Longer and shorter forms of Sendai virus C proteins play different roles in modulating the cellular antiviral response

C Proteins: Controllers of Orderly Paramyxovirus Replication and of the Innate Immune Response

Particles of many paramyxoviruses include small amounts of proteins with a molecular weight of about 20 kDa. These proteins, termed “C”, are basic, have low amino acid homology and some secondary structure conservation. C proteins are encoded in alternative reading frames of the phosphoprotein gene. Some viruses express nested sets of C proteins that exert their functions in different locations: In the nucleus, they interfere with cellular transcription factors that elicit innate immune responses; in the cytoplasm, they associate with viral ribonucleocapsids and control polymerase processivity and orderly replication, thereby minimizing the activation of innate immunity. In addition, certain C proteins can directly bind to, and interfere with the function of, several cytoplasmic proteins required for interferon induction, interferon signaling and inflammation. Some C proteins are also required for efficient virus particle assembly and budding. C-deficient viruses can be grown in certain transformed cell lines but are not pathogenic in natural hosts. C proteins affect the same host functions as other phosphoprotein gene-encoded proteins named V but use different strategies for this purpose. Multiple independent systems to counteract host defenses may ensure efficient immune evasion and facilitate virus adaptation to new hosts and tissue environments.

C Protein is Essential for Canine Distemper Virus Virulence and Pathogenicity in Ferrets

Paramyxoviruses, including members of the genus Morbillivirus, express accessory proteins with ancillary functions during viral replication. One of these, the C protein, is expressed from an alternate open reading frame (ORF) located in the P gene. The measles virus (MeV) C protein has been implicated in modulation of interferon signaling, but has more recently been shown to play a vital role in regulation of viral transcription and replication, preventing the excessive production of double-stranded RNA. Failure to do so, as seen with C-deficient MeV, leads to early activation of innate immune responses resulting in restriction of viral replication and attenuation in the host. One puzzling aspect of morbillivirus C protein biology has been the finding that a C-deficient canine distemper virus (CDV) generated with a similar mutagenesis strategy displayed no attenuation in ferrets, an animal model commonly used to evaluate CDV pathogenesis. To resolve how virus lacking this protein could maintain virulence, we re-visited the CDV C protein and found that truncated C proteins are expressed from the CDV gene using alternative downstream start codons even when the first start codon was disrupted. We introduced an additional point mutation abrogating expression of these truncated C proteins. A new CDV with this mutation was attenuated in vitro and led to increased activation of protein kinase R. It was also strongly attenuated in ferrets, inducing only mild disease in infected animals, thus replicating the phenotype of C-deficient MeV. Our results demonstrate the crucial role of morbillivirus C proteins in pathogenesis.IMPORTANCE The measles (MeV) and canine distemper viruses (CDV) express accessory proteins that regulate the host immune response and enhance replication. The MeV C protein is critical in preventing the generation of excess immunostimulatory double-stranded RNA. C protein-deficient MeV is strongly attenuated compared to wild-type virus, whereas CDV with a similarly disrupted C open reading frame is fully pathogenic. Here we show that CDV can compensate the disrupting mutations by expression of truncated, but apparently functional C proteins from several alternative start codons. We generated a new recombinant CDV that does not express these truncated C protein. This virus was attenuated both in cell culture and in ferrets, and finally resolves the paradox of the MeV and CDV C proteins, showing that both in fact have similar functions important for viral pathogenesis.

Respirovirus C protein inhibits activation of type I interferon receptor-associated kinases to block JAK-STAT signaling

Respirovirus C protein blocks the type I interferon (IFN)-stimulated activation of the JAK-STAT pathway. It has been reported that C protein inhibits IFN-α-stimulated tyrosine phosphorylation of STATs, but the underlying mechanism is poorly understood. Here, we show that the C protein of Sendai virus (SeV), a member of the Respirovirus genus, binds to the IFN receptor subunit IFN-α/β receptor subunit (IFNAR)2 and inhibits IFN-α-stimulated tyrosine phosphorylation of the upstream receptor-associated kinases, JAK1 and TYK2. Analysis of various SeV C mutant (Cm) proteins demonstrates the importance of the inhibitory effect on receptor-associated kinase phosphorylation for blockade of JAK-STAT signaling. Furthermore, this inhibitory effect and the IFNAR2 binding capacity are observed for all the respirovirus C proteins examined. Our results suggest that respirovirus C protein inhibits activation of the receptor-associated kinases JAK1 and TYK2 possibly through interaction with IFNAR2.

Evolution and structural organization of the C proteins of paramyxovirinae

The phosphoprotein (P) gene of most Paramyxovirinae encodes several proteins in overlapping frames: P and V, which share a common N-terminus (PNT), and C, which overlaps PNT. Overlapping genes are of particular interest because they encode proteins originated de novo, some of which have unknown structural folds, challenging the notion that nature utilizes only a limited, well-mapped area of fold space. The C proteins cluster in three groups, comprising measles, Nipah, and Sendai virus. We predicted that all C proteins have a similar organization: a variable, disordered N-terminus and a conserved, α-helical C-terminus. We confirmed this predicted organization by biophysically characterizing recombinant C proteins from Tupaia paramyxovirus (measles group) and human parainfluenza virus 1 (Sendai group). We also found that the C of the measles and Nipah groups have statistically significant sequence similarity, indicating a common origin. Although the C of the Sendai group lack sequence similarity with them, we speculate that they also have a common origin, given their similar genomic location and structural organization. Since C is dispensable for viral replication, unlike PNT, we hypothesize that C may have originated de novo by overprinting PNT in the ancestor of Paramyxovirinae. Intriguingly, in measles virus and Nipah virus, PNT encodes STAT1-binding sites that overlap different regions of the C-terminus of C, indicating they have probably originated independently. This arrangement, in which the same genetic region encodes simultaneously a crucial functional motif (a STAT1-binding site) and a highly constrained region (the C-terminus of C), seems paradoxical, since it should severely reduce the ability of the virus to adapt. The fact that it originated twice suggests that it must be balanced by an evolutionary advantage, perhaps from reducing the size of the genetic region vulnerable to mutations.

