Expression of the Major Viral Glycoprotein of Avian Tumor Virus in Cells of chf(+) Chicken Embryos

Role of methylation in the induced and spontaneous expression of the avian endogenous virus ev-1: DNA structure and gene products

The endogenous avian provirus ev-1 is widespread in white leghorn chickens. Although it has no major structural defects, ev-1 has not been associated with any phenotype and is ordinarily expressed at a very low level. In this report, we describe a chicken embryo (Number 1836) cell culture containing both ev-1 and ev-6 which spontaneously expressed the ev-1 provirus. This culture released a high level of noninfectious virions containing a full complement of virion structural (gag) proteins but devoid of reverse transcriptase activity or antigen. These virions contained 70S RNA closely related to the genome of Rous-associated virus type 0, but identifiable as the ev-1 genome by oligonucleotide mapping. A fraction of the RNA molecules in the 70S complex were unusual in that they were polyadenylated 100 to 200 nucleotides downstream of the usual polyadenylation site. Eight sibling embryo cultures did not share this unusual phenotype with 1836, indicating that it was not inherited. However, an identical phenotype was inducible in the sibling cultures by treatment with 5-azacytidine, an inhibitor of DNA methylation, and the induced expression was stable for more than 10 generations. Analysis of chromatin structure and DNA methylation of the ev-1 provirus in 1836 cells revealed the presence (in a fraction of the proviruses) of both DNase I hypersensitive sites in the long terminal repeats and in gag and a pattern of cleavage sites for methyl-sensitive restriction endonuclease not found in a nonexpressing sibling. These results lend strong support to the role of DNA methylation in the control of gene expression. Additionally, they explain the lack of phenotype associated with ev-1 as due to a combination of its low expression and defectiveness in pol and env.

Role of methylation in the induced and spontaneous expression of the avian endogenous virus ev-1: DNA structure and gene products

The endogenous avian provirus ev-1 is widespread in white leghorn chickens. Although it has no major structural defects, ev-1 has not been associated with any phenotype and is ordinarily expressed at a very low level. In this report, we describe a chicken embryo (Number 1836) cell culture containing both ev-1 and ev-6 which spontaneously expressed the ev-1 provirus. This culture released a high level of noninfectious virions containing a full complement of virion structural (gag) proteins but devoid of reverse transcriptase activity or antigen. These virions contained 70S RNA closely related to the genome of Rous-associated virus type 0, but identifiable as the ev-1 genome by oligonucleotide mapping. A fraction of the RNA molecules in the 70S complex were unusual in that they were polyadenylated 100 to 200 nucleotides downstream of the usual polyadenylation site. Eight sibling embryo cultures did not share this unusual phenotype with 1836, indicating that it was not inherited. However, an identical phenotype was inducible in the sibling cultures by treatment with 5-azacytidine, an inhibitor of DNA methylation, and the induced expression was stable for more than 10 generations. Analysis of chromatin structure and DNA methylation of the ev-1 provirus in 1836 cells revealed the presence (in a fraction of the proviruses) of both DNase I hypersensitive sites in the long terminal repeats and in gag and a pattern of cleavage sites for methyl-sensitive restriction endonuclease not found in a nonexpressing sibling. These results lend strong support to the role of DNA methylation in the control of gene expression. Additionally, they explain the lack of phenotype associated with ev-1 as due to a combination of its low expression and defectiveness in pol and env.

The influence of the ev 3 locus on the inducibility of serum antibody reactivity for envelope glycoprotein group-specific determinants

Chickens segregating for the ev 3 locus were bred by backcross matings of line 6(3) to line 15B. Analysis of RAV-1-infected segregants indicated that inducibility of antibody reactivity for envelope glycoprotein group-specific determinants correlated with the absence of ev 3, whereas noninducibility correlated with the presence of ev 3. Since the ev 3(+) and ev 3(-) segregants possessed similar genetic backgrounds, these results provide direct evidence that the ev 3 locus determines the phenotype of noninducibility.

