RNF39 facilitates antiviral immune responses by promoting K63-linked ubiquitination of STING.

The cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS)-dependent pathway is a key DNA-sensing pathway that recognizes cytosolic DNA and plays a crucial role in initiating innate immune responses against pathogenic microbes and cancer. Various molecules have been identified as regulators of the cGAS-dependent pathway that controls innate immune responses. However, despite the important roles of Stimulator-of-interferon genes (STING) in the cGAS-dependent pathway, the regulation of its activation has not been elucidated. Here, we show that the E3 ubiquitin ligase, RING finger protein 39 (RNF39), interacts with STING in macrophages and HERK293T cells. Moreover, RNF39 accelerates DNA-sensing pathways by promoting lysine (K)63-linked ubiquitination of STING, and then facilitating the formation of STING-TBK1 complex. Concordantly, Rnf39 deficiency inhibits innate immune responses triggered by DNA viral infection and accelerates viral replication. Furthermore, herpes simplex virus-1 (HSV-1) infection induces RNF39 expression in an IFN-I-dependent manner. Thus, we outline a novel mechanism for controlling STING activation and a feedback mechanism for controlling antiviral immune responses. RNF39 could be a priming intervention target for the prevention and treatment of viral diseases, especially DNA viral infections.

ISGylation by HERCs facilitates STING activation.

Optimal activation of stimulator of interferon genes (STING) protein is crucial for host defenses against pathogens and avoiding detrimental effects. Various post-translational modifications control STING activity. However, the function of interferon (IFN)-stimulated gene (ISG) 15 modification (ISGylation) in controlling STING stability and activation is unclear. Here, we show that the E3 ISGylation ligases HECT domain- and RCC1-like domain-containing proteins (HERCs; HERC5 in humans and HERC6 in mice) facilitate STING activation by mediating ISGylation of STING at K150, preventing its K48-linked ubiquitination and degradation. Concordantly, Herc6 deficiency suppresses herpes simplex virus 1 infection-induced type I IFN responses and facilitates viral replication both in vitro and in vivo. Notably, severe acute respiratory syndrome coronavirus 2 protein papain-like protease cleaves HERC5-mediated ISGylation of STING, suppressing host antiviral responses. These data identify a mechanism by which HERCs-mediated ISGylation controls STING stability and activation and uncover the correlations and interactions of ISGylation and ubiquitination during STING activation.

[In vitro study on the sealing effect of different shapes of cuff tracheal tubes under the lowest safe pressure].

