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Journal of Virology, December 2003, p. 12907-12913, Vol. 77, No. 23
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.23.12907-12913.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
International Centre for Genetic Engineering and Biotechnology, New Delhi 110067, India
Received 16 June 2003/ Accepted 1 August 2003
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Several laboratories worldwide are exploring multiple approaches towards developing dengue virus vaccines based on live attenuated viruses (1, 21, 36), inactivated viruses (35), infectious clone-derived intertypic (18, 26) and chimeric (5, 13, 14, 43) viruses, antigen-encoding plasmids (23, 24), recombinant proteins expressed in heterologous systems (2, 22, 38, 40), and live vaccinia virus vectors encoding antigen genes (9, 31, 32). However, the major focus is on the live, empirically attenuated (1, 21, 36), and infectious clone-derived ChimeriVax vaccines based on the attenuated YF17D yellow fever vaccine vector (13, 14). Alternative attenuated vector backbones based on dengue type 1 (DEN-1) (29, 45), DEN-2 (18), and DEN-4 (8) viruses are being developed in parallel. All these strategies rely on the creation of monovalent vaccine viruses, which are mixed together to generate tetravalent formulations.
Recent studies in which the tetravalent live attenuated (21) and ChimeriVax (13) vaccines were tested in humans and nonhuman primates, respectively, revealed that the tetravalent formulations elicited an unbalanced immune response, which was predominantly specific to a single serotype. This has been ascribed to viral interference that apparently comes into play when all four vaccine viruses are mixed together and coinjected (21). The observation that the tetravalent ChimeriVax vaccine formulation is also apparently associated with the phenomenon of viral interference (despite all four of its component viruses having identical YF17D backbones, unlike the live attenuated tetravalent vaccine) underscores the difficulties, and more importantly the risk, inherent in the current strategy of creating a tetravalent dengue vaccine. This warrants investigation of other recombinant viral vector systems that may permit the creation of a single tetravalent dengue virus vaccine vector. From such a perspective, the adenovirus (Ad) expression system appears worth investigating, as vectors are available that can accommodate inserts of up to
35 kb (16, 44), making it possible to envisage the creation of a single vaccine vector that encodes critical protective antigens of all four dengue virus serotypes to provide complete protection against dengue.
Ad vectors offer several important advantages from a vaccine perspective (reviewed in references 34 and 39). They have an exceptional safety record as live viral vaccines (10) and are not particularly pathogenic in humans (17). Two of the most promising recent reports pertaining to nonhuman primate models of the Ebola virus (41) and the human immunodeficiency virus (4, 37) emphasize the potential of Ad-based vaccination strategy. However, one concern regarding Ad vectors for human use is that preexisting immunity to Ad can compromise the efficacy of Ad-based vaccines. Recent work on Ad-based human immunodeficiency virus (4) and Ebola vaccines (46) has suggested that DNA priming followed by vector boosting can effectively overcome the effect of prior Ad immunity. Though poxvirus vectors can accommodate very large inserts, a comparison of attenuated poxvirus vectors such as NYVAC (11) and MVA (4, 37) with replication-defective Ad vectors have shown the latter to be safer and more efficacious for vaccine applications, particularly with reference to the primer-booster injection strategy.
To investigate the utility of the Ad vector in developing dengue virus vaccines, in this study we constructed a replication-defective recombinant Ad (rAd) vector by inserting the envelope (E) protein-encoding gene of DEN-2 virus into the early region 1 (E1) of the Ad genome. The E protein, which is exposed on the surface of the virus, is the dominant virus antigen (25) that interacts with host cell receptors (6) and represents the major target of neutralizing antibodies (7, 30). We used the ectodomain of the DEN-2E virus molecule obtained by deletion of the carboxy-terminal transmembrane domain. This truncation does not compromise the structural-antigenic integrity of the E protein. In fact, studies have shown that the truncated version is more immunogenic than the full-length molecule (32). We also included in our construct the putative secretory signal encoded by the carboxy terminus of the premembrane protein. It was necessary to include this signal sequence, as its deletion inhibited expression of the E protein (our unpublished data).
