Expression of measles virus V protein is associated with pathogenicity and control of viral RNA synthesis

Dynamics of viral RNA synthesis during measles virus infection

We propose a reference model of the kinetics of a viral RNA-dependent RNA polymerase (vRdRp) activities and its regulation during infection of eucaryotic cells. After measles virus infects a cell, mRNAs from all genes immediately start to accumulate linearly over the first 5 to 6 h and then exponentially until approximately 24 h. The change from a linear to an exponential accumulation correlates with de novo synthesis of vRdRp from the incoming template. Expression of the virus nucleoprotein (N) prior to infection shifts the balance in favor of replication. Conversely, inhibition of protein synthesis by cycloheximide favors the latter. The in vivo elongation speed of the viral polymerase is approximately 3 nucleotides/s. A similar profile with fivefold-slower kinetics can be obtained using a recombinant virus expressing a structurally altered polymerase. Finally, virions contain only encapsidated genomic, antigenomic, and 5'-end abortive replication fragment RNAs.

The Plowright vaccine strain of Rinderpest virus has attenuating mutations in most genes

The currently used vaccine strain of Rinderpest virus was derived by serial passage of the highly virulent Kabete ‘O’ strain (KO). A full-length cDNA copy of the KO strain was made from which a virus identical in pathogenicity to the wild-type virus was rescued. A series of chimeric viruses was prepared in which the coding sequences for the N, P, F, H or L proteins were replaced with the corresponding sequences from the vaccine strain. The KO-based virus with the vaccine strain H gene and that with the carboxy-terminal half of the L gene replaced with the corresponding sequence from the vaccine strain retained all or almost all of the virulence of the original KO virus. Animals infected with the KO-based virus containing the vaccine strain N, P or F gene, or the amino-terminal half of the L gene, developed high and prolonged pyrexia and leukopenia, but with reduced or absent lesions and other clinical signs; although partially attenuated, none was nearly as attenuated as the vaccine strain itself. These data indicate that the high attenuation and stability of the current vaccine are due to the accumulation of a number of separate mutations, none of which is itself so sufficiently debilitating that there is strong selective pressure in favour of the revertant.

Morbilliviruses and human disease

Morbilliviruses are a group of viruses that belong to the family Paramyxoviridae. The most instantly recognizable member is measles virus (MV) and individuals acutely infected with the virus exhibit a wide range of clinical symptoms ranging from a characteristic mild self-limiting infection to death. Canine distemper virus (CDV) and rinderpest virus (RPV) cause a similar but distinctive pathology in dogs and cattle, respectively, and these, alongside experimental MV infection of primates, have been useful models for MV pathogenesis. Traditionally, viruses were identified because a distinctive disease was observed in man or animals; an infectious agent was subsequently isolated, cultured, and this could be used to recapitulate the disease in an experimentally infected host. Thus, satisfying Koch's postulates has been the norm. More recently, particularly due to the advent of exceedingly sensitive molecular biological assays, many researchers have looked for infectious agents in disease conditions for which a viral aetiology has not been previously established. For these cases, the modified Koch's postulates of Bradford Hill have been developed as criteria to link a virus to a specific disease. Only in a few cases have these conditions been fulfilled. Therefore, many viruses have over the years been definitely and tentatively linked to human diseases and in this respect the morbilliviruses are no different. In this review, human diseases associated with morbillivirus infection have been grouped into three broad categories: (1) those which are definitely caused by the infection; (2) those which may be exacerbated or facilitated by an infection; and (3) those which currently have limited, weak, unsubstantiated or no credible scientific evidence to support any link to a morbillivirus. Thus, an attempt has been made to clarify the published data and separate human diseases actually linked to morbilliviruses from those that are merely anecdotally associated.

Measles virus phosphoprotein gene products: conformational flexibility of the P/V protein amino-terminal domain and C protein infectivity factor function

The measles virus (MV) P gene codes for three proteins: P, an essential polymerase cofactor, and V and C, which have multiple functions but are not strictly required for viral propagation in cultured cells. V shares the amino-terminal domain with P but has a zinc-binding carboxyl-terminal domain, whereas C is translated from an overlapping reading frame. During replication, the P protein binds incoming monomeric nucleocapsid (N) proteins with its amino-terminal domain and positions them for assembly into the nascent ribonucleocapsid. The P protein amino-terminal domain is natively unfolded; to probe its conformational flexibility, we fused it to the green fluorescent protein (GFP), thereby also silencing C protein expression. A recombinant virus (MV-GFP/P) expressing hybrid GFP/P and GFP/V proteins in place of standard P and V proteins and not expressing the C protein was rescued and produced normal ratios of mono-, bi-, and tricistronic RNAs, but its replication was slower than that of the parental virus. Thus, the P protein retained nearly intact polymerase cofactor function, even with a large domain added to its amino terminus. Having noted that titers of cell-associated and especially released MV-GFP/P were reduced and knowing that the C protein of the related Sendai virus has particle assembly and infectivity factor functions, we produced an MV-GFP/P derivative expressing C. Intracellular titers of this virus were almost completely restored, and those of released virus were partially restored. Thus, the MV C protein is an infectivity factor.

Overlap of interaction domains indicates a central role of the P protein in assembly and regulation of the Borna disease virus polymerase complex

The active polymerase complex of Borna disease virus is composed of the viral proteins N, P, and L. The viral X (negative regulatory factor) protein acts as a regulator of polymerase activity. Interactions of P with N and X were previously studied, but interactions with L were poorly defined. Using a mammalian two-hybrid system, we observed that L specifically interacts with P but not with N, X, or itself. Mapping of the L-binding domain in the P molecule revealed that it overlaps with two adjacent domains required for multimerization and interaction with N. Competition experiments showed that the interaction between L and P was inefficient when N was present, indicating that L may preferentially interact with free P in infected cells. Interestingly, a multimerization-defective P mutant maintained the ability to interact with L, N, and X but failed to support reporter gene expression from an artificial Borna disease virus minigenome. Furthermore, dominant negative effects on minigenome activity were only observed when P mutants with an intact multimerization domain were used, suggesting that P multimers, rather than monomers, exhibit biological activity. P mutants lacking functional interaction domains for L or N still formed complexes with these viral proteins when wild-type P was available as a bridging molecule, indicating that P multimers have the potential to act as scaffolds on which the RNA polymerase complex is assembled.

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.

