- Volume 71, Issue 3, 1990

The envelope protein E1 of hog cholera virus (HCV) was isolated by immunoaffinity purification with monoclonal antibodies (MAbs) directed against HCV. E1 consisted of a doublet of glycoproteins which varied in size from 51K to 56K between the three strains tested. E1 contains major antigenic determinants of HCV which are conserved, and are involved in neutralization by MAbs. In infected cells, E1 was found always connected with a glycoprotein of 31K. When N-linked glycans were removed, El had a polypeptide backbone of approximately 47K. After proteolytic cleavage of E1 with Staphylococcus protease V8 and after electrophoresis and electrotransfer, peptide fragments containing different antigenic domains of E1 were detected with MAbs directed against HCV.

In this report we present data indicating that the recently detected monomer covalently closed linear replicative form DNA (Mccl RF DNA) is an intermediate of Aleutian disease virus (ADV) DNA replication. This DNA molecule is characterized by covalently closed terminal hairpins on either end of the linear ds genomic molecule. Its first detection early after infection in vitro, the association with ADV-specific replication complexes and the de novo synthesis of Mccl RF DNA by isolated replication complexes point to an important role in parvoviral DNA replication. The presence of the Mccl RF DNA in extracts of virus- producing bone marrow cells of naturally infected mink rules out the possibility that this DNA form represents a mere cell culture artefact. The detection of the ADV non-structural protein 1 (NS1) in replication complexes supports the view that this protein is involved in ADV DNA replication.

The gene encoding the basic DNA-binding protein (p6·5) of the multicapsid nuclear polyhedrosis virus of Orgyia pseudotsugata (OpMNPV) was localized by Southern blot analysis using a eDNA probe containing the Autographa californica virus (AcMNPV) p6·9 gene. The OpMNPV p6·5 gene was mapped to the HindIII G fragment at map unit 67. Nucleotide sequence and transcriptional analysis of a 3·26 kb region encompassing this area revealed four open reading frames (ORFs 1 to 4) oriented in the same direction. ORF 1 demonstrated a seven codon overlap with ORF 2. Messenger RNAs initiated upstream of each of the four ORFs late in infection and were coterminal at a single site downstream of the fourth ORF. The conserved late gene promoter/mRNA start site sequence (ATAAG) was present upstream of all the ORFs, but did not appear to be the major site of mRNA initiation for the third ORF, as determined by primer extension analysis. The fourth ORF in this series encoded a predicted peptide of 51 amino acids (6·5K), which was 80% similar to the p6·9 basic DNA-binding protein of AcMNPV.

Egtved virus, the rhabdovirus causing viral haemorrhagic septicaemia in rainbow trout, was analysed at the antigen level with a future subunit vaccine in mind. Three monoclonal antibodies to the viral G protein were characterized with respect to neutralizing activity at the cell culture level, as well as their ability to protect rainbow trout fingerlings against virus infection following passive immunization. Two antibodies showed strong protective activity in fish. Only one of these antibodies was able to neutralize viral infectivity in vitro. Reduction of disulphide bonds in the G protein abolished reactivity of this antibody in immunoblot- ting, whereas antigen deglycosylation did not influence the binding ability of any of the antibodies. These data suggest that the G protein contains linear as well as non-linear, carbohydrate-free epitopes, which are involved in the protection against Egtved virus. However, an indirect influence of oligosaccharide side chains on epitope formation could not be excluded, since in situ inhibition of glycosylation prevented the binding of the protecting antibodies in immunofluorescence.

The nucleotide sequence of genome segment A cDNA of the STC strain of infectious bursal disease virus (IBDV) was determined and compared with sequences of the homologous genome segment of the 002–73 strain of IBDV and the Jasper strain of infectious pancreatic necrosis virus (IPNV). The STC-IBDV genome segment A was determined to be 3262 base pairs (bp), which is close to the estimated total length of 3300 bp for genome segment A in IBDV, although there is no proof that it is the real length of this genome segment. The STC-IBDV genome segment A contains two major overlapping open reading frames (ORFs). The large ORF of 3036 bp predicts a polyprotein of M r 109358, whereas the small ORF is 435 bp and predicts a protein of M r 16550 in STC-IBDV. STC-IBDV and 002–73-IBDV polyproteins are closely related (97·4% amino acid homology). Most of the amino acid mismatches are in VP2 sequences, mainly within the area of the conformation-dependent epitope. Comparison with the Jasper-IPNV polyprotein reveals levels of amino acid sequence homology of about 40% in VP2, 32% in VP3 and 21% in VP4. Within the VP2 molecule the conformation-dependent epitope area is again the least homologous, but the heterogeneity is more conspicuous than between the two IBDV strains, which is not surprising since IBDV and IPNV are serologically unrelated. The small ORF proteins have about 88% amino acid sequence homology between STC-IBDV and 002–73-IBDV, and 30% between each IBDV strain and Jasper-IPNV. There is no homology at all in the non-coding regions of IBDV and IPNV. These comparative sequence data will be useful for subgrouping the Birnaviridae family.

