Newcastle disease virus (NDV)-based assay demonstrates interferon-antagonist activity for the NDV V protein and the Nipah virus V, W, and C proteins

Newcastle disease virus exerts oncolysis by both intrinsic and extrinsic caspase-dependent pathways of cell death

Newcastle disease virus (NDV), an avian paramyxovirus, is tumor selective and intrinsically oncolytic. Here, we present evidence that genetically modified, recombinant NDV strains are cytotoxic to human tumor cell lines of ecto-, endo-, and mesodermal origin. We show that cytotoxicity against tumor cells is due to multiple caspase-dependent pathways of apoptosis independent of interferon signaling competence. The signaling pathways of NDV-induced, cancer cell-selective apoptosis are not well understood. We demonstrate that NDV triggers apoptosis by activating the mitochondrial/intrinsic pathway and that it acts independently of the death receptor/extrinsic pathway. Caspase-8-methylated SH-SY5Y neuroblastoma cells are as sensitive to NDV as other caspase-8-competent cells. This demonstrates that NDV is likely to act primarily through the mitochondrial death pathway. NDV infection results in the loss of mitochondrial membrane potential and the subsequent release of the mitochondrial protein cytochrome c, but the second mitochondrion-derived activator of caspase (Smac/DIABLO) is not released. In addition, we describe early activation of caspase-9 and caspase-3. In contrast, cleavage of caspase-8, which is predominantly activated by the death receptor pathway, is a TNF-related, apoptosis-inducing ligand (TRAIL)-induced late event in NDV-mediated apoptosis of tumor cells. Our data, therefore, indicate that the death signal(s) generated by NDV in tumor cells ultimately converges at the mitochondria and that it acts independently of the death receptor pathway. Our cytotoxicity studies demonstrate that recombinant NDV could be developed as a cancer virotherapy agent, either alone or in combination with therapeutic transgenes. We have also shown that trackable oncolytic NDV could be developed without any reduction in oncolytic efficacy.

Rinderpest virus blocks type I and type II interferon action: role of structural and nonstructural proteins

Rinderpest virus (RPV) is a paramyxovirus closely related to the human pathogen Measles virus. It causes severe disease in cattle, buffalo, and some wild animals; although it can infect humans, it does not cause disease. Here, we demonstrate that RPV blocks the action of both type I (alpha) and type II (gamma) interferons (IFNs) by blocking the phosphorylation and nuclear translocation of STAT1 and STAT2 and that this block is not related to species specificity. In addition, both wild-type virulent and vaccine strains of the virus blocked IFN action. Unlike the case with some other paramyxoviruses, neither STAT1 nor STAT2 is degraded upon virus infection. STAT1 is bound by both the viral structural protein P, and thereby recruited to concentrations of viral protein in the cell, and the nonstructural protein V. Although both P and V proteins bind to STAT1 and can block IFN action when expressed in transfected cells, the IFN antagonist activity of the P protein is weaker than that of the V protein. The viral C protein also seems to weakly block IFN-induced activation of STAT1 in transfection experiments. However, studies with knockout viruses showed that the viral V protein appears to be the dominant inhibitor of IFN signaling in the context of virus infection, since prevention of viral V expression restored the IFN sensitivity of infected cells. Although a change in the distribution pattern of STAT2 was observed in virus-infected cells, STAT2 was not bound by any viral protein.

Poly(I)-poly(C12U) but not ribavirin prevents death in a hamster model of Nipah virus infection

Clinical nonrandomized trials demonstrate some efficacy for ribavirin in the treatment of patients with severe Nipah virus-induced encephalitis. We report here that EICAR, the 5-ethynyl analogue of ribavirin, and the OMP-decarboxylase inhibitors 6-aza-uridine and pyrazofurin have strong antiviral activity against Nipah virus replication in vitro. Ribavirin and 6-aza-uridine were tested further in hamsters infected with a lethal dose of Nipah virus. The activity of these small-molecule inhibitors was compared with that of the interferon inducer poly(I)-poly(C(12)U). Both ribavirin and 6-aza-uridine were able to delay but not prevent Nipah virus-induced mortality. Poly(I)-poly(C(12)U), at 3 mg/kg of body weight daily from the day of infection to 10 days postinfection, prevented mortality in 5 of 6 infected animals.

Differentially regulated interferon response determines the outcome of Newcastle disease virus infection in normal and tumor cell lines

Newcastle disease virus (NDV) is a negative-strand RNA virus with oncolytic activity against human tumors. Its effectiveness against tumors and safety in normal tissue have been demonstrated in several clinical studies. Here we show that the spread of NDV infection is drastically different in normal cell lines than in tumor cell lines and that the two cell types respond differently to beta interferon (IFN-beta) treatment. NDV rapidly replicated and killed HT-1080 human fibrosarcoma cells but spread poorly in CCD-1122Sk human skin fibroblast cells. Pretreatment with endogenous or exogenous IFN-beta completely inhibited NDV replication in normal cells but had little or no effect in tumor cells. Thus, the outcome of NDV infection appeared to depend on the response of uninfected cells to IFN-beta. To investigate their differences in IFN responsiveness, we analyzed and compared the expression and activation of components of the IFN signal transduction pathway in these two types of cells. The levels of phosphorylated STAT1 and STAT2 and that of the ISGF3 complex were markedly reduced in IFN-beta-treated tumor cells. Moreover, cDNA microarray analysis revealed significantly fewer IFN-regulated genes in the HT-1080 cells than in the CDD-1122Sk cells. This finding suggests that tumor cells demonstrate a less-than-optimum antiviral response because of a lesion in their IFN signal transduction pathway. The rapid spread of NDV in HT-1080 cells appears to be caused by their deficient expression of anti-NDV proteins upon exposure to IFN-beta.

Ebola virus VP35 protein binds double-stranded RNA and inhibits alpha/beta interferon production induced by RIG-I signaling

The Ebola virus (EBOV) VP35 protein blocks the virus-induced phosphorylation and activation of interferon regulatory factor 3 (IRF-3), a transcription factor critical for the induction of alpha/beta interferon (IFN-alpha/beta) expression. However, the mechanism(s) by which this blockage occurs remains incompletely defined. We now provide evidence that VP35 possesses double-stranded RNA (dsRNA)-binding activity. Specifically, VP35 bound to poly(rI) . poly(rC)-coated Sepharose beads but not control beads. In contrast, two VP35 point mutants, R312A and K309A, were found to be greatly impaired in their dsRNA-binding activity. Competition assays showed that VP35 interacted specifically with poly(rI) . poly(rC), poly(rA) . poly(rU), or in vitro-transcribed dsRNAs derived from EBOV sequences, and not with single-stranded RNAs (ssRNAs) or double-stranded DNA. We then screened wild-type and mutant VP35s for their ability to target different components of the signaling pathways that activate IRF-3. These experiments indicate that VP35 blocks activation of IRF-3 induced by overexpression of RIG-I, a cellular helicase recently implicated in the activation of IRF-3 by either virus or dsRNA. Interestingly, the VP35 mutants impaired for dsRNA binding have a decreased but measurable IFN antagonist activity in these assays. Additionally, wild-type and dsRNA-binding-mutant VP35s were found to have equivalent abilities to inhibit activation of the IFN-beta promoter induced by overexpression of IPS-1, a recently identified signaling molecule downstream of RIG-I, or by overexpression of the IRF-3 kinases IKKepsilon and TBK-1. These data support the hypothesis that dsRNA binding may contribute to VP35 IFN antagonist function. However, additional mechanisms of inhibition, at a point proximal to the IRF-3 kinases, most likely also exist.

