STING: infection, inflammation and cancer

Multifaceted roles of TRIM38 in innate immune and inflammatory responses

The tripartite motif-containing (TRIM) proteins represent the largest E3 ubiquitin ligase family. The multifaceted roles of TRIM38 in innate immunity and inflammation have been intensively investigated in recent years. TRIM38 is essential for cytosolic RNA or DNA sensor-mediated innate immune responses to both RNA and DNA viruses, while negatively regulating TLR3/4- and TNF/IL-1β-triggered inflammatory responses. In these processes, TRIM38 acts as an E3 ubiquitin or SUMO ligase, which targets key cellular signaling components, or as an enzymatic activity-independent regulator. This review summarizes recent advances that highlight the critical roles of TRIM38 in the regulation of proper innate immune and inflammatory responses.

The expanding role of immunotherapy

The use of agents able to modulate the immune system to induce or potentiate its anti-tumour activity is not a new strategy in oncology. However, the development of new agents such as immune checkpoint inhibitors has achieved unprecedented efficacy results in a wide variety of tumours, dramatically changing the landscape of cancer treatment in recent years. Ipilimumab, nivolumab, pembrolizumab or atezolizumab are now standard of care options in several malignancies and new indications are being approved on a regular basis in different tumours. Moreover, there are many other novel immunotherapy strategies that are currently being assessed in clinical trials. Agonists of co-stimulatory signals, adoptive cell therapies, vaccines, virotherapy and others have raised interest as therapeutic options against cancer. In addition, many of these novel approaches are being developed both in monotherapy and as part of combinatory regimes in order to synergize their activity. The results from those studies will help to define the expanding role of immunotherapy in cancer treatment in a forthcoming future.

Decreased expression of STING predicts poor prognosis in patients with gastric cancer

STING (stimulator of interferon genes) has recently been found to play an important role in host defenses against virus and intracellular bacteria via the regulation of type-I IFN signaling and innate immunity. Chronic infection with Helicobacter pylori is identified as the strongest risk factor for gastric cancer. Thus, we aim to explore the function of STING signaling in the development of gastric cancer. Immunohistochemistry was used to detect STING expression in 217 gastric cancer patients who underwent surgical resection. STING protein expression was remarkably decreased in tumor tissues compared to non-tumor tissues, and low STING staining intensity was positively correlated with tumor size, tumor invasion depth, lymph mode metastasis, TNM stage, and reduced patients’ survival. Multivariate analysis identified STING as an independent prognostic factor, which could improve the predictive accuracy for overall survival when incorporated into TNM staging system. In vitro studies revealed that knock-down of STING promoted colony formation, viability, migration and invasion of gastric cancer cells, and also led to a defect in cytosolic DNA sensing. Besides, chronic H. pylori infection up-regulated STING expression and activated STING signaling in mice. In conclusion, STING was proposed as a novel independent prognostic factor and potential immunotherapeutic target for gastric cancer.

Liposomal Delivery Enhances Immune Activation by STING Agonists for Cancer Immunotherapy

Overcoming the immunosuppressive tumor microenvironment (TME) is critical to realizing the potential of cancer immunotherapy strategies. Agonists of stimulator of interferon genes (STING), a cytosolic immune adaptor protein, have been shown to induce potent anti-tumor activity when delivered into the TME. However, the anionic properties of STING agonists make them poorly membrane permeable, and limit their ability to engage STING in the cytosol of responding cells. In this study, cationic liposomes with varying surface polyethylene glycol (PEG) levels were used to encapsulate cGAMP to facilitate its cytosolic delivery. In vitro studies with antigen-presenting cells (APCs) revealed that liposomal formulations substantially improved the cellular uptake of cGAMP and pro-inflammatory gene induction relative to free drug. Liposomal encapsulation allowed cGAMP delivery to metastatic melanoma tumors in the lung, leading to anti-tumor activity, whereas free drug produced no effect at the same dose. Injection of liposomal cGAMP into orthotopic melanoma tumors showed retention of cGAMP at the tumor site and co-localization with tumor-associated APCs. Liposomal delivery induced regression of injected tumors and produced immunological memory that protected previously treated mice from rechallenge with tumor cells. These results show that liposomal delivery improves STING agonist activity, and could improve their utility in clinical oncology.

