Thymidine kinase deficient human cells have increased UV sensitivity in their capacity to support herpes simplex virus but normal UV sensitivity for colony formation

A thymidine kinase deficient (tk-) and two thymidine kinase proficient (tk+) human cell lines were compared for UV sensitivity using colony-forming ability as well as their capacity to support the plaque formation of herpes simplex type 1 (HSV-1). The tk- line (143 cells) was a derivative of one of the tk+ lines (R970-5), whereas the other tk+ line (AC4 cells) was a derivative of the 143 cells obtained by transfection with purified sheared HSV-2 DNA encoding the viral tk gene. 143, R970-5 and AC4 cells showed a similar UV sensitivity for colony-forming ability. In contrast, the capacity to support HSV-1 plaque formation immediately (within 1 h) after UV-irradiation was reduced to a greater extent in the 143 cells compared to the R970-5 and AC4 cells. Capacity curves for plaque formation of the HSV-1: KOS wild-type (tk+) strain were similar to those for the HSV-1: PTK3B mutant (tk-) strain in the 3 cell strains, indicating that the viral tk gene does not influence the ability of HSV-1 to form plaques in UV-irradiated compared to unirradiated human cells. Cellular capacity for HSV-1 plaque formation was found to recover in both tk- and tk+ cells for cultures infected 24 h after UV-irradiation. These results suggest that repair of UV-damaged DNA takes place to a similar extent in both tk- and tk+ human cells, but the kinetics of repair are initially slower in tk- compared to tk+ human cells.     

Role of the viral and cellular encoded thymidine kinase in the repair of UV-irradiated herpes simplex virus

A strain of herpes simplex type 1 (HSV-1:KOS) encoding a functional thymidine kinase (tk+) gene and a thymidine kinase deficient (tk-) mutant strain (HSV-1:PTK3B) were used as probes to examine the repair of UV-damaged viral DNA in one tk- (143) and two tk+ (R970-5 and AC4) human cell lines. UV survival for each HSV-1 strain was similar for infection of both tk- and tk+ cells suggesting that the repair of viral DNA was not dependent on the expression of a functional cellular tk gene. In contrast, UV survival of HSV-1:PTK3B was substantially reduced compared to HSV-1:KOS when infecting all 3 human cell lines, as well as Vero monkey kidney cells and LPM1A mouse cells. These results suggest that the repair of UV-irradiated HSV-1 in lytically infected mammalian cells depends, in part at least, on the expression of the viral encoded tk.     

Temperature-sensitive mutants of herpes simplex virus type 1 defective in lysis but not in transformation.

Twelve temperature-sensitive (ts) mutants of herpes simplex virus type 1 (HSV-1), representing seven complementation groups, were isolated subsequent to 5-bromodeoxyuridine mutagenesis. These mutants were identified by their inability to replicate in a line of monkey (CV-1) cells at 39 C. Seven of these mutants, representing six complementation groups, induced thymidine kinase (tk) and transformed Ltk- cells, a line of mouse L cells lacking tk, to a tk+ phenotype at both the permissive (34 C) and nonpermissive (39 C) temperatures. Thus, the defective cistrons in these six complementation groups, although necessary for lysis, have no essential function in this transformation system. Transformation by these 12 mutants was dependent on prior UV irradiation. Infection of cells with unirradiated virus under conditions which did not permit virus replication was not sufficient to allow cell transformation. Five mutants, representing two complementation groups, were tk- and were incapable of causing the tk--to-tk+ transformation at either 34 C of 39 C. The tk defects in these mutants are probably unrelated to the ts defects, since one of these complementation groups contains a tk+ member. Therefore, transformation of Ltk- cells to a tk+ phenotype by HSV-1 requires an active viral tk gene. One complementation group was represented by a single tk- member. The role of this cistron in transformation remains undetermined since the primary block to transformation is presumed to be the tk- phenotype. Mutants representing the seven complementation groups were unable to replicate at 39 C in two lines of HSV-1-transformed cells, indicating that the activities of resident wild-type copies of the defective cistrons, if present, could not be detected by complementation.     

