Hantavirus regulation of type I interferon responses

The Role of Long Noncoding RNA Negative Regulator of Interferon Response in the Regulation of Hantaan Virus Infection

Hantaan virus (HTNV) is prevalent in Eurasia. It causes hemorrhagic fever with renal syndrome (HFRS). Long noncoding RNAs (lncRNAs) play key roles in regulating innate immunity. Among these, lncRNA negative regulator of interferon response (NRIR) was reported as an inhibitor of several interferon (IFN)-stimulated genes. Our results showed that: NRIR expression was upregulated by HTNV infection in a type I IFN-dependent manner. The expression of NRIR in CD14+ monocytes from HFRS patients in acute phase was significantly higher than that in convalescent phase and healthy controls. HTNV infection in some HTNV-compatible cells was promoted by NRIR. NRIR negatively regulated innate immunity, especially IFITM3 expression. Localized in the nucleus, NRIR bound with HNRNPC, and knockdown of HNRNPC significantly weakened the effect of NRIR in promoting HTNV infection and restored IFITM3 expression. These results indicated that NRIR regulates the innate immune response against HTNV infection possibly through its interaction with HNRNPC and its influence on IFITM3.

Hantaan virus inhibits type I interferon response by targeting RLR signaling pathways through TRIM25

Hantaan virus (HTNV) is responsible for hemorrhagic fever with renal syndrome (HFRS), primarily due to its ability to inhibit host innate immune responses, such as type I interferon (IFN-I). In this study, we conducted a transcriptome analysis to identify host factors regulated by HTNV nucleocapsid protein (NP) and glycoprotein. Our findings demonstrate that NP and Gc proteins inhibit host IFN-I production by manipulating the retinoic acid-induced gene I (RIG-I)-like receptor (RLR) pathways. Further analysis reveals that HTNV NP and Gc proteins target upstream molecules of MAVS, such as RIG-I and MDA-5, with Gc exhibiting stronger inhibition of IFN-I responses than NP. Mechanistically, NP and Gc proteins interact with tripartite motif protein 25 (TRIM25) to competitively inhibit its interaction with RIG-I/MDA5, suppressing RLR signaling pathways. Our study unveils a cross-talk between HTNV NP/Gc proteins and host immune response, providing valuable insights into the pathogenic mechanism of HTNV.

STING strengthens host anti-hantaviral immunity through an interferon-independent pathway

Hantaan virus (HTNV), the prototype virus of hantavirus, could escape innate immunity by restraining type I interferon (IFN) responses. It is largely unknown whether there existed other efficient anti-hantaviral tactics in host cells. Here, we demonstrate that the stimulator of interferon genes (STING) strengthens the host IFN-independent anti-hantaviral immunity. HTNV infection activates RIG-I through IRE1-XBP 1-mediated ER stress, which further facilitates the subcellular translocation and activation of STING. During this process, STING triggers cellular autophagy by interacting with Rab7A, thus restricting viral replication. To note, the anti-hantaviral effects of STING are independent of canonical IFN signaling. Additionally, neither application of the pharmacological antagonist nor the agonist targeting STING could improve the outcomes of nude mice post HTNV challenge in vivo. However, the administration of plasmids exogenously expressing the mutant C-terminal tail (ΔCTT) STING, which would not trigger the type I IFN responses, protected the nude mice from lethal HTNV infection. In summary, our research revealed a novel antiviral pathway through the RIG-I-STING-autophagy pathway, which offered novel therapeutic strategies against hantavirus infection.

Orthohantavirus Replication in the Context of Innate Immunity

Orthohantaviruses are rodent-borne, negative-sense RNA viruses that are capable of causing severe vascular disease in humans. Over the course of viral evolution, these viruses have tailored their replication cycles in such a way as to avoid and/or antagonize host innate immune responses. In the rodent reservoir, this results in life long asymptomatic infections. However, in hosts other than its co-evolved reservoir, the mechanisms for subduing the innate immune response may be less efficient or absent, potentially leading to disease and/or viral clearance. In the case of human orthohantavirus infection, the interaction of the innate immune response with viral replication is thought to give rise to severe vascular disease. The orthohantavirus field has made significant advancements in understanding how these viruses replicate and interact with host innate immune responses since their identification by Dr. Ho Wang Lee and colleagues in 1976. Therefore, the purpose of this review, as part of this special issue dedicated to Dr. Lee, was to summarize the current knowledge of orthohantavirus replication, how viral replication activates innate immunity, and how the host antiviral response, in turn, impacts viral replication.

HTNV infection induces activation and deficiency of CD8+MAIT cells in HFRS patients

Hantaan virus (HTNV) infection causes an epidemic of hemorrhagic fever with renal syndrome (HFRS) mainly in Asia. Mucosal-associated invariant T (MAIT) cells are innate-like T lymphocytes known to play an important role in innate host defense during virus infection. However, their roles and phenotypes during HTNV infection have not yet been explored. We characterized CD8+MAIT cells from HFRS patients based on scRNA-seq data combined with flow cytometry data. We showed that HTNV infection caused the loss and activation of CD8+MAIT cells in the peripheral blood, which were correlated with disease severity. The production of granzyme B and IFN-γ from CD8+MAIT cells and the limitation of HTNV replication in endothelia cells indicated the anti-viral property of CD8+MAIT cells. In addition, in vitro infection of MAIT cells by HTNV or HTNV-exposed monocytes showed that the activation of MAIT cells was IL-18 mediated. In conclusion, this study identified, for the first time, gene expression profiles of MAIT cells, provided underlying molecular mechanisms for activation of MAIT cells during HTNV infection, and suggested a potential anti-viral role of MAIT cells in HFRS.