Synthesis of the adenovirus type 12 El A and E1B tumour antigens was compared in productively infected human (KB) cells and in abortively infected baby hamster kidney (BHK) cells. By the use of anti-peptide antibodies, the El A tumour antigens were easily detectable in infected KB extracts as early as 6 h postinfection, but were not detectable in infected BHK extracts until 12 h post-infection. The level of El A tumour antigens detected in BHK extracts was 10 to 15 % of that detected in KB extracts. The level of the E1B 163R (19K) tumour antigen was also lower in BHK extracts: 5 to 10 % of that detected in KB extracts. Stability of the El A tumour antigens was not significantly different in the two infected cell species, indicating that the lower El A level during abortive infection was due to a lower rate of synthesis of these proteins. These data suggest that early protein synthesis is not the same in abortively infected cells as it is in productively infected cells.

We have isolated drug-resistant variants from adenovirus-transformed rat cells that had concomitantly lost their transformed phenotype. Our aim was to determine the reason for reversion, to attempt retransformation with 5-azacytidine (5-AzaC) and to study the mechanism of retransformation. Of the three cell lines studied, one (G4F) had lost the integrated E1a genes, whereas the other two (G2a and G5) failed to synthesize E1a RNA or proteins. Incubation of these cell lines with 3 μm-5-AzaC for 2 days, followed by passaging in the absence of drug, gave rise to transformed foci in all of the cell lines. The efficiency of transformation was typical of each cell line. Surprisingly, retransformation was not accompanied by the reappearance of detectable levels of E1a gene activity in the G2aAza and G5Aza cell lines. In search of a mechanistic explanation for the loss of gene activity in the revertants and its reappearance in the retransformants, we examined the state of methylation of the E1a gene region in these cells. Neither the E1a promoter nor its upstream region was methylated in the revertants or the 5-AzaC retransformants. These results suggest that E1a transcription was suppressed by mechanisms other than DNA methylation and that 5-AzaC could retransform these cells without lifting the E1a-suppressed state.

Among antibodies to flaviviral proteins only those directed at the virion envelope protein (E) or the nonstructural glycoprotein NS1 are known to confer protection. To investigate the possible role of complement-mediated cytolysis (CMC) in protection we measured the capacity of anti-NSl, or E monospecific serum or monoclonal antibodies to bind to yellow fever virus (YFV)-infected cells and of anti-NSl or E serum to sensitize them to CMC. Although both anti-NSl and anti-E antibody bound to YFV-infected cells, CMC was observed only with anti-NSl antibody. Greater binding by anti-NS 1 antibody suggested the presence of larger amounts of NS1 than E associated with the cell membrane. Using the cell membrane-impermeable, cross-linking reagent BS3, cell surface NS1, but not E, was detected as a homopolymer, a form in which bound antibody might be expected to activate complement more efficiently. Peak titres of progeny virus were reduced 10- to 100-fold when infected cells were treated with complement-fixing, anti-NSl monoclonal antibody or monospecific, anti-NSl rabbit serum and complement. Taken together these results are consistent with the hypothesis that CMC subserved by anti- NSl antibody provides an alternative to direct neutralization of virus in the protective immune response to flaviviral infection.

The ability of passage in HeLa cells to attenuate flaviviruses was investigated for three different strains of the mosquito-borne West Nile (WN) virus and two tick-borne viruses, louping-ill and Langat. One strain of WN virus, Sarawak, was attenuated 4000-fold for adult mice by intraperitoneal or intranasal challenge after six HeLa passages. The HeLa-passaged virus was also found to be antigenically different and temperature-sensitive in its growth characteristics compared with the parent. After six HeLa cell passages the Egypt 101 and Smithburn strains of WN virus lost their ability to infect monkey kidney cells and no longer killed adult mice, although inoculated animals became sick for several days. In contrast, two tick-borne flaviviruses remained as virulent for mice after six HeLa passages as the parent non-HeLa-passaged virus. Neither of the tick-borne viruses exhibited characteristics associated with temperature sensitivity. The results, therefore, indicate that the mosquito-borne, but not tick-borne, flaviviruses can be attenuated by very few passages in HeLa cells. This observation may provide a model system with which to analyse the molecular basis of attenuation and/or virulence of mosquito-borne flaviviruses.