Cell tropism of wild-type measles virus is affected by amino acid substitutions in the P, V and M proteins, or by a truncation in the C protein

Two nucleotide differences in the P/C/V and M genes between B95a cell- and Vero cell-isolated wild-type measles viruses (MV) have previously been found from the same patient. The nucleotide difference in the P/C/V gene resulted in an amino acid difference (M175I) in the P and V proteins and a 19 aa deletion in the C protein. The nucleotide difference in the M gene resulted in an amino acid difference (P64H) in the M protein. To verify this result and to examine further whether the amino acid difference or truncation is important for MV cell tropism, recombinant MV strains containing one of the two nucleotide substitutions, or both, were generated. It was found that the P64H substitution in the M protein was important for efficient virus growth and dissemination in Vero cells and that the M175I substitution in the P and V protein or truncation of the C protein was required for optimal growth.

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.

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 conforms to the rule of six in a minigenome replication assay

To study the replication of Nipah virus (NiV), a minigenome replication assay that does not require the use of infectious virus was developed. The minigenome was constructed to encode a NiV vRNA analogue containing the gene for chloramphenicol acetyltransferase (CAT) under the control of putative NiV transcription motifs and flanked by the NiV genomic termini. CAT protein was detected only when plasmids encoding the NiV minigenome, nucleocapsid protein (N), phosphoprotein (P) and polymerase protein (L) were transfected into CV1 cells. To determine whether NiV conforms to the rule of six, a series of plasmids encoding minigenomes that differed in length by a single nucleotide was tested in the replication assay. CAT production was detected only with the minigenome whose length was an even multiple of six. The replication assay was also used to show that the N, P and L proteins of NiV recognize cis-acting sequences in the genomic termini of Hendra virus (HeV) but not measles virus. While these results suggest that NiV uses a replication strategy that is similar to those of other paramyxoviruses, they also support the inclusion of NiV and HeV in a separate genus within the subfamily Paramyxovirinae.

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.

Structural disorder and modular organization in Paramyxovirinae N and P

The existence and extent of disorder within the replicative complex (N, P and the polymerase, L) of Paramyxovirinae were investigated, drawing on the discovery that the N-terminal moiety of the phosphoprotein (P) and the C-terminal moiety of the nucleoprotein (N) of measles virus are intrinsically unstructured. We show that intrinsic disorder is a widespread property within Paramyxovirinae N and P, using a combination of different computational approaches relying on different physico-chemical concepts. Notably, experimental support that has often gone unnoticed for most of the predictions has been found in the literature. Identification of disordered regions allows the unveiling of a common organization in all Paramyxovirinae P, which are composed of six modules defined on the basis of structure or sequence conservation. The possible functional significance of intrinsic disorder is discussed in the light of experimental data, which show that unstructured regions of P and N are involved in numerous interactions with several protein and protein-RNA partners. This study provides a contribution to the rather poorly investigated field of intrinsically disordered proteins and helps in targeting protein domains for structural studies.

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.

Active borna disease virus polymerase complex requires a distinct nucleoprotein-to-phosphoprotein ratio but no viral X protein

Analysis of the composition and regulation of the Borna disease virus (BDV) polymerase complex has so far been limited by the lack of a functional assay. To establish such an assay on the basis of an artificial minigenome, we constructed expression vectors encoding either nucleoprotein (N), phosphoprotein (P), X protein, or polymerase (L) of BDV under the control of the chicken beta-actin promoter. A Flag-tagged version of L colocalized with virus-encoded N and P in characteristic nuclear dots of BDV-infected cells and increased viral N-protein levels in persistently infected Vero cells. Vector-driven expression of L, N, and P in BSR-T7 cells together with a negative-sense BDV minigenome carrying a chloramphenicol acetyltransferase (CAT) reporter gene resulted in efficient synthesis of CAT protein. Induction of CAT protein synthesis strongly depended on a 10- to 30-fold molar excess of the N-encoding plasmid over the P-encoding plasmid. Cotransfection of even small amounts of plasmid encoding the viral X protein reduced CAT synthesis to background levels. Thus, the N-to-P stoichiometry seems to play a central role in the regulation of the BDV polymerase complex. Our data further suggest a negative regulatory function for the X protein of BDV.

Measles Virus 1998–2002: Progress and Controversy

Despite the extensive media exposure that viruses such as West Nile, Norwalk, and Ebola have received lately, and the emerging threat that old pathogens may reappear as new agents of terrorism, measles virus (MV) persists as one of the leading causes of death by infectious agents worldwide, approaching the annual mortality rate of human immunodeficiency virus (HIV)-1. For most MV victims, fatality is indirect: Virus-induced transient immunosuppression predisposes the individual to opportunistic infections that, left untreated, can result in mortality. In rare cases, MV may also cause progressive neurodegenerative disease. During the past five years (1998-2002), development of animal models and the application of reverse genetics and immunological assays have collectively contributed to major progress in our understanding of MV biology and pathogenesis. Nevertheless, questions and controversies remain that are the basis for future research. In this review, major advances and current debates are discussed, including MV receptor usage, the cellular basis of immunosuppression, the suspected role of MV in "nonviral" diseases such as multiple sclerosis and Paget's disease, and the controversy surrounding MV vaccine safety.

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 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.

Dendritic cells and measles virus infection

Measles is a major cause of childhood mortality in developing countries which is mainly attributed to the ability of measles virus (MV) to suppress general immune responses. Paradoxically, virus-specific immunity is efficiently induced, which leads to viral clearance from the host and confers long-lasting protection against reinfection. As sensitisers of pathogen encounter and instructors of the adaptive immune response, dendritic cells (DCs) may play a decisive role in the induction and quality of the MV-specific immune activation. The ability of MV wild-type strains in particular to infect DCs in vitro is dearly established, and the receptor binding haemagglutinin protein of these viruses essentially determines this particular tropism. DC maturation as induced early after MV infection is likely to be of crucial importance for the induction of MV-specific immunity. DCs may, however, be instrumental in MV-induced immunosuppression. (1) T cell depletion could be brought about by DC-T cell fusion or TRAIL-mediated induction of apoptosis. (2) Inhibition of stimulated IL-12 production from MV-infected DCs might affect T cell responses in qualitative terms in favouring Th2 and suppressing Th1 responses. (3) The viral glycoprotein complex expressed at high levels on infected DCs late in infection is able to directly inhibit T cell proliferation by surface contact-dependent negative signalling. This most likely accounts for the failure of infected DC cultures to stimulate allogeneic and inhibit mitogen-stimulated T cell proliferation in vitro and the pronounced proliferative unresponsiveness of T cell ex vivo to polyclonal and antigen-specific stimulation which is a central finding of MV-induced immunosuppression.