Clinical and genomic characterisation of a fatal Puumala orthohantavirus case with low levels of neutralising antibodies

BACKGROUND

Orthohantaviruses are rodent-borne emerging viruses that cause haemorrhagic fever with renal syndrome (HFRS) in Eurasia and hantavirus pulmonary syndrome in America. Transmission between humans have been reported and the case-fatality rate ranges from 0.4% to 40% depending on virus strain. There is no specific and efficient treatment for patients with severe HFRS. Here, we characterised a fatal case of HFRS and sequenced the causing Puumala orthohantavirus (PUUV).

METHODS

PUUV RNA and virus specific neutralising antibodies were quantified in plasma samples from the fatal case and other patients with non-fatal PUUV infection. To investigate if the causing PUUV strain was different from previously known strains, Sanger sequencing was performed directly from the patient's plasma. Biopsies obtained from autopsy were stained for immunohistochemistry.

RESULTS

The patient had approximately tenfold lower levels of PUUV neutralising antibodies and twice higher viral load than was normally seen for patients with less severe PUUV infection. We could demonstrate unique mutations in the S and M segments of the virus that could have had an impact on the severity of infection. Due to the severe course of infection, the patient was treated with the bradykinin receptor inhibitor icatibant to reduce bradykinin-mediated vessel permeability and maintain vascular circulation.

CONCLUSIONS

Our data suggest that bradykinin receptor inhibitor may not be highly efficient to treat patients that are at an advanced stage of HFRS. Low neutralising antibodies and high viral load at admission to the hospital were associated with the fatal outcome and may be useful for future predictions of disease outcome.

OUP accepted manuscript

Hantavirus-induced diseases are emerging zoonoses with endemic appearances and frequent outbreaks in different parts of the world. In humans, hantaviral pathology is characterized by the disruption of the endothelial cell barrier followed by increased capillary permeability, thrombocytopenia due to platelet activation/depletion and an overactive immune response. Genetic vulnerability due to certain human leukocyte antigen haplotypes is associated with disease severity. Typically, two different hantavirus-caused clinical syndromes have been reported: hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). The primarily affected vascular beds differ in these two entities: renal medullary capillaries in HFRS caused by Old World hantaviruses and pulmonary capillaries in HCPS caused by New World hantaviruses. Disease severity in HFRS ranges from mild, e.g. Puumala virus-associated nephropathia epidemica, to moderate, e.g. Hantaan or Dobrava virus infections. HCPS leads to a severe acute respiratory distress syndrome with high mortality rates. Due to novel insights into organ tropism, hantavirus-associated pathophysiology and overlapping clinical features, HFRS and HCPS are believed to be interconnected syndromes frequently involving the kidneys. As there are no specific antiviral treatments or vaccines approved in Europe or the USA, only preventive measures and public awareness may minimize the risk of hantavirus infection. Treatment remains primarily supportive and, depending on disease severity, more invasive measures (e.g., renal replacement therapy, mechanical ventilation and extracorporeal membrane oxygenation) are needed.

Interactions of Viral Proteins from Pathogenic and Low or Non-Pathogenic Orthohantaviruses with Human Type I Interferon Signaling

Rodent-borne orthohantaviruses are asymptomatic in their natural reservoir, but they can cause severe diseases in humans. Although an exacerbated immune response relates to hantaviral pathologies, orthohantaviruses have to antagonize the antiviral interferon (IFN) response to successfully propagate in infected cells. We studied interactions of structural and nonstructural (NSs) proteins of pathogenic Puumala (PUUV), low-pathogenic Tula (TULV), and non-pathogenic Prospect Hill (PHV) viruses, with human type I and III IFN (IFN-I and IFN-III) pathways. The NSs proteins of all three viruses inhibited the RIG-I-activated IFNβ promoter, while only the glycoprotein precursor (GPC) of PUUV, or its cleavage product Gn/Gc, and the nucleocapsid (N) of TULV inhibited it. Moreover, the GPC of both PUUV and TULV antagonized the promoter of IFN-stimulated responsive elements (ISRE). Different viral proteins could thus contribute to inhibition of IFNβ response in a viral context. While PUUV and TULV strains replicated similarly, whether expressing entire or truncated NSs proteins, only PUUV encoding a wild type NSs protein led to late IFN expression and activation of IFN-stimulated genes (ISG). This, together with the identification of particular domains of NSs proteins and different biological processes that are associated with cellular proteins in complex with NSs proteins, suggested that the activation of IFN-I is probably not the only antiviral pathway to be counteracted by orthohantaviruses and that NSs proteins could have multiple inhibitory functions.

