Innate Immune Sensors Research Articles

Chronic interferon-stimulated gene transcription promotes oncogene-induced breast cancer.

The MRE11 complex (comprising MRE11, RAD50, and NBS1) is integral to the maintenance of genome stability. We previously showed that a hypomorphic Mre11 mutant mouse strain (Mre11 ATLD1/ATLD1 ) was highly susceptible to oncogene-induced breast cancer. Here we used a mammary organoid system to examine which MRE11-dependent responses are tumor-suppressive. We found that Mre11 ATLD1/ATLD1 organoids exhibited an elevated interferon-stimulated gene (ISG) signature and sustained changes in chromatin accessibility. This Mre11 ATLD1/ATLD1 phenotype depended on DNA binding of a nuclear innate immune sensor, IFI205. Ablation of Ifi205 in Mre11 ATLD1/ATLD1 organoids restored baseline and oncogene-induced chromatin accessibility patterns to those observed in WT. Implantation of Mre11 ATLD1/ATLD1 organoids and activation of the oncogene led to aggressive metastatic breast cancer. This outcome was reversed in implanted Ifi205 -/- Mre11 ATLD1/ATLD1 organoids. These data reveal a connection between innate immune signaling and tumor development in the mammary epithelium. Given the abundance of aberrant DNA structures that arise in the context of genome instability syndromes, the data further suggest that cancer predisposition in those contexts may be partially attributable to chronic innate immune transcriptional programs.

Retrotransposons are co-opted to activate hematopoietic stem cells and erythropoiesis

Hematopoietic stem cells (HSCs) and erythropoiesis are activated during pregnancy and after bleeding by the derepression of retrotransposons, including endogenous retroviruses and LINE elements. Retrotransposon transcription activates the innate immune sensors cyclic GMP-AMP synthase (cGAS) and stimulator of interferon (IFN) genes (STING), which induce IFN and IFN-regulated genes in HSCs, increasing HSC division and erythropoiesis. Inhibition of reverse transcriptase or deficiency for cGAS or STING had little or no effect on hematopoiesis in non-pregnant mice but depleted HSCs and erythroid progenitors in pregnant mice, reducing red blood cell counts. Retrotransposons and IFN regulated genes were also induced in mouse HSCs after serial bleeding and in human HSCs during pregnancy. Reverse transcriptase inhibitor use was associated with anemia in pregnant, but not non-pregnant, people suggesting conservation of these mechanisms from mice to humans.

Evaluation of immune sensor responses to a viral small noncoding RNA.

Gammaherpesviruses are widespread pathogens causing persistent infections linked to the development of numerous types of lymphomas in humans. During latency, most of the viral protein-coding genes are suppressed, facilitating evasion of adaptive immune recognition of protein antigens. In contrast, many noncoding RNA (ncRNA) molecules are expressed in infected cells and can regulate key cellular pathways while simultaneously evading adaptive immune recognition. To counteract this, many cells express internal pattern recognition receptors that can intrinsically sense ongoing infections and initiate cellular defenses. Murine gammaherpesvirus 68 (MHV68) is a valuable model to study in vivo aspects of gammaherpesvirus pathogenesis. The MHV68 ncRNA TMER4 (tRNA-miRNA-encoding RNA 4) promotes lymph node egress of infected B cells: in the absence of TMER4, MHV68-infected B cells accumulate in the lymph node in a manner similar to B cells activated through specific antigen encounter. We hypothesized that TMER4 may alter intrinsic immune activation. In research described here, we aimed to explore the immunomodulatory functions of TMER4 by evaluating its impact on signaling through the critical immune sensors Toll-like receptor 4 (TLR4), TLR3, TLR7, and retinoic acid-inducible gene I (RIG-I). To accomplish this, we developed a system to test noncoding RNAs using commercially available reporter cell lines. We optimized the experimental procedure to ensure ncRNA expression and to quantify immune sensory molecule induction or inhibition by the expressed ncRNA. Expression of TMER4 RNAs from plasmid constructs did not alter TLR or RIG-I signaling. This study provides a clear experimental framework that can be applied to test other small ncRNAs for their impact on various innate immune sensor proteins.

