Reverse guanine phosphoribosyltransferase selection of recombinant vaccinia viruses

Poxviruses as Gene Therapy Vectors: Generating Poxviral Vectors Expressing Therapeutic Transgenes

Treatments with poxvirus vectors can have long-lasting immunological impact in the host, and thus they have been extensively studied to treat diseases and for vaccine development. More importantly, the oncolytic properties of poxviruses have led to their development as cancer therapeutics. Two poxviruses, vaccinia virus (VACV) and myxoma virus (MYXV), have been extensively studied as virotherapeutics with promising results. Vaccinia virus vectors have advanced to the clinic and have been tested as oncolytic therapeutics for several cancer types with successes in phase I/II clinical trials. In addition to oncolytic applications, MYXV has been explored for additional applications including immunotherapeutics, purging of cancer progenitor cells, and treatments for graft-versus-host diseases. These novel therapeutic applications have encouraged its advancement into clinical trials. To meet the demands of different treatment needs, VACV and MYXV can be genetically engineered to express therapeutic transgenes. The engineering process used in poxvirus vectors can be very different from that of other DNA virus vectors (e.g., the herpesviruses). This chapter is intended to serve as a guide to those wishing to engineer poxvirus vectors for therapeutic transgene expression and to produce viral preparations for preclinical studies.

Structural and functional conservation of cis-acting RNA elements in coronavirus 5'-terminal genome regions

Structure predictions suggest a partial conservation of RNA structure elements in coronavirus terminal genome regions. Here, we determined the structures of stem-loops (SL) 1 and 2 of two alphacoronaviruses, human coronavirus (HCoV) 229E and NL63, by RNA structure probing and studied the functional relevance of these putative cis-acting elements. HCoV-229E SL1 and SL2 mutants generated by reverse genetics were used to study the effects on viral replication of single-nucleotide substitutions predicted to destabilize the SL1 and SL2 structures. The data provide conclusive evidence for the critical role of SL1 and SL2 in HCoV-229E replication and, in some cases, revealed parallels with previously characterized betacoronavirus SL1 and SL2 elements. Also, we were able to rescue viable HCoV-229E mutants carrying replacements of SL2 with equivalent betacoronavirus structural elements. The data obtained in this study reveal a remarkable degree of structural and functional conservation of 5′-terminal RNA structural elements across coronavirus genus boundaries.

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Conservation of 5'-terminal SL1 and SL2 elements in alpha- and betacoronaviruses.

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HCoV-229E reverse genetics data suggest critical role for SL1/2 in viral replication.

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Replacement of SL2 in HCoV-229E with betacoronavirus equivalents is tolerated.

Generation of Recombinant Vaccinia Viruses

This unit describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector or PCR fragment to generate a recombinant virus. Selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses are described. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented. © 2017 by John Wiley & Sons, Inc.

Production of a Chikungunya Vaccine Using a CHO Cell and Attenuated Viral-Based Platform Technology

Vaccinia-based systems have been extensively explored for the development of recombinant vaccines. Herein we describe an innovative vaccinia virus (VACV)-derived vaccine platform technology termed Sementis Copenhagen Vector (SCV), which was rendered multiplication-defective by targeted deletion of the essential viral assembly gene D13L. A SCV cell substrate line was developed for SCV vaccine production by engineering CHO cells to express D13 and the VACV host-range factor CP77, because CHO cells are routinely used for manufacture of biologics. To illustrate the utility of the platform technology, a SCV vaccine against chikungunya virus (SCV-CHIK) was developed and shown to be multiplication-defective in a range of human cell lines and in immunocompromised mice. A single vaccination of mice with SCV-CHIK induced antibody responses specific for chikungunya virus (CHIKV) that were similar to those raised following vaccination with a replication-competent VACV-CHIK and able to neutralize CHIKV. Vaccination also provided protection against CHIKV challenge, preventing both viremia and arthritis. Moreover, SCV retained capacity as an effective mouse smallpox vaccine. In summary, SCV represents a new and safe vaccine platform technology that can be manufactured in modified CHO cells, with pre-clinical evaluation illustrating utility for CHIKV vaccine design and construction.

Generation of Recombinant Vaccinia Viruses

This unit describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector or PCR fragment to generate a recombinant virus. Selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses are described. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented. © 2017 by John Wiley & Sons, Inc.

Modified Vaccinia Virus Ankara: History, Value in Basic Research, and Current Perspectives for Vaccine Development

Safety tested Modified Vaccinia virus Ankara (MVA) is licensed as third-generation vaccine against smallpox and serves as a potent vector system for development of new candidate vaccines against infectious diseases and cancer. Historically, MVA was developed by serial tissue culture passage in primary chicken cells of vaccinia virus strain Ankara, and clinically used to avoid the undesirable side effects of conventional smallpox vaccination. Adapted to growth in avian cells MVA lost the ability to replicate in mammalian hosts and lacks many of the genes orthopoxviruses use to conquer their host (cell) environment. As a biologically well-characterized mutant virus, MVA facilitates fundamental research to elucidate the functions of poxvirus host-interaction factors. As extremely safe viral vectors MVA vaccines have been found immunogenic and protective in various preclinical infection models. Multiple recombinant MVA currently undergo clinical testing for vaccination against human immunodeficiency viruses, Mycobacterium tuberculosis or Plasmodium falciparum. The versatility of the MVA vector vaccine platform is readily demonstrated by the swift development of experimental vaccines for immunization against emerging infections such as the Middle East Respiratory Syndrome. Recent advances include promising results from the clinical testing of recombinant MVA-producing antigens of highly pathogenic avian influenza virus H5N1 or Ebola virus. This review summarizes our current knowledge about MVA as a unique strain of vaccinia virus, and discusses the prospects of exploiting this virus as research tool in poxvirus biology or as safe viral vector vaccine to challenge existing and future bottlenecks in vaccinology.