Hantavirus Inhibits TRAIL-Mediated Killing of Infected Cells by Downregulating Death Receptor 5

Cytotoxic lymphocytes normally kill virus-infected cells by apoptosis induction. Cytotoxic granule-dependent apoptosis induction engages the intrinsic apoptosis pathway, whereas death receptor (DR)-dependent apoptosis triggers the extrinsic apoptosis pathway. Hantaviruses, single-stranded RNA viruses of the order Bunyavirales, induce strong cytotoxic lymphocyte responses in infected humans. Cytotoxic lymphocytes, however, are largely incapable of eradicating hantavirus-infected cells. Here, we show that the prototypic hantavirus, Hantaan virus (HTNV), induces TRAIL production but strongly inhibits TRAIL-mediated extrinsic apoptosis induction in infected cells by downregulating DR5 cell surface expression. Mechanistic analyses revealed that HTNV triggers both 26S proteasome-dependent degradation of DR5 through direct ubiquitination of DR5 and hampers DR5 transport to the cell surface. These results corroborate earlier findings, demonstrating that hantavirus also inhibits cytotoxic cell granule-dependent apoptosis induction. Together, these findings show that HTNV counteracts intrinsic and extrinsic apoptosis induction pathways, providing a defense mechanism utilized by hantaviruses to inhibit cytotoxic cell-mediated eradication of infected cells.

Biodefense Implications of New-World Hantaviruses

Hantaviruses, part of the Bunyaviridae family, are a genus of negative-sense, single-stranded RNA viruses that cause two major diseases: New-World Hantavirus Cardiopulmonary Syndrome and Old-World Hemorrhagic Fever with Renal Syndrome. Hantaviruses generally are found worldwide with each disease corresponding to their respective hemispheres. New-World Hantaviruses spread by specific rodent-host reservoirs and are categorized as emerging viruses that pose a threat to global health and security due to their high mortality rate and ease of transmission. Incidentally, reports of Hantavirus categorization as a bioweapon are often contradicted as both US National Institute of Allergy and Infectious Diseases and the Centers for Disease Control and Prevention refer to them as Category A and C bioagents respectively, each retaining qualitative levels of importance and severity. Concerns of Hantavirus being engineered into a novel bioagent has been thwarted by Hantaviruses being difficult to culture, isolate, and purify limiting its ability to be weaponized. However, the natural properties of Hantaviruses pose a threat that can be exploited by conventional and unconventional forces. This review seeks to clarify the categorization of Hantaviruses as a bioweapon, whilst defining the practicality of employing New-World Hantaviruses and their effect on armies, infrastructure, and civilian targets.

The Andes Orthohantavirus NSs Protein Antagonizes the Type I Interferon Response by Inhibiting MAVS Signaling

The small messenger RNA (SmRNA) of the Andes orthohantavirus (ANDV), a rodent-borne member of the Hantaviridae family of viruses of the Bunyavirales order, encodes a multifunctional nucleocapsid (N) protein and for a nonstructural (NSs) protein of unknown function. We have previously shown the expression of the ANDV-NSs, but only in infected cell cultures. In this study, we extend our early findings by confirming the expression of the ANDV-NSs protein in the lungs of experimentally infected golden Syrian hamsters. Next, we show, using a virus-free system, that the ANDV-NSs protein antagonizes the type I interferon (IFN) induction pathway by suppressing signals downstream of the melanoma differentiation-associated protein 5 (MDA5) and the retinoic acid-inducible gene 1 (RIG-I) and upstream of TBK1. Consistent with this observation, the ANDV-NSs protein antagonized mitochondrial antiviral-signaling protein (MAVS)-induced IFN-β, NF-κB, IFN-regulatory factor 3 (IRF3), and IFN-sensitive response element (ISRE) promoter activity. Results demonstrate that ANDV-NSs binds to MAVS in cells without disrupting the MAVS-TBK-1 interaction. However, in the presence of the ANDV-NSs ubiquitination of MAVS is reduced. In summary, this study provides evidence showing that the ANDV-NSs protein acts as an antagonist of the cellular innate immune system by suppressing MAVS downstream signaling by a yet not fully understand mechanism. Our findings reveal new insights into the molecular regulation of the hosts' innate immune response by the Andes orthohantavirus.IMPORTANCE Andes orthohantavirus (ANDV) is endemic in Argentina and Chile and is the primary etiological agent of hantavirus cardiopulmonary syndrome (HCPS) in South America. ANDV is distinguished from other hantaviruses by its unique ability to spread from person to person. In a previous report, we identified a novel ANDV protein, ANDV-NSs. Until now, ANDV-NSs had no known function. In this new study, we established that ANDV-NSs acts as an antagonist of cellular innate immunity, the first line of defense against invading pathogens, hindering the cellular antiviral response during infection. This study provides novel insights into the mechanisms used by ANDV to establish its infection.

