-8's absence, MLKL triggers NLPR3 inflammasome activation and IL-1β release. Altogether, our findings reveal that functionally interconnected but temporally and spatially distinct death complexes differentially mediate pyroptosis and IL-1β release to ensure robust host defense against pathogen blockade of TAK1 and IKK.

The Z-nucleic acid sensor ZBP1 in health and disease

Nucleic acid sensing is a central process in the immune system, with far-reaching roles in antiviral defense, autoinflammation, and cancer. Z-DNA binding protein 1 (ZBP1) is a sensor for double-stranded DNA and RNA helices in the unusual left-handed Z conformation termed Z-DNA and Z-RNA. Recent research established ZBP1 as a key upstream regulator of cell death and proinflammatory signaling. Recognition of Z-DNA/RNA by ZBP1 promotes host resistance to viral infection but can also drive detrimental autoinflammation. Additionally, ZBP1 has interesting roles in cancer and other disease settings and is emerging as an attractive target for therapy.

PANoptosome signaling and therapeutic implications in infection: central role for ZBP1 to activate the inflammasome and PANoptosis

The innate immune response provides the first line of defense against infection and disease. Regulated cell death (RCD) is a key component of innate immune activation, and RCD must be tightly controlled to clear pathogens while preventing excess inflammation. Recent studies have highlighted a central role for the innate immune sensor Z-DNA-binding protein 1 (ZBP1) as an activator of a form of inflammatory RCD called PANoptosis, which is regulated by a multifaceted cell death complex called the PANoptosome. In response to influenza A virus infection, ZBP1 activates the nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing protein 3 (NLRP3) inflammasome, which then acts as an integral component of the ZBP1-PANoptosome to drive inflammatory cell death, PANoptosis. In this context, the NLRP3 inflammasome is critical for caspase-1 activation and proinflammatory cytokine interleukin (IL)-1β and IL-18 maturation, but dispensable for cell death due to functional redundancies between PANoptosome molecules. Similarly, ZBP1 is also central to the absent in melanoma 2 (AIM2)-PANoptosome; this PANoptosome forms in response to Francisella novicida and herpes simplex virus 1 infection and incorporates the AIM2 inflammasome as an integral component. In this review, we will discuss the critical roles of ZBP1 in mediating innate immune responses through inflammasomes, PANoptosomes, and PANoptosis during infection. An improved understanding of the molecular mechanisms of innate immunity and cell death will be essential for the development of targeted modalities that can improve patient outcomes by mitigating severe disease.

New therapeutic concepts against ischemia-reperfusion injury in organ transplantation

INTRODUCTION

Ischemia-reperfusion injury (IRI) involves a positive amplification feedback loop that stimulates innate immune-driven tissue damage associated with organ procurement from deceased donors and during transplantation surgery. As our appreciation of its basic immune mechanisms has improved in recent years, translating putative biomarkers into therapeutic interventions in clinical transplantation remains challenging.

AREAS COVERED

This review presents advances in translational/clinical studies targeting immune responses to reactive oxygen species in IRI-stressed solid organ transplants, especially livers. Here we focus on novel concepts to rejuvenate suboptimal donor organs and improve transplant function using pharmacologic and machine perfusion (MP) strategies. Cellular damage induced by cold ischemia/warm reperfusion and the latest mechanistic insights into the microenvironment's role that leads to reperfusion-induced sterile inflammation is critically discussed.

EXPERT OPINION

Efforts to improve clinical outcomes and increase the donor organ pool will depend on improving donor management and our better appreciation of the complex mechanisms encompassing organ IRI that govern the innate-adaptive immune interface triggered in the peritransplant period and subsequent allo-Ag challenge. Computational techniques and deep machine learning incorporating the vast cellular and molecular mechanisms will predict which peri-transplant signals and immune interactions are essential for improving access to the long-term function of life-saving transplants.

Targeting Inflammasome Activation in Viral Infection: A Therapeutic Solution?

Inflammasome activation is exclusively involved in sensing activation of innate immunity and inflammatory response during viral infection. Accumulating evidence suggests that the manipulation of inflammasome assembly or its interaction with viral proteins are critical factors in viral pathogenesis. Results from pilot clinical trials show encouraging results of NLRP3 inflammasome suppression in reducing mortality and morbidity in SARS-CoV-2-infected patients. In this article, we summarize the up-to-date understanding of inflammasomes, including NLRP3, AIM2, NLRP1, NLRP6, and NLRC4 in various viral infections, with particular focus on RNA viruses such as SARS-CoV-2, HIV, IAV, and Zika virus and DNA viruses such as herpes simplex virus 1. We also discuss the current achievement of the mechanisms involved in viral infection-induced inflammatory response, host defense, and possible therapeutic solutions.

Programmed Necrosis in Host Defense

Host control over infectious disease relies on the ability of cells in multicellular organisms to detect and defend against pathogens to prevent disease. Evolution affords mammals with a wide variety of independent immune mechanisms to control or eliminate invading infectious agents. Many pathogens acquire functions to deflect these immune mechanisms and promote infection. Following successful invasion of a host, cell autonomous signaling pathways drive the production of inflammatory cytokines, deployment of restriction factors and induction of cell death. Combined, these innate immune mechanisms attract dendritic cells, neutrophils and macrophages as well as innate lymphoid cells such as natural killer cells that all help control infection. Eventually, the development of adaptive pathogen-specific immunity clears infection and provides immune memory of the encounter. For obligate intracellular pathogens such as viruses, diverse cell death pathways make a pivotal contribution to early control by eliminating host cells before progeny are produced. Pro-apoptotic caspase-8 activity (along with caspase-10 in humans) executes extrinsic apoptosis, a nonlytic form of cell death triggered by TNF family death receptors (DRs). Over the past two decades, alternate extrinsic apoptosis and necroptosis outcomes have been described. Programmed necrosis, or necroptosis, occurs when receptor interacting protein kinase 3 (RIPK3) activates mixed lineage kinase-like (MLKL), causing cell leakage. Thus, activation of DRs, toll-like receptors (TLRs) or pathogen sensor Z-nucleic acid binding protein 1 (ZBP1) initiates apoptosis as well as necroptosis if not blocked by virus-encoded inhibitors. Mammalian cell death pathways are blocked by herpesvirus- and poxvirus-encoded cell death suppressors. Growing evidence has revealed the importance of Z-nucleic acid sensor, ZBP1, in the cell autonomous recognition of both DNA and RNA virus infection. This volume will explore the detente between viruses and cells to manage death machinery and avoid elimination to support dissemination within the host animal.