A poxvirus bidirectional promoter element with early/late and late functions

The Development of Dual Vaccines against Lumpy Skin Disease (LSD) and Bovine Ephemeral Fever (BEF)

Dual vaccines (n = 6) against both lumpy skin disease (LSD) and bovine ephemeral fever (BEF) were constructed, based on the BEFV glycoprotein (G) gene, with or without the BEFV matrix (M) protein gene, inserted into one of two different LSDV backbones, nLSDV∆SOD-UCT or nLSDVSODis-UCT. The inserted gene cassettes were confirmed by PCR; and BEFV protein was shown to be expressed by immunofluorescence. The candidate dual vaccines were initially tested in a rabbit model; neutralization assays using the South African BEFV vaccine (B-Phemeral) strain showed an African consensus G protein gene (Gb) to give superior neutralization compared to the Australian (Ga) gene. The two LSDV backbones expressing both Gb and M BEFV genes were tested in cattle and shown to elicit neutralizing responses to LSDV as well as BEFV after two inoculations 4 weeks apart. The vaccines were safe in cattle and all vaccinated animals were protected against virulent LSDV challenge, unlike a group of control naïve animals, which developed clinical LSD. Both neutralizing and T cell responses to LSDV were stimulated upon challenge. After two inoculations, all vaccinated animals produced BEFV neutralizing antibodies ≥ 1/20, which is considered protective for BEF.

Advancements in the Growth and Construction of Recombinant Lumpy Skin Disease Virus (LSDV) for Use as a Vaccine Vector

Attenuated vaccine strains of lumpy skin disease virus (LSDV) have become increasingly popular as recombinant vaccine vectors, to target both LSDV, as well as other pathogens, including human infectious agents. Historically, these vaccine strains and recombinants were generated in primary (lamb) testis (LT) cells, Madin–Darby bovine kidney (MDBK) cells or in eggs. Growth in eggs is a laborious process, the use of primary cells has the potential to introduce pathogens and MDBK cells are known to harbor bovine viral diarrhea virus (BVDV). In this study, data is presented to show the growth of an attenuated LSDV strain in baby hamster kidney (BHK-21) cells. Subsequently, a recombinant LSDV vaccine was generated in BHK-21 cells. Partial growth was also observed in rabbit kidney cells (RK13), but only when the vaccinia virus host range gene K1L was expressed. Despite the limited growth, the expression of K1L was enough to serve as a positive selection marker for the generation of recombinant LSDV vaccines in RK13 cells. The simplification of generating (recombinant) LSDV vaccines shown here should increase the interest for this platform for future livestock vaccine development and, with BHK-21 cells approved for current good manufacturing practice, this can be expanded to human vaccines as well.

Genomic Characterisation of a Novel Avipoxvirus Isolated from an Endangered Yellow-Eyed Penguin (Megadyptes antipodes)

Emerging viral diseases have become a significant concern due to their potential consequences for animal and environmental health. Over the past few decades, it has become clear that viruses emerging in wildlife may pose a major threat to vulnerable or endangered species. Diphtheritic stomatitis, likely to be caused by an avipoxvirus, has been recognised as a significant cause of mortality for the endangered yellow-eyed penguin (Megadyptes antipodes) in New Zealand. However, the avipoxvirus that infects yellow-eyed penguins has remained uncharacterised. Here, we report the complete genome of a novel avipoxvirus, penguinpox virus 2 (PEPV2), which was derived from a virus isolate obtained from a skin lesion of a yellow-eyed penguin. The PEPV2 genome is 349.8 kbp in length and contains 327 predicted genes; five of these genes were found to be unique, while a further two genes were absent compared to shearwaterpox virus 2 (SWPV2). In comparison with penguinpox virus (PEPV) isolated from an African penguin, there was a lack of conservation within the central region of the genome. Subsequent phylogenetic analyses of the PEPV2 genome positioned it within a distinct subclade comprising the recently isolated avipoxvirus genome sequences from shearwater, canary, and magpie bird species, and demonstrated a high degree of sequence similarity with SWPV2 (96.27%). This is the first reported genome sequence of PEPV2 from a yellow-eyed penguin and will help to track the evolution of avipoxvirus infections in this rare and endangered species.

