Genome Replication Is Associated With Release of Immunogenic DNA Waste

The Exonuclease TREX1 Constitutes an Innate Immune Checkpoint Limiting cGAS/STING-Mediated Antitumor Immunity

The DNA exonuclease three-prime repair exonuclease 1 (TREX1) is critical for preventing autoimmunity in mice and humans by degrading endogenous cytosolic DNA, which otherwise triggers activation of the innate cGAS/STING pathway leading to the production of type I IFNs. As tumor cells are prone to aberrant cytosolic DNA accumulation, we hypothesized that they are critically dependent on TREX1 activity to limit their immunogenicity. Here, we show that in tumor cells, TREX1 restricts spontaneous activation of the cGAS/STING pathway, and the subsequent induction of a type I IFN response. As a result, TREX1 deficiency compromised in vivo tumor growth in mice. This delay in tumor growth depended on a functional immune system, systemic type I IFN signaling, and tumor-intrinsic cGAS expression. Mechanistically, we show that tumor TREX1 loss drove activation of CD8+ T cells and NK cells, prevented CD8+ T-cell exhaustion, and remodeled an immunosuppressive myeloid compartment. Consequently, TREX1 deficiency combined with T-cell-directed immune checkpoint blockade. Collectively, we conclude that TREX1 is essential to limit tumor immunogenicity, and that targeting this innate immune checkpoint remodels the tumor microenvironment and enhances antitumor immunity by itself and in combination with T-cell-targeted therapies. See related article by Toufektchan et al., p. 673.

When DNA-damage responses meet innate and adaptive immunity

When cells proliferate, stress on DNA replication or exposure to endogenous or external insults frequently results in DNA damage. DNA-Damage Response (DDR) networks are complex signaling pathways used by multicellular organisms to prevent DNA damage. Depending on the type of broken DNA, the various pathways, Base-Excision Repair (BER), Nucleotide Excision Repair (NER), Mismatch Repair (MMR), Homologous Recombination (HR), Non-Homologous End-Joining (NHEJ), Interstrand Crosslink (ICL) repair, and other direct repair pathways, can be activated separately or in combination to repair DNA damage. To preserve homeostasis, innate and adaptive immune responses are effective defenses against endogenous mutation or invasion by external pathogens. It is interesting to note that new research keeps showing how closely DDR components and the immune system are related. DDR and immunological response are linked by immune effectors such as the cyclic GMP-AMP synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway. These effectors act as sensors of DNA damage-caused immune response. Furthermore, DDR components themselves function in immune responses to trigger the generation of inflammatory cytokines in a cascade or even trigger programmed cell death. Defective DDR components are known to disrupt genomic stability and compromise immunological responses, aggravating immune imbalance and leading to serious diseases such as cancer and autoimmune disorders. This study examines the most recent developments in the interaction between DDR elements and immunological responses. The DDR network's immune modulators' dual roles may offer new perspectives on treating infectious disorders linked to DNA damage, including cancer, and on the development of target immunotherapy.

Cancer cell-specific cGAS/STING Signaling pathway in the era of advancing cancer cell biology

Pattern-recognition receptors (PRRs) are critical to recognizing endogenous and exogenous threats to mount a protective proinflammatory innate immune response. PRRs may be located on the outer cell membrane, cytosol, and nucleus. The cGAS/STING signaling pathway is a cytosolic PRR system. Notably, cGAS is also present in the nucleus. The cGAS-mediated recognition of cytosolic dsDNA and its cleavage into cGAMP activates STING. Furthermore, STING activation through its downstream signaling triggers different interferon-stimulating genes (ISGs), initiating the release of type 1 interferons (IFNs) and NF-κB-mediated release of proinflammatory cytokines and molecules. Activating cGAS/STING generates type 1 IFN, which may prevent cellular transformation and cancer development, growth, and metastasis. The current article delineates the impact of the cancer cell-specific cGAS/STING signaling pathway alteration in tumors and its impact on tumor growth and metastasis. This article further discusses different approaches to specifically target cGAS/STING signaling in cancer cells to inhibit tumor growth and metastasis in conjunction with existing anticancer therapies.

TREX1 cytosolic DNA degradation correlates with autoimmune disease and cancer immunity

The N-terminal domain of Three Prime Repair Exonuclease 1 (TREX1) is catalytically active and can degrade dsDNA or ssDNA in the cytosol, whereas the C-terminal domain is primarily involved in protein localization. TREX1 deficiency induces cytosolic DNA accumulation as well as activation of the cGAS-STING-IFN signaling pathway, which results in tissue inflammation and autoimmune diseases. Furthermore, TREX1 expression in cancer immunity can be adaptively regulated to promote tumor proliferation, making it a promising therapeutic target.

Deficiency for SAMHD1 activates MDA5 in a cGAS/STING-dependent manner

Defects in nucleic acid metabolizing enzymes can lead to spontaneous but selective activation of either cGAS/STING or RIG-like receptor (RLR) signaling, causing type I interferon-driven inflammatory diseases. In these pathophysiological conditions, activation of the DNA sensor cGAS and IFN production are linked to spontaneous DNA damage. Physiological, or tonic, IFN signaling on the other hand is essential to functionally prime nucleic acid sensing pathways. Here, we show that low-level chronic DNA damage in mice lacking the Aicardi-Goutières syndrome gene SAMHD1 reduced tumor-free survival when crossed to a p53-deficient, but not to a DNA mismatch repair-deficient background. Increased DNA damage did not result in higher levels of type I interferon. Instead, we found that the chronic interferon response in SAMHD1-deficient mice was driven by the MDA5/MAVS pathway but required functional priming through the cGAS/STING pathway. Our work positions cGAS/STING upstream of tonic IFN signaling in Samhd1-deficient mice and highlights an important role of the pathway in physiological and pathophysiological innate immune priming.