The porcine paramyxovirus is a newly identified agent of a fatal disease in piglets, endemic in Mexico since 1980, where it was seen around the town of La Piedad, Michoacan, Mexico (hence LPM virus). At least six [3 5 S] methionine-labelled proteins could be resolved by SDS-PAGE and five of them were clearly immuno- precipitated. Selective labelling of LPMV-infected cells with [3H]glucosamine revealed two bands with an M r of about 66K and 59K, corresponding to the two viral glycoproteins, the haemagglutinin-neuramini- dase protein and the fusion protein. Labelling of virus with [32P]orthophosphate disclosed one band with an M r of 52K, corresponding to the phosphoprotein.

Analysis of nucleocapsids obtained from purified virus or from a permanently infected cell line revealed one major band with an M r of 68K, the nucleoprotein. Two other proteins were also identified, the large protein and the matrix protein, with apparent M r of about 200K and 40K, respectively. The protein migration pattern of LPMV was compared, by SDS-PAGE, with that of Newcastle disease virus, bovine parainfluenza 3 virus and Sendai virus. Differences in the M r of LPMV proteins and the proteins of these paramyxoviruses were observed. We propose that LPMV should be classified as a novel member of the genus Paramyxovirus.

Evidence has been presented that generation of polycistronic readthrough RNAs in mumps virus- infected cells is not a simple stochastic process with strain-dependent variations in the generation of certain readthrough products, but that this process is affected by host as well as viral factors. RNAs extracted from infected Vero cells or chicken embryo fibroblast (CEF) cells have been analysed by Northern blotting with virus-specific probes for the nucleocapsid (N), phos- phoprotein (P), matrix (M), fusion (F), small hydrophobic (SH) and haemagglutinin-neuraminidase (HN) genes. Vero cells infected with tissue culture cell-adapted virus strains generate monocistronic as well as polycistronic RNAs. Transcription analysis of Vero cells infected with an egg-adapted strain reveal the absence of monocistronic M and F transcripts, with a concomitant increase in readthrough transcripts involving these genes. When the same virus infects CEF cells monocistronic RNAs accumulate. The presence of viral proteins in the various virus/host cell combinations assessed by immunofluorescence with mumps virus-specific monoclonal antibodies for the N, P, M, F and HN proteins correlates well with the patterns of transcription.

In this paper we report on the identification of nonessential genes in the terminal repeats of the avipox- virus fowlpox virus and the use of these as insertion sites in a vector system. Foreign genes inserted into these sites are shown to be present in two copies in the resultant recombinant virus. To test the potential use of this vector as a live vaccine the fusion gene of Newcastle disease virus has been inserted into a vaccine strain of fowlpox virus and inoculated into chickens. The experiments demonstrate the ability of the recombinant to protect chickens against challenge by a virulent strain of Newcastle disease virus and to elicit the formation of an anti-fusion protein antibody.

Neutralizing monoclonal antibodies (nMAbs) elicited against foot-and-mouth disease virus (FMDV) of serotype C were assayed with field isolates and variant FMDVs using several immunoassays. Of a total of 36 nMAbs tested, 23 recognized capsid protein VP1 and distinguished at least 13 virion conformation- independent epitopes involved in neutralization of FMDV C. Eleven epitopes of FMDV C-S8cl have been located in segments 138–156 or 192–209 of VP1 by quantifying the reactivity of nMAbs with synthetic peptides and with nMAb-resistant mutants of FMDV C-S8cl carrying defined amino acid substitutions. The main antigenic site of FMDV C-S8cl (VP1 residues 138 to 150) consists of multiple (at least 10), distinguishable, overlapping epitopes. Some amino acid replacements abolished one of the epitopes, whereas other replacements affected several epitopes in this region. The conservative substitution His(146)→ Arg, found in many nMAb-resistant mutants analysed, abolished the reactivity of the virus with all nMAbs that recognized epitopes in the main antigenic site of FMDV C-S8cl. This indicates that a minimum genetic change can result in a highly amplified phenotypic effect, as regards the antigenicity of FMDV.

The diabetogenic D variant of encephalomyocarditis virus (EMC-D) was previously shown to differ from the non-diabetogenic B variant (EMC-B) by 14 nucleotides out of 7829 bases. Similar approaches with a new nondiabetogenic variant, EMC-DVi, obtained by plaque purification of the EMC-D variant stock pool, enabled us to narrow down further the possible genomic area responsible for the diabetogenicity of EMC virus. EMC-DV ] does not induce interferon in vitro, differing from the highly interferon-inducing EMC-B. The complete nucleotide sequence of EMC-DY, was determined by RNA-dependent DNA sequencing and cDNA sequencing. The genomic size and organization of EMC-EMC-DV1 are similar to those of EMC-D and EMC-B, with a long open reading frame encoding a polyprotein of 2292 amino acids. Comparative analyses of sequence information as well as biological activities of EMC-EMC-DV1 with EMC-D and EMC-B suggest that (i) the diabetogenicity is apparently distinct from the ability to induce interferon, which is probably due to the single U base insertion at position 765 in EMC-B, and (ii) the diabetogenicity of EMC virus is most probably controlled by one or both of two amino acids, Phe 16 (on the leader peptide) and Ala 776 (152nd amino acid on the VP1) on the polyprotein.

