- Open Access
Characterisation of immune responses and protective efficacy in mice after immunisation with Rift Valley Fever virus cDNA constructs
© Lagerqvist et al; licensee BioMed Central Ltd. 2009
Received: 31 December 2008
Accepted: 17 January 2009
Published: 17 January 2009
Affecting both livestock and humans, Rift Valley Fever is considered as one of the most important viral zoonoses in Africa. However, no licensed vaccines or effective treatments are yet available for human use. Naked DNA vaccines are an interesting approach since the virus is highly infectious and existing attenuated Rift Valley Fever virus vaccine strains display adverse effects in animal trials. In this study, gene-gun immunisations with cDNA encoding structural proteins of the Rift Valley Fever virus were evaluated in mice. The induced immune responses were analysed for the ability to protect mice against virus challenge.
Immunisation with cDNA encoding the nucleocapsid protein induced strong humoral and lymphocyte proliferative immune responses, and virus neutralising antibodies were acquired after vaccination with cDNA encoding the glycoproteins. Even though complete protection was not achieved by genetic immunisation, four out of eight, and five out of eight mice vaccinated with cDNA encoding the nucleocapsid protein or the glycoproteins, respectively, displayed no clinical signs of infection after challenge. In contrast, all fourteen control animals displayed clinical manifestations of Rift Valley Fever after challenge.
The appearance of Rift Valley Fever associated clinical signs were significantly decreased among the DNA vaccinated mice and further adjustment of this strategy may result in full protection against Rift Valley Fever.
Rift Valley Fever virus (RVFV) is a mosquito-borne Phlebovirus in the Bunyaviridae family. RVFV infects domesticated ruminants and humans and regularly induces epizootics with concomitant epidemics throughout the African continent and on the Arabian Peninsula [1, 2]. Outbreaks among domesticated ruminants are characterised by a large increase of spontaneous abortions and the case fatality rate may reach 100% in young animals . While Rift Valley Fever (RVF) is generally benign in man, more severe clinical manifestations such as hemorrhagic fever, encephalitis and retinitis are regulary observed .
Despite the fact that RVF is an important viral zoonosis, and the risk for emergence in new susceptible areas has been emphasized , effective and safe vaccines are not commercially available. However, formalin inactivated vaccines have been developed for human use, but the distribution is limited to high-risk occupation staff [5, 6]. Currently there are a few vaccines available for use in livestock: vaccines based on the live-attenuated Smithburn strain  and formalin inactivated virus preparations . The Smithburn virus vaccine is suggested to induce lifelong protection, but has retained the ability to induce abortions and teratogenic effects in livestock [9, 10]. The inactivated virus vaccines are safe, but less immunogenic and require annual booster vaccinations . Previously, two vaccine candidates have been proposed and tested for their safety and efficacy in animal trials: a naturally attenuated RVFV isolate from a benign human case in the Central African Republic, Clone 13  and a human virus isolate of RVFV attenuated in cell culture by 5-fluorouracil treatment, MP12 [13, 14]. Although Clone 13 and MP12 were shown to be safe and immunogenic in mice and in cattle and sheep, respectively , the MP12 vaccine was found teratogenic for pregnant sheep if used during the first trimester .
In addition to the adverse effects previously shown for attenuated RVF vaccines, there are considerable safety concerns regarding viral vaccines based on highly pathogenic organisms due to the risk for exposure or escape of live agents during the manufacturing process. In addition, there is also a risk of insufficient inactivation or emergence of revertants, when large quantities of virulent virus strains are handled. Because of these shortcomings, new RVF vaccine strategies ought to be considered. Genetic immunisation is an attractive alternative, since the antigens are produced by the host cells and the presentation resembles natural infections by intracellular parasites. It is also cost-effective and circumvents the need for elevated biosafety level facilities . Genetic vaccines are also less vulnerable to elevated temperatures during storage and transportation, which are important factors when performing vaccinations in developing countries . These characteristics make DNA vaccines uniquely suited for vaccine production against highly pathogenic organisms, such as RVFV [18, 19].
The RVFV is a three segmented negative stranded RNA virus. The (L)arge segment encodes a RNA dependent RNA polymerase and the (M)edium segment encodes two glycoproteins (GN and GC), a 78 kDa protein as well as a non-structural protein (NSm). The (S)mall segment encodes a non-structural protein (NSs) and the immunogenic and highly expressed nucleocapsid protein (N) .