S100A9 regulates M1 macrophage polarization and exacerbates steatotic liver ischemia-reperfusion injury

ObjectiveSteatotic livers exhibit higher susceptibility to ischemia reperfusion (IR) injury, which increase the risk of primary graft non-function following liver transplantation. S100A9 is identified as a pivotal innate immune sensor that regulates the progression of liver diseases. However, its significance in steatotic liver IR injury remains under-investigated. MethodsIn mice model, we generated S100A9 knockout (S100A9 KO) mice to investigate the role of S100A9 in IR-stimulated steatotic livers. In vitro, primary bone marrow-derived macrophages were utilized to explore the effect of S100A9 in regulating macrophage polarization and inflammation. ResultsS100A9 expression was markedly increased in steatotic livers of mice subjected to IR insult. S100A9 deletion significantly attenuated liver inflammatory injury, as evidenced by the diminished infiltration of both monocytes/macrophages and neutrophils (p < 0.05). The expression of proinflammatory factors was reduced (p < 0.05) at the same time. Additionally, S100A9-deficient livers demonstrated M1 polarization decrease and Toll-like receptor 4 (TLR4) suppression (p < 0.05). In vitro, genetic TLR4 inhibition led to nuclear factor kappa B (NF-κB) inactivation and subsequent M1 polarization decrease (p < 0.05) in macrophages treated with recombinant S100A9. ConclusionIn this study, we highlight the pivotal role of TLR4/NF-κB as a critical mediator of S100A9 in inducing M1 macrophage polorization- dependent inflammation in steatotic livers IR injury.

MDA5 ISGylation is crucial for immune signaling to control viral replication and pathogenesis.

The posttranslational modification (PTM) of innate immune sensor proteins by ubiquitin or ubiquitin-like proteins is crucial for regulating antiviral host responses. The cytoplasmic dsRNA receptor melanoma differentiation-associated protein 5 (MDA5) undergoes several PTMs including ISGylation within its first caspase activation and recruitment domain (CARD), which promotes MDA5 signaling. However, the relevance of MDA5 ISGylation for antiviral immunity in an infected organism has been elusive. Here, we generated knock-in mice (MDA5K23R/K43R) in which the two major ISGylation sites, K23 and K43, in MDA5 were mutated. Primary cells derived from MDA5K23R/K43R mice exhibited abrogated endogenous MDA5 ISGylation and an impaired ability of MDA5 to form oligomeric assemblies leading to blunted cytokine responses to MDA5 RNA-agonist stimulation or infection with encephalomyocarditis virus (EMCV) or West Nile virus. Phenocopying MDA5-/- mice, the MDA5K23R/K43R mice infected with EMCV displayed increased mortality, elevated viral titers, and an ablated induction of cytokines and chemokines compared to WT mice. Molecular studies identified human HERC5 (and its functional murine homolog HERC6) as the primary E3 ligases responsible for MDA5 ISGylation and activation. Taken together, these findings establish the importance of CARD ISGylation for MDA5-mediated RNA virus restriction, promoting potential avenues for immunomodulatory drug design for antiviral or anti-inflammatory applications.

Pathogenesis of Inflammation in Skin Disease: From Molecular Mechanisms to Pathology.

Many skin diseases begin with inflammatory changes on a molecular level. To develop a more thorough understanding of skin pathology and to identify new targets for therapeutic advancements, molecular mechanisms of inflammation in the context of skin disease should be studied. Current research efforts to better understand skin disease have focused on examining the role of molecular processes at several stages of the inflammatory response such as the dysregulation of innate immunity sensors, disruption of both transcriptional and post-transcriptional regulation, and crosstalk between immune and neuronal processes (neuro-immune crosstalk). This review seeks to summarize recent developments in our understanding of inflammatory processes in skin disease and to highlight opportunities for therapeutic advancements. With a focus on publications within the past 5 years (2019-2024), the databases PubMed and EBSCOhost were used to search for peer-reviewed papers regarding inflammatory molecular mechanisms and skin disease. Several themes of research interest regarding inflammatory processes in skin disease were determined through extensive review and were included based on their relative representation in current research and their focus on therapeutic potential. Several skin diseases such as psoriasis, atopic dermatitis, hidradenitis suppurativa, and scleroderma were described in the paper to demonstrate the widespread influence of inflammation in skin disease.