Use of FTA filter paper for the molecular detection of Newcastle disease virus

The feasibility of using Flinders Technology Associates filter papers (FTA cards) to collect allantoic fluid and chicken tissue samples for Newcastle disease virus (NDV) molecular detection was evaluated. Trizol RNA extraction and one-step reverse transcriptase-polymerase chain reaction (RT-PCR) were used. FTA cards allowed NDV identification from allantoic fluid with a titre of 10(5.8) median embryo lethal doses/ml. The inactivated virus remained stable on the cards for 15 days. NDV was detected from FTA imprints of the trachea, lung, caecal tonsil and cloacal faeces of experimentally infected birds. RT-PCR detection from FTA cards was confirmed by homologous frozen-tissue RT-PCR and virus isolation. Direct nucleotide sequence of the amplified F gene allowed prediction of NDV virulence. No virus isolation was possible from the FTA inactivated samples, indicating viral inactivation upon contact. The FTA cards are suitable for collecting and transporting NDV-positive samples, providing a reliable source of RNA for molecular characterization and a hazard-free sample.

Developments towards effective treatments for Nipah and Hendra virus infection

Hendra and Nipah virus are closely related emerging viruses comprising the Henipavirus genus of the subfamily Paramyxovirinae and are distinguished by their ability to cause fatal disease in both animal and human hosts. In particular, the high mortality and person-to-person transmission associated with the most recent Nipah virus outbreaks, as well as the very recent re-emergence of Hendra virus, has confirmed the importance and necessity of developing effective therapeutic interventions. Much research conducted on the henipaviruses over the past several years has focused on virus entry, including the attachment of virus to the host cell, the identification of the virus receptor and the membrane fusion process between the viral and host cell membranes. These findings have led to the development of possible vaccine candidates, as well as potential antiviral therapeutics. The common link among all of the possible antiviral agents discussed here, which have also been developed and tested, is that they target very early stages of the infection process. The establishment and validation of suitable animal models of Henipavirus infection and pathogenesis are also discussed as they will be crucial in the assessment of the effectiveness of any treatments for Hendra and Nipah virus infection.

The interferon response circuit: induction and suppression by pathogenic viruses

Type I interferons (IFN-α/β) are potent antiviral cytokines and modulators of the adaptive immune system. They are induced by viral infection or by double-stranded RNA (dsRNA), a by-product of viral replication, and lead to the production of a broad range of antiviral proteins and immunoactive cytokines. Viruses, in turn, have evolved multiple strategies to counter the IFN system which would otherwise stop virus growth early in infection. Here we discuss the current view on the balancing act between virus-induced IFN responses and the viral counterplayers.

Human parainfluenza virus type 4 is incapable of evading the interferon-induced antiviral effect

The V proteins of some paramyxoviruses have developed the ability to efficiently inactivate STAT protein function as a countermeasure for evading interferon (IFN) responses. Human parainfluenza virus type 4 (hPIV4) is one of the rubulaviruses, which are members of the family Paramyxoviridae, and has a V protein with a highly conserved cysteine-rich domain that is the hallmark of paramyxovirus V proteins. In order to study the function of the hPIV4 V protein, we established HeLa cells expressing the hPIV4A V protein (HeLa/FlagPIV4V). The hPIV4 V protein had no ability to reduce the level of STAT1 or STAT2, although it associated with STAT1, STAT2, DDB1, and Cul4A. It interfered with neither STAT1 and STAT2 tyrosine phosphorylation nor IFN-induced STAT nuclear accumulation. In addition, HeLa/FlagPIV4V cells are fully sensitive to both beta interferon (IFN-beta) and IFN-gamma, indicating that the hPIV4 V protein has no ability to block IFN-induced signaling. We further established HeLa cells expressing various chimeric proteins between the hPIV2 and hPIV4A V proteins. The lack of IFN-antagonistic activity of the hPIV4 V protein is caused by both the P/V common and V-specific domains. At least two regions (amino acids [aa] 32 to 45 and aa 143 to 164) of hPIV4 V in the P/V common domain and one region (aa 200 to 212) of the C terminus are involved in the inability to evade the IFN-induced signaling. Moreover, we established HeLa cells persistently infected with hPIV4 to make sure of the inability to escape IFN and confirmed that hPIV4 is the only paramyxovirus analyzed to date that can't evade the IFN-induced antiviral responses.

Hendra and Nipah viruses: different and dangerous

Hendra virus and Nipah virus are highly pathogenic paramyxoviruses that have recently emerged from flying foxes to cause serious disease outbreaks in humans and livestock in Australia, Malaysia, Singapore and Bangladesh. Their unique genetic constitution, high virulence and wide host range set them apart from other paramyxoviruses. These features led to their classification into the new genus Henipavirus within the family Paramyxoviridae and to their designation as Biosafety Level 4 pathogens. This review provides an overview of henipaviruses and the types of infection they cause, and describes how studies on the structure and function of henipavirus proteins expressed from cloned genes have provided insights into the unique biological properties of these emerging human pathogens.

Nipah virus: an emergent paramyxovirus causing severe encephalitis in humans

Nipah virus is a recently emergent paramyxovirus that is capable of causing severe disease in both humans and animals. The first outbreak of Nipah virus occurred in Malaysia and Singapore in 1999 and, more recently, outbreaks were detected in Bangladesh. In humans, Nipah virus causes febrile encephalitis with respiratory syndrome that has a high mortality rate. The reservoir for Nipah virus is believed to be fruit bats, and humans are infected by contact with infected bats or by contact with an intermediate animal host such as pigs. Person to person spread of the virus has also been described. Nipah virus retains many of the genetic and biologic properties found in other paramyxoviruses, though it also has several unique characteristics. However, the virologic characteristics that allow the virus to cause severe disease over a broad host range, and the epidemiologic, environmental and virologic features that favor transmission to humans are unknown. This review summarizes what is known about the virology, epidemiology, pathology, diagnosis and control of this novel pathogen.

Inhibition of alpha/beta interferon signaling by the NS4B protein of flaviviruses

Flaviviruses are insect-borne, positive-strand RNA viruses that have been disseminated worldwide. Their genome is translated into a polyprotein, which is subsequently cleaved by a combination of viral and host proteases to produce three structural proteins and seven nonstructural proteins. The nonstructural protein NS4B of dengue 2 virus partially blocks activation of STAT1 and interferon-stimulated response element (ISRE) promoters in cells stimulated with interferon (IFN). We have found that this function of NS4B is conserved in West Nile and yellow fever viruses. Deletion analysis shows that that the first 125 amino acids of dengue virus NS4B are sufficient for inhibition of alpha/beta IFN (IFN-alpha/beta) signaling. The cleavable signal peptide at the N terminus of NS4B, a peptide with a molecular weight of 2,000, is required for IFN antagonism but can be replaced by an unrelated signal peptide. Coexpression of dengue virus NS4A and NS4B together results in enhanced inhibition of ISRE promoter activation in response to IFN-alpha/beta. In contrast, expression of the precursor NS4A/B fusion protein does not cause an inhibition of IFN signaling unless this product is cleaved by the viral peptidase NS2B/NS3, indicating that proper viral polyprotein processing is required for anti-interferon function.

Weapons of STAT destruction. Interferon evasion by paramyxovirus V protein

The signal transducer and activator of transcription (STAT) family of proteins function to activate gene transcription downstream of myriad cytokine and growth factor signals. The prototype STAT proteins, STAT1 and STAT2, are required for innate and adaptive antimicrobial immune responses that result from interferon signal transduction. While many viruses have evolved the ability to avoid these antiviral cytokines, the Paramyxoviruses are distinct in their abilities to interfere directly with STAT proteins. Individual paramyxovirus species differ greatly in their precise mechanism of STAT signaling evasion, but a virus-encoded protein called V plays a central role in this process. The theme of V-dependent interferon evasion and its variations provide significant insights into virus-host interactions and viral immune evasion that can help define targets for antiviral drug design. Exposure of the viral weapons of STAT destruction may also be instructive for application to STAT-directed therapeutics for diseases characterized by STAT hyperactivity.

