Thymidine Plaque Autoradiography of Thymidine Kinase-

Transcription

1 JOURNAL OF CLINICAL MICROBIOLOGY, Jan. 1983, p /83/ $02.00/0 Vol. 17, No. 1 Thymidine Plaque Autoradiography of Thymidine Kinase- Positive and Thymidine Kinase-Negative Herpesviruses RICHAR B. TENSER,* JOHN C. JONES, STEPHEN J. RESSEL, AN F. ANNE FRALISH epartments of Medicine (Neurology) and Microbiology and Cancer Research Center, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania Received 12 July 1982/Accepted 12 October 1982 Plaques formed by herpes simplex virus (HSV), pseudorabies virus, and varicella-zoster virus were studied by plaque autoradiography after [14C]thymidine labeling. Standard thymidine kinase-positive (TK+) viruses and TK- mutants of HSV types 1 and 2 and pseudorabies virus were studied, including cellcultured viruses and viruses isolated from animals. Autoradiography was performed with X-ray film with an exposure time of 5 days. After development of films, TK+ plaques showed dark rims due to isotope incorporation, whereas TKplaques were minimally labeled. Plaque autoradiography of stock TK- viruses showed reversion frequencies to the TK+ phenotype of <10-3. Autoradiography indicated that TK- virus retained the TK- phenotype after replication in vivo. In addition, it was shown that TK- HSV could be isolated from mouse trigeminal ganglion tissue after corneal inoculation of TK- HSV together with TK+ HSV. The plaque autoradiographic procedure was very useful to evaluate proportions of TK+ and TK- virus present in TK+-TK- virus mixtures. Herpes simplex virus (HSV) thymidine kinase (TK) expression has been of recent clinical interest, in part because of the development of a group of selective antiviral agents. The antiviral activity of drugs such as arabinosylthymine (ara- T), acycloguanosine (acyclovir), and bromovinyldeoxyuridine is apparently dependent on their phosphorylation by viral TK to an active form which then inhibits viral NA polymerase (2, 6, 8, 13). Studies have been most detailed for acyclovir (6, 13). It is clear, however, that HSV mutants resistant to inhibition by these drugs can be readily developed in cell culture (3, 10, 16). Resistance of HSV mutants to acyclovir is thought to be due to mutations in the viral TK so that the drug is not phosphorylated, or to mutations in the viral NA polymerase so that the phosphorylated drug is not inhibitory (3, 10). Most mutations probably occur in the viral TK so that TK-negative (TK-) virus develops. Recently, TK- HSV mutants have been isolated in humans after acyclovir therapy (1, 4, 11). Based on studies in experimental animals (15-17), we hypothesized that such mutants are likely to have limited pathogenicity in humans because of their relative inability to establish neuronal infections (15, 17). It remains important to determine the development of such mutants during the course(s) of antiviral therapy, as well as the possible presence of TK+ HSV during or after therapy. TK activity of isolates from patients is easily studied. However, biochemical TK assays on such specimens have two drawbacks: (i) a mixed TK+-TK- population may be present, which will probably result in a TK measurement of intermediate activity, and (ii) the specimen must first be cultured in vitro to produce a quantity of virus adequate for biochemical study. Replication under nonselective conditions may result in changes in the TK phenotype of the TK- mutants. We now report the use of a simple thymidine (TdR) plaque autoradiography procedure to study several herpesviruses, a procedure which minimized these problems and permitted direct assay of infectious virus. (A preliminary report of this work was presented at the 1982 Annual Meeting of the American Society for Microbiology [R. B. Tenser, Abstr. Annu. Meet. Am. Soc. Microbiol. 1982, S57, p. 244].) MATERIALS AN METHOS Viruses. TK+ and TK- HSV-1 (strain KOS) and the B2006 TK- mutant, provided by S. Kit (Baylor University College of Medicine, Houston, Tex.), were used (17), as were standard TK+ HSV-2 (strain 333) and TK- virus selected by passage through bromodeoxyuridine (BUdR; 100,ug/ml). TK+ and TK- pseudorabies virus (PRV) was obtained from T. Ben-Porat (Vanderbilt University, Nashville, Tenn.). TK- virus was selected by growth of TK+ virus in the presence of ara-t. TK+ varicella-zoster virus (VZV)-infected cells isolated from a recent clinical patient were obtained from R. Hyman, the Pennsylvania State Uni- 122