Despite an abundance of the N protein in the virus and in the infected cell, this protein is not generally associated with protective immunity. However, a recent study has shown that a proportion of mice inoculated with purified RVFV N proteins were protected against virus challenge . Although antibodies targeting the RVFV glycoproteins are recognized for their protective properties  contradictory results regarding the level of protection after DNA vaccination have been presented [20, 22, 23].
In this study we evaluate the induced immune responses and the conferred protection in mice after genetic immunisation with cDNA encoding the structural proteins of RVFV. The elicited immune responses towards the N, GN, GC and GN/GC proteins after gene-gun immunisation were analysed and the protective abilities of the N and the GN/GC construct were tested by virus challenge.
Cells and viruses
BHK-21 (ATCC number CCL-10) cells were maintained in Glasgow MEM (GIBCO, Invitrogen, Carlsbad, CA) supplemented with 5% FCS, 1.3 g/l Tryptose (Difco™, Becton, Dickinson and Company, Sparks, MD), 10 mM HEPES, 1 mM sodium pyruvate, 100 U penicillin/ml and 100 μg/ml streptomycin at 37°C/5% CO2. The working stocks of RVFV and cDNA constructs, originated from the ZH548 wild-type strain, isolated from a human case in Egypt in 1977 . Viral stocks were prepared and titrated on monolayers of BHK-21 cells and the cDNA sequences are found under the GenBank accession numbers AF134534 and DQ380206[25, 26].
Production of DNA vaccine
Forward primer sequences
Reverse primer sequences
Animal immunisation and infection
Female BALB/c mice, six to eight weeks old, were used in this study. Before immunisation the mice were thoroughly shaved on the abdomen and vaccinated with cDNA encoding the antigens using a gene-gun (Helios™, BioRad Laboratories). The cDNA was administrated four times with two to three week intervals. The primary immunisation was performed using four gene-gun cartridges and the following three boosters with two cartridges. Blood samples were collected three, five, seven and nine weeks after the primary immunisation. In order to study the immune responses post infection (p.i.) and the effectiveness of the genetic vaccines, mice were injected intraperitoneally (i.p.) with RVFV diluted in sterile PBS to a final volume of 100 μl. Infected animals were kept in micro-isolator cages inside an animal isolator (Bell Isolation Systems Ltd, Livingston, Scotland) and all manipulations involving infected animals or viable virus were performed within a BSL-3 laboratory. During the experimental procedures the animals were monitored daily and were kept with free access to food and water. Mice found in a moribund condition (fatigue and "hunchback-like posture") were instantly euthanized. This project was approved by The Animal Research Ethics Committee of Umeå University, Sweden.
Evaluation of immune response
To evaluate and compare the immune responses after vaccination and infection, eight animals were vaccinated with cDNA encoding N, four with cDNA containing the open reading frame of the GN/GC poly-protein and two groups, each containing four animals, were immunised with either the GN or the GC construct. To analyse the immune responses after infection, one group consisting of nine mice were infected with 2.4 × 104 PFU of RVFV. At day 14 p.i. the animals were euthanized and samples collected. As negative controls, four mice were immunised with the pcDNA3.1 vector without insert and another four mice were injected with sterile PBS and kept under the same conditions.
A total of 30 mice were used in the challenge study, eight of which were vaccinated with cDNA encoding the RVFV N protein and eight with the GN/GC construct. As controls, eight animals were vaccinated with an irrelevant gene (encoding the N protein of the Puumala virus, PUU-N) and six animals with pcDNA 3.1 vectors without insert. After four rounds of immunisations, half of the mice of each vaccination group were challenged with 2.4 × 103 and half with 2.4 × 104 PFU of RVFV. Blood samples were collected every alternate day until the end of the experiment at day 17 p.i.
Antigen production and purification
For antigen production and prokaryotic expression, cDNA encoding the full-length N protein (aa 1–245) of RVFV was ligated into pET-14b (Novagen, Darmstadt, Germany) and cDNA encoding truncated N derivatives, N1 (aa 1–100), N2 (aa 71–170), N3 (aa 141–245), N1/2 (aa 1–170) and N2/3 (aa 71–245), were inserted into pET101/D-TOPO® or pET151/D TOPO® (Invitrogen). The primer sequences are shown in Table 1.