OBJECTIVE: To compare the effectiveness of cylindrical-shaped and conical-shaped cuff catheters for airway closure using different pressure measurement methods at the lowest safe pressure and to guide the clinical application. METHODS: Twenty-four patients with endotracheal intubation admitted to the intensive care unit (ICU) of Guangxi Medical University Cancer Hospital from December 2021 to January 2022 were enrolled. Leakage test in vitro was performed on the secretion on the patients' cuff. The needle and plunger from 20 mL syringe was separated, the syringe was sealed with adhesive, and the syringe nozzle was filled thoroughly to create a tracheal model. Consecutively, both cylindrical-shaped and conical-shaped cuff catheters were inserted into the simulated trachea, and the cuff pressure was calibrated to 20 cmH2O (1 cmH2O ≈ 0.098 kPa) before commencing the experiment. The viscosity of the secretion on the patients' cuff was classified (grade I was watery subglottic secretion, grade II was thick subglottic secretion, grade III was gel-like subglottic secretion), and the same viscosity secretion was injected into the catheter cuff. Utilizing a self-control approach, intermittent pressure measurement was initially conducted on both the cylindrical-shaped and conical-shaped cuff by improved pressure measurement method (intermittent pressure measurement group), followed by continuous pressure measurement experiment (continuous pressure measurement group). The leakage volume of the three viscosity subglottic secretions and the values of cuff pressure measurement of different shaped cuff catheters at 4, 6, 8 hours of inflation were recorded. RESULTS: A total of 180 retention samples were extracted from 24 patients with tracheal intubation during ventilation, with 90 samples in each of the two groups using different pressure measurement methods, and 30 samples of retention materials with different viscosities in each group. In the intermittent pressure measurement group, at 4 hours of inflation, all samples of secretion with grade I and grade II on cylindrical-shaped cuff leaked, while 3 samples of secretion with grade III also leaked. For conical-shaped cuff, 28 samples of secretion with grade I leaked, only 2 samples of secretion with grade II leaked, and there was no leak for secretion with grade III. At 6 hours of inflation, all samples of the three viscosity secretions on different shaped cuffs leaked. The leakage was gradually increased with the prolongation of inflation time. In the continuous pressure measurement group, at 4 hours of inflation, all samples of secretion with grade I on cylindrical-shaped cuff leaked, while 29 samples of secretion with grade II leaked, and there was no leak for secretion with grade III. For the conical-shaped cuff, 26 samples of secretion with grade I leaked, and there was no leak for secretion with grade II and grade III. At 6 hours of inflation, the conical-shaped cuff still had no leak for secretion with grade III. As the inflation time prolonged, the leakage of subglottic secretion on different shaped cuffs in both groups was gradually increased. At 8 hours of inflation, all samples experienced leakage, but the leakage of subglottic secretion on different shaped cuffs in the continuous pressure measurement group was significantly reduced as compared with the intermittent pressure measurement group [leakage for secretion with grade III (mL): 1.00 (0.00, 1.25) vs. 2.00 (1.00, 2.00) on the cylindrical-shaped cuff, 1.00 (0.00, 1.00) vs. 2.00 (2.00, 2.00) on the conical-shaped cuff, both P < 0.01]. The values of pressure measurement of cuffs with different shapes at different time points of inflation in the continuous pressure measurement group were within the set range (20-21 cmH2O). The cuff pressure at 4 hours of inflation in the intermittent pressure measurement group was significantly lower than the initial value (cmH2O: 18.3±0.6 vs. 20.0±0.0 in the cylindrical-shaped cuff, 18.4±0.6 vs. 20.0±0.0 in the conical-shaped cuff, both P < 0.01), and the cuff pressure in both shaped cuffs showed a significant decrease tendency as inflation time prolonged. However, there was no statistically significant difference in values of pressure measurement between the different shaped cuff catheters. CONCLUSIONS: Continuous pressure monitoring devices can maintain the effective sealing of conical-shaped cuff catheters at the lowest safe pressure. When using an improved pressure measurement method for intermittent pressure measurement and/or using a cylindrical cuff catheter, the target pressure should be set at 25-30 cmH2O, and the cuff pressure should be adjusted regularly.

S-nitrosothiol homeostasis maintained by ADH5 facilitates STING-dependent host defense against pathogens.

Oxidative (or respiratory) burst confers host defense against pathogens by generating reactive species, including reactive nitrogen species (RNS). The microbial infection-induced excessive RNS damages many biological molecules via S-nitrosothiol (SNO) accumulation. However, the mechanism by which the host enables innate immunity activation during oxidative burst remains largely unknown. Here, we demonstrate that S-nitrosoglutathione (GSNO), the main endogenous SNO, attenuates innate immune responses against herpes simplex virus-1 (HSV-1) and Listeria monocytogenes infections. Mechanistically, GSNO induces the S-nitrosylation of stimulator of interferon genes (STING) at Cys257, inhibiting its binding to the second messenger cyclic guanosine monophosphate-adenosine monophosphate (cGAMP). Alcohol dehydrogenase 5 (ADH5), the key enzyme that metabolizes GSNO to decrease cellular SNOs, facilitates STING activation by inhibiting S-nitrosylation. Concordantly, Adh5 deficiency show defective STING-dependent immune responses upon microbial challenge and facilitates viral replication. Thus, cellular oxidative burst-induced RNS attenuates the STING-mediated innate immune responses to microbial infection, while ADH5 licenses STING activation by maintaining cellular SNO homeostasis.

Engineered exosomes with enhanced stability and delivery efficiency for glioblastoma therapy.

Due to the blood-brain barrier (BBB), the application of chemical drugs for glioblastoma treatment is severely limited. Recently, exosomes have been widely applied for drug delivery to the brain. However, the differences in brain targeting efficiency among exosomes derived from different cell sources, as well as the premature drug leakage during circulation, still limit the therapeutic efficacy. Here, we designed a functional oligopeptide-modified exosome loaded with doxorubicin (Pep2-Exos-DOX) for glioblastoma treatment. BV2 mouse microglial cell line was selected as the exosome source due to the favorable BBB penetration. To avoid drug release in the circulation, a redox-response oligopeptide was designed for incorporation into the membranes of exosomes to lock the drug during circulation. The enrichment of the drug in glioblastoma was confirmed. Pharmacodynamic evaluation showed Pep2-Exos-DOX possessed significant anti-cancer activity against glioblastoma as well as relative biosafety. This exosome-based drug delivery system modified with redox-response oligopeptides provides us a novel strategy for brain diseases treatment.