Changes in nucleosomal core histone variants during chicken development and maturation

The nucleosomal core histones H2A, H2B, and H3 of the chicken can be resolved by polyacrylamide gel electrophoresis in the presence of nonionic detergents into two primary structure variants each, which occur in different relative amounts in various adult tissues. Quantitative analysis of the histone components throughout embryonic development and posthatching maturation of the chicken revealed that the proportions of the three pairs of variants change independently. Thus, the two H2A variants occur in similar proportions throughout embryonic development and in all adult tissues. In contrast, only one variant each of H2B and H3 is detectable at the earliest stages (primitive streak). The second variant of these histones becomes detectable and increases gradually during somite formation (2-12 days of incubation) to reach a plateau at a level of about 3 and 10% of total H2B and H3 histones, respectively. After hatching, the relative amounts of the minor H2B and H3 variants remain at embryonic levels in those tissues which maintain a high mitotic activity such as blood-forming tissues, but increase with different kinetics in tissues which essentially stop cell division in adults (e.g., liver, kidney, etc.). However, while H2B.2 remains a very minor component in all tissues, H3.3 increases at a relatively high rate for more than a year to become the predominant H3 variant in the liver and kidney of older chickens. The changes in chicken core histone variant proportions appear to be related to changes in growth rate rather than cell differentiation. The extensive change of H3 variant proportions in nondividing adult tissues is most likely due to replication-independent incorporation of H3.3 into nucleosomes.

Differential reactivity of serum antibody from tumor-bearing 15I5 X 7(2) chickens for cross-reactive species of endogenous retroviral envelope glycoprotein

Experiments were undertaken to determine if the sera of avian sarcoma virus-infected 15I5 X 7(2) chickens exhibit antibody reactivity for species of endogenous retroviral envelope glycoprotein expressed in 15I5 X 7(2) fibroblasts. Two viruses were used for infection of the 15I5 X 7(2) chickens, Pr-A and cl. 85; the envelope glycoprotein of Pr-A, but not of cl. 85, is antigenically cross-reactive with 15I5 X 7(2) endogenous retroviral envelope glycoprotein. Both the Pr-A and cl. 85-infected 15I5 X 7(2) chickens exhibited serum antibody reactivity for the envelope glycoprotein of the endogenous retrovirus RAV-O. In contrast, neither group of infected chickens exhibited detectable serum antibody reactivity for a distinct species of endogenous retroviral envelope glycoprotein, one which though antigenically cross-reactive with the envelope glycoprotein of RAV-O is expressed to much higher levels in 15I5 X 7(2) fibroblasts. Possible mechanisms to account for the observed pattern of antibody reactivity are discussed.

Identification of a Rous sarcoma virus transformation-related protein in normal avian and mammalian cells

Avian sarcoma viruses (ASV) contain a gene (src) whose protein product mediates sarcomagenic transformation. This product is a 60,000-Mr phosphoprotein designated pp60src. We have found that normal uninfected frog, chicken, rat, and human cells contain a 60,000-Mr phosphoprotein related to the product of the ASV src gene and have designated that protein pp60. A phosphoprotein of similar size was not detectable in Drosophila cells. The pp60 proteins were detected by immunoprecipitation with rabbit antitumor serum containing broad spectrum antibodies to pp60src. Peptide maps of [35S]methionine-labeled pp60 and pp60src indicated major similarities as well as some differences in amino acid composition. Peptide maps of the 32P-labeled proteins demonstrated that the phosphopeptides of all endogenous pp60 molecules tested were identical. However, some differences were noted between the phosphopeptide patterns of pp60 and viral pp60src. The kinase activity associated with pp60src was measured in the immunocomplex and resulted in the transfer of radioactive phosphorus from [gamma-32P]ATP to the immunoglobulin heavy chain as well as to an 80,000-Mr phosphoprotein. The pp60 of chicken, rat, and human origin also contained an associated kinase activity. These results are consistent with the notion that the pp60 molecules are the protein products of endogenous sarc sequences found in vertebrate cells.

