TLR7/9 versus TLR3/MDA5 signaling during virus infections and diabetes

IFN-I are pleiotropic cytokines that impact innate and adaptive immune responses. In this article, we discuss TLR7/9 versus TLR3/MDA5 signaling in antiviral responses and diabetes. pDCs are thought to have a critical role in antiviral defense because of their ability to rapidly secrete large amounts of IFN-I through TLR7/9 signaling. A recent study demonstrates that although pDCs are a source of IFN-I in vivo, their overall contribution to viral containment is limited and time-dependent, such that additional cellular sources of IFN-I are required to fully control viral infections. dsRNA sensors, such as TLR3 and MDA5, provide another important trigger for antiviral IFN-I responses, which can be exploited to enhance immune responses to vaccines. In the absence of infection, IFN-I production by pDCs or from signaling through dsRNA sensors has been implicated in the pathogenesis of autoimmune diseases such as diabetes. However, recent data demonstrate that IFN-I production via TLR3 and MDA5 is critical to counter diabetes caused by a virus with preferential tropism for pancreatic β-cells. This highlights the complexity of the host antiviral response and how multiple cellular and molecular components balance protective versus pathological responses.     

Plasmacytoid dendritic cell ablation impacts early interferon responses and antiviral NK and CD8(+) T cell accrual

Plasmacytoid dendritic cells (pDCs) mediate type I interferon (IFN-I) responses to viruses that are recognized through the Toll-like receptor 7 (TLR7) or TLR9 signaling pathway. However, it is unclear how pDCs regulate the antiviral responses via innate and adaptive immune cells. We generated diphtheria toxin receptor transgenic mice to selectively deplete pDCs by administration of diphtheria toxin. pDC-depleted mice were challenged with viruses known to activate pDCs. In murine cytomegalovirus (MCMV) infection, pDC depletion reduced early IFN-I production and augmented viral burden facilitating the expansion of natural killer (NK) cells expressing the MCMV-specific receptor Ly49H. During vesicular stomatitis virus (VSV) infection, pDC depletion enhanced early viral replication and impaired the survival and accumulation of virus-specific cytotoxic T lymphocytes. We conclude that pDCs mediate early antiviral IFN-I responses and influence the accrual of virus-specific NK or CD8(+) T?cells in?a virus-dependent manner.     

Early type I interferon-mediated signals on B cells specifically enhance antiviral humoral responses

Type I interferons (IFN-I) limit viral spread by inducing antiviral genes in infected target cells and by shaping the adaptive response through induction of additional cytokines. Vesicular stomatitis virus (VSV) efficiently triggers the production of IFN-I in mice, and it is suggested that IFN-alpha is induced after binding of VSV to TLR7 in infected cells. Our study with virus-specific B cell receptor-transgenic mice demonstrates here that IFN-I directly fuel early humoral immune responses in vivo. VSV-specific B cells that lacked IFN-alpha/beta receptors were considerably impaired in plasma cell formation and in generating antiviral IgM. At low viral titers, production of IFN-alpha following VSV infection was independent of TLR7-mediated signals. Interestingly, however, TLR7 ligation in B cells increased the formation of early antiviral IgM. These findings indicate that IFN-alpha-mediated augmentation of specific B cell responses is a partially TLR7- and virus dose-dependent mechanism.     

Molecular characterizations of grass carp (Ctenopharyngodon idella) TBK1 gene and its roles in regulating IFN-I pathway

