Human innate immunity responds to viral infection by activating the production of interferons (IFNs) and proinflammatory cytokines. The mitochondrial adaptor molecule MAVS plays a critical role in innate immune response to viral infection. In this study, we show that TRIM21 (tripartite motif-containing protein 21) interacts with MAVS to positively regulate innate immunity. Under viral infection, TRIM21 is upregulated through the IFN/JAK/STAT signaling pathway. Knockdown of TRIM21 dramatically impairs innate immune response to viral infection. Moreover, TRIM21 interacts with MAVS and catalyzes its K27-linked polyubiquitination, thereby promoting the recruitment of TBK1 to MAVS. Specifically, the PRY-SPRY domain of TRIM21 is the key domain for its interaction with MAVS, while the RING domain of TRIM21 facilitates the polyubiquitination chains of MAVS. In addition, the MAVS-mediated innate immune response is enhanced by both the PRY-SPRY and RING domains of TRIM21. Mutation analyses of all the lysine residues of MAVS further revealed that Lys325 of MAVS is catalyzed by TRIM21 for the K27-linked polyubiquitination. Overall, this study reveals a novel mechanism by which TRIM21 promotes the K27-linked polyubiquitination of MAVS to positively regulate innate immune response, thereby inhibiting viral infection. IMPORTANCE Activation of innate immunity is essential for host cells to restrict the spread of invading viruses and other pathogens. MAVS plays a critical role in innate immune response to RNA viral infection. In this study, we demonstrated that TRIM21 targets MAVS to positively regulate innate immunity. Notably, TRIM21 targets and catalyzes K27-linked polyubiquitination of MAVS and then promotes the recruitment of TBK1 to MAVS, leading to upregulation of innate immunity. Our study outlines a novel mechanism by which the IFN signaling pathway blocks RNA virus to escape immune elimination.

The absence of a robust cell culture system for hepatitis B virus (HBV) and hepatitis C virus (HCV) infection has limited the analysis of the virus lifecycle and drug discovery. We have established a hepatoma cell line, HLCZ01, the first cell line, to the authors' knowledge, supporting the entire lifecycle of both HBV and HCV. HBV surface antigen (HBsAg)-positive particles can be observed in the supernatant and the lumen of the endoplasmic reticulum of the cells via electron microscopy. Interestingly, HBV and HCV clinical isolates propagate in HLCZ01 cells. Both viruses replicate in the cells without evidence of overt interference. HBV and HCV entry are blocked by antibodies against HBsAg and human CD81, respectively, and the replication of HBV and HCV is inhibited by antivirals. HLCZ01 cells mount an innate immune response to virus infection. The cell line provides a powerful tool for exploring the mechanisms of virus entry and replication and the interaction between host and virus, facilitating the development of novel antiviral agents and vaccines.cell culture model | primary human hepatocytes | cccDNA | interferon | ISGs M ore than 500 million people worldwide are persistently infected with hepatitis B virus (HBV) and/or hepatitis B virus (HCV) and are at risk of developing chronic liver diseases (1). There is no vaccine against HCV, and many patients who are persistently infected by HBV or HCV do not respond to currently available therapies (2, 3). Improved understanding of the biology and pathogenesis of these infections is required for the development of vaccine and antiviral drugs (4). The inability to grow HBV and HCV efficiently in cell culture has presented a major obstacle to understanding the virus lifecycle and pathogenesis and to developing improved therapeutics.HBV is a member of the hepadnavirus families, and its genome is a relaxed circular, partially double-stranded DNA molecule. The negative strand has an invariable length of ∼3.2 kb, and the positive strand is 50-100% of this length. Several key issues about the biology of HBV remain to be explored, including the identification of the cellular receptors, the role of the X gene, and the mechanisms by which the viral minichromosome is formed. Covalently closed circular DNA (cccDNA) is responsible for the establishment of viral infection and persistence. Understanding the mechanisms underlying cccDNA formation and regulation is critical for understanding the HBV pathogenesis and finding a cure for hepatitis B. HepG2.2.15 cells derived from the hepatoma cell line HepG2 transfected with the full genome of HBV have been used to study HBV replication (5). Primary human hepatocytes (PHH) are susceptible to HBV infection (6, 7), but the use of this model is hampered by the limited availability and unpredictable variability of human liver. Several human hepatoma cell lines support HBV replication after HBV DNA transfection, and overexpression of sodium-taurocholate cotransporting polypeptide (NTCP) in HepG2 and Huh7 cells can render these cells able t...

