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

Activation of the human immunodeficiency virus type 1 long terminal repeat by vaccinia virus

A variety of DNA viruses are known to activate gene expression directed by the long terminal repeat (LTR) of human immunodeficiency virus type 1 (HIV-1). In light of the proposed use of recombinant vaccinia virus for HIV-1 vaccines, evaluation of the role of vaccinia virus in HIV-1 activation is warranted. To investigate whether vaccinia virus induces HIV LTR-directed gene expression, transient expression assays in Jurkat cells persistently infected with vaccinia virus (Jvac) using plasmid DNA containing the LTR linked to the bacterial chloramphenicol acetyltransferase (CAT) gene were performed. CAT activity in Jvac cells was always recorded, although the level appears to fluctuate independently of virus titers. Dual intracytoplasmic staining and fluorescence-activated cell sorter analysis showed that CAT activity was expressed in the infected cells. CAT expression was not due to plasmid replication, since plasmid DNA extracted from Jvac cells 48 h after transfection was restricted only by enzymes which recognize methylated sequences, indicating a prokaryotic source for the DNA. These findings suggest that a factor(s) present in vaccinia virus-infected cells is capable of activating the LTR of HIV-1.

Stimulation of lymphokines in Jurkat cells persistently infected with vaccinia virus

The response of the human CD4+ T-cell line Jurkat to infection with vaccinia virus was investigated. Virus titers peaked approximately 3 to 4 days after infection, while cell growth paralleled that of uninfected cells, indicating that growth rates were not appreciably affected by viral infection. Results from plaque assays and fluorescence-activated cell sorter (FACS) analyses of virus antigens demonstrated that a persistent infection in which the percentage of infected cells and the virus titers fluctuated from passage to passage was established. Further characterization of the persistent infection revealed that the virus influences cellular functions. Induction of interleukin-2 (IL-2) and IL-2 receptor alpha (IL-2R alpha) in Jvac cells was shown by enzyme-linked immunosorbent assay and FACS analysis, respectively. Hybridization of cellular RNA with cloned probes confirmed the increased IL-2 expression and demonstrated that Jvac cells also expressed more IL-6 but not gamma interferon (IFN-gamma) or IL-1 beta. Dual-antibody staining and FACS analysis for vaccinia virus antigens and IL-2R alpha indicated that IL-2R alpha expression was restricted to the infected cells. Jvac cells were also resistant to superinfection, an additional proof that persistent infection elicited phenotypic changes in the cell population.

Changes in cell gene expression in human leukemic cells persistently infected with vaccinia virus

Persistent viral infections in vitro are useful systems to study the coevolution of virus and cell populations. Persistent infection of mouse Friend erythroleukemic cells (FEL) with vaccinia virus results in profound changes of the virus as well as of the cells. To investigate phenotypic changes of other cell types, we have established a persistent infection with vaccinia virus in a human leukemic cell line (K562). This cell line can be induced to differentiate along the erythroid pathway synthesizing embryonic and fetal globins, thus providing a system in which specific genes can be stimulated. After serial passage, the persistently infected cells (K562vac) became spontaneously differentiated, as shown by the increase in the number of cells producing hemoglobin (benzidine positive cells), and resistant to superinfection. These phenotypic changes of the cells were not accompanied by changes in the viral population. Hybridization of cellular RNA with cloned embryonic and fetal globin genes indicated that uninduced K562 cells do not express these genes, whereas cells induced by hemin or butyrate express G gamma (fetal globin) epsilon and zeta (embryonic globins) genes. By contrast vaccinia infected cells spontaneously express the G gamma gene. These results demonstrate that persistent infection with vaccinia virus elicited phenotypic changes in the infected cell population; in this case the constitutive expression of fetal hemoglobin.

Tumorigenic conversion of NIH 3T3 cells by transfection with Shope fibroma virus DNA

The properties of NIH 3T3 cells transfected with Shope fibroma virus (SFV) DNA were investigated. Six focus-derived cell lines were established that display the following properties: a) They all contained SFV DNA sequences at early passages, b) five of them also expressed SFV RNA at early passages and induced tumors in nude mice, and c) four lines were anchorage independent. Transfection with cloned DNA fragments containing terminal sequences did not induce foci, except with fragment C, which contains the growth-factor gene. However, these transfected cells failed to cause tumors, suggesting that the growth factor alone may not be responsible for tumorigenesis.

Characterization of vaccinia virus deletion mutants isolated from persistently infected Friend erythroleukemia cells

Persistent viral infections in vitro are useful to study the evolution of virus populations in the absence of immunological pressure. Several deletion mutants have been isolated in this laboratory from Friend erythroleukemia cells persistently infected with vaccinia virus strain IHD-W, designated SQAvac. Two of the mutants, which remain stable after serial passage in L cells, have been characterized. The deletion which range between 20 to 22 kb, has been localized at the left terminus comprising HindIII fragments C and N. In addition, HindIII B fragment lost the sequences that hybridize to pAG5, a plasmid containing the 3.5 kb terminal sequences and acquired different restriction sites. Phenotypic characterization of these mutants revealed that they were not replication defective since they grew in all cell lines tested and produced plaques of the same size as the wild-type. However, they were less effective in suppressing host protein synthesis. The LD50 for the mutants titered in NIH Swiss female mice was greater than 10(9) PFU as compared to 10(6) PFU for the wild-type, indicating reduced virulence in vivo. These mutants, which display different properties to previously described mutants with deletions at the left terminus, provide another valuable system to study the molecular basis of virulence of vaccinia.

Attenuated deletion mutants of vaccinia virus lacking the vaccinia growth factor are defective in replication in vivo

Understanding the molecular basis of virulence in poxvirus is of great importance for the development of recombinant vaccines using vaccinia virus as a vector. We have previously described mutants of vaccinia virus with deletions ranging from 20 to 21 kb at the left terminus and with attenuated phenotype. The virulence of these mutants was studied, using different routes of inoculation, for protection from wild-type challenge in mice and for replication in vivo. Regardless of the route of inoculation, the LD50 of the deletion mutants is at least 1000-fold higher than that of the wild-type. Results from protection experiments using viable and ultraviolet-inactivated viruses, and from determination of infectivity in different organs, indicated that the mutants were unable to replicate in vivo. Southern blot hybridization of viral DNA with pSC16, a plasmid containing the vaccinia growth factor (VGF) gene, revealed that in the IHD-W strain of vaccinia virus this gene is localized at the left terminus exclusively and that the mutants lack this gene. The results suggest that absence of the VGF gene is correlated with inability to replicate in vivo and decreased virulence.

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).