SENP7 Potentiates cGAS Activation by Relieving SUMO-Mediated Inhibition of Cytosolic DNA Sensing

Cyclic GMP-AMP (cGAMP) synthase (cGAS, a.k.a. MB21D1), a cytosolic DNA sensor, catalyzes formation of the second messenger 2’3’-cGAMP that activates the stimulator of interferon genes (STING) signaling. How the cGAS activity is modulated remains largely unknown. Here, we demonstrate that sentrin/SUMO-specific protease 7 (SENP7) interacted with and potentiated cGAS activation. The small ubiquitin-like modifier (SUMO) was conjugated onto the lysine residues 335, 372 and 382 of cGAS, which suppressed its DNA-binding, oligomerization and nucleotidyl-transferase activities. SENP7 reversed this inhibition via catalyzing the cGAS de-SUMOylation. Consistently, silencing of SENP7 markedly impaired the IRF3-responsive gene expression induced by cGAS-STING axis. SENP7-knockdown mice were more susceptible to herpes simplex virus 1 (HSV-1) infection. SENP7 was significantly up-regulated in patients with SLE. Our study highlights the temporal modulation of the cGAS activity via dynamic SUMOylation, uncovering a novel mechanism for fine-tuning the STING signaling in innate immunity.

The Cyclic GMP-AMP (cGAMP) synthase (cGAS, a.k.a. MB21D1) is critical for monitoring the pathogen-derived DNA upon microbial infection. Its activity should be dynamically modulated in case the inadvertent recognition of the aberrant self nucleic acids in cytosol leads to severe autoimmune diseases. Protein posttranslational modifications dynamically shape the strength and duration of the immune signaling pathways. It is intriguing to explore whether SUMOylation could modulate the cGAS-initiated signaling. In this study, we characterized sentrin/SUMO-specific protease 7 (SENP7) to specifically potentiate the cGAS activation. Upon microbial DNA challenge, the small ubiquitin-like modifier (SUMO) was conjugated onto cGAS, which suppressed its DNA-binding, oligomerization and nucleotidyl-transferase activities. SENP7 reversed this inhibition via catalyzing the de-SUMOylation of cGAS. Our study sheds new light on the dynamic function of the SUMOylation in cytosolic DNAs-triggered innate immunity response.

Disease-associated mutations identify a novel region in human STING necessary for the control of type I interferon signaling

BACKGROUND

Gain-of-function mutations in transmembrane protein 173 (TMEM173) encoding stimulator of interferon genes (STING) underlie a recently described type I interferonopathy called STING-associated vasculopathy with onset in infancy (SAVI).

OBJECTIVES

We sought to define the molecular and cellular pathology relating to 3 individuals variably exhibiting the core features of the SAVI phenotype including systemic inflammation, destructive skin lesions, and interstitial lung disease.

METHODS

Genetic analysis, conformational studies, in vitro assays and ex vivo flow-cytometry were performed.

RESULTS

Molecular and in vitro data demonstrate that the pathology in these patients is due to amino acid substitutions at positions 206, 281, and 284 of the human STING protein. These mutations confer cGAMP-independent constitutive activation of type I interferon signaling through TBK1 (TANK-binding kinase), independent from the alternative STING pathway triggered by membrane fusion of enveloped RNA viruses. This constitutive activation was abrogated by ex vivo treatment with the janus kinase 1/2 inhibitor ruxolitinib.

CONCLUSIONS

Structural analysis indicates that the 3 disease-associated mutations at positions 206, 281, and 284 of the STING protein define a novel cluster of amino acids with functional importance in the regulation of type I interferon signaling.

Methods of Assessing STING Activation and Trafficking

The signaling adapter protein STING is crucial for the host immune response to cytosolic DNA and cyclic dinucleotides. Under basal conditions, STING resides on the endoplasmic reticulum (ER ) , but upon activation, it traffics through secretory pathway to cytoplasmic vesicles, where STING activates downstream immune signaling. Classical STING activation and trafficking are triggered by binding of cyclic dinucleotide ligands. STING signaling can also be activated by gain-of-function mutations that lead to constitutive trafficking of STING. These gain-of-function mutations are associated with several human diseases such as STING-associated vasculopathy with onset in infancy (SAVI), systemic lupus erythematosus (SLE), or familial chilblain lupus (FCL). This dynamic activation pathway presents a challenge to study. We describe methods here for measuring ligand-dependent and ligand-independent activation of STING signaling in HEK293T cells. We also describe a retroviral-based reconstitution assay to study STING protein trafficking and activation in immune competent cells such as mouse embryonic fibroblasts (MEF), which avoids the use of plasmid DNA. These methods will expedite research regarding STING trafficking and signaling dynamics in the settings of infection and autoimmune diseases.