The E protein-encoding gene (DEN-2E virus gene) was isolated as a
1.3-kb cDNA fragment by reverse transcription-PCR using DEN-2 virus (NGC strain) genomic RNA as the template. The cloned gene was sequenced and verified by comparison with published data (20) to correspond to the carboxy-terminal 31-amino-acid-encoding portion (putative secretory signal sequence) of the premembrane gene followed by the amino-terminal 395-amino-acid-encoding portion (the ectodomain) of the E gene. The DEN-2E virus gene was placed under the transcriptional control of the cytomegalovirus (CMV) early promoter/enhancer in an appropriate shuttle vector, and the resultant expression cassette was then inserted into the E1 region of the Ad type 5 (Ad5) genome by using in vivo recombination in Escherichia coli and rescuing the resultant E1-lacking recombinant virus, rAdD2E, in the E1 trans-complementing human 293 cell line as described previously (15). The genome structure of rAdD2E (verified by PCR and extensive restriction analyses) is depicted in Fig. 1A.
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FIG. 1. rAd vector rAdD2E for the expression of DEN-2 virus E protein in mammalian cells. (A) Schematic representation of the linear genome of rAdDen2E. E1 denotes early region 1 of the Ad genome, which had been replaced by the E ectodomain expression cassette (CMV, CMV promoter/enhancer; D2E, cDNA encoding the E ectodomain of DEN-2 virus; pA, polyadenylation signal) in the recombinant virus. E3 indicates a 2.7-kb deletion in the nonessential E3 region of the Ad genome. The gray-shaded boxes at either end of the viral genome represent the cis-acting elements critical for viral DNA replication (L-ITR and R-ITR, left and right inverted terminal repeats, respectively; , packaging signal). (B) Immunoprecipitation (using DEN-2 virus-specific 3H5 mAb) of DEN-2E virus protein from infected BHK cells. Cells were either mock infected (lane 1) or infected with cAd (lane 2), rAdD2E (rAd; lane 3), or DEN-2 virus (lane 4). Infected cells were metabolically labeled with [35S]methionine, lysed, immunoprecipitated with 3H5 mAb, and analyzed on a denaturing 15% polyacrylamide gel. The positions and sizes (in kilodaltons) of prestained protein markers run on the same gel are indicated on the left of the panel. The arrows on the right indicate the positions of the full-length E protein expressed by DEN-2 virus (dotted arrow) and the C-terminally truncated version (E ectodomain) expressed by the rAd vector (solid arrow). (C) Indirect immunofluorescence micrograph of infected BHK cells. Cells were either mock infected or infected with the same viruses as described for panel B, fixed in cold acetone, and incubated with 3H5 mAb followed by fluorescein-labeled anti-mouse antibody to visualize E protein expression.
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53 kDa) from rAdD2E-infected cells (Fig. 1B, lane 3). The positive-control experiment carried out in parallel showed that DEN-2 virus-infected BHK cells expressed the full-length E protein (Fig. 1B, lane 4). We obtained similar results using COS cells in place of BHK cells. Interestingly, when we used 293 and HeLa cells, we detected the expression of rAd-encoded E ectodomain protein but not of DEN-2 virus-encoded full-length E protein (data not shown), presumably reflecting the inability of DEN-2 virus to successfully infect these cell types.
From a comparison of the expression levels attained (Fig. 1B; compare lanes 3 and 4), it is evident that rAdD2E can be used to achieve high levels of DEN-2E virus expression in infected cells. Using a purified preparation of recombinant E protein mutant containing the 3H5 epitope as a reference in a quantitative immunoblot assay, we estimated the levels of rAd-mediated E protein expression in infected BHK cells to be
2 µg/106 cells; in 293 cells, the levels were
8- to 10-fold higher (data not shown). Protein expression was also analyzed by immunofluorescence, as shown in Fig. 1C. Infected BHK cells were fixed in acetone and probed with 3H5 mAb. Antigen-antibody interactions were detected using fluorescein-tagged anti-mouse immunoglobulin G. Mock-infected as well as cAd-infected BHK cells failed to show any fluorescence in this assay. DEN-2 virus-infected BHK cells displayed intense fluorescence and served as the positive control. The rAdD2E-infected BHK cells manifested fluorescence comparable to that of the positive control. Interestingly, the difference in E protein expression levels evident in the immunoprecipitation experiment was not discernible in the immunofluorescence experiment. As the same antibody (3H5 mAb) was used in both techniques, the apparent discrepancy may be a reflection of differences in the detection methodologies.
