Isolation of complementary DNA unique to the genome of avian myeloblastosis virus (AMV)

Visna-maedi virus induces interleukin-8 in sheep alveolar macrophages through a tyrosine-kinase signaling pathway

The mechanisms leading to the severe lung damage seen in some sheep naturally infected with the visna-maedi virus, and to pulmonary lesions in other lentiviral diseases, appear to involve the recruitment of large numbers of uninfected inflammatory cells. Only a few alveolar macrophages from experimentally infected lambs express virus, but high levels of interleukin (IL)-8 mRNA are present in the macrophage population. In vitro infection with visna-maedi virus at low multiplicity of alveolar macrophages from uninfected sheep also strongly induced the expression of IL-8 mRNA and the accumulation of IL-8 in the extracellular medium. An initial peak of IL-8 mRNA expression at 3 or 6 h after infection was followed by a fall, then a more persistent expression lasting at least 48 h after infection. The early peak was accompanied by expression of mRNA for IL-1beta, and a possible rise in tumor necrosis factor alpha (TNFalpha) mRNA, although this was frequently elevated in uninfected ovine alveolar macrophages. Interestingly, these events occurred identically in cells treated with non-infectious heat-treated virus, suggesting that interaction between viral components and cellular membrane receptors could suffice for both early and late IL-8 induction. The level of IL-8 mRNA induced by treatment with live or inactivated virus could be severely reduced by pretreatment of the macrophages with genistein but not with staurosporine, suggesting the involvement of a tyrosine-kinase signaling pathway. The early induction of IL-1beta and possibly of TNFalpha may explain the occurrence of a later persistent expression of IL-8 mRNA through an autocrine mechanism.

Characterization of the lymphocytic alveolitis in visna-maedi virus-induced interstitial lung disease of sheep

In order to investigate the contribution of lymphocytes to interstitial lung disease in animals with visna-maedi infection, we studied in parallel bronchoalveolar cells and lung tissue from slaughter-house animals (n = 29) and from colostrum-deprived lambs transtracheally inoculated with field isolates of visna-maedi virus (n = 9) or saline (n = 6). Lymphocyte subpopulations were identified in bronchoalveolar lavage by immunofluorescence and flow cytometry analysis and in lung tissue using indirect immunohistochemistry. In infected animals a lymphocytic alveolitis containing CD4 and CD8 lymphocytes was observed. Peribronchovascular lymphoid nodules comprise mostly CD4 lymphocytes. Alveolar lymphocytes of both subsets displayed increased expression of MHC class II antigens in animals with naturally occurring maedi but not in experimentally infected ones. A sequential process of lymphocyte attraction and activation is likely to occur in vivo as part of the alveolitis.

The proto-oncogene c-ets is preferentially expressed in lymphoid cells

The transforming sequences of the avian acute leukemia virus, E26, contain two distinct oncogenes, v-mybE and v-ets, fused together. By using a probe containing v-ets sequences, polyadenylated transcripts of the c-ets proto-oncogene were detected in avian tissues; they included a major 7.0-kilobase and a minor 2.0-kilobase species. These c-ets mRNAs were detected at high levels only in lymphoid organs and in avian T and B lymphoid cell lines. A similar pattern of c-ets transcription was observed in human hematopoietic cell lines, with transcripts detected in lymphoid B and T cells but not in erythroid or myeloid cells. The E26 oncogene was inserted into an inducible expression vector, and a 90-kilodalton protein (bp90) was produced in bacteria. Rabbit antisera raised to purified bp90 precipitated P135gag-mybE-ets, the v-mybE-ets polyprotein expressed in E26-transformed cells, and also reacted with p50v-mybA, the transforming protein of the avian myeloblastosis virus. Antiserum to bp90 was absorbed with a bacterially synthesized v-mybA protein to remove anti-myb activity. The absorbed anti-bp90 serum retained the ability to immunoprecipitate P135gag-mybE-ets from E26-transformed cells and specifically reacted with a 56-kilodalton polypeptide (p56) detected in chicken lymphoid organs and in T and B lymphocytes of both avian and human origin. The data suggest that p56 is a translational product of the c-ets proto-oncogene and imply that p56 may be involved in regulating the growth of lymphoid cells.