Viral 2A-peptides mediate continuous transcription and self-cleavage of multiple heterologous genes in fowlpox virus vector

Applicability of viral 2A-peptides in generation of multi-cistronic transcripts to deliver separate self-cleaved proteins is well established. However, the use of viral 2A-peptides in fowlpox virus vector construction to co-express multiple heterologous genes has not been explored. To evaluate the same, a recombinant transfer plasmid pJFWPVt was constructed through two intermediate plasmid constructs, pJF7F9 and pJFHNGFP. The construction of pJF7F9 involved cloning of F7 and F9 genes of FWPV into pCI-neo with modifications in the F7-F9 intergenic region. For the construction of pJFHNGFP, a synthetic DNA adapter consisting of one synthetic early late promoter (PE/L), two viral 2A-peptides (P2A and T2A) and three multiple cloning sites (MCS1, MCS2 and MCS3) was synthesized chemically and was cloned into pUC19 to obtain pJFHNGFPi. Heterologous genes fusion (F) and haemagglutininneuraminidase (HN) of Avian Avulavirus-1 (AAv1) and marker gene AcGFP were cloned sequentially into MCS1, MCS2 and MCS3 of pJFHNGFPi to obtain pJFHNGFP. The insert (PE/L-F-P2A-HN-T2A-AcGFP) in pJFHNGFP was cloned into pJF7F9 to obtain pJFWPVt, which upon transfection in FWPV infected chicken embryo fibroblast (CEF) cells resulted in fluorescence. This confirmed the expression of AcGFP and the continuous transcription ability of viral 2A-peptides. Further, western blotting of CEF pellet showed separate protein bands of F and HN protein at 66 kDa and 74 kDa respectively, which confirmed the self-cleaving ability of viral 2A-peptides. Herein, in FWPV vector construction, continuous transcription and self-cleaving ability of viral 2A-peptides in FWPV vector construction was confirmed. This warrants scope for future viral 2A-peptide based FWPV vector construction.

Identification and functional analysis of the GTPV bidirectional promoter region

The goat pox chick embryo-attenuated virus (GTPV) has been developed as an effective vaccine that can elicit protective immune responses. It possesses a large genome and a robust ability to express exogenous genes. Thus, this virus is an ideal vector for recombinant live vaccines for infectious diseases in ruminant animals. In this study, we identified a novel bidirectional promoter region of GTPV through screening named PbVV(±). PbVV(±) is located between ETF-l and VITF-3, which are transcribed in opposite directions. A new recombinant goat pox virus (rGTPV) was constructed, in which duplicate PbVV(+) was used as a promoter element to enhance Brucella OMP31 expression, and duplicate PbVV(-) was used as a promoter element to regulate enhanced green fluorescent protein (EGFP) at the same time as the selection marker. PbVV(-) promoter activity was compared to that of the P7.5 promoter of vaccinia virus, as measured by EGFP expression; the fluorescence intensity of EGFP expressed in cells was confirmed by fluorescence microscopy and flow cytometry. PbVV(+) promoter activity was measured by Brucella OMP31 expression. Interaction with the anti-Brucella-OMP31 monoclonal antibody was confirmed by western blotting, and OMP31 mRNA expression was assessed by qRT-PCR. The results of this study will be useful for the further study of effective multivalent vaccines based on rGTPV. This study also provides a theoretical basis for overcoming the problem of low expression of exogenous genes.

