Generation of high-titer RCAS virus from DF1 chicken fibroblasts

RCAS viruses are replication-competent in avian cells, but are replication-deficient in mammalian cells. Therefore, high-titer RCAS virus stocks can be generated only in avian cells. The chicken fibroblast cell line DF1 is well suited for this purpose. Successful infection of target mammalian cells, particularly in vivo, is dependent on the production of high titer viruses by DF1 cells. Moreover, consistency in viral titer helps to ensure uniformity in results produced following the use of independent lots of virus producer cells. Therefore, it is critical to determine the viral titer before initiating these experiments. Because several factors, including insert size and the effect of the inserted gene product on the viability of DF1 cells, influence viral titer, the production of high virus titers cannot be assumed. For RCASBP-A-based viruses, a titer of >1 × 10(7) IU/mL is considered appropriate. Importantly, the virus reverse transcriptase is error prone; errors will accumulate in the virus produced over time. Therefore, virus producer cells should not be cultured for >4-6 wk before being replaced with fresh producer cells. Low passage virus producer cells may be frozen and stored at -80°C; thawed cells will not display a reduction in virus titer. Virus can be collected regularly, concentrated, and stored at -80°C for long-term use; thawed viral stocks typically show a 10-fold decrease in titer.

A threshold level of gag expression is required for particle formation in rat cells transformed by avian retroviruses

To test the hypothesis that the block of polyprotein precursor processing and particle formation in RSV-transformed mammalian cells is due to a low level of pr76gag expression, rat tumor cell lines with different amounts of precursor molecules were used. The wild-type forms of pr76gag have been expressed at a high level by use of SV40-based vector and thirty-two stable transfected cell clones were isolated. The gag protein expression was detected in the cell lysate by immunoblotting. Untransfected cells released no proteins that could be detected by immunoprecipitation with anti-RSV serum. Membrane-enclosed gag precursor-polyprotein molecules and infectious virus particles from different stably transfected clones have been found in the medium. Both immature and mature virions of type C morphology were directly detected by transmission electron microscopy. Surprisingly, virus-like particles of morphology similar to mature type C retroviruses were found enclosed within intracellular membranes in a stably transfected nonproducing clone.

High prevalence of "Simkania Z," a novel Chlamydia-like bacterium, in infants with acute bronchiolitis

The newly described microorganism "Simkania Z" ("Z"), an obligate intracellular, penicillin-resistant microorganism most closely related to the chlamydiae, has been associated with adult community-acquired pneumonia. The possible involvement of "Z" in bronchiolitis in infants was examined in a prospective study of 239 infants with bronchiolitis and 78 controls. Other potential etiologic agents sought were respiratory syncytial virus (RSV), adenovirus, and cytomegalovirus. Evidence for the presence of "Z" in nasopharyngeal wash specimens (polymerase chain reaction and/or culture) was found in 25% of infants with bronchiolitis, while controls were all negative (P < .001). A serum IgA response to "Z" infection was detected by immunoperoxidase assay in 15% of infants with bronchiolitis versus 1.3% of controls (P < .001). Clinical findings were not different for infants with bronchiolitis associated with RSV alone, "Z" alone, or RSV and "Z" together. The high prevalence of "Z" in infants with bronchiolitis, often accompanied by an immune response, suggests a possible etiologic role of this agent in the disease.

Selection of an avian retrovirus mutant with extended receptor usage

Receptor recognition by avian retroviruses is thought to involve the interaction of two regions of the SU protein, hr1 and hr2, with the host cell surface receptor. These regions exhibit considerable variation, concordant with differences in receptor usage among the many avian leukosis virus subgroups. We hypothesize that some retroviruses have altered receptor usage in response to selective pressures imposed by receptor polymorphisms in their hosts. To test this hypothesis, we passaged td-Pr-RSV-B on cocultured permissive chicken (C/E) and nonpermissive quail (QT6/BD) cells. A variant virus with an expanded host range was identified at passage 29 and ultimately shown to be identical in sequence to td-Pr-RSV-B, except for changes at codons 155 and 156 of SU amino acid corresponding to two amino acid changes within hr1. Superinfection resistance studies suggest that the variant virus recognizes the subgroup B receptor on chicken cells and the subgroup E receptor on quail cells. These findings indicate that altered receptor usage can be conferred by small changes in env and may point to a key region for receptor interaction. Further, they demonstrate the evolutionary potential of retroviral env genes to alter receptor usage in response to appropriate selective pressure.

Injury models of the vascular endothelium: apoptosis and loss of thromboresistance induced by a viral protein

Endothelial injury caused by viruses usually involves viral replication or transformation. We report a novel mechanism of endothelial damage by a toxic viral protein. We have isolated a new retrovirus from hemangiosarcomas which appeared among layer hens. The isolated avian hemangiosarcoma virus (AHV) is capable of inducing hemangiomas in hens in-vivo and causes a cytopathic effect (CPE) and loss of thromboresistance in cultured bovine aortic endothelial cells (BAEC). These effects do not require viral replication and can be induced by purified AHV envelop glycoprotein (gp85). AHV causes CPE in BAEC through a typical programmed cell death (apoptosis). Quiescent G0/G1-BAEC are much more sensitive to AHV induced apoptosis than actively dividing cells. These experiments demonstrate the capacity of viral proteins to affect the integrity and functionality of vascular endothelial cells.