Defective viral genomes arising in vivo provide critical danger signals for the triggering of lung antiviral immunity

The innate immune response to viruses is initiated when specialized cellular sensors recognize viral danger signals. Here we show that truncated forms of viral genomes that accumulate in infected cells potently trigger the sustained activation of the transcription factors IRF3 and NF-κB and the production type I IFNs through a mechanism independent of IFN signaling. We demonstrate that these defective viral genomes (DVGs) are generated naturally during respiratory infections in vivo even in mice lacking the type I IFN receptor, and their appearance coincides with the production of cytokines during infections with Sendai virus (SeV) or influenza virus. Remarkably, the hallmark antiviral cytokine IFNβ is only expressed in lung epithelial cells containing DVGs, while cells within the lung that contain standard viral genomes alone do not express this cytokine. Together, our data indicate that DVGs generated during viral replication are a primary source of danger signals for the initiation of the host immune response to infection.

Clustered basic amino acids of the small sendai virus C protein Y1 are critical to its RAN GTPase-mediated nuclear localization

The Sendai virus (SeV) C proteins are shown to exert multiple functions during the course of infection. Perhaps reflecting their many functions, they occur at multiple sites of the cell. In this study, we focused on the nuclear-localizing ability of the smaller C protein, Y1, and found that this translocation is mediated by Ran GTPase but not by passive diffusion, and that basic residues within the 149-157 amino acid region are critical for that. The mechanism of inhibition of interferon (IFN)-signaling seemed to differ between the C and Y1 proteins, since deletion of 12 C-terminal amino acids resulted in a loss of the function for the C but not for the Y1 protein. The ability of Y1 mutants to inhibit IFN-α-induced, ISRE-driven expression of a reporter gene almost paralleled with that to localize in the nucleus. These results suggest that nuclear localization of the Y1 protein might be important for the inhibitory effect on type-I IFN-stimulated gene expression.

Antagonism to human BST-2/tetherin by Sendai virus glycoproteins

Tetherin is an interferon-inducible factor that restricts viral particle production. We show here that Sendai virus (SeV) induces a drastic decrease in tetherin levels in infected HeLa cells. Using ectopic expression of tetherin in Madin-Darby canine kidney cells, we find that infectious SeV production is sensitive to restriction by tetherin, suggesting that SeV downregulates tetherin to counter this form of cellular restriction. By using radioactive tetherin in pulse-chase experiments, applying conditions that limit protein degradation, and by estimating tetherin mRNA levels, we find that tetherin degradation is the mechanism of downregulation. Suppression of the virus envelope proteins matrix, fusion (F) or haemagglutinin-neuraminidase protein (HN) during the course of infection demonstrates that F and HN, in concert, are responsible for tetherin degradation. The mechanism(s) by which these two viral glycoproteins participate in degrading tetherin remains to be determined.

Passage of a Sendai virus recombinant in embryonated chicken eggs leads to markedly rapid accumulation of U-to-C transitions in a limited region of the viral genome

The P gene of paramyxoviruses is unique in producing not only P but also “accessory” C and/or V proteins. Successful generation of C- or V-deficient recombinant viruses using a reverse genetics technique has been revealing their importance in viral pathogenesis as well as replication. As for Sendai virus (SeV), the C proteins, a nested set of four polypeptides C’, C, Y1, and Y2, have been shown to exert multiple functions in escaping from the host innate immunity, inhibiting virus-induced apoptosis, promoting virus assembly and budding, and regulating viral RNA synthesis. In this study, we subjected the 4C(-) recombinant lacking expression of all four C proteins to serial passages through eggs, and found the rapid emergence of a C-recovered revertant virus. Unlike the SeV strains or the recombinants reported previously or tested in this study, this was caused by an exceptionally quick accumulation of U-to-C transitions in a limited region of the 4C(-) genome causing recovery of the C protein expression. These results suggest that a lack of C proteins could lead unexpectedly to strong selective pressures, and that the C proteins might play more critical roles in SeV replication than ever reported.

VIGOR extended to annotate genomes for additional 12 different viruses

A gene prediction program, VIGOR (Viral Genome ORF Reader), was developed at J. Craig Venter Institute in 2010 and has been successfully performing gene calling in coronavirus, influenza, rhinovirus and rotavirus for projects at the Genome Sequencing Center for Infectious Diseases. VIGOR uses sequence similarity search against custom protein databases to identify protein coding regions, start and stop codons and other gene features. Ribonucleicacid editing and other features are accurately identified based on sequence similarity and signature residues. VIGOR produces four output files: a gene prediction file, a complementary DNA file, an alignment file, and a gene feature table file. The gene feature table can be used to create GenBank submission. VIGOR takes a single input: viral genomic sequences in FASTA format. VIGOR has been extended to predict genes for 12 viruses: measles virus, mumps virus, rubella virus, respiratory syncytial virus, alphavirus and Venezuelan equine encephalitis virus, norovirus, metapneumovirus, yellow fever virus, Japanese encephalitis virus, parainfluenza virus and Sendai virus. VIGOR accurately detects the complex gene features like ribonucleicacid editing, stop codon leakage and ribosomal shunting. Precisely identifying the mat_peptide cleavage for some viruses is a built-in feature of VIGOR. The gene predictions for these viruses have been evaluated by testing from 27 to 240 genomes from GenBank.

Nipah and hendra virus interactions with the innate immune system

Nipah virus and Hendra virus are related, highly pathogenic paramyxoviruses with unusually broad host ranges. Henipaviruses encode several proteins that block innate immune responses, and these are likely to serve as virulence factors. Specfically, four virus-encoded proteins, the phosphoprotein (P), the V protein, the W protein, and the C protein have each been demonstrated to counteract aspects of the interferon (IFN)-α/β response, a key component of the innate immune response to virus infection. The available data indicate that V and W can inhibit the production of IFNα/β in response to various stimuli, while the P, V, and W proteins also block the ability of IFNs to signal and induce an antiviral state in cells. The C protein also inhibits the antiviral effects of IFNα/β by a poorly characterized mechanism. Reverse genetics systems, which allow the generation of recombinant viruses bearing specific mutations, have demonstrated the importance of the viral IFN-antagonists for replication. With these systems in hand, the field is now poised to define how specific viral IFN-antagonist functions influence viral pathogenesis.