In infected cattle, bovine herpesvirus type 4 (BHV-4) induces an immune response with low neutralizing antibody levels or in the absence of such antibodies. For the study of this phenomenon, monoclonal antibodies (MAbs) raised against two BHV-4 glycoproteins identified previously (150K7120K/51K and 120K/16·5K) were used in neutralization tests. None of the MAbs except for MAb 16 could neutralize alone; pairs of MAbs against the 150K/120K/51K and 120K/16·5K glycoproteins were able to neutralize BHV-4 infectivity. MAbs involved in neutralization were used in competitive binding assays to identify epitopes relevant for BHV-4 neutralization. These MAbs showed a low avidity and a weak neutralizing activity, and they partially decreased BHV-4 attachment to cells. These results suggest that the BHV-4 glycoprotein domains involved in viral infectivity are poorly exposed to the immune system.

Monoclonal antibodies (MAbs) and polyclonal antibodies raised in mice to murine cytomegalovirus (MCMV) were characterized in vitro by their virusneutralizing activity and their reactivity with MCMV polypeptides and MCMV-infected mouse embryo fibroblasts (MEF). MCMV was neutralized by the MAbs prior to virus adsorption to MEF by a complement-dependent mechanism. Although the neutralization of MCMV prior to virus adsorption to MEF by polyclonal antibodies was enhanced in the presence of complement, MCMV was also neutralized by a complement-independent mechanism after virus adsorption. No correlation was observed between the level of neutralization of MCMV and the ability of MAbs or polyclonal antibodies to interfere with the binding of the virus to MEF. As the neutralization of MCMV with polyclonal antibodies by both complement-dependent and -independent mechanisms may reflect the interaction of antibodies with different specificities, the ability of each MAb to interact with a second MAb was investigated. One MAb, 1E8, inhibited the neutralizing activity of several other MAbs. Several MAbs reacted with multiple polypeptides by immunoprecipitation and Western blotting analysis; MAb AC1 cross-reacted with a neutralizing 92K/98K MCMV domain and a 70K ribonucleoprotein, the latter with which sera from patients with connective tissue diseases also reacted. This suggests that an homology exists between these proteins, which may lead to the development of autoimmune manifestations in vivo.

Epstein-Barr (EB) virus-immortalized B lymphocytes coexpress the EB viral latent gene products (EB viral nuclear antigens 1 to 6, the latent membrane protein and the terminal protein gene products) and the cellular activation antigen CD23. Immortalized B cells can be separated from those which are infected but not immortalized on the basis of CD23 expression as early as 2 days after in vitro infection. In the present report we have confirmed these data, but show that if left in culture for 7 days after infection before separation the CD23-negative cells show a donor-related ability to become CD23-positive and immortalize. CD23-nega- tive cells separated 2 days after infection can be induced to immortalize by the addition of low Mr B cell growth factor but not by the addition of recombinant interleukin 1, 4 or soluble CD23. At 2 to 3 days after infection the EB viral nuclear antigens 1, 2 and the high Mr species 3, 4 and 6, as well as the latent membrane protein can be detected in the CD23-positive fraction. In contrast at this time only nuclear antigens 1 and 2 could be detected in the CD23-negative fraction. This difference in gene expression may account for the inability of the CD23-negative fraction to immortalize. In the light of these observations the mechanism of viral persistence in vivo is discussed.

The initial velocity of thymidine uptake was measured in HeLa S3 cells infected with herpes simplex virus type 1 (HSV-1). The rate of nucleoside influx into the cells was shown to increase from as early as 1 h postinfection (p.i.) up to 8 h p.i. This increased uptake was shown to be attributable to a progressively increasing contribution from passive diffusion superimposed upon normal transport. Thus, the specific nucleoside transport system was still operating with unaltered kinetic parameters 8 h after infection. Despite the inhibition of host cell protein synthesis and its replacement by the synthesis of virus-specified proteins, the numbers and affinity of the nucleoside transporters in cells 8 h after infection were virtually unchanged. The increased transport of thymidine in infected cultures was resistant to the nucleoside transport inhibitor dipyridamole, and was correlated with entry of a normally impermeant solute (sucrose) into infected cells. These data suggest that the system for the carrier-mediated facilitated diffusion of nucleosides remains intact in HSV-infected cells, but that progressively increasing passive diffusion takes place. Passive diffusion is the major process operating late after virus infection.