The UAF1-USP1 Deubiquitinase Complex Stabilizes cGAS and Facilitates Antiviral Responses.

Cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) detects cytoplasmic microbial DNA and self-DNA from genomic instability, initiates innate immunity, and plays fundamental roles in defense against viruses and the development of various diseases. The cellular cGAS level determines the magnitude of the response to DNA. However, the underlying mechanisms of the control of cGAS stability, especially its feedback regulation during viral infection, remain largely unknown. In this study, we show that viral infection induces the expression of the UAF1-USP1 deubiquitinase complex in primary peritoneal macrophages (PMs) of C57BL/6J mice. UAF1-USP interacts with cGAS, selectively cleaves its K48-linked polyubiquitination, and thus stabilizes its protein expression in PMs and HEK293T cells. Concordantly, the UAF1-USP1 deubiquitinase complex enhances cGAS-dependent type I IFN responses in PMs. Uaf1 deficiency and ML323 (a specific inhibitor of UAF1-USP1 deubiquitinase complex) attenuates cGAS-triggered antiviral responses and facilitates viral replication both in vitro and in vivo. Thus, our study uncovers a positive feedback mechanism of cGAS-dependent antiviral responses and suggests the UAF1-USP1 complex as a potential target for the treatment of diseases caused by aberrant cGAS activation.

Resveratrol, a novel inhibitor of fatty acid binding protein 5, inhibits cervical cancer metastasis by suppressing fatty acid transport into nucleus and downstream pathways.

BACKGROUND AND PURPOSE: Because of cervical cancer (CC) metastasis, the prognosis of diagnosed patients is poor. However, the molecular mechanisms and therapeutic approach for metastatic CC remain elusive. EXPERIMENTAL APPROACH: In this study, we first evaluated the effect of resveratrol (RSV) on CC cell migration and metastasis. Via an activity-based protein profiling (ABPP) approach, a photoaffinity probe of RSV (RSV-P) was synthesized, and the protein targets of RSV in HeLa cells were identified. Based on target information and subsequent in vivo and in vitro validation experiments, we finally elucidated the mechanism of RSV corresponding to its antimetastatic activity. KEY RESULTS: The results showed that RSV concentration-dependently suppressed CC cell migration and metastasis. A list of proteins was identified as the targets of RSV, through the ABPP approach with RSV-P, among which fatty acid binding protein 5 (FABP5) attracted our attention based on The Cancer Genome Atlas (TCGA) database analysis. Subsequent knockout and overexpression experiments confirmed that RSV directly interacted with FABP5 to inhibit fatty acid transport into the nucleus, thereby suppressing downstream matrix metalloproteinase-2 (MMP2) and matrix metalloproteinase-9 (MMP9) expression, thus inhibiting CC metastasis. CONCLUSIONS AND IMPLICATIONS: Our study confirmed the key role of FABP5 in CC metastasis and provided important target information for the design of therapeutic lead compounds for metastatic CC.

Polyamine metabolism controls B-to-Z DNA transition to orchestrate DNA sensor cGAS activity.

Cyclic guanosine monophosphate (GMP)-AMP (cGAMP) synthase (cGAS) is a universal double-stranded DNA (dsDNA) sensor that recognizes foreign and self-DNA in the cytoplasm and initiates innate immune responses and has been implicated in various infectious and non-infectious contexts. cGAS binds to the backbone of dsDNA and generates the second messenger, cGAMP, which activates the stimulator of interferon genes (STING). Here, we show that the endogenous polyamines spermine and spermidine attenuated cGAS activity and innate immune responses. Mechanistically, spermine and spermidine induced the transition of B-form DNA to Z-form DNA (Z-DNA), thereby decreasing its binding affinity with cGAS. Spermidine/spermine N1-acetyltransferase 1 (SAT1), the rate-limiting enzyme in polyamine catabolism that decreases the cellular concentrations of spermine and spermidine, enhanced cGAS activation by inhibiting cellular Z-DNA accumulation; SAT1 deficiency promoted herpes simplex virus 1 (HSV-1) replication in vivo. The results indicate that spermine and spermidine induce dsDNA to adopt the Z-form conformation and that SAT1-mediated polyamine metabolism orchestrates cGAS activity.

Integrative analysis of immune-related signature profiles in eosinophilic chronic rhinosinusitis with nasal polyposis.

