[Biology, serology and biochemistry of leucemogenic viruses derived from a radiation-induced tumor of C57BL mice]

Two distinct rat propagates of a radiation leukemia virus (RadLV-Rs) from the C57BL mouse respectively induced characteristic leukemogenic effects. These were found to be related with the infection titers of the isolates, but not with either their antigenic specificities or their viral and proviral genome sequences.

Generation of oncogenic mouse type C viruses: in vitro selection of carcinoma-inducing variants

Type C retroviruses are endogenous in most vertebrate species. These viruses generally are of low pathogenicity in their natural hosts. Variants that contain cell-derived transforming genes have been isolated infrequently in the past upon continued passage in vivo. We report here a procedure that allows the isolation of new mouse leukemia-, sarcoma-, and carcinoma-inducing type C viruses entirely in cell culture. The viruses generated after passage in chemically transformed mouse cells and selectron in epithelial mink lung cells produced pulmonary adenomas and adenocarcinomas and also ovarian carcinomas. Viruses with cell-transforming capacity, as determined by their ability to induce normal murine or mink cells to form progressively growing colonies in soft agar, appeared only transiently (2-4 days) after acute infection of "spontaneously" or chemically transformed mouse cells by nontransforming helper viruses. These transiently appearing transforming viruses can be "captured" by selecting the cells that respond to their newly acquired growth-stimulating ability. This system may lend itself to the systematic isolation of tissue-specific transforming functions from any cell that can be efficiently infected by retroviruses.

In vivo infectivity of the fibrotropic C-type viral isolates from C57BL/Ka mice

Of the three fibrotropic C-type viral isolates from C57BL/Ka mice, only the BL/Ka(B) virus is capable of infecting normal hematopoietic and lymphoid cell populations of C57BL/Ka mice in vivo, and none are tumorigenic. Inoculation of this virus alone into neonates resulted in transient replication in the bone marrow, spleen, and occasionally the thymus. Thymocytes could, however, be permanently infected in such animals if BL/Ka(B) were coinoculated with the xenotropic BL/Ka(X) virus. Neonatal injection of BL/Ka(B) prior to fractionated wholebody irradiation yielded an increase in the percentage of virus-productive radiogenic lymphomas and a decrease in incidence of such tumors. Injection of BL/Ka(B) into normal adult C57BL/Ka mice did not yield overt expression of virus replication in any of the tissues tested; latent infection could, however, be detected in the marrow and in the reticuloepithelium of the thymus. Whole-body X-irradiation of adults with 400 rads partially restored the neonatal susceptibility of bone marrow cells to infection by the isolate. BL/Ka(B) injection after fractionated whole-body irradiation of weanling C57BL/Ka mice increased the percentage of virus-positive lymphomas and revealed that a bone marrow cell subpopulation permissive for infection by the virus increases greatly in abundance soon after irradiation.

Studies on the infectivity of viral nucleic acids performed in the "Stefan S. Nicolau" Institute of Virology

The investigations on viral nucleic acids (NA) performed in the Institute of Virology since 1957 refer to the infectivity of NA (RNA or DNA) from large, complex viruses, from RNA viruses (ECHO, MM Coxsackie, etc.), from modified viruses, as well as to the biological activity (oncogenicity and infectivity) of NA from tumors and leukemic tissues of human, murine, bovine and avian origin. Studies are reviewed as regards the action of physical and chemical agents on the behaviour of viral and cellular NA and the implications of the use of such agents in viral vaccine preparation are discussed.

Establishment and characterization of C-type RNA virus-producing cell lines from radiation-induced murine osteosarcomas

Eight cell lines were established from murine osteosarcomas induced in vivo with the radionuclides 224Ra and 227Th. They have been compared by light and electron microscopy, by karyology, and by their growth properties. The morphology, the growth pattern, and the ability to induce tumors in mice indicate that five of them are tumor cell lines. Chromosome studies demonstrated that the five cell lines have marker chromosomes. The other cell lines only showed some criteria generally used to score for transformation of fibroblasts and they may be derived from stromal cells. All cell lines release virus particles in the culture fluid which have the typical properties of RNA tumor viruses. They possess C-type morphology, a density of 1.16--1.18 g/cm3, a 60--70 S RNA, a RNA dependent DNA polymerase and they induce syncytia in rat XC cells. The possible significance of these virus particles in radiation osteosarcomagenesis is discussed.