Fowlpox virus as a recombinant vaccine vector for use in mammals and poultry

Live vaccines against fowlpox virus, which causes moderate pathology in poultry and is the type species of the Avipoxvirus genus, were developed in the 1920s. Development of recombinant fowlpox virus vector vaccines began in the 1980s, for use not only in poultry, but also in mammals including humans. In common with other avipoxviruses, such as canarypox virus, fowlpox virus enters mammalian cells and expresses proteins, but replicates abortively. The use of fowlpox virus as a safe vehicle for expression of foreign antigens and host immunomodulators, is being evaluated in numerous clinical trials of vaccines against cancer, malaria, tuberculosis and AIDS, notably in heterologous prime-boost regimens. In this article, technical approaches to, and issues surrounding, the use of fowlpox virus as a recombinant vaccine vector in poultry and mammals are reviewed.

Antigen-specific T-cell responses to a recombinant fowlpox virus are dependent on MyD88 and interleukin-18 and independent of Toll-like receptor 7 (TLR7)- and TLR9-mediated innate immune recognition

Fowlpox virus (FWPV) is a double-stranded DNA virus long used as a live-attenuated vaccine against poultry diseases, but more recent interest has focused on its use as a mammalian vaccine vector. Here, in a mouse model system using FWPV encoding the nominal target antigen chicken ovalbumin (OVA) (FWPV(OVA)), we describe for the first time some of the fundamental processes by which FWPV engages both the innate and adaptive immune systems. We show that Toll-like receptor 7 (TLR7) and TLR9 are important for type I interferon secretion by dendritic cells, while TLR9 is solely required for proinflammatory cytokine secretion. Despite this functional role for TLR7 and TLR9 in vitro, only the adapter protein myeloid differentiation primary response gene 88 (MyD88) was shown to be essential for the formation of adaptive immunity to FWPV(OVA) in vivo. The dependence on MyD88 was confined only to the T-cell compartment and was not related to its contribution to TLR signaling, dendritic cell maturation, or the capture and presentation of FWPV-derived OVA antigen. We demonstrate that this is not by means of mediating T-cell-dependent interleukin-1 (IL-1) signaling, but rather, we suggest that MyD88 functions to support T-cell-specific IL-18 receptor signaling, which in turn is essential for the formation of adaptive immunity to FWPV-encoded OVA.

Construction of recombinant fowlpox viruses carrying multiple vaccine antigens and immunomodulatory molecules

Here we describe plasmid vectors and selection protocols developed to allow the construction of recombinant fowlpox viruses (rFPVs) with up to three insertions of foreign DNA in the viral genome. Transient dominant selection allows the construction of recombinant viruses that do not retain the selection markers and can therefore be used for the insertion of additional genes at other sites in the viral genome. A SYBR Green real-time PCR sequence detection assay was applied to the identification of recombinant viruses from individual plaques, eliminating the need for amplification and hybridization from the transient dominant protocol and resulting in significant savings in time at each round of plaque purification. Dominant selection techniques allow more rapid recombinant virus construction; however, as the markers are retained along with the gene of interest, they can only be used to generate the final recombinant. rFPV vaccines constructed using these techniques have reached preclinical nonhuman primate and phase I human clinical trials in prime/boost vaccination studies as human immunodeficiency virus (HIV) therapeutic andprophylactic vaccines.

Replication and expression of a swinepox virus vector delivering feline leukemia virus Gag and Env to cell lines of swine and feline origin

The host range of swinepox virus (SPV) is restricted to swine, although SPV has been shown to infect mammalian, non-swine cells, without recovery of infectious virus. SPV is a reasonable candidate for development as a non-productively replicating viral vector for use in non-swine, mammalian species, such as the cat. A novel SPV gene deletion (SPV 043) was created and found to be non-attenuating. This deletion was utilized to generate a stable recombinant virus expressing the Gag-Pro and Env proteins of feline leukemia virus (FeLV). Expression and replication of this vector was studied in embryonic swine kidney cells (ESK-4), and two feline cell lines, Crandell feline kidney cells (CRFK) and feline skin fibroblasts (FSF). Our results showed that feline cells were susceptible to infection by SPV and supported expression of foreign genes driven by synthetic poxvirus promoters, however, SPV viral DNA was not replicated in feline cells and infectious virus was not recovered. In addition, FeLV Gag virus-like particles were produced from both ESK-4 and CRFK cells and foreign antigens were incorporated into infectious SPV intracellular mature virions (IMV). These results suggest that SPV may have potential as a safe vaccine delivery vector for cats.