TANK-binding kinase 1 (TBK1), a kinase at the crossroads of multiple IFN-inducing signaling pathways, plays essential roles in both antiviral and antibacterial innate immunity in mammals. Here, TBK1 gene (10339 bp) was identified and characterized from grass carp Ctenopharyngodon idella (CiTBK1). The genomic sequence is shorter than other orthologs in vertebrate, and a promoter region is found in intron 1. mRNA expression of CiTBK1 was widespread in fifteen tissues investigated, and was up-regulated post GCRV challenge in vivo and in vitro, as well as after stimulation of viral/bacterial PAMPs in vitro. CiTBK1 mediates IFN-I signal pathway through over-expression experiment. Post GCRV challenge, CiTBK1 over-expression inhibits viral infection by induction of CiIFN-I and CiMx1 mainly via CiIRF7. In CiTBK1 over-expression cells, mRNA expressions of CiIRF3, CiIRF7 and CiIFN-I were inhibited, whereas CiMx1 was facilitated after poly I:C stimulation, comparing to those in control group. The result indicated that CiMx1 expression mediated by CiTBK1 is in IFN-I independent way after poly I:C stimulation. However, over-expression of CiTBK1 diminishes LPS-induced expressions of CiIRF3 and CiIRF7 but promotes the induction of CiIFN-I and CiMx1 in comparison with the control, which suggests that CiTBK1-triggered IFN-I activation is in IRF3/IRF7-independent manner after LPS stimulation. Notably, over-expression of CiTBK1 negatively regulated PGN-induced IRF3, IRF7, IFN-I and Mx1 immune response. Taken together, CiTBK1 participates in broad antiviral and antibacterial immune responses in different manners, and keeps regulatory balance that prevents harmful effects from excessive activation.     

Type 1 interferon-induced IL-7 maintains CD8+ T-cell responses and homeostasis by suppressing PD-1 expression in viral hepatitis

Type 1 interferon (IFN-I) promotes antigen-presenting cell maturation and was recently shown to induce hepatic IL-7 production during infection. Herein, we further explored the underlying mechanisms used by IFN-I to orchestrate antiviral immune responses in the liver. Acute viral hepatitis was induced by i.v. injection of adenovirus (Ad) in IFN-α receptor knockout (IFNAR^−/−) and control mice. To disrupt signaling, monoclonal antibodies (mAbs) against IL-7 receptor alpha (IL-7Rα) or PD-L1 were i.p. injected. We found that CD8⁺ T cells in IFNAR^−/− mice were less effective than those in control mice. The reduced T-cell function was accompanied by increased levels of PD-1 expression, apoptosis and decreased IFN-γ production. The lack of IFN-I signaling also impaired the expression of accessory molecules in both intrahepatic dendritic cell (DCs) and hepatocytes. PD-L1 was comparably and highly expressed on hepatocytes in both IFNAR^−/− and control mice. Injection of PD-L1-specific mAb in IFNAR^−/− mice reversed the compromised immune responses in the liver. Further investigation showed that hepatic IL-7 elevation was less pronounced in IFNAR^−/− mice compared to the controls. A treatment with recombinant IL-7 suppressed PD-1 expression on CD8⁺ T cells in vitro. Accordingly, blocking IL-7R signaling in vivo resulted in increased PD-1 expression on CD8⁺ T cells in Ad-infected mice. Collectively, the results suggest that IFN-I-induced hepatic IL-7 production maintains antiviral CD8⁺ T-cell responses and homeostasis by suppressing PD-1 expression in acute viral hepatitis.     

SARS-CoV-2 ORF10 suppresses the antiviral innate immune response by degrading MAVS through mitophagy

The global coronavirus disease 2019(COVID-19)pandemic caused by severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)has caused severe morbidity and mortality in humans.It is urgent to understand the function of viral genes.However,the function of open reading frame 10(ORF10),which is uniquely expressed by SARS-CoV-2,remains unclear.In this study,we showed that overexpression of ORF10 markedly suppressed the expression of type I interferon(IFN-I)genes and IFN-stimulated genes.Then,mitochondrial antiviral signaling protein(MAVS)was identified as the target via which ORF10 suppresses the IFN-I signaling pathway,and MAVS was found to be degraded through the ORF10-induced autophagy pathway.Furthermore,overexpression of ORF10 promoted the accumulation of LC3 in mitochondria and induced mitophagy.Mechanistically,ORF10 was translocated to mitochondria by interacting with the mitophagy receptor Nip3-like protein X(NIX)and induced mitophagy through its interaction with both NIX and LC3B.Moreover,knockdown of NIX expression blocked mitophagy activation,MAVS degradation,and IFN-I signaling pathway inhibition by ORF10.Consistent with our observations,in the context of SARS-CoV-2 infection,ORF10 inhibited MAVS expression and facilitated viral replication.In brief,our results reveal a novel mechanism by which SARS-CoV-2 inhibits the innate immune response;that is,ORF10 induces mitophagy-mediated MAVS degradation by binding to NIX.     

