Hantaviruses primarily infect the endothelial cell lining of capillaries and cause two vascular permeability-based diseases. The ability of pathogenic hantaviruses to regulate the early induction of interferon determines whether hantaviruses replicate in endothelial cells. Tula virus (TULV) and Prospect Hill virus (PHV) are hantaviruses which infect human endothelial cells but fail to cause human disease. PHV is unable to inhibit early interferon (IFN) responses and fails to replicate within human endothelial cells. However, TULV replicates successfully in human endothelial cells, suggesting that TULV is capable of regulating cellular IFN responses. We observed a >300-fold reduction in the IFN-stimulated genes (ISGs) MxA and ISG56 following TULV versus PHV infection of endothelial cells 1 day postinfection. Similar to results with pathogenic hantaviruses, expressing the TULV Gn protein cytoplasmic tail (Gn-T) blocked RIG-I- and TBK1-directed transcription from IFN-stimulated response elements (ISREs) and IFN-β promoters (>90%) but not transcription directed by constitutively active IFN regulatory factor-3 (IRF3). In contrast, expressing the PHV Gn-T had no effect on TBK1-induced transcriptional responses. Analysis of Gn-T truncations demonstrated that the C-terminal 42 residues of the Gn-T (Gn-T-C42) from TULV, but not PHV, inhibited IFN induction >70%. These findings demonstrate that the TULV Gn-T inhibits IFN- and ISRE-directed responses upstream of IRF3 at the level of the TBK1 complex and further define a 42-residue domain of the TULV Gn-T that inhibits IFN induction. In contrast to pathogenic hantavirus Gn-Ts, the TULV Gn-T lacks a C-terminal degron domain and failed to bind tumor necrosis factor (TNF) receptor-associated factor 3 (TRAF3), a TBK1 complex component required for IRF3 activation. These findings indicate that the nonpathogenic TULV Gn-T regulates IFN induction but accomplishes this via unique interactions with cellular TBK1 complexes. These findings fundamentally distinguish nonpathogenic hantaviruses, PHV and TULV, and demonstrate that IFN regulation alone is insufficient for hantaviruses to cause disease. Yet regulating the early IFN response is necessary for hantaviruses to replicate within human endothelial cells and to be pathogenic. Thus, in addition to IFN regulation, hantaviruses contain discrete virulence determinants which permit them to be human pathogens.
Hantaviruses cause two diseases with prominent vascular permeability defects, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. All hantaviruses infect human endothelial cells, although it is unclear what differentiates pathogenic from nonpathogenic hantaviruses. We observed dramatic differences in interferon-specific transcriptional responses between pathogenic and nonpathogenic hantaviruses at 1 day postinfection, suggesting that hantavirus pathogenesis may in part be determined by viral regulation of cellular interferon responses. In contrast to pathogenic NY-1 virus (NY-1V) and Hantaan virus (HTNV), nonpathogenic Prospect Hill virus (PHV) elicits early interferon responses following infection of human endothelial cells. We determined that PHV replication is blocked in human endothelial cells and that RNA and protein synthesis by PHV, but not NY-1V or HTNV, is inhibited at 2 to 4 days postinfection. The addition of antibodies to beta interferon (IFN-β) blocked interferon-directed MxA induction by >90% and demonstrated that hantavirus infection induces the secretion of IFN-β from endothelial cells. Coinfecting endothelial cells with NY-1V and PHV resulted in a 60% decrease in the induction of interferon-responsive MxA transcripts by PHV and further suggested the potential for NY-1V to regulate early IFN responses. Expression of the NY-1V G1 cytoplasmic tail inhibited by >90% RIG-I- and downstream TBK-1-directed transcription from interferon-stimulated response elements or β-interferon promoters in a dose-dependent manner. In contrast, expression of the NY-1V nucleocapsid or PHV G1 tail had no effect on RIG-I- or TBK-1-directed transcriptional responses. Further, neither the NY-1V nor PHV G1 tails inhibited transcriptional responses directed by a constitutively active form of interferon regulatory factor 3 (IRF-3 5D), and IRF-3 is a direct target of TBK-1 phosphorylation. These findings indicate that the pathogenic NY-1V G1 protein regulates cellular IFN responses upstream of IRF-3 phosphorylation at the level of the TBK-1 complex. These findings further suggest that the G1 cytoplasmic tail contains a virulence element which determines the ability of hantaviruses to bypass innate cellular immune responses and delineates a mechanism for pathogenic hantaviruses to successfully replicate within human endothelial cells.