Virulence of Newcastle disease virus is determined by the cleavage site of the fusion protein and by both the stem region and globular head of the haemagglutinin-neuraminidase protein

Virulence of Newcastle disease virus (NDV) is mainly determined by the amino acid sequence surrounding the fusion (F) protein cleavage site, since host proteases that cleave the F protein of virulent strains are present in more tissues than those that cleave the F protein of non-virulent strains. Nevertheless, comparison of NDV strains that carry exactly the same F protein cleavage site shows that significant differences in virulence still exist. For instance, virulent field strain Herts/33 with the F cleavage site 112RRQRRF117 had an intracerebral pathogenicity index of 1.88 compared with 1.28 for strain NDFLtag, which has the same cleavage site. This implies that additional factors contribute to virulence. After generating an infectious clone of Herts/33 (FL-Herts), we were able to map the location of additional virulence factors by exchanging sequences between FL-Herts and NDFLtag. The results showed that, in addition to the F protein cleavage site, the haemagglutinin-neuraminidase (HN) protein also contributed to virulence. The effect of the HN protein on virulence was most prominent after intravenous inoculation. Interestingly, both the stem region and the globular head of the HN protein seem to be involved in determining virulence.

Identification of naturally occurring amino acid variations that affect the ability of the measles virus C protein to regulate genome replication and transcription

The C protein of measles virus (MV C) is a basic protein of 186 amino acids (aa) that plays at least two roles in infected cells, interference with the innate immune response and modulation of viral polymerase activity. In this study, Northern blots were used to demonstrate that C proteins from three vaccine strains and three wild-type isolates of MV downregulated both mRNA transcription and genome replication in a plasmid-based mini-genome assay. The effect on transcription always paralleled the effect on replication; however, the six MV C proteins varied considerably in their ability to inhibit polymerase activity. Though the amino-terminal 45 aa of the C protein are more variable among different MV strains than the remaining 75% of the protein, the ability of the MV C proteins to inhibit polymerase activity was not regulated by substitutions in the amino terminus, but rather by the more conserved region containing aa 46-167. Naturally occurring substitutions at positions 147 and 166, but not 88 and 186, were found to regulate MV C protein activity. Deletion of the carboxyl-terminal 19 aa did not affect the polymerase-modulating activity. Though we did not find a link between the aa changes in MV C and attenuation, these data provide new information regarding the functions of this non-structural protein.

Nuclear localization of the Nipah virus W protein allows for inhibition of both virus- and toll-like receptor 3-triggered signaling pathways

The Nipah virus V and W proteins, which are encoded by the P gene via RNA editing, have a common N-terminal domain but unique C-terminal domains. They localize to the cytoplasm and nucleus, respectively, and have both been shown to function as inhibitors of JAK/STAT signaling. Here we report that V and W proteins also block virus activation of the beta interferon (IFN-beta) promoter and the IFN regulatory factor 3 (IRF3)-responsive IFN-stimulated gene 54 promoter. Surprisingly, only W protein shows strong inhibition of promoter activation in response to stimulation of Toll-like receptor 3 (TLR3) by extracellular double-stranded RNA. This activity is dependent on the nuclear localization of W protein. Within the unique C-terminal domain of W protein, we have identified a nuclear localization signal (NLS) that requires basic residues at positions 439, 440, and 442. This NLS is responsible for mediating the preferential interaction of W protein with karyopherin-alpha 3 and karyopherin-alpha 4. Nuclear localization of W protein therefore enables it to target both virus and TLR3 pathways, whereas the cytoplasmic V protein is restricted to inhibiting the virus pathway. We propose that this discrepancy is in part due to the V protein being less able to block signaling in response to the kinase, TBK-1, whereas both V and W can prevent promoter activation in response to IKKepsilon. We demonstrate that, when the TLR3 pathway is stimulated, the levels of phosphorylated IRF3 are reduced in the presence of W protein but not V protein, confirming the differential effects of these proteins and illustrating that W protein-mediated inhibition is due to a loss of active IRF3.

Stringent requirement for the C protein of wild-type measles virus for growth both in vitro and in macaques

The P gene of measles virus (MV) encodes the P protein and three accessory proteins (C, V, and R). However, the role of these accessory proteins in the natural course of MV infection remains unclear. For this study, we generated a recombinant wild-type MV lacking the C protein, called wtMV(C-), by using a reverse genetics system (M. Takeda, K. Takeuchi, N. Miyajima, F. Kobune, Y. Ami, N. Nagata, Y. Suzaki, Y. Nagai, and M. Tashiro, J. Virol. 74:6643-6647). When 293 cells expressing the MV receptor SLAM (293/hSLAM) were infected with wtMV(C-) or parental wild-type MV (wtMV), the growth of wtMV(C-) was restricted, particularly during late stages. Enhanced green fluorescent protein-expressing wtMV(C-) consistently induced late-stage cell rounding and cell death in the presence of a fusion-inhibiting peptide, suggesting that the C protein can prevent cell death and is required for long-term MV infection. Neutralizing antibodies against alpha/beta interferon did not restore the growth restriction of wtMV(C-) in 293/hSLAM cells. When cynomolgus monkeys were infected with wtMV(C-) or wtMV, the number of MV-infected cells in the thymus was >1,000-fold smaller for wtMV(C-) than for wtMV. Immunohistochemical analyses showed strong expression of an MV antigen in the spleen, lymph nodes, tonsils, and larynx of a cynomolgus monkey infected with wtMV but dramatically reduced expression in the same tissues in a cynomolgus monkey infected with wtMV(C-). These data indicate that the MV C protein is necessary for efficient MV replication both in vitro and in cynomolgus monkeys.

Mumps virus V protein antagonizes interferon without the complete degradation of STAT1

Mumps virus (MuV) has been shown to antagonize the antiviral effects of interferon (IFN) through proteasome-mediated complete degradation of STAT1 by using the viral V protein (T. Kubota et al., Biochem. Biophys. Res. Commun. 283:255-259, 2001). However, we found that MuV could inhibit IFN signaling and the generation of a subsequent antiviral state long before the complete degradation of cellular STAT1 in infected cells. In MuV-infected cells, nuclear translocation and phosphorylation of STAT1 and STAT2 tyrosine residue (Y) at 701 and 689, respectively, by IFN-beta were significantly inhibited but the phosphorylation of Jak1 and Tyk2 was not inhibited. The transiently expressed MuV V protein also inhibited IFN-beta-induced Y701-STAT1 and Y689-STAT2 phosphorylation, suggesting that the V protein could block IFN-beta-induced signal transduction without the aid of other viral components. Finally, a substitution of an alanine residue in place of a cysteine residue in the C-terminal V-unique region known to be required for STAT1 degradation and inhibition of anti-IFN signaling resulted in the loss of V protein function to inhibit the Y701-STAT1 and Y689-STAT2 phosphorylation.

[Sendai virus proteins counteracting the host innate immunity]

The nucleotide sequence of Sendai virus (SeV) genome was determined in the 1980's. During the analysis of its cDNA, two mRNAs were found to be transcribed from the P gene; one encoding P protein, the other encoding V protein. In addition, C protein was found to be translated from both/ mRNAs. Though the function of V and C proteins was being unknown for a while, the reverse-genetic technique of paramyxoviruses developed at the latter half of the 1990's gave the light on studying them. The V or C protein-knockout-SeV can be made successfully, indicating that the V and C proteins are nonessential for virus growth, However, V knockout-SeV was cleared from the mouse lungs at the one day post inoculation, and C knockout-SeV was cleared immediately after the inoculation. Both V and C proteins were thus appeared to be important for counteracting host innate immunity generated in the early phase of viral infection.

Transcriptional response of avian cells to infection with Newcastle disease virus

Newcastle disease virus (NDV) causes widespread disease in poultry and wild-birds throughout the world. cDNA microarray analysis was used to examine the effect of NDV infection on host cell transcription. The results show that NDV infection causes an apparent suppression of the interferon response genes during the early stages of infection. In addition, the results reveal transcriptional silencing of cytoskeletal proteins such as the alpha, beta, and gamma types of actin, and a downregulation of the thioredoxin gene, a likely mediator of apoptosis with possible implications in NDV pathogenesis. Comparative analyses show that a majority of genes that were transcriptionally regulated during infection with another common respiratory pathogen of poultry, the avian pneumovirus, remained unaltered during NDV infection, suggesting that even phylogenetically related viruses elicit unique or "signature" patterns of host transcriptional profiles during infection of host cells.