A consideration of previously uncharacterized fowl poxvirus unidirectional and bidirectional late promoters for inclusion in homologous recombinant vaccines

Because of the limited analysis of fowl poxvirus (FPV) promoters, expression of foreign proteins by recombinant FPV has usually been directed by heterologous vaccinia virus or synthetic poxvirus promoters. Thus, the impact of completely homologous recombinant virus vaccines has yet to be realized by the poultry industry. In an effort to increase the availability of such transcriptional regulatory elements, the modulation of gene expression by six previously uncharacterized FPV late promoters was examined. To simplify this comparison, each promoter region was separately coupled to the same reporter gene (lacZ) in individual plasmid constructs, and their activities in transfected, virus-infected cells were monitored. In each of the four selected unidirectional transcriptional regulatory elements as well as a 30-base pair representative of the bidirectional promoter region, the predicted temporal specificity of expressing at late stages of virus replicative cycle was verified. Stable lacZ gene transcripts arising from each plasmid varied less than threefold in quantity, whereas the amounts of beta-galactosidase product ranged within a 130-fold interval. Only the promoter that naturally regulates expression of the A type inclusion body protein gene directed production of beta-galactosidase at a level comparable with that associated with the strong vaccinia virus P11 promoter. Because one of the remaining unidirectional transcriptional regulatory elements, P174, was only 2.4-fold less efficient, both of these promoters, P174 and P190, should be satisfactory for directing the expression of poultry pathogen genes inserted into the genomes of FPV recombinant vaccines.

The genome of fowlpox virus

Here we present the genomic sequence, with analysis, of a pathogenic fowlpox virus (FPV). The 288-kbp FPV genome consists of a central coding region bounded by identical 9.5-kbp inverted terminal repeats and contains 260 open reading frames, of which 101 exhibit similarity to genes of known function. Comparison of the FPV genome with those of other chordopoxviruses (ChPVs) revealed 65 conserved gene homologues, encoding proteins involved in transcription and mRNA biogenesis, nucleotide metabolism, DNA replication and repair, protein processing, and virion structure. Comparison of the FPV genome with those of other ChPVs revealed extensive genome colinearity which is interrupted in FPV by a translocation and a major inversion, the presence of multiple and in some cases large gene families, and novel cellular homologues. Large numbers of cellular homologues together with 10 multigene families largely account for the marked size difference between the FPV genome (260 to 309 kbp) and other known ChPV genomes (178 to 191 kbp). Predicted proteins with putative functions involving immune evasion included eight natural killer cell receptors, four CC chemokines, three G-protein-coupled receptors, two beta nerve growth factors, transforming growth factor beta, interleukin-18-binding protein, semaphorin, and five serine proteinase inhibitors (serpins). Other potential FPV host range proteins included homologues of those involved in apoptosis (e.g., Bcl-2 protein), cell growth (e.g., epidermal growth factor domain protein), tissue tropism (e.g., ankyrin repeat-containing gene family, N1R/p28 gene family, and a T10 homologue), and avian host range (e.g., a protein present in both fowl adenovirus and Marek's disease virus). The presence of homologues of genes encoding proteins involved in steroid biogenesis (e.g., hydroxysteroid dehydrogenase), antioxidant functions (e.g., glutathione peroxidase), vesicle trafficking (e.g., two alpha-type soluble NSF attachment proteins), and other, unknown conserved cellular processes (e.g., Hal3 domain protein and GSN1/SUR4) suggests that significant modification of host cell function occurs upon viral infection. The presence of a cyclobutane pyrimidine dimer photolyase homologue in FPV suggests the presence of a photoreactivation DNA repair pathway. This diverse complement of genes with likely host range functions in FPV suggests significant viral adaptation to the avian host.