2 VOL. 17, 1983 THYMIINE PLAQUE AUTORAIOGRAPHY 123 A *49 c E t w #, le *# r ae C~~* iv _E.k T.Tl" 4 A I' -Ir YTCI't T FIG. 1. Autoradiographs of HSV-1-infected plates (left, A, C, E) and monolayer cultures from which the autoradiographs were made (right, B,, F). Autoradiographs and plates are similarly oriented to permit comparison of individual plaques. (A) and (B) show plaques produced by infection with KOS TK+ stock virus; (C) and () show plaques with KOS TK- stock virus; and (E) and (F) show plaques with B2006 TKvirus. Arrows point to the same plaque in monolayer cultures and the appropriate autoradiographs. TdR plaque autoradiography. The TdR plaque autoradiography assay was modified from methods de- 4 B scribed by Jamieson and Subak-Sharpe (7) and Summers and Summers (14). Confluent primary rabbit kidney cells (60- or 150-ml culture plates) were infected with appropriately diluted stock virus or with virus isolated from animals. After incubation at 37 C in a humidified atmosphere for 1 h, monolayers were overlaid with 0.5% methylcellulose and incubated for 3 to 4 days at 37 C. In some cases, ara-t (50 to 150 gg/ml) was added to the methylcellulose. Methylcellulose was removed, and medium containing [14C]TdR (0.5,iCi/ml; 53.4 mci/mmol, New England Nuclear Corp., Boston, Mass.) was added. Plates were incubated at 37 C for an additional 4 to 6 h, after which time the medium was discarded. Monolayers were stained with 0.5% neutral red and, after washing, were fixed with 10o Formalin. After plaques were counted, the monolayers were dried overnight, and the vertical circumferential rims of the plastic plates were removed. Monolayers were then placed in contact with X-ray film (Kodak L, Rochester, N.Y.) for 5 days. Exposures at -70 C, 4 C, and room temperature were equally satisfactory. Films were developed, and F plaques with clear evidence of labeled rims were counted. Such TK+ plaques were easily distinguished from TK- plaques, and the total number of plaques observed by autoradiography was compared with that 4 in the fixed, stained monolayer cultures. This procedure was slightly modified for study of VZV; medium without methylcellulose was used for the initial overlay, and VZV-infected cells were used for titrations <- ḷ, v rather than cell-free virus. A 4s versity (5). HSV and PRV stock viruses were grown in primary rabbit kidney cells, and stock VZV was grown in Flow 5000 fibroblasts (Flow Laboratories, Inc., Rockville, Md.). etermination of TK phenotype of HSV and PRV stock viruses was performed by methods used previously, in which lysates of lytically infected TK- mouse cells were tested (15). In assays run at 37 C, TK+ HSV-1 phosphorylated 54.1 pmol of TdR per mg of protein, and the TK- virus value was 1.3%. HSV-2 phosphorylated 41.9 pmol/mg, and the TK- virus value was 1.1%. PRV TK+ virus phosphorylated 45.3 pmol/mg, and the TK- value was <1%. Inoculation and isolation of virus from animals. Fiveto 7-week-old randomly bred male and female mice (C-1, Charles River Breeding Laboratories, Inc., Wilmington, Mass.) were anesthetized and corneally inoculated with 25,ul of virus suspension. Virus stocks were diluted to 1 x 105 to 5 x 10i PFU/ml before use. Eyes were swabbed, and trigeminal ganglion homogenates were prepared as described previously (17). Virus present in these preparations was titrated by standard methods. C FIG. 2. Autoradiographs of HSV-2-infected plates (left, A, C) and monolayer cultures from which the autoradiographs were made (right, B, ). (A) and (B) show plaques of 333 TK+ stock virus, and (C) and () show plaques of the 333 TK- mutant. The arrows point to two adjacent TK+ plaques in the autoradiograph and in the monolayer culture.