Eosinophilic chronic rhinosinusitis with nasal polyps (ECRSwNP) is a subtype of chronic rhinosinusitis (CRS) that is associated with the nasal cavity and sinus polyps, elevated levels of eosinophils, and dysregulated immune responses to environmental triggers. The underlying cause of ECRSwNP is not well understood, and few studies have focused on the unique features of this subtype of CRS. Our study integrated proteomic and transcriptomic data with multi-omic bioinformatics analyses. We collected nasal polyps from three ECRSwNP patients and three control patients and identified 360 differentially expressed (DE) proteins, including 119 upregulated and 241 downregulated proteins. Functional analyses revealed several significant associations with ECRSwNP, including focal adhesion, hypertrophic cardiomyopathy, and extracellular matrix (ECM)-receptor interactions. Additionally, a protein-protein interaction (PPI) network revealed seven hub proteins that may play crucial roles in the development of ECRSwNP. We also compared the proteomic data with publicly available transcriptomic data and identified a total of 1077 DE genes. Pathways enriched by the DE genes involved angiogenesis, positive regulation of cell motility, and immune responses. Furthermore, we investigated immune cell infiltration and identified biomarkers associated with eosinophil and M2 macrophage infiltration using CIBERSORT and Weighted Gene Correlation Network Analysis (WGCNA). Our results provide a more complete picture of the immune-related mechanisms underlying ECRSwNP, which could contribute to the development of more precise treatment strategies for this condition.

Machine-Learning-Based Calibration of Temperature Sensors.

Temperature sensors are widely used in industrial production and scientific research, and accurate temperature measurement is crucial for ensuring the quality and safety of production processes. To improve the accuracy and stability of temperature sensors, this paper proposed using an artificial neural network (ANN) model for calibration and explored the feasibility and effectiveness of using ANNs to calibrate temperature sensors. The experiment collected multiple sets of temperature data from standard temperature sensors in different environments and compared the calibration results of the ANN model, linear regression, and polynomial regression. The experimental results show that calibration using the ANN improved the accuracy of the temperature sensors. Compared with traditional linear regression and polynomial regression, the ANN model produced more accurate calibration. However, overfitting may occur due to a small sample size or a large amount of noise. Therefore, the key to improving calibration using the ANN model is to design reasonable training samples and adjust the model parameters. The results of this study are important for practical applications and provide reliable technical support for industrial production and scientific research.

Posttranslational ISGylation of NLRP3 by HERC enzymes facilitates inflammasome activation in models of inflammation.

The NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome is a crucial component of the innate immune system that initiates inflammatory responses. Posttranslational modifications (PTMs) of NLRP3, including ubiquitination and phosphorylation, control inflammasome activation and determine the intensity of inflammation. However, the role of other PTMs in controlling NLRP3 inflammasome activation remains unclear. This study found that TLR priming induced NLRP3 ISGylation (a type of PTM in which ISG15 covalently binds to the target protein) to stabilize the NLRP3 protein. Viral infection, represented by SARS-COV-2 infection, and type I IFNs induced expression of ISG15 and the predominant E3 ISGylation ligases HECT domain- and RCC1-like domain-containing proteins (HERCs; HERC5 in humans and HERC6 in mice). HERCs promoted NLRP3 ISGylation and inhibited K48-linked ubiquitination and proteasomal degradation, resulting in the enhancement of NLRP3 inflammasome activation. Concordantly, Herc6 deficiency ameliorated NLRP3-dependent inflammation as well as hyperinflammation caused by viral infection. The results illustrate the mechanism by which type I IFNs responses control inflammasome activation and viral infection-induced aberrant NLRP3 activation. This work identifies ISGylation as a PTM of NLRP3, revealing a priming target that modulates NLRP3-dependent immunopathology.

Selenoprotein K enhances STING oligomerization to facilitate antiviral response.

Stimulator-of-interferon gene (STING) is a vital element of the innate immune system against DNA viruses. Optimal activation of STING is crucial for maintaining immune homeostasis and eliminating invading viruses, and the oligomerization of STING is an essential prerequisite for STING activation. However, the mechanism of cGAMP-induced STING oligomerization in ER remains unclear. Selenoproteins are crucial for various physiological processes. Here, we identified that the endoplasmic reticulum (ER)-located transmembrane selenoprotein K (SELENOK) was induced during virus infection and facilitated innate immune responses against herpes simplex virus-1 (HSV-1). Mechanistically, SELENOK interacts with STING in the ER and promotes STING oligomerization, which in turn promotes its translocation from the ER to the Golgi. Consequently, Selenok deficiency suppresses STING-dependent innate responses and facilitates viral replication in vivo. Thus, the control of STING activation by selenium-mediated SELENOK expression will be a priming therapeutic strategy for the treatment of STING-associated diseases.