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.

Hepatitis B virus X protein and simian virus 5 V protein exhibit similar UV-DDB1 binding properties to mediate distinct activities

The UV-damaged DNA-binding activity protein (UV-DDB) consists of two subunits, DDB1 and DDB2, and functions in DNA repair and cell cycle regulation. The DDB1 subunit is a target for the hepatitis B virus X protein (HBx). Binding of HBx to DDB1 interferes with cell growth and viability in culture and has been implicated in the establishment of viral infection. DDB1 also interacts with the V proteins encoded by several paramyxoviruses including simian virus 5 (SV5), which prevent interferon signaling by targeting either STAT1 or STAT2 proteins for proteolysis. The role of V binding to DDB1, however, remains unclear. Here we show that the V protein of SV5 (SV5-V) and HBx exhibit strikingly similar DDB1 binding properties. Thus, SV5-V and HBx bind to DDB1 in a mutually exclusive manner, and SV5-V shares with HBx the ability to enhance the steady-state levels of DDB1 and to inhibit its association with DDB2. Yet only HBx induces cell death, and SV5-V can prevent HBx from doing so by blocking its interaction with DDB1. Binding of SV5-V to DDB1 may serve another function, since SV5-V shows a decreased ability to induce STAT1 degradation in cells expressing reduced amounts of DDB1. These findings demonstrate that HBx performs a unique function through its association with DDB1 for which SV5-V cannot substitute and suggest that SV5-V and HBx have evolved to bind DDB1 to achieve distinct functions, both by a mechanism that does not involve DDB2.

Oral immunization with recombinant Yersinia enterocolitica expressing a measles virus CD4 T cell epitope protects against measles virus-induced encephalitis

Immunization via the oral route with an attenuated Yersinia enterocolitica strain expressing a fragment of the measles virus nucleocapsid protein (aa 79-161) via its type III protein secretion system induced a T helper type 1 response in immunized C3H mice, which conferred protection against measles virus-induced encephalitis in a time- and dose-dependent manner.

The V proteins of simian virus 5 and other paramyxoviruses inhibit induction of interferon-beta

In this article we show that the paramyxovirus SV5 is a poor inducer of interferon-beta (IFN-beta). This inefficient induction is a consequence of the expression of an intact viral V protein. In the absence of the viral V protein cysteine-rich C-terminal domain, IFN-beta mRNA is strongly induced and the transcription factors NF-kappaB and IRF-3 are activated significantly. The V protein can work in isolation from SV5 to block intracellular dsRNA signaling. The mechanism of block to dsRNA signaling is distinct from that previously observed for blocking IFN signaling in that proteolysis of candidate factors cannot be detected, and furthermore, the respective blocks require distinct protein domains. Blocking of the induction of IFN-beta by dsRNA requires the C-terminal cysteine-rich domain, a feature that is highly conserved among paramyxoviruses. We demonstrate that the V proteins from other paramyxoviruses have equivalent functions and speculate that limiting the yield of IFN-beta during infection may be a general property of paramyxoviruses.

Recovery of paramyxovirus simian virus 5 with a V protein lacking the conserved cysteine-rich domain: the multifunctional V protein blocks both interferon-beta induction and interferon signaling

The V protein of the Paramyxovirus simian virus 5 (SV5) is a multifunctional protein containing an N-terminal 164 residue domain that is shared with the P protein and a distinct C-terminal domain that is cysteine-rich and which is highly conserved among Paramyxoviruses. We report the recovery from Vero cells [interferon (IFN) nonproducing cells] of a recombinant SV5 (rSV5) that lacks the V protein C-terminal specific domain (rSV5VDeltaC). In Vero cells rSV5VDeltaC forms large plaques and grows at a rate and titer similar to those of rSV5. In BHK or CV-1 cells rSV5VDeltaC forms small plaques and grows poorly. However, even when grown in Vero cells rSV5VDeltaC reverts to pseudo-wild-type virus in four to five passages, indicating the importance of the V protein for successful replication of SV5. Whereas rSV5 grows in many cell types with minimal cytopathic effect (CPE), rSV5VDeltaC causes extensive CPE in the same cell types. To overcome the antiviral state induced by IFN, many viruses have evolved mechanisms to counteract the effects of IFN by blocking the production of IFN and abrogating IFN signaling. Whereas rSV5 blocks IFN signaling by mediating the degradation of STAT1, rSV5VDeltaC does not cause the degradation of STAT1 and IFN signaling occurs through formation of the ISGF3 transcription complex. Furthermore, we find that rSV5 infection of cells prevents production of IFN-beta. The transcription factor IRF-3 which is required for transcription of the IFN-beta gene is not translocated from the cytoplasm to the nucleus in rSV5-infected cells. In contrast, in rSV5VDeltaC-infected cells IRF-3 is localized predominantly in the nucleus and IFN-beta is produced. By using ectopic expression of IRF-3, it was shown that after dsRNA treatment and expression of the V protein IRF-3 remained in the cytoplasm, whereas after dsRNA treatment and expression of the P protein (which lacks the C-terminal cysteine-rich domain) IRF-3 was localized predominantly in the nucleus. Thus, SV5 blocks two distinct pathways of the innate immune response, both of which require the presence of the C-terminal specific cysteine-rich domain of the multifunctional SV5 V protein.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

SLAM (CD150)-independent measles virus entry as revealed by recombinant virus expressing green fluorescent protein

Wild-type measles virus (MV) strains use human signaling lymphocyte activation molecule (SLAM) as a cellular receptor, while vaccine strains such as the Edmonston strain can use both SLAM and CD46 as receptors. Although the expression of SLAM is restricted to cells of the immune system (lymphocytes, dendritic cells, and monocytes), histopathological studies with humans and experimentally infected monkeys have shown that MV also infects SLAM-negative cells, including epithelial, endothelial, and neuronal cells. In an attempt to explain these findings, we produced the enhanced green fluorescent protein (EGFP)-expressing recombinant MV (IC323-EGFP) based on the wild-type IC-B strain. IC323-EGFP showed almost the same growth kinetics as the parental recombinant MV and produced large syncytia exhibiting green autofluorescence in SLAM-positive cells. Interestingly, all SLAM-negative cell lines examined also showed green autofluorescence after infection with IC323-EGFP, although the virus hardly spread from the originally infected individual cells and thus did not induce syncytia. When the number of EGFP-expressing cells after infection was taken as an indicator, the infectivities of IC323-EGFP for SLAM-negative cells were 2 to 3 logs lower than those for SLAM-positive cells. Anti-MV hemagglutinin antibody or fusion block peptide, but not anti-CD46 antibody, blocked IC323-EGFP infection of SLAM-negative cells. This infection occurred under conditions in which entry via endocytosis was inhibited. These results indicate that MV can infect a variety of cells, albeit with a low efficiency, by using an as yet unidentified receptor(s) other than SLAM or CD46, in part explaining the observed MV infection of SLAM-negative cells in vivo.