Inhibition of parainfluenza virus type 3 and Newcastle disease virus hemagglutinin-neuraminidase receptor binding: effect of receptor avidity and steric hindrance at the inhibitor binding sites

Zanamivir (4-guanidino-Neu5Ac2en [4-GU-DANA]) inhibits not only the neuraminidase activity but also the receptor interaction of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN), blocking receptor binding and subsequent fusion promotion. All activities of the HPIV3 variant ZM1 HN (T193I/I567V) are less sensitive to 4-GU-DANA's effects. The T193I mutation in HN confers both increased receptor binding and increased neuraminidase activity, as well as reduced sensitivities of both activities to 4-GU-DANA inhibition, consistent with a single site on the HN molecule carrying out both catalysis and binding. We now provide evidence that the HPIV3 variant's resistance to receptor-binding inhibition by 4-GU-DANA is related to a reduced affinity of the HN receptor-binding site for this compound as well as to an increase in the avidity of HN for the receptor. Newcastle disease virus (NDV) HN and HPIV3 HN respond differently to inhibition in ways that suggest a fundamental distinction between them. NDV HN-receptor binding is less sensitive than HPIV3 HN-receptor binding to 4-GU-DANA, while its neuraminidase activity is highly sensitive. Both HPIV3 and NDV HNs are sensitive to receptor-binding inhibition by the smaller molecule DANA. However, for NDV HN, some receptor binding cannot be inhibited. These data are consistent with the presence in NDV HN of a second receptor-binding site that is devoid of enzyme activity and has a negligible, if any, affinity for 4-GU-DANA. Avidity for the receptor contributes to resistance by allowing the receptor to compete effectively with inhibitors for interaction with HN, while the further determinant of resistance is the reduced binding of the inhibitor molecule to the binding pocket on HN. Based upon our data and recent three-dimensional structural information on the HPIV3 and NDV HNs, we propose mechanisms for the observed sensitivity and resistance of HN to receptor-binding inhibition and discuss the implications of these mechanisms for the distribution of HN functions.

Molecular constraints to interspecies transmission of viral pathogens

The successful replication of a viral pathogen in a host is a complex process involving many interactions. These interactions develop from the coevolution of pathogen and host and often lead to a species specificity of the virus that can make interspecies transmissions difficult. Nevertheless, viruses do sporadically cross species barriers into other host populations, including humans. In zoonotic infections, many of these interspecies transfer events are dead end, where transmission is confined only to the animal-to-human route but sometimes viruses adapt to enable spread from human to human. A pathogen must overcome many hurdles to replicate successfully in a foreign host. The viral pathogen must enter the host cell, replicate with the assistance of host factors, evade inhibitory host products, exit the first cell and move on to the next, and possibly leave the initial host and transmit to another. Each of these stages may require adaptive changes in the pathogen. Although the factors that influence each stage of the replication and transmission of most agents have not been resolved, the genomics of both hosts and pathogens are now at hand and we have begun to understand some of the molecular changes that enable some viruses to adapt to a new host.

The N- and C-terminal domains of the NS1 protein of influenza B virus can independently inhibit IRF-3 and beta interferon promoter activation

The NS1 proteins of influenza A and B viruses (A/NS1 and B/NS1 proteins) have only approximately 20% amino acid sequence identity. Nevertheless, these proteins show several functional similarities, such as their ability to bind to the same RNA targets and to inhibit the activation of protein kinase R in vitro. A critical function of the A/NS1 protein is the inhibition of synthesis of alpha/beta interferon (IFN-alpha/beta) during viral infection. Recently, it was also found that the B/NS1 protein inhibits IFN-alpha/beta synthesis in virus-infected cells. We have now found that the expression of the B/NS1 protein complements the growth of an influenza A virus with A/NS1 deleted. Expression of the full-length B/NS1 protein (281 amino acids), as well as either its N-terminal RNA-binding domain (amino acids 1 to 93) or C-terminal domain (amino acids 94 to 281), in the absence of any other influenza B virus proteins resulted in the inhibition of IRF-3 nuclear translocation and IFN-beta promoter activation. A mutational analysis of the truncated B/NS1(1-93) protein showed that RNA-binding activity correlated with IFN-beta promoter inhibition. In addition, a recombinant influenza B virus with NS1 deleted induces higher levels of IRF-3 activation, as determined by its nuclear translocation, and of IFN-alpha/beta synthesis than wild-type influenza B virus. Our results support the hypothesis that the NS1 protein of influenza B virus plays an important role in antagonizing the IRF-3- and IFN-induced antiviral host responses to virus infection.

Major human cytomegalovirus structural protein pp65 (ppUL83) prevents interferon response factor 3 activation in the interferon response

We have identified a cytomegalovirus virion protein capable of modulating the rapid induction of an interferon-like response in cells that follows virus binding and penetration. Functional genomics revealed a role for the major cytomegalovirus structural protein, pp65 (ppUL83), in counteracting this response. The underlying mechanism involves a differential impact of this structural protein on the regulation of interferon response factor 3 (IRF-3). In contrast, NF-kappaB is activated independent of pp65, and neither STAT1 nor STAT3 becomes activated by either virus. pp65 is sufficient to prevent the activation of IRF-3 when introduced alone into cells. pp65 acts by inhibiting nuclear accumulation of IRF-3 and is associated with a reduced IRF-3 phosphorylation state. Thus, this investigation shows that the major structural protein of cytomegalovirus is committed to the modulation of the IRF-3 response, a primary mediator of the type I interferon response. By subverting IRF-3, the virus escapes throwing a central alarm devoted to both immediate antiviral control and regulation of the immune response.

Dissection of measles virus V protein in relation to its ability to block alpha/beta interferon signal transduction

Interferon (IFN)-alpha and -beta are the main cytokines for innate immune responses against viral infections. To replicate efficiently in the hosts, viruses have evolved various countermeasures to the IFN response. The V protein of measles virus (MV) has been shown to block IFN-alpha/beta signalling. Here, the wild-type IC-B strain of MV was shown to grow comparably in the presence and absence of IFN-alpha, whereas replication of the Edmonston tag strain recovered from cloned DNA was strongly suppressed in its presence. The V protein of the IC-B strain, but not the Edmonston tag strain, blocked IFN-alpha signalling. The V protein of the Edmonston strain from the ATCC also inhibited IFN-alpha signalling. There were three amino acid differences between the V proteins of the Edmonston ATCC and tag strains, and substitutions of both residues at positions 110 and 272 were required for the Edmonston ATCC V protein to lose IFN-antagonist activity. The P protein of the IC-B strain, which shares the N-terminal 231 aa residues with the V protein, also inhibited IFN-alpha signalling. Indeed, fragments comprising only those 231 residues of the IC-B and Edmonston ATCC V proteins, but not the Edmonston tag V protein, were able to block IFN-alpha signalling. However, the N-terminal region of the Edmonston tag V protein, when attached to the C-terminal region of the Edmonston ATCC V protein, inhibited IFN-alpha signalling. Taken together, our results indicate that both the N- and C-terminal regions contribute to the IFN-antagonist activity of the MV V protein.

Synthesis of IFN-beta by virus-infected chicken embryo cells demonstrated with specific antisera and a new bioassay

Transcripts of interferon-alpha (IFN-alpha) and IFN-beta genes are present in virus-infected chicken cells, but because of a lack of appropriate assays and reagents, it was unclear if biologically active IFN-beta is secreted. We have established a nonviral bioassay for the sensitive detection of chicken IFN (ChIFN). This assay is based on a quail cell line that carries a luciferase gene that is controlled by the IFN-responsive chicken Mx promoter. Luciferase activity was strongly stimulated when the indicator cells were incubated with ChIFN-alpha, ChIFN-beta, or ChIFN-gamma but not with chicken interleukin-1beta (ChIL-1beta). Unlike the classic antiviral assay that preferentially detects ChIFN-alpha, the Mx-luciferase assay detected ChIFN-alpha and ChIFN-beta with similar sensitivity. With the help of this novel assay and with rabbit antisera specific for either IFN-alpha or IFN-beta, we analyzed the composition of IFN in supernatants of virus-infected chicken embryo cells. Virtually all IFN produced in response to Newcastle disease virus (NDV) was IFN-alpha. However, IFN produced in response to influenza A or vaccinia virus (VV) was a mixture of usually more than 80% IFN-alpha and up to 20% IFN-beta. Thus, IFN-alpha and IFN-beta both contribute to the cytokine activity in supernatants of virus-infected chicken cells. Furthermore, the infecting virus appears to determine the IFN subtype composition.