Expression of the Sendai (murine parainfluenza) virus C protein alleviates restriction of measles virus growth in mouse cells

Measles virus (MV), a human pathogen, uses the signaling lymphocyte activation molecule (SLAM) or CD46 as an entry receptor. Although several transgenic mice expressing these receptors have been generated as small animal models for measles, these mice usually have to be made defective in IFN-α/β signaling to facilitate MV replication. Similarly, when functional receptors are expressed by transfection, mouse cells do not allow MV growth as efficiently as primate cells. In this study, we demonstrate that MV efficiently grows in SLAM-expressing mouse cells in which the Sendai virus (SeV) C protein is transiently expressed. We developed a SLAM-expressing mouse cell line whose genome also encodes the SeV C protein downstream of the sequence flanked with loxP sequences. When this cell line was infected with the recombinant MV expressing the Cre recombinase, the SeV C protein was readily expressed. Importantly, the Cre recombinase-encoding MV grew in this cell line much more efficiently than it did in the parental cell. The minigenome assay demonstrated that the SeV C protein does not modulate MV RNA synthesis. Analyses using the mutant proteins with the defined functional defects revealed that the IFN-antagonist function, but not the budding-accelerating function, of the SeV C protein was critical for supporting efficient MV growth in mouse cells. Our results indicate that insufficient IFN antagonism can be an important determinant of the host range of viruses, and the system described here may be useful to overcome the species barrier of other human viruses.

Cytopathogenesis of Sendai virus in well-differentiated primary pediatric bronchial epithelial cells

Sendai virus (SeV) is a murine respiratory virus of considerable interest as a gene therapy or vaccine vector, as it is considered nonpathogenic in humans. However, little is known about its interaction with the human respiratory tract. To address this, we developed a model of respiratory virus infection based on well-differentiated primary pediatric bronchial epithelial cells (WD-PBECs). These physiologically authentic cultures are comprised of polarized pseudostratified multilayered epithelium containing ciliated, goblet, and basal cells and intact tight junctions. To facilitate our studies, we rescued a replication-competent recombinant SeV expressing enhanced green fluorescent protein (rSeV/eGFP). rSeV/eGFP infected WD-PBECs efficiently and progressively and was restricted to ciliated and nonciliated cells, not goblet cells, on the apical surface. Considerable cytopathology was evident in the rSeV/eGFP-infected cultures postinfection. This manifested itself by ciliostasis, cell sloughing, apoptosis, and extensive degeneration of WD-PBEC cultures. Syncytia were also evident, along with significant basolateral secretion of proinflammatory chemokines, including IP-10, RANTES, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), interleukin 6 (IL-6), and IL-8. Such deleterious responses are difficult to reconcile with a lack of pathogenesis in humans and suggest that caution may be required in exploiting replication-competent SeV as a vaccine vector. Alternatively, such robust responses might constitute appropriate normal host responses to viral infection and be a prerequisite for the induction of efficient immune responses.

Conserved charged amino acids within Sendai virus C protein play multiple roles in the evasion of innate immune responses

One of the accessory proteins of Sendai virus (SeV), C, translated from an alternate reading frame of P/V mRNA has been shown to function at multiple stages of infection in cell cultures as well as in mice. C protein has been reported to counteract signal transduction by interferon (IFN), inhibit apoptosis induced by the infection, enhance the efficiency of budding of viral particles, and regulate the polarity of viral genome-length RNA synthesis to maximize production of infectious particles. In this study, we have generated a series of SeV recombinants containing substitutions of highly conserved, charged residues within the C protein, and characterized them together with previously-reported C′/C(−), 4C(−), and F170S recombinant viruses in infected cell cultures in terms of viral replication, cytopathogenicity, and antagonizing effects on host innate immunity. Unexpectedly, the amino acid substitutions had no or minimal effect on viral growth and viral RNA synthesis. However, all the substitutions of charged amino acids resulted in the loss of a counteracting effect against the establishment of an IFN-α-mediated anti-viral state. Infection by the virus (Cm2′) containing mutations at K77 and D80 induced significant IFN-β production, severe cytopathic effects, and detectable amounts of viral dsRNA production. In addition to the Cm2′ virus, the virus containing mutations at E114 and E115 did not inhibit the poly(I:C)-triggered translocation of cellular IRF-3 to the nucleus. These results suggest that the C protein play important roles in viral escape from induction of IFN-β and cell death triggered by infection by means of counteracting the pathway leading to activation of IRF-3 as well as of minimizing viral dsRNA production.

Antagonism of innate immunity by paramyxovirus accessory proteins

Paramyxovirinae, a subfamily of Paramyxoviridae, are negative strand RNA viruses comprised of many important human and animal pathogens, which share a high degree of genetic and structural homology. The accessory proteins expressed from the P/V/C gene are major factors in the pathogenicity of the viruses, because of their ability to abrogate various facets of type I interferon (IFN) induction and signaling. Most of the paramyxoviruses exhibit a commonality in their ability to antagonize innate immunity by blocking IFN induction and the Jak/STAT pathway. However, the manner in which the accessory proteins inhibit the pathway differs among viruses. Similarly, there are variations in the capability of the viruses to counteract intracellular detectors (RNA helicases, mda-5 and RIG-I). Furthermore, a functional specificity in the antagonism of the IFN response has been reported, suggesting that specificity in the circumvention of innate immunity restricts viral host range. Available evidence indicates that paramyxoviruses employ specific strategies to antagonize the IFN response of their specific hosts, which is one of the major factors that determine viral pathogenicity and host range.

Determination of the henipavirus phosphoprotein gene mRNA editing frequencies and detection of the C, V and W proteins of Nipah virus in virus-infected cells

The henipaviruses, Nipah virus (NiV) and Hendra virus (HeV), are highly pathogenic zoonotic paramyxoviruses. Like many other paramyxoviruses, henipaviruses employ a process of co-transcriptional mRNA editing during transcription of the phosphoprotein (P) gene to generate additional mRNAs encoding the V and W proteins. The C protein is translated from the P mRNA, but in an alternate reading frame. Sequence analysis of multiple, cloned mRNAs showed that the mRNA editing frequencies of the P genes of the henipaviruses are higher than those reported for other paramyxoviruses. Antisera to synthetic peptides from the P, V, W and C proteins of NiV were generated to study their expression in infected cells. All proteins were detected in both infected cells and purified virions. In infected cells, the W protein was detected in the nucleus while P, V and C were found in the cytoplasm.