Hantavirus pulmonary syndrome (HPS) is a severe respiratory disease which is thought to result from a dysregulated immune response to infection with pathogenic hantaviruses, such as Sin Nombre virus or Andes virus (ANDV). Other New World hantaviruses, such as Prospect Hill virus (PHV), have not been associated with human disease. Activation of an antiviral state and cell signaling in response to hantavirus infection were examined using human primary lung endothelial cells, the main target cell infected in HPS patients. PHV, but not ANDV, was found to induce a robust beta interferon (IFN-β) response early after infection of primary lung endothelial cells. The level of IFN induction correlated with IFN regulatory factor 3 (IRF-3) activation, in that IRF-3 dimerization and nuclear translocation were detected in PHV but not ANDV infection. In addition, phosphorylated Stat-1/2 levels were significantly lower in the ANDV-infected cells relative to PHV. Presumably, this reflects the lower level of IRF-3 activation and initial IFN induced by ANDV relative to PHV. To determine whether, in addition, ANDV interference with IFN signaling also contributed to the low Stat-1/2 activation seen in ANDV infection, the levels of exogenous IFN-β-induced Stat-1/2 activation detectable in uninfected versus ANDV- or PHV-infected Vero-E6 cells were examined. Surprisingly, both viruses were found to downregulate IFN-induced Stat-1/2 activation. Analysis of cells transiently expressing only ANDV or PHV glycoproteins implicated these proteins in this downregulation. In conclusion, while both viruses can interfere with IFN signaling, there is a major difference in the initial interferon induction via IRF-3 activation between ANDV and PHV in infected primary endothelial cells, and this correlates with the reported differences in pathogenicity of these viruses.
Pathogenic hantaviruses replicate within human endothelial cells and cause two diseases, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. In order to replicate in endothelial cells pathogenic hantaviruses inhibit the early induction of beta interferon (IFN-β). Expression of the cytoplasmic tail of the pathogenic NY-1 hantavirus Gn protein is sufficient to inhibit RIG-I- and TBK1-directed IFN responses. The formation of TBK1-TRAF3 complexes directs IRF-3 phosphorylation, and both IRF-3 and NF-κB activation are required for transcription from the IFN-β promoter. Here we report that the NY-1 virus (NY-1V) Gn tail inhibits both TBK1-directed NF-κB activation and TBK1-directed transcription from promoters containing IFN-stimulated response elements. The NY-1V Gn tail coprecipitated TRAF3 from cellular lysates, and analysis of TRAF3 deletion mutants demonstrated that the TRAF3 N terminus is sufficient for interacting with the NY-1V Gn tail. In contrast, the Gn tail of the nonpathogenic hantavirus Prospect Hill virus (PHV) failed to coprecipitate TRAF3 or inhibit NF-κB or IFN-β transcriptional responses. Further, expression of the NY-1V Gn tail blocked TBK1 coprecipitation of TRAF3 and infection by NY-1V, but not PHV, blocked the formation of TBK1-TRAF3 complexes. These findings indicate that the NY-1V Gn cytoplasmic tail forms a complex with TRAF3 which disrupts the formation of TBK1-TRAF3 complexes and downstream signaling responses required for IFN-β transcription.
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.
Hantaviruses represent important human pathogens and can induce hemorrhagic fever with renal syndrome (HFRS), which is characterized by endothelial dysfunction. Both pathogenic and nonpathogenic hantaviruses replicate without causing any apparent cytopathic effect, suggesting that immunopathological mechanisms play an important role in pathogenesis. We compared the antiviral responses triggered by Hantaan virus (HTNV), a pathogenic hantavirus associated with HFRS, and Tula virus (TULV), a rather nonpathogenic hantavirus, in human umbilical vein endothelial cells (HUVECs). Both HTNV- and TULV-infected cells showed increased levels of molecules involved in antigen presentation. However, TULV-infected HUVECs upregulated HLA class I molecules more rapidly. Interestingly, HTNV clearly induced the production of beta interferon (IFN-β), whereas expression of this cytokine was barely detectable in the supernatant or in extracts from TULV-infected HUVECs. Nevertheless, the upregulation of HLA class I on both TULV- and HTNV-infected cells could be blocked by neutralizing anti-IFN-β antibodies. Most strikingly, the antiviral MxA protein, which interferes with hantavirus replication, was already induced 16 h after infection with TULV. In contrast, HTNV-infected HUVECs showed no expression of MxA until 48 h postinfection. In accordance with the kinetics of MxA expression, TULV replicated only inefficiently in HUVECs, whereas HTNV-infected cells produced high titers of virus particles that decreased after 48 h postinfection. Both hantavirus species, however, could replicate equally well in Vero E6 cells, which lack an IFN-induced MxA response. Thus, delayed induction of antiviral MxA in endothelial cells after infection with HTNV could allow viral dissemination and contribute to the pathogenesis leading to HFRS.