Post-transcriptional regulation of IFI16 promotes inflammatory endothelial pathophenotypes observed in pulmonary arterial hypertension.

Pulmonary arterial hypertension (PAH) is a progressive disease driven by endothelial cell inflammation and dysfunction, resulting in the pathological remodeling of the pulmonary vasculature. Innate immune activation has been linked to PAH development; however, the regulation, propagation, and reversibility of the induction of inflammation in PAH is poorly understood. Here, we demonstrate a role for interferon inducible protein 16 (IFI16), an innate immune sensor, as a modulator of endothelial inflammation in pulmonary hypertension, utilizing human pulmonary artery endothelial cells (PAECs). Inflammatory stimulus of PAECs with IL-1β up-regulates IFI16 expression, inducing proinflammatory cytokine up-regulation and cellular apoptosis. IFI16 mRNA stability is regulated by post-transcriptional m6A modification, mediated by Wilms' tumor 1-associated protein (WTAP), a structural stabilizer of the methyltransferase complex, via regulation of m6A methylation of IFI16. Additionally, m6A levels are increased in the peripheral blood mononuclear cells of PAH patients compared to control, indicating that quantifying this epigenetic change in patients may hold potential as a biomarker for disease identification. In summary, our study demonstrates IFI16 mediates inflammatory endothelial pathophenotypes seen in pulmonary arterial hypertension.

A Selection Protocol to Identify Therapeutics to Target NLRP3-Associated Sensory Hearing Loss.

We propose a selection process to identify a small molecule inhibitor to treat NLRP3-associated sensory hearing loss. The NLRP3 inflammasome is an innate immune sensor and present in monocytes and macrophages. Once the inflammasome is activated, a cleavage cascade is initiated leading to the release of proinflammatory cytokines IL-1β and IL-18. The NLRP3 inflammasome has been implicated in many causes of hearing loss, including autoimmune disease, tumors, and chronic suppurative otitis media. Although the target has been identified, there is a lack of available therapeutics to treat NLRP3-associated hearing loss. We created a target product profile with specific characteristics that are required for a compound to treat sensory hearing loss. We then looked at available small molecule NLRP3 inhibitors at different stages of development and selected compounds that fit that profile best. Those compounds were then tested for cell toxicity in MTT assays to determine the dosage to be used for efficacy testing. We tested efficacy of a known NLRP3 inhibitor, MCC950, in a proof-of-concept screen on reporter monocytes. Six compounds were selected that fulfilled our selection criteria for further testing. We found the maximum tolerated dose for each of those compounds that will be used for further efficacy testing. The proof-of-concept efficacy screen on reporter monocytes confirmed that those cells can be used for further efficacy testing. Our selection process and preliminary results provide a promising concept to develop small molecule NLRP3 inhibitors to treat sensory hearing loss.

Myddosomes in Toll-like receptor signaling-one to bind and rule them all.

Toll-like receptors (TLRs) are innate immune sensors for the presence of pathogens and endogenous danger signals. TLR activation results in conserved intracellular signaling events that orchestrate inflammation and antimicrobial defense. While the identity and interplay of key TLR signaling components are well established, how these largely cytosolic proteins are physically connected is not well understood. For the activation of conserved intracellular signaling events, most TLRs engage the adapter MyD88 (myeloid differentiation primary response 88), which assembles into higher-order protein complexes, myddosomes. In their recent publication, Fisch etal. present evidence that oligomeric myddosomes detach from initiating TLRs and evolve into larger scaffolds that dynamically assemble not only proximal but also distal cytosolic elements required to execute the entire cascade of the TLR-MyD88 signaling pathway. Coinciding with decline in TLR signaling over time, myddosomes progressively recruit autophagy machinery that mediates myddosome clearance. These findings expand the current understanding of TLR signaling by positioning myddosomes as the central structural element that physically assembles the key executors and regulators of TLR-MyD88-dependent intracellular signaling cascades.