Importance of Zika Virus NS5 Protein for Viral Replication

Zika virus is the latest addition to an ever-growing list of arboviruses that are causing outbreaks with serious consequences. A few mild cases were recorded between 1960 and 1980 until the first major outbreak in 2007 on Yap Island. This was followed by more severe outbreaks in French Polynesia (2013) and Brazil (2015), which significantly increased both Guillain-Barre syndrome and microcephaly cases. No current vaccines or treatments are available, however, recent studies have taken interest in the NS5 protein which encodes both the viral methyltransferase and RNA-dependent RNA polymerase. This makes it important for viral replication alongside other important functions such as inhibiting the innate immune system thus ensuring virus survival and replication. Structural studies can help design inhibitors, while biochemical studies can help understand the various mechanisms utilized by NS5 thus counteracting them might inhibit or abolish the viral infection. Drug repurposing targeting the NS5 protein has also proven to be an effective tool since hundreds of thousands of compounds can be screened therefore saving time and resources, moreover information on these compounds might already be available especially if they are used to treat other ailments.

Puumala and Tula Virus Differ in Replication Kinetics and Innate Immune Stimulation in Human Endothelial Cells and Macrophages

Old world hantaviruses cause hemorrhagic fever with renal syndrome (HFRS) upon zoonotic transmission to humans. In Europe, the Puumala virus (PUUV) is the main causative agent of HFRS. Tula virus (TULV) is also widely distributed in Europe, but there is little knowledge about the pathogenicity of TULV for humans, as reported cases are rare. We studied the replication of TULV in different cell types in comparison to the pathogenic PUUV and analyzed differences in stimulation of innate immunity. While both viruses replicated to a similar extent in interferon (IFN)-deficient Vero E6 cells, TULV replication in human lung epithelial (A549) cells was slower and less efficient when compared to PUUV. In contrast to PUUV, no replication of TULV could be detected in human microvascular endothelial cells and in macrophages. While a strong innate immune response towards PUUV infection was evident at 48 h post infection, TULV infection triggered only a weak IFN response late after infection of A549 cells. Using appropriate in vitro cell culture models for the orthohantavirus infection, we could demonstrate major differences in host cell tropism, replication kinetics, and innate immune induction between pathogenic PUUV and the presumably non- or low-pathogenic TULV that are not observed in Vero E6 cells and may contribute to differences in virulence.

Hantavirus entry: Perspectives and recent advances

Hantaviruses are important zoonotic pathogens of public health importance that are found on all continents except Antarctica and are associated with hemorrhagic fever with renal syndrome (HFRS) in the Old World and hantavirus pulmonary syndrome (HPS) in the New World. Despite the significant disease burden they cause, no FDA-approved specific therapeutics or vaccines exist against these lethal viruses. The lack of available interventions is largely due to an incomplete understanding of hantavirus pathogenesis and molecular mechanisms of virus replication, including cellular entry. Hantavirus Gn/Gc glycoproteins are the only viral proteins exposed on the surface of virions and are necessary and sufficient to orchestrate virus attachment and entry. In vitro studies have implicated integrins (β_1-3), DAF/CD55, and gC1qR as candidate receptors that mediate viral attachment for both Old World and New World hantaviruses. Recently, protocadherin-1 (PCDH1) was demonstrated as a requirement for cellular attachment and entry of New World hantaviruses in vitro and lethal HPS in vivo, making it the first clade-specific host factor to be identified. Attachment of hantavirus particles to cellular receptors induces their internalization by clathrin-mediated, dynamin-independent, or macropinocytosis-like mechanisms, followed by particle trafficking to an endosomal compartment where the fusion of viral and endosomal membranes can occur. Following membrane fusion, which requires cholesterol and acid pH, viral nucleocapsids escape into the cytoplasm and launch genome replication. In this review, we discuss the current mechanistic understanding of hantavirus entry, highlight gaps in our existing knowledge, and suggest areas for future inquiry.

Tula orthohantavirus nucleocapsid protein is cleaved in infected cells and may sequester activated caspase-3 during persistent infection to suppress apoptosis

The family Hantaviridae mostly comprises rodent-borne segmented negative-sense RNA viruses, many of which are capable of causing devastating disease in humans. In contrast, hantavirus infection of rodent hosts results in a persistent and inapparent infection through their ability to evade immune detection and inhibit apoptosis. In this study, we used Tula hantavirus (TULV) to investigate the interplay between viral and host apoptotic responses during early, peak and persistent phases of virus infection in cell culture. Examination of early-phase TULV infection revealed that infected cells were refractory to apoptosis, as evidenced by the complete lack of cleaved caspase-3 (casp-3C) staining, whereas in non-infected bystander cells casp-3C was highly abundant. Interestingly, at later time points, casp-3C was abundant in infected cells, but the cells remained viable and able to continue shedding infectious virus, and together these observations were suggestive of a TULV-associated apoptotic block. To investigate this block, we viewed TULV-infected cells using laser scanning confocal and wide-field deconvolution microscopy, which revealed that TULV nucleocapsid protein (NP) colocalized with, and sequestered, casp-3C within cytoplasmic ultrastructures. Consistent with casp-3C colocalization, we showed for the first time that TULV NP was cleaved in cells and that TULV NP and casp-3C could be co-immunoprecipitated, suggesting that this interaction was stable and thus unlikely to be solely confined to NP binding as a substrate to the casp-3C active site. To account for these findings, we propose a novel mechanism by which TULV NP inhibits apoptosis by spatially sequestering casp-3C from its downstream apoptotic targets within the cytosol.