Hantaviruses indigenous to the New World are the etiologic agents of hantavirus cardiopulmonary syndrome (HCPS). These viruses induce a strong interferon-stimulated gene (ISG) response in human endothelial cells. African green monkey-derived Vero E6 cells are used to propagate hantaviruses as well as many other viruses. The utility of the Vero E6 cell line for virus production is thought to owe to their lack of genes encoding type I interferons (IFN), rendering them unable to mount an efficient innate immune response to virus infection. Interferon λ, a more recently characterized type III IFN, is transcriptionally controlled much like the type I IFNs, and activates the innate immune system in a similar manner.
We show that Vero E6 cells respond to hantavirus infection by secreting abundant IFNλ. Three New World hantaviruses were similarly able to induce IFNλ expression in this cell line. The IFNλ contained within virus preparations generated with Vero E6 cells independently activates ISGs when used to infect several non-endothelial cell lines, whereas innate immune responses by endothelial cells are specifically due to viral infection. We show further that Sin Nombre virus replicates to high titer in human hepatoma cells (Huh7) without inducing ISGs.
Herein we report that Vero E6 cells respond to viral infection with a highly active antiviral response, including secretion of abundant IFNλ. This cytokine is biologically active, and when contained within viral preparations and presented to human epithelioid cell lines, results in the robust activation of innate immune responses. We also show that both Huh7 and A549 cell lines do not respond to hantavirus infection, confirming that the cytoplasmic RNA helicase pathways possessed by these cells are not involved in hantavirus recognition. We demonstrate that Vero E6 actively respond to virus infection and inhibiting IFNλ production in these cells might increase their utility for virus propagation.
Maladaptive immune responses are considered to be important factors in the pathogenesis of the two diseases caused by hantaviruses, hemorrhagic fever with renal syndrome and hantavirus cardiopulmonary syndrome (HCPS). While the intensity of adaptive antiviral T-cell responses seems to correlate with the severity of HCPS, there is increasing evidence that innate antiviral responses by endothelial cells, the native targets for hantavirus infection in vivo, are induced within hours of exposure to infectious hantaviruses. To investigate early events in the innate response to Sin Nombre virus (SNV), the principal etiologic agent of HCPS in North America, we treated human endothelial cells with live virus, or virus subjected to inactivation by UV irradiation at minimal doses required to inhibit replication, and assayed host expression of interferon-stimulated genes (ISG) by microarray and reverse transcription-PCR. We show herein that a variety of ISG are induced between 4 and 24 h after exposure to both live and killed virus. The levels of such induction at early time points (before 24 h) were generally higher in cells treated with SNV particles that had been killed by exposure to UV irradiation. Additionally, SNV exposed to increasing doses of UV irradiation induced ISG better than live virus despite increased disruption of viral RNA integrity. However, SNV replication was required for continued ISG overexpression by 3 days posttreatment. These results suggest that hantavirus particles may themselves be capable of early induction of ISG and that ongoing production of viral particles during infection could contribute to the pathogenic process.
Hantaviruses, causing hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS), are known to be sensitive to nitric oxide (NO) and to pretreatment with type I and II interferons (alpha interferon [IFN-α]/IFN-β and IFN-γ, respectively). Elevated serum levels of NO and IFN-γ have been observed in HFRS patients, but little is known regarding the systemic levels of other IFNs and the possible effects of hantaviruses on innate antiviral immune responses. In Puumala virus-infected HFRS patients (n = 18), we report that the levels of IFN-α and IFN-β are similar, whereas the level of IFN-λ (type III IFN) is significantly decreased, during acute (day of hospitalization) compared to the convalescent phase. The possible antiviral effects of IFN-λ on the prototypic hantavirus Hantaan virus (HTNV) replication was then investigated. Pretreatment of A549 cells with IFN-λ alone inhibited HTNV replication, and IFN-λ combined with IFN-γ induced additive antiviral effects. We then studied the effect of postinfection treatment with IFNs. Interestingly, an already-established HTNV infection was insensitive to subsequent IFN-α, -β, -γ, and -λ stimulation, and HTNV-infected cells produced less NO compared to noninfected cells when stimulated with IFN-γ and IL-1β. Furthermore, less phosphorylated STAT1 after IFN treatment was observed in the nuclei of infected cells than in those of noninfected cells. The results suggest that hantavirus can interfere with the activation of antiviral innate immune responses in patients and inhibit the antiviral effects of all IFNs. We believe that future studies addressing the mechanisms by which hantaviruses interfere with the activation and shaping of immune responses may bring more knowledge regarding HFRS and HCPS pathogenesis.