Classical apoptotic stimulus, staurosporine, induces lytic inflammatory cell death, PANoptosis

Innate immunity is the body’s first line of defense against disease, and regulated cell death is a central component of this response that balances pathogen clearance and inflammation. Cell death pathways are generally categorized as non-lytic and lytic. While non-lytic apoptosis has been extensively studied in health and disease, lytic cell death pathways are increasingly implicated in infectious and inflammatory diseases and cancers. Staurosporine (STS) is a well-known inducer of non-lytic apoptosis. However, in this study, we observed that STS also induces lytic cell death at later timepoints. Using biochemical assessments with genetic knockouts, pharmacological inhibitors, and gene silencing, we identified that STS triggered PANoptosis via the caspase-8/RIPK3 axis, which was mediated by RIPK1. PANoptosis is a unique, lytic, innate immune cell death pathway initiated by innate immune sensors and driven by caspases and RIPKs through PANoptosome complexes. Deletion of caspase-8 and RIPK3, core components of the PANoptosome complex, protected against STS-induced lytic cell death. Overall, our study identifies STS as a time-dependent inducer of lytic inflammatory cell death, PANoptosis. These findings emphasize the importance of understanding trigger- and time-specific activation of distinct cell death pathways to advance our understanding of the molecular mechanisms of innate immunity and cell death for clinical translation.

Chromatin Regulator SMARCA4 Is Essential for MHV-Induced Inflammatory Cell Death, PANoptosis.

The innate immune system serves as the first line of defense against β-coronaviruses (β-CoVs), a family of viruses that includes SARS-CoV-2. Viral sensing via pattern recognition receptors triggers inflammation and cell death, which are essential components of the innate immune response that facilitate viral clearance. However, excessive activation of the innate immune system and inflammatory cell death can result in uncontrolled release of proinflammatory cytokines, resulting in cytokine storm and pathology. PANoptosis, innate immune, inflammatory cell death initiated by innate immune sensors and driven by caspases and RIPKs through PANoptosome complexes, has been implicated in the pathology of viral infections. Therefore, understanding the molecular mechanisms regulating PANoptosis in response to β-CoV infection is critical for identifying new therapeutic targets that can mitigate disease severity. In the current study, we analyzed findings from a cell death-based CRISPR screen with archetypal β-CoV mouse hepatitis virus (MHV) as the trigger to characterize host molecules required for inflammatory cell death. As a result, we identified SMARCA4, a chromatin regulator, as a putative host factor required for PANoptosis in response to MHV. Furthermore, we observed that gRNA-mediated deletion of Smarca4 inhibited MHV-induced PANoptotic cell death in macrophages. These findings have potential translational and clinical implications for the advancement of treatment strategies for β-CoVs and other infections.

Heterozygous de novo dominant negative mutation of REXO2 results in interferonopathy

Mitochondrial RNA (mtRNA) in the cytosol can trigger the innate immune sensor MDA5, and autoinflammatory disease due to type I IFN. Here, we show that a dominant negative mutation in the gene encoding the mitochondrial exonuclease REXO2 may cause interferonopathy by triggering the MDA5 pathway. A patient characterized by this heterozygous de novo mutation (p.T132A) presented with persistent skin rash featuring hyperkeratosis, parakeratosis and acanthosis, with infiltration of lymphocytes and eosinophils around small blood vessels. In addition, circulating IgE levels and inflammatory cytokines, including IFNα, are found consistently elevated. Transcriptional analysis highlights a type I IFN gene signature in PBMC. Mechanistically, REXO2 (T132A) lacks the ability to cleave RNA and inhibits the activity of wild-type REXO2. This leads to an accumulation of mitochondrial dsRNA in the cytosol, which is recognized by MDA5, leading to the associated type I IFN gene signature. These results demonstrate that in the absence of appropriate regulation by REXO2, aberrant cellular nucleic acids may accumulate and continuously trigger innate sensors, resulting in an inborn error of immunity.