Characterization of a small (25-kilodalton) derivative of the Rous sarcoma virus Gag protein competent for particle release

Retroviral Gag proteins have the ability to induce budding and particle release from the plasma membrane when expressed in the absence of all of the other virus-encoded components; however, the locations of the functional domains within the Gag protein that are important for this process are poorly understood. It was shown previously that the protease sequence of the Rous sarcoma virus (RSV) Gag protein can be replaced with a foreign polypeptide, iso-1-cytochrome c from a yeast, without disrupting particle assembly (R. A. Weldon, Jr., C. R. Erdie, M. G. Oliver, and J. W. Wills, J. Virol. 64:4169-4179, 1990). An unexpected product of the chimeric gag gene is a small, Gag-related protein named p25C. This product was of interest because of its high efficiency of packaging into particles. The goal of the experiments described here was to determine the mechanism by which p25C is synthesized and packaged into particles. The results demonstrate that it is not the product of proteolytic processing of the Gag-cytochrome precursor but is derived from an unusual spliced mRNA. cDNA clones of the spliced mRNA were obtained, and each expressed a product of approximately 25 kDa, designated p25M1, which was released into the growth medium in membrane-enclosed particles that were much lighter than authentic retrovirions as measured in sucrose density gradients. DNA sequencing revealed that the clones encode the first 180 of the 701 amino acids of the RSV Gag protein and no residues from iso-1-cytochrome c. This suggested that a domain in the carboxy-terminal half of Gag is important for the packaging of Gag proteins into dense arrays within the particles. In support of this hypothesis, particles of the correct density were obtained when a small segment from the carboxy terminus of the RSV Gag protein (residues 417 to 584) was included on the end of p25.

Retroviral integrase functions as a multimer and can turn over catalytically

A number of studies have demonstrated that the retroviral protein integrase (IN) alone is sufficient to carry out two discrete steps required for retroviral integration: the endonucleolytic processing of viral DNA ends and the cleavage and joining of host DNA to the processed viral DNA termini. Little is known about the biochemical and biophysical mechanisms involved in these reactions. Here, we employ in vitro assays of Rous sarcoma virus IN to demonstrate for the first time that IN is capable of multiple turnover in both the processing and joining reactions. The turnover number calculated for the processing reaction is 0.26 cleavages/min/mol of IN. Our steady state kinetic studies indicate that both the processing and joining activities require a multimeric form of IN. Ultracentrifugation analyses reveal a substrate-independent reversible equilibrium among the monomeric, dimeric, and tetrameric forms of this protein. From these results we conclude that the minimal functional unit for both the processing and joining of each viral DNA end is an IN dimer.

Multiple complex families of endogenous retroviruses are highly conserved in the genus Gallus

We have analyzed the genome of the domestic chicken for the presence of genetic sequences related to the envelope protein-encoding genes of avian sarcoma/leukosis retroviruses to determine the organization, structure, potential functionality, and distribution of such sequences. We have previously identified in the genus Gallus an extensive group of endogenous avian retroviruses termed EAV-0. Southern blot and sequence analysis presented here of EAV-0 elements revealed that the majority of the EAV-0 elements in the domestic chicken genome have large deletions in their env genes. Screening of a line 0 chicken genomic DNA library for potential full-length env gene-containing endogenous elements yielded three provirus clones of a previously unrecognized group of endogenous retroviruses. These three clones, E13, E33, and E51, are more closely related to each other (80% or more sequence identity) than to other avian retroviruses (70% or less sequence identity). The E13 element has a large deletion in env, but the E51 element has full-length and highly divergent SU- and TM-coding domains. Complete sequence analysis of the E51 env gene region revealed a defective SU-coding domain and an intact TM-coding domain. Sequence analysis of the E51, E33, and E13 3' termini revealed highly distinctive long terminal repeats of approximately 360 bp which appear to be the products, in part, of long terminal repeat domain shuffling. Hybridization analysis with E51 and E33 env gene probes indicated that they are members of an extensive group of elements present in all Gallus species, and at least one element, E51, could be shown by polymerase chain reaction amplification and direct sequencing to have integrated prior to Gallus speciation.

Phosphorylation of integrin in differentiating ts-Rous sarcoma virus-infected myogenic cells

The differentiation of primary myogenic cultures requires the attachment of the cells to an extracellular matrix substrate using an integrin family receptor. These integrin receptors can be phosphorylated on both their alpha and beta chains, and it has been postulated that phosphorylation regulates the receptor function. Quail myogenic clones transformed with ts-LA24A differentiated into mature myotubes following a temperature shift to nonpermissive temperature which inactivates the viral src kinas. Phosphorylation of integrin beta-1 chain and of at least one alpha chain was detected on both serine and tyrosine. An additional alpha chain(s) with a mobility similar to alpha 5 was not phosphorylated at either temperature. Following the induction of differentiation by a temperature shift, there was a marked decrease in integrin phosphorylation of both alpha and beta integrin chains. This decrease was more prominent for serine than for tyrosine, suggesting that src could not be the only kinase involved. The drop in integrin phosphorylation correlated with the initiation of differentiation, suggesting that integrin phosphorylation could be at least part of the mechanism by which myogenic differentiation is blocked by v-src.