A Kaposi's sarcoma-associated herpesvirus protein that forms inhibitory complexes with type I interferon receptor subunits, Jak and STAT proteins, and blocks interferon-mediated signal transduction

Type I interferons (IFNs) are important mediators of innate antiviral defense and function by activating a signaling pathway through their cognate type I receptor (IFNAR). Here we report that lytic replication of Kaposi's sarcoma-associated herpesvirus (KSHV) efficiently blocks type I IFN signaling and that an important effector of this blockade is the viral protein RIF, the product of open reading frame 10. RIF blocks IFN signaling by formation of inhibitory complexes that contain IFNAR subunits, the Janus kinases Jak1 and Tyk2, and the STAT2 transcription factor. Activation of both Tyk2 and Jak1 is inhibited, and abnormal recruitment of STAT2 to IFNAR1 occurs despite the decrement in Tyk2 activity. As a result of these actions, phosphorylation of both STAT2 and STAT1 is impaired, with subsequent failure of ISGF3 accumulation in the nucleus. The presence in the viral genome of potent inhibitors of type I IFN signaling, along with several viral genes that block IFN induction, highlights the importance of the IFN pathway in the control of this human tumor virus infection.

Reverse genetics of measles virus and resulting multivalent recombinant vaccines: applications of recombinant measles viruses

An overview is given on the development of technologies to allow reverse genetics of RNA viruses, i.e., the rescue of viruses from cDNA, with emphasis on nonsegmented negative-strand RNA viruses ( Mononegavirales ), as exemplified for measles virus (MV). Primarily, these technologies allowed site-directed mutagenesis, enabling important insights into a variety of aspects of the biology of these viruses. Concomitantly, foreign coding sequences were inserted to (a) allow localization of virus replication in vivo through marker gene expression, (b) develop candidate multivalent vaccines against measles and other pathogens, and (c) create candidate oncolytic viruses. The vector use of these viruses was experimentally encouraged by the pronounced genetic stability of the recombinants unexpected for RNA viruses, and by the high load of insertable genetic material, in excess of 6 kb. The known assets, such as the small genome size of the vector in comparison to DNA viruses proposed as vectors, the extensive clinical experience of attenuated MV as vaccine with a proven record of high safety and efficacy, and the low production cost per vaccination dose are thus favorably complemented.

Sendai virus C protein plays a role in restricting PKR activation by limiting the generation of intracellular double-stranded RNA

Sendai virus (SeV) C protein is a multifunctional protein that plays important roles in regulating viral genome replication and transcription, antagonizing the host interferon system, suppressing virus-induced apoptosis, and facilitating virus assembly and budding. We here report a novel role of SeV C protein, the limitation of double-stranded RNA (dsRNA) generation for maintaining the rate of protein synthesis in infected cells. It was found that the intracellular protein synthesis rate was maintained even after wild-type (wt) SeV infection, but markedly suppressed following C-knockout SeV infection. This indicates the requirement of C protein for maintaining protein synthesis after infection. In contrast to wt SeV infection, C-knockout SeV infection caused phosphorylation of both the translation initiation factor eIF2alpha and dsRNA-dependent protein kinase (PKR). Phosphorylation of eIF2alpha occurred mainly due to the action of PKR, since knockdown of PKR by small interfering RNA limited eIF2alpha phosphorylation. C protein, however, could inhibit neither poly(I):poly(C)-activated nor Newcastle disease virus-induced phosphorylation of PKR and eIF2alpha, suggesting that C protein does not target common pathways leading to PKR activation. Immunofluorescent staining experiments with a monoclonal antibody specifically recognizing dsRNA revealed generation of a large amount of dsRNA in cells infected with C-knockout SeV but not wt SeV. The dsRNA generation as well as phosphorylation of PKR and eIF2alpha induced by C-knockout SeV was markedly suppressed in cells constitutively expressing C protein. Taken together, these results demonstrate that the SeV C protein limits generation of dsRNA, thereby keeping PKR inactive to maintain intracellular protein synthesis.

Human parainfluenza virus type 1 C proteins are nonessential proteins that inhibit the host interferon and apoptotic responses and are required for efficient replication in nonhuman primates

Recombinant human parainfluenza virus type 1 (rHPIV1) was modified to create rHPIV1-P(C-), a virus in which expression of the C proteins (C', C, Y1, and Y2) was silenced without affecting the amino acid sequence of the P protein. Infectious rHPIV1-P(C-) was readily recovered from cDNA, indicating that the four C proteins were not essential for virus replication. Early during infection in vitro, rHPIV1-P(C-) replicated as efficiently as wild-type (wt) HPIV1, but its titer subsequently decreased coincident with the onset of an extensive cytopathic effect not observed with wt rHPIV1. rHPIV1-P(C-) infection, but not wt rHPIV1 infection, induced caspase 3 activation and nuclear fragmentation in LLC-MK2 cells, identifying the HPIV1 C proteins as inhibitors of apoptosis. In contrast to wt rHPIV1, rHPIV1-P(C-) and rHPIV1-C(F170S), a mutant encoding an F170S substitution in C, induced interferon (IFN) and did not inhibit IFN signaling in vitro. However, only rHPIV1-P(C-) induced apoptosis. Thus, the anti-IFN and antiapoptosis activities of HPIV1 were separable: both activities are disabled in rHPIV1-P(C-), whereas only the anti-IFN activity is disabled in rHPIV1-C(F170S). In African green monkeys (AGMs), rHPIV1-P(C-) was considerably more attenuated than rHPIV1-C(F170S), suggesting that disabling the anti-IFN and antiapoptotic activities of HPIV1 had additive effects on attenuation in vivo. Although rHPIV1-P(C-) protected against challenge with wt HPIV1, its highly restricted replication in AGMs and in primary human airway epithelial cell cultures suggests that it might be overattenuated for use as a vaccine. Thus, the C proteins of HPIV1 are nonessential but have anti-IFN and antiapoptosis activities required for virulence in primates.

Virus growth and antibody responses following respiratory tract infection of ferrets and mice with WT and P/V mutants of the paramyxovirus Simian Virus 5

P/V gene substitutions convert the non-cytopathic paramyxovirus Simian Virus 5 (SV5), which is a poor inducer of host cell responses in human tissue culture cells, into a mutant (P/V-CPI−) that induces high levels of apoptosis, interferon (IFN)-beta, and proinflammatory cytokines. However, the effect of SV5-P/V gene mutations on virus growth and adaptive immune responses in animals has not been determined. Here, we used two distinct animal model systems to test the hypothesis that SV5-P/V mutants which are more potent activators of innate responses in tissue culture will also elicit higher antiviral antibody responses. In mouse cells, in vitro studies identified a panel of SV5-P/V mutants that ranged in their ability to limit IFN responses. Intranasal infection of mice with these WT and P/V mutant viruses elicited equivalent anti-SV5 IgG responses at all doses tested, and viral titers recovered from the respiratory tract were indistinguishable. In primary cultures of ferret lung fibroblasts, WT rSV5 and P/V-CPI− viruses had phenotypes similar to those established in human cell lines, including differential induction of IFN secretion, IFN signaling and apoptosis. Intranasal infection of ferrets with a low dose of WT rSV5 elicited ~ 500 fold higher anti-SV5 serum IgG responses compared to the P/V-CPI− mutant, and this correlated with overall higher viral titers for the WT virus in tracheal tissues. There was a dose-dependent increase in antibody response to infection of ferrets with P/V-CPI−, but not with WT rSV5. Together our data indicate that WT rSV5 and P/V mutants can elicit distinct innate and adaptive immunity phenotypes in the ferret animal model system, but not in the mouse system. We present a model for the effect of P/V gene substitutions on SV5 growth and immune responses in vivo.

