Simian virus 40-permissive cell interactions: selection and characterization of spontaneously arising monkey cells that are resistant to simian virus 40 infection.

A fraction of permissive cells survive simian virus 40 (SV40) infection. The frequency of such surviving cells depends only upon the concentration of infecting virus, both parental and progeny, to which the cells are exposed during the course of selection. Surviving clones, which can be freed of virus by cloning in the presence of SV40 antiserum, are indistinguishable from parental cells in their growth of characteristics and display no SV40 antigen; thus they are not transformed. Most surviving clones are less than 10% as susceptible as parental cells to SV40 infection; 5 to 10% are less than 1% as susceptible. None of these SV40-resistant clones is absolutely resistant to SV40 infection. Analysis of 16 independently arising resistant clones indicates that they all block SV40 infection at an early stage after adsorption and eclipse but before full uncoating. Viral mutants have been isolated that partially overcome the block to infection in these cells; these host range viruses plaque on resistant lines fivefold more efficiently than wild-type SV40 and have a characteristic plaque morphology. Fluctuation analysis indicates that resistant cells arise spontaneously during the growth of normally susceptible permissive cells. Thus, SV40-resistant cells are selected for, not induced by, SV40 infection.     

An early event in murine cytomegalovirus replication inhibits presentation of cellular antigens to cytotoxic T lymphocytes.

Cytomegalovirus (CMV) infection of simian virus 40 (SV40)-immune mice inhibits priming of SV40-specific helper and cytotoxic T lymphocytes (CTL) in vivo (A. E. Campbell, J. S. Slater, and W. S. Futch, Virology 173:268-275, 1989; J. S. Slater, W. S. Futch, V. J. Cavanaugh and A. E. Campbell, Virology 185:132-139, 1991). We now demonstrate that murine CMV (MCMV) infection of SV40-transformed macrophages and fibroblasts prevents presentation of SV40 T antigen to SV40-specific CTL. MCMV-infected macrophages failed to stimulate SV40-immune CTL precursors in vitro. In addition, MCMV-infected, SV40-transformed macrophage and fibroblast target cells lost their susceptibility to lysis by major histocompatibility complex class I-restricted, SV40-specific CTL clones. MCMV infection did not alter the synthesis of SV40 T antigen in the target cells. MCMV early gene expression was required for inhibition of SV40 T-antigen presentation; immediate-early gene expression was insufficient for this effect. Early viral gene expression also resulted in significant reduction of H-2K and H-2D molecules on the surface of MCMV-infected fibroblasts. However, this reduction occurred independently from suppression of antigen presentation to CTL. The same target cells which were resistant to lysis by SV40 CTL were susceptible to lysis by MCMV-specific CTL. MCMV early gene products therefore interfere with the processing and/or presentation of SV40 T-antigen determinants to CTL independent of alterations in the major histocompatibility complex.     

Characterization of simian virus 40 transformed African green monkey cells (CV-1). I. Defective virion and viral genome.

Several clones of SV40 transformed CV-1 cells have been characterized for the production of T- and V-antigens and for the state of viral genome. The transformed CV-1 cells failed to produce infectious virions as assayed after sonication or cocultivation and fusion with normal CV-1 cells, and were resistant to super-infection by SV40. Some clones of the transformed cells contained V-antigens. The population of V-antigen positive cells varied from 0 to 100% depending on the passage number while the T-antigen positive cells were always 100%. The virions isolated from the transformed cells were similar in morphology to complete SV40, but lighter in density than complete SV40. In one clone, a small amount of SV40 DNA was detectable in a free state while a large proportion of the DNA hybridizable with SV40 3H cRNA was linearly integrated into the cell DNA. The free SV40 DNA was noninfectious, closed circular DNA with a size smaller than infectious SV40 DNA component I. Since the cell extracts of the transformed cells contained an agent(s) which induced T- and V-antigens in normal CV-1 cells, it was suggested that the SV40 transformed CV-1 cells contained free as well as integrated defective SV40 genomes responsible for the synthesis of T- and V-antigens.     