Multiple sclerosis-associated agent. Failure of MS brain and serum to depress the polymorph count in normal mice and mice inoculated with cells containing C/type particles

The groups that originally reported and confirmed the demonstration of a multiple sclerosis associated agent (MSAA) are now, along with others, unable to reproduce this effect. In view of this confusion and the potential importance of this work for multiple sclerosis (MS) we have done a strict double-blind trial using larger groups of mice (10) and counting more cells (900) than in previous reports to offset the high variability of mouse polymorphonuclear neutrophil (PMN) counts. Sera from 5 active MS patients and 4 normal subjects were tested in mice, half of which had previously been injected with PAM line cells (containing C-type particles and subject to reduced cell yield when cultured with MS brain extract). No significant PMN depression was found in either MS or normals on any basis of comparison. However, a significant depression was seen following PAM cell injection irrespective of serum origin. Higher counting accuracy did not reduce PMN variability. A single MS brain specimen was also without effect. consequently we have been unable to confirm the existence of an MSAA as defined by PMN depression in mice.

Viral studies in streptozotocin-induced pancreatic insulitis

Multiple injections of streptozotocin to Charles River (CD-1) Laboratory mice resulted in a syndrome characterised by diabetes mellitus, insulitis and the induction of endogenous type C viruses in pancreatic beta cells. Within one week after the completion of five intraperitoneal injections of streptozotocin, the CD-1 mice exhibited irreversible hyperglycaemia and insulinopaenia. Light microscopic studies of pancreata from mice sacrificed at this time demonstrated insulitis and beta cell necrosis. Electron microscopic studies revealed spherical and atypical cylindrical type C viruses and occasional clusters of intracisternal type A viruses exclusively within beta cells. To clarify the identification of the type C viruses and their role in the genesis of the insulitis, type C virus specific antigens were identified within islet cells by immune fluorescence at various intervals after streptozotocin administration. Immune fluorescence studies demonstrated the presence of type C virus antigens within islets from streptozotocin treated mice but not in buffer-injected controls. Time course studies suggested that type C virus induction may precede the appearance of insulitis by two days and that insulitis is consistently accompanied by the presence of virus positive islet cells.

Generation of new mouse sarcoma viruses in cell culture

Endogenous nontumor-producing type C viruses from C3H mice were used to generate rapid, solid tumor-inducing variants in cell culture. The new mouse sarcoma viruses induce undifferentiated sarcomas with a short latency period upon inoculation into newborn NIH Swiss mice. Transforming viruses appear only transiently, at a time when the virus-infected cells show morphologic alterations; both before and after this time, transforming viruses cannot be detected. These results show that variants of endogenous type C virus which contain transforming genes (oncogenes) can arise during spread of the endogenous virus in fibroblast lines in vitro as well as in susceptible tissues in vivo.

Generation of oncogenic type C viruses: rapidly leukemogenic viruses derived from C3H mouse cells in vivo and in vitro

A type C virus was isolated from C3H/10T1/2 mouse cells in culture after activation with iododeoxyuridine. This virus was poorly infectious for mouse cells and did not cause tumors upon inoculation into newborn NIH Swiss mice. Variants with increased infectivity for mouse cells were then derived both in vivo and in vitro by selecting for variants able to grow to high titers. The highly infectious variants were found to induce mouse fibroblasts to grow in soft agar. When the viruses were inoculated into newborn NIH Swiss mice, 100% of the animals died of leukemia within 4 months. Solid tumors developed at the injection site. Both mouse-tropic and dualtropic viruses were isolated from the leukemic mice and plaque purified. The first group of viruses produced large syncytial plaques on rat XC cells and did not grow in mink cells. The viruses of the other group replicated well in both mouse and mink cells, producing morphologic changes similar to transformation but not XC syncytia; they, therefore, are members of the newly described MCF class of mouse type C viruses. Isolates from either group were highly leukemogenic on retesting, the mean latent period being 67 days for a mouse-tropic virus and 105 days for one of the dual-tropic viruses. The results led to the conclusion that the better a mouse type C virus grows in cell culture the more effective it is as a leukemogen. Further, it is possible to start with a weakly infectious, nonleukemogenic virus and to convert it to a rapidly replicating, highly leukemogenic virus by passage either in cell culture or in the animal. The availability of a defined series of viruses from a low-leukemia mouse strain that differ greatly in their biologic properties should facilitate studies of the molecular basis for the acquisition of type C virus oncogenicity.

Why cell biologists should be aware of genetically transmitted viruses

Retrovirus genomes exist as endogenous genetic elements in the cells of many species used in biomedical research. Many cell lines spontaneously release virus, and other cells are induced to do so by procedures commonly used in research laboratories. The expression of endogenous retroviruses can affect the results of seemingly unrelated experiments. Some retroviruses endogenous to animals grow avidly in human cells. They are not known to be hazardous to man, but further studies are necessary.