Phosphorylation of a novel SOCS-box regulates assembly of the HIV-1 Vif-Cul5 complex that promotes APOBEC3G degradation

HIV-1 Vif (viral infectivity factor) protein overcomes the antiviral activity of the DNA deaminase APOBEC3G by targeting it for proteasomal degradation. We report here that Vif targets APOBEC3G for degradation by forming an SCF-like E3 ubiquitin ligase containing Cullin 5 and Elongins B and C (Cul5-EloB-EloC) through a novel SOCS (suppressor of cytokine signaling)-box that binds EloC. Vif binding to EloC is negatively regulated by serine phosphorylation in the BC-box motif of the SOCS-box. Vif ubiquitination is promoted by Cul5 in vitro and in vivo, and requires an intact SOCS-box. Thus, autoubiquitination of Vif occurs within the assembled Vif-Cul5 complex, analogous to F-box proteins that are autoubiquitinated within their SCF (Skp1-Cullin-F-box) complex. These findings suggest mechanisms that regulate the assembly and activity of Cul5 E3 complexes through phosphorylation or autoubiquitination of the SOCS-box protein, and identify interactions between Vif and host cell proteins that may be therapeutic targets.     

SARS-CoV-2, HIV,and HPV: Convergent evolution of selective regulation of cGAS–STING signaling

Recognizing aberrant cytoplasmic double-stranded DNA and stimulating innate immunity is essential for the host's defense against viruses and tumors. Cyclic GMP–AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that synthesizes the second messenger 2′3′-cGAMP and subsequently activates stimulator of interferon genes (STING)-mediated activation of TANK-binding kinase 1 (TBK1)/interferon regulatory factor 3 (IRF3) and the production of type I interferon (IFN-I). Both the cGAS–STING-mediated IFN-I antiviral defense and the countermeasures developed by diverse viruses have been extensively studied. However, recent studies have revealed a convergent evolutionary feature of severe acute respiratory syndrome coronavirus 2 and human immunodeficiency virus (HIV) viral proteins in terms of the selective regulation of cGAS–STING-mediated nuclear factor-κB (NF-κB) signaling without any effect on cGAS–STING-mediated TBK1/IRF3 activation and IFN production. The potential beneficial effect of this cGAS–STING-mediated, NF-κB-dependent antiviral effect, and the possible detrimental effect of IFN-I in the pathogenesis of coronavirus disease 2019 and HIV infection deserve more attention and future investigation.     

USP14 promotes K63‐linked RIG‐I deubiquitination and suppresses antiviral immune responses

Retinoic acid‐inducible gene I (RIG‐I) is a critical RNA virus sensor that initiates antiviral immune response through K63‐linked ubiquitination. In this study, we demonstrated USP14, a deubiquitinating enzyme, as a negative regulator in antiviral responses by directly deubiquitinating K63‐linked RIG‐I. USP14 knockdown significantly enhanced RIG‐I‐triggered type I IFN signaling and inhibited vesicular stomatitis virus (VSV) replication both in mouse peritoneal macrophages and THP1 cells. USP14 overexpression in HeLa cells attenuated RIG‐I‐triggered IFN‐β expression and promoted VSV replication. Besides, USP14‐specific inhibitor, IU1, increased RIG‐I‐mediated type I IFN production and antiviral responses in vitro and in vivo. In addition, USP14 could interact with RIG‐I and remove RIG‐I K63‐linked polyubiquitination chains. This article is the first to report that USP14 acts as a negative regulator in antiviral response through deubiquitinating K63‐linked RIG‐I. These findings provide insights into a potential new therapy targeting USP14 for RNA virus‐related diseases.     