Mutagenesis by simian virus 40. I. detection of mutations in Chinese hamster cell lines using different resistance markers.

The mutagenic action of SV40 in permanent lines of Chinese hamster cells (CHO-K1 and V79) was investigated with the aid of different resistance markers. The markers studied had resistance to 8-azaguanine (25 and 30 mug/ml), aminopterin (3.3--5.5X10(-3) mug/ml), colchicine (6.5 and 7.0X10(-2) mug/ml) and 5-bromodeoxyuridine (50--120 mug/ml), respectively. After virus infection the mutation frequencies were increased by one (azaguanine, aminopterin) and two (colchicine) orders of magnitude as compared with spontaneous mutation frequencies. In contrast, it was not possible to enhance the frequency of mutation to BUdR resistance. On the other hand, the ability to proliferate in HAT medium was induced in three of five BUdR-resistant cell clones by infection with SV40. The resistance induced by SV40 was stable when isolated clones were cultured under non-selective conditions. Mechanisms are proposed that may be responsible for the mutagenic action of SV40.     

Cold-sensitive growth of simian virus 40 in semipermissive variants of CV1 cells.

Two cell clones were isolated from the simian line CV1, permissive for simian virus 40 (SV40), by selection at low temperature with the tsA239 mutant of SV40. These clones exhibited cold-sensitive semipermissivity to both SV40 virions and SV40 DNA. On the basis of virus yields, their resistance to viral DNA was increased approximately 15 times over that of CV1 cells when the incubation temperature was lowered from 38.5 to 33.5 degrees C. A further 30- to 40-fold resistance increase was exhibited at both temperatures upon infection with SV40 virions. Partial characterization of these clones indicated that the cold sensitivity affected an early function in viral growth, between viral uncoating and the appearance of T-antigen positivity, with a burst-size decrease in all cells at the restricted temperature. This conditional defect appeared to be superimposed upon a temperature-independent uncoating defect, presumably carried in a CV1 subpopulation from which the two clones were ultimately selected.     

Quantification of expression of linked cloned genes in a simian virus 40-transformed xeroderma pigmentosum cell line.

We wished to determine whether simian virus 40 (SV40)-transformed xeroderma pigmentosum cells, despite their defective DNA repair, were suitable for DNA-mediated gene transfer experiments with linked genes. Expression of a nonselectable gene (cat, coding for chloramphenicol acetyltransferase [CAT]) linked to a selectable gene (gpt, coding for xanthine-guanine phosphoribosyltransferase [XPRT]) in the plasmid pSV2catSVgpt was quantified after transfection of SV40-transformed xeroderma pigmentosum [XP20s(SV40)] and normal human [GM0637(SV40)] fibroblast cell lines. A novel autoradiographic assay with [3H]xanthine incorporation showed 0.5 to 0.7% phenotypic expression of XPRT in both cell lines. Without selection, transient CAT activity was 20 times greater in the GM0637(SV40) than in the XP20s(SV40) cells, and transient XPRT activity was 5 times greater. Both of these transient activities were increased and equalized in both cell lines by transfection with pRSVcat or pRSVgpt. Genotypic transformation to gpt+ occurred at a frequency of 2 X 10(-4) to 4 X 10(-4) in both cell lines with pSV2catSVgpt. After 2 to 3 months in selective medium, stable expression of the (nonselected) cat gene was found in 11 (92%) of 12 gpt-containing clones derived from GM0637(SV40) cells and in 13 (81%) of 16 gpt-containing clones from XP20s(SV40) cells. However, the levels of CAT activity did not correlate with those of XPRT activity, and both of these activities varied more than 100-fold among different clones. Copies (1 to 4) of the gpt gene were integrated in four clones of the GM0637(SV40) cells having an XPRT activity of 1 to 5 nmol/min per mg, but 5 to 80 copies were integrated in four XP20s(SV40) clones with an XPRT activity of 0.8 to 1.8 nmol/min per mg. This study shows that XP20s(SV40) is as suitable for gene transfer experiments as the normal human line GM0637(SV40).     