Mutant of herpes simplex virus type 1 conditionally able to transform thymidine kinaseless L cells to a tk+ phenotype.

After nitrous acid mutagenesis of herpes simplex virus type 1 (HSV-1), a mutant, 1093, was isolated which, during productive infection, induced very low levels of thymidine kinase (tk). The mutant virus was found, after UV irradiation, to be unable to transform L cells lacking tk (Ltk-) to a tk+ phenotype as chararcterized by growth of the cells in a modified HAT-selective medium containing 1.6 X 10(-5) M thymidine. Cells transformed by wild-type virus grew vigorously under the same conditions. The mutant was able to transform Ltk- cells if the medium contained 10(-3) M thymidine. These transformed cells maintained their conditional character and would not grow in low concentrations of thymidine in selective medium. Therefore, this mutant is conditional on the thymidine concentration in the selection medium in its ability to transform Ltk- cells to a tk+ phenotype. The conditionally transformed cells could be supertransformed with wild-type UV-irradiated HSV-1 to a phenotype which would grow in low-thymidine selective medium. The frequency of supertransformation closely approximated the frequency of transformation of Ltk- cells by wild-type virus. Supertransformation at high frequency could not be effected by mutant 1093 or the tk- mutant B2006. These results indicate that the presence of HSV-1 genetic information in HSV-1-transformed cells does not preclude the acquisition by these cells of at least one additional HSV-1 gene, that for tk.     

Expression and regulation of glycoprotein C gene of herpes simplex virus 1 resident in a clonal L-cell line.

Ltk- cells were transfected with a plasmid containing the entire domain of glycoprotein C (gC), a true gamma or gamma 2 gene of herpes simplex virus 1 (HSV-1) and the methotrexate-resistant mouse dihydrofolate reductase mutant gene. The resulting methotrexate-resistant cell line was cloned; of the 39 clonal lines tested only 1, L3153(28), expressed gC after infection with HSV-1(MP), a gC- mutant, and none expressed gC constitutively. The induction of gC was optimal at multiplicities ranging between 0.5 and 2 PFU per cell, and the quantities produced were equivalent to or higher than those made by methotrexate-resistant gC- L cells infected with wild-type (gC+) virus. The gC gene resident in the L3153(28) cells was regulated as a beta gene inasmuch as the amounts of gC made in infected L3153(28) cells exposed to concentrations of phosphonoacetate that inhibited viral DNA synthesis were higher than those made in the absence of the drug, gC was induced at both permissive and nonpermissive temperatures by the DNA- mutant tsHA1 carrying a lesion in the gene specifying the major DNA-binding protein and which does not express gamma 2 genes at the nonpermissive temperature, and gC was induced only at the permissive temperature in cells infected with ts502 containing a mutation in the alpha 4 gene. The gC induced in L3153(28) cells was made earlier and processed faster to the mature form than that induced in a gC- clone of methotrexate-resistant cells infected with wild-type virus. Unlike virus stocks made in gC- cells, HSV-1(MP) made in L3153(28) cells was susceptible to neutralization by anti-gC monoclonal antibody.     

Identification of the herpes simplex virus DNA sequences present in six herpes simplex virus thymidine kinase-transformed mouse cell lines