Hitting the right button: MAVS-mediated defense against HAV infection

Studying the immune response against infection with hepatitis viruses is hampered by the lack of suitable preclinical model systems. A recent publication in Science identifies the cytosolic adapter molecule MAVS as being responsible for species restriction of infection with hepatitis A virus as well as linking cytosolic immune sensing in infected hepatocytes with innate effector functions and protective adaptive immunity.

Innate recognition of microbial-derived signals in immunity and inflammation

Microbes generate a vast array of different types of conserved structural components called pathogen-associated molecular patterns (PAMPs), which can be recognized by cells of the innate immune system. This recognition of "nonself" signatures occurs through host pattern recognition receptors (PRRs), suggesting that microbial-derived signals are good targets for innate immunity to discriminate between self- and nonself. Such PAMP-PRR interactions trigger multiple but distinct downstream signaling cascades, subsequently leading to production of proinflammatory cytokines and interferons that tailor immune responses to particular microbes. Aberrant PRR signals have been associated with various inflammatory diseases and fine regulation of PRR signaling is essential for avoiding excessive inflammatory immune responses and maintaining immune homeostasis. In this review we summarize the ligands and signal transduction pathways of PRRs and highlight recent progress of the mechanisms involved in microbe-specific innate immune recognition during immune responses and inflammation, which may provide new targets for therapeutic intervention to the inflammatory disorders.

Design, Synthesis, and Biological Evaluation of Novel Cyclic Adenosine-Inosine Monophosphate (cAIMP) Analogs That Activate Stimulator of Interferon Genes (STING)

We describe novel STING-activating cyclic dinucleotides whose constituent nucleosides are adenosine and inosine and that vary by ribose substitution, internucleotide linkage position, and phosphate modification. In mammalian cells in vitro, some of these cAIMP analogs induce greater STING-dependent IRF and NF-κB pathway signaling than do the reference agonists for murine (DMXAA) or human (2',3'-cGAMP) STING. In human blood ex vivo, they induce type I interferons (IFNs) and proinflammatory cytokines: for the former, 3',3'-cAIMP (9; EC₅₀ of 6.4 μM) and analogs 52-56 (EC₅₀ of 0.4-4.7 μM), which contain one or two 2'-fluoro-2'-deoxyriboses and/or bis-phosphorothioate linkages, are more potent than 2',3'-cGAMP (EC₅₀ of 19.6 μM). Interestingly, 9 induces type I IFNs more strongly than do its linkage isomers 2',3'-cAIMP (10), 3',2'-cAIMP (23), and 2',2'-cAIMP (27). Lastly, some of the cAIMP analogs are more resistant than 2',3'-cGAMP to enzymatic cleavage in vitro. We hope to exploit our findings to develop STING-targeted immunotherapies.

Functions and Malfunctions of Mammalian DNA-Cytosine Deaminases

The AID/APOBEC family enzymes convert cytosines in single-stranded DNA to uracils, causing base substitutions and strand breaks. They are induced by cytokines produced during the body's inflammatory response to infections, and they help combat infections through diverse mechanisms. AID is essential for the maturation of antibodies and causes mutations and deletions in antibody genes through somatic hypermutation (SHM) and class-switch recombination (CSR) processes. One member of the APOBEC family, APOBEC1, edits mRNA for a protein involved in lipid transport. Members of the APOBEC3 subfamily in humans (APOBEC3A, APOBEC3B, APOBEC3C, APOBEC3D, APOBEC3F, APOBEC3G, and APOBEC3H) inhibit infections of viruses such as HIV-1, HBV, and HCV, and retrotransposition of endogenous retroelements through mutagenic and nonmutagenic mechanisms. There is emerging consensus that these enzymes can cause mutations in the cellular genome at replication forks or within transcription bubbles depending on the physiological state of the cell and the phase of the cell cycle during which they are expressed. We describe here the state of knowledge about the structures of these enzymes, regulation of their expression, and both the advantageous and deleterious consequences of their expression, including carcinogenesis. We highlight similarities among them and present a holistic view of their regulation and function.