DNA constructs expressing the N protein and truncated N derivatives were expressed in Escherichia coli (E. coli) BL21 DE3 (Invitrogen). Briefly, transformed bacteria were grown in Luria-Bertani media supplemented with 100 μg/ml carbencillin to OD A600 of 0.7. Expression of the antigens was induced by the addition of isopropyl-beta-D-thiogalactopyranoside (IPTG) at a final concentration of 0.5 mM. The purification of the full length N protein expressed from a poly-histidine-fusion vector was performed with metal chelating chromatography using Ni-NTA Agarose (Qiagen GmbH, Hilden, Germany), essentially as described previously . N protein preparations used for the lymphocyte proliferation assay were purified further with Triton X-114 (Sigma-Aldrich Inc., St. Louis, MO) to remove contaminating amounts of endotoxins . Each batch was tested for unspecific stimulation of splenocytes before use.
Enzyme-linked immunosorbent assay (ELISA), Western blot and Immunofluorescence analysis (IFA)
Indirect ELISA (total Ig) was performed using microtiter plates (NUNC-immuno™ MaxiSorp, Nalgene Nunc International, Rochester, NY) coated with 3 μg/ml of purified recombinant N protein as previously described . Wells lacking the primary antibody were used to establish the background levels and negative or pre-immune sera were used to determine unspecific binding.
Western blot was performed using E. coli extracts containing the complete N protein or truncated variants thereof (N1, N2, N3, N1/2, N2/3). The separated proteins were transferred to Immobilon TMP transfer membranes (type PVDF, Millipore Co., USA). Membranes containing the antigens were incubated with serum samples from individual mice at dilution 1:600 in parallel with internal controls, either an anti-V5 antibody (Invitrogen) diluted 1:5000 or a mouse anti-poly-histidine antibody (ZYMED® Laboratories, S. San Francisco, CA) diluted 1:3000. A horseradish peroxidase (HRP) conjugated rabbit anti-mouse Ig antibody (DacoCytomation, Glostrup, Denmark) diluted 1:2000 was used as secondary antibody. The antibody-antigen complexes were visualised with enhanced chemiluminescence (ECL, Amersham Bioscience, Uppsala, Sweden). The blotting and incubation procedures have previously been described in detail .
For IFA, BHK-21 cells were grown on cover slips and infected with ZH548 at MOI 1, or transfected with cDNA constructs using FuGene™ reagent according to the manufacturer's instructions (Roche Diagnostics, Basel, Switzerland). At 36 h p.i. or 48 h post transfection the cells were fixed with 3% paraformaldehyde in PBS (for anti-glycoprotein antibody detection) or methanol (for anti-N antibody detection). Labelling was performed with mouse sera diluted 1:200, followed by visualisation with an Alexa Fluor™ 488 (Molecular probes, Invitrogen) secondary antibody at dilution 1:5000. The expression of the antigens was verified using an anti-V5 antibody (Invitrogen) diluted 1:5000, positive sera from previously infected mice or monoclonal antibodies directed against the GN and GC proteins, kindly provided by Dr. George Ludwig (USAMRIID, Fort Detrick, MD) at predetermined dilutions.
Lymphocyte proliferation test
The lymphocyte proliferation assay was performed as described earlier . Briefly, spleen cells of five mice vaccinated with cDNA encoding the full length N protein of RVFV were prepared in RPMI 1640 (GIBCO, Invitrogen) supplemented with 5% FCS, 2 mM sodium pyruvat, 2.5 × 10-5 M β-Mercaptoethanol and 50 μg/ml gentamicin sulphate. After washing the spleen cells three times in cell culture media by centrifugation at 600 × g, the lymphocytes were resuspended to 4 × 105 cells/ml. Aliquots (100 μl) of the cells were seeded to 96-wells flat-bottom microplates (Nalgene Nunc International) in cell culture media containing the antigen at different concentrations. After two days incubation at 37°C/5% CO2, 1 μCi of 3HTdR (5'-3H Thymidine spec.act 14.4 Ci/mmol, Amersham Biosciences) was added. After an additional 16–18 hr of metabolic labelling, the cells were harvested on GF/C filters (Inotech AG, Basle, Switzerland) and analysed for incorporated radioactivity using a liquid scintillation counter (TriCarb 2500 TR, Packard Instruments, Meriden, CT). Spleen cells obtained from four mice immunised with the plasmid vector without insert constituted the negative control. The stimulation index (SI) was calculated as the ratio of radioactivity incorporated into cells from vaccinated mice and the count rate in cells from control mice.