Response of Simian Virus 40-Transformed Cell Lines and Cell Hybrids to Superinfection with Simian Virus 40 and Its Deoxyribonucleic Acid

Whereas normal human and monkey cells were susceptible both to intact simian virus 40 (SV40) and to SV40 deoxyribonucleic acid (DNA), human and monkey cells transformed by SV40 were incapable of producing infectious virus after exposure to SV40, but displayed susceptibility to SV40 DNA. On the other hand, mouse and hamster cells, either normal or SV40-transformed, were resistant both to the virus and to SV40 DNA. Hybrids between permissive and nonpermissive parental cells revealed a complex response: whereas most hybrids tested were resistant, three of them produced a small amount of infectious virus upon challenge with SV40 DNA. All were resistant to whole virus challenge. The persistence of infectious SV40 DNA in permissive and nonpermissive cells up to 96 hr after infection was ascertained by cell fusion. The decay kinetics proved to be quite different in permissive and nonpermissive cells. Adsorption of SV40 varied widely among the different cell lines. Very low adsorption of SV40 was detected in nonsusceptible cells with the exception of the mKS-BU100 cell line. A strong increase in SV40 adsorption was produced by pretreating cells with polyoma virus. In spite of this increased adsorption, the resistance displayed by SV40-transformed cells to superinfection with the virus was maintained.     

Induction of gene mutations and chromosomal aberrations by simian virus 40 in cultured mammalian cells

• Induction of gene mutations by SV40 was studied in aneuploid human and Chinese hamster cells. In Chinese hamster cells SV40-induced chromosome aberrations were also studied.

• SV40 penetrated into the cells of both lines and induced synthesis of the T antigen. The efficiency of infection in Chinese hamster cells was tested additionallby their ability to form colonies in medium lacking the serum growth factor. The maximal number of cells with growth factor independence was observed on the first day after infection. When hamster cells had been maintained in “factor-free medium” for the first two passages after infection a sub-line was isolated, which synthesized T antigen 60 days after exposure to SV40. This was considered to be an indirect proof of the integration of viral genome into host chromosome.

• A significant increase in the frequency of chromosomal aberrations was detected in SV40-infected Chinese hamster cells. It was observed on the first and second days after treatment. The most numerous were the chromosome and chromatid breaks, which were distributed randomly in 5 morphological groups according to the chromosome length.

• SV40-induced mutations of resistance to 8-AG and 6-MP in human and Chinese hamster cells respectively were detected, when cells were plated in selective medium one to five days after infection. Induction was detected in all the 4 experiments with human cells and in 9 out of 11 experiments with Chinese hamsters cells. Induction was highly significant according to the Wilcoxon test (P>0.99), when the results of all experiments carried out in human and Chinese hamster cells were summarized. Resistance was stable after prolonged cultivation of 13 isolated clones under non-selective conditions.

• It is suggested that viral genome integration, gene mutations and chromosomal aberrations may have common molecular mechanisms. The role of gene mutations in virus-induced carcinogenesis is discussed.

Increased mutation rates in doubly viral transformed Chinese hamster cells.

An untransformed parental Chinese hamster cell line, one polyoma transformed clone, and two clones containing both SV40 and polyoma DNA were tested for the appearance of variants resistant to 6-thioguanine or oubain. The frequency of such variants was found to be highest in doubly transformed cells. The mutation rate at these loci was correlated with the level of transformation. The mutants observed were stable and had low reversion frequencies with no gross cytogenetic changes. These results establish a quantitative basis for the evaluation of phenotypic variability observed in transformed cell populations.     

DNA polymerase and simian virus 40 infection of resting monkey cells: induction of aphidicolin resistant alpha-polymerase.