Delayed Interferon Type 1-Induced Antiviral State Is a Potential Factor for Hemorrhagic Fever With Renal Syndrome Severity

Hantaviruses cause hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia. Interferon (IFN) responses play an important role in HFRS pathogenesis and early IFN-β response is delayed by pathogenic hantaviruses. The severity of HFRS caused by Dobrava virus (DOBV) and Puumala virus (PUUV) varies. Our aim was to determine whether differences in early activation of IFN type 1-induced antiviral state influence HFRS severity. Peripheral blood mononuclear cells (PBMCs) from healthy donors and HFRS patients were stimulated with DOBV or PUUV and expression of selected genes was measured. PUUV, but not DOBV, activated IFN type 1-induced antiviral state in stimulated PBMCs, and IFNβ, STAT-1, and MxA were highly upregulated. Upregulation of MxA was earlier in acute-phase PBMCs and higher in convalescent-phase PBMCs from patients with mild compared with severe PUUV infection. Our study showed that delayed IFN type 1-induced antiviral state could contribute to HFRS severity, particularly in PUUV infection.

Unique Interferon Pathway Regulation by the Andes Virus Nucleocapsid Protein Is Conferred by Phosphorylation of Serine 386

Andes virus (ANDV) causes hantavirus pulmonary syndrome (HPS) and is the only hantavirus shown to spread person to person and cause a highly lethal HPS-like disease in Syrian hamsters. The unique ability of ANDV N protein to inhibit beta interferon (IFNβ) induction may contribute to its virulence and spread. Here we analyzed IFNβ regulation by ANDV N protein substituted with divergent residues from the nearly identical Maporal virus (MAPV) N protein. We found that MAPV N fails to inhibit IFNβ signaling and that replacing ANDV residues 252 to 296 with a hypervariable domain (HVD) from MAPV N prevents IFNβ regulation. In addition, changing ANDV residue S386 to the histidine present in MAPV N or the alanine present in other hantaviruses prevented ANDV N from regulating IFNβ induction. In contrast, replacing serine with phosphoserine-mimetic aspartic acid (S386D) in ANDV N robustly inhibited interferon regulatory factor 3 (IRF3) phosphorylation and IFNβ induction. Additionally, the MAPV N protein gained the ability to inhibit IRF3 phosphorylation and IFNβ induction when ANDV HVD and H386D replaced MAPV residues. Mass spectroscopy analysis of N protein from ANDV-infected cells revealed that S386 is phosphorylated, newly classifying ANDV N as a phosphoprotein and phosphorylated S386 as a unique determinant of IFN regulation. In this context, the finding that the ANDV HVD is required for IFN regulation by S386 but dispensable for IFN regulation by D386 suggests a role for HVD in kinase recruitment and S386 phosphorylation. These findings delineate elements within the ANDV N protein that can be targeted to attenuate ANDV and suggest targeting cellular kinases as potential ANDV therapeutics.IMPORTANCE ANDV contains virulence determinants that uniquely permit it to spread person to person and cause highly lethal HPS in immunocompetent hamsters. We discovered that ANDV S386 and an ANDV-specific hypervariable domain permit ANDV N to inhibit IFN induction and that IFN regulation is directed by phosphomimetic S386D substitutions in ANDV N. In addition, MAPV N proteins containing D386 and ANDV HVD gained the ability to inhibit IFN induction. Validating these findings, mass spectroscopy analysis revealed that S386 of ANDV N protein is uniquely phosphorylated during ANDV infection. Collectively, these findings reveal new paradigms for ANDV N protein as a phosphoprotein and IFN pathway regulator and suggest new mechanisms for hantavirus regulation of cellular kinases and signaling pathways. Our findings define novel IFN-regulating virulence determinants of ANDV, identify residues that can be modified to attenuate ANDV for vaccine development, and suggest the potential for kinase inhibitors to therapeutically restrict ANDV replication.

TRAF3 enhances STING-mediated antiviral signaling during the innate immune activation of black carp

Tumor necrosis factor receptor-associated factor 3 (TRAF3) is a main regulator of antiviral and anti-inflammatory pathways in mammals, which is considered to induce type I interferon (IFN) activation and negatively regulate the activation of the canonical and non-canonical NF-κB pathways. To elucidate its function in teleost fish, TRAF3 homologue of black carp (Mylopharyngodon piceus) has been cloned and characterized in this study. The open reading frame (ORF) of black carp TRAF3 (bcTRAF3) consists of 1722 nucleotides and bcTRAF3 contains 574 amino acids. bcTRAF3 protein migrated around 65 KDa in immunoblot analysis of both EPC and HEK293T cells. bcTRAF3 was identified as a cytosolic protein and suggested to form aggregates or be associated with vesicles scattering in the cytoplasm. It was interesting that both NF-κB and IFN transcription was activated by bcTRAF3 in reporter assay. When co-expressed with black carp STING (bcSTING), bcTRAF3 was redistributed in the cytoplasm and its subcellular location overlapped with that of bcSTING no matter what the cells was infected with GCRV or not, which suggested the association between these two molecules. bcSTING-mediated IFN production was up-regulated by bcTRAF3 in a dose dependent manner in reporter assay. Accordingly, EPC cells transfected with both bcSTING and bcTRAF3 showed enhanced antiviral activity comparing EPC cells expressing bcSTING alone. Taken together, the data generated in this paper supported the conclusion that bcTRAF3 was recruited into host innate immune activation and positively regulated bcSTING-mediated antiviral signaling.