The proto-oncogene c-ets is preferentially expressed in lymphoid cells

The transforming sequences of the avian acute leukemia virus, E26, contain two distinct oncogenes, v-mybE and v-ets, fused together. By using a probe containing v-ets sequences, polyadenylated transcripts of the c-ets proto-oncogene were detected in avian tissues; they included a major 7.0-kilobase and a minor 2.0-kilobase species. These c-ets mRNAs were detected at high levels only in lymphoid organs and in avian T and B lymphoid cell lines. A similar pattern of c-ets transcription was observed in human hematopoietic cell lines, with transcripts detected in lymphoid B and T cells but not in erythroid or myeloid cells. The E26 oncogene was inserted into an inducible expression vector, and a 90-kilodalton protein (bp90) was produced in bacteria. Rabbit antisera raised to purified bp90 precipitated P135gag-mybE-ets, the v-mybE-ets polyprotein expressed in E26-transformed cells, and also reacted with p50v-mybA, the transforming protein of the avian myeloblastosis virus. Antiserum to bp90 was absorbed with a bacterially synthesized v-mybA protein to remove anti-myb activity. The absorbed anti-bp90 serum retained the ability to immunoprecipitate P135gag-mybE-ets from E26-transformed cells and specifically reacted with a 56-kilodalton polypeptide (p56) detected in chicken lymphoid organs and in T and B lymphocytes of both avian and human origin. The data suggest that p56 is a translational product of the c-ets proto-oncogene and imply that p56 may be involved in regulating the growth of lymphoid cells.

Depression of reverse transcriptase activity by hybridoma supernatants: a potential problem in screening for retroviral contamination

Murine hybridoma cell lines generally produce retroviral particles (type A and/or type C), often in large numbers. We have measured reverse transcriptase activity in the supernatant of some 30 hybridoma lines of murine origin and found that the observed activity expressed as pmoles of [3H]-dGMP incorporated into an acid insoluble polymer, is frequently much lower than would be expected from the amount of retrovirus seen by electron microscopy in the corresponding cells. We demonstrate that this reduction is due to the presence of a nuclease which degrades the high molecular weight product but is not due to a change in the reverse transcriptase activity. This nuclease activity may be associated with mycoplasma contamination of the cell lines.

Target cells infected by avian erythroblastosis virus differentiate and become transformed

Transformation in vitro of bone marrow cells by avian erythroblastosis virus (AEV) gives rise to rapidly growing cells of erythroid nature. Target cells of neoplastic transformation by AEV are recruited among the early progenitors of the erythroid lineage, the burst-forming units-erythroid (BFU-E). They express a brain-related antigen at a high level and an immature antigen at a low level. We show that AEV-transformed cells express low levels of the brain antigen and high levels of the immature antigen. Their response to specific factors regulating the erythroid differentiation indicates that they are very sensitive to erythropoietin. Furthermore, cells transformed by a temperature-sensitive mutant of AEV differentiate into hemoglobin-synthesizing cells 4 days after being shifted to the nonpermissive temperature. All these properties are similar to those of late progenitors of the erythroid lineage, the colony-forming units-erythroid (CFU-E). These results indicate that the AEV-transformed cells are blocked in their differentiation at the CFU-E stage.

Proposal for naming host cell-derived inserts in retrovirus genomes

We propose a system for naming inserted sequences in transforming retroviruses (i.e., onc genes), based on using trivial names derived from a prototype strain of virus.