Tightly confluent monkey cell lines BSC-1 and CV-1 held in stale medium for several days exhibited an extremely low level of thymidine incorporation into cellular DNA. Yet, these cells contained a level of alpha-polymerase equal to about 15% of the level in rapidly dividing cells, and they still were capable of supporting replication of SV40 DNA. SV40 infection and culture in stale medium resulted in a four-fold induction of alpha-polymerase in CV-1 cells, whereas no change in alpha-polymerase level was observed in BSC-1 cells. Characterization of alpha-polymerase partially purified from infected CV-1 cells revealed that 80-90% of the enzyme activity was aphidicolin resistant. SV40 DNA replication in resting CV-1 cells, however, was aphidicolin sensitive. SV40 infection of resting CV-1 cells may induce an aphidicolin-resistant enzyme or lead to a modified alpha-polymerase species.     

Integration of SV40 DNA into the cell genome and viral mutagenesis

Integration of DNA of a temperature-sensitive SV40 mutant (tsA239) into the cell genome was studied. The viral A gene (the oncogene) encodes the tumour T antigen which is ts in the mutant and is devoid of mutagenic and transforming activity under non-permissive conditions (40 degrees C). Clones of Chinese hamster cells infected by tsA239 mutant were analysed. Those infected by wild-type SV40 served as controls. As shown by dot-hybridization, SV40 DNA was detected in cells of 14 out of 18 clones infected by tsA mutant and incubated at 40.5 degrees C, and in all 20 clones infected by tsA mutant and incubated under permissive conditions (33 degrees C), the difference between the two groups being insignificant (p greater than 0.05). By means of blot-hybridization it was established that viral DNA was integrated into the cell genome of all 12 clones analysed, belonging to the three experimental series: infection by tsA mutant, incubation at 40.5 and 33 degrees C, infection by wt SV40, incubation at 40.5 degrees C. The number of integration sites ranged from one to four in different clones. Integration of SV40 DNA in tandems was observed. The data presented allow to conclude that integration per se does not play a crucial role in determining the mutagenic and transforming effect of the virus. Obviously, what matters is the activity of viral oncogene product - the T antigen.     

Resistance of teratocarcinoma stem cells to infection with simian virus 40: early events.

Multipotential stem cells of a murine teratocarcinoma are resistant to typical infection with either polyoma virus (PV) or Simian virus 40 (SV40). Differentiated progeny of the stem cells are susceptible to infection in a manner identical to other mouse somatic cells, i.e., they are permissive for PV and nonpermissive for SV40. The early interactions between the stem cells (embryonal carcinoma or EC cells) and SV40 and PV were studied. Virions adsorbed to and penetrated into the cytoplasm and nucleus of EC cells, but did not induce expression of T antigen in the EC nuclei. Purified SV40 DNA was capable of inducing T antigen in differentiated teratocarcinoma cells but not in EC cells. Virus could not be rescued from EC cells previously exposed to SV40. The resistance of the stem cells to infection apparently involves a block in the infectious cycle after adsorption and penetration but before T antigen induction.     

Acquisition of Sequences Homologous to Host Deoxyribonucleic Acid by Closed Circular Simian Virus 40 Deoxyribonucleic Acid

The synthesis of closed circular simian virus 40 (SV40) deoxyribonucleic acid (DNA) containing sequences homologous to host cell DNA depends upon the conditions under which the cells are infected. When BS-C-1 monkey cells were infected with non-plaque-purified virus at low multiplicity of infection [MOI, 0.032 plaque-forming units (PFU)/cell], little, if any, of the SV40 DNA extracted from the infected cells hybridized to host DNA; but when increasingly higher multiplicities were used (in the range 0.16 to 3,000 PFU/cell), an increasingly greater amount of the extracted SV40 DNA hybridized to host DNA. The same effect was observed when the closed circular SV40 DNA was extracted from purified virions (grown at low and high MOI) rather than from the infected cell complex. When the cells were infected at high MOI with plaque-purified virus (11 viral clones were tested), none of the SV40 DNA extracted from the cells hybridized detectably with host cell DNA. However, plaque-purified virus that was serially passaged, undiluted, induced the synthesis of virus DNA which again showed extensive homology to host DNA. It is suggested that, under certain circumstances, recombination occurs between viral and host DNA during lytic infection which results in the incorporation of host DNA sequences into closed circular SV40 DNA.     