Interferons and viruses: an interplay between induction, signalling, antiviral responses and virus countermeasures

The interferon (IFN) system is an extremely powerful antiviral response that is capable of controlling most, if not all, virus infections in the absence of adaptive immunity. However, viruses can still replicate and cause disease in vivo, because they have some strategy for at least partially circumventing the IFN response. We reviewed this topic in 2000 [Goodbourn, S., Didcock, L. & Randall, R. E. (2000). J Gen Virol 81, 2341-2364] but, since then, a great deal has been discovered about the molecular mechanisms of the IFN response and how different viruses circumvent it. This information is of fundamental interest, but may also have practical application in the design and manufacture of attenuated virus vaccines and the development of novel antiviral drugs. In the first part of this review, we describe how viruses activate the IFN system, how IFNs induce transcription of their target genes and the mechanism of action of IFN-induced proteins with antiviral action. In the second part, we describe how viruses circumvent the IFN response. Here, we reflect upon possible consequences for both the virus and host of the different strategies that viruses have evolved and discuss whether certain viruses have exploited the IFN response to modulate their life cycle (e.g. to establish and maintain persistent/latent infections), whether perturbation of the IFN response by persistent infections can lead to chronic disease, and the importance of the IFN system as a species barrier to virus infections. Lastly, we briefly describe applied aspects that arise from an increase in our knowledge in this area, including vaccine design and manufacture, the development of novel antiviral drugs and the use of IFN-sensitive oncolytic viruses in the treatment of cancer.

Recruitment of Alix/AIP1 to the plasma membrane by Sendai virus C protein facilitates budding of virus-like particles

Sendai virus (SeV) is unique in that one of the viral accessory proteins, C, enhances budding of virus-like particles (VLPs) formed by SeV matrix protein M by physically interacting with Alix/AIP1. C protein itself does not have the ability to form VLPs, while M protein provides viral budding force, like other enveloped viruses. Here we show that SeV C protein recruits Alix/AIP1 to the plasma membrane (PM) to facilitate VLP budding. SeV M-VLP budding is sensitive to overexpression of a dominant-negative (DN) form of VPS4A only in the presence of the C proteins, which is able to recruit Alix/AIP1 to the PM. Our results indicate that SeV M and C proteins play separate roles in the budding process: M protein drives budding and C protein enhances the efficiency of the utilization of cellular MVB sorting machinery for efficient VLP budding.

Sendai virus budding in the course of an infection does not require Alix and VPS4A host factors

Closing the Sendai virus C protein open reading frames (rSeV-DeltaC virus) results in the production of virus particles with highly reduced infectivity. Besides, the Sendai virus C proteins interact with Alix/AIP1 and Alix suppression negatively affects Sendai virus like particle (VLP) budding. Similarly, the Sendai virus M protein has been shown to interact with Alix. On this basis, it has been suggested that Sendai virus budding involves recruitment of the multivesicular body formation machinery. We follow, here, the production of SeV particles upon regular virus infection. We find that neither Alix suppression nor dominant negative-VPS4A expression, applied separately or in combination, affects physical or infectious virion production. This contrasts with the observed decrease of SV5 virion production upon dominant negative-VPS4A expression. Finally, we show that suppression of more than 70% of a GFP/C protein in the background of a rSeV-DeltaC virus infection has no effect either on SeV particle production or on virus particle infectivity. Our results contrast with what has been published before. Possible explanations for this discrepancy are discussed.

Importance of the anti-interferon capacity of Sendai virus C protein for pathogenicity in mice

The Sendai virus (SeV) C protein blocks signal transduction of interferon (IFN), thereby counteracting the antiviral actions of IFN. Using HeLa cell lines expressing truncated or mutated SeV C proteins, we found that the C-terminal half has anti-IFN capacity, and that K(151)A, E(153)A, and R(154)A substitutions in the C protein eliminated this capacity. Here, we further created the mutant virus SeV Cm*, in which K(151)A, E(153)K, and R(157)L substitutions in the C protein were introduced without changing the amino acid sequence of overlapped P, V, and W proteins. SeV Cm* was found to lack anti-IFN capacity, as expected. While the growth rate and final yield of SeV Cm* were inferior to those of the wild-type SeV in IFN-responsive, STAT1-positive 2fTGH cells, SeV Cm* grew equivalently to the wild-type SeV in IFN-nonresponsive, STAT1-deficient U3A cells. SeV Cm* was thus shown to maintain multiplication capacity, except that it lacked anti-IFN capacity. Intranasally inoculated SeV Cm* could propagate in the lungs of STAT1(-/-) mice but was cleared from those of STAT1(+/+) mice without propagation. It was found that the anti-IFN capacity of the SeV C protein was indispensable for pathogenicity in mice. Conversely, the results show that the innate immunity contributed to elimination of SeV in early stages of infection in the absence of anti-IFN capacity.

Overproduction of double-stranded RNA in vesicular stomatitis virus-infected cells activates a constitutive cell-type-specific antiviral response

In a companion paper (D. Ostertag, T. M. Hoblitzell-Ostertag, and J. Perrault, J. Virol. 81:492-502, 2007), we provided indirect evidence that cell-type-specific growth restriction of the vesicular stomatitis virus (VSV) polR mutants may be due to enhanced production of double-stranded RNA (dsRNA). We show here that polR growth in mouse L-929 cells was rescued by vaccinia virus coinfection and that sole expression of the vaccinia virus dsRNA-binding E3L protein, via coinfection with an engineered VSV minigenome, also restored polR growth. Expression of dsRNA-binding protein NS1A or NS1B from influenza virus, but not C protein from Sendai virus, which does not bind dsRNA, likewise effected polR rescue. The N-terminal dsRNA-binding domain of NS1A, only 73 amino acids in length, but not a full-size mutant NS1A lacking dsRNA-binding activity, restored polR growth. Both key aspects of polR growth restriction, namely inhibition of genome replication and release of low-infectivity virus particles, were countered by expression of the dsRNA-binding proteins. We tested the effects of overproducing dsRNA in wild-type VSV infections by coinfecting cells with a VSV recombinant expressing the sense strand of the enhanced green fluorescent protein gene (VSV-GFP) and one expressing the antisense strand (VSV-PFG). These coinfections mimicked all aspects of polR restriction, including host range, lack of effect on transcription, reduced virus particle infectivity, and insensitivity to inhibition of host gene transcription or dsRNA-activated protein kinase activity. We conclude that, for some cell types, overproduction of dsRNA during VSV infection triggers an immediate and constitutive host cell antiviral effector response independent of interferon induction or signaling.