In vitro translation of avian myeloblastosis virus RNA

Avian myeloblastosis virus (AMV)-infected cells contain two viral mRNA's, a genome-sized 34S (7.5-kilobase) mRNA and a 21S (2.5-kilobase) subgenomic mRNA, which contains the AMV-specific sequences (myb sequences). We found that AMV virions packaged both the 7.5-kilobase full-length genomic RNA and the 2.5-kilobase subgenomic RNA. In vitro translation of AMV virion RNA sized by sucrose density gradient centrifugation yielded 76,000-, 56,000-, 48,500-, 47,000-, and 32,000-dalton products. The 76,000-dalton protein was coded for by RNA throughout the gradient, but the peak of activity was at 34S to 35S. The 56,000-, and 48,500-, and 32,000-dalton proteins were encoded in a 21S RNA, and 47,000-dalton protein was encoded in an RNA of approximately 24S. The 76,000-dalton protein was identified as Pr76gag, based upon immunoprecipitation with specific antiserum and the presence of the 19* dipeptide. 7-Methylguanosine triphosphate inhibited the syntheses of Pr76gag and the 56,000-, 48,500-, and 32,000-dalton proteins, but not the synthesis of the 47,000-dalton protein. The 56,000-, 48,500-, 47,000-, and 32,000-dalton proteins were not immunoprecipitated by anti-gag, anti-reverse transcriptase, or anti-gp85 antiserum. Two-dimensional peptide maps of the 56,000- and 48,500-dalton proteins indicated that they were unique. In vitro translational products of myeloblastosis-associated virus 1 were also analyzed to aid in the identification of the AMV myb gene product(s); the translational products analyzed included Pr76gag, p60env, and a 56,000-dalton polypeptide which apparently was not identical to the 56,000-dalton AMV translational product, as determined by two-dimensional peptide mapping. Our data indicated that one of these proteins (56,000, 48,500, or 32,000 daltons) may represent the product of the AMV myb gene and, therefore, the putative transforming protein(s) of AMV.

Differential effects of transforming avian RNA tumor viruses on avian macrophages

Functionally differentiated chicken macrophages were derived by in vitro differentiation of embryonic yolk sac cells and were characterized by several macrophage-specific cell markers. Uniform, infected, virus-producing cultures were obtained after exposure of these macrophages to avian myoblastosis virus (AMV), avian myelocytomatosis virus (MC29), myeloblastosis-associated virus (MAV-2), and Prague strain of Rous sarcoma virus (PR-B RSV). Both AMV and MC29 induced morphological transformation typical of the in vivo leukemias induced by these virus strains. Analysis of the expression of macrophage-specific markers in these two transformed cell types demonstrated that different markers of the mature macrophage were suppressed by each virus, even though the parental cell immediately preceding the transformation event was a mature macrophage in both cases. Cells infected with PR-B RSV and MAV-2 showed no observable difference from uninfected macrophages in terms of morphological characteristics, growth rate, or expression of the differentiated functions of macrophages. Ths system provides demonstrations of a cell type that produces infectious, transforming RSV but fails to respond by functional alterations induced by the transforming gene, src.

The genome and the intracellular RNAs of avian myeloblastosis virus

Avian myeloblastosis virus (AMV) is an acute leukemia virus which causes a myeloblastic leukemia in birds and transforms myeloid hematopoietic cells in vitro. We have analyzed RNA from AMV virions and from AMV-transformed producer and nonproducer cells by gel electrophoresis followed by transfer to chemically activated paper and hybridization to several complementary DNA (cDNA) probes. Using a cDNA probe specific for AMV, we identified two RNA species of 7.2 and 2.3 kb, which were present in all AMV-transformed cells and in all AMV virion preparations examined. The 7.2 kb species, which is presumably the genome of AMV, appears to contain the entire retroviral gag gene and at least part of the pol gene, but lacks much (or all) of the env gene. Thus AMV differs from other acute leukemia viruses described to date, since the latter have genomes of 5.5 to 5.6 kb, have only part of the gag gene and lack pol sequences. The smaller RNA does not contain gag-, pol- or env-specific nucleotide sequences but does carry nucleotide sequences from both the 5' and 3' termini of the genome, suggesting that it may be a subgenomic mRNA. Both the 7.2 and 2.3 kb species were associated with the 70S RNA complex in virions. These results suggest that AMV, unlike other acute leukemia viruses, does not express its transforming gene via a gag-related "fusion" protein but rather as a (so far unidentified) protein translated from a subgenomic mRNA.