Emerging Mechanisms in Initiating and Terminating Autophagy

Autophagy is a major degradative process activated in a rapid and transient manner to cope with stress conditions. Whether autophagy is beneficial or detrimental depends upon the rate of induction and the appropriateness of the duration. Alterations in both autophagy initiation and termination predispose the cell to death, and affect the execution of other inducible processes such as inflammation. In this review we discuss how stress signaling pathways dynamically control the activity of the autophagy machinery by mediating post-translational modifications and regulatory protein interactions. In particular, we highlight the emerging role of TRIM and CULLIN families of ubiquitin ligases which play opposite roles in the autophagy response by promoting or inhibiting, respectively, the activity of the autophagy initiation complex.

cGAS-cGAMP-STING: The three musketeers of cytosolic DNA sensing and signaling

Innate immunity is the first line of host defense against invading pathogens. The detection of aberrant nucleic acids which represent some conserved PAMPs triggers robust type I IFN-mediated innate immune responses. Host- or pathogen-derived cytosolic DNA binds and activates the DNA sensor cGAS, which synthesizes the second messenger 2'3'-cGAMP and triggers STING-dependent downstream signaling. Here, we highlight recent progress in cGAS-cGAMP-STING, the Three Musketeers of cytosolic DNA sensing and signaling, and their essential roles in infection, autoimmune diseases, and cancer. We also focus on the regulation of these critical signal components by variant host/pathogen proteins and update our understanding of this indispensable pathway to provide new insights for drug discovery. © 2016 IUBMB Life, 68(11):858-870, 2016.

STING Requires the Adaptor TRIF to Trigger Innate Immune Responses to Microbial Infection

The intracellular microbial nucleic acid sensors, TLR3 and STING, recognize pathogen molecules and signal to activate the interferon pathway. The TIR-domain containing protein TRIF is the sole adaptor of TLR3. Here, we report an essential role for TRIF in STING signaling: various activators of STING could not induce genes in the absence of TRIF. TRIF and STING interacted directly, through their carboxy-terminal domains, to promote STING dimerization, intermembrane translocation, and signaling. Herpes simplex virus (HSV), which triggers the STING signaling pathway and is controlled by it, replicated more efficiently in the absence of TRIF, and HSV-infected TRIF(-/-) mice displayed pronounced pathology. Our results indicate that defective STING signaling may be responsible for the observed genetic association between TRIF mutations and herpes simplex encephalitis in patients.

Sumoylation Promotes the Stability of the DNA Sensor cGAS and the Adaptor STING to Regulate the Kinetics of Response to DNA Virus

During viral infection, sensing of cytosolic DNA by the cyclic GMP-AMP synthase (cGAS) activates the adaptor protein STING and triggers an antiviral response. Little is known about the mechanisms that determine the kinetics of activation and deactivation of the cGAS-STING pathway, ensuring effective but controlled innate antiviral responses. Here we found that the ubiquitin ligase Trim38 targets cGas for sumoylation in uninfected cells and during the early phase of viral infection. Sumoylation of cGas prevented its polyubiquitination and degradation. Trim38 also sumoylated Sting during the early phase of viral infection, promoting both Sting activation and protein stability. In the late phase of infection, cGas and Sting were desumoylated by Senp2 and subsequently degraded via proteasomal and chaperone-mediated autophagy pathways, respectively. Our findings reveal an essential role for Trim38 in the innate immune response to DNA virus and provide insight into the mechanisms that ensure optimal activation and deactivation of the cGAS-STING pathway.

Sensing of dangerous DNA

The presence of damaged and microbial DNA can pose a threat to the survival of organisms. Cells express various sensors that recognize specific aspects of such potentially dangerous DNA. Recognition of damaged or microbial DNA by sensors induces cellular processes that are important for DNA repair and inflammation. Here, we review recent evidence that the cellular response to DNA damage and microbial DNA are tightly intertwined. We also discuss insights into the parameters that enable DNA sensors to distinguish damaged and microbial DNA from DNA present in healthy cells.