A variant Schmidt-Ruppin strain of Rous sarcoma virus with increased affinity for mammalian cells

SR-RSV-D(H), a variant virus with extremely high tropism for mammalian cells, was isolated by passage of the Schmidt-Ruppin strain of Rous sarcoma virus of subgroup D (SR-RSV-D) through hamster cells. This variant virus has acquired an altered envelope glycoprotein, encoded by the env gene, that has high affinity for receptors on the surface of mammalian cells. The variant virus transforms rat cells at about 100 times the efficiency of the parental virus, SR-RSV-D(S), as assayed by focus formation. Addition of amphotericin B (Fungizone) to the medium at a concentration of 0.2 micrograms/ml completely inhibited rat cell transformation by SR-RSV-D(H), possibly by blocking virus penetration into the cells, whereas the drug showed no inhibitory effect on transformation of chick embryo fibroblast (CEF) cells by the variant virus or on transformation of rat cells by the parental virus. The efficiency of transformation of rat cells by the variant virus was much less than its efficiency of transformation of CEF cells. Analysis of infection of rat cells suggested that the virus can infect rat cells as efficiently as CEF cells but that rat cells were not transformed by the virus as fully as CEF cells because of inefficiency of some post-penetrational step involved in viral gene expression. The finding that E1AY cells, rat cells expressing adenovirus E1A gene, were transformed by SR-RSV-D(H) as efficiently as CEF cells supports this conclusion and suggests that expression of the E1A gene in rat cells may overcome the defect in the transforming step(s) in rat cells.

Evaluation of the cocarcinogenic effect of wounding in Rous sarcoma virus tumorigenesis

Chickens given injections of Rous sarcoma virus form sarcomas at the site of inoculation (primary tumor) and at the site of experimentally introduced wounds (wound tumor). This latter finding provides a model system to study systematically the mechanisms underlying the cocarcinogenic effects of wounding. Our experiments show the following. (a) Chickens inoculated with a Rous sarcoma virus-derived, replication-defective virus construct fail to elaborate wound tumors in spite of aggressively growing primary tumors. We thus rule out metastasis as a mechanism and conclude that infectious virus is required for wound tumor formation; (b) using bromodeoxyuridine incorporation and immunofluorescence on frozen sections we demonstrate proliferation in the unwounded wing in cell types which are normally targets for Rous sarcoma virus infection and transformation and conclude that proliferation per se is not sufficient to induce wound tumors; (c) using immunohistochemistry for the viral protein p19gag we show that wounding induces virus expression in fibroblasts of newly forming granulation tissue 2 days after injury. We also demonstrate expression of viral mRNA in wound tumors by in situ hybridization with a v-src probe. We discuss the possibility of activation of integrated, silent virus or the preferential infection of a special target cell population as a result of wounding as well as the potential role of wound factors in transformation.

A newly isolated avian sarcoma virus, ASV-1, carries the crk oncogene

Avian sarcoma virus 1 (ASV-1) was isolated from a spontaneous sarcoma of an adult chicken. It is a replication-defective virus that induces fibrosarcomas in young chickens and oncogenic transformation in cultured chick embryo fibroblasts. The ASV-1 genome has been cloned in the lambda gt WES.lambda B vector from closed circular viral DNA extracted from infected cells. The nucleotide sequence of the oncogene insert in the ASV-1 genome has been determined. It is virtually identical to the sequence of the oncogene crk recently discovered in the avian sarcoma virus CT10. ASV-1 and CT10 are two independent retrovirus isolates carrying the crk oncogene.

Rescue of infectious virus from nonproducer Rous cells by chick cellular DNA

The rescue of infectious virus from nonproducer BH RSV(-) cells by chick cellular DNA was attempted in order to investigate the functional state of endogenous and exogenous retroviral genes integrated within the cellular DNA. No infectious virus was rescued by transfection with DNAs of chick helper factor (chf)-negative chick embryo cells (CEC), chf-positive CEC or uninfected CEC producing endogenous Rous associated virus (RAV-0). On the other hand, infectious Rous viruses with the phenotype of RAV-0 and RAV-1 were rescued by transfection with DNAs of CEC which had been infected with RAV-0 and RAV-1. From these results, it seems that exogenous retroviral genes integrated in the cellular DNA are expressed rather easily by transfection while those present endogenously are not.

Cellular immunity in chicks expressing fibrosarcomatous liver tumor following embryo infection with subgroup A Rous sarcoma virus

Bryan standard strain of Rous sarcoma virus (BS-RSV) of subgroup A was inoculated into heavy and light breeds of chicken embryos via chorioallantoic membrane (CAM) to ascertain cell-mediated immune response, as measured by a leukocyte migration inhibition (LMI) test. Chicks hatched from eggs with pock-positive CAMs were more likely to develop liver tumors than those hatched from eggs with pock-negative CAMs. Chicks that developed tumors usually had a positive cell-mediated immune response, and those that were negative for liver tumor were negative, based on the LMI test.