Plaque reduction neutralisation test (PRNT)
Heat-inactivated mouse sera including positive and negative controls, were serially diluted three-fold in PBS and incubated with a virus suspension containing about 30 plaque forming units (PFU) of RVFV. The mixtures were incubated for 90 min at 37°C and thereafter used to infect monolayers of BHK-21 cells in 6-well tissue culture plates (NUNC tissue culture, Nalgene Nunc International). After an adsorption period of 30 min at 37°C, the cells were rinsed with PBS and incubated with cell culture media containing 1% Carboxy-Methyl Cellulose (Aquacide II, Calbiochem®, Merck, CA) for six days at 37°C/5%CO2. The cells were subsequently fixed with 10% formaldehyde, washed with water and counter-stained with 1% crystal violet in water containing 20% ethanol and 0.7% NaCl. The PRNT50 titer was calculated as the reciprocal of the highest serum dilution that reduced the number of plaques by 50%, as compared to the virus control.
The outcome of the challenge was evaluated using the Fisher exact test (Epi Info™, Version 3.5). Quantitative variables were based on measurements of at least two independent experiments containing duplicate samples. Variables are expressed as means and the error bars represent the standard deviation.
Antibody response after immunisation with cDNA encoding the N protein
Proliferative response subsequent immunisation with cDNA encoding the N protein
Humoral response after immunisation with cDNA encoding the glycoproteins
All mice sero-converted after immunisation with cDNA encoding the GN/GC proteins or the GN protein but only two out of four after vaccination with cDNA encoding the GC protein, as detected by IFA performed on infected cells. The virus neutralising antibody titers after GC and GN vaccination were in the lower range, less than 25 and between 25 to 75, respectively. However, the GN/GC vaccinated mice acquired considerably higher titers, up to 225 (data not shown). These results indicate that vaccination with the GN/GC construct resulted in higher virus neutralising antibody titers than the use of cDNA encoding for the individual glycoproteins.
Challenge of gene-gun vaccinated mice
Neutralising antibody titers and outcome after challenge after DNA vaccination against RVFV
No. of animals
PRNT50 titers a
Outcome after challenge with RVFV
Clinical signs b
2.4 × 103
2.4 × 104
25 – 75
2.4 × 103
25 – 75
2.4 × 104
2.4 × 103
2.4 × 104
2.4 × 103
2.4 × 104
Since differences in clinical signs could not be ascribed to the different challenge doses, the two subgroups within each vaccine group were consolidated and evaluated together. In the groups of mice immunised with the N or the GN/GC constructs, four of eight and five of eight animals, respectively, displayed no clinical signs during the entire experiment (Table 2). Despite the large proportion of animals without RVF clinical signs in the GN/GC vaccination group, extensive viral replication after infection was indicated by high N specific antibody titers, similar to the titers observed for the control animals (data not shown). Apart from one casualty, due to a moribund condition, in the N vaccinated group, no major differences in the severity of the clinical manifestations were observed between the GN/GC and N vaccinated mice after challenge. In contrast, all animals in the two control groups displayed either clinical signs of infection followed by complete recovery (12/14) or were sacrificed due to a moribund condition (2/14) (Table 2). Significant protection against RVF clinical signs was observed among the N vaccinated mice (p = 0.0096, Fisher exact test) and the GN/GC vaccinated mice (p = 0.0021, Fisher exact test) as compared to the controls.
RVF is an important emerging zoonotic infection and early efforts to protect animals and humans resulted in development of attenuated and inactivated virus vaccines. Vaccines based on live attenuated RVFV strains have shown to induce long-lasting protection in contrast to inactivated virus vaccines, which require multiple booster doses to retain a protective immunity . Unfortunately, teratogenic effects and the ability to cause abortions limit the likelihood for wide use and distribution of the current vaccines based on attenuated RVFV strains. As the existing vaccines have such shortcomings, efforts to design safer and more efficient RVF vaccines need to be undertaken.
