Vaccinia virus-mediated expression of African swine fever virus genes

African swine fever virus transcription

African swine fever virus (ASFV), a large, enveloped, icosahedral dsDNA virus, is currently the only known DNA-containing arbovirus and the only recognized member of the family Asfarviridae. Its genome encodes more than 150 open reading frames that are densely distributed, separated by short intergenic regions. ASFV gene expression follows a complex temporal programming. Four classes of mRNAs have been identified by its distinctive accumulation kinetics. Gene transcription is coordinated with DNA replication that acts as the main switch on ASFV gene expression. Immediate early and early genes are expressed before the onset of DNA replication, whereas intermediate and late genes are expressed afterwards. ASFV mRNAs have a cap 1 structure at its 5'-end and a short poly(A) tail on its 3'-end. Transcription initiation and termination occurs at very precise positions within the genome, producing transcripts of definite length throughout the expression program. ASFV devotes approximately 20% of its genome to encode the 20 genes currently considered to be involved in the transcription and modification of its mRNAs. This transcriptional machinery gives to ASFV a remarkable independence from its host and an accurate positional and temporal control of its gene expression. Here, we review the components of the ASFV transcriptional apparatus, its expression strategies and the relevant data about the transcriptional cis-acting control sequences.

African swine fever virus

African swine fever virus (ASFV) is a large, intracytoplasmically-replicating DNA arbovirus and the sole member of the family Asfarviridae. It is the etiologic agent of a highly lethal hemorrhagic disease of domestic swine and therefore extensively studied to elucidate the structures, genes, and mechanisms affecting viral replication in the host, virus-host interactions, and viral virulence. Increasingly apparent is the complexity with which ASFV replicates and interacts with the host cell during infection. ASFV encodes novel genes involved in host immune response modulation, viral virulence for domestic swine, and in the ability of ASFV to replicate and spread in its tick vector. The unique nature of ASFV has contributed to a broader understanding of DNA virus/host interactions.

The cellular immune recognition of proteins expressed by an African swine fever virus random genomic library

The cellular immune recognition of peptides expressed by an African swine fever virus (ASFV) random genomic library has been studied. DNA from the Malawi (LIL20/1) ASFV isolate was randomly sheared by sonication, cloned into a plasmid vector downstream of a bacteriophage T7 promoter, and 72 recombinant plasmids were arbitrarily selected. These plasmids were transiently expressed following transfection into major histocompatibility complex (MHC) class I(+) class II(-) matched pig skin cells, which had been co-infected with vTF7-3, a recombinant vaccinia virus encoding bacteriophage T7 RNA polymerase. Such cells served as antigen presenting cells and each recombinant plasmid was screened in a proliferation assay for recognition by CD8(+) lymphocytes from inbred pigs previously exposed to ASFV. This assay was demonstrated to measure CD8(+) T cell proliferation, as predicted by the phenotype of the antigen presenting cell. Of the 72 randomly selected clones, 14 were reproducibly recognised by immune pig lymphocytes and 10 corresponded to non-overlapping and distinct nucleic acid sequences. This high frequency of ASFV encoded antigenic epitopes supports the concept that cellular immunity to the virus may play an important role in resistance to ASF.

A novel strategy for determining protective antigens of the parapoxvirus, orf virus

We investigated the feasibility of using vaccinia virus (VAC) recombinants containing large multigene fragments of orf virus DNA to identify protective antigens of orf virus (OV). Sixteen OV strain NZ2 DNA fragments with an average size of 11.4 kb were recombined into VAC strain Lister. Each fragment was mapped relative to OV restriction endonuclease maps but was otherwise uncharacterized. Together the recombinants represent 95% of the OV genome in an overlapping manner. Immunofluorescence showed all 16 constructs expressed products recognized by OV antiserum and radioimmune precipitation with the same antiserum allowed the localization of the major antigens of OV to specific recombinants. These data indicated the approximate genomic locations of the genes encoding the OV major antigens and showed that their expression was authentic rather than resulting from read through from VAC sequences adjacent to the site of recombination. Vaccination of OV-naive sheep with the recombinant library provided protection against a subsequent challenge with virulent OV. These data confirm the feasibility of the proposed strategy.