The amino-terminal half of Sendai virus C protein is not responsible for either counteracting the antiviral action of interferons or down-regulating viral RNA synthesis

The Sendai virus C proteins, C', C, Y1, and Y2, are a nested set of independently initiated carboxy-coterminal proteins translated from a reading frame overlapping the P frame on the P mRNA. The C proteins are extremely versatile and have been shown to counteract the antiviral action of interferons (IFNs), to down-regulate viral RNA synthesis, and to promote virus assembly. Using the stable cell lines expressing the C, Y1, Y2, or truncated C protein, we investigated the region responsible for anti-IFN action and for down-regulating viral RNA synthesis. Truncation from the amino terminus to the middle of the C protein maintained the inhibition of the signal transduction of IFNs, the formation of IFN-stimulated gene factor 3 (ISGF3) complex, the generation of the anti-vesicular stomatitis virus state, and the synthesis of viral RNA, but further truncation resulted in the simultaneous loss of all of these inhibitory activities. A relatively small truncation from the carboxy terminus also abolished all of these inhibitory activities. These data indicated that the activities of the C protein to counteract the antiviral action of IFNs and to down-regulate viral RNA synthesis were not encoded within a region of at least 98 amino acids in its amino-terminal half.

Activity of polymerase proteins of vaccine and wild-type measles virus strains in a minigenome replication assay

The relative activities of five measles virus (MV) polymerase (L) proteins were compared in an intracellular, plasmid-based replication assay. When coexpressed with N and P proteins from an attenuated strain, L proteins from two attenuated viruses directed the production of up to eight times more reporter protein from an MV minigenome than the three wild-type L proteins. Northern blot analysis demonstrated that the differences in reporter protein production correlated with mRNA transcription levels. Increased activity of polymerases from attenuated viruses equally affected mRNA transcription and minigenome replication. The higher level of transcription may be a consequence of increased template availability or may be an independent effect of the elevated activity of the attenuated polymerases. Coexpression of wild-type L proteins with homologous N and P proteins did not affect the activity of the wild-type polymerases, indicating that the differential activity was a function of the L proteins alone. Use of a minigenome that incorporated two nucleotide changes found in the genomic leader of the three wild-type viruses did not raise the activity of the wild-type L proteins. These data demonstrate that increased polymerase activity differentiates attenuated from wild-type viruses and suggest that functions involved in RNA synthesis contribute to the attenuated phenotype of MV vaccine strains.

Restriction of measles virus RNA synthesis by a mouse host cell line: trans-complementation by polymerase components or a human cellular factor(s)

The mouse epithelial MODE-K cell line expressing human CD46 or CD150 cellular receptors was found to be nonpermissive for measles virus (MV) replication. The virus binding and membrane fusion steps were unimpaired, but only very limited amounts of virus protein and RNA synthesized were detected after the infection. In a minigenome chloramphenicol acetyltransferase assay, MODE-K cells were as able as the permissive HeLa cells in supporting MV polymerase activity. The restriction phenotype of MODE-K cells could be alleviated by providing, in trans, either N-P-L or N-P functional protein complexes but not by P-L complexes or individual N, P, and L proteins. Several human x mouse (HeLa x MODE-K) somatic hybrid clones expressing human CD46 were isolated and found to be either nonpermissive or permissive according to their human chromosomal contents. The MV-restricted phenotype exhibited by the MODE-K cell line suggests that a cellular factor(s) can control MV transcription, possibly by stabilizing the incoming virus polymerase templates.

The N-terminal domain of the phosphoprotein of Morbilliviruses belongs to the natively unfolded class of proteins

We report the bacterial expression, purification, and characterization of the N-terminal domain (PNT) of the measles virus phosphoprotein. Using nuclear magnetic resonance, circular dichroism, gel filtration, and light scattering, we show that PNT is not structured in solution. We show by two complementary computational approaches that PNT belongs to the recently described class of natively unfolded proteins, further confirming its reported similarity with acidic activation domains of cellular transcription factors. We extend these results to the N-terminal domains of other Morbillivirus phosphoproteins and to the corresponding protein W of Sendai virus, a Paramyxovirus. Unstructured proteins may undergo some degree of folding upon binding to their partners, a process termed "induced folding." Using limited proteolysis in the presence of trifluoroethanol, we identified residues 27 to 38 as a putative secondary structure element of PNT arising upon induced folding.

Expression of a foreign gene by recombinant canine distemper virus recovered from cloned DNAs

A canine distemper virus (CDV) genomic cDNA clone and expression plasmids required to establish a CDV rescue system were generated from a laboratory-adapted strain of the Onderstepoort vaccine virus. In addition, a CDV minireplicon was prepared and used in transient expression studies performed to identify optimal virus rescue conditions. Results from the transient expression experiments indicated that minireplicon-encoded reporter gene activity was increased when transfected cell cultures were maintained at 32 rather than 37 degrees C, and when the cellular stress response was induced by heat shock. Applying these findings to rescue of recombinant CDV (rCDV) resulted in efficient recovery of virus after transfected HEp2 or A549 cells were co-cultured with Vero cell monolayers. Nucleotide sequence determination and analysis of restriction site polymorphisms confirmed that rescued virus was rCDV. A rCDV strain also was engineered that contained the luciferase gene inserted between the P and M genes; this virus directed high levels of luciferase expression in infected cells.