We have used a novel filter hybridization approach to detect and map the herpes simplex virus (HSV) DNA sequences which are present in four HSV thymidine kinase (HSVtk+)-transformed cell lines which were derived by exposure of thymidine kinase negative (tk-) mouse cells to UV light-irradiated HSV type 2 (HSV-2). In addition, we have mapped the HSV-1 DNA sequences which are present in two HSV-1tk+-transformed cell lines produced by transfection of tk- mouse cells with sheared HSV-1 DNA. The results of these studies can be summarized as follows. (i) The only HSV DNA sequences which were common to all HSVtk+-transformed cells were those located between map coordinates 0.28 and 0.32. Thus, this region contains all of the viral DNA sequences which are necessary for the expression of HSV-mediated tk transformation. (ii) Many of the cell lines also contained variable amounts of non-tk gene viral DNA sequences located between map coordinates 0.11 to 0.57 and 0.82 to 1.00, suggesting that incorporation of the viral DNA sequences located between these map coordinates is a relatively random event. (iii) The viral DNA sequences located between map coordinates 0 to 0.11 and 0.57 to 0.82 were uniformly absent from all of the HSVtk+ cell lines tested, suggesting that there is a strong negative selective pressure against incorporation of these viral DNA sequences.     

High-frequency homologous recombination in vaccinia virus DNA.

A recombinant vaccinia virus genome was constructed in which the viral thymidine kinase (tk) gene was placed between direct repeats of a 1.5-kilobase-pair DNA sequence of heterologous origin. When forced to replicate in tk- cells in the presence of methotrexate (i.e., under tk+-selective conditions), the recombinant maintained its tk+ phenotype. Under nonselective conditions, however, the tk gene was frequently excised by both inter- and intramolecular recombination events because the repeated sequences provided substantial targets for homologous DNA recombination. Unique DNA products of intramolecular recombination were detected in the cytoplasm of infected cells soon after the onset of viral DNA replication, and their appearance was blocked by inhibitors of DNA synthesis. During repeated passage of the virus under nonselective conditions, the tk+ fraction decreased with first-order kinetics at a rate that reflected the frequency of recombination per cycle of virus replication. Eventually, a residual population of stable tk+ viruses remained, and analyses of the genome structures of individual members of this population showed that some of them appeared to be the products of nonhomologous DNA recombination.     

Hybrid plasmids containing an active thymidine kinase gene of Herpes simplex virus 1.

The gene for the thymidine kinase (TK) of Herpes simplex virus type 1 (HSV-1) is located in the KpnI m and BamHI p fragments of the genome (Wigler et al., Cell 11, 223-232 (1977)). These fragments have been inserted into the EcoRI and BamHI sites, respectively, of plasmid pBR322, and propagated in E.coli. The TK gene contained in the recombinant plasmids was shown to be biologically active when introduced into TK- mouse L cells. Detailed restriction site maps of the BamHI p fragment have been constructed and the approximate location of the TK gene has been determined. Mouse cells transformed with cloned HSV-1 tk+ DNA produced HSV-1-specific thymidine kinase; superinfection with HSV-1 tk- virus increased the level of TK activity tenfold, suggesting that the BamHI p sequences present in transformed cells respond to virus-encoded regulatory gene product(s).     

Characterization of mutants of herpes simplex viruses, types 1 and 2, that produce fragments of the thymidine kinase polypeptide

Seven tk- mutants of herpes simplex virus, type 2 (HSV-2), and three tk- mutants of herpes simplex virus, type 1 (HSV-1), were isolated which did not produce the thymidine kinase (TK) polypeptides but formed smaller polypeptides not seen in wild-type infected cells. Positive TK mRNA selection by hybridization to the cloned tk genes followed by in vitro translation identified the TK polypeptides. Comparisons of the products of partial proteolysis of the polypeptides of four HSV-2 and two HSV-1 tk- mutants to those of the parental TK polypeptides indicated that, in each case, the novel polypeptide was a fragment of the TK polypeptide, showing that these mutants have defects in the structural gene for tk. HSV-2 mutants of this sort have not been previously described. They and the HSV-1 mutants are similar to HSV-1 mutants reported previously. In addition, it was found that TK mRNA was present early in infection but was absent late in infection, suggesting that the shutoff of TK synthesis is due to message degradation. Also, HSV-2 TK mRNA did not hybridize to the cloned HSV-1 tk gene indicating that these genes have extensively diverged.     