Induction of humoral and cellular immune responses in mice by a recombinant fowlpox virus expressing the E2 protein of bovine viral diarrhea virus

A recombinant fowlpox virus (rFPV/E2) expressing the E2 protein of bovine viral diarrhea virus (BVDV) was constructed and characterized. Mice were immunized with recombinant virus and both humoral and cellular immune responses were studied. rFPV/E2 induced BVDV-specific antibodies which were detected by ELISA. In addition, mouse sera were shown to neutralize BVDV. A cytokine ELISA assay revealed that mice vaccinated with rFPV/E2 induced 7-fold more interferon-gamma than parental fowlpox virus.

The 131-amino-acid repeat region of the essential 39-kilodalton core protein of fowlpox virus FP9, equivalent to vaccinia virus A4L protein, is nonessential and highly immunogenic

The immunodominant, 39,000-molecular weight core protein (39K protein) of fowlpox virus (FP9 strain), equivalent to the vaccinia virus A4L gene product, contains highly charged domains at each end of the protein and multiple copies of a 12-amino-acid serine-rich repeat sequence in the middle of the protein. Similar repeats were also detected in other fowlpox virus strains, suggesting that they might confer a selective advantage to the virus. The molloscum contagiosum virus homolog (MC107L) also contains repeats, unlike the vaccinia virus protein. The number of repeats in the fowlpox virus protein does not seem to be crucial, since some strains have a different number of repeats, as shown by the difference in the size of the protein in these strains. The repeat region could be deleted, indicating that it is not essential for replication in vitro. It was not possible to delete the entire 39K protein, indicating that it was essential (transcriptional control signals for the flanking genes were left intact). The repeat region is partly responsible for the immunodominance of the protein, but the C-terminal part of the protein also contains highly antigenic linear epitopes. A role for the 39K protein in immune system modulation is discussed.

Selective induction of immune responses by cytokines coexpressed in recombinant fowlpox virus

Avipoxviruses have recently been studied as potential vectors for the delivery of heterologous vaccine antigen. Because these viruses abortively infect mammalian cells yet still effectively present encoded foreign genes to the host immune system, they offer a safer but effective alternative to other live virus vectors. We have examined the effect of coexpressing the cytokine interleukin-6 or gamma interferon on immune responses to a recombinant fowlpox virus expressing influenza virus hemagglutinin. The encoded cytokine was expressed for prolonged periods in infected cell culture with little cytopathic effect due to the abortive nature of the infection. In mice, vector-expressed cytokine dramatically altered immune responses induced by the coexpressed hemagglutinin antigen. Expression of interleukin-6 augmented both primary systemic and mucosal antibody responses and primed for enhanced recall responses. In contrast, expression of gamma interferon markedly inhibited antibody responses without affecting the generation of cell-mediated immunity. The safety of these constructs was demonstrated in mice with severe combined immunodeficiency, and no side effects due to cytokine expression were observed. In summary, fowlpox virus vectors encoding cytokines represent a safe and effective vaccine strategy which may be used to selectively manipulate the immune response.

Modification of infectious bursal disease virus antigen VP2 for cell surface location fails to enhance immunogenicity

The host protective antigen gene VP2 of infectious bursal disease virus (IBDV) was genetically modified and expressed by recombinant fowlpox viruses (rFPV). To achieve cell surface localization, VP2 was expressed as a hybrid protein with signal sequence and membrane anchors of influenza virus hemagglutinin or neuraminidase. Native VP2 was expressed as VP2 alone or as self-processing VP2-VP4-VP3 polyprotein for coexpression of IBDV structural proteins. VP2 hybrid protein containing the carboxy-terminal membrane anchor sequence of influenza virus hemagglutinin was located on the cell surface and was N-glycosylated. The expression of VP2 fused to the N-terminal signal/anchor sequence of influenza virus neuraminidase led to cell lysis and the VP2 protein remained mainly unglycosylated. Cell surface localization of VP2 reduced immunogenicity (antibody induction) and abolished protection in poultry in comparison with the native VP2 expressed by FPV as VP2 alone or as the self-processing VP2-VP4-VP3. Vaccination of poultry with rFPV expressing native VP2 protein alone provided better protection from IBDV infection than VP2 derived from the VP2-VP4-VP3 polyprotein.