What Do We Know about How Hantaviruses Interact with Their Different Hosts?

Hantaviruses, like other members of the Bunyaviridae family, are emerging viruses that are able to cause hemorrhagic fevers. Occasional transmission to humans is due to inhalation of contaminated aerosolized excreta from infected rodents. Hantaviruses are asymptomatic in their rodent or insectivore natural hosts with which they have co-evolved for millions of years. In contrast, hantaviruses cause different pathologies in humans with varying mortality rates, depending on the hantavirus species and its geographic origin. Cases of hemorrhagic fever with renal syndrome (HFRS) have been reported in Europe and Asia, while hantavirus cardiopulmonary syndromes (HCPS) are observed in the Americas. In some cases, diseases caused by Old World hantaviruses exhibit HCPS-like symptoms. Although the etiologic agents of HFRS were identified in the early 1980s, the way hantaviruses interact with their different hosts still remains elusive. What are the entry receptors? How do hantaviruses propagate in the organism and how do they cope with the immune system? This review summarizes recent data documenting interactions established by pathogenic and nonpathogenic hantaviruses with their natural or human hosts that could highlight their different outcomes.

Infection of human airway epithelial cells by different subtypes of Dobrava-Belgrade virus reveals gene expression patterns corresponding to their virulence potential

Dobrava-Belgrade virus (DOBV) is a pathogen causing hemorrhagic fever with renal syndrome in Europe. Virulence and case fatality rate are associated with virus genotype; however the reasons for these differences are not well understood. In this work we present virus-specific effects on the gene expression profiles of human lung epithelial cells (A549) infected with different genotypes of DOBV (Dobrava, Kurkino, and Sochi), as well as the low-virulent Tula virus (TULV). The data was collected by whole-genome gene expression microarrays and confirmed by quantitative real-time PCR. Despite their close genetic relationship, the expression profiles induced by infection with different hantaviruses are significantly varying. Major differences were observed in regulation of immune response genes, which were especially induced by highly virulent DOBV genotypes Dobrava and Sochi in contrast to less virulent DOBV-Kurkino and TULV. This work gives first insights into the differences of virus - host interactions of DOBV on genotype level.

Effects of Different Doses of Nucleocapsid Protein from Hantaan Virus A9 Strain on Regulation of Interferon Signaling

Hantaan virus A9 strain (HTNV A9) is an etiologic agent of hemorrhagic fever with renal syndrome in China. The virulence of the pathogenic hantaviruses is determined by their ability to alter key signaling pathways of early interferon (IFN) induction within cells. The potential role of HTNV A9 structural proteins, such as nucleocapsid (N) and envelope glycoproteins (Gn and Gc), in regulating human's innate antiviral immune response has not yet been clarified. In this study, we investigated the effect of HTNV A9 N protein on the regulation of the IFN pathway. We found that A9 N protein can influence the host innate immune response by regulating the activation of IFNβ. The A9 N protein stimulates IFN response in low doses, whereas significantly inhibits IFNβ production at high doses. Furthermore, A9 N protein constitutively inhibits nuclear factor kappa B activation. A high dose of A9 N protein could inhibit either Poly IC-induced IFNβ or vesicular stomatitis virus-induced IFNβ and interferon-stimulated gene production. Our results indicate that HTNV A9 N protein helps virus establish successful infection by downregulating the IFN response and shed new light to the understanding of the interaction between the host innate immunity and virus during Hantaan virus infection.

The fundamental role of endothelial cells in hantavirus pathogenesis

Hantavirus, a genus of rodent- and insectivore-borne viruses in the family Bunyaviridae, is a group of emerging zoonotic pathogens. Hantaviruses cause hemorrhagic fever with renal syndrome and hantavirus cardiopulmonary syndrome in man, often with severe consequences. Vascular leakage is evident in severe hantavirus infections, and increased permeability contributes to the pathogenesis. This review summarizes the current knowledge on hantavirus interactions with hematopoietic and endothelial cells, and their effects on the increased vascular permeability.

SARS coronavirus papain-like protease inhibits the type I interferon signaling pathway through interaction with the STING-TRAF3-TBK1 complex

SARS coronavirus (SARS-CoV) develops an antagonistic mechanism by which to evade the antiviral activities of interferon (IFN). Previous studies suggested that SARS-CoV papain-like protease (PLpro) inhibits activation of the IRF3 pathway, which would normally elicit a robust IFN response, but the mechanism(s) used by SARS PLpro to inhibit activation of the IRF3 pathway is not fully known. In this study, we uncovered a novel mechanism that may explain how SARS PLpro efficiently inhibits activation of the IRF3 pathway. We found that expression of the membrane-anchored PLpro domain (PLpro-TM) from SARS-CoV inhibits STING/TBK1/IKKε-mediated activation of type I IFNs and disrupts the phosphorylation and dimerization of IRF3, which are activated by STING and TBK1. Meanwhile, we showed that PLpro-TM physically interacts with TRAF3, TBK1, IKKε, STING, and IRF3, the key components that assemble the STING-TRAF3-TBK1 complex for activation of IFN expression. However, the interaction between the components in STING-TRAF3-TBK1 complex is disrupted by PLpro-TM. Furthermore, SARS PLpro-TM reduces the levels of ubiquitinated forms of RIG-I, STING, TRAF3, TBK1, and IRF3 in the STING-TRAF3-TBK1 complex. These results collectively point to a new mechanism used by SARS-CoV through which PLpro negatively regulates IRF3 activation by interaction with STING-TRAF3-TBK1 complex, yielding a SARS-CoV countermeasure against host innate immunity.