RNA viruses represent a major global health threat, and the visualization of their RNA genome in infected cells is essential for virological research and clinical diagnosis. Due to the lack of chemical toolkits for the live-cell imaging of viral RNA genomes, especially native viral genomes without labeling and genetic modification, studies on native virus infection at the single-live-cell level are challenging. Herein, taking hepatitis C virus (HCV) as a representative RNA virus, we propose that the innate noncanonical G-quadruplex (G4) structure of viral RNA can serve as a specific imaging target and report a new benzothiazole-based G4-targeted fluorescence light-up probe, ThT-NE, for the direct visualization of the native RNA genome of HCV in living host cells. We demonstrate the use of the ThT-NE probe for several previously intractable applications, including the sensitive detection of individual virus-infected cells by small-molecule staining, real-time monitoring of the subcellular distribution of the viral RNA genome in live cells, and continuous live-cell tracking of the infection and propagation of clinically isolated native HCV. The fluorogenic-probe-based viral RNA light-up system opens up a promising chemical strategy for cutting-edge live-cell viral analysis, providing a potentially powerful tool for viral biology, medical diagnosis, and drug development.

Activation of innate immunity is essential for host cells to restrict the spread of invading viruses and other pathogens. However, attenuation or termination of signaling is also necessary for preventing immune-mediated tissue damage and spontaneous autoimmunity. Here, we identify NLRX1 as a negative regulator for MAVS-mediated signaling pathway during HCV infection. Depletion of NLRX1 enhances HCV-triggered activation of IFN signaling and causes the suppression of HCV propagation in hepatocytes. NLRX1, a HCV-inducible protein, interacts with MAVS and mediates the K48-linked polyubiquitination and the subsequent degradation of MAVS via proteasomal pathway. Moreover, PCBP2 interacts with NLRX1 to participate in NLRX1-induced degradation of MAVS and inhibition of antiviral responses during HCV infection. Mutagenic analyses further revealed that the NOD domain of NLRX1 is essential for NLRX1 to interact with PCBP2 and subsequently induce MAVS degradation. Our study unlocks a key mechanism of fine tuning of innate immunity by which NLRX1 restrains RLRs-MAVS signaling cascade by recruiting PCBP2 to MAVS for inducing MAVS degradation through the proteasomal pathway. NLRX1, a negative regulator of innate immunity, is a pivotal host factor for HCV to establish persistent infection. Innate immunity needs to be tightly regulated to maximize antiviral response and minimize immune-mediated pathology. Whereas, the underlying mechanisms are poorly understood. In this study, we report that NLRX1 is a proviral host factor for HCV infection and functions as a negative regulator in HCV-triggered innate immune response. NLRX1 recruits PCBP2 to MAVS and induces the K48-linked polyubiquitination and degradation of MAVS, leading to the negative regulation of IFN signaling pathway and promoting HCV infection. Overall, this study provides intriguing insights into how innate immunity is regulated during viral infection.

Emerging evidence indicates that long noncoding RNAs (lncRNAs) regulate various biological processes, especially innate and adaptive immunity. However, the relationship between lncRNAs and the interferon (IFN) pathway remains largely unknown. Here, we report that lncRNA ITPRIP-1 (lncITPRIP-1) is involved in viral infection and plays a crucial role in the virus-triggered IFN signaling pathway through the targeting of melanoma differentiation-associated gene 5 (MDA5). LncITPRIP-1 can be induced by viral infection, which is not entirely dependent on the IFN signal. Besides, there is no coding potential found in the lncITPRIP-1 transcript. LncITPRIP-1 binds to the C terminus of MDA5, and it possesses the ability to boost the oligomerization of both the full length and the 2 caspase activation and recruitment domains of MDA5 in a K63-linked polyubiquitination-independent manner. Amazingly, we also found that MDA5 can suppress hepatitis C virus (HCV) replication independently of IFN signaling through its C-terminal-deficient domain bound to viral RNA, in which lncITPRIP-1 plays a role as an assistant. In addition, the expression of lncITPRIP-1 is highly consistent with MDA5 expression, indicating that lncITPRIP-1 may function as a cofactor of MDA5. All the data suggest that lncITPRIP-1 enhances the innate immune response to viral infection through the promotion of oligomerization and activation of MDA5. Our study discovers the first lncRNA ITPRIP-1 involved in MDA5 activation. Hepatitis C virus infection is a global health issue, and there is still no available vaccine, which makes it urgent to reveal the underlying mechanisms of HCV and host factors. Although RIG-I has been recognized as the leading cytoplasmic sensor against HCV for a long time, recent findings that MDA5 regulates the IFN response to HCV have emerged. Our work validates the significant role of MDA5 in IFN signaling and HCV infection and proposes the first lncRNA inhibiting HCV replication by promoting the activation of MDA5 and mediating the association between MDA5 and HCV RNA, the study of which may shed light on the MDA5 function and treatment for hepatitis C patients. Our suggested model of how lncITPRIP-1 orchestrates signal transduction for IFN production illustrates the essential role of lncRNAs in virus elimination.