3 S....;P r. A+. m.... if. *..... ' 124 TENSER ET AL. J. CLIN. MICROBIOL. plaques and 2,008 TK- PRV plaques by autora- :B diography, and no TK+ revertants were noted. Therefore, reversion frequencies of these mutants were <10-. PRV TK+ plaques were A&.* (*4V= ;'". greatly diminished in size by adding ara-t and BUdR to overlays, although plaques continued to show the TK+ phenotype. The small plaques were difficult to identify against cellular background activity when ara-t was used; however, a;; with BUdR, which decreased cellular TdR uptake, these pinpoint plaques were easily discerned (Fig. 3B and C). VZV plaques were identified as TK+, al- though VZV plaques were not as heavily labeled as HSV and PRV plaques. For this reason, a longer period of isotope labeling was often utilized (Fig. 4). Use of methylcellulose overlay containing ara-t (50 jig/ml) eliminated the VZV plaques. VZV reportedly induces a virus-specific TK with a substrate specificity similar to that for HSV and is inhibited by ara-t (9). Mouse and FIG. 3. Autoradiographs of PIRV-infected plates. human cytomegalovirus plaques were TK- by (A) shows TK+ PRV plaques, alnd () shows TK- plaque autoradiography (data not shown). It PRV plaques. The TK+-infected culture from which appears that although cytomegalovirus infection the autoradiograph (B) was madle had been treated may have induced cell protein synthesis, enculture for (C) was zyme activity was not sufficient to have plaques with ara-t (100,g/ml), and the treated with BUdR (100,ug/ml). Piinpoint TK+ plaques appear TK' by autoradiography. are present in both but are easier tc see in (C), in which As determined by plaque autoradiography, cellular background activity had b)een decreased. TK- HSV-1, HSV-2, and PRV did not change in TK phenotype after replication in mice. The lack of plaque label of virus isolated from eye swabs RESULTS 3 days after corneal inoculation of TK- HSV-1 In the autoradiograph (Fig. IIA) of an infected and HSV-2 is shown in Fig. 5. These viral monolayer culture (Fig. 1 E3), TK+ HSV-1 plaques were due to virus replication in vivo plaques showed dark rims irndicative of TdR rather than the residual of inoculated virus (Ta- accounted for ble 1). Virus was barely detectable 1 h after uptake. The labeled plaques seeen all plaques in the monolayer ciulture. TK- KOS inoculation, and eye swab titers increased to 5.5 and B2006 plaques showed minimal, if any, x 103 to 2.5 x 104 PFU per eye after 3 days labeling (Fig. 1C and E). Figures 1 and F show (Table 1). From this, it is apparent that the the plates from which the atatoradiographs in viruses replicated well in vivo. Replication of Fig. 1C and E were made. Off more than 2,000 TK- mutants of HSV-1, HSV-2, and PRV in plaques of standard HSV-1 K,OS examined, all vivo in ocular tissues and the conserved TKshowed the TK+ phenotype. From stock TK- phenotype are shown in Table 1. KOS, a single TK+ plaque wa,s identified out of Evaluation of an in vivo infection by a mixed more than 3,520 plaques exammined. This is con- TK+ and TK- HSV-1 population, including the sistent with a reversion freque-ncy of <1o-3 for the TK- mutant. At times, bliarred plaques and apparent isotope streaming artifacts were pres- -I B ent, but differentiation of TK+ from TK- Although I plaques was still straightfoirward. [14C]TdR was used, [3H]TdR could also be used, although exposure times of 3 to 4 weeks were L necessary (data not shown). a TK+ HSV-2 was also readi ly distinguishable from TK- mutant virus. Autoradiographs and the monolayers from which th ey were made are shown for TK+ HSV-2 (Fig. 2} k and B) and TKvirus (Fig. 2C and ). PRV TK+ plaques were FIG. 4. Autoradiograph of VZV-infected plate (A) also easily differentiated from IrK- plaques (Fig. and the culture from which the autoradiograph was 3A and ). We examined 1,,072 HSV-2 TK- made (B). Isotope labeling was for 12 h.

4 VOL. 17, 1983 A C FIG. 5. Autoradiographs (left, A, C) and monolayer cultures (right, B, ) from which the autoradiographs were made. Infecting virus was isolated from eye swabs taken 3 days after corneal inoculation of TK- HSV-1 (A, B) and TK- HSV-2 (C, ). The TKphenotype of these animal isolates is apparent. proportion of TK+ and TK- virus present, was also done by TdR plaque autoradiography. The component TK+ and TK- titers were established by comparing the plaque counts in the monolayer culture with those in the autoradiograph of that monolayer. Results from two animals are shown in Fig. 6. The total HSV titer, indicated by the presence of 81 plaques in the monolayer culture (Fig. 6B), was 8.1 x IO' PFU. The autoradiograph of that culture (Fig. 6A) shows 39 of the 81 plaques to be TK+; therefore, 42 plaques