Evasion of interferon (IFN)-mediated antiviral signaling is a common defense strategy for pathogenic RNA viruses. To date, research on IFN antagonism by hantaviruses is limited and has focused on only a subset of the numerous recognized hantavirus species. The host IFN response has two phases, an initiation phase, resulting in the induction of alpha/beta IFN (IFN-α/β), and an amplification phase, whereby IFN-α/β signals through the Jak/STAT pathway, resulting in the establishment of the cellular antiviral state. We examined interactions between these critical host responses and the New World hantaviruses. We observed delayed cellular responses in both Andes virus (ANDV)- and Sin Nombre virus (SNV)-infected A549 and Huh7-TLR3 cells. We found that IFN-β induction is inhibited by coexpression of ANDV nucleocapsid protein (NP) and glycoprotein precursor (GPC) and is robustly inhibited by SNV GPC alone. Downstream amplification by Jak/STAT signaling is also inhibited by SNV GPC and by either NP or GPC of ANDV. Therefore, ANDV- and SNV-encoded proteins have the potential for inhibiting both IFN-β induction and signaling, with SNV exhibiting the more potent antagonism ability. Herein we identify ANDV NP, a previously unrecognized inhibitor of Jak/STAT signaling, and show that IFN antagonism by ANDV relies on expression of both the glycoproteins and NP, whereas the glycoproteins appear to be sufficient for antagonism by SNV. These data suggest that IFN antagonism strategies by hantaviruses are quite variable, even between species with similar disease phenotypes, and may help to better elucidate species-specific pathogenesis.
Although hantaviruses have been previously considered as rodent-borne pathogens, recent studies demonstrate genetically distinct hantaviruses in evolutionarily distant non-rodent reservoirs, including shrews, moles and bats. The immunological responses to these newfound hantaviruses in humans are unknown. We compared the innate immune responses to Imjin virus (MJNV) and Thottapalayam virus (TPMV), two shrew-borne hantaviruses, with that toward two rodent-borne hantaviruses, pathogenic Hantann virus (HTNV) and nonpathogenic Prospect Hill virus (PHV). Infection of human macrophages and endothelial cells with either HTNV or MJNV triggered productive viral replication and up-regulation of anti-viral responsive gene expression from day 1 to day 3 postinfection, compared with PHV and TPMV. Furthermore, HTNV, MJNV and TPMV infection led to prolonged increased production of pro-inflammatory cytokines from days 3 to 7 postinfection. By contrast, PHV infection failed to induce pro-inflammatory responses. Distinct patterns of innate immune activation caused by MJNV suggest that it might be pathogenic to humans.
hantavirus; Imjin virus; shrew; innate immune response; anti-viral response
Type III interferons ([IFNs]IFN-λ and interleukin-28 and -29 [IL-28/29]) are recently recognized cytokines with innate antiviral effects similar to those of type I IFNs (IFN-α/β). Like IFN-α/β, IFN-λ-expression can be induced by viruses, and it is believed that type I and III IFNs are regulated in the same manner. Hantaviruses are weak IFN-α/β inducers and have surprisingly been shown to activate IFN-α/β-independent IFN-stimulated gene (ISG) expression. Here, we show that in Hantaan virus (HTNV)-infected human epithelial A549 cells, induction of IFN-λ1 preceded induction of MxA and IFN-β by 12 and 24 h, respectively, and IFN-α was not induced at all. Furthermore, induction of IFN-λ1 and MxA was observed in HTNV-infected African green monkey epithelial Vero E6 cells, a cell line that cannot produce type I IFNs, clearly showing that HTNV can induce IFN-λ1 and ISGs in the complete absence of IFN-α/β. In HTNV-infected human fibroblast MRC-5 cells, which lack the IFN-λ receptor, induction of MxA coincided in time with IFN-β-induction. UV-inactivated HTNV did not induce any IFNs or MxA in any cell line, showing that activation of IFN-λ1 is dependent on replicating virus. Induction of both IFN-β and IFN-λ1 in A549 cells after poly(I:C)-stimulation was strongly inhibited in HTNV-infected cells, suggesting that HTNV can inhibit signaling pathways used to simultaneously activate types I and III IFNs. In conclusion, we show that HTNV can cause type I IFN-independent IFN-λ1 induction and IFN-λ1-specific ISG induction. Importantly, the results suggest the existence of specific signaling pathways that induce IFN-λ1 without simultaneous type I IFN induction during virus infection.
Hantaviruses primarily infect endothelial cells (ECs) and nonlytically cause vascular changes that result in hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Acute pulmonary edema during HPS may be caused by capillary leakage and failure of lymphatic vessels to clear fluids. Uniquely regulated lymphatic ECs (LECs) control fluid clearance, although roles for lymphatics in hantavirus disease remain undetermined. Here we report that hantaviruses productively infect LECs and that LEC infection by HPS causing Andes virus (ANDV) and HFRS causing Hantaan virus (HTNV) are inhibited by αvβ3 integrin antibodies. Although αvβ3 integrins regulate permeabilizing responses directed by vascular endothelial growth factor receptor 2 (VEGFR2), we found that only ANDV-infected LECs were hyperpermeabilized by the addition of VEGF-A. However, VEGF-C activation of LEC-specific VEGFR3 receptors blocked ANDV- and VEGF-A-induced LEC permeability. In addition, ∼75% of ANDV-infected LECs became viable mononuclear giant cells, >4 times larger than normal, in response to VEGF-A. Giant cells are associated with constitutive mammalian target of rapamycin (mTOR) activation, and we found that both giant LECs and LEC permeability were sensitive to rapamycin, an mTOR inhibitor, and VEGF-C addition. These findings indicate that ANDV uniquely alters VEGFR2-mTOR signaling responses of LECs, resulting in giant cell and LEC permeability responses. This suggests that ANDV infection alters normal LEC and lymphatic vessel functions which may contribute to edematous fluid accumulation during HPS. Moreover, the ability of VEGF-C and rapamycin to normalize LEC responses suggests a potential therapeutic approach for reducing pulmonary edema and the severity of HPS following ANDV infection.