Myristic acid as a checkpoint to regulate STING-dependent autophagy and interferon responses by promoting N-myristoylation.

Stimulator of interferon gene (STING)-triggered autophagy is crucial for the host to eliminate invading pathogens and serves as a self-limiting mechanism of STING-induced interferon (IFN) responses. Thus, the mechanisms that ensure the beneficial effects of STING activation are of particular importance. Herein, we show that myristic acid, a type of long-chain saturated fatty acid (SFA), specifically attenuates cGAS-STING-induced IFN responses in macrophages, while enhancing STING-dependent autophagy. Myristic acid inhibits HSV-1 infection-induced innate antiviral immune responses and promotes HSV-1 replication in mice in vivo. Mechanistically, myristic acid enhances N-myristoylation of ARF1, a master regulator that controls STING membrane trafficking. Consequently, myristic acid facilitates STING activation-triggered autophagy degradation of the STING complex. Thus, our work identifies myristic acid as a metabolic checkpoint that contributes to immune homeostasis by balancing STING-dependent autophagy and IFN responses. This suggests that myristic acid and N-myristoylation are promising targets for the treatment of diseases caused by aberrant STING activation.

TOB1 attenuates IRF3-directed antiviral responses by recruiting HDAC8 to specifically suppress IFN-ß expression.

Interferon regulatory factor 3 (IRF3) is a key transcription factor required for the secretion of type I interferons (IFN-α/ß) and initiation of antiviral immune response. However, the negative feedback regulator of IRF3-directed antiviral response remains unknown. In this study, we demonstrated that viral infection induced the interaction of the transducer of ERBB2.1 (TOB1) with IRF3, which bound to the promoter region of Ifnb1 in macrophages. TOB1 inhibited Ifnb1 transcription by disrupting IRF3 binding and recruiting histone deacetylase 8 (HDAC8) to the Ifnb1 promoter region. Consequently, TOB1 attenuated IRF3-directed IFN-ß expression in virus-infected macrophages. Tob1 deficiency enhanced antiviral response and suppressed viral replication in vivo. Thus, we identified TOB1 as a feedback inhibitor of host antiviral innate immune response and revealed a mechanism underlying viral immune escape.

KAP1-Mediated Epigenetic Suppression in Anti-RNA Viral Responses by Direct Targeting RIG-I and MDA5.

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), including RIG-I (encoded by Ddx58) and melanoma differentiation-associated gene 5 (MDA5) (encoded by Ifih1), are crucial for initiating antiviral responses. Endogenous retroviral elements (ERVs) are transposable elements derived from exogenous retroviruses that are integrated into the genome. KRAB-associated protein 1 (KAP1) is a key epigenetic suppressor of ERVs that protects cells from detrimental genome instability. Increased ERV transcripts are sensed by RLRs and trigger innate immune signaling. However, whether KAP1 directly controls RLRs activity remains unclear. In this study, we show that KAP1 attenuates RNA viral infection-induced type I IFNs and facilitates viral replication by inhibiting RIG-I/MDA5 expression in primary peritoneal macrophages (PMs) of C57BL/6J mice. Kap1 deficiency increases IFN-ß expression and inhibits vesicular stomatitis virus replication in C57BL/6J mice in vivo. Mechanistically, KAP1 binds to the promoter regions of Ddx58 and Ifih1 and promotes the establishment of repressive histone marks in primary PMs of C57BL/6J mice. Concordantly, KAP1 suppresses the expression of RIG-I and MDA5 at the transcriptional level in primary PMs of C57BL/6J mice. Our results establish that KAP1 epigenetically suppresses host antiviral responses by directly targeting RIG-1 and MDA5, thus facilitating the immune escape of RNA viruses.

TRIM28 SUMOylates and stabilizes NLRP3 to facilitate inflammasome activation.