Biological and biochemical characterisation of purine analogue resistant clones of V79 Chinese hamster cells

Purine analogue resistant clones have been selected from the closely related Chinese hamster lines V79A and V79S. Clones were of either spontaneous origin or induced by EMS or ultraviolet light. The majority of clones selected in 8-azaguanine showed stable cross resistance to 6-thioguanine. Clones derived from V79A and selected for 6-thioguanine resistance were cross resistant to 8-azaguanine: however a group of 6-thioguanine resistant mutants selected from V79S cells were 8-azaguanine sensitive. All clones except two were unable to grow in HAT medium. The two exceptions were 8-azaguanine resistant, showed partial sensitivity to 6-thioguanine, and also differed in other biochemical characteristics. HGPRT activity was measurable in extracts of all clones under standard conditions. In many clones, HGPRT activity increased as the hypoxanthine concentration was reduced. Whole cell uptake of [14C] hypoxanthine was low in all cases examined and was not modified by incubation in the presence of amethopterin. The heat sensitivity and electrophoretic mobility of HGPRT in extracts of some clones was compared to that in wild-type extracts. All clones tested except one, which was consistently HAT positive, showed enhanced heat sensitivity and reduced electrophoretic mobility. None of the mutants reverted spontaneously at detectable frequency but some could be induced to revert by EMS. The presence of measurable enzyme with altered properties in all clones suggests that these revertable drug resistant clones represent missense mutants.     

Simian virus 40 gene A regulation of cellular DNA synthesis. I. In permissive cells.

The kinetics of host cellular DNA stimulation by simian virus 40 (SV40) tsA58 infection was studied by flow microfluorometry and autoradiography in two types of productively infected monkey kidney cells (AGMK, secondary passage, and the TC-7 cell line). Prior to infection, the cell populations were maintained predominantly in G0-G1 hase of the cell cycle by low (0.25%) serum concentration. Infection of TC-7 or AGMK cells by wild-type SV40, viable deletion mutant dl890, or by SV40 tsA58 at 33 degrees C induced cells through S phase after which they were blocked with a 4N DNA content in the G2 phase. The infection of TC-7 cells by tsA58 at 41 degrees C, which was a nonpermissive temperature for viral DNA replication, induced a round of cell DNA synthesis in approximately 30% of the cell population. These cells proceeded through S phase but then re-entered the G1 resting state. In contrast, infection of AGMK cells by tsA58 at 41 degrees C induced DNA synthesis in approximately 50% of the cells, but this population remained blocked in the G2 phase. These results indicate that the mitogenic effect of the A gene product upon cellular DNA is more heat resistant than its regulating activity on viral DNA synthesis and that the extent of induction of cell DNA synthesis by the A gene product may be influenced by the host cell.     

Spontaneous Virus Production by Clonal Lines of Simian Virus 40-Transformed Cells and Effects of Superinfection by Deoxyribonucleic Acid from Mutant Simian Virus 40 Strains