Studying experimental measles virus vaccines in the presence of maternal antibodies in the cotton rat model (Sigmodon hispidus)

The inhibition of vaccine-induced seroconversion after vaccination is one of the problems associated with measles virus (MV) immunization. In cotton rats, after transfer of human MV specific antibodies, vaccine-induced seroconversion is inhibited. With this model, it was shown that plasmid immunization (although successful in seronegative animals) was inhibited by maternal antibodies. In contrast, immunization via a mucosal surface with a vesicular stomatitis virus expressing the MV hemagglutinin induced seroconversion in the presence of maternal antibodies and subsequent protection.

Rinderpest virus C and V proteins interact with the major (L) component of the viral polymerase

Rinderpest virus, like other Morbilliviruses, expresses three proteins from the single P gene. In addition to the P protein, which interacts both with the viral polymerase (L) and the nucleocapsid (N) protein, the virus expresses a C and a V protein from the same gene. The functions of these two proteins in the viral life cycle are not clear. Although both C and V proteins are dispensable, in that viable viruses can be made that express neither, each seems to play a role in optimum viral replication. We have used the yeast-two hybrid system, binding to coexpressed fusions of C and V to glutathione-S-transferase, and studies of the native size of these proteins to investigate interactions of the rinderpest virus C and V proteins with other virus-encoded proteins. The V protein was found to interact with both the N and L proteins, while the C protein was found to bind to the L protein, and to self-associate in high-molecular-weight aggregates.

The Bunyamwera virus nonstructural protein NSs inhibits viral RNA synthesis in a minireplicon system

The small (S) genomic segment of Bunyamwera virus (family Bunyaviridae, genus Bunyavirus) encodes the nucleocapsid protein, N, and a nonstructural protein, NSs, in overlapping reading frames. In order to elucidate the function of NSs, we established a plasmid-based minireplicon system using mammalian cells that express large amounts of T7 RNA polymerase. Expression of N, the viral polymerase protein (L), and a minireplicon containing a reporter gene was sufficient to reconstitute functional virus nucleocapsids. Coexpression of NSs, however, led to a dose-dependent decrease in reporter activity without affecting expression of controls. The inhibition could not be reversed by overexpression of N, L or the minireplicon, indicating that the NSs effect was not caused by a reduction in virus gene expression. The NSs proteins of two other members of the Bunyavirus genus, Guaroa virus and Lumbo virus, were also inhibitory in our system. The intracellular localisation of Bunyamwera virus NSs was investigated and found to be predominantly cytoplasmic, but intranuclear inclusion was also detected. Taken together, these data suggest that, in mammalian cells, the bunyavirus NSs protein controls the activity of the viral polymerase by a highly conserved mechanism.

Comparison of predicted amino acid sequences of measles virus strains in the Edmonston vaccine lineage

Protein-encoding nucleotide sequences of the N, P, M, F, H, and L genes were determined for a low-passage isolate of the Edmonston wild-type (wt) measles virus and five Edmonston-derived vaccine virus strains, including AIK-C, Moraten, Schwarz, Rubeovax, and Zagreb. Comparative analysis demonstrated a high degree of nucleotide sequence homology; vaccine viruses differed at most by 0. 3% from the Edmonston wt strain. Deduced amino acid sequences predicted substitutions in all viral polypetides. Eight amino acid coding changes were common to all vaccine viruses; an additional two were conserved in all vaccine strains except Zagreb. Comparisons made between vaccine strains indicated that commercial vaccine lots of Moraten and Schwarz had identical coding regions and were closely related to Rubeovax, while AIK-C and Zagreb diverged from the Edmonston wt along slightly different paths. These comparisons also revealed amino acid coding substitutions in Moraten and Schwarz that were absent from the closely related reactogenic Rubeovax strain. All of the vaccine viruses contained amino acid coding changes in the core components of the virus-encoded transcription and replication apparatus. This observation, combined with identification of noncoding region nucleotide changes in potential cis-acting sequences of the vaccine strains (C. L. Parks, R. A. Lerch, P. Walpita, H.-P. Wang, M. S. Sidhu, and S. A. Udem, J. Virol. 75:921-933, 2001), suggest that modulation of transcription and replication plays an important role in attenuation.

Analysis of the noncoding regions of measles virus strains in the Edmonston vaccine lineage

The noncoding sequence of five Edmonston vaccine viruses (AIK-C, Moraten, Rubeovax, Schwarz, and Zagreb) and those of a low-passage Edmonston wild-type (wt) measles virus have been determined and compared. Twenty-one nucleotide positions were identified at which Edmonston wt and one or more vaccine strains differed. The location of some of these nucleotide substitutions suggests that they may influence the efficiency of mRNA synthesis, processing, and translation, as well as genome replication and encapsidation. Five nucleotide substitutions were conserved in all of the vaccine strains. Two of these were in the genomic 3'-terminal transcriptional control region and could affect RNA synthesis or encapsidation. Three were found within the 5'-untranslated region of the F mRNA, potentially altering translation control sequences. The remaining vaccine virus base changes were found in one to four vaccine strains. Their genomic localization suggests that some may modify cis-acting regulatory domains, including the Kozak consensus element of the P and M genes, the F gene-end signal, and the F mRNA 5'-untranslated sequence.

The paramyxovirus simian virus 5 V protein slows progression of the cell cycle

Infection of cells by many viruses affects the cell division cycle of the host cell to favor viral replication. We examined the ability of the paramyxovirus simian parainfluenza virus 5 (SV5) to affect cell cycle progression, and we found that SV5 slows the rate of proliferation of HeLa T4 cells. The SV5-infected cells had a delayed transition from G(1) to S phase and prolonged progression through S phase, and some of the infected cells were arrested in G(2) or M phase. The levels of p53 and p21(CIP1) were not increased in SV5-infected cells compared to mock-infected cells, suggesting that the changes in the cell cycle occur through a p53-independent mechanism. However, the phosphorylation of the retinoblastoma protein (pRB) was delayed and prolonged in SV5-infected cells. The changes in the cell cycle were also observed in cells expressing the SV5 V protein but not in the cells expressing the SV5 P protein or the V protein lacking its unique C terminus (VDeltaC). The unique C terminus of the V protein of SV5 was shown previously to interact with DDB1, which is the 127-kDa subunit of the multifunctional damage-specific DNA-binding protein (DDB) heterodimer. The coexpression of DDB1 with V can partially restore the changes in the cell cycle caused by expression of the V protein.