Generation of Recombinant Vaccinia Viruses

This unit describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector or PCR fragment to generate a recombinant virus. Selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses are described. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented.

A selectable and excisable marker system for the rapid creation of recombinant poxviruses

BACKGROUND

Genetic manipulation of poxvirus genomes through attenuation, or insertion of therapeutic genes has led to a number of vector candidates for the treatment of a variety of human diseases. The development of recombinant poxviruses often involves the genomic insertion of a selectable marker for purification and selection purposes. The use of marker genes however inevitably results in a vector that contains unwanted genetic information of no therapeutic value.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe an improved strategy that allows for the creation of marker-free recombinant poxviruses of any species. The Selectable and Excisable Marker (SEM) system incorporates a unique fusion marker gene for the efficient selection of poxvirus recombinants and the Cre/loxP system to facilitate the subsequent removal of the marker. We have defined and characterized this new methodological tool by insertion of a foreign gene into vaccinia virus, with the subsequent removal of the selectable marker. We then analyzed the importance of loxP orientation during Cre recombination, and show that the SEM system can be used to introduce site-specific deletions or inversions into the viral genome. Finally, we demonstrate that the SEM strategy is amenable to other poxviruses, as demonstrated here with the creation of an ectromelia virus recombinant lacking the EVM002 gene.

CONCLUSION/SIGNIFICANCE

The system described here thus provides a faster, simpler and more efficient means to create clinic-ready recombinant poxviruses for therapeutic gene therapy applications.

Generation of recombinant vaccinia viruses

This unit first describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector to generate a recombinant virus. Methods are also presented for purifying vaccinia virus and for isolating viral DNA, which can be used during transfection. Also presented are selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented.

Generation of recombinant vaccinia viruses

This unit first describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector to generate a recombinant virus. Methods are also presented for purifying vaccinia virus and for isolating viral DNA, which can be used during transfection. Also presented are selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented.

Cell surface expression of the vaccinia virus complement control protein is mediated by interaction with the viral A56 protein and protects infected cells from complement attack

The vaccinia virus (VACV) complement control protein (VCP) is the major protein secreted from VACV-infected cells. It has been reported that VCP binds to the surfaces of uninfected cells by interacting with heparan sulfate proteoglycans (HSPGs). In this study, we show that VCP is also expressed on the surfaces of infected cells and demonstrate that surface localization occurs independently of HSPGs. Since VCP does not contain a transmembrane domain, we hypothesized that VCP interacts with a membrane protein that localizes to the infected-cell surface. We show that the VACV A56 membrane protein is necessary for the cell surface expression of VCP and demonstrate that VCP and A56 interact in VACV-infected cells. Since the surface expression of VCP was abrogated by reducing agents, we examined the contribution of an unpaired cysteine residue on VCP to VCP surface expression and VCP's interaction with A56. To do this, we mutated the unpaired cysteine in VCP and generated a recombinant virus expressing the altered form of VCP. Following the infection of cells with the mutant virus, VCP was neither expressed on the cell surface nor able to interact with A56. Importantly, the cell surface expression of VCP was found to protect infected cells from complement-mediated lysis. Our findings suggest a new function for VCP that may be important for poxvirus pathogenesis and impact immune responses to VACV-based vaccines.

Characterization of the vaccinia virus A35R protein and its role in virulence

The vaccinia virus A35R gene is highly conserved among poxviruses and encodes a previously uncharacterized hydrophobic acidic protein. Western blotting with anti-A35R peptide antibodies indicated that the protein is expressed early in infection and resolved as a single sharp band of approximately 23 kDa, slightly higher than the 20 kDa predicted from its sequence. The protein band appeared to be the same molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, whether expressed in an in vitro transcription/translation system without microsomes or expressed in infected cells, suggesting that it was not glycosylated. A mutant virus with the A35R gene deleted (vA35Delta) formed wild-type-sized plaques on all cell lines tested (human, monkey, mouse, and rabbit); thus, A35R is not required for replication and does not appear to be a host range gene. Although the A35R protein is hydrophobic, it is unlikely to be an integral membrane protein, as it partitioned to the aqueous phase during TX-114 partitioning. The protein could not be detected in virus-infected cell supernatants. A35R localized intracellularly to the virus factories, where the first stages of morphogenesis occur. The vA35Delta mutant formed near-normal levels of the various morphogenic stages of infectious virus particles and supported normal acid-induced fusion of virus-infected cells. Despite normal growth and morphogenesis in vitro, the vA35Delta mutant virus was attenuated in intranasal challenge of mice compared to wild-type and A35R rescue virus. Thus, the intracellular A35R protein plays a role in virulence. The A35R has little homology to any protein outside of poxviruses, suggesting a novel virulence mechanism.