[Occurrence of viral particles of avian sarcoma viruses in sarcoma tissue]

An electron-microscopic examination was performed of chicken fibrosarcoma caused by avian sarcoma virus (ASV), strain B 77, to investigate virus budding and release through the cytoplasmic membrane. The virus particles of type C- were 90-100 nm in size, the electron-optically denser nucleoids being clearly differentiated from the outer membrane.

Transduction of the cellular src gene and 3' adjacent sequences in avian sarcoma virus PR2257

When injected into chickens, a transformation-defective mutant of the Prague C strain of Rous sarcoma virus induced tumors at low incidence and after a long latency. One such tumor released a replication-defective virus designated PR2257. We molecularly cloned and sequenced the proviral DNA from quail fibroblasts transformed by PR2257. Comparison of PR2257 sequence with that of Prague C, cellular src, and 3' adjacent cellular DNA showed that the spliced version of the c-src gene and about 950 base pairs (bp) of 3'-flanking cellular DNA were transduced into PR2257. This transduction eliminated nearly all replicative genes, since the gag gene splice donor site was linked to the splice acceptor site of the src gene and, on the 3' side, recombination occurred in the end of env gene. Insertion of two extra cytosines 23 bp before and 19 bp after the c-src stop codon resulted in an extension of the coding portion up to 587 amino acids, divergence of sequences after Pro-525 and replacement of Tyr-527 by a valine residue. In addition, it appears that the 5' and 3' untranslated regions of PR2257 result from multiple recombinations between exogenous and endogenous virus genomes. Limited digestion of p66src encoded by PR2257 with Staphylococcus aureus V8 protease yielded a V2 peptide (C-terminal moiety) with an apparent molecular mass of 31 kilodaltons, consistent with the 5.7-kilodalton increase expected from the DNA sequence. The structure of PR2257 suggests that the first step in the capture of c-src gene by avian lymphomatosis viruses is the trans splicing of the viral leader mRNA to exon 1 of c-src.

The LTR, v-src, LTR provirus generated in the mammalian genome by src mRNA reverse transcription and integration

Different types of altered proviruses of Rous sarcoma virus (RSV) have been detected in mammalian tumor cell lines. We cloned and sequenced one of these altered proviruses with the structure LTR, v-src, LTR. The presence of an intact viral splice junction, as well as duplications of the chromosomal sequence GCGGGG flanking the two 2-base-pair-deleted LTRs, demonstrated reverse transcription and normal retroviral integration of src mRNA in mammalian cells. In addition, a 1-nucleotide deletion 2 bases upstream from the AAUAAA polyadenylation signal is suspected to be responsible for the absence of a poly(A) track in the src mRNA present in virions of rescued viruses.

Tissue tropism and temporal expression of Rous sarcoma virus in embryonic avian limb in ovo

Embryonic avian tissue is resistant to the transforming potential of Rous sarcoma virus (RSV) in ovo. Analysis of the pattern of host-viral interactions in the first semester of chick development has demonstrated that RSV is first expressed in a limited population of muscle precursor cells and proceeds to spread throughout the developing dorsal and ventral limb musculature. The number of non-muscle cells participating in the infection is initially low but gradually increases as development continues. The data show that RSV infection in ovo is both compatible with the process of differentiation and the maintenance of the differentiated state of the limb. The kinetics of viral spreading and competence for transformation are developmentally regulated in the embryo. The contrasting properties of embryonic cells in ovo as compared with those of the adult provide an opportunity for evaluating host related regulatory factors that are of significance to the expression of viral transforming function.

Characterization of transforming viruses rescued from a hamster tumour cell line harbouring the v-src gene flanked by long terminal repeats

The organization of proviruses derived from infecting transforming viruses rescued from hamster tumour cells was studied. Southern blot analysis indicated that the provirus from the F6 cell line was organized as long terminal repeat (LTR)-src-LTR, and S1 mapping experiments suggested that it was probably derived by reverse transcription of src mRNA followed by integration. In the E6 cell line, the provirus unit was arranged as LTR-delta gag-src-LTR, indicating a recombination event between the rescued transforming virus and the helper virus. These results suggest that transforming defective viruses containing only the src gene can be rescued from nonpermissive mammalian cells.

[Characteristics of new loci of endogenous proviruses in chickens]

The composition and structure of endogenous proviruses in the genome of Italian partridge chicken was studied by the blot-hybridization method using 32P-DNA of RAV-2 virus as a probe. Among 34 fowls examined, 5-contained no endogenous proviruses related to fowl leukemia viruses. DNA of the other fowls contained various combinations of 4 previously undescribed endogenous proviruses varying in structure and localization. None of them was identical in its structure with DNA of endogenous fowl virus RAV-0, all the four discovered loci have deletions. The origin, modes of genetic variability, and function of endogenous proviruses are discussed.