Next, we investigated the immunogenicity of the rAd vector. Groups of six BALB/c mice were immunized intraperitoneally with rAdD2E or cAd (107 PFU/mouse) at 0, 1, and 2 months. Sera (collected 1 week after each inoculation) were analyzed by enzyme-linked immunosorbent assays (ELISA) using live DEN-2 virus as the capture antigen. The data are depicted in Fig. 2. Sera collected after the initial injection had no significant antibody titers in either cAd- or rAdD2E-immunized animals. However, the presence of DEN-2 virus-specific antibodies could be discerned in the rAdD2E-immunized sera after the second injection; the levels detected increased quite significantly after the final injection (Fig. 2B). There was no change in the observed ELISA titers of cAd-immunized (Fig. 2A) or phosphate-buffered saline (PBS) (mock)-immunized (data not shown) mice after the second and third inoculations. The rAdD2E-induced antibodies specifically recognized the DEN-2 virus-expressed full-length E protein, as is evident from the data presented in Fig. 3. In this experiment, culture supernatants of DEN-2 virus-infected, [35S]methionine-labeled COS cells were immunoprecipitated with mouse immune sera collected after the first, second, and third immunizations. A control immunoprecipitation carried out in parallel using 3H5 mAb in place of mouse sera revealed the full-length E protein (Fig. 3, lane 2). The rAdD2E-immunized animals did not show any discernible anti-DEN-2 virus antibody response after the initial injection (Fig. 3, lane 5). However, after the booster immunizations an anti-DEN-2 antibody response was readily detectable in rAdD2E-immunized mouse sera (Fig. 3, lanes 8 and 11). The immunoprecipitated protein bands shown in lanes 8 and 11 comigrated with the 3H5 immunoprecipitated band shown in lane 2, indicating that the antibodies elicited by rAdD2E can efficiently recognize and immunoprecipitate the full-length E protein expressed by DEN-2 virus.
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FIG. 2. Anti-DEN-2 virus antibodies in rAd-immunized mouse sera. Groups of BALB/c mice (n = 6) were immunized intraperitoneally with either cAd (A) or rAdD2E (B) as described in the text. Anti-DEN-2 virus antibody titers in sera obtained after the first (triangles), second (circles), and third (squares) injections were determined with DEN-2 virus as the capture antigen and revealed with horseradish peroxidase-conjugated anti-mouse antibody. ELISA plates (96 well) coated overnight with DEN-2 virus (250 50% tissue culture infective doses [TCID50]) at 4°C were blocked (in 1x PBS-1% polyvinylpyrrolidone-0.2% horse serum for 2 to 4 h at 4°C), washed four times (using 1x PBS-0.1% Tween 20), and incubated for 45 min at 37°C with serial twofold dilutions of the individual mouse serum samples. The wells were washed (using 1x PBS-0.25% Tween 20) and incubated with anti-mouse immunoglobulin G-horseradish peroxidase conjugate (at a 1:5,000 dilution) for 45 min at 37°C, washed again, and incubated with 3,3'5,5'tetramethyl benzidine substrate for 25 min at 37°C. The color reaction was terminated by the addition of 1 M H2SO4, and the optical density (OD) was read at 450 nm. Each data point represents the average of six independent determinations (error bars represent standard deviations [SD]).
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FIG. 3. Analysis of rAdD2E-induced DEN-2 virus-specific antibodies in murine sera by radioimmunoprecipitation. Pooled sera (5 µl from each group) obtained from PBS (lanes 3, 6, and 9)-, cAd (lanes 4, 7, and 10)-, and rAd (lanes 5, 8, and 11)-immunized mice collected after the first (A), second (B), and third (C) vaccinations were incubated separately (overnight at 4°C) with 500 µl of DEN-2 virus-infected, [35S]methionine-radiolabeled COS cell culture supernatant. The resultant antigen-antibody complexes were captured using protein G Sepharose, washed, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A control immunoprecipitation using 3H5 mAb (1 µl) instead of murine antiserum (lane 2) was performed in parallel. Lane 1 shows prestained markers whose positions were traced with radioactive ink on the dried gel prior to autoradiography. The sizes of these markers are indicated (in kilodaltons) on the left side of the panel. The arrows on the right side of each panel indicate the positions of the DEN-2 virus-expressed full-length E protein.