We have investigated the prospect of employing genetic immunisation against RVF. The DNA vaccine platform has been extensively studied during the last decade. However, the breakthrough has been on halt until recently when the first licensed products became available, such as the vaccine against West Nile virus infection in horses and a vaccine for use in salmon against the hematopoietic necrosis virus . The DNA vaccine technology is especially suitable against pathogens such as RVFV, since the need of elevated biosafety facilities are circumvented and the stability of these vaccines allow distribution in developing countries lacking the logistics to maintain a "cold-chain".
In this study, the immune responses in mice after genetic immunisation with RVFV cDNA encoding the N protein, the glycopolyprotein GN/GC, and the separate GC and GN proteins were analysed. The N and the GN/GC constructs displayed the most promising results regarding the elicited immune response and were evaluated further for the ability to confer protection in a subsequent challenge study.
After gene-gun vaccination with the N construct, high antibody titers were repeatedly induced along with an antigen induced proliferative cellular response. Interestingly, no clinical signs were observed after challenge in 50% of the animals (compared to 100% in the control group) despite the lack of detectable levels of neutralising antibodies after vaccination. The observed protection might be explained by cell-mediated immune factors as indicated by the dose-dependent proliferation of spleen cells from the immunised animals. Nevertheless, the characteristics of the proliferating cells remain to be investigated further. Analogous results were previously found after vaccination with the purified RVFV N protein when protection was obtained in 60% of the vaccinated mice . Also, a recent study using the Toscana virus (Phlebovirus, Bunyaviridae) reported approximately 60% survival upon challenge after immunisation with the recombinant N protein, probably due to a cellular mediated immune response .
Previous studies of N proteins of Hantaviruses revealed that strong B-cells epitopes are located near the amino-terminus [33, 34]. However, this does not seem to be the case for RVFV N. Genetic immunisations are in general believed to mimic the natural presentation of antigens , but interestingly, while the sera of immunised mice recognized the amino-terminal part (aa 1–100) of the N-protein, sera of the infected animals did not. The lack of reactivity towards the central (N2) and the C-terminal (N3) parts could either be explained by a distorted conformation of the encoded antigens or disruption of epitope-regions within the N protein.
In this study, antibodies towards the glycoproteins were induced after genetic vaccination, but virus neutralisation was only observed in sera of mice immunised with cDNA containing the GN gene. This observation is in accordance with earlier findings, where GN has been shown to possess antigenic determinants important for protection, while GC does not [38, 39]. However Besselar and co-workers found neutralising epitopes associated with protection in the GC, as well as in the GN protein . The absence of neutralising antibodies after gene-gun vaccination using the GC construct alone might be explained by incorrect folding of the expressed antigen, since neutralising antibodies elicited by the glycoproteins are often found to be conformation dependent .
The RVFV glycoproteins have been used in several protection studies, utilizing different vaccination strategies and animal models. The protective effect varied from no/low to complete protection depending on the administration strategy, antigen and animal model used [20–22, 38–40, 42]. In this study, the majority of the GN/GC vaccinated mice were protected against RVF. However, the incomplete protection found was unexpected as a similar study, using analogous GN/GC constructs (RVFV-NSm), reported complete protection of mice after challenge . On the other hand, intramuscular inoculation of cDNA encoding the GN/GC polyprotein did not induce neutralising antibodies and did not protect against RVFV challenge . Interestingly, a recent study reported that dual expression of the N and the GN/GC proteins may generate RVF Virus-Like Particles (VLPs) , and the formation of VLPs after genetic immunisation is hypothesised to be the reason for the high virus neutralising antibody titers induced by the genetic West Nile virus vaccine . Perhaps, by using a similar approach, and introducing cDNA encoding the N and the GN/GC proteins of RVFV, a fully protective immune response might be induced.
In summary, while DNA vaccination against RVF induced strong humoral and proliferative immune responses in vaccinated mice, complete protection after challenge was not achieved. Nevertheless, naked DNA vaccines may constitute a promising strategy for vaccine development and this study provides insight for the basis of a future development of an efficacious DNA vaccine against RVF.
Dr. Bo Lilliehöök is greatly acknowledged for interesting discussions and valuable contributions. This study was supported by the Swedish Defence Agency, the Medical Faculty of Umeå University and grants from the County Council of Västerbotten. This project was also partially supported by grants from the Swedish Research Council (project 12177) and the European Community (contract no. QLK2-CT-2002-01358).