Small amounts of infectious simian virus 40 (SV40) were recovered from parental cultures of SV40-transformed human embryonic lung (WI38 Va13A) cells, from 12 primary clones, from 17 secondary clones, and from 18 tertiary clones. The cloning experiments demonstrated that the capacity for spontaneous virus production is a hereditary property of WI38 Va13A cells. Infectious virus was not recovered from every clone at every passage. Repeated trials at different passage levels were necessary to detect virus production. Approximately one in 10(5) to 10(6) of the cells of the clonal lines initiated plaque formation when plated on the CV-1 line of African green monkey kidney cells. No increase in infectious center formation was observed after the clonal lines were treated with bromodeoxyuridine, iododeoxyuridine, or mitomycin C or after heterokaryon formation of treated cells with CV-1 cells. The clonal lines of WI38 Va13A cells were susceptible to superinfection by SV40 deoxyribonucleic acid (DNA). To determine whether only those cells which spontaneously produced virus supported the replication of superinfecting SV40 DNA, cultures were infected with DNA from a plaque morphology mutant and a temperature-sensitive mutant of SV40. After infection by SV40 DNA, approximately 100 to 4,400 times more transformed cells formed infectious centers than were spontaneously producing virus. To determine whether the resident SV40 genome or the superinfecting SV40 genome was replicating, infectious centers produced by SV40 DNA-infected WI38 Va13A cells on CV-1 monolayers were picked and the progeny virus was analyzed. Only the superinfecting SV40 was recovered from the infectious centers, indicating that in the majority of superinfected cells the resident SV40 was not induced to replicate.     

Genetic analysis of simian virus 40 from brains and kidneys of macaque monkeys.

Simian virus 40 (SV40) was isolated from the brains of three rhesus monkeys and the kidneys of two other rhesus monkeys with simian immunodeficiency virus-induced immunodeficiency. A striking feature of these five cases was the tissue specificity of the SV40 replication. SV40 was also isolated from the kidney of a Taiwanese rock macaque with immunodeficiency probably caused by type D retrovirus infection. Multiple full-length clones were derived from all six fresh SV40 isolates, and two separate regions of their genomes were sequenced: the origin (ori)-enhancer region and the coding region for the carboxy terminus of T antigen (T-ag). None of the 23 clones analyzed had two 72-bp enhancer elements as are present in the commonly used laboratory strain 776 of SV40; 22 of these 23 clones were identical in their ori-enhancer sequences, and these had only a single 72-bp enhancer element. We found no evidence for differences in ori-enhancer sequences associated with tissue-specific SV40 replication. The T-ag coding sequence that was analyzed was identical in all clones from kidney. However, significant variation was observed in the carboxy-terminal region of T-ag in SV40 isolated from brain tissues. This sequence variation was located in a region previously reported to be responsible for SV40 host range in cultured cell lines. Thus, SV40 appears to be an opportunistic pathogen in the setting of simian immunodeficiency virus-induced immunodeficiency, similarly to JC virus in human immunodeficiency virus-infected humans, the enhancer sequence organization generally attributed to SV40 is not representative of natural SV40 isolates, and sequence variation near the carboxy terminus of T-ag may play a role in tissue-specific replication of SV40.     

Transformation of isolated rat hepatocytes with simian virus 40

Rat hepatocytes were transformed by simian virus 40 (SV40). Hepatocytes from two different strains of rats and a temperature-sensitive mutant (SV40tsA 1609), as well as wild-type virus were used. In all cases, transformed cells arose from approximately 50% of the cultures containing hepatocytes on collagen gels or a collagen gel-nylon mesh substratum. Cells did not proliferate in mock-infected cultures. SV40-transformed hepatocytes were epithelial in morphology, retained large numbers of mitochondria, acquired an increased nucleus to cytoplasm ratio, and contained cytoplasmic vacuoles. Evidence that these cells were transformed by SV40 came from the findings that transformants were 100% positive for SV40 tumor antigen expression, and that SV40 was rescued when transformed hepatocytes were fused with monkey cells. All SV40-transformed cell lines tested formed clones in soft agarose. Several cell lines transformed by SV40tsA 1609 were temperature dependent for colony formation on plastic dishes. Transformants were diverse in the expression of characteristic liver gene functions. Of eight cell lines tested, one secreted 24% of total protein as albumin, which was comparable to albumin production by freshly plated hepatocytes; two other cell lines produced 4.2 and 5.7%, respectively. Tyrosine aminotransferase activity was present in five cell lines tested but was inducible by dexamethasone treatment in only two. We conclude from these studies that adult, nonproliferating rat hepatocytes are competent for virus transformation.