Inhibition of MHC class I is a virulence factor in herpes simplex virus infection of mice

Herpes simplex virus (HSV) has a number of genes devoted to immune evasion. One such gene, ICP47, binds to the transporter associated with antigen presentation (TAP) 1/2 thereby preventing transport of viral peptides into the endoplasmic reticulum, loading of peptides onto nascent major histocompatibility complex (MHC) class I molecules, and presentation of peptides to CD8 T cells. However, ICP47 binds poorly to murine TAP1/2 and so inhibits antigen presentation by MHC class I in mice much less efficiently than in humans, limiting the utility of murine models to address the importance of MHC class I inhibition in HSV immunopathogenesis. To address this limitation, we generated recombinant HSVs that efficiently inhibit antigen presentation by murine MHC class I. These recombinant viruses prevented cytotoxic T lymphocyte killing of infected cells in vitro, replicated to higher titers in the central nervous system, and induced paralysis more frequently than control HSV. This increase in virulence was due to inhibition of antigen presentation to CD8 T cells, since these differences were not evident in MHC class I-deficient mice or in mice in which CD8 T cells were depleted. Inhibition of MHC class I by the recombinant viruses did not impair the induction of the HSV-specific CD8 T-cell response, indicating that cross-presentation is the principal mechanism by which HSV-specific CD8 T cells are induced. This inhibition in turn facilitates greater viral entry, replication, and/or survival in the central nervous system, leading to an increased incidence of paralysis.

While animal models are often instructive in understanding human diseases, many factors that influence disease differ between mouse and man. Although mice can be experimentally infected with HSV-1, this virus has evolved as a human pathogen. One facet of this evolution is HSV's mechanisms to evade the immune response, allowing the virus to persist for the lifetime of the human host. This evasion includes preventing CD8 T cells from recognizing and killing infected cells by inhibiting the expression of the molecule that presents viral peptides to CD8 T cells: major histocompatibility complex (MHC) class I. HSV is unable to inhibit mouse MHC class I, thus rendering this immune-evasion strategy inoperative in the mouse. To better understand the biology of HSV infection and the immune response to this virus in humans, the authors corrected this deficiency by inserting a gene which inhibits murine MHC class I. This recombinant virus demonstrates that MHC class I inhibition is an important determinant of disease progression. The authors found that the recombinant HSV still effectively elicits a CD8 T-cell response, but this response is ineffective in controlling the infection. This finding reveals the important distinction between the size of the immune response and the effectiveness of the response, which may be important to HSV vaccine studies.

Rapid identification of coronavirus replicase inhibitors using a selectable replicon RNA

A previously unknown coronavirus (CoV) is the aetiological agent causing severe acute respiratory syndrome (SARS), for which an effective antiviral treatment is urgently needed. To enable the rapid and biosafe identification of coronavirus replicase inhibitors, we have generated a non-cytopathic, selectable replicon RNA (based on human CoV 229E) that can be stably maintained in eukaryotic cells. Most importantly, the replicon RNA mediates reporter gene expression as a marker for coronavirus replication. We have used a replicon RNA-containing cell line to test the inhibitory effect of several compounds that are currently being assessed for SARS treatment. Amongst those, interferon-alpha displayed the strongest inhibitory activity. Our results demonstrate that coronavirus replicon cell lines provide a versatile and safe assay for the identification of coronavirus replicase inhibitors. Once this technology is adapted to SARS-CoV replicon RNAs, it will allow high throughput screening for SARS-CoV replicase inhibitors without the need to grow infectious SARS-CoV.

Improved protection conferred by vaccination with a recombinant vaccinia virus that incorporates a foreign antigen into the extracellular enveloped virion

Recombinant poxviruses have shown promise as vaccine vectors. We hypothesized that improved cellular immune responses could be developed to a foreign antigen by incorporating it as part of the extracellular enveloped virion (EEV). We therefore constructed a recombinant vaccinia virus that replaced the cytoplasmic domain of the B5R protein with a test antigen, HIV-1 Gag. Mice immunized with the virus expressing Gag fused to B5R had significantly better primary CD4 T-cell responses than recombinant virus expressing HIV-Gag from the TK-locus. The CD8 T-cell responses were less different between the two groups. Importantly, although we saw differences in the immune response to the test antigen, the vaccinia virus-specific immune responses were similar with both constructs. When groups of vaccinated mice were challenged 30 days later with a recombinant Listeria monocytogenes that expresses HIV-Gag, mice inoculated with the virus that expresses the B5R-Gag fusion protein had lower colony counts of Listeria in the liver and spleen than mice vaccinated with the standard recombinant. Thus, vaccinia virus expressing foreign antigen incorporated into EEV may be a better vaccine strategy than standard recombinant vaccinia virus.

Plasma membrane localization and fusion inhibitory activity of the cowpox virus serpin SPI-3 require a functional signal sequence and the virus encoded hemagglutinin

The cowpox virus (CPV) glycoprotein serpin SPI-3, a functional protease inhibitor, and the viral hemagglutinin (HA) are required to prevent fusion of wt CPV infected cells. SPI-3 and HA from CPV infected cells co-localize to the plasma membrane and are found in extracellular enveloped virus (EEV). We also show that an N-terminal SPI-3 signal sequence, but not glycosylation, is required for membrane localization and fusion inhibition. In the absence of HA (CPVDeltaHA), no SPI-3 is found on the membrane and infected cells fuse. Conversely, HA from both wt CPV and CPVDeltaSPI-3 infections is on the membrane, indicating a requirement of HA for SPI-3 plasma membrane localization. In the absence of HA, secretion of SPI-3 or SPI-3 N-glyc(-) was markedly enhanced, suggesting HA serves to retain SPI-3 on the plasma membrane,thereby preventing cell fusion.

