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Antimicrobial Agents and Chemotherapy, September 2006, p. 3142-3145, Vol. 50, No. 9
0066-4804/06/$08.00+0 doi:10.1128/AAC.00342-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Tatsuo Suzutani,2
Yumiko Yamamoto,1
Yoshiko Fukui,1
Naoki Nozawa,1
D. Scott Schmid,3
Ichiro Kurane,1 and
Naoki Inoue1*
Department of Virology I, National Institute of Infectious Diseases, Tokyo, Japan,1 Department of Microbiology, Fukushima Medical University, Fukushima, Japan,2 National Varicella-Zoster Virus Laboratory, Centers for Disease Control and Prevention, Atlanta, Georgia3
Received 22 March 2006/ Returned for modification 27 April 2006/ Accepted 5 July 2006
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Previously, we established a reporter cell line for human herpesvirus 8 and demonstrated its practical uses (10, 11, 14). We report here the generation of a similar cell line for VZV by use of the following approach: (i) selection of the most suitable promoter from various promoters, (ii) identification of the minimum sequence length for efficient activation of the selected promoter to avoid nonspecific activation, (iii) establishment of a cell line that contains the reporter gene under the control of the minimum promoter, and (iv) characterization of the clone that performed best.
Characterization of individual VZV promoters and global VZV gene expression have been reported previously (4, 5, 12). On the basis of those studies, various DNA fragments containing known or putative VZV promoters (Table 1) were amplified from the nucleocapsid DNA of VZV pOka strain by PCR, cloned into a luciferase vector pGL3-Basic, and then analyzed for their responses to VZV infection in transient transfection experiments using BSC40 cells (3) and MeWo cells (9). Among the promoters examined, the ORF9 promoter was most strongly activated in both cell lines (Fig. 1). Activation of each promoter was also analyzed by cotransfection with a plasmid expressing VZV IE62. The correlation between infection- and IE62-mediated activation (Fig. 1C) was good, indicating that infection-mediated activation was VZV specific.
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TABLE 1. Comparison of VZV promoter activities
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FIG. 1. Comparison of VZV promoter activities. MeWo (7 x 104 cells/well) (A) and BSC40 (2.4 x 104 cells/well) cells (B) in 96-well plates were transfected with 125 ng of a reporter plasmid containing the promoter for the indicated open reading frame (ORF) along with 4 ng of a Renilla luciferase-expressing plasmid (pRL-CMV; Promega) as an internal control, by using FuGENE 6 (Roche). pGL3-Basic, a vector plasmid used to clone the promoters, was used as a negative control (). At 24 h after transfection, the cells were infected with VZV at an multiplicity of infection of 0.02, and 1 day later, both firefly and Renilla luciferase activities were determined by a chemiluminescent assay reaction (Dual-Glo luciferase assay system; Promega) followed by measurement of relative light units (RLU) with a luminometer (JNR AB2300; ATTO, Japan). Means and standard deviations (SDs) of RLU from triplicate wells are shown. (C) Correlation of IE62- and infection-dependent promoter activation. MeWo cells were transfected with each reporter plasmid along with an IE62-expressing plasmid, and 2 days later their luciferase activities were measured and compared with those represented in panel A. Each circle represents one promoter-reporter construct. An arrow indicates ORF9.
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FIG. 2. Minimum ORF9 promoter region for responses to VZV infection and to IE62. (A) Reporter plasmids containing truncated ORF9 promoter sequences, 9C, 9D, 9F, 9G, and 9I were constructed. A duplicated form of the minimum essential 93-bp sequence (9G) was cloned into BamHI site of pGL3-Basic, resulting in pGL-T9G. (B) The sequence of the minimum essential region of the ORF9 promoter (9G) and the USF binding motif in the region are shown. A 2-bp alteration (CACGTG to CAtaTG) of the USF binding motif (9U) was introduced into the sequence. (C and D) Activities of the modified promoters represented in panels A and B were analyzed in transient transfection experiments. Means and SDs of luciferase activities from triplicate wells are shown. pGL3-Basic (vector) used for cloning of the promoters was used as a negative control.
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The luciferase activity in MV9G increased gradually after infection, and the activity increased in a dose-dependent manner with a detection limit of 50 PFU (Fig. 3). Coculturing of VZV-infected cells with MV9G cells induced luciferase activities with a detection limit of fewer than 10 infected cells (Fig. 3C). No luciferase activity was detected in MV9G cells cocultured with cells infected with human cytomegalovirus (Towne), human herpesvirus 6 (Z29) (15), or human herpesvirus 7 (SB) (2) or in MV9G cells directly infected with cell-free human cytomegalovirus. In contrast, MV9G cells infected with herpes simplex virus type 1 (HSV-1) resulted in luciferase activities similar to those seen with VZV-infected cells at 24 h after infection. However, at 48 h after infection, the infected cells showed strong cytopathic effect and their luciferase activities decreased, making it practical to distinguish HSV-1 infection from VZV infection.