Attenuating mutations in the P/C gene of human parainfluenza virus type 1 (HPIV1) vaccine candidates abrogate the inhibition of both induction and signaling of type I interferon (IFN) by wild-type HPIV1

Recombinant human parainfluenza virus type 1 (HPIV1) and mutants containing point and deletion (Delta) mutations in the P/C gene (r-CDelta10-15HNT553A, r-CR84G, r-CF170S and r-CDelta170), which have previously been evaluated as HPIV1 vaccine candidates, were evaluated for their effect on the type I interferon (IFN) response in vitro. HPIV1 wt infection inhibited the IFN response by inhibiting IFN regulatory factor-3 (IRF-3) activation and IFN production in A549 cells and IFN signaling in Vero cells. In contrast, r-CR84G, r-CF170S and r-CDelta170 were defective for inhibition of IRF-3 activation and IFN production and r-CF170S and r-CDelta170 did not inhibit IFN signaling. Thus, HPIV1 antagonizes the IFN response at both the level of induction and signaling, and antagonism at both levels was disrupted by mutations in the P/C gene. Because CF170S affects C and not P, the anti-IFN function can be attributed to the C proteins. These data, in the context of previous in vivo studies, suggest that the loss of antagonism of the IFN response at both the level of induction and signaling, observed with the P/C mutants, r-CF170S and r-CDelta170, was necessary for significant attenuation in African green monkeys (AGMs).

Sendai virus, the mouse parainfluenza type 1: a longstanding pathogen that remains up-to-date

Biologically speaking, Sendai virus (SeV), the murine parainfluenza virus type 1, is perceived as a common respiratory pathogen that is endemic in many rodent colonies throughout the world. Currently it is believed that SeV is the leading cause of pneumonia in mice and together with the mouse hepatitis viruses, is the most prevalent and important of the naturally occurring infections of mice. The scientific community also considers SeV as the archetype organism of the Paramyxoviridae family because most of the basic biochemical, molecular and biologic properties of the whole family were derived from its own characteristics. Recently, scientific interest for this old pathogen has re-emerged, this time because of its potential value as a vector for gene transfer. This review aimed at drawing an exhaustive picture of this multifaceted pathogen.

Human parainfluenza virus type 1 but not Sendai virus replicates in human respiratory cells despite IFN treatment

Sendai virus (SeV) and human parainfluenza virus type I (hPIV1) are highly homologous but have distinct host ranges, murine versus human. To identify the factors that affect the host specificity of parainfluenza viruses, we determined the infectivity and anti-IFN activities of SeV and hPIV1 in human and murine culture cells. SeV infected normal human lung MRC-5 and murine lung MM14.Lu or MLg2908 cells efficiently. Infection with SeV induced the release of IFN-beta into culture medium in MRC-5 cells at similar levels with that of cells infected with hPIV1. SeV or hPIV1 infections, as well as expression of SeV or hPIV1 C proteins, inhibited the nuclear localization of STAT1 induced by IFN-beta, suggesting that both SeV and hPIV1 C proteins block the IFN Jak/STAT pathway in MRC-5 cells. Pretreatment of MRC-5 cells with IFN suppressed replication of SeV and hPIV1 at an early stage of infection. However, hPIV1 overcame this suppression while SeV did not. SeV replication was restored in IFN-beta pretreated murine MM14.Lu cells, suggesting SeV anti-IFN activity is species specific. These results suggest that SeV is less effective than hPIV1 in overcoming antiviral activity in human cells, which could be one of the factors that restrict the host range of SeV.

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.

[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.

Biological and clinical basis for molecular studies of interferons

The cytokine family of interferons (IFNs) has multiple functions, including antiviral, anti-tumor, and immunomodulatory effects and regulation of cell differentiation. The multiple functions of the IFN system are thought to be an innate defense against microbes and foreign substances. The IFN system consists first of cells that produce IFNs in response to viral infection or other foreign stimuli and second of cells that establish the antiviral state in response to IFNs. This process of innate immunity involves multiple signaling mechanisms and activation of various host genes. Viruses have evolved to develop mechanisms that circumvent this system. IFNs have also been used clinically in the treatment of viral diseases. Improved treatments will be possible with better understanding of the IFN system and its interactions with viral factors. In addition, IFNs have direct and indirect effects on tumor cell proliferation, effector leukocytes and on apoptosis and have been used in the treatment of some cancers. Improved knowledge of how IFNs affect tumors and the mechanism that lead to a lack of response to IFNs would help the development of better IFN treatments for malignancies.

TLR-Independent Induction of Dendritic Cell Maturation and Adaptive Immunity by Negative-Strand RNA Viruses

TLR signaling leads to dendritic cell (DC) maturation and immunity to diverse pathogens. The stimulation of TLRs by conserved viral structures is the only described mechanism leading to DC maturation after a virus infection. In this report, we demonstrate that mouse myeloid DCs mature normally after in vivo and in vitro infection with Sendai virus (SeV) in the absence of TLR3, 7, 8, or 9 signaling. DC maturation by SeV requires virus replication not necessary for TLR-mediated triggering. Moreover, DCs deficient in TLR signaling efficiently prime for Th1 immunity after infection with influenza or SeV, generating IFN-gamma-producing T cells, CTLs and antiviral Abs. We have previously demonstrated that SeV induces DC maturation independently of the presence of type I IFN, which has been reported to mature DCs in a TLR-independent manner. The data presented here provide evidence for the existence of a novel intracellular pathway independent of TLR-mediated signaling responsible for live virus triggering of DC maturation and demonstrate its critical role in the onset of antiviral immunity. The revelation of this pathway should stimulate invigorating research into the mechanism for virus-induced DC maturation and immunity.