An innate immunity-regulating virulence determinant is uniquely encoded by the Andes virus nucleocapsid protein

Andes virus (ANDV) is the only hantavirus known to spread from person to person and shown to cause highly lethal hantavirus pulmonary syndrome (HPS) in patients and Syrian hamsters. Hantaviruses replicate in human endothelial cells and accomplish this by restricting the early induction of beta interferon (IFN-β)- and IFN-stimulated genes (ISGs). Our studies reveal that the ANDV nucleocapsid (N) protein uniquely inhibits IFN signaling responses directed by cytoplasmic double-stranded RNA (dsRNA) sensors RIG-I and MDA5. In contrast, N proteins from Sin Nombre, New York-1, and Prospect Hill hantaviruses had no effect on RIG-I/MDA5-directed transcriptional responses from IFN-β-, IFN-stimulated response element (ISRE)-, or κB-containing promoters. Ablating a potential S-segment nonstructural open reading frame (ORF) (NSs) within the ANDV plasmid expressing N protein failed to alter IFN regulation by ANDV N protein. Further analysis demonstrated that expressing the ANDV N protein inhibited downstream IFN pathway activation directed by MAVS, TBK1, and IκB kinase ε (IKKε) but failed to inhibit transcriptional responses directed by constitutive expression of active interferon regulatory factor IRF3-5D or after stimulation by alpha interferon (IFN-α) or tumor necrosis factor alpha (TNF-α). Consistent with IFN pathway-specific regulation, the ANDV N protein inhibited TBK1-directed IRF3 phosphorylation (phosphorylation of serine 396 [pS396]) and TBK1 autophosphorylation (pS172). Collectively, these findings indicate that the ANDV N inhibits IFN signaling responses by interfering with TBK1 activation, upstream of IRF3 phosphorylation and NF-κB activation. Moreover, our findings reveal that ANDV uniquely carries a gene encoding a virulence determinant within its N protein that is capable of restricting ISG and IFN-β induction and provide a rationale for the novel pathogenesis and spread of ANDV.

Andes virus (ANDV) is distinguished from other hantaviruses by its unique ability to spread from person to person and cause lethal hantavirus pulmonary syndrome (HPS)-like disease in Syrian hamsters. However, virulence determinants that distinguish ANDV from other pathogenic hantaviruses have yet to be defined. Here we reveal that ANDV uniquely contains a virulence determinant within its nucleocapsid (N) protein that potently inhibits innate cellular signaling pathways. This novel function of the N protein provides a new mechanism for hantaviruses to regulate interferon (IFN) and IFN-stimulated gene (ISG) induction that is likely to contribute to the enhanced ability of ANDV to replicate, spread, and cause disease. These findings differentiate ANDV from other HPS-causing hantaviruses and provide a potential target for viral attenuation that needs to be considered in vaccine development.

Hantavirus interferon regulation and virulence determinants

Hantaviruses predominantly replicate in primary human endothelial cells and cause 2 diseases characterized by altered barrier functions of vascular endothelium. Most hantaviruses restrict the early induction of interferon-β (IFNβ) and interferon stimulated genes (ISGs) within human endothelial cells to permit their successful replication. PHV fails to regulate IFN induction within human endothelial cells which self-limits PHV replication and its potential as a human pathogen. These findings, and the altered regulation of endothelial cell barrier functions by pathogenic hantaviruses, suggest that virulence is determined by the ability of hantaviruses to alter key signaling pathways within human endothelial cells. Our findings indicate that the Gn protein from ANDV, but not PHV, inhibits TBK1 directed ISRE, kB and IFNβ induction through virulence determinants in the Gn cytoplasmic tail (GnT) that inhibit TBK1 directed IRF3 phosphorylation. Further studies indicate that in response to hypoxia induced VEGF, ANDV infection enhances the permeability and adherens junction internalization of microvascular and lymphatic endothelial cells. These hypoxia/VEGF directed responses are rapamycin sensitive and directed by mTOR signaling pathways. These results demonstrate the presence of at least two hantavirus virulence determinants that act on endothelial cell signaling pathways: one that regulates antiviral IFN signaling responses, and a second that enhances normal hypoxia-VEGF-mTOR signaling pathways to facilitate endothelial cell permeability. These findings suggest signaling pathways as potential targets for therapeutic regulation of vascular deficits that contribute to hantavirus diseases and viral protein targets for attenuating pathogenic hantaviruses.