Expression of unselected adenovirus genes in human cells co-transformed with the HSV-1 tk gene and adenovirus 2 DNA

We have introduced adenovirus 2 genes into high molecular weight DNA of permissive human cells by co-transformation of tk- human 143 cells with Ad2 restriction enzyme fragments and a cloned Bam HI fragment that carries the HSV-1 thymidine kinase gene. Tk+ cells were isolated after selection and maintenance in HAT medium. Several co-transformed lines are able to complement the growth of Ad5 dl312 (delta 1.2--3.7) and Ad5 dl434 (delta 2.6--8.7), deletion mutants that lack sequences from the left end of the viral genome. The amount and arrangement of viral sequences in the co-transformed cell lines have been analyzed by restriction endonuclease digestion and filter hybridization. Most of the cell lines contain a single insertion of the HSV-1 tk fragment and a single insert of adenoviral DNA. However, one line (B1) contains at least four different insertions, two of which are present in multiple copies. The adenoviral DNA in all cell lines is composed of sequences from the left end of the genome and extends for varying lengths in different lines. Two cell lines that complement deletion mutants efficiently synthesize both early region 1a and 1b mRNAs. The B1 line synthesizes low levels of 1a mRNA, higher levels of 1b mRNA and a unique mRNA that maps to the right of the 1b gene family. When grown continuously in HAT medium, some cell lines are quite stable while others are fairly unstable. Some tk+ subclones support the growth of viral mutants as well as the parental line while others give reduced levels of complementation. For all tk+ subclones examined, the alteration or reduction in viral gene expression is independent of changes in the pattern of integration of viral DNA.     

Transfer of purified herpes virus thymidine kinase gene to cultured mouse cells.

Treatment of Ltk^?, mouse L cells deficient in thymidine kinase (tk), with Bam I restriction endonuclease cleaved DNA from herpes simplex virus-1 (HSV-1) produced tk⁺ clones with a frequency of 10^?6/2 μg of HSV-1 DNA. Untreated cells or cells treated with Eco RI restriction endonuclease fragments produced no tk+ clones under the same conditions. The thymidine kinase activities of four independently derived clones were characterized by biochemical and serological techniques. By these criteria, the tk activities were found to be identical to HSV-1 tk and different from host wildtype tk. The tk⁺ phenotype was stable over several hundred cell generations, although the rate of reversion to the tk^? phenotype, as judged by cloning efficiency in the presence of bromodeoxyuridine, was high (1–5 × 10^?3). HSV-1 DNA Bam restriction fragments were separated by gel electrophoresis, and virtually all activity, as assayed by transfection, was found to reside in a 3.4 kb fragment. Transformation efficiency with the isolated fragment is 20 fold higher per gene equivalent than with the unfractionated total Bam digest. These results prove the usefulness of transfection assays as a means for the bioassay and isolation of restriction fragments carrying specific genetic information. Cells expressing HSV-1 tk may also provide a useful model system for the detailed analysis of eucaryotic and viral gene regulation.     

The transfer and stable integration of the HSV thymidine kinase gene into mouse cells.

Treatment of mutant mouse cells (Ltk-) deficient in thymidine kinase with Bam I restriction endonuclease-cleaved HSV-1 DNA results in the appearance of numerous surviving colonies which stably express thte tk+ phenotype. Through a series of electrophoretic fractionations in concert with transfection assays, we isolated a 3.4 kb fragment which contains the thymidine kinase gene and which alone is competent in the biochemical transformation of Ltk- cells. In this report, we have examined the distribution of tk sequences in the DNA of several transformed clones following stable gene transfer. A series of complementary experiments involving reassociation kinetics in solution and annealings with tk DNA to restriction-cleaved cellular DNA following electrophoresis and transfer to filters allow us to make the following general conclusions concerning the fate of the tk gene in all clones examined: the tk gene is present in all cells at a frequency of one copy per chromosomal complement; the tk gene is stably integrated in the DNA of all transformants; and integration is not site-specific and occurs at different loci in the DNA of all transformants examined. The existence of a single active tk gene in tk+ transformants now facilitates an analysis of the sequence organization of tk- mutant cells and provides a useful model system for studies on the transfer of cellular genes.     