Gs, an allele of chickens for endogenous avian leukosis viral antigens, segregates with ev 3, a genetic locus that contains structural genes for virus

Gs is an allele of chickens for the expression of endogenous avian leukosis virus-related core (gs) and envelope (chf) antigens. Progeny of a genetic cross in which Gs was segregating were analyzed for endogenous viral DNA as well as for the expression of endogenous viral antigens. Viral genetic information was identified by cleavage of embryo DNA with restriction endonucleases, electrophoretic separation of the resulting fragments, and identification of bands containing viral sequences by hybridization of the DNA to 32P-labeled viral RNA. Four different chromosomal sites of residence of endogenous viral sequences were identified by this method. These sites were the same as those previously assigned to the endogenous viral loci ev 1, ev 3, ev 4, and ev 5. ev 1 was present in all of the progeny of the cross. ev 3, ev 4, and ev5 were present in various combinations with ev 1. ev 3 cosegregated with the gs+chf+ phenotpye. Cells which did not contain ev 3 but contained ev 1, ev 4, and/or ev 5, did not express detectable levels of viral antigens. We suggest that Gs contains the structural genes for endogenous virus which reside at ev 3 and that these structural genes code for gs and chf in gs+chf+ cells.

Endogenous oncornaviral antigen in the bursa of Fabricius of 15B X 7(2) chickens

Oncornaviral antigen was detected in the bursal epithelium and in a subpopulation of bursal follicular cells of 15B X 72 chickens. This antigen is present in the bursal epithelium at 11 days of embryogenesis and persists there for at least 3 weeks after hatching. The absence of detectable antigen in the intestinal epithelium contiguous to the bursal epithelium indicates that the accumulation of viral antigen is a specific property of the bursal epithelium. The observation of C-type particles in the intraepithelial spaces suggests that the viral antigen in synthesized and assembled into virions by the bursal epithelial cells. In embryonic bursas, viral antigen-positve cells radiate from the surface epithelium toward the central region of the follicles. In bursas from post-hatch chickens, viral antigen-positive cells, including intrafollicular epithelial cells and cells resembling lymphocytes, are confined to the medullary region of the follicles.

Assays for endogenous and exogenous lymphoid leukosis viruses and chick helper factor with RSV(--) cell lines

Japanese quail cells transformed by the envelope-defective Bryan high-titer strain of Rous sarcoma virus [R(-)Q] were used as a source of the Rous sarcoma virus genome in three kinds of assays. (i) The simplest and most sensitive assay for infectious, endogenous viruses of the chicken belonging to subgroup E involved infection of a mixture of R(-)Q cells and turkey cells with the sample and assay of supernatants of these cells for focus formation on subgroup E susceptible cells. (ii) Inactivated Sendai virus-induced fusion of R(-)Q cells with live test cells was found to be a specific method for detection of chick helper factor. Focus formation by supernatant of the fused cells on subgroup E susceptible cells was correlated with the presence of subgroup E envelope glycoprotein on the plasma membranes of test cells. Whole blood cells as well as fibroblasts could be used in this assay. (iii) A method of assay for exogenous lymphoid leukosis viruses in which mixed cultures of R(-)Q cells and C/E cells and assay of supernatants for focus formation on C/E cells was as sensitive as assays presently used for exogenous lymphoid leukosis virus. Because no infectious Rous sarcoma virus was used as part of the procedure, the assays for infectious virus described here yielded pure pseudotypes of the input virus, an advantage for determining purity and subgroup of the input virus.

Antigenic determinants specific to the envelope glycoprotein of exogenous avian tumor viruses

Antibody in the sera of 15B X 7(2) chickens was measured for reactivity to determinants of the envelope glycoprotein of endogenous and exogenous avian tumor viruses. The pattern of reactivity of animals infected with exogenous sarcoma virus served to operationally define a class of determinants present on the envelope glycoprotein of exogenous viruses and absent from that of the endogenous virus Rous-associated virus-0.