Protein concentrations and immunosuppressive properties of serum in chickens experimentally infected with avian tumor viruses

Sera from chickens affected by Marek's disease or developing Rous sarcoma were investigated. There were changes in the protein fractions, and the amount of alpha and beta fractions was consistently increased. At the same time, immunosuppressive factors were found to inhibit the number of plaque-forming cells in the spleen of mice immunized with sheep red blood cells.

Isolation of three new avian sarcoma viruses: ASV 9, ASV 17, and ASV 25

The newly isolated avian sarcoma viruses, ASV 9, 17, and 25, cause fibrosarcomas in young chickens and induce foci of transformed cells in chick embryo fibroblast cultures. They are defective in replication and belong to envelope subgroup A. The sizes of their genomes are 6 kb (ASV 9), 5 kb (ASV 17), and 6 kb (ASV 25), respectively. All three contain long terminal repeat (LTR) and gag sequences but lack pol. env is absent from ASV 9 and ASV 25, but some env sequences are detectable in ASV 17. None of the defective viral genomes hybridized to selected onc probes representing src, fps, yes, myc, myb, and erb A. erb B appears absent from ASV 9 and ASV 17, but some hybridization between the erb B probe and the RNA of ASV 25 was detected. ASV 9 codes for a transformation-specific gag-linked protein of 130kDa. Multiple gag-linked transformation-specific proteins are seen in ASV 17 and 25; they require further study.

Characterization of two strains of avian sarcoma virus isolated from avian lymphatic leukosis virus-induced sarcomas

Two replication-defective avian sarcoma viruses, S1 and S2, which were independently isolated from tumors of chickens inoculated with avian lymphatic leukosis virus (LLV) were characterized. The genomes of S1 and S2 contain src-related sequences and are, respectively, about 3.9 and 4.5 kilobases long. pp60src-related proteins with molecular weights of 62,000 (p62) were detected in cells infected with these viruses, and protein kinase activity was found to be associated with these proteins. No other viral proteins, such as gag, pol, and env gene products, were detected. These results suggested that the c-src sequence in normal chicken cells was incorporated into LLV genomes by recombination at the expense of most of the viral genes to generate highly defective new sarcoma viruses.

Characterization of a replication-defective temperature-sensitive mutant of Rous sarcoma virus

A temperature-sensitive mutant (LA83) of Rous sarcoma virus defective both in the transformation and replication function has been isolated and partially characterized. Temperature-shift experiments showed that the defects in both the focus-forming and replication functions were late and continuous. The mutant LA83 was complemented by avian leukosis viruses. Complementation of LA83 replication was also observed with the glycoprotein-deletion mutant, Brian high-titer RSV(-) suggesting that the env gene in LA83 was not defective. At the nonpermissive temperature LA83-infected cells produced noninfectious particles with a yield of about 30%. The noninfectious particles had only about 3% of reverse-transcriptase activity as the infectious LA83 produced at the permissive temperature. However, the LA83 virions were as thermolabile as the parent wild-type PR-B virions.

Two strains of avian sarcoma virus newly isolated from chick fibrosarcomas induced by lymphatic leukemia virus subgroup A in two lines of chickens

Two strains of avian sarcoma virus, designated S1 and S2, have been newly isolated from avian lymphatic leukemia virus (LLV) subgroup A [LLV(A)]-induced fibrosarcomas in 2 chickens from 2 White Leghorn flocks of lines 151 and BK. Each stock of S1 and S2 contained 2-3 X 10(3) focus-forming units of virus/ml and 10(6) tissue culture infective dose of LLV(A)/ml. The focus-forming titer of S1 and S2 estimated in chick embryo fibroblast cultures was almost consistent with that estimated by tumor (fibrosarcoma) formation in the chicks. All of the chicks bearing wingweb tumors (fibrosarcomas at the site of inoculation) had macroscopically neoplastic lesions (fibrosarcomas) in a number of visceral organs, such as the lung, liver, and spleen. Each stock of S1 and S2 was cloned by picking a single focus, and each focus was examined for the production of focus-forming virus and LLV. The results suggest that S1 and S2 are heterogenous stocks of sarcoma viruses that are replication-defective to various degrees. The low titer of focus-forming virus in the stocks of S1 and S2 was considered to be due to the small amounts of avian sarcoma virus that could be complemented by LLV.

Isolation and characterization of host range mutants of avian sarcoma virus

Two host range mutants of avian sarcoma virus (ASV) were isolated from a clone of the Bratislava 77 strain, subgroup C (B77-C). An HR- mutant was obtained by adaptation to duck cells and showed almost the same efficiencies of transformation (e.o.t.) and growth in cultures of chick embryo fibroblasts (CEF) and duck embryo fibroblasts (DEF). An HR+ mutant showed lower e.o.t. and slower growth on DEF than on CEF. Characterization of these mutants and wild-type B77-C showed that (i) unlike the host range mutants reported previously, these mutants and wild-type B77-C have common antigenicity, (ii) the number of infective centres is almost the same for the two mutants and the wild-type, and so the adsorption and integration of these viruses are similar, and (iii) the content of viral RNA in DEF infected with HR- is increased and decreased in the case of HR+ compared with the wild-type.