were TK-. From this, the TK+ titer was 3.9 x 103 PFU, and the TK- titer was 4.2 x THYMIINE PLAQUE AUTORAIOGRAPHY PFU. In the other animal, the total titer was B 7.9 x 103 PFU, and the TK+ and TK- titers were 2.7 x 103 and 5.2 x 103 PFU, respectively. In these examples, TK+ and TK- HSV were isolated from trigeminal ganglion tissue after corneal inoculation of an equal mixture of both viruses. This supported previous results showing that large amounts of TK- HSV-1 could be isolated from trigeminal ganglion tissue after mixed TK+-TK- virus inoculation (17); it is not yet clear whether the TK- virus can then be maintained in a latent state. ISCUSSION TdR plaque autoradiography performed by standard titration procedures indicated the quantities of TK+ and TK- HSV present in virus stocks and in animal isolates. The TK+ phenotypes of plaques were clearly indicated when plaque-purified stock virus and virus mixtures were assayed and when cell-cultured virus or animal isolates were tested. Of importance in evaluating the role of viral TK expression in pathogenesis is the observation that the TKphenotype of mutants was shown to be main- A t 4 a * B TABLE 1. TK phenotype by plaque autoradiography of ocular isolates after corneal inoculation of TK- HSV-1, HSV-2, and PRV' Virus titer at time [14C]TdR plaque after corneal autoradiography of TK- virus inoculation (PFU isolates 3 days after inoculated per eye): inoculation (TKplaques/plaques 1 h 3 days examined) HSV x 104 1,380 /1,380 HSV x 104 3,281b/3,292 PRV x 103 2,559 /2,559 a Stock viruses with titers of 1 x 105 to 5 x 105 PFU per 0.1 ml were corneally inoculated. Isolates were obtained from eye swabbings 3 days after inoculation and cultivated on primary rabbit kidney cells. b Eleven lightly labeled plaques of possible intermediate TK activity were observed. It r';p x FIG. 6. Autoradiographs (left, A, C) and monolayer cultures (right, B, ) from which the autoradiographs were made. Infecting virus was isolated from trigeminal ganglion tissue 3 days after corneal inoculation of equal amounts of TK+ and TK- HSV-1 (strain KOS). Open arrows show a TK- plaque, and closed arrows show a TK+ plaque. Virus isolated from one mouse is shown in A and B (81 total plaques when the ganglion homogenate was diluted to 10-2; 39 TK+ plaques are seen in the autoradiograph, indicating 42 plaques were TK-). Results from another mouse are seen in C and (79 total plaques, 27 are TK+, indicating 52 are TK-).

5 126 TENSER ET AL. tained even when the viruses were passed in animals. The plaque autoradiography procedure is based on the phosphorylation of TdR well above cellular background levels by the TKs of HSV-1, HSV-2, PRV, and, to a lesser degree, VZV. As shown for PRV, TK+ virus may be inhibited by ara-t and BUdR, although the TK phenotype of virus plaques appeared to remain TK+. By performing duplicate titrations with and without these inhibitors, and by studying the duplicate titrations by plaque autoradiography, an analysis of viral TK phenotype after replication under selective and nonselective conditions can be provided. Mutants with intermediate amounts of TK activity could be differentiated from TK+ and TK- viruses when compared side by side. Although this was not extensively studied, it appeared that mutants with 10 to 25% of parental TK activity could be differentiated from viruses with <5 or >65% of TK activity. Jamieson and Subak-Sharpe had used a [3H]TdR incorporation method in which infected monolayers were overlaid with photographic emulsion after infection (7), and Smiley et al. used [125I]deoxycytidine ([125I]dCdR) and photographic emulsion (12). Summers and Summers used [125I]dCdR incorporation and X-ray film autoradiography but did not report the observation of individual plaques (14). In the present study, X-ray film required shorter exposure times than those reported for photographic emulsion and was easier to use than emulsion. In addition, [14C]TdR is much simpler to use than [125I]dCdR. By placing X-ray film in direct contact with fixed monolayer cells, sufficient [14C]TdR signal could be detected, and individual plaques could be easily studied. Isotope-containing medium was added several hours before plaques were counted, and because thoroughly confluent monolayers were used, background cellular uptake was minimal. Cellular uptake could be further decreased by adding BUdR to the methylcellulose overlay. Use of TK- mouse cells for plaque autoradiography also resulted in further decreased background activity, but clear HSV plaques were more difficult to identify (data not shown). From previous investigations with several HSV-1 TK- mutants, it has become apparent that expression of this virus-specified enzyme