Bidirectional transcription from actin promoters in Dictyostelium

Numerous transformation vectors for Dictyostelium discoideum have been constructed by fusing actin 6 and 15 promoters to selectable markers such as G418 and hygromycin. Strand specific probing of RNAs from transformed cells demonstrates that cells containing these vectors accumulate transcripts representing both strands of the vector. The evidence is consistent with bidirectional transcription initiating in the truncated actin promoter fragments. A larger fragment of the actin 15 promoter largely eliminates this problem.

Infectious bursal disease virus structural protein VP2 expressed by a fowlpox virus recombinant confers protection against disease in chickens

Two fowlpox virus recombinants were constructed which expressed the host-protective antigen, VP2, of infectious bursal disease virus (IBDV). Recombinant FPV-VP 2.4.3 contained the gene for the VP 2-VP4-VP3 polyprotein under the control of the vaccinia virus late promoter P.L 11 inserted within the thymidine kinase (TK) gene of FPV. In infected chicken embryo skin (CES) cells VP2 and VP3 proteins were correctly processed from the polyprotein precursor molecule. Recombinant FPV-VP2 contained only the VP2 encoding region under the control of the fowlpox early/late promoter P.E/L inserted immediately downstream of the TK gene. The expression level of VP2 from FPV-VP2 was approximately 5 times higher than from FPV-VP2.4.3. Wing web inoculation of birds resulted in the development of typical fowlpox lesions and the development of antibodies to FPV with either of the recombinants, but only birds vaccinated with FPV-VP2 developed antibodies to IBDV. When challenged with IBDV (strain 002-73), a significant level of protection was provided by FPV-VP2 vaccination, although the level was lower than the protection provided by an oil adjuvanted inactivated whole IBDV vaccine. Birds vaccinated with FPV-VP2.4.3 were not protected from infection as assessed by ELISA for the presence of IBD virus in bursae.

Characterization and temporal regulation of mRNAs encoded by vaccinia virus intermediate-stage genes

The steady-state levels of mRNAs encoded by three intermediate-stage genes of vaccinia virus, A1L, A2L, and G8R, were compared with those encoded by well-characterized early- and late-stage genes. After synchronous infection of HeLa cells, the early mRNA was detected within 20 min and peaked at about 100 min; all three intermediate mRNAs were detected at 100 min and peaked at about 120 min; and the late mRNA was detected at 140 min and increased thereafter. Upon reaching maximum levels, the early and intermediate mRNAs declined at rates consistent with half-lives of about 30 min, providing the basis for rapid changes in gene expression. Intermediate mRNA was not detected when viral DNA synthesis was prevented, whereas its accumulation was enhanced by blocking translation after removal of the replication inhibitor. The 5' ends of the mRNAs initiated within a TAAAT or TAAAAT sequence in the coding DNA strand but contained a poly(A) leader of up to 30 additional bases. Diffuse bands of A1L and G8R RNA, equal to and longer than the coding region, were resolved by agarose gel electrophoresis, suggesting preferred sites of 3'-end formation that did not correlate with early gene termination signals. The cis-regulatory sequences were investigated by constructing recombinant viruses containing mutated intermediate promoters preceding the beta-galactosidase reporter gene. The effects of mutations on expression were similar to those previously obtained by transfection studies (C.J. Baldick, Jr., J.G. Keck, and B. Moss, J. Virol. 66:4710-4719, 1992), providing further evidence for functional core, spacer, and initiator regions. In addition, an up-regulated bifunctional early/intermediate promoter was created by making four single-base substitutions in the G8R promoter.