Identification of the orthopoxvirus p4c gene, which encodes a structural protein that directs intracellular mature virus particles into A-type inclusions

The orthopoxvirus gene p4c has been identified in the genome of the vaccinia virus strain Western Reserve. This gene encodes the 58-kDa structural protein P4c present on the surfaces of the intracellular mature virus (IMV) particles. The gene is disrupted in the genome of cowpox virus Brighton Red (BR), demonstrating that although the P4c protein may be advantageous for virus replication in vivo, it is not essential for virus replication in vitro. Complementation and recombination analyses with the p4c gene have shown that the P4c protein is required to direct the IMV into the A-type inclusions (ATIs) produced by cowpox virus BR. The p4c gene is highly conserved among most members of the orthopoxvirus genus, including viruses that produce ATIs, such as cowpox, ectromelia, and raccoonpox viruses, as well as those such as variola, monkeypox, vaccinia, and camelpox viruses, which do not. The conservation of the p4c gene among the orthopoxviruses, irrespective of their capacities to produce ATIs, suggests that the P4c protein provides functions in addition to that of directing IMV into ATIs. These findings, and the presence of the P4c protein in IMV but not extracellular enveloped virus (D. Ulaeto, D. Grosenbach, and D. E. Hruby, J. Virol. 70:3372-3377, 1996), suggest a model in which the P4c protein may play a role in the retrograde movement of IMV particles, thereby contributing to the retention of IMV particles within the cytoplasm and within ATIs when they are present. In this way, the P4c protein may affect both viral morphogenesis and processes of virus dissemination.

The vaccinia virus B9R protein is a 6 kDa intracellular protein that is non-essential for virus replication and virulence

Vaccinia virus (VV) strain Western Reserve gene B9R is shown to encode an intracellular 6 kDa protein that is expressed late during the infectious cycle. In vitro transcription and translation produced two polypeptides in the presence of microsomal membranes, but only the larger protein in the absence of membranes. The smaller protein sedimented with microsomes during centrifugation, suggesting it was inserted into the lipid membrane or into the microsomal lumen via the N-terminal hydrophobic signal sequence that was subsequently cleaved proteolytically. A VV mutant lacking B9R was constructed and found to replicate normally in cell culture and two in vivo models.

The vaccinia virus superoxide dismutase-like protein (A45R) is a virion component that is nonessential for virus replication

A characterization of the A45R gene from vaccinia virus (VV) strain Western Reserve is presented. The open reading frame is predicted to encode a 125-amino-acid protein (M(r), of 13,600) with 39% amino acid identity to copper-zinc superoxide dismutase (Cu-Zn SOD). Sequencing of the A45R gene from other orthopoxviruses, here and by others, showed that the protein is highly conserved in all viruses sequenced, including 16 strains of VV, 2 strains of cowpox virus, camelpox virus, and 4 strains of variola virus. In all cases the protein lacks key residues involved in metal ion binding that are important for the catalytic activity. The A45R protein was expressed in Escherichia coli, purified, and tested for SOD activity, but neither enzymatic nor inhibitory SOD activity was detected. Additionally, no virus-encoded SOD activity was detected in infected cells or purified virions. A monoclonal antibody raised against the A45R protein expressed in E. coli identified the A45R gene product as a 13.5-kDa protein that is expressed late during VV infection. Confocal microscopy of VV-infected cells indicated that the A45R protein accumulated predominantly in cytoplasmic viral factories. Electron microscopy and biochemical analyses showed that the A45R protein is incorporated into the virion core. A deletion mutant lacking the majority of the A45R gene and a revertant virus in which the deleted gene was restored were constructed and characterized. The growth properties of the deletion mutant virus were indistinguishable from those of wild-type and revertant viruses in all cell lines tested, including macrophages. Additionally, the virulence and pathogenicity of the three viruses were also comparable in murine and rabbit models of infection. A45R is unusual in being the first VV core protein described that affects neither virus replication nor virulence.

Repression of vaccinia virus Holliday junction resolvase inhibits processing of viral DNA into unit-length genomes

The vaccinia virus A22R gene encodes a protein that is homologous to the bacterial enzyme RuvC and specifically cleaves and resolves four-way DNA Holliday junctions into linear duplex products. To investigate the role of the vaccinia virus Holliday junction resolvase during an infection, we constructed two recombinant viruses: vA22-HA, which has a short C-terminal epitope tag appended to the A22R open reading frame, and vA22i, in which the original A22R gene is deleted and replaced by an inducible copy. Polyacrylamide gel electrophoresis and Western blot analysis of extracts and purified virions from cells infected with vA22-HA revealed that the resolvase was expressed after the onset of DNA replication and incorporated into virion cores. vA22i exhibited a conditional replication defect. In the absence of an inducer, (i) viral protein synthesis was unaffected, (ii) late-stage viral DNA replication was reduced, (iii) most of the newly synthesized viral DNA remained in a branched or concatemeric form that caused it to be trapped at the application site during pulsed-field gel electrophoresis, (iv) cleavage of concatemer junctions was inhibited, and (v) virion morphogenesis was arrested at an immature stage. These data indicated multiple roles for the vaccinia virus Holliday junction resolvase in the replication and processing of viral DNA into unit-length genomes.