The cellular NLRP3 protein level is crucial for assembly and activation of the NLRP3 inflammasome. Various posttranslational modifications (PTMs), including phosphorylation and ubiquitination, control NLRP3 protein degradation and inflammasome activation; however, the function of small ubiquitin-like modifier (SUMO) modification (called SUMOylation) in controlling NLRP3 stability and subsequent inflammasome activation is unclear. Here, we show that the E3 SUMO ligase tripartite motif-containing protein 28 (TRIM28) is an enhancer of NLRP3 inflammasome activation by facilitating NLRP3 expression. TRIM28 binds NLRP3, promotes SUMO1, SUMO2 and SUMO3 modification of NLRP3, and thereby inhibits NLRP3 ubiquitination and proteasomal degradation. Concordantly, Trim28 deficiency attenuates NLRP3 inflammasome activation both in vitro and in vivo. These data identify a mechanism by which SUMOylation controls the cellular NLRP3 level and inflammasome activation, and reveal correlations and interactions of NLRP3 SUMOylation and ubiquitination during inflammasome activation.

Emerging Roles of MHC Class I Region-Encoded E3 Ubiquitin Ligases in Innate Immunity.

The major histocompatibility complex (MHC) class I (MHC-I) region contains a multitude of genes relevant to immune response. Multiple E3 ubiquitin ligase genes, including tripartite motif 10 (TRIM10), TRIM15, TRIM26, TRIM27, TRIM31, TRIM38, TRIM39, TRIM40, and RING finger protein 39 (RNF39), are organized in a tight cluster, and an additional two TRIM genes (namely TRIM38 and TRIM27) telomeric of the cluster within the MHC-I region. The E3 ubiquitin ligases encoded by these genes possess important roles in controlling the intensity of innate immune responses. In this review, we discuss the E3 ubiquitin ligases encoded within the MHC-I region, highlight their regulatory roles in innate immunity, and outline their potential functions in infection, inflammatory and autoimmune diseases.

Galectin-9 Targets NLRP3 for Autophagic Degradation to Limit Inflammation.

NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome has been implicated in a variety of inflammatory disorders, and its activation should be tightly controlled to avoid detrimental effects. NLRP3 protein expression is considered as the rate-limiting step for NLRP3 inflammasome activation. In this study, we show that galectin-9 (encoded by lgals9) attenuated NLRP3 inflammasome activation by promoting the protein degradation of NLRP3 in primary peritoneal macrophages of C57BL/6J mice. Lgals9 deficiency enhances NLRP3 inflammasome activation and promotes NLRP3-dependent inflammation in C57BL/6J mice in vivo. Mechanistically, galectin-9 interacts with NLRP3, promotes the formation of NLRP3/p62 (an autophagic cargo receptor, also known as SQSTM1) complex, and thus facilitates p62-dependent autophagic degradation of NLRP3 in primary peritoneal macrophages of C57BL/6J mice and HEK293T cells. Therefore, we identify galectin-9 as an "eat-me" signal for selective autophagy of NLRP3 and uncover the potential roles of galectins in controlling host protein degradation. Furthermore, our work suggests galectin-9 as a priming therapeutic target for the diseases caused by improper NLRP3 inflammasome activation.

Virus Caused Imbalance of Type I IFN Responses and Inflammation in COVID-19.

The global expansion of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has emerged as one of the greatest public health challenges and imposes a great threat to human health. Innate immunity plays vital roles in eliminating viruses through initiating type I interferons (IFNs)-dependent antiviral responses and inducing inflammation. Therefore, optimal activation of innate immunity and balanced type I IFN responses and inflammation are beneficial for efficient elimination of invading viruses. However, SARS-CoV-2 manipulates the host's innate immune system by multiple mechanisms, leading to aberrant type I IFN responses and excessive inflammation. In this review, we will emphasize the recent advances in the understanding of the crosstalk between host innate immunity and SARS-CoV-2 to explain the imbalance between inflammation and type I IFN responses caused by viral infection, and explore potential therapeutic targets for COVID-19.

RNF39 mediates K48-linked ubiquitination of DDX3X and inhibits RLR-dependent antiviral immunity.

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) are major cytosolic RNA sensors and play crucial roles in initiating antiviral innate immunity. Furthermore, RLRs have been implicated in multiple autoimmune disorders. Thus, RLR activation should be tightly controlled to avoid detrimental effects. "DEAD-box RNA helicase 3, X-linked" (DDX3X) is a key adaptor in RLR signaling, but its regulatory mechanisms remain unknown. Here, we show that the E3 ubiquitin ligase RNF39 inhibits RLR pathways through mediating K48-linked ubiquitination and proteasomal degradation of DDX3X. Concordantly, Rnf39 deficiency enhances RNA virus-triggered innate immune responses and attenuates viral replication. Thus, our results uncover a previously unknown mechanism for the control of DDX3X activity and suggest RNF39 as a priming intervention target for diseases caused by aberrant RLR activation.