Panax ginseng induces production of proinflammatory cytokines via toll-like receptor

Ginseng radix, the dried root of Panax ginseng C. A. Meyer, has been shown to enhance the ability to resist microbial infections. Proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), IL-6, and interferon-gamma (IFN-gamma) are produced by macrophages treated with ginseng radix extract (GRE) in vitro as well as in vivo. However, the molecular mechanisms of the production are still not clear. In the present study, we demonstrated that production of TNF-alpha and IFN-gamma was induced by GRE in spleen cells and peritoneal macrophages from C3H/HeN mice but was impaired in C3H/HeJ mice carrying a defective toll-like receptor-4 (TLR-4) gene. In addition to these cytokines, the expression of IFN-beta and inducible nitric oxide synthase (iNOS) mRNAs was also increased in GRE-treated C3H/HeN spleen cells. We investigated the possibility that GRE contains a lipopolysaccharide (LPS)-like component. However, GRE induced production of TNF-alpha and IFN-gamma in the presence of polymyxin B, an LPS inhibitor. Furthermore, a Limulus amebocyte lysate assay showed that the endotoxin content of GRE was below the threshold level of 1 ng/ml LPS. These results suggest that GRE contains a non-LPS agent that enhances innate immunity through production of proinflammatory cytokines via TLR-4.

Induction of cellular immune responses to simian immunodeficiency virus gag by two recombinant negative-strand RNA virus vectors

A recombinant Newcastle disease virus (rNDV) expressing simian immunodeficiency virus (SIV) Gag protein (rNDV/SIVgag) was generated. The rNDV/SIVgag virus induced Gag-specific cellular immune responses in mice, leading to a specific anti-Gag antiviral immunity. This was evidenced by the inhibition of growth of recombinant vaccinia virus expressing an identical Gag antigen (rVac/SIVgag) but not of wild-type vaccinia virus in rNDV/SIVgag-immunized mice. Among intravenous, intraperitoneal, or intranasal immunization routes, intranasal administration induced the strongest protective response against challenge with rVac/SIVgag. We further demonstrated that these immune responses were greatly enhanced after booster immunization with recombinant influenza viruses expressing immunogenic portions of SIV Gag. The magnitude of the protective immune response correlated with the levels of cellular immune responses to Gag, which were still evident 9 weeks after immunization. These results suggest that rNDV and influenza virus vectors are suitable candidate vaccines against AIDS as well as against other infectious diseases.

C and V proteins of Sendai virus target signaling pathways leading to IRF-3 activation for the negative regulation of interferon-beta production

We here report a molecular basis for downregulation of interferon (IFN)-beta production by V and C proteins of Sendai virus (SeV). The infection of HeLa cells with SeV poorly induced IFN-beta even if the expression of C/C' was disrupted. In contrast, when the expression of C/C'/Y1/Y2 or V/W was disrupted, SeV infection strongly induced IFN-beta production and significantly activated the interferon regulatory factor (IRF)-3 pathway. The independent expression of C or V inhibited the double-stranded (ds) RNA- or Newcastle disease virus (NDV)-induced activation of IRF-3 and NF-kappa B, as well as the IFN-beta promoter. This inhibitory effect was also observed when Y1, Y2, or a C-terminal half fragment (aa 85-204) of C was independently expressed. Phosphorylation and homodimer formation of IRF-3 were suppressed not only in cells infected with SeV capable of expressing both C/C'/Y1/Y2 (or Y1/Y2) and V/W, but also in HeLa cells constitutively expressing Y1. These results suggest that C, Y1, Y2, and V block signaling pathways leading to IRF-3 activation to downregulate IFN-beta production.

Characterization of the amino acid residues of sendai virus C protein that are critically involved in its interferon antagonism and RNA synthesis down-regulation

Sendai virus (SeV) encodes two accessory proteins, V and C, in the alternative reading frames in the P gene that are accessed transcriptionally (V) or translationally (C). The C protein is expressed as a nested set of four C-coterminal proteins, C', C, Y1, and Y2, that use different initiation codons. Using HeLa cell lines constitutively expressing the various C proteins, we previously found that the smallest (the 175-residue Y2) of the four C proteins was fully capable of counteracting the antiviral action of interferons (IFNs) and inhibiting viral RNA synthesis and that the C-terminal half of 106 residues was sufficient for both of these inhibitory functions (A. Kato et al., J. Virol. 75:3802-3810, 2001, and A. Kato et al., J. Virol. 76:7114-7124, 2002). Here, we further generated HeLa cell lines expressing the mutated C (Cm) proteins with charged amino acids substituted for alanine residues at either positions 77 and 80; 114 and 115; 139 and 142; 151, 153, and 154; 156; or 173, 175, and 176. We found that only the mutations at positions 151, 153, and 154 abolished IFN antagonism. All the Cm proteins lost the ability to bind with STAT1 under our assay conditions, regardless of their ability to inhibit IFN signaling. On the other hand, the Cm proteins that altered the tyrosine phosphorylation and dephosphorylation of STAT1 and STAT2 always retained IFN antagonism. Thus, the abnormality of phosphorylation or dephosphorylation appeared to be a cause of the IFN antagonism by SeV C. Regarding viral RNA synthesis inhibition, all mutants but the mutant with replacements at positions 114 and 115 greatly reduced the inhibitory activity, indicating that anti-RNA synthesis by the C protein is governed by amino acids scattered across its C-terminal half. Thus, amino acid sequence requirements differ greatly between IFN antagonism and RNA synthesis inhibition. In addition, we confirmed that another SeV accessory protein, V, does not antagonize IFN.

Inhibition of severe acute respiratory syndrome virus replication by small interfering RNAs in mammalian cells

Severe acute respiratory syndrome (SARS) is an acute respiratory infectious disease that spread worldwide in early 2003. The cause was determined as a novel coronavirus (CoV), SARS-associated CoV (SARS-CoV), with a single-stranded, plus-sense RNA. To date, no effective specific treatment has been identified. To exploit the possibility of using RNA interference as a therapeutic approach to fight the disease, plasmid-mediated small interfering RNAs (siRNAs) were generated to target the SARS-CoV genome. The expression of siRNAs from two plasmids, which specifically target the viral RNA polymerase, effectively blocked the cytopathic effects of SARS-CoV on Vero cells. These two plasmids also inhibited viral replication as shown by titer assays and by an examination of viral RNA and protein levels. Thus, our results demonstrated the feasibility of developing siRNAs as effective anti-SARS drugs.

The P gene of Newcastle disease virus does not encode an accessory X protein

Many paramyxoviruses encode non-essential accessory proteins that are involved in the regulation of virus replication and inhibition of cellular antiviral responses. It has been suggested that the P gene mRNA of Newcastle disease virus (NDV) encodes an accessory protein – the so-called X protein – by translation initiation at a conserved in-frame AUG codon at position 120. Using a monoclonal antibody that specifically detected the P and X proteins, it was shown that an accessory X protein was not expressed in NDV-infected cells. Recombinant NDV strains in which the AUG was changed into a GCC (Ala) or GUC (Val) codon were viable but showed a reduction in virulence, probably because the amino acid change affected the function of the P and/or V protein.