Vaccinia virus F12L protein is required for actin tail formation, normal plaque size, and virulence

Vaccinia virus gene F12L is shown to encode a 65-kDa protein that is synthesized early and late during infection and that is not modified by glycosylation. Computational sequence comparison revealed that related proteins are encoded by all sequenced chordopoxviruses. A virus deletion mutant lacking the F12L gene (vDeltaF12L) and a revertant virus with the F12L gene reinserted into the deletion mutant (vF12L-rev) were constructed and analyzed. A version of the F12L gene with a C-terminal amino acid tag derived from the influenza virus hemagglutinin and that is recognized by a monoclonal antibody was also inserted into the F12L locus of vDeltaF12L. Loss of the F12L protein reduced the formation of IMV 2-fold, but there was a dramatic (99.5%) reduction in actin tail formation, and the levels of cell-associated enveloped virus and extracellular enveloped virus were reduced 8- to 11-fold and 7-fold, respectively. Consistent with the lack of actin tail formation, vDeltaF12L produced a very small plaque. The vDeltaF12L virus was severely attenuated in vivo, such that a dose of vDeltaF12L 10,000-fold greater than the dose of wild-type virus that induced severe disease was unable to induce disease in mice infected intranasally.

The cowpox virus SPI-3 and myxoma virus SERP1 serpins are not functionally interchangeable despite their similar proteinase inhibition profiles in vitro

The myxoma virus (MYX) serpin SERP1 is a secreted glycoprotein with anti-inflammatory activity that is required for full MYX virulence in vivo. The cowpox virus (CPV) serpin SPI-3 (vaccinia virus ORF K2L) is a nonsecreted glycoprotein that blocks cell-cell fusion, independent of serpin activity, and is not required for virulence of vaccinia virus or CPV in mice. Although SPI-3 has only 29% overall identity to SERP1, both serpins have arginine at the P1 position in the reactive center loop, and SPI-3 has a proteinase inhibitory profile strikingly similar to that of SERP1 [Turner, P. C., Baquero, M. T., Yuan, S., Thoennes, S. R., and Moyer, R. W. (2000) Virology 272, 267-280]. To determine whether SPI-3 and SERP1 were functionally equivalent, a CPV variant was constructed where the SPI-3 gene was deleted and replaced with the SERP1 gene regulated by the SPI-3 promoter. Cells infected with CPVDeltaSPI-3::SERP1 secrete SERP1 and show extensive fusion, suggesting that SERP1 is unable to functionally substitute for SPI-3 in fusion inhibition. In the reciprocal experiment, both copies of SERP1 were deleted from MYX and replaced with SPI-3 under the control of the SERP1 promoter. Cells infected with the MYXDeltaSERP1::SPI-3 recombinant unexpectedly secreted SPI-3, suggesting either that the cellular secretory pathway is enhanced by MYX or that CPV encodes a protein that prevents SPI-3 secretion. MYXDeltaSERP1::SPI-3 was as attenuated in rabbits as MYXDeltaSERP1::lacZ, indicating that SPI-3 cannot substitute for SERP1 in MYX pathogenesis.

Vaccinia virus gene B7R encodes an 18-kDa protein that is resident in the endoplasmic reticulum and affects virus virulence

This paper presents a characterisation of vaccinia virus (VV) gene B7R that was predicted to encode a polypeptide of 182 amino acids with an N-terminal signal peptide. In vitro transcription and translation analysis showed the B7R gene product was a 21-kDa protein that, in the presence of microsomes, was processed into an 18-kDa mature form. The 18-kDa form associated with the microsomal membranes and was within the lumen of the vesicle where it was inaccessible to exogenous protease or an antibody raised against the B7R C terminus. Within VV-infected cells, the 18-kDa form of B7R was detected late during infection in the endoplasmic reticulum where it colocalised with protein disulphide isomerase. The B7R protein was detected neither in the culture supernatant nor associated with virus particles. A virus deletion mutant lacking the B7R gene and a revertant virus were constructed. Compared to wild-type and revertant viruses, the deletion mutant replicated normally in cell culture and had unaltered virulence in a murine intranasal model of infection. However, the deletion mutant was attenuated in a murine intradermal model where it induced a smaller lesion than the control viruses.

Recombinant vaccinia viruses. Design, generation, and isolation

The technologies of recombinant gene expression have greatly enhanced the structural and functional analyses of genetic elements and proteins. Vaccinia virus, a large double-stranded DNA virus and the prototypic and best characterized member of the poxvirus family, has been an instrumental tool among these technologies and the recombinant vaccinia virus system has been widely employed to express genes from eukaryotic, prokaryotic, and viral origins. Vaccinia virus is also the prototype live viral vaccine and serves as the basis for well established viral vectors which have been successfully evaluated as human and animal vaccines for infectious diseases and as anticancer vaccines in a variety of animal model systems. Vaccinia virus technology has also been instrumental in a number of unique applications, from the discovery of new viral receptors to the synthesis and assembly of other viruses in culture. Here we provide a simple and detailed outline of the processes involved in the generation of a typical recombinant vaccinia virus, along with an up to date review of relevant literature.