- Gerdes G: Rift Valley fever. Rev Sci Tech 2004, 23: 613-623.PubMedGoogle Scholar
- Balkhy H, Memish Z: Rift Valley fever: an uninvited zoonosis in the Arabian peninsula. Int J Antimicrob Agents 2003, 21: 153-157. 10.1016/S0924-8579(02)00295-9View ArticlePubMedGoogle Scholar
- Elliott R: The Bunyaviridae. New York: Plenum Press; 1996.View ArticleGoogle Scholar
- Flick R, Bouloy M: Rift Valley fever virus. Curr Mol Med 2005, 5: 827-834. 10.2174/156652405774962263View ArticlePubMedGoogle Scholar
- Pittman P, Liu C, Cannon T, Makuch R, Mangiafico J, Gibbs P, Peters C: Immunogenicity of an inactivated Rift Valley fever vaccine in humans: a 12-year experience. Vaccine 1999, 18: 181-189. 10.1016/S0264-410X(99)00218-2View ArticlePubMedGoogle Scholar
- Kark J, Aynor Y, Peters C: A rift Valley fever vaccine trial. I. Side effects and serologic response over a six-month follow-up. Am J Epidemiol 1982, 116: 808-820.PubMedGoogle Scholar
- Smithburn KC: Rift Valley Fever: The neurotropic adaptation of the virus and the experimental use of this modified virus as a vaccine. Br J Exp Pathol 1949, 1-16.Google Scholar
- Metwally S: Foreign animal diseases. 7th edition. Edited by: Brown C, Torres A. Boca Raton, FL: Boca Publications Group, Inc; 2008:369-376.Google Scholar
- Botros B, Omar A, Elian K, Mohamed G, Soliman A, Salib A, Salman D, Saad M, Earhart K: Adverse response of non-indigenous cattle of European breeds to live attenuated Smithburn Rift Valley fever vaccine. J Med Virol 2006, 78: 787-791. 10.1002/jmv.20624View ArticlePubMedGoogle Scholar
- Coetzer J, Barnard B: Hydrops amnii in sheep associated with hydranencephaly and arthrogryposis with wesselsbron disease and rift valley fever viruses as aetiological agents. Onderstepoort J Vet Res 1977, 44: 119-126.PubMedGoogle Scholar
- Lubroth J, Rweyemamu M, Viljoen G, Diallo A, Dungu B, Amanfu W: Veterinary vaccines and their use in developing countries. Rev Sci Tech 2007, 26: 179-201.PubMedGoogle Scholar
- Muller R, Saluzzo J, Lopez N, Dreier T, Turell M, Smith J, Bouloy M: Characterization of clone 13, a naturally attenuated avirulent isolate of Rift Valley fever virus, which is altered in the small segment. Am J Trop Med Hyg 1995, 53: 405-411.PubMedGoogle Scholar
- Caplen H, Peters C, Bishop D: Mutagen-directed attenuation of Rift Valley fever virus as a method for vaccine development. J Gen Virol 1985, 66: 2271-2277. 10.1099/0022-1317-66-10-2271View ArticlePubMedGoogle Scholar
- Vialat P, Muller R, Vu T, Prehaud C, Bouloy M: Mapping of the mutations present in the genome of the Rift Valley fever virus attenuated MP12 strain and their putative role in attenuation. Virus Res 1997, 52: 43-50. 10.1016/S0168-1702(97)00097-XView ArticlePubMedGoogle Scholar
- Hunter P, Erasmus B, Vorster J: Teratogenicity of a mutagenised Rift Valley fever virus (MVP 12) in sheep. Onderstepoort J Vet Res 2002, 69: 95-98.PubMedGoogle Scholar
- Beláková J, Horynová M, Krupka M, Weigl E, Raska M: DNA vaccines: are they still just a powerful tool for the future? Arch Immunol Ther Exp (Warsz) 2007, 55: 387-398. 10.1007/s00005-007-0044-4View ArticleGoogle Scholar
- Giese M: DNA-antiviral vaccines: new developments and approaches-a review. Virus Genes 1998, 17: 219-232. 10.1023/A:1008013720032View ArticlePubMedGoogle Scholar
- Donnelly J, Wahren B, Liu M: DNA vaccines: progress and challenges. J Immunol 2005, 175: 633-639.View ArticlePubMedGoogle Scholar