A Cool Look at Positive-Strand RNA Virus Replication Organelles: New Insights from Cryo-Electron Microscopy.

Positive-strand RNA viruses encompass a variety of established and emerging eukaryotic pathogens. Their genome replication is confined to specialized cytoplasmic membrane compartments known as replication organelles (ROs). These ROs derive from host membranes, transformed into distinct structures such as invaginated spherules or intricate membrane networks including single- and/or double-membrane vesicles. ROs play a vital role in orchestrating viral RNA synthesis and evading detection by innate immune sensors of the host. In recent years, groundbreaking cryo-electron microscopy studies conducted with several prototypic viruses have significantly advanced our understanding of RO structure and function. Notably, these studies unveiled the presence of crown-shaped multimeric viral protein complexes that seem to actively participate in viral RNA synthesis and regulate the release of newly synthesized RNA into the cytosol for translation and packaging. These findings have shed light on novel viral functions and fascinating macromolecular complexes that delineate promising new avenues for future research.

Evolutionary analysis of ZAP and its cofactors identifies intrinsically disordered regions as central elements in host-pathogen interactions

The zinc-finger antiviral protein (ZAP) is an innate immunity sensor of non-self nucleic acids. Its antiviral activity is exerted through the physical interaction with different cofactors, including TRIM25, Riplet and KHNYN. Cellular proteins that interact with infectious agents are expected to be engaged in genetic conflicts that often result in their rapid evolution. To test this possibility and to identify the regions most strongly targeted by natural selection, we applied in silico molecular evolution tools to analyze the evolutionary history of ZAP and cofactors in four mammalian groups. We report evidence of positive selection in all genes and in most mammalian groups. On average, the intrinsically disordered regions (IDRs) embedded in the four proteins evolve significantly faster than folded domains and most positively selected sites fall within IDRs. In ZAP, the PARP domain also shows abundant signals of selection, and independent evolution in different mammalian groups suggests modulation of its ADP-ribose binding ability. Detailed analyses of the biophysical properties of IDRs revealed that chain compaction and conformational entropy are conserved across mammals. The IDRs in ZAP and KHNYN are particularly compact, indicating that they may promote phase separation (PS). In line with this hypothesis, we predicted several PS-promoting regions in ZAP and KHNYN, as well as in TRIM25. Positively selected sites are abundant in these regions, suggesting that PS may be important for the antiviral functions of these proteins and the evolutionary arms race with viruses. Our data shed light into the evolution of ZAP and cofactors and indicate that IDRs represent central elements in host-pathogen interactions.

Targeting toll-like receptor 7 as a therapeutic development strategy for systemic lupus erythematosus

Endosomal TLRs (TLR3/7/8/9) are highly analogous innate immunity sensors for various viral or bacterial RNA/DNA molecular patterns. Among them, TLR7, in particular, has been suggested to be a target for various inflammatory disorders and autoimmune diseases including systemic lupus erythematosus (SLE); but few small-molecule inhibitors with elaborated mechanism have been reported in literature. Here, we reported a well-characterized human TLR7-specific small-molecule inhibitor, TH-407b, with promising potency and negligible cytotoxicity through a novel binding mechanism. Notably, TH-407b not only effectively inhibited TLR7-mediated pro-inflammatory signaling in a variety of cultured cell lines but also demonstrated potent inflammation suppressing activities in primary peripheral blood mononuclear cells (PBMCs) derived from SLE patients. Furthermore, TH-407b showed prominent efficacy in vivo, improved survival rate and ameliorated symptoms of SLE model mice. To obtain molecular insights into the TH-407b derivatives’ inhibition mechanism, we performed the structural analysis of TLR7/TH-407b complex using cryogenic electron microscopy (cryo-EM) method. As an atomistic resolution cryo-EM structure of the TLR family, it not only of value to facilitate structure-based drug design, but also shed light to methodology development of small proteins using EM. Significantly, TH-407b has unveiled an inhibition strategy for TLR7 via stabilizing its resting/inactivated state. Such a resting state could be generally applicable to all TLRs, rendering a useful method for targeting this group of important immunological receptors.