Nipah virus V and W proteins have a common STAT1-binding domain yet inhibit STAT1 activation from the cytoplasmic and nuclear compartments, respectively

In previous reports it was demonstrated that the Nipah virus V and W proteins have interferon (IFN) antagonist activity due to their ability to block signaling from the IFN-alpha/beta receptor (J. J. Rodriguez, J. P. Parisien, and C. M. Horvath, J. Virol. 76:11476-11483, 2002; M. S. Park et al., J. Virol. 77:1501-1511, 2003). The V, W, and P proteins are all encoded by the same viral gene and share an identical 407-amino-acid N-terminal region but have distinct C-terminal sequences. We now show that the P protein also has anti-IFN function, confirming that the common N-terminal domain is responsible for the antagonist activity. Truncation of this N-terminal domain revealed that amino acids 50 to 150 retain the ability to block IFN and to bind STAT1, a key component of the IFN signaling pathway. Subcellular localization studies demonstrate that the V and P proteins are predominantly cytoplasmic whereas the W protein is localized to the nucleus. In all cases, STAT1 colocalizes with the corresponding Nipah virus protein. These interactions are sufficient to inhibit STAT1 activation, as demonstrated by the lack of STAT1 phosphorylation on tyrosine 701 in IFN-stimulated cells expressing P, V, or W. Therefore, despite their common STAT1-binding domain, the Nipah virus V and P proteins act by retaining STAT1 in the cytoplasm while the W protein sequesters STAT1 in the nucleus, creating both a cytoplasmic and a nuclear block for STAT1. We also show that the IFN antagonist activity of the P protein is not as strong as that of V or W, perhaps explaining why Nipah virus has evolved to express these two edited products.

Host Evasion by Emerging Paramyxoviruses: Hendra Virus and Nipah Virus V Proteins Inhibit Interferon Signaling

Interferon (IFN) can activate Signal Transducer and Activator of Transcription (STAT) proteins to establish a cellular antiviral response and inhibit virus replication. Many viruses have evolved strategies to inhibit this antiviral mechanism, but paramyxoviruses are unique in their abilities to directly target the IFN-responsive STAT proteins. Hendra virus and Nipah virus (Henipaviruses) are recently emerged paramyxoviruses that are the causative agents of fatal disease outbreaks in Australia and peninsular Malaysia. Similar to other paramyxoviruses, Henipaviruses inhibit IFN signal transduction through a virus-encoded protein called V. Recent studies have shown that Henipavirus V proteins target STAT proteins by inducing the formation of cytoplasmically localized high molecular weight STAT-containing complexes. This sequestration of STAT1 and STAT2 prevents STAT activation and blocks antiviral IFN signaling. As the V proteins are important factors for host evasion, they represent logical targets for therapeutics directed against Henipavirus epidemics.

Identification of the nuclear export signal and STAT-binding domains of the Nipah virus V protein reveals mechanisms underlying interferon evasion

The V proteins of Nipah virus and Hendra virus have been demonstrated to bind to cellular STAT1 and STAT2 proteins to form high-molecular-weight complexes that inhibit interferon (IFN)-induced antiviral transcription by preventing STAT nuclear accumulation. Analysis of the Nipah virus V protein has revealed a region between amino acids 174 and 192 that functions as a CRM1-dependent nuclear export signal (NES). This peptide is sufficient to complement an export-defective human immunodeficiency virus Rev protein, and deletion and substitution mutagenesis revealed that this peptide is necessary for both V protein shuttling and cytoplasmic retention of STAT1 and STAT2 proteins. However, the NES is not required for V-dependent IFN signaling inhibition. IFN signaling is blocked primarily by interaction between Nipah virus V residues 100 to 160 and STAT1 residues 509 to 712. Interaction with STAT2 requires a larger Nipah virus V segment between amino acids 100 and 300, but deletion of residues 230 to 237 greatly reduced STAT2 coprecipitation. Further, V protein interactions with cellular STAT1 is a prerequisite for STAT2 binding, and sequential immunoprecipitations demonstrate that V, STAT1, and STAT2 can form a tripartite complex. These findings characterize essential regions for Henipavirus V proteins that represent potential targets for therapeutic intervention.

Essential role of the NV protein of Novirhabdovirus for pathogenicity in rainbow trout

Novirhabdovirus, infectious hematopoietic necrosis virus (IHNV), and viral hemorrhagic septicemia virus (VHSV) are fish rhabdoviruses that, in comparison to the other rhabdoviruses, contain an additional gene coding for a small nonvirion (NV) protein of unassigned function. A recombinant IHNV with the NV gene deleted but expressing the green fluorescent protein (rIHNV-Delta NV) has previously been shown to be efficiently recovered by reverse genetics (S. Biacchesi et al., J. Virol. 74:11247-11253, 2000). However, preliminary experiments suggested that the growth in cell culture of rIHNV-Delta NV was affected by the NV deletion. In the present study, we show that the growth in cell culture of rIHNV-Delta NV is indeed severely impaired but that a normal growth of rIHNV-Delta NV can be restored when NV is provided in trans by using fish cell clones constitutively expressing the NV protein. These results indicate that NV is a protein that has a crucial biological role for optimal replication of IHNV in cell culture. Although IHNV and VHSV NV proteins do not share any significant identity, we show here that both NV proteins play a similar role since a recombinant IHNV virus, rIHNV-NV(VHSV), in which the IHNV NV open reading frame has been replaced by that of VHSV, was shown to replicate as well as the wild-type (wt) IHNV into fish cells. Finally, data provided by experimental fish infections with the various recombinant viruses strongly suggest an essential role of the NV protein for the pathogenicity of IHNV. Furthermore, we show that juvenile trout immunized with NV-knockout IHNV were protected against challenge with wt IHNV. That opens a new perspective for the development of IHNV attenuated live vaccines.

Innate immune response against nonsegmented negative strand RNA viruses

Innate immune response represents the hallmark of host defense against foreign pathogens, including viruses. Not only does this response combat viruses during initial stages of infection, but it shapes the adaptive immune response as well. This review focuses on this critical host defense mechanism, the innate immune response, in the context of infection by nonsegmented negative strand RNA viruses of the Paramyxoviridae family. We specifically focus on the two critical transcription factors, nuclear factor-kappaB (NF-kappaB) and interferon (IFN) regulatory factor-3 (IRF-3), that play an important role in establishing an innate antiviral state. The antiviral cytokine IFN-alpha/beta (IFN type I) produced following viral infection as a result of activation of NF-kappaB or IRF-3 or both exerts an antiviral state by inducing the Janus kinases/signal transducer and activator (Jak-Stat) pathway. In that context, our review discusses various strategies adopted by these viruses to counteract and evade the antiviral action of IFN I for replicative advantages, especially after modulation of the Jak-Stat antiviral pathway. Understanding this interplay between the innate immune response and viral replication is fundamental to probing into the molecular basis of host-virus interaction.

Role of fusion protein cleavage site in the virulence of Newcastle disease virus

Newcastle disease virus (NDV) causes a highly contagious and economically important disease in poultry. Viral determinants of NDV virulence are not completely understood. The amino acid sequence at the protease cleavage site of the fusion (F) protein has been postulated as a major determinant of NDV virulence. In this study, we have examined the role of F protein cleavage site sequence in NDV virulence using reverse genetics technology. The sequence G-R-Q-G-R present at the cleavage site of the F protein of avirulent strain LaSota was mutated to R-R-Q-K-R, which is present in the F cleavage site of neurovirulent strain Beaudette C (BC). The resultant mutated LaSota V.F. virus did not require exogenous protease for infectivity in cell culture, indicating that the F protein was cleaved by intracellular proteases. The virulence of the mutant and parental viruses was evaluated in vivo by intracerebral pathogenicity index (ICPI) and intravenous pathogenicity index (IVPI) tests in chickens. Our results showed that the modification of the F protein cleavage site resulted in a dramatic increase in virulence from an ICPI value of 0.00 for LaSota to a value of 1.12 for LaSota V.F. However, the ICPI value of LaSota V.F. was lower than that of BC, which had a value of 1.58. Interestingly, the IVPI tests showed values of 0.00 for both LaSota and LaSota V.F. viruses, compared to the IVPI value of 1.45 of BC. In vitro characteristics of the viruses were also studied. Our results demonstrate that the efficiency of cleavage of the F protein plays an important role if the NDV is delivered directly into the brains of chicks, but there could be other viral factors that probably affect peripheral replication, viremia, or entry into the central nervous system.