Chicken ovalbumin gene fused to a herpes simplex virus alpha promoter and linked to a thymidine kinase gene is regulated like a viral gene.

We are describing a system for the introduction, selection, and expression of eucaryotic genes in higher eucaryotic cells. The carrier consisted of the herpes simplex virus 1 (HSV-1) tk gene covalently linked to an HSV-1 alpha promoter directed away from the tk gene. In this study we fused to the alpha promoter the 5' transcribed noncoding sequences and the coding sequences of the chicken oviduct ovalbumin gene. Cells converted to the TK+ phenotype with this chimeric fragment produced an ovalbumin precursor which was processed and secreted into the extracellular fluid. The ovalbumin gene utilized the HSV-1 alpha promoter and was regulated as a viral gene inasmuch as inversion of the genomic DNA relative to the alpha promoter resulted in no ovalbumin synthesis, and production of ovalbumin was enhanced after superinfection with HSV-1. Synthesis of ovalbumin was not detected when cDNA was linked to the HSV-1 alpha promoter. The carrier system described in this study is suitable for introduction, selection, and expression of eucaryotic genes whose natural promoter is either weak or requires the presence of regulatory elements which may be absent from undifferentiated cells in culture.     

Mapping of the herpes simplex virus DNA sequences present in herpes simplex virus type-1 thymidine kinase-transformed cells

Analyses of the herpes simplex virus (HSV) DNA sequences which are present in three HSV thymidine kinase-transformed (HSVtk+) mouse cell lines have revealed that these cells contain relatively large and variable portions of the viral genome. Two of these cell lines do not contain the viral DNA sequences known to encode the early viral genes normally responsible for regulating tk gene expression during lytic HSV infections. This finding suggests that cell-associated viral tk gene expression may be regulated by cellular rather than viral control mechanisms. In addition, we have compared the viral DNA sequences present in one unstable HSVtk+ cell line to those present in tk- revertant and tk+ rerevertant cell lines sequentially derived from it. Our results have shown that within the limits of sensitivity of our mapping approach, these three related cell lines contain the same set of viral DNA sequences. Thus, gross changes in viral DNA content do not appear to be responsible for the different tk phenotypes of these cells.     

Clearance of herpes simplex virus type 2 by CD8+ T cells requires gamma interferon and either perforin- or Fas-mediated cytolytic mechanisms

The T-cell-mediated resolution of herpes simplex virus type 2 (HSV-2) genital infections is not fully understood. In these studies, the mechanisms by which CD8+ T cells clear virus from the genital epithelium were examined. Ovalbumin (OVA)-specific CD8+ T cells from OT-I transgenic mice cleared a thymidine kinase-deficient, ovalbumin-expressing HSV-2 virus (HSV-2 tk- OVA) from the genital epithelium of recipient mice, and clearance was abrogated by in vivo neutralization of gamma interferon (IFN-gamma). Further, CD8+ OT-I T cells deficient in IFN-gamma were unable to clear HSV-2 tk- OVA from the vaginal epithelium. The requirement for cytolytic mechanisms in HSV-2 tk- OVA clearance was tested in radiation chimeras by adoptive transfer of wild-type or perforin-deficient OT-I T cells to irradiated Fas-defective or wild-type recipients. Although a dramatic decrease in viral load was observed early after challenge with HSV-2 tk- OVA, full resolution of the infection was not achieved in recipients lacking both perforin- and Fas-mediated cytolytic pathways. These results suggest that IFN-gamma was responsible for an early rapid decrease in HSV-2 virus titer. However, either perforin- or Fas-mediated cytolytic mechanisms were required to achieve complete clearance of HSV-2 from the genital epithelium.     