Expression of endogenous avian myeloblastosis virus information in different chicken cells

Uninfected chicken cells were found to contain endogenous avian myeloblastosis virus (AMV)-specific information. Different tissues from chicken embryos and chickens expressed different amounts of the AMV-specific information. The endogenous AMV-related RNA was most abundant in bone marrow cells, which contained about 20 copies per cell. About 5 to 10 copies of AMV endogenous RNA per cell were found in embryonic yolk sac cells and bursa cells. The spleen, muscle, liver, and kidney cells of chickens and the fibroblasts of chicken embryos contained about two copies per cell. The amounts of AMV endogenous RNA in bone marrow, yolk sac, and bursa varied with age. From 19-day-old embryos to 2-week-old chickens, the bone marrow contained 20 copies of AMV RNA per cell. Bone marrow cells from 2-year-old chickens contained five copies per cell. Yolk sac cells of 10-day-old embryos and 1-day-old chickens were found to contain two copies per cell, whereas in 15- to 17-day-old embryos, these cells contained 5 to 10 copies. These results indicate that the level of endogenous AMV expression correlates with the development of granulopoiesis of the chicken hemopoietic system. The results of experiments on the thermostability of RNA-DNA hybrids indicated that the endogenous AMV RNA is closely related to viral AMV RNA. The expression of endogenous AMV information is independent of the activity of the chick helper factor. This endogenous AMV information is expressed as 20 to 21S RNA in both bone marrow and yolk sac cells.

Genetic structure of avian myeloblastosis virus, released from transformed myeloblasts as a defective virus particle

Chicken myeloblasts transformed by avian myeloblastosis virus (AMV) in the absence of nondefective helper virus (termed nonproducer cells) were found to release a defective virus particle (DVP) that contains avian tumor viral gag proteins but lacks envelope glycoprotein and a DNA polymerase. Nonproducer cells contain a Pr76 gag precursor protein and also a protein that is indistinguishable from the Pr180 gag-pol protein of nondefective viruses. The RNA of the DVP is 7.5 kilobases (kb) long and is 0.7 kb shorter than the 8.2-kb RNAs of the helper viruses of AMV, MAV-1 and MAV-2. Comparisons based on RNA.cDNA hybridization and mapping of RNase T1-resistant oligonucleotides indicated that DVP RNA shares with MAV RNAs nearly isogenic 5'-terminal gag and pol-related sequences of 5.3 kb and a 3'-terminal c-region of 0.7 kb that is different from that found in other avian tumor viruses. Adjacent to the c-region, DVP RNA contains a contiguous specific sequence of 1.5 kb defined by 14 specific oligonucleotides. Except for two of these oligonucleotides that map at its 5' end, this sequence is unrelated to any sequences of nondefective avian tumor viruses of four different envelope subgroups as well as to the specific sequences of fibroblast-transforming avian acute leukemia and sarcoma viruses of four different RNA subgroups. The specific sequence of the DVP RNA is present in infectious stocks of AMV from this and other laboratories in an AMV-transformed myeloblast line from another laboratory, and it is about 70% related to nucleotide sequences of E26 virus, an independent isolate of an AMV-like virus. Preliminary experiments show DVP to be leukemogenic if fused into susceptible cells in the presence of helper virus. We conclude that DVP RNA is the leukemogenic component of infectious AMV and that its specific sequence, termed AMV, may carry genetic information for oncogenicity. Thus we have found here a transformation-specific RNA sequence, unrelated to helper virus, in a highly oncogenic virus that does not transform fibroblasts.