A recombinant influenza A virus expressing an RNA-binding-defective NS1 protein induces high levels of beta interferon and is attenuated in mice

Previously we found that the amino-terminal region of the NS1 protein of influenza A virus plays a key role in preventing the induction of beta interferon (IFN-beta) in virus-infected cells. This region is characterized by its ability to bind to different RNA species, including double-stranded RNA (dsRNA), a known potent inducer of IFNs. In order to investigate whether the NS1 RNA-binding activity is required for its IFN antagonist properties, we have generated a recombinant influenza A virus which expresses a mutant NS1 protein defective in dsRNA binding. For this purpose, we substituted alanines for two basic amino acids within NS1 (R38 and K41) that were previously found to be required for RNA binding. Cells infected with the resulting recombinant virus showed increased IFN-beta production, demonstrating that these two amino acids play a critical role in the inhibition of IFN production by the NS1 protein during viral infection. In addition, this virus grew to lower titers than wild-type virus in MDCK cells, and it was attenuated in mice. Interestingly, passaging in MDCK cells resulted in the selection of a mutant virus containing a third mutation at amino acid residue 42 of the NS1 protein (S42G). This mutation did not result in a gain in dsRNA-binding activity by the NS1 protein, as measured by an in vitro assay. Nevertheless, the NS1 R38AK41AS42G mutant virus was able to replicate in MDCK cells to titers close to those of wild-type virus. This mutant virus had intermediate virulence in mice, between those of the wild-type and parental NS1 R38AK41A viruses. These results suggest not only that the IFN antagonist properties of the NS1 protein depend on its ability to bind dsRNA but also that they can be modulated by amino acid residues not involved in RNA binding.

Persistent infection and suppression of host response by alphaviruses

Alphaviruses cause chronic noncytopathic infection in mosquito cells and develop a highly cytopathic infection in a wide variety of cells of vertebrate origin. Upon infection, alphaviruses modify cellular processes to meet the virus needs for propagation. Downregulation of translation and transcription caused by viral infection appears to reduce interferon (IFN) and cytokine gene expression and allows more efficient dissemination of infection. Alphaviruses with mutations in nonstructural protein nsP2 can become less cytopathic and capable of persisting in some vertebrate cell lines for a number of passages. nsP2 likely functions as an important regulator of virus-host cell interactions and plays a significant role in suppressing the antiviral response. Mammalian cells having no defects in type I IFN system react to replication of the nsP2 viral mutants by more efficient activation of IFN and IFN-dependent genes and are capable of eliminating established alphavirus infection. Blocking of IFN-alpha/beta signaling makes mouse fibroblasts unable to stop replication of Sindbis virus (SINV) with mutated nsP2 and leads to persistent infection. Downregulation of transcription and translation during alphavirus infection are quite independent events, and both probably are involved in inhibition of the antiviral response.

Hendra virus V protein inhibits interferon signaling by preventing STAT1 and STAT2 nuclear accumulation

The V protein of the recently emerged paramyxovirus, Nipah virus, has been shown to inhibit interferon (IFN) signal transduction through cytoplasmic sequestration of cellular STAT1 and STAT2 in high-molecular-weight complexes. Here we demonstrate that the closely related Hendra virus V protein also inhibits cellular responses to IFN through binding and cytoplasmic sequestration of both STAT1 and STAT2, but not STAT3. These findings demonstrate a V protein-mediated IFN signal evasion mechanism that is a general property of the known Henipavirus species.

Inhibition of interferon signaling by dengue virus

Dengue virus is a worldwide-distributed mosquito-borne flavivirus with a positive strand RNA genome. Its transcribed polyprotein is cleaved by host- and virus-encoded peptidases into 10 proteins, some of which are of unknown function. Although dengue virus-infected cells seem to be resistant to the antiviral action of IFN, the viral products that mediate this resistance are unknown. Therefore, we have analyzed the ability of the 10 dengue virus-encoded proteins to antagonize the IFN response. We found that expression in human A549 cells of the dengue virus nonstructural proteins NS2A, NS4A, or NS4B enhances replication of an IFN-sensitive virus. Moreover, expression of NS4B and, to a lesser extent, of NS2A and NS4A proteins results in down-regulation of IFN-beta-stimulated gene expression. Cells expressing NS4B or infected with dengue virus do not exhibit nuclear signal transducer and activator of transcription (STAT) 1 on treatment with IFN-beta or IFN-gamma, indicating that NS4B might be involved in blocking IFN signaling during dengue virus infections. This protein, encoded by a positive strand RNA virus, is implicated as an IFN-signaling inhibitor.

Innate immune activation as a broad-spectrum biodefense strategy: prospects and research challenges

Biodefense strategies require protection against a broad and largely unforeseen spectrum of pathogens—the forte of innate immune system defenses—that have evolved over millennia to function within moments of encountering either ancient or newly emerging pathogens. Although constitutive, the innate immune system is activated by the presence of microbes or their products, providing a rationale for a potential biodefense strategy. Both prophylactic and postexposure strategies involving innate immune stimulation have been shown to be plausible to prevent or ameliorate infections in animal models. Innate immune-activating compounds based on conserved microbial components recognized by toll-like molecules and other receptors could be synthesized and delivered like drugs by using an entirely different strategy from conventional vaccination. However, important theoretic and practical questions emerge about developing and deploying innate immune protective strategies for biodefense. This rostrum discusses prospects and problems in the overall approach itself. Important topics include microbe-specific issues about innate immune system effectiveness against highly virulent pathogens and general questions, such as whether innate immune responses will be safe and effective if used in a diverse human population of different age groups and with different genetic makeups.

Identification of a mutation in editing of defective Newcastle disease virus recombinants that modulates P-gene mRNA editing and restores virus replication and pathogenicity in chicken embryos

Editing of P-gene mRNA of Newcastle disease virus (NDV) enables the formation of two additional proteins (V and W) by inserting one or two nontemplated G residues at a conserved editing site (5'-AAAAAGGG). The V protein of NDV plays an important role in virus replication and is also a virulence factor presumably due to its ability to counteract the antiviral effects of interferon. A recombinant virus possessing a nucleotide substitution within the A-stretch (5'-AAgAAGGG) produced 20-fold-less V protein and, in consequence, was impaired in replication capacity and completely attenuated in pathogenicity for chicken embryos. However, in a total of seven serial passages, restoration of replication and pathogenic capacity in 9- to 11-day-old chicken embryos was noticed. Determining the sequence around the editing site of the virus at passage 7 revealed a C-to-U mutation at the second nucleotide immediately upstream of the 5'-A(5) stretch (5'-GuUAAgAAGGG). The V mRNA increased from an undetectable level at passage 5 to ca. 1 and 5% at passages 6 and 7, respectively. In addition, similar defects in another mutant possessing a different substitution mutation (5'-AAAcAGGG) were restored in an identical manner within a total of seven serial passages. Introduction of the above C-to-U mutation into the parent virus (5'-GuUAAAAAGGG) altered the frequency of P, V, and W mRNAs from 68, 28, and 4% to 15, 44, and 41%, respectively, demonstrating that the U at this position is a key determinant in modulating P-gene mRNA editing. The results indicate that this second-site mutation is required to compensate for the drop in edited mRNAs and consequently to restore the replication capacity, as well as the pathogenic potential, of editing-defective NDV recombinants.