Induction of interleukin-8 gene expression is associated with herpes simplex virus infection of human corneal keratocytes but not human corneal epithelial cells.

Interleukin-8 (IL-8) is a proinflammatory cytokine released at sites of tissue damage by various cell types. One important function of IL-8 is to recruit neutrophils into sites of inflammation and to activate their biological activity. Stromal keratitis induced by herpes simplex virus type 1 (HSV-1) is characterized by an initial infiltration of neutrophils. This study was carried out to determine whether cells resident in the cornea synthesize IL-8 after virus infection. Pure cultures of epithelial cells and keratocytes established from human corneas were infected with HSV-1, and the medium overlying the cells was subsequently assayed for IL-8 by an enzyme-linked immunosorbent assay. Cytokine mRNA levels in cell lysates were monitored by Northern (RNA) blot analysis. It was found that virus infection of keratocyte cultures led to the synthesis of IL-8-specific mRNA with more than 30 ng of IL-8 made per 10(6) cells. Neither UV-inactivated virus nor virus-free filtrates collected from HSV-1-infected keratocytes could induce IL-8 protein or mRNA, suggesting that viral gene expression was needed for induction of IL-8 gene expression. Unlike keratocytes, HSV-1-infected epithelial cells failed to synthesize IL-8 protein or mRNA. However, these cells readily produced both molecules following tumor necrosis factor alpha stimulation. HSV-1 had similar titers in both cell types. Thus, the failure to induce IL-8 synthesis was not due to an inability of the virus to replicate in epithelial cells. The capacity of HSV-1-infected corneal keratocytes to synthesize IL-8 suggests that these cells can contribute to the induction of the acute inflammatory response seen in herpes stromal keratitis.     

gamma 2-Thymidine kinase chimeras are identically transcribed but regulated a gamma 2 genes in herpes simplex virus genomes and as beta genes in cell genomes.

True gamma or gamma 2 genes, unlike alpha, beta, and gamma 1 (beta gamma) genes of herpes simplex virus 1 (HSV-1), stringently require viral DNA synthesis for their expression. We report that gamma 2 genes resident in cells were induced in trans by infection with HSV-1 but that the induction did not require amplification of either the resident gene or the infecting viral genome. Specifically, to test the hypothesis that expression of these genes is amplification dependent, we constructed two sets of gamma 2-thymidine kinase (TK) chimeric genes. The first (pRB3038) consisted of the promoter-regulatory region and a portion of 5'-transcribed noncoding region of the domain of a gamma 2 gene identified by Hall et al. (J. Virol. 43:594-607) in the HSV-1(F) BamHI fragment D' to the 5'-transcribed noncoding and coding regions of the TK gene. The second (pRB3048) contained, in addition, an origin of HSV-1 DNA replication. Cells transfected with either the first or second construct and selected for the TK+ phenotype were then tested for TK induction after superinfection with HSV-1(F) delta 305, containing a deletion in the coding sequences of the TK gene, and viruses containing, in addition, a ts lesion in the alpha 4 regulatory protein (ts502 delta 305) or in the beta 8 major DNA-binding protein (tsHA1 delta 305). The results were as follows: induction by infection with TK- virus of chimeric TK genes with or without an origin of DNA replication was dependent on functional alpha 4 protein but not on viral DNA synthesis; the resident chimeric gene in cells selected for G418 (neomycin) resistance was regulated in the same fashion; the chimeric gene recombined into the viral DNA was regulated as a gamma 2 gene in that its expression in infected cells was dependent on viral DNA synthesis; the gamma 2-chimeric genes resident in the host and in viral genomes were transcribed from the donor BamHI fragment D' containing the promoter-regulatory domain of the gamma 2 gene. The significance of the differential regulation of gamma 2 genes in the environments of host and viral genomes by viral trans-acting factors is discussed.