Telltale tumor infiltrating lymphocytes (TIL) in oral, head & neck cancer

Evidence gleaned from recent studies on the role of tumor-infiltrating lymphocytes (TILs) suggests that cancer is not only a genetic disease but also an immunologic disease. Head and Neck Squamous Cell Carcinoma (HNSCC) has been a significant model to study cancer cell-immune cell interactions. First, immune cell infiltration is an important feature of these tumors. Second, HNSCC frequently develops resistance to immunogenic cytotoxicity, which provides a window to decipher how tumors engage the immune system to establish immune tolerance. Finally, chemoradiation therapy, as a central modality for HNSCC treatment, has been shown to elicit immune activation. The presence of effector immune cells in the tumor microenvironment is often associated with superior clinical response to adjuvant therapy. On the other hand, an activated immune system, in addition to limiting tumor initiation and progression, could also exert selective pressure to promote the growth of less immunogenic tumors, as a pivotal immunoediting process. But it remains unclear how cancer cell signaling regulates tumor immunogenicity and how to mitigate HNSCC-potentiated TIL suppression. In this review, we will revisit the prognostic role of TILs in HNSCC, and collectively discuss how cancer cell machinery impacts upon the plasticity of TILs.

The expanding regulatory network of STING-mediated signaling

The identification and characterization of DNA-sensing pathways has been a subject of intensive investigation for the last decade. This interest, in part, is supported by the fact that the main outcome of DNA-responses is production of type I interferon (IFN-I), which, if produced in excessive amounts, leads to various pathologies. STING (stimulator of interferon genes) is positioned in the center of these responses and is activated either via direct sensing of second messengers or via interaction with upstream sensors of dsDNA. STING mediates responses to pathogens as well as host-derived DNA and is, therefore, linked to various autoimmune diseases, cancer predisposition and ageing. Recent mouse models of DNA damage showed the adaptor STING to be crucial for heightened resting levels of IFN-I. In this review, we will focus on recent advances in understanding the regulation of STING-signaling and identification of its novel components.

The host STING pathway at the interface of cancer and immunity

A major subset of human cancers shows evidence for spontaneous adaptive immunity, which is reflected by the presence of infiltrating CD8+ T cells specific for tumor antigens within the tumor microenvironment. This observation has raised the question of which innate immune sensing pathway might detect the presence of cancer and lead to a natural adaptive antitumor immune response in the absence of exogenous infectious pathogens. Evidence for a critical functional role for type I IFNs led to interrogation of candidate innate immune sensing pathways that might be triggered by tumor presence and induce type I IFN production. Such analyses have revealed a major role for the stimulator of IFN genes pathway (STING pathway), which senses cytosolic tumor-derived DNA within the cytosol of tumor-infiltrating DCs. Activation of this pathway is correlated with IFN-β production and induction of antitumor T cells. Based on the biology of this natural immune response, pharmacologic agonists of the STING pathway are being developed to augment and optimize STING activation as a cancer therapy. Intratumoral administration of STING agonists results in remarkable therapeutic activity in mouse models, and STING agonists are being carried forward into phase I clinical testing.

The role of myeloid cells in cancer therapies

Recent clinical trials have demonstrated the ability to durably control cancer in some patients by manipulating T lymphocytes. These immunotherapies are revolutionizing cancer treatment but benefit only a minority of patients. It is thus a crucial time for clinicians, cancer scientists and immunologists to determine the next steps in shifting cancer treatment towards better cancer control. This Review describes recent advances in our understanding of tumour-associated myeloid cells. These cells remain less studied than T lymphocytes but have attracted particular attention because their presence in tumours is often linked to altered patient survival. Also, experimental studies indicate that myeloid cells modulate key cancer-associated activities, including immune evasion, and affect virtually all types of cancer therapy. Consequently, targeting myeloid cells could overcome limitations of current treatment options.

Development of a multifunctional envelope-type nano device and its application to nanomedicine

Successful nanomedicines should be based on sound drug delivery systems (DDS) the permit intracellular trafficking as well as the biodistribution of cargos to be controlled. We have been developing new types of DDS that are multifunctional envelope-type nano devices referred to as MENDs. First, we will focus the in vivo delivery of siRNA to hepatocytes using a YSK-MEND which is composed of pH-responsive cationic lipids. The YSK-MEND is capable of inducing efficient silencing activity in hepatocytes and can be used to cure mice that are infected with hepatitis C or B. The YSK-MEND can also be applied to cancer immunotherapy through the activation of immune cells by delivering different compounds such as cyclic-di-GMP, siRNA or alpha-galactosylceramide as a lipid antigen. The findings indicate that, as predicted, these compounds, when encapsulated in the YSK-MEND, can be delivered to the site of action and induced immune activation through different mechanisms. Finally, a MITO-Porter, a membrane fusion-based delivery system to mitochondria, is introduced as an organelle targeting DDS and a new strategy for cancer therapy is proposed by delivering gentamicin to mitochondria of cancer cells. These new technologies are expected to extend the therapeutic area of Nanomedicine by increasing the power of DDS, especially from the view point of controlled intracellular trafficking.