Defining PANoptosis: Biochemical and Mechanistic Evaluation of Innate Immune Cell Death Activation.

The innate immune system is the first line of host defense. Innate immune activation utilizes pattern recognition receptors to detect pathogens, pathogen-associated and damage-associated molecular patterns (PAMPs and DAMPs), and homeostatic alterations and drives inflammatory signaling pathways and regulated cell death. Cell death activation is critical to eliminate pathogens and aberrant or damaged cells, while excess activation can be linked to inflammation, tissue damage, and disease. Therefore, there is increasing interest in studying cell death mechanisms to understand the underlying biology and identify therapeutic strategies. However, there are significant technical challenges, as many cell death pathways share key molecules with each other, and genetic models where these cell death molecules are deleted remain the gold standard for evaluation. Furthermore, extensive crosstalk has been identified between the cell death pathways pyroptosis, apoptosis, necroptosis, and the more recently characterized PANoptosis, which is defined as a prominent, unique innate immune, lytic, and inflammatory cell death pathway initiated by innate immune sensors and driven by caspases and RIPKs through PANoptosomes. PANoptosomes are multi-protein complexes assembled by innate immune sensor(s) in response to pathogens, PAMPs, DAMPs, cytokines, and homeostatic changes that drive PANoptosis. In this article, we provide methods for molecularly defining distinct cell death pathways, including PANoptosis, using both genetic and chemical approaches through western blot, LDH assay, and microscopy readouts. This procedure allows for the assessment of cell death on the cell population and single-cell levels even without access to genetic models. Having this comprehensive workflow that is more accessible to all labs will improve our ability as a scientific community to accelerate discovery. Using these protocols will help identify new innate immune sensors that drive PANoptosis and define the molecular mechanisms and regulators involved to establish new targetsfor clinical translation. © 2024 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Induction and quantification of cell death using live cell imaging Alternate Protocol 1: Quantification of cell death using LDH Alternate Protocol 2: Assessment of cell death complexes in single cells using immunofluorescence staining Basic Protocol 2: Analysis of cell death mechanisms by immunoblots (western blots).

Overexpression of NLRP12 enhances antiviral immunity and alleviates herpes simplex keratitis via pyroptosis/IL-18/IFN-γ signaling

Herpes simplex keratitis (HSK) is a blinding disease caused by herpes simplex virus type 1 (HSV-1) infection, and rapid eradication of the virus from the affected cornea is imperative. Nod-like receptors (NLRs) are intracellular innate immune sensors closely associated with cell death, inflammation and immune responses. In this study, we investigated the role of NLRP12 in the antiviral immunology in HSK and the underlying mechanisms. We found that NLRP12 expression was significantly decreased in HSV-1-infected human corneal epithelial cells (HCE-Ts) and HSK mouse corneas. Overexpression of NLRP12 significantly reduced viral replication in infected HCE-Ts and functioned through inflammasome-mediated pyroptosis and downstream IL-18-IFN-γ axis. In HSK mouse models, overexpression of NLRP12 reduced viral replication in the cornea and alleviated HSK symptoms. This resulted from enhanced antiviral immune responses including the activation of specific immune cells in both the cornea and the draining lymph nodes. Specifically, the NLRP12-IL-18-IFN-γ axis regulated the interaction between infected corneal epithelial cells and macrophages. In conclusion, our study identified a role of NLRP12 in mediating pyroptosis and regulating antiviral immune responses. This novel finding opens the possibilities of NLRP12 as a viable target in the therapeutic strategies for HSV-1 infection.