The vaccinia virus A4OR gene product is a nonstructural, type II membrane glycoprotein that is expressed at the cell surface

Gene A40R from vaccinia virus (VV) strain Western Reserve has been characterized. The open reading frame (ORF) was predicted to encode a 159 amino acid, 18152 Da protein with amino acid similarity to C-type animal lectins and to the VV A34R protein, a component of extracellular enveloped virus (EEV). Northern blotting and S1 nuclease mapping showed that gene A40R is transcribed early during infection from a position 12 nucleotides upstream of the ORF, producing a transcript of approximately 600 nucleotides. Rabbit anti-sera were raised against bacterial fusion proteins containing parts of the A40R protein. These were used to identify an 18 kDa primary translation product and N- and O-glycosylated forms of 28, 35 and 38 kDa. The A40R proteins were detected early during infection, formed higher molecular mass complexes under non-reducing conditions and were present on the cell surface but absent from virions. The proteins partitioned with integral membrane proteins in Triton X-114. Canine pancreatic microsomal membranes protected in vitro-translated A40R from proteinase K digestion, suggesting the A40R protein has type II membrane topology. A mutant virus with the A40R gene disrupted after amino acid 50, so as to remove the entire lectin-like domain, and a revertant virus were constructed. Disruption of the A40R gene did not affect virus plaque size, in vitro growth rate and titre, EEV formation, or virus virulence in a murine intranasal model.

Protection against herpes B virus infection in rabbits with a recombinant vaccinia virus expressing glycoprotein D

Herpes B virus infects naturally monkeys of the macaque genus in whom it can cause recurrent oral and genital lesions. However, when the virus infects humans it causes a neurological illness with a high case fatality rate. Successful treatment is possible but this depends on diagnosis prior to the onset of respiratory arrest, and fatalities over the last 10 years have been the result of late or no diagnostic data on which to base anti-viral intervention. An effective vaccine would be an ideal way to combat the risk of herpes B virus disease in humans working with potentially infected monkeys or their tissues. A recombinant vaccinia virus expressing herpes B virus glycoprotein D (gD) was constructed and rabbits inoculated with the chimeric virus were tested for immunoglobulin responses to herpes B virus by virus neutralisation, ELISA and Western blot analyses. Anti-gD humoral responses were detected in all vaccinated animals by ELISA and Western blot but neutralising antibody was not detected prior to challenge with herpes B virus. Non-vaccinated rabbits died within 8 days of challenge while 10/11 vaccinated animals were protected against herpes B virus disease. No antibodies to herpes B virus proteins other than gD were detectable in surviving animals, suggesting minimal herpes B virus replication post challenge. Autopsies were carried out on 4/10 rabbits which had remained healthy at 31 days post challenge and the dorsal root ganglia adjacent to the inoculation site were removed. Attempts to detect herpes B virus DNA by PCR followed by hybridisation proved negative suggesting protection against latent herpes B virus infection.

Functional analysis of vaccinia virus B5R protein: role of the cytoplasmic tail

Vaccinia extracellular enveloped virus (EEV) is important for cell-to-cell and long-range virus spread both in vitro and in vivo. Six genes have been identified that encode protein constituents of the EEV outer membrane, and some of these proteins are critical for EEV formation. The B5R gene encodes an EEV-specific type I membrane protein, and deletion of this gene markedly decreases EEV formation and results in a small plaque phenotype. Data suggest that the transmembrane domain, cytoplasmic tail, or both contain the EEV localization signals that are required for targeting of the B5R protein to EEV and for EEV formation. Here, we report the construction of mutant vaccinia viruses in which the wild-type B5R gene was replaced with a mutated one that encodes a protein with the putative cytoplasmic tail deleted. The mutated protein showed normal intracellular distribution and was properly incorporated into EEV. Vaccinia viruses expressing the B5R protein lacking the cytoplasmic tail formed plaques that were similar in type and size to those formed by wild-type viruses and produced equivalent amounts of infectious EEV. These results indicate that the B5R cytoplasmic tail is not necessary for EEV formation and points to the transmembrane domain as the major determinant for targeting the B5R protein to the outer membrane of EEV and for supporting EEV formation.

Serp2, an inhibitor of the interleukin-1beta-converting enzyme, is critical in the pathobiology of myxoma virus

Recently, myxoma virus was shown to encode an additional member of the serpin superfamily. The viral gene, called serp2, was cloned, and the Serp2 protein was shown to specifically bind to interleukin-1beta (IL-1beta)-converting enzyme (ICE), thus inhibiting the cleavage of pro-IL-1beta by the protease (F. Petit, S. Bertagnoli, J. Gelfi, F. Fassy, C. Boucraut-Baralon, and A. Milon, J. Virol. 70:5860-5866, 1996). Here, we address the role of Serp2 in the development of myxomatosis, a lethal infectious disease of the European rabbit. A Serp2 mutant myxoma virus was constructed by disruption of the single-copy serp2 gene and insertion of the Escherichia coli gpt gene serving as the selectable marker. A revertant virus was obtained by replacing the E. coli gpt gene by the intact serp2 open reading frame. The Serp2(-) mutant virus replicated with wild-type kinetics both in rabbit fibroblasts and a rabbit CD4(+) T-cell line (RL5). Moderate reduction of cell surface levels of major histocompatibility complex I was observed after infection with wild-type or Serp2(-) mutant myxoma virus, and both produced white pocks on the chorioallantoic membrane of the chick embryo. After the infection of European rabbits, the Serp2(-) mutant virus proved to be highly attenuated compared to wild-type myxoma virus, as demonstrated by the clinical course of myxomatosis and the survival rates of infected animals. Pathohistological examinations revealed that infection with wild-type myxoma virus resulted in a blockade of the inflammatory response at the vascular level. In contrast, rapid inflammatory reactions occurred upon infection with the Serp2(-) mutant virus. Furthermore, lymphocytes in lymph nodes derived from animals inoculated with Serp2 mutant virus were shown to rapidly undergo apoptosis. We postulate that the virulence of myxoma virus in the European rabbit can be partially attributed to an impairment of host inflammatory processes and to the prevention of apoptosis in lymphocytes. The weakening of host defense is directly linked to serp2 gene function and is likely to involve the inhibition of IL-1beta-converting-enzyme-dependent pathways.

