DNA-Stimulated Cell Death: Implications for Host Defence, Inflammatory Diseases and Cancer

Purpose: DNA in abnormal cellular compartments (cytosol, endosomes, micronuclei) is a potent danger signal for the innate immune system, primarily known for inducing type I interferons and IL-1β. More recently, it has become clear that cytosolic DNA also triggers various forms of programmed cell death (PCD), but the pathways and biological implications of DNA-stimulated cell death remain incompletely understood. This review addresses the gap in understanding how DNA-sensing pathways engage PCD and how these processes shape host defence, inflammatory diseases, and cancer. ---

Hypothesis: The review’s central thesis is: if cytosolic DNA is sensed by innate immune receptors (AIM2, cGAS-STING, TLR9), then this triggers not only cytokine responses but also multiple forms of programmed cell death—apoptosis, pyroptosis, necroptosis, and lysosomal cell death—through distinct molecular pathways, with outcomes ranging from protective host defence to pathological inflammation and cancer progression. ---

Aims: - Describe key DNA sensors (TLR9, AIM2, cGAS-STING) and their canonical cytokine outputs (type I IFN and IL-1β). - Review the pathways and molecular mechanisms of DNA-stimulated cell death, including apoptosis, pyroptosis, necroptosis, lysosomal cell death, and potential autophagy-dependent cell death. - Discuss the biological functions of different types of cell death and their inflammatory consequences. - Examine the impact of DNA-stimulated PCD on microbial infections, sterile inflammatory diseases, and cancer. - Identify outstanding questions and future directions for targeting DNA-stimulated PCD therapeutically. ---

Delivery system: This review does not describe an engineered delivery system. Its “platform” is the endogenous DNA-sensing and cell death machinery: DNA sensors: - TLR9: endosomal sensor of unmethylated CpG DNA; signals via MYD88-IRF7 to induce IFNα in pDCs and B cells. - AIM2: cytosolic PYHIN protein; detects dsDNA >70–80 bp in a sequence-independent manner; forms inflammasome with ASC and caspase-1. - cGAS: cytosolic sensor of dsDNA; produces 2′3′ cGAMP, which activates STING → TBK1 → IRF3 → type I IFN. - IFI16, RNA polymerase III, NLRP3: accessory or subordinate DNA-sensing pathways. Cell death pathways: - Apoptosis: caspase-3/7-dependent; non-inflammatory; intrinsic (mitochondrial) or extrinsic (death receptor) pathways. - Pyroptosis: gasdermin D/E pore formation; caspase-1/4/5/11-dependent; inflammatory. - Necroptosis: RIPK3-MLKL-dependent; inflammatory. - Lysosomal cell death: STING translocation to lysosomes; lysosomal membrane permeabilization; inflammatory. - Autophagy-dependent cell death: no direct experimental evidence for DNA-activated form yet. Key molecular executors: - Caspase-1, caspase-8, caspase-3/7, gasdermin D, gasdermin E (DFNA5), MLKL, RIPK3, BAX, IRF3, ASC, NLRP3, PERK, CHOP, TREX1, DNase I/II. ---

Approach: Narrative review of published literature. No primary experimental groups. Model systems discussed include: - In vitro: mouse macrophages, human myeloid cells, fibroblasts, T cells, neuronal-like cell lines, keratinocytes. - In vivo: mouse models of viral infection (HSV-1, influenza A, HIV, HTLV-1, vaccinia virus, MHV68), bacterial infection (Listeria monocytogenes, Francisella tularensis, Mycobacterium bovis, Legionella pneumophila, Streptococcus pneumoniae, Brucella abortus), sterile inflammatory diseases (alcoholic liver disease, radiation injury, aortic aneurysm, inflammatory bowel disease), and cancer (T cell leukemia, multiple myeloma, colorectal carcinoma). - Human disease: Aicardi-Goutières syndrome (TREX1 mutations), SAVI (STING gain-of-function), systemic lupus erythematosus, Sjögren’s syndrome, psoriasis. ---