- Liu M, Wahren B, Karlsson Hedestam G: DNA vaccines: recent developments and future possibilities. Hum Gene Ther 2006, 17: 1051-1061. 10.1089/hum.2006.17.1051View ArticlePubMedGoogle Scholar
- Wallace D, Ellis C, Espach A, Smith S, Greyling R, Viljoen G: Protective immune responses induced by different recombinant vaccine regimes to Rift Valley fever. Vaccine 2006, 24: 7181-7189. 10.1016/j.vaccine.2006.06.041View ArticlePubMedGoogle Scholar
- Schmaljohn C, Parker M, Ennis W, Dalrymple J, Collett M, Suzich J, Schmaljohn A: Baculovirus expression of the M genome segment of Rift Valley fever virus and examination of antigenic and immunogenic properties of the expressed proteins. Virology 1989, 170: 184-192. 10.1016/0042-6822(89)90365-6View ArticlePubMedGoogle Scholar
- Spik K, Shurtleff A, McElroy A, Guttieri M, Hooper J, SchmalJohn C: Immunogenicity of combination DNA vaccines for Rift Valley fever virus, tick-borne encephalitis virus, Hantaan virus, and Crimean Congo hemorrhagic fever virus. Vaccine 2006, 24: 4657-4666. 10.1016/j.vaccine.2005.08.034View ArticlePubMedGoogle Scholar
- Wang Q, Wang X, Hu S, Ge J, Bu Z: Study on DNA immune of envelope protein gene of Rift Valley Fever Virus. Wei Sheng Wu Xue Bao 2007, 47: 677-681. [abstract, article in Chinese]PubMedGoogle Scholar
- Meegan J: The Rift Valley fever epizootic in Egypt 1977–78. 1. Description of the epizzotic and virological studies. Trans R Soc Trop Med Hyg 1979, 73: 618-623. 10.1016/0035-9203(79)90004-XView ArticlePubMedGoogle Scholar
- Sall A, Zanotto P, Sene O, Zeller H, Digoutte J, Thiongane Y, Bouloy M: Genetic reassortment of Rift Valley fever virus in nature. J Virol 1999, 73: 8196-8200.PubMed CentralPubMedGoogle Scholar
- Bird B, Khristova M, Rollin P, Ksiazek T, Nichol S: Complete genome analysis of 33 ecologically and biologically diverse Rift Valley fever virus strains reveals widespread virus movement and low genetic diversity due to recent common ancestry. J Virol 2007, 81: 2805-2816. 10.1128/JVI.02095-06PubMed CentralView ArticlePubMedGoogle Scholar
- Johansson P, Olsson M, Lindgren L, Ahlm C, Elgh F, Holmström A, Bucht G: Complete gene sequence of a human Puumala hantavirus isolate, Puumala Umeå/hu: sequence comparison and characterisation of encoded gene products. Virus Res 2004, 105: 147-155. 10.1016/j.virusres.2004.05.005View ArticlePubMedGoogle Scholar
- Lindkvist M, Lahti K, Lilliehöök B, Holmström A, Ahlm C, Bucht G: Cross-reactive immune responses in mice after genetic vaccination with cDNA encoding hantavirus nucleocapsid proteins. Vaccine 2007, 25: 1690-1699. 10.1016/j.vaccine.2006.09.082View ArticlePubMedGoogle Scholar
- Johansson P, Lindgren T, Lundström M, Holmström A, Elgh F, Bucht G: PCR-generated linear DNA fragments utilized as a hantavirus DNA vaccine. Vaccine 2002, 20: 3379-3388. 10.1016/S0264-410X(02)00265-7View ArticlePubMedGoogle Scholar
- Aida Y, Pabst M: Removal of endotoxin from protein solutions by phase separation using Triton X-114. J Immunol Methods 1990, 132: 191-195. 10.1016/0022-1759(90)90029-UView ArticlePubMedGoogle Scholar
- Näslund J, Lagerqvist N, Lundkvist A, Evander M, Ahlm C, Bucht G: Kinetics of Rift Valley Fever Virus in experimentally infected mice using quantitative real-time RT-PCR. J Virol Methods 2008, 151: 277-282. 10.1016/j.jviromet.2008.04.007View ArticlePubMedGoogle Scholar