Dominant host range selection of vaccinia recombinants by rescue of an essential gene

We report the rescue of a defective vaccinia virus, forming the basis for a stringent selection protocol to generate replicating recombinant virus without the need for marker cassettes and selection agents. Plaques of recombinant virus could be isolated solely by their ability to grow in wild-type cells normally supporting the growth of vaccinia virus. All growth-competent clones analyzed contained the gene of interest in the intended genomic locus and displayed foreign gene expression to the same levels as was seen with classical recombinants obtained by insertion into the vaccinia virus thymidine kinase locus. The system is based on a defective vaccinia virus, expressing exclusively early genes, termed eVAC-1, and an insertion plasmid vector providing the essential function, the uracil DNA glycosylase gene. In addition, the defective virus is free of selection and color marker genes, thus also representing a basic vector for the generation of defective recombinants.

Functional analysis of vaccinia virus B5R protein: essential role in virus envelopment is independent of a large portion of the extracellular domain

Vaccinia virus has two forms of infectious virions: the intracellular mature virus and the extracellular enveloped virus (EEV). EEV is critical for cell-to-cell and long-range spread of the virus. The B5R open reading frame (ORF) encodes a membrane protein that is essential for EEV formation. Deletion of the B5R ORF results in a dramatic reduction of EEV, and as a consequence, the virus produces small plaques in vitro and is highly attenuated in vivo. The extracellular portion of B5R is composed mainly of four domains that are similar to the short consensus repeats (SCRs) present in complement regulatory proteins. To determine the contribution of these putative SCR domains to EEV formation, we constructed recombinant vaccinia viruses that replaced the wild-type B5R gene with a mutated gene encoding a B5R protein lacking the SCRs. The resulting recombinant viruses produced large plaques, indicating efficient cell-to-cell spread in vitro, and gradient centrifugation of supernatants from infected cells confirmed that EEV was formed. In contrast, phalloidin staining of infected cells showed that the virus lacking the SCR domains was deficient in the induction of thick actin bundles. Thus, the highly conserved SCR domains present in the extracellular portion of the B5R protein are dispensable for EEV formation. This indicates that the B5R protein is a key viral protein with multiple functions in the process of virus envelopment and release. In addition, given the similarity of the extracellular domain to complement control proteins, the B5R protein may be involved in viral evasion from host immune responses.

Poxviruses as expression vectors

Poxviruses are widely used for the cytoplasmic expression of recombinant genes in mammalian cells. Recent improvements allow high expression and simplify the integration of multiple foreign genes. Vaccinia virus mutants and other poxviruses that undergo abortive infection in mammalian cells are receiving special attention because of their diminished cytopathic effects and increased safety. New replicating and 'non-replicating' vectors, encoding the bacteriophage T7 RNA polymerase for transcription of recombinant genes, have been engineered.

Genetically engineered poxviruses for recombinant gene expression, vaccination, and safety

Vaccinia virus, no longer required for immunization against smallpox, now serves as a unique vector for expressing genes within the cytoplasm of mammalian cells. As a research tool, recombinant vaccinia viruses are used to synthesize and analyze the structure-function relationships of proteins, determine the targets of humoral and cell-mediated immunity, and investigate the types of immune response needed for protection against specific infectious diseases and cancer. The vaccine potential of recombinant vaccinia virus has been realized in the form of an effective oral wild-life rabies vaccine, although no product for humans has been licensed. A genetically altered vaccinia virus that is unable to replicate in mammalian cells and produces diminished cytopathic effects retains the capacity for high-level gene expression and immunogenicity while promising exceptional safety for laboratory workers and potential vaccine recipients.

A mechanism for the inhibition of fever by a virus

Poxviruses encode proteins that block the activity of cytokines. Here we show that the study of such virulence factors can contribute to our understanding of not only virus pathogenesis but also the physiological role of cytokines. Fever is a nonspecific response to infection that contributes to host defense. Several cytokines induce an elevation of body temperature when injected into animals, but in naturally occurring fever it has been difficult to show that any cytokine has a critical role. We describe the first example of the suppression of fever by a virus and the molecular mechanism leading to it. Several vaccinia virus strains including smallpox vaccines express soluble interleukin 1 (IL-1) receptors, which bind IL-1 beta but not IL-1 alpha. These viruses prevent the febrile response in infected mice, whereas strains that naturally or through genetic engineering lack the receptor induce fever. Repair of the defective IL-1 beta inhibitor in the smallpox vaccine Copenhagen, a more virulent virus than the widely used vaccine strains Wyeth and Lister, suppresses fever and attenuates the disease. The vaccinia-induced fever was inhibited with antibodies to IL-1 beta. These findings provide strong evidence that IL-1 beta, and not other cytokines, is the major endogenous pyrogen in a poxvirus infection.