Varicella-zoster virus (VZV) is an alphaherpesvirus that is restricted to humans. VZV infection of differentiated cells within the host and establishment of latency likely require evasion of innate immunity and limited secretion of antiviral cytokines. Since interferons (IFNs) severely limit VZV replication, we examined the ability of VZV to modulate the induction of the type I IFN response in primary human embryonic lung fibroblasts (HELF). IFN-β production was not detected, and transcription of two interferon response factor 3 (IRF3)-dependent interferon-stimulated genes (ISGs), ISG54 and ISG56, in response to poly(I:C) stimulation was downregulated in VZV-infected HELF. Inhibition of IRF3 function did not require VZV replication; the viral immediate-early protein 62 (IE62) alone was sufficient to produce this effect. IE62 blocked TBK1-mediated IFN-β secretion and IRF3 function, as shown in an IFN-stimulated response element (ISRE)-luciferase reporter assay. However, IRF3 function was preserved if constitutively active IRF3 (IRF3-5D) was expressed in VZV-infected or IE62-transfected cells, indicating that VZV interferes with IRF3 phosphorylation. IE62-mediated inhibition was mapped to blocking phosphorylation of at least three serine residues on IRF3. However, IE62 binding to TBK1 or IRF3 was not detected and IE62 did not perturb TBK1-IRF3 complex formation. IE62-mediated inhibition of IRF3 function was maintained even if IE62 transactivator activity was disrupted. Thus, IE62 has two critical but discrete roles following VZV entry: to induce expression of VZV genes and to disarm the IFN-dependent antiviral defense through a novel mechanism that prevents IRF3 phosphorylation.
Hantaviruses infect endothelial cells and cause 2 vascular permeability-based diseases. Pathogenic hantaviruses enhance the permeability of endothelial cells in response to vascular endothelial growth factor (VEGF). However, the mechanism by which hantaviruses hyperpermeabilize endothelial cells has not been defined. The paracellular permeability of endothelial cells is uniquely determined by the homophilic assembly of vascular endothelial cadherin (VE-cadherin) within adherens junctions, which is regulated by VEGF receptor-2 (VEGFR2) responses. Here, we investigated VEGFR2 phosphorylation and the internalization of VE-cadherin within endothelial cells infected by pathogenic Andes virus (ANDV) and Hantaan virus (HTNV) and nonpathogenic Tula virus (TULV) hantaviruses. We found that VEGF addition to ANDV- and HTNV-infected endothelial cells results in the hyperphosphorylation of VEGFR2, while TULV infection failed to increase VEGFR2 phosphorylation. Concomitant with the VEGFR2 hyperphosphorylation, VE-cadherin was internalized to intracellular vesicles within ANDV- or HTNV-, but not TULV-, infected endothelial cells. Addition of angiopoietin-1 (Ang-1) or sphingosine-1-phosphate (S1P) to ANDV- or HTNV-infected cells blocked VE-cadherin internalization in response to VEGF. These findings are consistent with the ability of Ang-1 and S1P to inhibit hantavirus-induced endothelial cell permeability. Our results suggest that pathogenic hantaviruses disrupt fluid barrier properties of endothelial cell adherens junctions by enhancing VEGFR2-VE-cadherin pathway responses which increase paracellular permeability. These results provide a pathway-specific mechanism for the enhanced permeability of hantavirus-infected endothelial cells and suggest that stabilizing VE-cadherin within adherens junctions is a primary target for regulating endothelial cell permeability during pathogenic hantavirus infection.
The competitiveness of two Tula hantavirus (TULV) isolates, TULV/Lodz and TULV/Moravia, was evaluated in interferon (IFN) -competent and IFN-deficient cells. The two isolates differ in the length of the open reading frame (ORF) encoding the nonstructural protein NSs, which has previously been shown to inhibit IFN response in infected cells.
In IFN-deficient Vero E6 cells both TULV isolates survived equally well. In contrast, in IFN-competent MRC5 cells TULV/Lodz isolate, that possesses the NSs ORF for the full-length protein of 90 aa, survived for more consequent passages than TULV/Moravia isolate, which contains the ORF for truncated NSs protein (66–67 aa).