- Bucht G, Sjölander K, Eriksson S, Lindgren L: Modifying the cellular transport of DNA-based vaccines alters the immune response to hantavirus nucleocapsid protein. Vaccine 2001, 19: 3820-3829. 10.1016/S0264-410X(01)00151-7View ArticlePubMedGoogle Scholar
- Gött P, Zöller L, Darai G, Bautz E: A major antigenic domain of hantaviruses is located on the aminoproximal site of the viral nucleocapsid protein. Virus Genes 1997, 14: 31-40. 10.1023/A:1007983306341View ArticlePubMedGoogle Scholar
- Lundkvist A, Meisel H, Koletzki D, Lankinen H, Cifire F, Geldmacher A, Sibold C, Gött P, Vaheri A, Krüger D, Ulrich R: Mapping of B-cell epitopes in the nucleocapsid protein of Puumala hantavirus. Viral Immunol 2002, 15: 177-192. 10.1089/088282402317340323View ArticlePubMedGoogle Scholar
- Kutzler M, Weiner D: DNA vaccines: ready for prime time? Nat Rev Genet 2008, 9: 776-788. 10.1038/nrg2432PubMed CentralView ArticlePubMedGoogle Scholar
- Gori Savellini G, Di Genova G, Terrosi C, Di Bonito P, Giorgi C, Valentini M, Docquier J, Cusi M: Immunization with Toscana virus N-Gc proteins protects mice against virus challenge. Virology 2008, 375: 521-528. 10.1016/j.virol.2008.02.006View ArticlePubMedGoogle Scholar
- Dean H, Haynes J, Schmaljohn C: The role of particle-mediated DNA vaccines in biodefense preparedness. Adv Drug Deliv Rev 2005, 57: 1315-1342. 10.1016/j.addr.2005.01.012View ArticlePubMedGoogle Scholar
- Dalrymple JMSEH, Kakach LT, Collet MS: Mapping protective determinants of Rift Valley Fever Virus using recombinant vaccinia viruses. Vaccines 89 1989, 371-375.Google Scholar
- Collett MS, Keegan K, Hu SL, Sridhar P, Purchio AF, Ennis WH, Dalrymple JM: Protective subunit immunogens to RVFV from bacteria and recombinant vaccinia virus. In The biology of negative stranded viruses. Edited by: Mahy B, Kolakofsky D. New York, NY: Elsevier; 1987:321-329.Google Scholar
- Besselaar T, Blackburn N: Topological mapping of antigenic sites on the Rift Valley fever virus envelope glycoproteins using monoclonal antibodies. Arch Virol 1991, 121: 111-124. 10.1007/BF01316748View ArticlePubMedGoogle Scholar
- Besselaar T, Blackburn N: The effect of neutralizing monoclonal antibodies on early events in Rift Valley fever virus infectivity. Res Virol 1994, 145: 13-19. 10.1016/S0923-2516(07)80002-1View ArticlePubMedGoogle Scholar
- Lorenzo G, Martín-Folgar R, Rodríguez F, Brun A: Priming with DNA plasmids encoding the nucleocapsid protein and glycoprotein precursors from Rift Valley fever virus accelerates the immune responses induced by an attenuated vaccine in sheep. Vaccine 2008, 26: 5255-5262. 10.1016/j.vaccine.2008.07.042View ArticlePubMedGoogle Scholar
- Liu L, Celma C, Roy P: Rift Valley fever virus structural proteins: expression, characterization and assembly of recombinant proteins. Virol J 2008, 5: 82. 10.1186/1743-422X-5-82PubMed CentralView ArticlePubMedGoogle Scholar
- Martin J, Pierson T, Hubka S, Rucker S, Gordon I, Enama M, Andrews C, Xu Q, Davis B, Nason M, Fay M, Koup Roederer M, Bailer R, Gomez P, Mascola J, Chang G-J, Nabel G, Graham B: A West Nile virus DNA vaccine induces neutralizing antibody in healthy adults during a phase 1 clinical trial. J Infect Dis 2007, 196: 1732-1740. 10.1086/523650PubMed CentralView ArticlePubMedGoogle Scholar
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