Quelling the storm: utilization of sphingosine-1-phosphate receptor signaling to ameliorate influenza virus-induced cytokine storm

Initial and early tissue injury associated with severe influenza virus infection is the result of both virus-mediated lysis of infected pulmonary cells coupled with an exuberant immune response generated against the virus. The excessive host immune response associated with influenza virus infection has been termed “cytokine storm.” Therapies that target virus replication are available; however, the selective pressure by such antiviral drugs on the virus often results in mutation and the escape of virus progeny now resistant to the antiviral regimen, thereby rendering such treatments ineffective. This event highlights the necessity for developing novel methods to combat morbidity and mortality caused by influenza virus infection. One potential method is restricting the host’s immune response. However, prior treatment regimens employing drugs like corticosteroids that globally suppress the host’s immune response were found unsatisfactory in large part because they disrupted the host’s ability to control virus replication. Here, we discuss a novel therapy that utilizes sphingosine-1-phosphate (S1P) receptor signaling that has the ability to significantly limit immunopathologic injury caused by the host’s innate and adaptive immune response, thereby significantly aborting morbidity and mortality associated with influenza virus infection. Moreover, S1P analog therapy allows for sufficient anti-influenza T cell and antibody formation to control infection. We review the anti-inflammatory effects of S1P signaling pathways and how modulation of these pathways during influenza virus infection restricts immunopathology. Finally, we discuss that combinatorial administration of S1P simultaneously with a current antiviral enhances the treatment efficacy for virulent influenza virus infections above that of either drug treatment alone. Interestingly, the scope of S1P receptor therapy reported here is likely to extend beyond influenza virus infection and could prove useful for the treatment of multiple maladies like other viral infections and autoimmune diseases where the host’s inflammatory response is a major component in the disease process.     

Downregulation of SOCS gene expression can inhibit the formation of acute and persistent BDV infections

High expression of suppressors of cytokine signalling (SOCS) has been detected during various viral infections. As a negative feedback regulator, SOCS participates in the regulation of multiple signalling pathways. In this study, to study the related mechanism between SOCS and BDV and to explore the effect of SOCS on IFN pathways in nerve cells, downregulated of SOCS1/3 in oligodendroglial (OL) cells and OL cells persistently infected with BDV (OL/BDV) were constructed with RNA interference technology. An interferon inducer (poly I:C, PIC) and an IFN‐α/β R1 antibody were used as stimulation in the SOCS1/3 low‐expression cell models, qRT‐PCR was used to detect type I IFN and BDV nucleic acid expression, Western blot was used to detect the expression of BDV P40 protein. After BDV acute infection with OL cells which with downregulated SOCS expression, the virus accounting was not detected, and the viral protein expression was lower than that of OL/BDV cells; the OL/BDV cells with downregulated SOCS expression had lower virus nucleic acid and protein expression than OL/BDV cells. Stimulated by IFN‐α/β R1 antibody, the expression of type I interferon in OL/BDV cells decreased, and the content of BDV nucleic acid and protein increased, which was higher than that of OL/BDV cells. From the results, it was concluded that downregulating SOCS1/3 can inhibit the formation of acute BDV infection and virus replication in persistent BDV infection by promoting the expression of IFN‐α/β and that SOCS can be used as a new target for antiviral therapy.     