Isolation of 16L virus: a rapidly transforming sarcoma virus from an avian leukosis virus-induced sarcoma

We have isolated a replication-defective rapidly transforming sarcoma virus (designated 16L virus) from a fibro-sarcoma in a chicken infected with td107A, a transformation-defective deletion mutant of subgroup A Schmidt-Ruppin Rous sarcoma virus. 16L virus transforms fibroblasts and causes sarcomas in infected chickens within 2 wk. Its genomic RNA is 6.0 kilobases and contains sequences homologous to the transforming gene (fps) of Fujinami sarcoma virus (FSV). RNase T1 oligonucleotide analysis shows that the 5' and 3' terminal sequences of 16L virus are indistinguishable from (and presumably derived from) td107A RNA. The central part of 16L viral RNA consists of fps-related sequences. These oligonucleotides fall into four classes: (i) oligonucleotides common to the putative transforming regions of FSV and another fps-containing avian sarcoma virus, UR1; (ii) an oligonucleotide also present in FSV but not in UR1; (iii) an oligonucleotide also present in UR1 but not in FSV; and (iv) an oligonucleotide not present in either FSV, UR1, or td107A. Cells infected with 16L virus synthesize a protein of Mr 142,000 that is immunoprecipitated with anti-gag antiserum. This protein has protein kinase activity. These results suggest that 16L virus arose by recombination between td107A and the cellular fps gene.

Fusion injection of Rous Sarcoma virus proteins into Rous sarcoma virus-transformed, non-producing hamster cells causes release of infectious virus

Purified virus proteins from transformation-defective (td) mutants of Rous sarcoma virus PrA or PrB were trapped in human erythrocyte ghosts which, after resealing, were fusion-injected into hamster RBH cells or rat TWERC cells. These cell lines are non-productively transformed by subgroup C Rous sarcoma virus. After fusion injection the hamster RBH cells released transforming subgroup C Rous sarcoma virus. No infectious virus could be rescued from rat TWERC cells. Since previous experiments have shown that fusion injection of the purified Rous sarcoma virus protein p15 into hamster RBH cells caused cleavage of the precursor protein pr76 to form the virus group-specific antigen (gag) but did not induce infectious virus, we conclude that in addition to p15 other virus proteins are required to induce virus rescue in hamster RBH cells.

The effects of T-cell growth factor and virus purification on virus-mediated inhibition of lymphocyte mitogenesis

Many different types of virus particles are able to non-specifically impede the ability of human peripheral blood lymphocytes to respond to mitogenic ro alloantigenic stimuli. This result is not obtained if ultra-purified virus is employed, although virus which has been banded only once through sucrose generally retains inhibitory potential. Ultra-pure virus is relatively unable to bind to cell surfaces, suggesting the importance of physical contact between viruses and cells in order for the observed inhibition to occur. Addition of exogenous T-cell growth factor (TCGF) to cultures containing virus, cells and stimulus causes a dose-dependent reversal of the usual inhibitory effect.

A ts T mutant of Schmidt Ruppin strain of Rous sarcoma virus restricted at 39.5 degrees C for the morphological transformation and the tumorigenicity of chicken embryo fibroblasts

In order to investigate a possible correlation between in vitro transformation and tumorigenicity in ovo, a new temperature-sensitive class T mutant of Rous Sarcoma Virus was isolated with a lower (39 degrees 5C) restrictive temperature for morphological transformation. This lower restrictive temperature was compatible with the survival of chicken and duck eggs for the tumorigenicity studies. In chicken embryo fibroblasts (CEF) infected by this new mutant, PA 17, and cultured at 39 degrees 5C, increase of hexose uptake, plasminogen activator production and anchorage-independent growth were only partially restricted, requiring incubation at 41 degrees 5C for a complete shut-off. Tumorigenicity in chicken and duck eggs inoculated with CEF infected and transformed by PA 17 was restricted at 39 degrees 5C, correlating well with the restriction of morphological transformation at this temperature. The kinase activity of the transforming protein pp60src in lysates of PA 17 infected cells cultured at permissive or restrictive temperatures was labile in RIPA buffer, as in the case of some previously examined ts T mutants. In the non-ionic detergent NP40 buffer, the kinase activity of PA17 infected cell lysates was better conserved and showed a moderate temperature dependence. These results suggest that, in spite of the correlations between the transformed cell phenotype in vitro and cell tumorigenicity in ovo, it is difficult to establish a quantitative relationship.

Spontaneous conversion of nontransformed avian sarcoma virus-infected rat cells to the transformed phenotype