Recombinant respiratory syncytial viruses with deletions in the NS1, NS2, SH, and M2-2 genes are attenuated in vitro and in vivo

Respiratory syncytial virus (RSV) encodes several proteins that lack well-defined functions; these include NS1, NS2, SH, and M2-2. Previous work has demonstrated that NS2, SH, and M2-2 can each be deleted from RSV genome and thus are considered as accessory proteins. To determine whether RSV can replicate efficiently when two or more transcriptional units are deleted, we removed NS1, NS2, SH, and M2-2 genes individually and in different combinations from an infectious cDNA clone derived from human RSV A2 strain. The following six mutants with two or more genes deleted were obtained: DeltaNS1NS2, DeltaM2-2SH, DeltaM2-2NS2, DeltaSHNS1, DeltaSHNS2, and DeltaSHNS1NS2. Deletion of M2-2 together with NS1 was detrimental to RSV replication. It was not possible to obtain a recombinant RSV when all four genes were deleted. All of the double and triple deletion mutants exhibited reduced replication and small plaque morphology in vitro. Replication of these deletion mutants was more reduced in HEp-2 cells than in Vero cells. Among the 10 single and multiple gene deletion mutants obtained, DeltaM2-2NS2 was most attenuated. DeltaM2-2NS2 formed barely visible plaques in HEp-2 cells and had a reduction of titer of 3 log(10) compared with the wild-type recombinant RSV in infected HEp-2 cells. When inoculated intranasally into cotton rats, all of the deletion mutants were attenuated in the respiratory tract. Our data indicated that the NS1, NS2, SH, and M2-2 proteins, although dispensable for virus replication in vitro, provide auxiliary functions for efficient RSV replication.

Recovery of pathogenic measles virus from cloned cDNA

Reverse genetics technology so far established for measles virus (MeV) is based on the Edmonston strain, which was isolated several decades ago, has been passaged in nonlymphoid cell lines, and is no longer pathogenic in monkey models. On the other hand, MeVs isolated and passaged in the Epstein-Barr virus-transformed marmoset B-lymphoblastoid cell line B95a would retain their original pathogenicity (F. Kobune et al., J. Virol. 64:700-705, 1990). Here we have developed MeV reverse genetics systems based on the highly pathogenic IC-B strain isolated in B95a cells. Infectious viruses were successfully recovered from the cloned cDNA of IC-B strain by two different approaches. One was simple cotransfection of B95a cells, with three plasmids each encoding the nucleocapsid (N), phospho (P), or large (L) protein, respectively, and their expression was driven by the bacteriophage T7 RNA polymerase supplied by coinfecting recombinant vaccinia virus vTF7-3. The second approach was transfection with the L-encoding plasmid of a helper cell line constitutively expressing the MeV N and P proteins and the T7 polymerase (F. Radecke et al., EMBO J. 14:5773-5784, 1995) on which B95a cells were overlaid. Virus clones recovered by both methods possessed RNA genomes identical to that of the parental IC-B strain and were indistinguishable from the IC-B strain with respect to growth phenotypes in vitro and the clinical course and histopathology of experimentally infected cynomolgus monkeys. Thus, the systems developed here could be useful for studying viral gene functions in the context of the natural course of MeV pathogenesis.

Identification and phylogenetic comparison of Salem virus, a novel paramyxovirus of horses

A virus that could not be identified as a previously known equine virus was isolated from the mononuclear cells of a horse. Electron microscopy revealed enveloped virions with nucleocapsid structures characteristic of viruses in the Paramyxoviridae family. The virus failed to hemabsorb chicken or guinea pig red blood cells and lacked neuraminidase activity. Two viral genes were isolated from a cDNA expression library. Multiple sequence alignments of one gene indicated an average identity of 45% as compared to Morbillivirus N protein sequences. A weaker relationship was found with Tupaia paramyxovirus (TPMV) and Hendra virus (HeV) N proteins. In the second gene, multiple open reading frames (ORFs) were identified, corresponding to the arrangement of the P, V, and C ORFs in the Morbillivirus and Respirovirus viruses. Short stretches in the C-terminal regions of the P and C proteins showed limited homologies to viruses in the Morbillivirus genus but no obvious relationship to viruses in other genera. The V ORF translation product contained a highly conserved, cysteine-rich domain that is common to most viruses in the Paramyxovirinae subfamily. Sequencing of P gene cDNA clones confirmed the use of a cotranscriptional editing mechanism for the regulation of P/V expression. Based on the location of its origin it has been named Salem virus (SalV).

Rinderpest viruses lacking the C and V proteins show specific defects in growth and transcription of viral RNAs

Rinderpest virus is a morbillivirus and the causative agent of an important disease of cattle and wild bovids. The P genes of all morbilliviruses give rise to two proteins in addition to the P protein itself: use of an alternate start translation site, in a second open reading frame, gives rise to the C protein, while cotranscriptional insertion of an extra base gives rise to the V protein, a fusion of the amino-terminal half of P to a short, highly conserved, cysteine-rich zinc binding domain. Little is known about the function of either of these two proteins in the rinderpest virus life cycle. We have constructed recombinant rinderpest viruses in which the expression of these proteins has been suppressed, individually and together, and studied the replication of these viruses in tissue culture. We show that the absence of the V protein has little effect on the replication rate of the virus but does lead to an increase in synthesis of genome and antigenome RNAs and a change in cytopathic effect to a more syncytium-forming phenotype. Virus that does not express the C protein, on the other hand, is clearly defective in growth in all cell lines tested, and this defect appears to be related to a decreased transcription of mRNA from viral genes. The phenotypes of both individual mutant virus types are both expressed in the double mutant expressing neither V nor C.