Newcastle disease virus V protein is a determinant of host range restriction

It has been demonstrated that the V protein of Newcastle disease virus (NDV) functions as an alpha/beta interferon (IFN-alpha/beta) antagonist (M. S. Park, M. L. Shaw, J. Muñoz-Jordan, J. F. Cros, T. Nakaya, N. Bouvier, P. Palese, A. García-Sastre, and C. F. Basler, J. Virol. 77:1501-1511, 2003). We now show that the NDV V protein plays an important role in host range restriction. In order to study V functions in vivo, recombinant NDV (rNDV) mutants, defective in the expression of the V protein, were generated. These rNDV mutants grow poorly in both embryonated chicken eggs and chicken embryo fibroblasts (CEFs) compared to the wild-type (wt) rNDV. However, insertion of the NS1 gene of influenza virus A/PR8/34 into the NDV V(-) genome [rNDV V(-)/NS1] restores impaired growth to wt levels in embryonated chicken eggs and CEFs. These data indicate that for viruses infecting avian cells, the NDV V protein and the influenza NS1 protein are functionally interchangeable, even though there are no sequence similarities between the two proteins. Interestingly, in human cells, the titer of wt rNDV is 10 times lower than that of rNDV V(-)/NS1. Correspondingly, the level of IFN secreted by human cells infected with wt rNDV is much higher than that secreted by cells infected with the NS1-expressing rNDV. This suggests that the IFN antagonist activity of the NDV V protein is species specific. Finally, the NDV V protein plays an important role in preventing apoptosis in a species-specific manner. The rNDV defective in V induces apoptotic cell death more rapidly in CEFs than does wt rNDV. Taken together, these data suggest that the host range of NDV is limited by the ability of its V protein to efficiently prevent innate host defenses, such as the IFN response and apoptosis.

Newcastle disease virus V protein is associated with viral pathogenesis and functions as an alpha interferon antagonist

Newcastle disease virus (NDV) edits its P gene by inserting one or two G residues at the conserved editing site (UUUUUCCC, genome sense) and transcribes the P mRNA (unedited), the V mRNA (with a +1 frameshift), and the W mRNA (with a +2 frameshift). All three proteins are amino coterminal but vary at their carboxyl terminus in length and amino acid composition. Little is known about the role of the V and W proteins in NDV replication and pathogenesis. We have constructed and recovered two recombinant viruses in which the expression of the V or both the V and W proteins has been abolished. Compared to the parental virus, the mutant viruses showed impaired growth in cell cultures, except in Vero cells. However, transient expression of the carboxyl-terminal portion of the V protein enhanced the growth of the mutant viruses. In embryonated chicken eggs, the parental virus grew to high titers in embryos of different gestational ages, whereas the mutant viruses showed an age-dependent phenomenon, growing to lower titer in more-developed embryos. An interferon (IFN) sensitivity assay showed that the parental virus was more resistant to the antiviral effect of IFN than the mutant viruses. Moreover, infection with the parental virus resulted in STAT1 protein degradation, but not with the mutant viruses. These findings indicate that the V protein of NDV possesses the ability to inhibit alpha IFN and that the IFN inhibitory function lies in the carboxyl-terminal domain. Pathogenicity studies showed that the V protein of NDV significantly contributes to the virus virulence.

The Ebola virus VP35 protein inhibits activation of interferon regulatory factor 3

The Ebola virus VP35 protein was previously found to act as an interferon (IFN) antagonist which could complement growth of influenza delNS1 virus, a mutant influenza virus lacking the influenza virus IFN antagonist protein, NS1. The Ebola virus VP35 could also prevent the virus- or double-stranded RNA-mediated transcriptional activation of both the beta IFN (IFN-beta) promoter and the IFN-stimulated ISG54 promoter (C. Basler et al., Proc. Natl. Acad. Sci. USA 97:12289-12294, 2000). We now show that VP35 inhibits virus infection-induced transcriptional activation of IFN regulatory factor 3 (IRF-3)-responsive mammalian promoters and that VP35 does not block signaling from the IFN-alpha/beta receptor. The ability of VP35 to inhibit this virus-induced transcription correlates with its ability to block activation of IRF-3, a cellular transcription factor of central importance in initiating the host cell IFN response. We demonstrate that VP35 blocks the Sendai virus-induced activation of two promoters which can be directly activated by IRF-3, namely, the ISG54 promoter and the ISG56 promoter. Further, expression of VP35 prevents the IRF-3-dependent activation of the IFN-alpha4 promoter in response to viral infection. The inhibition of IRF-3 appears to occur through an inhibition of IRF-3 phosphorylation. VP35 blocks virus-induced IRF-3 phosphorylation and subsequent IRF-3 dimerization and nuclear translocation. Consistent with these observations, Ebola virus infection of Vero cells activated neither transcription from the ISG54 promoter nor nuclear accumulation of IRF-3. These data suggest that in Ebola virus-infected cells, VP35 inhibits the induction of antiviral genes, including the IFN-beta gene, by blocking IRF-3 activation.

STAT protein interference and suppression of cytokine signal transduction by measles virus V protein

Measles virus, a paramyxovirus of the Morbillivirus genus, is responsible for an acute childhood illness that infects over 40 million people and leads to the deaths of more than 1 million people annually (C. J. Murray and A. D. Lopez, Lancet 349:1269-1276, 1997). Measles virus infection is characterized by virus-induced immune suppression that creates susceptibility to opportunistic infections. Here we demonstrate that measles virus can inhibit cytokine responses by direct interference with host STAT protein-dependent signaling systems. Expression of the measles V protein prevents alpha, beta, and gamma interferon-induced transcriptional responses. Furthermore, it can interfere with signaling by interleukin-6 and the non-receptor tyrosine kinase, v-Src. Affinity purification demonstrates that the measles V protein associates with cellular STAT1, STAT2, STAT3, and IRF9, as well as several unidentified partners. Mechanistic studies indicate that while the measles V protein does not interfere with STAT1 or STAT2 tyrosine phosphorylation, it causes a defect in IFN-induced STAT nuclear accumulation. The defective STAT nuclear redistribution is also observed in measles virus-infected cells, where some of the STAT protein is detected in cytoplasmic bodies that contain viral nucleocapsid protein and nucleic acids. Interference with STAT-inducible transcription may provide a novel intracellular mechanism for measles virus-induced cytokine inhibition that links innate immune evasion to adaptive immune suppression.

Measles virus V protein blocks interferon (IFN)-alpha/beta but not IFN-gamma signaling by inhibiting STAT1 and STAT2 phosphorylation

Measles virus (MV), a member of the family Paramyxoviridae, encodes C and V non-structural proteins. To clarify the functions of MV C and V proteins, HeLa cell lines constitutively expressing C or V protein were established. We found that expression of V protein inhibited interferon (IFN)-alpha/beta signaling but not IFN-gamma signaling. C protein had no inhibitory effect on IFN signaling in our experimental condition. Degradation of selective signal transducers and activators of transcription (STAT) proteins was not observed in HeLa cells expressing V protein. In contrast, tyrosine phosphorylation of both STAT1 and STAT2 was inhibited in these cells after IFN-beta stimulation.

Involvement of the zinc-binding capacity of Sendai virus V protein in viral pathogenesis

The V protein of Sendai virus (SeV) is nonessential to virus replication in cell culture but indispensable to viral pathogenicity in mice. The highly conserved cysteine-rich zinc finger-like domain in its carboxyl terminus is believed to be responsible for this viral pathogenicity. In the present study, we showed that the cysteine-rich domain of the SeV V protein could actually bind zinc by using glutathione-S-transferase fusion proteins. When the seven conserved cysteine residues at positions 337, 341, 353, 355, 358, 362, and 365 were replaced individually, the zinc-binding capacities of the mutant proteins were greatly impaired, ranging from 22 to 68% of that of the wild type. We then recovered two mutant SeVs from cDNA, which have V-C(341)S and V-C(365)R mutations and represent maximal and minimal zinc-binding capacities among the corresponding mutant fusion proteins, respectively. The mutant viruses showed viral protein synthesis and growth patterns similar to those of wild-type SeV in cultured cells. However, the mutant viruses were strongly attenuated in mice in a way similar to that of SeV V(DeltaC), which has a truncated V protein lacking the cysteine-rich domain, by exhibiting earlier viral clearance from the mouse lung and less virulence to mice. We therefore conclude that the zinc-binding capacity of the V protein is involved in viral pathogenesis.