Normal rat kidney (NRK) fibroblasts were infected with the Schmidt-Ruppin strain (SR-D) of avian sarcoma virus (ASV) and cloned 20 h after infection without selection for the transformed phenotype. Most infected clones initially exhibited the flat, nontransformed morphology that is characteristic of uninfected NRK cells. In long-term culture, however, the majority of the SR-D NRK clones began segregating typical ASV-transformed cells. Transforming ASV could be rescued by fusion with chicken embryo fibroblasts from most of the infected clones tested. Three predominantly flat, independently infected clones were further analyzed by subcloning 8 to 10 weeks after infection. Most flat progeny subclones derived at random from two of these "parental" SR-D NRK clonal lines did not yield virus upon fusion with chicken embryo fibroblasts, although a nondefective transforming ASV was repeatedly recovered from the parental clones. This observation suggested that most, but not all, daughter cells in these SR-D NRK clones lost the ASV provirus after cloning. The progeny of the third independent parental cell clone, c17, gave rise to both flat and transformed subclones that carried ASV. In this case, ASV recovery by fusion and transfection from the progeny subclones was equally efficient regardless of the transformation phenotype of the cells. The 60,000-dalton phosphoprotein product of the ASV src gene was, however, expressed at high level only in the transformed variants. The results of a Luria-Delbruck fluctuation analysis and of Newcombe's respreading test indicated that the event leading to the spontaneous conversion to the transformed state occurred at random in dividing cultures of these flat ASV NRK cells at a rate predicted for somatic mutation.

Potential diagnostic and prognostic significance of the transformation-enhancing factor(s) in the plasma cryoprecipitate of tumor patients

A biological activity which enhances cell transformation (TEF) in Rous sarcoma virus temperature-sensitive mutant-infected cultures has been evidenced in the plasma cryoprecipitate from patients affected with different types of neoplastic disease. In the present paper we report data on the analysis of TEF activity in the plasma cryoprecipitates from leukemic and tumor patients tested either before or during specific antineoplastic treatments. The screening of 57 cases of different neoplastic diseases and of 57 controls, healthy subjects or patients affected with other non-neoplastic diseases indicates that TEF activity is generally related to the presence of neoplasia. Furthermore, a follow-up of patients from the onset of the disease through its evolution during therapy suggests that variations of TEF activity in the plasma cryoprecipitate correlate well with the clinical and pathological conditions, thus indicating the TEF as a potential marker for monitoring cancer patients.

Lymphoid neoplasms in chicken flocks free of infection with exogenous avian tumor viruses

More than 4,500 breeding female chickens of nine inbred lines maintained under specific-pathogen-free conditions to approximately 500 days of age were studied. Routine monitoring and special assays indicated that they were free of infection by exogenous viruses of the leukosis-sarcoma and the reticuloendotheliosis groups. Some birds were maintained free of Marek's disease (MD) virus infection in plastic isolators, and others were maintained in conventional chicken houses and vaccinated with the herpesvirus of turkeys to prevent the lesions of MD. Ten birds bearing lymphoid tumors were observed in two sublines of one line of chickens known to produce embryos that spontaneously produce Rous-associated virus, type 0 (RAV-O), an endogenous virus of the chicken. Four tumors were found in chickens of one subline maintained free of MD virus infection in isolators. These tumors did not involve the bursa and had some histologic features different from those typical of lymphoid leukosis. Six tumors were found in chickens of the other subline that were vaccinated to prevent MD; these tumors involved the bursa and were typical of lymphoid leukosis but not MD. These results suggest that two types of tumors may have been observed. The fact that DNA extracted from both types of tumors did not contain exogenous lymphoid leukosis virus sequences confirms the virologic evidence that exogenous viruses were not involved. The fact that endogenous viral sequences were not increased in copy number suggests that the endogenous virus RAV-O did not directly induce the tumors. Two birds with tumors not involving the bursa were found alive, and transplantable lymphoid tumors were developed. These tumors were of T-cell origin rather than of bursa cell origin as would be expected of lymphoid leukosis. These are the first reported lymphoid tumors that have been observed in the absence of known exogenous tumor virus infection in chickens. Our evidence suggests that the endogenous virus RAV-O did not play a primary role in the induction of these tumors.

The infectivity of fluid from regressing Rous sarcomas

Fluid was withdrawn from the site of regressing Rous sarcomas in chickens and inoculated into the wing-webs of untreated chickens from three strains of chickens with divergent degrees of resistance to Rous sarcomas. The transfer of fluid initiated progressive tumors in all three strains of chickens. The infectivity of the fluid was apparently due to tumor cells and not to virus.

Complementation rescue of Rous sarcoma virus from transformed mammalian cells by polyethylene glycol-mediated cell fusion

Polyethylene glycol (PEG) is effective as a fusing agent for the rescue of virus from Rous sarcoma virus-transformed mammalian cells. The procedure of PEG-mediated rescue of virus from virogenic cell lines is described, and the technique is compared with that of Sendai virus-mediated rescue. Virus may be rescued quantitatively from virogenic cell lines by plating mitomycin C-killed transformed mammalian cells with chicken embryo cells, treating the monolayers with 50% PEG and overlaying the monolayers with focus agar. The number of foci that appeared reflected the number of heterokaryons in the fusion mixtures that released infectious virus. PEG gave reproducible results in virus rescue experiments with an efficiency equal to the best Sendai virus preparations. In addition to the description of the technique for PEG-mediated virus rescue from virogenic cell lines, a method for virus rescue from nonvirogenic lines is presented. Preinfection of the chicken embryo cells with helper avian leukosis virus (Rous-associated virus) prior to fusion with mammalian cells transformed by defective viruses complements the virus defect. We examined four nonvirogenic cell lines, and all released infectious virus in the complementation rescue assay.