Nucleotide sequence relationships between the genomes of an endogenous and an exogenous avian tumor virus

We have used mapping of large T1 oligonucleotides to examine the genome of Rous-associated virus-O (RAV-O), an endogenous virus of chickens, and to compare it with that of Prague strain Rous sarcoma virus, subgroup B, (Pr-RSV-B), an exogenous sarcoma virus. To extend the sensitivity of such comparisons, we have developed a system of nucleic acid hybridization and hybridization-competition combined with fingerprinting. This method allows us to estimate the relative degree of relatedness of various portions of the viral genomes. From the results of this study, we have concluded that the genomes of Pr-RSV-B and RAV-O are related in the following way. The 5'-terminal half of the genomes (corresponding to the gag and pol regions) is virtually identical, with only scattered single nucleotide differences. This region is followed by a region comprising 25 to 30% of the genome (the env region) which contains substantial nucleotide sequence differences, most or all of which are due to single base changes. The env-coding region can be further subdivided into three regions: a more variable region probably containing sequences coding for subgroup specificity, flanked by relatively common sequences on each side. To the 3' side of the env region, the RAV-O genome contains a very short sequence not found in Pr-RSV-B, whereas the Pr-RSV-B genome contains a much longer unrelated sequence. The central portion of this sequence comprises the src gene as defined by transformation-defective mutants. Particularly striking is the absence, in the RAV-O genome, of any nucleotide sequence related to the "c region" found very near the 3' end of all exogenous tumor viruses. Both the Pr-RSV-B and RAV-O genomes contain the identical terminally redundant sequence of 21 nucleotides near each end of the genome.

Comparison of the RNA-dependent DNA polymerase of an endogenous avian leukosis virus to the polymerase of an exogenous avian leukosis virus

RNA-dependent DNA polymerases from Rous-associated virus-O and avian myeloblastosis virus were partially purified by affinity chromatography and compared to each other. The enzymes are indistinguishable in the immunoglobulin inhibition test and by several enzymological criteria, such as optimum curves for the concentrations of Mg2+, K+, H+; monophasic Lineweaver-Burk plot for dTTP and biphasic Lineweaver-Burk plot for dGTP. In thermal inactivation studies a small difference can be observed, suggesting a minor difference in the physical structures of the enzymes. Our findings are consistent with the idea that the RNA-dpendent DNA polymerases of endogenous and exogenous avian leukosis viruses are very closely related to each other and therefore may be regarded as one group of polymerases.

env Gene of chicken RNA tumor viruses: extent of conservation in cellular and viral genomes

The env gene of avian sarcoma-leukosis viruses codes for envelope glycoproteins that determine viral host range, antigenic specificity, and interference patterns. We used molecular hybridization to analyze the natural distribution and possible origins of the nucleotide sequences that encode env; our work exploited the availability of radioactive DNA (cDNA(gp)) complementary to most or all of env. env sequences were detectable in the DNAs of chickens which synthesized an env gene product (chick helper factor positive) encoded by an endogenous viral gene and also in the DNAs of chickens which synthesized little or no env gene product (chick helper factor negative). env sequences were not detectable in DNAs from Japanese quail, ring-necked pheasant, golden pheasant, duck, squab, salmon sperm, or calf thymus. The detection of sequences closely related to viral env only in chicken DNA contrasts sharply with the demonstration that the transforming gene (src) of avian sarcoma viruses has readily detectable homologues in the DNAs of all avian species tested [D. Stehelin, H. E. Varmus, J. M. Bishop, and P. K. Vogt, Nature (London) 260: 170-173, 1976] and in the DNAs of other vertebrates (D. Spector, personal communication). Thermal denaturation studies on duplexes formed between cDNA(gp) and chicken DNA and also between cDNA(gp) and RNAs of subgroup A to E viruses derived from chickens indicated that these duplexes were well matched. In contrast, cDNA(gp) did not form stable hybrids with RNAs of viruses which were isolated from ring-necked and golden pheasants. We conclude that substantial portions of nucleotide sequences within the env genes of viruses of subgroups A to E are closely related and that these genes probably have a common, perhaps cellular, evolutionary origin.