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1.5-fold of each other (73 versus 106). These data demonstrate that the rAdD2E is immunogenic and can elicit antibodies that can specifically recognize and neutralize DEN-2 virus. |
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TABLE 1. Virus-neutralizing antibody titers in sera of immunized micea
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] and interleukin-4 [IL-4]) production. At all dilutions of DEN-2 virus used for in vitro restimulation, cAd-primed mouse splenocytes did not exhibit any proliferative response (Fig. 4A). In contrast, cells from rAdD2E-primed mouse spleens demonstrated a significant proliferation response (as evidenced by the magnitude of [3H]thymidine uptake) at all dilutions of DEN-2 virus used for in vitro restimulation (Fig. 4; compare panels B and A). Mock-stimulated spleen cell cultures, obtained from rAdD2E-primed mice, displayed a consistently minimal response (Fig. 4B). The proliferative response displayed by rAdD2E-primed mouse splenocytes was accompanied by IFN-
secretion. These cells displayed significantly elevated levels of IFN-
after 5 days of restimulation with DEN-2 virus; mock-stimulated cells did not secrete IFN-
into the culture medium (Fig. 5B). When cAd-primed mouse spleen cell cultures were tested, IFN-
was barely detectable in mock- and DEN-2 virus-stimulated culture supernatants (Fig. 5A). In contrast to the situation with IFN-
, IL-4 was detected in the splenocyte culture supernatants during the entire course of the 5-day experiment. However, there were only marginal differences in the levels of secreted IL-4 between mock- and DEN-2 virus-stimulated spleen cell cultures regardless of whether they had been obtained from cAd- or rAdD2E-primed animals (Fig. 5, panels C and D). This leads to the conclusion that splenocytes from rAdD2E-primed mice respond to in vitro restimulation with DEN-2 virus by secreting elevated levels of IFN-
without any significant change in the basal levels of IL-4 production. This may be indicative of a Th1 response, which is considered to be crucial for maturation of cytotoxic T lymphocytes that are necessary to eliminate virus-infected cells.
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FIG. 4. Determination of T-cell proliferative response. Splenocytes obtained from cAd-primed (A) and rAd-primed (B) mice were seeded in 96-well plates in Dulbecco's modified Eagle's medium-10% heat-inactivated fetal calf serum (2.5 x 105 cells in 0.1 ml/well). Cells were either mock stimulated (solid bars) or stimulated with live DEN-2 virus (hatched bars) (at several dilutions as indicated) for 5 days (U, undiluted virus equivalent to 1,000 TCID50). Each dilution was assayed in triplicate. Cells were pulsed with [3H]thymidine (1µCi/well) for 16 h at the end of the 5-day incubation period, and proliferation was quantitated by measuring the uptake of the radioisotope in a scintillation counter. Data depicted represent the mean values of three determinations (error bars represent SD).
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FIG. 5. IFN- and IL-4 responses to DEN-2 virus stimulation. Splenocytes obtained from cAd-primed (A and C) and rAd-primed (B and D) mice were either mock stimulated (solid bars) or stimulated with 1000 TCID50 of live DEN-2 virus (hatched bars) for 5 days. Aliquots of the culture supernatant were withdrawn at the time points shown, and the levels of IFN- (A and B) and IL-4 (C and D) were determined by solid-phase ELISA using commercially available murine IFN- and IL-4 kits (Quantikine M; R&D systems). Data depicted represent the mean values of three separate determinations (error bars represent SD).
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7.5 kb, of which the CMV promoter-driven DEN-2E virus expression cassette occupies
2.3 kb. A vector lacking E1/E3/E4 (with an insert capacity of
11 kb) which can be propagated in 911E4 cells is available (15). Also, there are Ad vectors lacking all viral genes that have a maximum insert capacity of
35 kb and can be propagated in the presence of packaging-defective mutant Ad helpers (44). It is thus feasible to insert the major structural genes of all four of the dengue viruses into a single rAd vector. This aspect would make a rAd-based dengue virus vaccine an attractive alternative, especially since this strategy has the potential to eliminate the phenomenon of viral interference that tends to skew the immune response predominantly towards one serotype when the tetravalent vaccine is constituted from four monovalent viral vaccines (13, 21). Currently, work is under way to develop such a tetravalent rAd vector.
We thank Andrew Falconar and Bert Vogelstein for providing DEN-2 virus and the components of the AdEasy system, respectively. We are grateful to Radha Padmanabhan for advice on culturing dengue viruses and Pawan Sharma for his helpful suggestions. S.J. is a Senior Research Fellow supported by the Council of Scientific and Industrial Research, Government of India.
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