Our data show that expression of a full-length NSs protein is beneficial for the virus survival and competitiveness in IFN-competent cells and not essential in IFN-deficient cells. These results suggest that the N-terminal aa residues are important for the full activity of the NSs protein.
The innate immune system recognizes virus infection and evokes antiviral responses which include producing type I interferons (IFNs). The induction of IFN provides a crucial mechanism of antiviral defense by upregulating interferon-stimulated genes (ISGs) that restrict viral replication. ISGs inhibit the replication of many viruses by acting at different steps of their viral cycle. Specifically, IFN treatment prior to in vitro human immunodeficiency virus (HIV) infection stops or significantly delays HIV-1 production indicating that potent inhibitory factors are generated. We report that HIV-1 infection of primary human macrophages decreases tumor necrosis factor receptor-associated factor 6 (TRAF6) and virus-induced signaling adaptor (VISA) expression, which are both components of the IFN signaling pathway controlling viral replication. Knocking down the expression of TRAF6 in macrophages increased HIV-1 replication and augmented the expression of IRF7 but not IRF3. Suppressing VISA had no impact on viral replication. Overexpression of IRF7 resulted in enhanced viral replication while knocking down IRF7 expression in macrophages significantly reduced viral output. These findings are the first demonstration that TRAF6 can regulate HIV-1 production and furthermore that expression of IRF7 promotes HIV-1 replication.
The type I interferon system plays a critical role in limiting the spread of viral infection. Viruses induce the production of interferon (IFN), which after binding to the IFN-α/β receptor (IFNAR), and triggering of the JAK/STAT signaling cascade, results in the induction of interferon-stimulated genes (ISGs). These ISGs function to inhibit viral replication and to regulate the host immune response. Among these ISGs, the ubiquitin-like molecule, ISG15, is one of the most strongly induced proteins. Similar to ubiquitin, through an IFN induced conjugation cascade, ISG15 is covalently linked to a variety of cellular proteins, suggesting regulation of different cellular processes. Studies performed over the past several years have shown that ISG15 plays a central role in the host’s antiviral response against many viruses. Mice lacking ISG15 display increased susceptibility to multiple viruses. Furthermore, several viruses have developed immune evasion strategies that directly target the ISG15 pathway. Work is now underway to determine the mechanism by which ISG15 functions as an antiviral molecule, such that therapies targeting this pathway can be developed in the future.
ISG15; interferon; antiviral; ubiquitin-like molecule
Pathogenic hantaviruses are a closely related group of rodent-borne viruses which are responsible for two distinct diseases in humans, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome (HPS, otherwise known as hantavirus cardiopulmonary syndrome, HCPS). The antiviral effect of ribavirin against Old World hantaviruses, most notably Hantaan virus, is well documented; however, only a few studies have addressed its inhibitory effect on New World hantaviruses. In the present study, we demonstrate that ribavirin is highly active against Andes virus (ANDV), an important etiological agent of HPS, both in vitro and in vivo using a lethal hamster model of HPS. Treatment of ANDV infected Vero E6 cells with ribavirin resulted in dose-dependent reductions in viral RNA and protein as well as virus yields with a half maximal inhibitory concentration between 5 and 12.5 µg ml−1. In hamsters, treatment with as little as 5 mg kg−1 day−1 was 100% effective at preventing lethal HPS disease when therapy was administered by intraperitoneal injection from day 1 through day 10 post-infection. Significant reductions were observed in ANDV RNA and antigen positive cells in lung and liver tissues. Ribavirin remained completely protective when administered by intraperitoneal injections up to three days post-infection. In addition, we show that daily oral ribavirin therapy initiated 1 day post-infection and continuing for ten days is also protective against lethal ANDV disease, even at doses of 5 mg kg−1 day−1. Our results suggest ribavirin treatment is beneficial for postexposure prophylaxis against HPS-causing hantaviruses and should be considered in scenarios where exposure to the virus is probable. The similarities between the results obtained in this study and those from previous clinical evaluations of ribavirin against HPS, further validate the hamster model of lethal HPS and demonstrate its usefulness in screening antiviral agents against this disease.