Immunogenicity of the envelope glycoprotein of avian sarcoma virus

The expression of type E-specific glycoprotein in chick-helper factor-positive [chf(+)] chickens is not restricted to a certain stage of embryogenesis but persists in postembryonic life. This finding prompted an investigation of the immunogenicity of the viral envelope glycoprotein of exogenous avian sarcoma virus in chf(+) and chf(-) chickens. Sensitization of both classes of chickens resulted in the induction of detectable antibody reactivity to determinants of the glycoprotein that were type-specific as well as group-specific. Because group-specific determinants are present on type E-specific glycoprotein, these results link immunity to exogenous viral envelope glycoprotein in chf(+) chickens with autoreactivity. A model is proposed to rationalize the induction of reactivity in this system.

Marker rescue of endogenous cellular genetic information related to the avian leukosis virus gene encoding RNA-directed DNA polymerase

Endogenous cellular genetic information related to the avian leukosis virus gene encoding RNA-directed DNA polymerase was studied, using a marker rescue assay to detect biological activity of subgenomic fragments of virus-related DNAs of uninfected avian cells. Recipient cultures of chicken embryo fibroblasts were treated with sonicated DNA fragments and were infected with a temperature-sensitive mutant of Rous sarcoma virus that encoded a thermolabile DNA polymerase. Wild-type progeny viruses were isolated by marker rescue with fragments of DNA of uninfected chicken, pheasant, quail, and turkey cells. The DNAs of these uninfected avian cells, therefore, appeared to contain endogenous genetic information related to the avian leukosis virus DNA polymerase gene.

Immunoprecipitation of herpes simplex virus type 1 antigens with different antisera and human cerebrospinal fluids

Rabbit convalescent and hyperimmune sera, human patient and blood donor sera, as well as cerebrospinal fluids of humans with herpes simplex virus encephalitis all recognize similar major antigenic components in herpes simplex virus infected rabbit or human cells as shown by electrophoretic analysis of immunoprecipitates. Besides the main glycoproteins with an apparent molecular weight of 100,000 (peak I) the antisera precipitate glycoproteins in a region of an apparent mol. wt. of 60,000--80,000 (peak II), which were resolved into distinct glycoprotein species only by antibody-containing cerebrospinal fluids. The peak II glycoproteins appear on the surface of the infected cell early, and absorb neutralizing antibodies, whereas the peak I glycoproteins are less accessible. Both antigens can be demonstrated in the cell as early as about 2 hours post infection. All major antigenic components studied were found to be glycosylated except one protein with an apparent mol. wt. of 110,000. The herpesvirus specificity of these antigens is demonstrated by a variety of control experiments. The antigens detected are virion components.

Expression of viral proteins in mammalian cells transformed by avian sarcoma viruses

The expression of viral proteins in nine lines of hamster and rat cells transformed by avian sarcoma viruses (ASV) was studied by indirect immunofluorescence with monospecific antisera to purified gp85 and p27 of AMV-B and a polyvalent antiserum to all the p proteins of this same virus. The lines of ASV-transformed cells were either low virus producers (VP) or inducible or non-inducible non producers (NP). Cytoplasmic expression of p proteins was observed in all the cell lines except the least inducible NP cell line, and cytoplasmic expression of gp85 in all the cell lines. The degree of expression varied widely with the lines and was not related to the class of permissiveness or inducibility. However, in the inducible NP class, the expression of p proteins and gp85 was higher in the most inducible cell lines. The data also suggest that the expression of the p proteins must be uncoordinate in at least some cell lines and must also be uncoordinate with the expression of gp85. In the VP cell lines and the most inducible NP lines, g85 and some p proteins other than p27 were also expressed on the cell membrane. The membrane expression of gp85 and the p proteins which were expressed appeared to be coordinate and to parallel the degree of cytoplasmic expression. In contrast, no, or a negligible expression of viral proteins was observed on the membrane of the least inducible and the non-inducible cell lines. These results suggest that there may exist translational and/or post-translational controls of the expression of viral proteins in the ASV-transformed mammalian cells and that the permissiveness and the inducibility of the cells may depend on the insertion of viral proteins in the cell membrane. The failure of p27 to insert in the cell membrane could account for the low permissiveness or the non-permissiveness of the cells.