Three adjacent genes of African swine fever virus with similarity to essential poxvirus genes

Nucleotide sequencing of the right end of the SalIj fragment of the highly virulent Malawi Lil20/1 strain of African swine fever virus (ASFV) has revealed three adjacent genes with similarity to: serine-threonine protein kinases; members of the putative helicase superfamily SF2; and the vaccinia virus 56 kDa abortive late protein. All three genes are transcribed to the left with respect to the orientation of the ASFV genome. Gene L19IL predicts a protein similar to serine-threonine protein kinases including vaccinia virus gene B1R. Gene L19KL predicts a protein that is likely to be a nucleic acid-dependent ATPase, as it has similarity to both the poxvirus 70 kDa early transcription factor subunit and the poxvirus nucleoside triphosphatase I gene. Gene L19LL has extensive similarity to the vaccinia virus 56 kDa abortive late protein.

Approaches to the identification of non-essential genes of African swine fever virus

It is poorly understood why vaccines could not be developed for the control and prevention of African swine fever (ASF) virus infection. The aim of our study was to identify genes non-essential for ASF virus replication because there were indications that certain viral gene products, which apparently are non-essential for viral replication, conferred protection from death due to ASF. A cosmid library representing the genome of ASF virus strain France 64 was established and characterized. Then, in order to inactivate viral genes by insertion, the beta-galactosidase (beta-gal) gene was introduced either randomly or at specific locations of selected cloned DNA fragments. These constructions were transfected into cells which had been previously infected with a cell-culture-adapted viral strain in order to allow the generation of recombinant progeny virus. Viable recombinant progeny was identified by at least one of the following means: (1) expression of beta-gal; (2) detection of beta-gal specific DNA by plaque hybridization, and (3) absence of a functional product of the inactivated gene. Presently, we are characterizing a recombinant virus with an insertionally inactivated thymidine kinase gene.

Transcriptional analysis of multigene family 110 of African swine fever virus

A transcriptional analysis of the 3.2-kb region of the African swine fever virus genome containing the five members of the multigene family 110 is presented. The mRNAs corresponding to the genes studied have short leader sequences with no intervening AUG codons before the translational start site, and their 3' ends map within a conserved sequence motif formed by a stretch of seven or more consecutive thymidylate residues. The possible role of this sequence as a signal for the 3'-end formation of African swine fever virus mRNAs is discussed. While four of the genes studied are actively transcribed from the beginning of the infection until the onset of virus DNA replication, the transcription of one of the members of the multigene family 110, the L270 gene, is silenced at an earlier time. A detailed analysis, including in vitro translation of mRNAs isolated from infected Vero cells, revealed that the L270 gene belongs to a small subset of early genes, designated immediate early, whose transcription is silenced before the onset of virus DNA replication. The transcriptional data obtained enabled us to generate the first detailed transcriptional map of a region of the African swine fever virus genome, thus opening the possibility of studying the cis-acting sequences involved in transcriptional control of the viral genes.

Identification of an African swine fever virus gene with similarity to a myeloid differentiation primary response gene and a neurovirulence-associated gene of herpes simplex virus

Here we describe an open reading frame (LMW23-NL) in the African swine fever virus genome that possesses striking similarity to a murine myeloid differentiation primary response gene (MyD116) and the neurovirulence-associated gene (ICP34.5) of herpes simplex virus. In all three proteins, a centrally located acidic region precedes a highly conserved, hydrophilic 56-amino-acid domain located at the carboxy terminus. LMW23-NL predicts a highly basic protein of 184 amino acids with an estimated molecular mass of 21.3 kDa. The similarity of LMW23-NL to genes involved in myeloid cell differentiation and viral host range suggests a role for it in African swine fever virus host range.

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.

Characterization of p30, a highly antigenic membrane and secreted protein of African swine fever virus

We have identified and characterized a 30-kDa phosphoprotein (p30) of African Swine Fever Virus (ASFV) that is synthesized, membrane localized, and released into the culture medium at early times after infection. Sequence analysis of the p30 open reading frame predicts a highly antigenic protein with putative phosphorylation, glycosylation, and membrane attachment sites.

An African swine fever virus gene with homology to DNA ligases

Sequence analysis of the SalI g region of the genome of a virulent isolate of ASFV (Malawi Lil 20/1) has revealed an open reading frame with the potential to encode a 48 kilodalton (kD) polypeptide which has significant homology with eukaryotic and prokaryotic DNA ligases. This ASFV encoded gene also contains the putative active site region of DNA ligases including the lysine residue which is necessary for enzyme-adenylate adduct formation, but lacks the C-terminal basic region conserved in other eukaryotic DNA ligases. A novel [32P]-labelled potential DNA ligase-adenylate adduct of M(r) 45 kD was observed upon incubation of ASFV infected cell cytoplasmic extracts with alpha-[32P]-ATP and subsequent analysis of products by SDS/PAGE. These data together suggest that ASFV encodes its own DNA ligase.