Hantaviruses infect human endothelial cells and cause two vascular permeability-based diseases: hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. Hantavirus infection alone does not permeabilize endothelial cell monolayers. However, pathogenic hantaviruses inhibit the function of αvβ3 integrins on endothelial cells, and hemorrhagic disease and vascular permeability deficits are consequences of dysfunctional β3 integrins that normally regulate permeabilizing vascular endothelial growth factor (VEGF) responses. Here we show that pathogenic Hantaan, Andes, and New York-1 hantaviruses dramatically enhance the permeability of endothelial cells in response to VEGF, while the nonpathogenic hantaviruses Prospect Hill and Tula have no effect on endothelial cell permeability. Pathogenic hantaviruses directed endothelial cell permeability 2 to 3 days postinfection, coincident with pathogenic hantavirus inhibition of αvβ3 integrin functions, and hantavirus-directed permeability was inhibited by antibodies to VEGF receptor 2 (VEGFR2). These studies demonstrate that pathogenic hantaviruses, similar to αvβ3 integrin-deficient cells, specifically enhance VEGF-directed permeabilizing responses. Using the hantavirus permeability assay we further demonstrate that the endothelial-cell-specific growth factor angiopoietin 1 (Ang-1) and the platelet-derived lipid mediator sphingosine 1-phosphate (S1P) inhibit hantavirus directed endothelial cell permeability at physiologic concentrations. These results demonstrate the utility of a hantavirus permeability assay and rationalize the testing of Ang-1, S1P, and antibodies to VEGFR2 as potential hantavirus therapeutics. The central importance of β3 integrins and VEGF responses in vascular leak and hemorrhagic disease further suggest that altering β3 or VEGF responses may be a common feature of additional viral hemorrhagic diseases. As a result, our findings provide a potential mechanism for vascular leakage after infection by pathogenic hantaviruses and the means to inhibit hantavirus-directed endothelial cell permeability that may be applicable to additional vascular leak syndromes.
Innate immune sensing of viral nucleic acids triggers type I interferon (IFN) production, which activates interferon-stimulated genes (ISGs) and directs a multifaceted antiviral response. ISGs can also be activated through IFN-independent pathways, although the precise mechanisms remain elusive. Here we found that the cytosolic exonuclease Trex1 regulates the activation of a subset of ISGs independently of IFN. Both Trex1−/− mouse and TREX1-mutant human cells express high levels of antiviral genes and are refractory to viral infections. The IFN-independent activation of antiviral genes in Trex1−/− cells requires STING, TBK1 and IRF3 and IRF7. We also found that Trex1-deficient cells display expanded lysosomal compartment, altered subcellular localization of the transcription factor EB (TFEB), and reduced mTORC1 activity. Together, our data identify Trex1 as a regulator of lysosomal biogenesis and IFN-independent activation of antiviral genes, and shows dysregulation of lysosomes can elicit innate immune responses.
Hantaviruses cause hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardio-pulmonary syndrome (HCPS; also called hantavirus pulmonary syndrome (HPS)), both human diseases with high case-fatality rates. Endothelial cells are the main targets for hantaviruses. An intriguing observation in patients with HFRS and HCPS is that on one hand the virus infection leads to strong activation of CD8 T cells and NK cells, on the other hand no obvious destruction of infected endothelial cells is observed. Here, we provide an explanation for this dichotomy by showing that hantavirus-infected endothelial cells are protected from cytotoxic lymphocyte-mediated induction of apoptosis. When dissecting potential mechanisms behind this phenomenon, we discovered that the hantavirus nucleocapsid protein inhibits the enzymatic activity of both granzyme B and caspase 3. This provides a tentative explanation for the hantavirus-mediated block of cytotoxic granule-mediated apoptosis-induction, and hence the protection of infected cells from cytotoxic lymphocytes. These findings may explain why infected endothelial cells in hantavirus-infected patients are not destroyed by the strong cytotoxic lymphocyte response.
Rodent-born hantaviruses cause two severe emerging diseases with high case-fatality rates in humans; hemorrhagic fever with renal syndrome (HFRS) in Eurasia and hantavirus cardio-pulmonary syndrome (HCPS; also called hantavirus pulmonary syndrome (HPS)) in the Americas. A hallmark of HFRS/HCPS is increased vascular permeability. While endothelial cells are the main targets for hantaviruses, infection per se is not lytic. Patients suffering from HFRS and HCPS show remarkable strong cytotoxic lymphocyte responses including high numbers of activated NK cells and antigen-specific CD8 T cells. Hence, it has been suggested that cytotoxic lymphocyte-mediated killing of hantavirus-infected endothelial cells might contribute to HFRS/HCPS-pathogenesis. Here, we show that hantaviruses protect infected endothelial cells from being killed by cytotoxic lymphocytes. Further, we also show that hantaviruses inhibit apoptosis in general. Hantaviruses are negative-stranded RNA viruses encoding four structural proteins. Interestingly, the nucleocapsid protein was shown to inhibit the enzymatic functions of both granzyme B and caspase 3, two enzymes crucial for cytotoxic lymphocyte-mediated killing of virus-infected cells. Our study provides new insights into the interactions between hantaviruses, infected cells, and cytotoxic lymphocytes, and argues against a role for cytotoxic lymphocyte-mediated killing of virus-infected endothelial cells in causing HFRS/HCPS.
