Gene-transfer, stability, and biochemical properties of animal cells transformed with vaccinia DNA

Regulated expression of nuclear genes by T3 RNA polymerase and lac repressor, using recombinant vaccinia virus vectors

Recombinant vaccinia viruses that express the bacteriophage T3 RNA polymerase (VV-T3pol) or the Escherichia coli lac repressor (VV-lacI) under control of the early-late vaccinia promoter P7.5 were constructed. To determine whether phage polymerase and lac repressor can function in the nucleus of mammalian cells, the bacterial chloramphenicol acetyltransferase (CAT) gene was cloned downstream of a T3 promoter (PT3-CAT) or downstream of a T3 promoter-lac operator fusion element (PT3Olac-CAT), and these reporter gene cassettes were introduced stably into NIH 3T3 or Ltk- cells. Infection of 3T3/PT3-CAT or Ltk-/PT3-CAT cells by VV-T3pol led to rapid expression of CAT (greater than 20 ng of CAT protein per 10(6) cells). The presence of hydroxyurea (which blocks virus DNA replication) did not prevent CAT production. When 3T3/PT3Olac-CAT cells were infected with both VV-T3pol and VV-lacI (multiplicities of infection of 2.5 and 10, respectively), greater than 30-fold repression of CAT gene activity by lac repressor was observed. This could be reversed to unrepressed levels by the presence of 10 mM o-nitrophenyl-beta-D-galactoside (IPTG) in the medium. Regulated expression of the target gene was observed with cell lines that had been maintained for over 1 year (greater than 50 passages in culture), and Southern blot analysis revealed the presence of the CAT gene only in the nuclear fraction in these cells, demonstrating the stability of the target gene. These results indicate that vaccinia virus-encoded proteins can function in the mammalian nucleus and provide the basis for a genetic system in which essential vaccinia virus genes, placed in the chromosome of a cell, can be used to complement defective virus particles. This approach may prove useful for other virus systems.

Characterization of the transformation properties of Shope fibroma virus

To investigate if Shope fibroma virus (SFV), a leporipoxvirus that induces benign tumors in adult rabbits, can trigger the second step of carcinogenesis in vitro or malignant transformation, an already immortalized rabbit cell line (SIRC) was inoculated with ultraviolet-irradiated virus. The resulting cell transformants displayed the characteristic properties of the malignant phenotype: lack of infectious particles, low serum requirement, high efficiency of cloning, resistance to superinfection, presence of viral DNA sequences in the nucleus, expression of viral proteins and induction of tumors in rabbits. However, this transformation was not stable since in all cell lines studied, a loss of the malignant phenotype was recorded close to the 50th passage. To assess the oncogenic potential of SFV, NIH 3T3 cells were transfected with SFV DNA. The results of these experiments indicate that SFV DNA can induce the formation of foci in certain NIH 3T3 cell lines. Taken together these results support the notion that SFV can elicit the transformation of cells in vitro.

The role of the host cell nucleus in vaccinia virus morphogenesis

Despite the fact that cells infected with wild type vaccinia virus synthesize viral DNA and assemble progeny virus particles within the cytoplasm, the host cell nucleus is required for a productive infection. Recent evidence suggests that vaccinia virus selectively recruits components from the host cell nucleus into the cytoplasm for use by the developing virus. One of these components is the largest subunit of the cellular RNA polymerase II (Pol II).

Vaccinia virus DNA sequences in the nucleus of persistently infected Friend erythroleukemia cells

FL vac cell lines are Friend erythroleukemia cells persistently infected with vaccinia virus. These cells produce attenuated leukemia virus, virulent poxvirus, resist superinfection with vaccinia, and show high levels of spontaneous erythrodifferentiation and decreased tumorigenicity in syngeneic hosts (Pogo, G.T. and Friend, C. (1982) Proc. Natl. Acad. Sci. USA 79, 4805-4809). To determine whether resistance to superinfection was associated with the presence of vaccinia DNA in the nucleus, DNA from cells at different passage levels was hybridized to a vaccinia DNA probe. Vaccinia DNA sequences were detected in the nucleus of cells of lines that were productively infected with vaccinia. No such sequences were detected in productively infected with vaccinia. No such sequences were detected in productively infected L cells nor in persistently infected cell lines that no longer produced infectious particles but were resistant to superinfection. Although no evidence of integration of vaccinia DNA was observed, differences in the restriction patterns were detected at some passage levels. The presence of vaccinia virus DNA sequences in the nucleus apparently did not affect the size of the provirus, the integration pattern or the expression of the leukemia virus.

Interferon prevents the generation of spontaneous deletions at the left terminus of vaccinia virus DNA

In this report we have shown that Friend erythroleukemia cells persistently infected with vaccinia virus maintain the persistent infection even after 1 year of continuous interferon (IFN) treatment. The persistently infected cultures were responsive to IFN as determined by their ability to induce 2-5A synthetase, to increase the intracellular levels of 2-5A, and to cause rRNA cleavage. While large deletions at the left terminus of vaccinia DNA occurred readily in the virus population from untreated cells, IFN completely suppressed the generation of these spontaneous deletions. Removal of IFN from these cultures led to the appearance of similar deletions at the left terminus of the viral genome. The regions deleted contain more than half of the left-end inverted terminal repetition of the vaccinia genome. These findings show that IFN alters specific events associated with the generation of vaccinia DNA deletions.

Generation of a dominant 8-MDa deletion at the left terminus of vaccinia virus DNA

Vaccinia virus mutants were obtained in high frequency from mouse Friend erythroleukemia (FEL) cells persistently infected with this virus, which contains a large (122-MDa) DNA. During long-term cell passages viral particles with deletions of the DNA are generated in FEL cells. These mutants have a major 8-MDa deletion starting between 2.2 and 3.2 MDa from the left terminus of the viral genome. More than half of the left end terminal repetition is deleted. These mutants have reduced infectivity compared to wild-type virus. The ease with which vaccinia virus mutants are obtained in FEL cells should provide a suitable system for generating mutants with other poxvirus and permit study of the genetic basis of virulence for this group of viruses.

Vaccinia virus RNA polymerase associated with nuclei of infected HeLa cells

Though vaccinia virus DNA and RNA replication take place predominantly in the cytoplasm of an infected cell, virus formation requires the presence of a functional nucleus in a yet undefined manner. When the nuclei from cells infected for 3 h are isolated and purified, they are found to synthesize five times more RNA in vitro than do corresponding nuclei from noninfected cells. Fifty percent of the RNA synthesized in vitro by nuclei from infected cells is vaccinia specific, and this vaccinia RNA synthesis is resistant to alpha-amanitin concentrations up to 100 micrograms/ml. Furthermore, when the RNA polymerase activities of these nuclei are separated on DEAE-Sephadex columns, 56% of the total nuclear enzyme activity is found to be the vaccinia-specific RNA polymerase known to be alpha-amanitin resistant. The nucleus associated vaccinia RNA polymerase represents 18% of the total cellular vaccinia RNA polymerase. This synthesis of vaccinia RNA in the nucleus may explain the nuclear requirement for vaccinia virus maturation.