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FIG. 3. Characterization of VZV reporter cell line MV9G. (A) Time-dependent activation of the reporter cell line after VZV infection. MV9G cells in 96-well plates were infected with 60 and 600 PFU of VZV and incubated for 2 h. After removal of the inoculums, cells were cultured for the indicated period (in hours), rinsed with phosphate-buffered saline, and stored at 80°C until luciferase activity was measured. Means and SDs of luciferase activities from triplicate wells are shown. (B) MV9G cells were infected with the indicated PFU of VZV and incubated for 2 h. After removal of the inoculums, the cells were cultured for an additional 46 h. Means and SDs of luciferase activities from triplicate wells are shown. (C) MV9G cells were infected with cell-associated virus, and their luciferase activities were measured 2 days later. MeWo cells were infected with the same cell-associated virus, and the numbers of the infected cells used for the inoculation were obtained by immunostaining using anti-VZV IE62 monoclonal antibody (MAb8616; Chemicon International). The luciferase activities were compared with the numbers of the infected cells.
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2 µg/ml) (13, 18, 28). The EC50 against infection with cell-associated virus was 10 to 20 µg/ml. Note that the EC50 of ACV is known to be higher when infections are induced using cell-associated virus (24). ACV treatment decreased luciferase activity only slightly in MV9G cells infected directly with cell-free virus; this was not unexpected, since the reporter gene expression in MV9G depends on a promoter that is active during the early phase of infection.
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FIG. 4. Applications of the reporter cell assay. (A) MeWo cells (7 x 104 cells/well) in 96-well plates were inoculated with cell-associated VZV ( 1 x 103 MeWo cells infected with VZV; cytopathic effect > 70%) ( , "cell-associated") or with 250 PFU of cell-free VZV (, "cell-free") and cultured in the presence or absence of the indicated concentrations of ACV. Two days later, MV9G cells (7 x 104 cells/well) were added to each well and cocultured in the presence of ACV for 1 day. The cells were harvested, and their luciferase activities were measured. Means and SDs of luciferase activities from triplicate wells are shown. (B) MeWo cells (7 x 104 cells/well) in 96-well plates were inoculated with approximately 200 of MeWo cells infected with TK-positive (pOka [] and CaQu [ ]) or -negative (rOkaYSR [ ] and Kanno [ ]) strains (22, 23, 28), and cultured in the presence of ACV for 2 days. Then, MV9G cells were overlaid and cocultured for 1 day. The luciferase activities in the ACV-treated cultures were expressed as percent RLU, with the activities obtained in the culture without inhibitor treatment used as a standard. Means and SDs of luciferase activities from triplicate wells are shown. (C) MeWo cells (7 x 104 cells/well) were infected with cell-associated VZV, cultured for 2 days, and then cocultured with MV9G for 1 day (, "coculture"). MV9G cells (7 x 104 cells/well) were directly infected with 400 PFU of cell-free VZV, and cultured for 2 days ( , "direct infection"). Roscovitine was added at the indicated concentrations throughout the processes. Means and SDs of luciferase activities from triplicate wells are shown.
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40 µg/ml, which was much higher than that observed for TK-positive strains. The use of a reporter cell assay thus eliminates the need for laborious preparation of cell-free VZV stocks for the detection of ACV-resistant strains, providing a strong practical advantage. Roscovitine, a kinase inhibitor, decreased luciferase activities, both in directly infected MV9G cells and in MV9G cells cocultured with MeWo cells that were infected in the presence of roscovitine (Fig. 4C). This suggests that at least part of its inhibitory effect occurs in the early phase of infection, which is consistent with previous reports (30). Cellular factors required for virus infection are attractive targets for the development of novel antiviral drugs. High-throughput screening of compounds in a cell-based assay is essential for that purpose, and MV9G appears to be ideally suited for that role.
In conclusion, the VZV reporter cell assays developed here are expected to foster the efficient evaluation of new antiviral compounds for the treatment of VZV-associated diseases.
This work was supported by a Grant-on-Aid for Science from the Ministry of Education, Science, Technology and Sports, Japan (N.I.), and by the Research on Health Sciences focusing on Drug Innovation program of Japanese Human Science Foundation (T.S., D.S.S., N.I.). G.-Q.W. was a fellow supported by the Japan Society for the Promotion of Science.
Present address: Department of Dermatology, Zhongshan Hospital of Xiamen University, and Department of Dermatology, Medical College of Xiamen University, Xiamen, China. ![]()
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