Accessory genes of the paramyxoviridae, a large family of nonsegmented negative-strand RNA viruses, as a focus of active investigation by reverse genetics

The Paramyxoviridae, a large family of nonsegmented negative-strand RNA viruses, comprises several genera each containing important human and animal pathogens. They possess in common six basal genes essential for viral replication and, in addition, a subset of accessory genes that are largely unique to each genus. These accessory genes are either encoded in one or more alternative overlapping frames of a basal gene, which are accessed transcriptionally or translationally, or inserted before or between the basal genes as one or more extra genes. However, the question of how the individual accessory genes contribute to actual viral replication and pathogenesis remained unanswered. It was not even established whether they are dispensable or indispensable for the viral life cycle. The plasmid-based reverse genetics of the full-length viral genome has now come into wide use to demonstrate that most, if not all, of these putative accessory genes can be disrupted without destroying viral infectivity, conclusively defining them as indeed dispensable accessory genes. Studies on the phenotypes of the resulting gene knockout viruses have revealed that the individual accessory genes greatly contribute specifically and additively to the overall viral fitness both in vitro and in vivo.

A short peptide at the amino terminus of the Sendai virus C protein acts as an independent element that induces STAT1 instability

The Sendai virus C protein acts to dismantle the interferon-induced cellular antiviral state in an MG132-sensitive manner, in part by inducing STAT1 instability. This activity of C maps to the first 23 amino acids (C(1-23)) of the 204-amino-acid (aa)-long protein (C(1-204)). C(1-23) was found to act as an independent viral element that induces STAT1 instability, since this peptide fused to green fluorescent protein (C(1-23)/GFP) is at least as active as C(1-204) in this respect. This peptide also induces the degradation of C(1-23)/GFP and other proteins to which it is fused. Most of C(1-204), and particularly its amino-terminal half, is predicted to be structurally disordered. C(1-23) as a peptide was found to be disordered by circular dichroism, and the first 11 aa have a strong potential to form an amphipathic alpha-helix in low concentrations of trifluoroethanol, which is thought to mimic protein-protein interaction. The critical degradation-determining sequence of C(1-23) was mapped by mutation to eight residues near its N terminus: (4)FLKKILKL(11). All the large hydrophobic residues of (4)FLKKILKL(11), plus its ability to form an amphipathic alpha-helix, were found to be critical for STAT1 degradation. In contrast, C(1-23)/GFP self-degradation did not require (8)ILKL(11), nor the ability to form an alpha-helix throughout this region. Remarkably, C(1-23)/GFP also stimulated C(1-204) degradation, and this degradation in trans required the same peptide determinants as for STAT1. Our results suggest that C(1-204) coordinates its dual activities of regulating viral RNA synthesis and counteracting the host innate antiviral response by sensing both its own intracellular concentration and that of STAT1.

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.

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.

Rinderpest virus phosphoprotein gene is a major determinant of species-specific pathogenicity

We previously demonstrated that the rinderpest virus (RPV) hemagglutinin (H) protein plays an important role in determining host range but that other viral proteins are clearly required for full RPV pathogenicity to be manifest in different species. To examine the effects of the RPV nucleocapsid (N) protein and phosphoprotein (P) genes on RPV cross-species pathogenicity, we constructed two new recombinant viruses in which the H and P or the H, N, and P genes of the cattle-derived RPV RBOK vaccine were replaced with those from the rabbit-adapted RPV-Lv strain, which is highly pathogenic in rabbits. The viruses rescued were designated recombinant RPV-lapPH (rRPV-lapPH) and rRPV-lapNPH, respectively. Rabbits inoculated with RPV-Lv become feverish and show leukopenia and a decrease in body weight gain, while clinical signs of infection are never observed in rabbits inoculated with RPV-RBOK or with rRPV-lapH. However, rabbits inoculated with either rRPV-lapPH or rRPV-lapNPH became pyrexic and showed leukopenia. Further, histopathological lesions and high virus titers were clearly observed in the lymphoid tissues from animals infected with rRPV-lapPH or rRPV-lapNPH, although they were not observed in rabbits infected with RPV-RBOK or rRPV-lapH. The clinical, virological, and histopathological signs in rabbits infected with the two new recombinant viruses did not differ significantly; therefore, the RPV P gene was considered to be a key determinant of cross-species pathogenicity.

Inhibition of the gamma interferon response by a Sendai virus C protein mutant with no STAT1-binding ability

Sendai virus C protein interacts with the signal transducer and activator of transcription (STAT) 1. This interaction is believed to be essential for the Sendai virus inhibition of the interferon (IFN) response. We here analyzed C(F170S) (a C protein mutant with the F170S mutation) with no STAT1-binding ability. C(F170S) lacked the ability to inhibit the IFN-alpha response, but retained the ability to inhibit the IFN-gamma response. IFN-gamma stimulation caused STAT1 phosphorylation, formation of the gamma-activated factor capable of binding to a gamma-activated sequence DNA probe, and STAT1 nuclear translocation, even in the presence of C(F170S). These results suggest that C protein has the STAT1-binding-independent anti-IFN-gamma mechanism, which targets processes after the STAT1 nuclear translocation event.

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.

The human parainfluenza virus type 3 (HPIV 3) C protein inhibits viral transcription

The C protein of human parainfluenza virus type 3 (HPIV 3), like other paramyxovirus C proteins, is synthesized from an alternate open reading frame (ORF) encoded within the phosphoprotein (P) mRNA, in addition, to two other proteins, namely D and V, which arise from the same mRNA by a process of transcriptional editing. The precise role of the C, D, and V proteins in viral transcription and replication, and their interaction, if any, with other viral proteins remains unknown. To ascertain the role of the C protein, we have examined its effect on transcription using an HPIV 3 minigenome construct and monitoring the luciferase reporter gene expression. Our results demonstrate that the HPIV 3 C protein effectively inhibits minigenome transcription in a dose-dependent manner. Interestingly, the Sendai virus (Se-V) C protein was also capable of inducing an inhibitory effect on the HPIV 3 minigenome transcription, thus demonstrating a heterologous interaction. A coiled-coil motif within the C protein has been identified, and a deletion mutant within this motif abrogated the inhibitory effect significantly thereby implying that oligomerization of the C protein may be involved in inhibition of transcription.