抗病毒治疗对低病毒载量的乙肝ACLF患者生存率的影响

目的 探讨抗病毒治疗对低病毒载量的乙型肝炎相关慢加急性肝衰竭（ACLF）患者转归的影响.方法 352例乙型肝炎相关ACLF患者,其中低病毒载量组175例、高病毒载量组177例,各组分为护肝治疗（对照）组及抗病毒治疗组,比较两组患者临床特征、生存率及抗病毒治疗短期疗效差异.结果 观察24周,抗病毒治疗组的乙型肝炎相关ACLF患者总体生存率高于护肝治疗者（P=0.010）,低病毒载量组中抗病毒治疗组的生存率高于护肝治疗组（P=0.001）,高病毒载量组中抗病毒治疗组与护肝治疗组生存率差异无统计学意义（P=0.856）,抗病毒治疗组中低病毒载量组与高病毒载量组的生存率差异无统计学意义（P=0.755）.结论 抗病毒治疗可提高低病毒复制的乙型肝炎相关ACLF患者的生存率.     

TRIM10 binds to IFN-α/β receptor 1 to negatively regulate type I IFN signal transduction

The type I interferon (IFN-I) system is important for antiviral and anticancer immunity. Prolonged activation of IFN/JAK/STAT signaling is closely associated with autoimmune diseases. TRIM10 dysfunction may be associated closely with certain autoimmune disorders. Here, we observed that the serum TRIM10 protein level is lower in patients with systemic lupus erythematosus than in healthy control subjects. We speculated the possible involvement of TRIM10-induced modulation of the IFN/JAK/STAT signaling pathway in systemic lupus erythematosus. In line with our hypothesis, TRIM10 inhibited the activation of JAK/STAT signaling pathway triggered by various stimuli. TRIM10 restricted the IFN-I/JAK/STAT signaling pathway, which was independent of its E3 ligase activity. Mechanistically, TRIM10 interacted with the intracellular domain of IFNAR1 and blocked the association of IFNAR1 with TYK2. These data suggest the possible TRIM10 suppresses IFN/JAK/STAT signaling pathway through blocking the interaction between IFNAR1 and TYK2. Targeting TRIM10 is a potential strategy for treating autoimmune diseases.     

Prospects of HIV-1 entry inhibitors as novel therapeutics

A great deal of progress has been made in understanding the mechanism of human immunodeficiency virus entry into target cells. Landmark discoveries such as the identification of viral coreceptors and the structure of a portion of the viral envelope protein (Env) bound to its receptor provided important insight into how Env mediates fusion of the viral and cellular membranes. This knowledge has been successfully applied to the development of inhibitors that target discrete steps of the entry process. Some of these compounds efficiently block HIV-1 replication in vitro and are currently being evaluated in clinical trials. In this review, we will introduce the challenges of antiviral therapy and highlight the need for novel therapeutics, such as entry inhibitors, to complement current antiviral regimens. The mechanism by which Env mediates HIV-1 entry and the therapeutic potential of small molecule inhibitors of this dynamic process will be discussed in detail.     

BRCC36 functions noncatalytically to promote antiviral response by maintaining STAT1 protein stability

Viral infection is a serious threat to both normal population and clinical patients. STAT1 plays central roles in host defense against viral infection. How STAT1 protein maintains stable in different conditions remains largely unknown. Here, we identified BRCC36 as a potent regulator of STAT1 protein stability. Mechanistically, BRCC36 maintains STAT1 levels by utilizing USP13 to form a balanced complex for antagonizing Smurf1-mediated degradation. Importantly, cellular BRCC36 deficiency results in rapid downregulation of STAT1 during viral infection, whereas a supplement of BRCC36 maintains STAT1 protein levels and host antiviral immunity in vivo. Moreover, we revealed that BRCC36 expression was downregulated in allogeneic HSC transplantation (allo-HSCT) mice that showed increased susceptibility to viral infection. Supplementing BRCC36 enhanced antiviral response of allo-HSCT mice by maintaining STAT1 stability. This study uncovers a critical role of BRCC36 in STAT1 protein stability and could provide potential strategies for enhancing clinical antiviral therapy.