Identification and functional analysis of the fowlpox virus homolog of the vaccinia virus p37K major envelope antigen gene

A fowlpox virus (FPV) gene with homology to the vaccinia virus p37K major envelope antigen gene was identified and sequenced. The predicted product has a molecular weight of 43,018 Da (p43K). The FPV p43K gene has 37.5% identity with its vaccinia counterpart and higher homology with a molluscum contagiosum virus gene (42.6% identity). Based on upstream sequences, p43K appears to be regulated as a late gene. Recombinant FPV were generated in which a large portion of p43K was replaced by the Escherichia coli lacZ gene. These recombinants failed to produce visible plaques under standard conditions. After prolonged incubation the microplaques developed into small macroscopic plaques. Plaques were purified on the basis of lacZ expression. Single-cycle growth curves comparing the p43K-deleted recombinant (designated fJd43Z) with parental FPV showed that the two viruses produce identical amounts of intracellular virions, but that fJd43Z released 20-fold fewer infectious particles into the medium. CsCl gradient centrifugation of [3H]thymidine-labeled virus was employed to examine differences in the production of physical particles. The two viruses produced equivalent levels of intracellular virions, but fJd43Z failed to produce detectable levels of released particles. FPV p43K is therefore involved in the release of virions from infected cells.

Quantitative assessment of poxvirus promoters in fowlpox and vaccinia virus recombinants

A comparison was undertaken of poxvirus promoters in vaccinia and fowlpox virus (FPV) recombinants using the level of beta-galactosidase expressed from the LacZ gene as a measure of promoter function. In this study a comparison was made of the vaccinia virus promoters, P 7.5 and P L11, the major late promoter of cowpox virus, P CPX (expressing the abundant inclusion body protein), and the FPV promoters, P E/L and P L. In vaccinia virus recombinants the FPV P E/L promoter expressed one-third to one-half the level of beta-galactosidase expressed by the P L11 promoter. In comparison with the P 7.5 promoter, the FPV P E/L promoter expressed four to five times the level of beta-galactosidase. In FPV recombinants beta-galactosidase activity expressed was equal for the P E/L and P CPX promoters. Levels expressed by P L11 and P L were one-half and one-fifth that level, respectively. The temporal regulation of the promoters was maintained in both vaccinia virus and FPV recombinants. The P E/L promoter of FPV has the TAAATG sequence characteristic of late poxvirus promoters at the transcription initiation site. In an attempt to enhance the utility of this promoter for the expression of foreign genes in FPV and vaccinia virus recombinants, the effect upon promoter function of changing the G of the ATG to A, T, or C was determined using transient expression assays with vaccinia virus. Substitution of A, T, or C for the G abolished promoter function. Because of its early/late function, the level of expression and the presence of the oppositely oriented late P L promoter, the FPV P E/L promoter will be valuable for the expression of foreign genes in poxvirus recombinants.

Expression of bovine leukaemia virus envelope gene by recombinant vaccinia viruses

Recombinant vaccinia viruses (VV) containing the envelope gene of bovine leukaemia virus (BLV) were constructed. Three virus constructs were designed: VV-BLV1 which contained the open reading frame for envelope glycoprotein gp51 alone, under control of VVP7.5 promoter; VV-BLV2 and VV-BLV3 contained the entire gene (gp51 + gp30) coding sequence downstream of VP7.5 and the fowlpox virus early/late promoter (PFE/L) respectively. All three VV recombinants expressed envelope glycoproteins as determined by the agar gel diffusion assay. By immunofluorescence techniques it was shown that while VV-BLV2 and VV-BLV3 expressed envelope glycoprotein on the surface of virus-infected cells, VV-BLV1 failed to do so. Rabbits inoculated with VV-BLV1 failed to show an anti envelope glycoprotein antibody response, however, significant levels of antibodies against envelope glycoprotein were detected in sera from rabbits inoculated with VV-BLV2 and VV-BLV3.