V and C proteins of measles virus function as virulence factors in vivo

The measles virus (MV) P gene encodes three proteins: the P protein and two nonstructural proteins, C and V. Because the functions of both the C and V protein are unknown, we used MV C (C-) and V (V-) deletion recombinants generated by the MV reverse genetics system (F. Radecke, P. Spielhofer, H. Schnieder, K. Kaelin, M. Huber, C. Dotsch, G. Christiansen, and M. A. Billeter 1995. EMBO J. 14, 5773-5784). Compared to parental vaccine strain, Edmonston (Ed) MV, both had normal growth and cytopathic effects in Vero cells and showed similar growth kinetics in human neuroblastoma SK-N-MC cells and in primary mouse neurons expressing the MV receptor, CD46. However, in vivo, using YAC-CD46 transgenic mice as a model for MV induced CNS disease (M. B. A. Oldstone, H. Lewicki, D. Thomas, A. Tishon, S. Dales, J. Patterson, M. Manchester, D. Homann, D. Naniche, and A. Holz 1999. Cell 98, 629-640), C- and V- viruses differed markedly from wt Ed(V(+)C(+)) virus. Newborn mice inoculated with as little as 10(3) PFU of Ed strain became ill and died after 10-15 days. In contrast, those inoculated with 10(3) or 10(4) PFU of MV C- or MV V- showed significantly fewer and milder clinical symptoms and had a lower mortality. A total of 10(5) PFU V- virus were required to kill most YAC-CD46 mice, and less than half (44%) were killed with a corresponding dose of MV C-. Immunohistochemical staining for MV antigens showed similar extents of spread for MV C- and MV Ed but restricted spread for MV V- throughout the brain. Viral load and transcription were markedly reduced for V- but not for C-. Multiple cytokines and chemokines were equivalently upregulated for all three viruses. Therefore, MV C and V proteins encode virulence functions in vivo and likely operate via separate mechanisms.

Lymphatic dissemination and comparative pathology of recombinant measles viruses in genetically modified mice

The dissemination of the Edmonston measles virus (Ed-MV) vaccine strain was studied with genetically modified mice defective for the alpha/beta interferon receptor and expressing human CD46 with human-like tissue specificity and efficiency. A few days after intranasal infection, macrophages expressing Ed-MV RNA were detected in the lungs, in draining lymph nodes, and in the thymus. In lymph nodes, large syncytia which stained positive for viral RNA and for macrophage surface marker proteins were found and apoptotic cell death was monitored. In the thymus, smaller syncytia which stained positive for macrophage and dendritic cell markers were detected. Thus, macrophages appear to be the main vectors for dissemination of MV infection in these mice; human macrophages may have a similar function in the natural host. We then compared the pathogenicities of two recombinant viruses lacking the C or V nonstructural proteins to that of the parental strain, Ed-MV. These viruses were less effective in spreading through the lymphatic system and, unlike Ed-MV, were not detected in the liver. After intracerebral inoculation the recombinant viruses caused lethal disease less often than Ed-MV and induced distinctive patterns of gliosis and inflammation. Ed-MV was reisolated from brain tissue, but its derivatives were not. C- and V-defective viruses should be considered as more-attenuated MV vaccine candidates.

Sequence analysis of the genes encoding the phosphoprotein of recent isolates of canine distemper virus in Japan

The nucleotide sequences of the phosphoprotein (P) of canine distemper virus (CDV) strains isolated between 1992 and 1996 in Japan were determined. This is the first report of the complete sequences of the P genes of recently prevalent CDV strains. The deduced amino acid sequences of the P, C and V proteins showed that in the new Japanese isolates, these proteins have approximately 93%, 90-91% and 92% identities with those of the Onderstepoort vaccine strain, respectively. The predicted functional regions were conserved. RNA editing resulting in a shift to the open reading frame (ORF) of the V protein was shown to occur with the same efficiency in both the field isolates and vaccine strain.

Respiratory syncytial virus that lacks open reading frame 2 of the M2 gene (M2-2) has altered growth characteristics and is attenuated in rodents

The M2 gene of respiratory syncytial virus (RSV) encodes two putative proteins: M2-1 and M2-2; both are believed to be involved in the RNA transcription or replication process. To understand the function of the M2-2 protein in virus replication, we deleted the majority of the M2-2 open reading frame from an infectious cDNA clone derived from the human RSV A2 strain. Transfection of HEp-2 cells with the cDNA clone containing the M2-2 deletion, together with plasmids that encoded the RSV N, P, and L proteins, produced a recombinant RSV that lacked the M2-2 protein (rA2DeltaM2-2). Recombinant virus rA2DeltaM2-2 was recovered and characterized. The levels of viral mRNA expression for 10 RSV genes examined were unchanged in cells infected with rA2DeltaM2-2, except that a shorter M2 mRNA was detected. However, the ratio of viral genomic or antigenomic RNA to mRNA was reduced in rA2DeltaM2-2-infected cells. By use of an antibody directed against the bacterially expressed M2-2 protein, the putative M2-2 protein was detected in cells infected with wild-type RSV but not in cells infected with rA2DeltaM2-2. rA2DeltaM2-2 displayed a small-plaque morphology and grew much more slowly than wild-type RSV in HEp-2 cells. In infected Vero cells, rA2DeltaM2-2 exhibited very large syncytium formation compared to that of wild-type recombinant RSV. rA2DeltaM2-2 appeared to be a host range mutant, since it replicated poorly in HEp-2, HeLa, and MRC5 cells but replicated efficiently in Vero and LLC-MK2 cells. Replication of rA2DeltaM2-2 in the upper and lower respiratory tracts of mice and cotton rats was highly restricted. Despite its attenuated replication in rodents, rA2DeltaM2-2 was able to provide protection against challenge with wild-type RSV A2. The genotype and phenotype of the M2-2 deletion mutant were stably maintained after extensive in vitro passages. The attenuated phenotype of rA2DeltaM2-2 suggested that rA2DeltaM2-2 may be a potential candidate for use as a live attenuated vaccine.

Molecular evolution of the Paramyxoviridae and Rhabdoviridae multiple-protein-encoding P gene

Presented here is an analysis of the molecular evolutionary dynamics of the P gene among 76 representative sequences of the Paramyxoviridae and Rhabdoviridae RNA virus families. In a number of Paramyxoviridae taxa, as well as in vesicular stomatitis viruses of the Rhabdoviridae, the P gene encodes multiple proteins from a single genomic RNA sequence. These products include the phosphoprotein (P), as well as the C and V proteins. The complexity of the P gene makes it an intriguing locus to study from an evolutionary perspective. Amino acid sequence alignments of the proteins encoded at the P and N loci were used in independent phylogenetic reconstructions of the Paramyxoviridae and Rhabdoviridae families. P-gene-coding capacities were mapped onto the Paramyxoviridae phylogeny, and the most parsimonious path of multiple-coding-capacity evolution was determined. Levels of amino acid variation for Paramyxoviridae and Rhabdoviridae P-gene-encoded products were also analyzed. Proteins encoded in overlapping reading frames from the same nucleotides have different levels of amino acid variation. The nucleotide architecture that underlies the amino acid variation was determined in order to evaluate the role of selection in the evolution of the P gene overlapping reading frames. In every case, the evolution of one of the proteins encoded in the overlapping reading frames has been constrained by negative selection while the other has evolved more rapidly. The integrity of the overlapping reading frame that represents a derived state is generally maintained at the expense of the ancestral reading frame encoded by the same nucleotides. The evolution of such multicoding sequences is likely a response by RNA viruses to selective pressure to maximize genomic information content while maintaining small genome size. The ability to evolve such a complex genomic strategy is intimately related to the dynamics of the viral quasispecies, which allow enhanced exploration of the adaptive landscape.