Selection of recombinant vaccinia viruses on the basis of plaque formation

We developed a procedure for isolation of recombinant vaccinia viruses (re-VV) based solely on plaque formation, without a requirement for specific cell lines, selective medium or special staining. The system consists of two components: (i) a mutant non-plaque-forming VV and (ii) a plasmid vector that, through homologous recombination, can simultaneously introduce a foreign gene and repair mutation in the VV genome. The mutant VV contains a deletion of the vp37 gene, encoding a 37-kDa protein component of the viral outer envelope that is required for efficient viral spread on cell monolayers. The plasmid vector contains a functional vp37, a strong synthetic VV early/late promoter, unique restriction sites for gene insertion, and flanking segments of VV DNA for homologous recombination. Following infection and transfection of cells with the mutant VV and plasmid vector, respectively, re-VV are identified and isolated by their ability to form plaques. To evaluate the system, a re-VV that expresses the gene encoding influenza virus hemagglutinin (HA) was isolated simply by picking visible plaques.

Vaccinia virus encodes a soluble type I interferon receptor of novel structure and broad species specificity

Vaccinia virus (VV) and other orthopoxviruses express a soluble type I interferon (IFN) receptor that for VV strain Western Reserve is encoded by gene B18R. The 60-65 kDa glycoprotein is related to the interleukin-1 receptors and is a member of the immunoglobulin superfamily, unlike other type I IFN receptors, which belong to the class II cytokine receptor family. The receptor has high affinity (KD, 174 pM) for human IFN alpha and, unlike other type I IFN receptors, has broad species specificity, binding to human, rabbit, bovine, rat, and mouse type I IFNs. This may have aided VV replication in multiple host species during evolution. A VV B18R deletion mutant is attenuated in a murine intranasal model. This type I IFN receptor represents the fourth VV protein that interferes with IFN and the fourth soluble cytokine receptor expressed by poxviruses.

Characterization of a vaccinia virus-encoded 42-kilodalton class I membrane glycoprotein component of the extracellular virus envelope

Using a reverse genetic approach, we have demonstrated that the product of the B5R open reading frame (ORF), which has homology with members of the family of complement control proteins, is a membrane glycoprotein present in the extracellular enveloped (EEV) form of vaccinia virus but absent from the intracellular naked (INV) form. An antibody (C'-B5R) raised to a 15-amino-acid peptide from the translated B5R ORF reacted with a 42-kDa protein (gp42) found in vaccinia virus-infected cells and cesium chloride-banded EEV but not INV. Under nonreducing conditions, an 85-kDa component, possibly representing a hetero- or homodimeric form of gp42, was detected by both immunoprecipitation and Western immunoblot analysis. Metabolic labeling with [3H]glucosamine and [3H]palmitate revealed that the B5R product is glycosylated and acylated. The C-terminal transmembrane domain of the protein was identified by constructing a recombinant vaccinia virus that overexpressed a truncated, secreted form of the B5R ORF product. By N-terminal sequence analysis of this secreted protein, the site of signal peptide cleavage of gp42 was determined. A previously described monoclonal antibody (MAb 20) raised to EEV, which immunoprecipitated a protein with biochemical characteristics similar to those of wild-type gp42, reacted with the recombinant, secreted product of the B5R ORF. Immunofluorescence of wild-type vaccinia virus-infected cells by using either MAb 20 or C'-B5R revealed that the protein is expressed on the cell surface and within the cytoplasm. Immunogold labeling of EEV and INV with MAb 20 demonstrated that the protein was found exclusively on the EEV membrane.

A soluble receptor for interleukin-1 beta encoded by vaccinia virus: a novel mechanism of virus modulation of the host response to infection

Vaccinia virus gene B15R is shown to encode an abundant, secretory glycoprotein that functions as a soluble interleukin-1 (IL-1) receptor. This IL-1 receptor has novel specificity since, in contrast with cellular counterparts, it binds only IL-1 beta and not IL-1 alpha or the natural competitor IL-1 receptor antagonist. The vaccinia IL-1 beta receptor is secreted when expressed in a baculovirus system and competitively inhibited binding of IL-1 beta to the natural receptor on T cells. Deletion of B15R from vaccinia virus accelerated the appearance of symptoms of illness and mortality in intranasally infected mice, suggesting that the blockade of IL-1 beta by vaccinia virus can diminish the systemic acute phase response to infection and modulate the severity of the disease. The IL-1 beta binding activity is present in other orthopoxviruses.

Vaccinia virus vectors for gene expression

Improvements to vaccinia virus expression vectors continue to be made. In particular, there are new methods for the construction of recombinant viruses, ways of increasing the level of gene expression, and vectors that allow the inducible expression of selected genes.

Vaccinia virus: a tool for research and vaccine development

Vaccinia virus is no longer needed for smallpox immunization, but now serves as a useful vector for expressing genes within the cytoplasm of eukaryotic cells. As a research tool, recombinant vaccinia viruses are used to synthesize biologically active proteins and analyze structure-function relations, determine the targets of humoral- and cell-mediated immunity, and investigate the immune responses needed for protection against specific infectious diseases. When more data on safety and efficacy are available, recombinant vaccinia and related poxviruses may be candidates for live vaccines and for cancer immunotherapy.