Infection of resistant avian cells by subgroup B Rous sarcoma virus

Chicken fibroblasts derived from the H & N flock, which have been characterized as resistant to subgroup B avian oncornaviruses in focus assays, can be infected in suspension shortly after trypsinization by subgroup B sarcoma and leukosis viruses. Once cells are plated, resistance to infection reappears rapidly. C/BE cell suspensions obtained by treatment with EDTA instead of trypsin are not as sensitive to infection. Late interference established by preinfection with subgroup B leukosis viruses is not overcome by trypsinization. In addition to C/BE H & N chicken cells, C/ABE RPRL line 7 cells can also be infected by subgroup B viruses shortly after trypsinization; however, none of the cell types can be made sensitive to subgroup E infection. These results are discussed in relation to current information on the genetic control of resistance to avian oncornaviruses.

Subunit structure of the glycoprotein complex of avian tumor virus

Envelope glycoprotein of avian tumor virus is linked by disulfide bonds in a structure that we have designated VGP to stand for viral glycoprotein. VGP appears to contain one molecule of gp85 and one of gp37. Under nonreducing conditions, VGP is the only glycoprotein component that is stable in the presence of ionic detergent, although in the presence of nonionic detergent two or more VGPs are associated in discrete complexes. The disulfide bonds linking viral glycoprotein are formed before release of virus from infected cells.

Viral glycoprotein synthesis studies in an established line of Japanese quail embryo cells infected with the Bryan high-titer strain of Rous sarcoma virus

Although a glycoprotein with an approximate molecular weight of 43,000 is associated with purified virions of the Bryan high-titer strain of Rous sarcoma virus propagated on R(-)Q cells, these virions lack gp85, the major glycoprotein of the avian tumor virus envelope. As measured by immune precipitation with a specific antiserum, gp85 does not accumulate to detectable levels in R(-)Q cells.

Formation of an infectious virus-antibody complex with Rous sarcoma virus and antibodies directed against the major virus glycoprotein

Preparations of Rous sarcoma virus (RSV) can form an infectious viral-antibody complex with antibodies raised against the major glycoprotein, gp85, isolated from avian myeloblastosis virus and Prague-RSV subgroup C. Binding of anti-gp85 antibodies to RSV can be demonstrated by the inhibition of focus-forming activity after addition of goat anti-rabbit immunoglobulin and by a shift in density of virions treated with anti-gp85 serum. Group- rather than subgroup- specific regions of viral gp85 appear to be the site of binding for infectious complex.

Lack of infectivity of the endogenous avian leukosis virus-related genes in the DNA of uninfected chicken cells

The infectivity of the avian leukosis virus-related genes in the DNA of four genetically distinct types of chicken cells was determined. Infectious DNA of Rous-associated virus-O(RAV-O) was obtained from V- chicken cells which were experimentally infected with RAV-O and from V+tvbs chicken cells, which spontaneously produced RAV-O and were sensitive to exogenous RAV-O infection. However, infectious DNA of RAV-O was not obtained from uninfected V- chicken cells or from V+tvbr chicken cells, which spontaneously produced a low titer of RAV-O but were resistant to exogenous RAV-O infection. No detectable amplification of the RAV-O related DNA sequences in the V+tvbs cells was found by hybridization of RAV-O 125I-labeled RNA to the DNAs of V+tvbs and uninfected V- cells. These results indicate that the endogenous avian leukosis virus-related genes in uninfected V- and V+tvbr cells differ from the RAV-O proviruses in RAV-O-infected V- and V+tvbs cells. The lack of infectivity of the DNA of V+tvbr cells is consistent with the hypothesis that the endogenous RAV-O genome in V+tvbr cells is linked to a cis-acting control element, which results in its inefficient expression.