Interferons (IFNs) restrict various kinds of viral infection via induction of hundreds of IFN-stimulated genes (ISGs), while the functions of the majority of ISGs are broadly unclear. Here, we show that a high-IFN-inducible gene, ISG12a (also known as IFI27), exhibits a nonapoptotic antiviral effect on hepatitis C virus (HCV) infection. Viral NS5A protein is targeted specifically by ISG12a, which mediates NS5A degradation via a ubiquitination-dependent proteasomal pathway. K374R mutation in NS5A domain III abrogates ISG12a-induced ubiquitination and degradation of NS5A. S-phase kinase-associated protein 2 (SKP2) is identified as an ubiquitin E3 ligase for NS5A. ISG12a functions as a crucial adaptor that promotes SKP2 to interact with and degrade viral protein. Moreover, the antiviral effect of ISG12a is dependent on the E3 ligase activity of SKP2. These findings uncover an intriguing mechanism by which ISG12a restricts viral infection and provide clues for understanding the actions of innate immunity. IMPORTANCE Upon virus invasion, IFNs induce numerous ISGs to control viral spread, while the functions of the majority of ISGs are broadlyunclear. The present study shows a novel antiviral mechanism of ISGs and elucidated that ISG12a recruits an E3 ligase, SKP2, for ubiquitination and degradation of viral protein and restricts viral infection. These findings provide important insights into exploring the working principles of innate immunity.

The ability to harness innate immunity is a promising solution for improving cancer immunotherapy. Interferon (IFN) induces expression of IFN-stimulated genes (ISGs) by activating the JAK-STAT signaling pathway to promote innate immunity and inhibit malignant tumor growth, but the functions and mechanisms of most ISGs in cancer regulation are unknown. As an innate immune effector, ISG12a promotes the innate immune response to viral infection. In this study, ISG12a was found to be expressed at low levels in gastrointestinal cancer, represented by hepatocellular cancer (HCC) and gastric cancer (GC), and it identified as a tumor suppressor that affects clinical prognosis. ISG12a silencing accelerated the malignant transformation and epithelial-mesenchymal transition of cancer cells. Mechanistically, ISG12a promoted β-catenin proteasomal degradation by inhibiting the degradation of ubiquitinated Axin, thereby suppressing the canonical Wnt/β-catenin signaling pathway. Notably, β-catenin was identified as a transcription factor for PD-L1. Inhibition of Wnt/β-catenin signaling by ISG12a suppressed expression of the immune checkpoint PD-L1, rendering cancer cells sensitive to NK cell-mediated killing. This study reveals a mechanism underlying the anticancer effects of IFN. Some ISGs, as represented by ISG12a, may be useful in cancer therapy and prevention. The identified interrelations among innate immunity, Wnt/β-catenin signaling, and cancer immunity may provide new insight into strategies that will improve the efficiency of immunotherapy.

GFP-like fluorescent proteins and their molecular mimics have revolutionized bioimaging research, but their emissions are largely limited in the visible to far-red region, hampering the in vivo applications in intact animals. Herein, we structurally modulate GFP-like chromophores using a donor–acceptor–acceptor (D–A–A′) molecular configuration to discover a set of novel fluorogenic derivatives with infrared-shifted spectra. These chromophores can be fluorescently elicited by their specific interaction with G-quadruplex (G4), a unique noncanonical nucleic acid secondary structure, via inhibition of the chromophores’ twisted-intramolecular charge transfer. This feature allows us to create, for the first time, FP mimics with tunable emission in the near-infrared (NIR) region (Emmax = 664–705 nm), namely, infrared G-quadruplex mimics of FPs (igMFP). Compared with their FP counterparts, igMFPs exhibit remarkably higher quantum yields, larger Stokes shift, and better photostability. In a proof-of-concept application using pathogen-related G4s as the target, we exploited igMFPs to directly visualize native hepatitis C virus (HCV) RNA genome in living cells via their in situ formation by the chromophore-bound viral G4 structure in the HCV core gene. Furthermore, igMFPs are capable of high contrast HCV RNA imaging in living mice bearing a HCV RNA-presenting mini-organ, providing the first application of FP mimics in whole-animal imaging.