Hantavirus GnT elements mediate TRAF3 binding and inhibit RIG-I/TBK1-directed beta interferon transcription by blocking IRF3 phosphorylation

Hantaviruses successfully replicate in primary human endothelial cells by restricting the early induction of beta interferon (IFN-β) and interferon-stimulated genes (ISGs). Gn proteins from NY-1V, ANDV, and TULV, but not PHV, harbor elements in their 142-residue cytoplasmic tails (GnTs) that inhibit RIG-I/MAVS/TBK1-TRAF3-directed IFN-β induction. Here, we define GnT interactions and residues required to inhibit TRAF3-TBK1-directed IFN-β induction and IRF3 phosphorylation. We observed that GnTs bind TRAF3 via residues within the TRAF-N domain (residues 392 to 415) and that binding is independent of the MAVS-interactive TRAF-C domain (residues 415 to 568). We determined that GnT binding to TRAF3 is mediated by C-terminal degrons within NY-1V or ANDV GnTs and that mutations that add degrons to TULV or PHV GnTs confer TRAF3 binding. Further analysis of GnT domains revealed that TRAF3 binding is a discrete GnT function, independent of IFN regulation, and that residues 15 to 42 from the NY-1V GnT C terminus are required for inhibiting TBK1-directed IFN-β transcription. Mutagenesis of the NY-1V GnT revealed that altering tyrosine 627 (Y627A/S/F) abolished GnT regulation of RIG-I/TBK1-directed IRF3 phosphorylation and transcriptional responses of ISRE, κB, and IFN-β promoters. Moreover, GnTs from NY-1V, ANDV, and TULV, but not PHV, inhibited RIG-I-directed IRF3 phosphorylation. Collectively, these findings suggest a novel role for GnTs in regulating RIG-I/TBK1 pathway-directed IRF3 phosphorylation and IFN-β induction and define virulence determinants within GnTs that may permit the attenuation of pathogenic hantaviruses. IMPORTANCE These findings provide a mechanism for selected hantavirus GnT interactions to regulate RIG-I/TBK1 signaling responses required for IFN-β induction by inhibiting TBK1 phosphorylation of IRF3. These studies culminate in showing that a single GnT residue, Y627, is required for the NY-1V GnT to inhibit RIG-I/TBK1-directed IRF3 phosphorylation and IFN-β induction. These findings define a potential virulence determinant within the NY-1V GnT that may permit hantavirus attenuation.

Hantaviruses: past, present and future

ABSTRACT: 

Hantaviruses productively infect endothelial cells in their rodent reservoirs and humans, but the infection only causes disease in humans – hantavirus pulmonary syndrome and hemorrhagic fever with renal syndrome. Despite the enormous progress that has been made in understanding the pathogenesis and immune responses of hantavirus infection, there is a large gap in our molecular-based knowledge of hantaviral proteins in their structures, functions and the mechanisms that facilitate their entry, replication and assembly. Importantly, we know little about the specific viral determinants and viral protein–host interactions that drive differences noted in immune responses between the reservoir and humans. This review discusses our current understanding and future work needed for unraveling the biology of these viruses in their reservoirs and in humans.

Strategies of highly pathogenic RNA viruses to block dsRNA detection by RIG-I-like receptors: hide, mask, hit

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dsRNA species are byproducts of RNA virus replication and/or transcription.

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Prompt detection of dsRNA by RIG-I like receptors (RLRs) is a hallmark of the innate immune response.

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RLRs activation triggers production of the type I interferon (IFN)-based antiviral response.

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Highly pathogenic RNA viruses encode proteins that block the RLRs pathway.

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Hide, mask and hit are 3 strategies of RNA viruses to avoid immune system activation.

Double-stranded RNA (dsRNA) is synthesized during the course of infection by RNA viruses as a byproduct of replication and transcription and acts as a potent trigger of the host innate antiviral response. In the cytoplasm of the infected cell, recognition of the presence of viral dsRNA as a signature of “non-self” nucleic acid is carried out by RIG-I-like receptors (RLRs), a set of dedicated helicases whose activation leads to the production of type I interferon α/β (IFN-α/β). To overcome the innate antiviral response, RNA viruses encode suppressors of IFN-α/β induction, which block RLRs recognition of dsRNA by means of different mechanisms that can be categorized into: (i) dsRNA binding and/or shielding (“hide”), (ii) dsRNA termini processing (“mask”) and (iii) direct interaction with components of the RLRs pathway (“hit”). In light of recent functional, biochemical and structural findings, we review the inhibition mechanisms of RLRs recognition of dsRNA displayed by a number of highly pathogenic RNA viruses with different disease phenotypes such as haemorrhagic fever (Ebola, Marburg, Lassa fever, Lujo, Machupo, Junin, Guanarito, Crimean-Congo, Rift Valley fever, dengue), severe respiratory disease (influenza, SARS, Hendra, Hantaan, Sin Nombre, Andes) and encephalitis (Nipah, West Nile).

Uncovering the mysteries of hantavirus infections

Hantaviruses are negative-sense single-stranded RNA viruses that infect many species of rodents, shrews, moles and bats. Infection in these reservoir hosts is almost asymptomatic, but some rodent-borne hantaviruses also infect humans, causing either haemorrhagic fever with renal syndrome (HFRS) or hantavirus cardiopulmonary syndrome (HCPS). In this Review, we discuss the basic molecular properties and cell biology of hantaviruses and offer an overview of virus-induced pathology, in particular vascular leakage and immunopathology.