Hantaviruses replicate primarily in the vascular endothelium and cause two human diseases, hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). In this report, we demonstrate that the cellular entry of HFRS-associated hantaviruses is facilitated by specific integrins expressed on platelets, endothelial cells, and macrophages. Infection of human umbilical vein endothelial cells and Vero E6 cells by the HFRS-causing hantaviruses Hantaan (HTN), Seoul (SEO), and Puumala (PUU) is inhibited by antibodies to αvβ3 integrins and by the integrin ligand vitronectin. The cellular entry of HTN, SEO, and PUU viruses, but not the nonpathogenic Prospect Hill (PH) hantavirus (i.e., a virus with no associated human disease), was also mediated by introducting recombinant αIIbβ3 or αvβ3 integrins into β3-integrin-deficient CHO cells. In addition, PH infectivity was not inhibited by αvβ3-specific sera or vitronectin but was blocked by α5β1-specific sera and the integrin ligand fibronectin. RGD tripeptides, which are required for many integrin-ligand interactions, are absent from all hantavirus G1 and G2 surface glycoproteins, and GRGDSP peptides did not inhibit hantavirus infectivity. Further, a mouse-human hybrid β3 integrin-specific Fab fragment, c7E3 (ReoPro), also inhibited the infectivity of HTN, SEO, and PUU as well as HPS-associated hantaviruses, Sin Nombre (SN) and New York-1 (NY-1). These findings indicate that pathogenic HPS- and HFRS-causing hantaviruses enter cells via β3 integrins, which are present on the surfaces of platelets, endothelial cells, and macrophages. Since β3 integrins regulate vascular permeability and platelet function, these findings also correlate β3 integrin usage with common elements of hantavirus pathogenesis.
Hantaviruses, similar to several emerging zoonotic viruses, persistently infect their natural reservoir hosts, without causing overt signs of disease. Spillover to incidental human hosts results in morbidity and mortality mediated by excessive proinflammatory and cellular immune responses. The mechanisms mediating the persistence of hantaviruses and the absence of clinical symptoms in rodent reservoirs are only starting to be uncovered. Recent studies indicate that during hantavirus infection, proinflammatory and antiviral responses are reduced and regulatory responses are elevated at sites of increased virus replication in rodents. The recent discovery of structural and non-structural proteins that suppress type I interferon responses in humans suggests that immune responses in rodent hosts could be mediated directly by the virus. Alternatively, several host factors, including sex steroids, glucocorticoids, and genetic factors, are reported to alter host susceptibility and may contribute to persistence of hantaviruses in rodents. Humans and reservoir hosts differ in infection outcomes and in immune responses to hantavirus infection; thus, understanding the mechanisms mediating viral persistence and the absence of disease in rodents may provide insight into the prevention and treatment of disease in humans. Consideration of the coevolutionary mechanisms mediating hantaviral persistence and rodent host survival is providing insight into the mechanisms by which zoonotic viruses have remained in the environment for millions of years and continue to be transmitted to humans.
Hantaviruses predominantly infect human endothelial cells and, in the absence of cell lysis, cause two diseases resulting from increased vascular permeability. Andes virus (ANDV) causes a highly lethal acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). ANDV infection enhances the permeability of endothelial cells in response to vascular endothelial growth factor (VEGF) by increasing signaling responses directed by the VEGFR2-Src-VE-cadherin pathway, which directs adherens junction (AJ) disassembly. Here we demonstrate that inhibiting pathway-specific VEGFR2 and Src family kinases (SFKs) blocks ANDV-induced endothelial cell permeability. Small interfering RNA (siRNA) knockdown of Src within ANDV-infected endothelial cells resulted in an ∼70% decrease in endothelial cell permeability compared to that for siRNA controls. This finding suggested that existing FDA-approved small-molecule kinase inhibitors might similarly block ANDV-induced permeability. The VEGFR2 kinase inhibitor pazopanib as well as SFK inhibitors dasatinib, PP1, bosutinib, and Src inhibitor 1 dramatically inhibited ANDV-induced endothelial cell permeability. Consistent with their kinase-inhibitory concentrations, dasatinib, PP1, and pazopanib inhibited ANDV-induced permeability at 1, 10, and 100 nanomolar 50% inhibitory concentrations (IC50s), respectively. We further demonstrated that dasatinib and pazopanib blocked VE-cadherin dissociation from the AJs of ANDV-infected endothelial cells by >90%. These findings indicate that VEGFR2 and Src kinases are potential targets for therapeutically reducing ANDV-induced endothelial cell permeability and, as a result, capillary permeability during HPS. Since the functions of VEGFR2 and SFK inhibitors are already well defined and FDA approved for clinical use, these findings rationalize their therapeutic evaluation for efficacy in reducing HPS disease. Endothelial cell barrier functions are disrupted by a number of viruses that cause hemorrhagic, edematous, or neurologic disease, and as a result, our findings suggest that VEGFR2 and SFK inhibitors should be considered for regulating endothelial cell barrier functions altered by additional viral pathogens.