Proteolytic processing and translation initiation: two independent mechanisms for the expression of the Sendai virus Y proteins

The four Sendai virus C-proteins (C', C, Y1, and Y2) represent an N-terminal nested set of non-structural proteins whose expression modulates both the readout of the viral genome and the host cell response. In particular, they modulate the innate immune response by perturbing the signaling of type 1 interferons. The initiation codons for the four C-proteins have been mapped in vitro, and it has been proposed that the Y proteins are initiated by ribosomal shunting. A number of mutations were reported that significantly enhanced Y expression, and this was attributed to increased shunt-mediated initiation. However, we demonstrate that this arises due to enhanced proteolytic processing of C', an event that requires its very N terminus. Curiously, although Y expression in vitro is mediated almost exclusively by initiation, Y proteins in vivo can arise both by translation initiation and processing of the C' protein. To our knowledge this is the first example of two apparently independent pathways leading to the expression of the same polypeptide chain. This dual pathway explains several features of Y expression.

Identification and characterization of viral antagonists of type I interferon in negative-strand RNA viruses

Interferons are cytokines secreted in response to viral infections with potent antiviral activity, and they represent a critical component of the innate immune response against viruses. It has now become apparent that many viruses have evolved different mechanisms to counteract the interferon response, allowing their efficient replication and propagation in their hosts. This review discusses how the development of reverse genetics techniques and the increase in our knowledge of the interferon response have led to the discovery of interferon-antagonistic functions of different genes of viruses belonging to the negative-strand RNA virus group. In many cases, these viral genes encode accessory pro- teins that are not required for viral infectivity but are critical for optimal replication and for virulence in the host.

The C protein of measles virus inhibits the type I interferon response

Type I interferons (IFNalpha/beta) are an important part of innate immunity to viral infections because they induce an antiviral response and limit viral replication until the adaptive response clears the infection. Since the nonstructural proteins of several paramyxoviruses inhibit the IFNalpha/beta response, we chose to explore the role of the C protein of measles virus (MV) in such inhibition. Previous studies have suggested that the MV C protein may serve as a virulence factor, but its role in the pathogenesis of MV remains undefined. In the present study, a recombinant MV strain that does not express the C protein (MV C-) and its parental strain (Ed Tag) were used. Growth of MV C- was restricted in human peripheral blood mononuclear cells and HeLa cells, but in the presence of neutralizing antibodies to IFNalpha/beta, MV C- produced titers that were equivalent to those of Ed Tag. In addition, expression of the MV C protein from plasmid DNA inhibited the production of an IFNalpha/beta responsive reporter gene and, to a lesser extent, inhibited an IFNgamma responsive reporter gene. The ability of the MV C protein to suppress the IFNalpha/beta response was confirmed using a biologic assay. After IFNbeta stimulation, HeLa cells infected with Ed Tag produced five-fold less IFNalpha/beta than cells infected with MV C-. While the mechanism of inhibition remains unclear, these data suggest that the MV C protein plays an important role in the pathogenesis of MV by inhibiting IFNalpha/beta signaling.

The C-terminal half-fragment of the Sendai virus C protein prevents the gamma-activated factor from binding to a gamma-activated sequence site

Sendai virus C protein associates with the signal transducer and activator of transcription (STAT) 1 and inhibits the interferon (IFN) response. We report a molecular basis for the anti-IFN-gamma mechanism of Sendai virus. The C-terminal half-fragment of the C protein (D1) retains both the STAT1-binding and the anti-IFN-gamma abilities comparable to those of the full-size C. IFN-gamma stimulation generates phosphorylated-STAT1 even in the presence of the C or the D1. The phosphorylated-STAT1 generated in the D1-expressing cells forms an aberrant complex, which does not bind to a gamma-activated sequence (GAS) probe. Purified D1, indeed, inhibits in vitro the binding of the phosphorylated-STAT1 dimer to the GAS probe. The D1, however, binds to the STAT1 N-terminal domain, but not the DNA binding domain. These results suggest the possibility that the C protein prevents the gamma-activated factor from binding to GAS elements through its interaction with the STAT1 N-terminal domain.

Sequence analysis of the Washington/1964 strain of human parainfluenza virus type 1 (HPIV1) and recovery and characterization of wild-type recombinant HPIV1 produced by reverse genetics

A complete consensus sequence was determined for the genomic RNA of human parainfluenza virus type 1 (HPIV1) strain Washington/20993/1964 (HPIV1 WASH/64), a clinical isolate that previously was shown to be virulent in adults. The sequence exhibited a high degree of relatedness to both Sendai virus, a PIV1 virus recovered from mice, and human PIV3 (HPIV3) with regard to cis-acting regulatory regions and protein-coding sequences. This consensus sequence was used to generate a full-length antigenomic cDNA and to recover a recombinant wild-type HPIV1 (rHPIV1). Interestingly, the rHPIV1 could be rescued from full-length antigenomic rHPIV1 cDNA using HPIV3 support plasmids, HPIV1 support plasmids, or a mixture thereof. The replication of rHPIV1 in vitro and in the respiratory tract of hamsters was similar to that of its biologically derived parent virus. The similar biological properties of rHPIV1 and HPIV1 WASH/64 in vitro and in vivo, together with the previous demonstration of the virulence of this specific isolate in humans, authenticates the rHPIV1 sequence as that of a wild-type virus. This rHPIV1 can now be used to study the biological properties of HPIV1 and as a substrate to introduce attenuating mutations for the generation of live-attenuated HPIV1 vaccine candidates.

Sendai virus targets inflammatory responses, as well as the interferon-induced antiviral state, in a multifaceted manner

We have used cDNA arrays to compare the activation of various cellular genes in response to infection with Sendai viruses (SeV) that contain specific mutations. Three groups of cellular genes activated by mutant SeV infection, but not by wild-type SeV, were identified in this way. While some of these genes are well known interferon (IFN)-stimulated genes, others, such as those for interleukin-6 (IL-6) and IL-8, are not directly induced by IFN. The gene for beta IFN (IFN-beta), which is critical for initiating an antiviral response, was also specifically activated in mutant SeV infections. The SeV-induced activation of IFN-beta was found to depend on IFN regulatory factor 3, and the activation of all three cellular genes was independent of IFN signaling. Mutations that disrupt four distinct elements in the SeV genome (the leader RNA, two regions of the C protein, and the V protein) all lead to enhanced levels of IFN-beta mRNA, and at least three of these viral genes also appear to be involved in preventing activation of IL-8. Our results suggest that SeV targets the inflammatory and adaptive immune responses as well as the IFN-induced intracellular antiviral state by using a multifaceted approach.