Beyond autophagy: New roles for ULK1 in immune signaling and interferon responses

The human serine/threonine kinase ULK1 is the human homolog of the Caenorhabditis elegans Unc-51 kinase and of the Saccharomyces cerevisiae autophagy-related protein kinase Atg1. As Unc-51 and Atg1, ULK1 regulates both axon growth and autophagy, respectively, in mammalian cells. However, a novel immunoregulatory role of ULK1 has been recently described. This kinase was shown to be required for regulation of both type I interferon (IFN) production and induction of type I IFN signaling. Optimal regulation of IFN production is crucial for generation of effective IFN-immune responses, and defects in such networks can be detrimental for the host leading to uncontrolled pathogen infection, tumor growth, or autoimmune diseases. Thus, ULK1 plays a central role in IFN-dependent immunity. Here we review the diverse roles of ULK1, with special focus on its importance to type I IFN signaling, and highlight important future study questions.

The DNA Structure-Specific Endonuclease MUS81 Mediates DNA Sensor STING-Dependent Host Rejection of Prostate Cancer Cells

Self-DNA is present in the cytosol of many cancer cells and can promote effective immune rejection of tumor cells, but the mechanisms leading to the presence of cytosolic DNA are unknown. Here, we report that the cleavage of genomic DNA by DNA structure-specific endonuclease MUS81 and PARP-dependent DNA repair pathways leads to the accumulation of cytosolic DNA in prostate cancer cells. The number of nuclear MUS81 foci and the amount of cytosolic dsDNA increased in tandem from hyperplasia to clinical stage II prostate cancers and decreased at stage III. Cytosolic DNA generated by MUS81 stimulated DNA sensor STING-dependent type I interferon (IFN) expression and promoted phagocytic and T cell responses, resulting in type I and II IFN-mediated rejection of prostate tumor cells via mechanisms that partly depended on macrophages. Our results demonstrate that the tumor suppressor MUS81 alerts the immune system to the presence of transformed host cells.

Cre-dependent DNA recombination activates a STING-dependent innate immune response

Gene-recombinase technologies, such as Cre/loxP-mediated DNA recombination, are important tools in the study of gene function, but have potential side effects due to damaging activity on DNA. Here we show that DNA recombination by Cre instigates a robust antiviral response in mammalian cells, independent of legitimate loxP recombination. This is due to the recruitment of the cytosolic DNA sensor STING, concurrent with Cre-dependent DNA damage and the accumulation of cytoplasmic DNA. Importantly, we establish a direct interplay between this antiviral response and cell–cell interactions, indicating that low cell densities in vitro could be useful to help mitigate these effects of Cre. Taking into account the wide range of interferon stimulated genes that may be induced by the STING pathway, these results have broad implications in fields such as immunology, cancer biology, metabolism and stem cell research. Further, this study sets a precedent in the field of gene-engineering, possibly applicable to other enzymatic-based genome editing technologies.

Unique Loss of the PYHIN Gene Family in Bats Amongst Mammals: Implications for Inflammasome Sensing

Recent genomic analysis of two bat species (Pteropus alecto and Myotis davidii) revealed the absence of the PYHIN gene family. This family is recognized as important immune sensors of intracellular self and foreign DNA and activators of the inflammasome and/or interferon pathways. Further assessment of a wider range of bat genomes was necessary to determine if this is a universal pattern for this large mammalian group. Here we expanded genomic analysis of this gene family to include ten bat species. We confirmed the complete loss of this gene family, with only a truncated AIM2 remaining in one species (Pteronotus parnellii). Divergence of the PYHIN gene loci between the bat lineages infers different loss-of-function histories during bat evolution. While all other major groups of placental mammals have at least one gene member, only bats have lost the entire family. This removal of inflammasome DNA sensors may indicate an important adaptation that is flight-induced and related, at least in part, to pathogen-host co-existence.