is important in the establishment of acute and latent trigeminal ganglion (neuron) infections (15, 17). The restriction of TK- HSV replication in ganglion tissue was thought to be based on the inability of the virus to replicate in neurons, a nondividing cell population, even though virus replication in ocular tissues, a source of dividing or potentially dividing cells, was intact (15). Based in part on these observations, we specu- J. CLIN. MICROBIOL. late that from a teleological standpoint, TK phenotype is important in the pathogenesis of HSV infections because it affects the ability of the virus to infect sensory ganglion neurons. TdR plaque autoradiography may prove to be a useful procedure in investigations in experimental animals as well as in the evaluation of virus isolates from humans. With this procedure, plaques derived from large numbers of infectious particles could be individually analyzed for TK activity. This technique may also be of use in following patients treated with drugs that inhibit TK+ HSV and which may result in the selection of TK- mutants (4, 11). ACKNOWLEGMENTS This investigation was supported by Public Health Service Teacher-Investigator Award NS from the National Institute of Neurological and Communicative isorders and Stroke (NINCS), by grant NS awarded by the NINCS, and by grants CA 18450, CA 09124, and CA from the National Cancer Institute. The editorial assistance of Melissa Reese and the secretarial assistance of Kelly Fortna and Carol Buck are gratefully acknowledged. LITERATURE CITE 1. Burns, W. H., R. Saral, G. W. Santos, 0. L. Laskin, P. S. Lietman, C. McLaren, and. W. Barry Isolation and characterization of resistant herpes simplex virus after acyclovir therapy. Lancet i: Cheng, Y.-C., G. utschman, J. J. Fox, K. A. Watanabe, and H. Machida ifferential activity of potential antiviral nucleoside analogs on herpes simplex virusinduced and human cellular thymidine kinases. Antimicrob. Agents. Chemother. 20: Coen,. M., and P. A. Schaffer Two distinct loci confer resistance to acycloguanosine in herpes simplex virus type 1. Proc. Natl. Acad. Sci. U.S.A. 77: Crumpacker, C. S., L. E. Schnipper, S. I. Marlowe, P. N. Kowalsky, B. J. Hershey, and M. J. Levin Resistance to antiviral drugs of herpes simplex virus isolated from a patient treated with acyclovir. N. Engl. J. Med. 306: Ecker, J. R., and R. W. Hyman Varicella-zoster virus NA exists as two steric isomers. Proc. Natl. Acad. Sci. U.S.A. 79: Furman, P. A., M. H. St. Clair, J. A. Fyfe, J. L. Rideout, P. M. Keller, and G. B. Elion Inhibition of herpes simplex virus-induced NA polymerase activity and viral NA replication by 9-(2-hydroxyethoxymethyl)guanine and its triphosphate. J. Virol. 32: Jamieson, A. T., and J. H. Subak-Sharpe Interallelic complementation of mutants of herpes simplex virus deficient in deoxypyrimidine kinase activity. Virology 85: Larsson, A., and B. Oberg Selective inhibition of herpesvirus deoxyribonucleic acid synthesis by acycloguanosine, 2'-fluoro-5-iodo-aracytosine, and (E)-5-(2-bromovinyl)-2'-deoxyuridine. Antimicrob. Agents Chemother. 19: Miller, R. L., J. P. Iltis, and F. Rapp ifferential effect of arabinofuranosylthymine on the replication of human herpesviruses. J. Virol. 23: Schnipper, L. E., and C. S. Crumpacker Resistance of herpes simplex virus to acycloguanosine: role of viral thymidine kinase and NA polymerase loci. Proc. Natl. Acad. Sci. U.S.A. 77:

6 VOL. 17, 1983 THYMIINE PLAQUE AUTORAIOGRAPHY Sibrack, C.., L. T. Gutman, C. M. Wilfert, C. McLaren, and. Barry Altered pathogenicity of acyclovir resistant HSV from an immune deficient child. Pediatr. Res. 15: Smiley, J. R., M. J. Wagner, W. P. Summers, and W. C. Summers Genetic and physical evidence for the polarity of transcription of the thymidine kinase gene of herpes simplex virus. Virology 102: St. Chir, M. H., P. A. Furman, C. M. Lubbers, and G. B. Elion Inhibition of cellular a and virally induced deoxyribonucleic acid polymerases by the triphosphate of acyclovir. Antimicrob. Agents Chemother. 18: Summers, W. C., and W. P. Summers [125IJdeoxycytidine used in a rapid, sensitive, and specific assay for herpes simplex virus type 1 thymidine kinase. J. Virol. 24: Tenser, R. B., and M. E. unstan Herpes simplex virus thymidine kinase expression in infection of the trigeminal ganglion. Virology 99: Tenser, R. B., R. L. Miller, and F. Rapp Trigeminal ganglion infection by thymidine kinase-negative mutants of herpes simplex virus. Science 205: Tenser, R. B., S. Ressel, and M. E. unstan Herpes simplex virus thymidine kinase expression in trigeminal ganglion infection: correlation of enzyme activity with ganglion virus titer and evidence of in vivo complementation. Virology 112: