Avian leukosis viruses of different subgroups and types isolated after passage of Rous sarcoma virus-Rous-associated virus-0 in cells from different ring-necked pheasant embryos

The location of v-src in a retrovirus vector determines whether the virus is toxic or transforming

We prepared infectious stocks of an avian retrovirus, a modified spleen necrosis virus, containing the herpes simplex virus type 1 thymidine kinase gene and the avian sarcoma virus v-src gene. Viruses were recovered after cotransfection of chicken cells with DNA of recombinants between cloned spleen necrosis virus thymidine kinase and v-src and with DNA of cloned reticuloendotheliosis virus strain A. When v-src was inserted near the 5'end of the viral genome, only low titers of recombinant virus were recovered. Most of the recovered viruses were smaller than expected and did not transform the morphology of rat or chicken cells. A very small amount of virus of the expected structure was recovered; this virus transformed rat cells and expressed v-src. Cotransfection data indicated that one reason we failed to recover a significant titer of recombinant virus is that efficient expression of v-src is acutely toxic to chicken and dog cells. Insertion of v-src near the 3' end of the viral genome, such that it was expressed at a lower level compared with the 5'-v-src-containing virus, yielded a higher titer of recombinant virus, and this virus was transforming. The differences in the recovery and transforming activity of these viruses indicate that the location of an oncogene in the viral genome is an important factor regulating the level of its expression and whether or not this expression is toxic or transforming to cells.

The location of v-src in a retrovirus vector determines whether the virus is toxic or transforming

We prepared infectious stocks of an avian retrovirus, a modified spleen necrosis virus, containing the herpes simplex virus type 1 thymidine kinase gene and the avian sarcoma virus v-src gene. Viruses were recovered after cotransfection of chicken cells with DNA of recombinants between cloned spleen necrosis virus thymidine kinase and v-src and with DNA of cloned reticuloendotheliosis virus strain A. When v-src was inserted near the 5'end of the viral genome, only low titers of recombinant virus were recovered. Most of the recovered viruses were smaller than expected and did not transform the morphology of rat or chicken cells. A very small amount of virus of the expected structure was recovered; this virus transformed rat cells and expressed v-src. Cotransfection data indicated that one reason we failed to recover a significant titer of recombinant virus is that efficient expression of v-src is acutely toxic to chicken and dog cells. Insertion of v-src near the 3' end of the viral genome, such that it was expressed at a lower level compared with the 5'-v-src-containing virus, yielded a higher titer of recombinant virus, and this virus was transforming. The differences in the recovery and transforming activity of these viruses indicate that the location of an oncogene in the viral genome is an important factor regulating the level of its expression and whether or not this expression is toxic or transforming to cells.

Mechanisms of oncogenesis by subgroup F avian leukosis viruses

Subgroup F avian leukosis viruses, such as RAV-61 and ring-necked pheasant virus, are recombinants between exogenous chicken retroviruses and endogenous pheasant viruses and contain new envelope (env) genes. Chickens infected as 10-day-old embryos with subgroup F viruses develop fibrosarcomas, nephroblastomas, osteopetrosis, B-cell lymphomas, and a high incidence of a proliferative disorder involving the lung. Fibrosarcomas, nephroblastomas, and lymphomas appear after long latent periods (3 to 12 months). They contain discrete virus-cell junction fragments and are therefore clonal outgrowths of a single infected cell. Two ring-necked pheasant virus-induced B-cell lymphomas and an adenocarcinoma of the abdomen contained proviruses integrated at the c-myc locus and elevated levels of myc mRNA. At least four of the fibrosarcomas appeared to contain proviruses integrated at a common site, suggesting that a specific cellular gene may be involved in these tumors. The host gene has not been identified, however; 16 different oncogene probes failed to hybridize to fibrosarcoma junction fragments. In contrast to these neoplasms, lung lesions appeared rapidly (4 to 5 weeks), showed no evidence of clonality, and lacked long terminal repeat-initiated transcripts other than viral 35S and 21S mRNA. We conclude, therefore, that subgroup F retroviruses induce the proliferative disorder of the lung by a different mechanism.

Expression from an internal AUG codon of herpes simplex thymidine kinase gene inserted in a retrovirus vector

We identified structural features that affect the expression of an exogenous gene inserted into a retrovirus vector constructed by using spleen necrosis virus, an avian retrovirus. The thymidine kinase gene from herpes simplex virus type 1 containing deletions in the promoter and terminal sequences of the mRNA was inserted into spleen necrosis virus. We found that synthesis of thymidine kinase by the recovered virus was apparently initiated from internal AUG residues. At least in some cases, however, the level of expression depended on the number of AUGs and the nucleotide sequence around the AUGs that preceded the initiator codon of the thymidine kinase gene.

Expression from an internal AUG codon of herpes simplex thymidine kinase gene inserted in a retrovirus vector

We identified structural features that affect the expression of an exogenous gene inserted into a retrovirus vector constructed by using spleen necrosis virus, an avian retrovirus. The thymidine kinase gene from herpes simplex virus type 1 containing deletions in the promoter and terminal sequences of the mRNA was inserted into spleen necrosis virus. We found that synthesis of thymidine kinase by the recovered virus was apparently initiated from internal AUG residues. At least in some cases, however, the level of expression depended on the number of AUGs and the nucleotide sequence around the AUGs that preceded the initiator codon of the thymidine kinase gene.

Cell killing by avian leukosis viruses

Infection of chicken cells with a cytopathic avian leukosis virus resulted in the detachment of killed cells from the culture dish. The detached, dead cells contained more unintegrated viral DNA than the attached cells. These results confirm the hypothesis that cell killing after infection with a cytopathic avian leukosis virus is associated with accumulation of large amounts of unintegrated viral DNA. No accumulation of large amounts of integrated viral DNA was found in cells infected with cytopathic avian leukosis viruses.

Correlation between cell killing and massive second-round superinfection by members of some subgroups of avian leukosis virus

Avian leukosis viruses of subgroups B, D, and F are cytopathic for chicken cells, whereas viruses of subgroups A, C, and E are not. The amounts of unintegrated linear viral DNA in cells at different times after infection with cytopathic or noncytopathic viruses were determined by hybridization and transfection assays. Shortly after infection, there is a transient accumulation of unintegrated linear viral DNA in cells infected with cytopathic avian leukosis viruses. By 10 days after infection, the majority of this unintegrated viral DNA is not present in the infected cells. The transient cytopathic effect seen in these infected cells also disappears by this time. Low amounts of unintegrated linear viral DNA persist in these cells. Cells infected with noncytopathic viruses do not show this transient accumulation of unintegrated viral DNA. Cells infected with cytopathic viruses and subsequently grown in the presence of neutralizing antibody do not show the transient accumulation of unintegrated viral DNA or cytopathic effects. These results demonstrate a correlation between envelope subgroup, transient accumulation of unintegrated linear viral DNA, and transient cell killing by avian leukosis viruses. The cell killing appears to be the result of massive second-round superinfection by the cytopathic avian leukosis viruses.

The specificity of neutralizing antibodies to feline leukaemia viruses

Feline leukaemia viruses (FeLV) of subgroups A (FeLV-A), B(FeLV-B) and C(FeLV-C) were examined by neutralization. The FeLV-A isolates were monotypic. By contrast, there was antigenic variation within subgroups B and C. There was also considerable cross-reactivity between subgroups. Thus, FeLV-A viruses showed weak cross-reactivity to the standard FeLV-C isolate; four of the subgroup B viruses were identical and cross-reactied with the standard FeLV-C, but not with FeLV-A or the fifth FeLV-B, and this latter virus weakly cros-reacted with FeLV-A but not with the FeLV-C; within subgroup C, one virus was indistinguishable from the standard FeLV-C strain but the other two were very similar to FeLV-A. Therefore, subgroup classification based on interference did not altogether correspond with neutralizing behaviour for subgroups B and C.

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.

Nucleotide sequences derived from pheasant DNA in the genome of recombinant avian leukosis viruses with subgroup F specificity

Recombination between viral and cellular genes can give rise to new strains of retroviruses. For example, Rous-associated virus 61 (RAV-61) is a recombinant between the Bryan high-titer strain of Rous sarcoma virus (RSV) and normal pheasant DNA. Nucleic acid hybridization techniques were used to study the genome of RAV-61 and another RAV with subgroup F specificity (RAV-F) obtained by passage of RSV-RAV-0 in cells from a ring-necked pheasant embryo. The nucleotide sequences acquired by these two independent isolates of RAV-F that were not shared with the parental virus comprised 20 to 25% of the RAV-F genomes and were indistinguishable by nucleic acid hybridization. (In addition, RAV-F genomes had another set of nucleotide sequences that were homologous to some pheasant nucleotide sequences and also were present in the parental viruses.) A specific complementary DNA, containing only nucleotide sequences complementary to those acquired by RAV-61 through recombination, was prepared. These nucleotide sequences were pheasant derived and were not present in the genomes of reticuloendotheliosis viruses, pheasant viruses, and avian leukosis-sarcoma viruses of subgroups A, B, C, D, and E. They were partially endogenous, however, to avian DNA other than pheasant. The fraction of these nucleotide sequences present in other avian DNAs generally paralleled the genetic relatedness of these avian species to pheasants. However, there was a high degree of homology between these pheasant nucleotide sequences and related nucleotide sequences in the DNA of normal chickens as indicated by the identical melting profiles of the respective hybrids.