[Persistence of the Sindbis virus in cultures producing oncornavirus]

Continuous lines were obtained from primary cultures of BALB/C mouse embryo cells which were found by electron microscope and reverse transcriptase reaction to produce permanently oncoronavirus type C after exogenous infection with Rauscher leukemia virus (RLV). Sindbis virus (SV) was inoculated into virogenic cultures 398 days after infection with RLV. The system in characterized by rapid (3-21 days) disappearance of the infectious arbovirus from the medium and the cells, long-term (over 5 months) persistence on SV noninfectious antigen and signs of stimulation of oncornavirus activity. The level of reverse transcriptase activity in cultures in the presence of persisting arbovirus was 1.5-3.3-fold higher than in cultures infected with RLV alone. Two variants of the course of mixed chronic infection of the cultures with oncornavirus and arbovirus differing in the rate of transition of the arbovirus into the noninfectious form and inhibition or stimulation of oncornavirus functions are discussed.

[Chronic infection caused by the simian Mason-Pfizer virus in primate cell cultures]

Cell cultures of monkey prepuce (Rhfs) and African green monkey kidney (BSC-1) were infected once with simian Mason-Pfizer virus (MPV) and virus expression in the course of establishment of chronic infection was studied. The productive infection was characterized by changes in the cell metabolism (DNA synthesis increased 2-3-fold as early as the "zero" passage), the appearance of gs-antigen, formation of virions of type D and high activity of RNA-dependent DNA-polymerase. Multinuclear giant cells appeared only in the infected Rhfs cell culture most sensitive to MPV. In human embryo kidney culture (HEK) productive infection was also established however, HEK cultures did not survive after 3-4 passages. No signs of transformation could be found in any of the cultures studied.

[Study of human diploid cell strains for the presence of oncornavirus determinants]

The WI-38 strain of human diploid cells (HDC) and two strains, L-58 and L-63 isolated in Moscow Research Institute of Virus Preparations were examined in the course of sequential passages for the presence of primate oncornaviruses and their antigens. In parallel, the tumorigenic capacity of the cells was tested by inoculating them into the cheek pouch tissue of Syrian hamsters. In all cases the examined HDC cultures contained no oncornavirus particles or their antigens and in no case showed any infectious activity determined in the KC-test. Only on one subculture of the L-63 strain cells at the 44th passage was a positive result obtained in immunoprecipitation test with antiserum to the LPV oncornavirus. Cells of the L-58 strain which had undergone 27 and 43 passages showed tumorigenicity in hamster experiments. In subsequent passages of these HDC strains no such properties were found. The regular control for the absence of oncogenic message is a necessary condition for the use of HDC strains in virological practice.

[Model studies on virus-induced tumors and their immunological treatment (author's transl)]

After a review of the general biological properties of C-type oncorna viruses, results are presented on the structure of an exogenous murine leukemia virus (FLV) and on the serobiological properties of its structural proteins. Our findings suggested a major role of the viral surface glycoprotein gp71 in immunological defense mechanisms. This was confirmed by vaccination experiments with isolated gp71 in mice. The induced immunity was highly specific and not operative against endogenous murine C-viruses belonging to other serotypes. Surprisingly the latter were found to be activated by the vaccination with gp71 of FLV. In heterologous animal species isolated FLV-gp71 induced the formation of broadly reacting antibodies. They were found to be effective in the therapy of infections with FLV in mice as well as with feline leukaemia virus in cats. Most impressive results were obtained with an antiserum prepared against feline leukaemia virus in a goat. This serum completely suppressed sarcomas induced by infection with feline sarcoma virus.

Oncogenicity in marmosets of HL-23V, a type C oncornavirus isolated from human leukemic cells, and comparison with simian sarcoma virus type 1 (SSV-1/SSAV-1)

Type C virus produced by dog thymus cells (A7573) that were infected with virus (HL-23V), isolated from cultured leukocytes of an acute myelogenous leukemia patient, transformed marmoset and horse cells in vitro and induced virus-producing fibromas in marmosets. The tumors and transformed foci were indistinguishable morphologically from those induced by simian sarcoma virus, type 1 (SSV-1/SSAV-1). HL-23V was indistinguishable from SSV-1/SSAV-1 by immunofluorescence and neutralization tests, and the nontransforming virus associated with HL-23V completely inhibited SSV-1 focus induction in interference tests. Cell cultures established from a marmoset fibroma produced transforming and nontransforming virus biologically and antigenically indistinguishable from HL-23V and SSV-1/SSAV-1.