Key methods: Techniques and endpoints highlighted across cited studies: - Cell viability and cytotoxicity assays. - Flow cytometry for cell death quantification. - Western blotting for caspase cleavage (caspase-1, -3, -7, -8), PARP cleavage, MLKL phosphorylation, gasdermin cleavage. - ELISA for IL-1β, IL-18, type I IFN. - Microscopy for morphological features (apoptotic bodies, membrane rupture, pore formation). - Genetic knockout/knockdown mouse models (Aim2−/−, Sting−/−, Casp1−/−, Ripk3−/−, Mlkl−/−, Irf3−/−, Ifnar−/−). - In vivo disease models with irradiation, infection, or chemical induction. - Inflammasome reconstitution assays. ---

Key results: - AIM2-mediated cell death: Cytosolic DNA triggers AIM2-ASC-dependent pyroptosis via caspase-1 and gasdermin D; in the absence of caspase-1 or with low DNA, ASC recruits caspase-8 to induce apoptosis. - STING-mediated apoptosis: STING can induce apoptosis via an IRF3-BAX complex (RIPA pathway) independent of IRF3 transcriptional activity but requiring TBK1; this pathway operates in fibroblasts and T cells. - STING-mediated lysosomal cell death: In human myeloid cells, STING activation leads to lysosomal membrane permeabilization and cell death, secondary to NLRP3 inflammasome activation; this is TBK1/IKKα-independent. - STING-mediated necroptosis: DNA and STING agonists activate necroptosis in mouse fibroblasts and macrophages under pan-caspase inhibition, dependent on STING-induced TNF and IFNα/β; high-dose STING ligand causes lethal shock in mice rescued by RIPK3 or MLKL deficiency. - Tonic type I IFN and necroptosis threshold: Constitutive cGAS-STING-dependent type I IFN maintains MLKL expression, lowering the threshold for necroptosis. - Apoptotic caspases block IFN induction: Apoptotic caspases prevent cGAS-STING-mediated type I IFN induction by mitochondrial DNA during apoptosis, maintaining the non-inflammatory nature of apoptosis. - Radiation-induced injury: Aim2−/− mice are protected against subtotal body irradiation-induced gastrointestinal syndrome and total body irradiation-induced hematopoietic failure, largely via pyroptotic cell death. - Aortic aneurysm: STING deficiency or TBK1 inhibition blocks TBK1-RIPK3-MLKL signaling in smooth muscle cells and improves disease outcome in mice. - Cancer therapy potential: STING agonists induce apoptosis in STING-expressing tumor cells (T cell leukemia, multiple myeloma) and promote tumor control. - HIV pathogenesis: IFI16 senses DNA in abortively infected CD4+ T cells, inducing pyroptosis that may contribute to T cell depletion and chronic immune activation. ---

Interpretation: The authors conclude that DNA-stimulated cell death pathways are central to immune responses to infections and cancer, but can also drive inflammatory pathology. The type of cell death—apoptotic (non-inflammatory) versus necrotic (inflammatory)—critically determines the biological outcome. Mechanistic understanding of these pathways may allow development of new treatments targeting cGAS-STING and cell death machinery, with agonists and antagonists already in development. The field is in a favorable position to rapidly translate these discoveries into therapies for infections, inflammatory diseases, and cancer. ---

Limitations: - Difficulty in discerning direct PRR signaling effects from PCD cascades in vivo; genetic tools to uncouple these responses are often unavailable. - Challenges in ascertaining specific cell death types in in vivo settings. - Limited knowledge on factors determining which death pathway is activated in a given cell type. - No experimental evidence for DNA-activated autophagy-dependent cell death. - Species-specific and cell-specific differences (e.g., human vs. mouse myeloid cells) complicate generalization. - Most in vitro necroptosis experiments require concomitant caspase inhibition. - Conflicting results on AIM2’s role in influenza A virus infection (protective vs. deleterious). - Limited direct evidence for DNA-driven cell death in human inflammatory diseases; most data are indirect. - As a review, this article does not provide primary data or systematic meta-analysis. This response is AI-generated, for reference only.