Observation of measles virus cell-to-cell spread in astrocytoma cells by using a green fluorescent protein-expressing recombinant virus

A recombinant measles virus (MV) which expresses enhanced green fluorescent protein (EGFP) has been rescued. This virus, MVeGFP, expresses the reporter gene from an additional transcription unit which is located prior to the gene encoding the measles virus nucleocapsid protein. The recombinant virus was used to infect human astrocytoma cells (GCCM). Immunocytochemistry (ICC) together with EGFP autofluorescence showed that EGFP is both an early and very sensitive indicator of cell infection. Cells that were EGFP-positive and ICC-negative were frequently observed. Confocal microscopy was used to indirectly visualize MV infection of GCCM cells and to subsequently follow cell-to-cell spread in real time. These astrocytoma cells have extended processes, which in many cases are intimately associated. The processes appear to have an important role in cell-to-cell spread, and MVeGFP was observed to utilize them in the infection of surrounding cells. Heterogeneity was seen in cell-to-cell spread in what was expected to be a homogeneous monolayer. In tissue culture, physical constraints govern the integrity of the syncytia which are formed upon extensive cell fusion. When around 50 cells were fused, the syncytia rapidly disintegrated and many of the infected cells detached. Residual adherent EGFP-positive cells were seen to either continue to be involved in the infection of surrounding cells or to remain EGFP positive but no longer participate in the transmission of MV infection to neighboring cells.

The H gene of rodent brain-adapted measles virus confers neurovirulence to the Edmonston vaccine strain

Molecular determinants of neuropathogenesis have been shown to be present in the hemagglutinin (H) protein of measles virus (MV). An H gene insertion vector has been generated from the Edmonston B vaccine full-length infectious clone of MV. Using this vector, it is possible to insert complete H open reading frames into the parental (Edtag) background. The H gene from a rodent brain-adapted MV strain (CAM/RB) was inserted into this vector, and a recombinant virus (EdtagCAMH) was rescued by using a modified vaccinia virus which expresses T7 RNA polymerase (MVA-T7). The recombinant virus grew at an equivalent rate and to similar titers as the CAM/RB and Edtag parental viruses. Neurovirulence was assayed in a mouse model for MV encephalitis. Viruses were injected intracerebrally into the right cortex of C57/BL/6 suckling mice. After infection mice inoculated with the CAM/RB strain developed hind limb paralysis and ataxia. Clinical symptoms were never observed with an equivalent dose of Edtag virus or in sham infections. Immunohistochemistry (IHC) was used to detect viral antigen in formalin-fixed brain sections. Measles antigen was observed in neurons and neuronal processes of the hippocampus, frontal, temporal, and olfactory cortices and neostriatum on both sides of symmetrical structures. Viral antigen was not detected in mice infected with Edtag virus. Mice infected with the recombinant virus, EdtagCAMH, became clinically ill, and virus was detected by IHC in regions of the brain similar to those in which it was detected in animals infected with CAM/RB. The EdtagCAMH infection had, however, progressed much less than the CAM/RB virus at 4 days postinfection. It therefore appears that additional determinants are encoded in other regions of the MV genome which are required for full neurovirulence equivalent to CAM/RB. Nevertheless, replacement of the H gene alone is sufficient to cause neuropathology.

The genome nucleotide sequence of a contemporary wild strain of measles virus and its comparison with the classical Edmonston strain genome

The only complete genome nucleotide sequences of measles virus (MeV) reported to date have been for the Edmonston (Ed) strain and derivatives, which were isolated decades ago, passaged extensively under laboratory conditions, and appeared to be nonpathogenic. Partial sequencing of many other strains has identified >/=15 genotypes. Most recent isolates, including those typically pathogenic, belong to genotypes distinct from the Edmonston type. Therefore, the sequence of Ed and related strains may not be representative of those of pathological measles circulating at that or any time in human populations. Taking into account these issues as well as the fact that so many studies have been based upon Ed-related strains, we have sequenced the entire genome of a recently isolated pathogenic strain, 9301B. Between this recent isolate and the classical Ed strain, there were 465 nucleotide differences (2.93%) and 114 amino acid differences (2.19%). Computation of nonsynonymous and synonymous substitutions in open reading frames as well as direct comparisons of noncoding regions of each gene and extracistronic regulatory regions clearly revealed the regions where changes have been permissible and nonpermissible. Notably, considerable nonsynonymous substitutions appeared to be permissible for the P frame to maintain a high degree of sequence conservation for the overlapping C frame. However, the cause and the effect were largely unclear for any substitution, indicating that there is a considerable gap between the two strains that cannot be filled. The sequence reported here would be useful as a reference of contemporary wild-type MeV.

Nonstructural C protein is required for efficient measles virus replication in human peripheral blood cells

The P gene of measles virus (MV) encodes the phosphoprotein, a component of the virus ribonucleoprotein complex, and two nonstructural proteins, C and V, with unknown functions. Growth of recombinant MV, defective in C or V expression, was explored in human peripheral blood mononuclear cells (PBMC). The production of infectious recombinant MV V- was comparable to that of parental MV tag in simian Vero fibroblasts and in PBMC. In contrast, MV C- progeny was strongly reduced in PBMC but not in Vero cells. Consistently, the expression of both hemagglutinin and fusion proteins, as well as that of nucleoprotein mRNA, was lower in MV C--infected PBMC. Thus, efficient replication of MV in natural host cells requires the expression of the nonstructural C protein. The immunosuppression that accompanies MV infection is associated with a decrease in the in vitro lymphoproliferative response to mitogens. MV C- was as potent as MV tag or MV V- in inhibiting the phytohemagglutinin-induced proliferation of PBMC, indicating that neither the C protein nor the V protein is directly involved in this effect.