A comparison of four methods used to concentrate Rous sarcoma virus from tissue culture fluids

Three methods of pelleting, pelleting followed by Pronase treatment, polyethylene glycol (PEG)-Pronase, and diaflo ultrafiltration (diafiltration) were used to concentrate RSV(RAV-1) from tissue culture fluids. Sucrose-gradient fractions containing virus preparations which had been concentrated by diafiltration or pelleting were heavily contaminated with amorphous debris. This debris was not present in similar, gradient-purified preparations that had been concentrated by the PEG-Pronase or pellet-Pronase methods. Maximum recovery of radiolabelled virus particles and virion-associated RNA-dependent DNA polymerase activity was obtained in gradient fractions containing virus concentrates prepared by the pellet-Pronase and PEG-Pronase methods. Although there were slight differences in recovery by these two methods, the advantages of the PEG-Pronase method make it the preferred method, especially when large volumes of tissue culture fluids are used.

High spontaneous mutation rate of an avian sarcoma virus

Three genetically distinct types of chicken sarcoma virus Bratislava 77 (B77 virus) differing in their ability to infect duck cells were identified. B77 virus type I does not infect duck cells; B77 virus type II has a low efficiency of infection of duck cells; and B77 virus type III has a high efficiency of infection of duck cells. B77 viruses type I and III are produced by spontaneous mutation during the growth of B77 virus type II in chicken cells. The spontaneous mutation of B77 virus type II TO B77 virus type III occurs with a high rate (approximately 1 mutation per 50 infected cell generations), requires cell replication, and neither occurs during the synthesis of viral DNA on an RNA template nor during the transcription of progeny viral RNA from the provirus. The rate of spontaneous mutation of B77 virus type II to B77 virus type I is greater than the rate of spontaneous mutation of B77 virus type II to B77 virus type III.

Inhibition by methioninyl adenylate of focus formation by Rous sarcoma virus

Methioninyl adenylate is a specific and potent inhibitor of the enzyme methionyl-tRNA synthetase and, consequently, of protein biosynthesis. In cultures of chick embryo fibroblasts infected with Rous sarcoma virus, incubation for a 2-day period with 1 to 3 mM concentrations of this inhibitor, as late as 4 days after infection, irreversibly prevented subsequent formation of foci of transformed cells. Later addition could also elicit the irreversible disappearance of already existing foci, by phenotypic reversion and/or cell killing. Virus production in transformed cells and replication in newly infected cells were also inhibited but to a lesser degree. Under the same conditions, the same concentrations of methioninyl adenylate caused only a reversible growth arrest of normal cells. The selective toxicity of the inhibitor for transformed cells is not due to a greater affinity for the target enzyme, but it may be due to the fact that inhibition of protein biosynthesis is only partially reversible in these cells, whereas it is fully reversible in normal cells.

Screen for type-C ribonucleic acid viruses in vaccines using the ribonucleic acid-dependent deoxyribonucleic acid polymerase assay

The ribonucleic acid-dependent deoxyribonucleic acid polymerase assay was used to detect type-C viruses in live virus vaccines. Conditions were first established to maximize the sensitivity of the assay. Vaccines tested included live poliomyelitis, rubella, measles, mumps, and yellow fever. Only yellow fever and measles vaccines known to have been produced in avian leukosis-contaminated cells showed evidence of type-C viruses using the assay. The result of the survey show that the assay has direct practical application to the problem of detecting latent agents in biological products intended for human use.

Contamination of some avian species with viruses of the leukosis-sarcoma complex

Contamination of 8 avian species with leukosis viruses was studied. An insignificant portion of sera from 4 and 24 months old chickens contained neutralizing antibody to 3 sero-types (A, B and D) of Rous sarcoma virus (RSV). Sera from guinea-fowl reacted positively in the neutralization test only with the RSV-RAV-1 strain. None of the Peking duck and semi-domestic Maran fowl sera was found to contain antibody to RSV strains. Twenty-five and 6.6% of embryos derived from 7-8 and 24 to 30 months old chickens respectively, 30% of liver specimens from 3-4 months old chickens 3.3% of liver specimens from adult guinea-fowl and 72% of liver specimens from Maran fowl contained the group-specific leukosis complement-fixing antigen (gs-antigen). Duck embryos and livers from 24-30 months old chickens, crows, sparrows, rooks or jackdaws contained no gs-antigen. Duck embryos did not react and ducklings reacted poorly to inoculation with RSV whereas guinea-fowl embryos and chickens proved to be highly susceptible to RSV.

Infectivity of Rous sarcoma cell DNA: comparison of two techniques of transfection assay

A DNA extracted from a clone of chicken cells transformed by the Schmidt-Ruppin strain of Rous sarcoma virus, subgroup D(SR-RSV-D), was assayed for infectivity by means of DEAE-dextran and calcium techniques. The calcium technique like the previously described DEAE-dextran procedure gave rise to viruses in transfection assays with both native and denatured (S1 nuclease susceptible) DNAs. The efficiency of these transfection techniques with native DNA was compared and found to be about the same provided that with the calcium technique carrier DNA was used to complement DNA concentrations lower than 2.5 mug/ml.