Innate Immune Sensing of Adeno-Associated Virus Vectors.

Adeno-associated virus (AAV) based viral vectors are widely used in human gene therapy and form the basis of approved treatments for several genetic diseases. Immune responses to vector and transgene products, however, substantially complicate these applications in clinical practice. The role of innate immune recognition of AAV vectors was initially unclear, given that inflammatory responses early after vector administration were typically mild in animal models. However, more recent research continues to identify innate immune pathways that are triggered by AAV vectors and that serve to provide activation signals for antigen-presenting cells and initiation of adaptive immune responses. Sensing of the AAV genome by the endosomal DNA receptor toll-like receptor 9 (TLR9) promotes early inflammatory response and interferon expression. Thus, activation of the TLR9>MyD88 pathway in plasmacytoid dendritic cells (pDCs) leads to the conditioning of antigen cross-presenting DCs through type I interferon (IFN-I) and ultimately CD8+ T cell activation. Alternatively, pDCs may also promote CD8+ T cell responses in a TLR9-independent manner by the production of IL-1 cytokines, thereby activating the IL-1R1>MyD88 signaling pathway. AAV can induce cytokine expression in monocyte-derived DCs, which in turn increases antibody formation. Binding of AAV capsid to complement components likely further elevates B cell activation. At high systemic vector doses in humans and in non-human primates, AAV vectors can trigger complement activation, with contributions by classical and alternative pathways, leading to severe toxicities. Finally, evidence for activation of TLR2 by the capsid and of additional innate receptors for nucleic acids has been presented. These observations show that AAV vectors can initiate several and likely redundant innate immune pathways resulting in an exaggerated adaptive immune response.

Nucleic Acid Sensor-Mediated PANoptosis in Viral Infection.

Innate immunity, the first line of host defense against viral infections, recognizes viral components through different pattern-recognition receptors. Nucleic acids derived from viruses are mainly recognized by Toll-like receptors, nucleotide-binding domain leucine-rich repeat-containing receptors, absent in melanoma 2-like receptors, and cytosolic DNA sensors (e.g., Z-DNA-binding protein 1 and cyclic GMP-AMP synthase). Different types of nucleic acid sensors can recognize specific viruses due to their unique structures. PANoptosis is a unique form of inflammatory cell death pathway that is triggered by innate immune sensors and driven by caspases and receptor-interacting serine/threonine kinases through PANoptosome complexes. Nucleic acid sensors (e.g., Z-DNA-binding protein 1 and absent in melanoma 2) not only detect viruses, but also mediate PANoptosis through providing scaffold for the assembly of PANoptosomes. This review summarizes the structures of different nucleic acid sensors, discusses their roles in viral infections by driving PANoptosis, and highlights the crosstalk between different nucleic acid sensors. It also underscores the promising prospect of manipulating nucleic acid sensors as a therapeutic approach for viral infections.

NLRC5 senses NAD+ depletion, forming a PANoptosome and driving PANoptosis and inflammation

NLRs constitute a large, highly conserved family of cytosolic pattern recognition receptors that are central to health and disease, making them key therapeutic targets. NLRC5 is an enigmatic NLR with mutations associated with inflammatory and infectious diseases, but little is known about its function as an innate immune sensor and cell death regulator. Therefore, we screened for NLRC5's role in response to infections, PAMPs, DAMPs, and cytokines. We identified that NLRC5 acts as an innate immune sensor to drive inflammatory cell death, PANoptosis, in response to specific ligands, including PAMP/heme and heme/cytokine combinations. NLRC5 interacted with NLRP12 and PANoptosome components to form a cell death complex, suggesting an NLR network forms similar to those in plants. Mechanistically, TLR signaling and NAD+ levels regulated NLRC5 expression and ROS production to control cell death. Furthermore, NLRC5-deficient mice were protected in hemolytic and inflammatory models, suggesting that NLRC5 could be a potential therapeutic target.