ChimeriVax-West Nile Virus Live-Attenuated Vaccine ...

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ChimeriVax-West Nile Virus Live-Attenuated Vaccine ...

  1. 1. JOURNAL OF VIROLOGY, Nov. 2004, p. 12497–12507 Vol. 78, No. 22 0022-538X/04/$08.00 0 DOI: 10.1128/JVI.78.22.12497–12507.2004 Copyright © 2004, American Society for Microbiology. All Rights Reserved. ChimeriVax-West Nile Virus Live-Attenuated Vaccine: Preclinical Evaluation of Safety, Immunogenicity, and Efficacy Juan Arroyo,1,2 Chuck Miller,1 John Catalan,1 Gwendolyn A. Myers,1 Marion S. Ratterree,3 Dennis W. Trent,1,4 and Thomas P. Monath1* Acambis, Inc., Cambridge, Massachusetts1; DynPort Vaccine Co. LLC, Frederick, Maryland2; Tulane National Primate Research Center, Covington, Louisiana3; and Vaxin, Inc., Birmingham, Alabama4 Received 31 March 2004/Accepted 9 July 2004 The availability of ChimeriVax vaccine technology for delivery of flavivirus protective antigens at the time West Nile (WN) virus was first detected in North America in 1999 contributed to the rapid development of the vaccine candidate against WN virus described here. ChimeriVax-Japanese encephalitis (JE), the first live-at- tenuated vaccine developed with this technology has successfully undergone phase I and II clinical trials. The ChimeriVax technology utilizes yellow fever virus (YF) 17D vaccine strain capsid and nonstructural genes to deliver the envelope gene of other flaviviruses as live-attenuated chimeric viruses. Amino acid sequence homology between the envelope protein (E) of JE and WN viruses facilitated targeting attenuating mutation sites to develop the WN vaccine. Here we discuss preclinical studies with the ChimeriVax-WN virus in mice and macaques. ChimeriVax-WN virus vaccine is less neurovirulent than the commercial YF 17D vaccine in mice and nonhu- man primates. Attenuation of the virus is determined by the chimeric nature of the construct containing attenu- Downloaded from jvi.asm.org by on April 13, 2007 ating mutations in the YF 17D virus backbone and three point mutations introduced to alter residues 107, 316, and 440 in the WN virus E protein gene. The safety, immunogenicity, and efficacy of the ChimeriVax-WN02 vaccine in the macaque model indicate the vaccine candidate is expected to be safe and immunogenic for humans. Following isolation of West Nile (WN) virus in New York in both mouse and monkey models. Because WN virus, like other 1999, the virus rapidly spread across North America, causing flaviviruses in the genus, is neurotropic for mammals (21, 29), disease in wild birds, horses, and humans. The number of attenuating point mutations were later introduced in the en- human cases increased dramatically in 2002 and 2003, when velope of the YF/WN chimera to further reduce its virulence. 4,145 and 8,977 cases were reported, respectively (7, 8). WN Mutation sites were targeted only to regions of the envelope virus is transmitted principally between wild birds and Culex (E) protein gene and were based on previous observations by mosquitoes (7). Recently, WN virus has been isolated in the others (1, 3, 28, 32) pertaining to attenuation phenotypes in West Indies and serosurveys have identified neutralizing anti- related flaviviruses: specifically JE and tick-borne encephali- body-positive avian species in Mexico (14), Jamaica, and the tis viruses. Site-directed mutations in the WN virus E gene Dominican Republic (13, 17). The rapid geographic expansion of the chimeric prototype vaccine, ChimeriVax-West Nile01, of the virus is attributed to movement by viremic birds during (ChimeriVax-WN01) resulted in a significant reduction in virus local and migratory flight behavior. To date, there is no effec- neurovirulence. Here we discuss a vaccine in a YF vaccine back- tive drug treatment against WN virus infection and surveil- bone; the WN virus envelope (E) protein mutagenesis ratio- lance and mosquito control measures have not significantly nale; and the assessment of the safety, immunogenicity, effi- influenced the number of human infections (27). A vaccine cacy, and genetic stability of these ChimeriVax-WN vaccine against WN virus represents an important approach to the candidates in the mouse and macaque models. prevention and control of this emerging disease. MATERIALS AND METHODS The ChimeriVax technology has been successfully used to YF/WN chimeric clones and molecular procedures for virus assembly. Chi- develop a live vaccine against Japanese encephalitis (JE) virus meric flaviviruses were constructed with the ChimeriVax two-plasmid technology that is now in phase II trials (23). JE virus is a close genetic previously described (9). Briefly, the two-plasmid system provides plasmid sta- relative of WN virus (31), a fact that expedited use of this bility in Escherichia coli by dividing the cloned YF backbone into two plasmids. technology to develop multiple WN virus vaccine candidates. This provides smaller plasmids that are more stable to manipulate the YF The ChimeriVax technology employs the yellow fever (YF) sequences facilitating replacement of the prM and E genes of the flavivirus target vaccine. The WN virus prM and E genes used were cloned from the WN fla- 17D vaccine capsid and nonstructural genes to deliver the mingo isolate 383-99 sequence (GenBank accession no. AF196835; kindly provid- envelope genes (prM and E) of other flaviviruses. In the work ed by John Roehrig, Centers for Disease Control and Prevention, Fort Collins, presented here, the envelope genes of YF 17D were replaced Colo.). Virus prME sequence cDNA was obtained by reverse transcription-PCR with the corresponding genes of the wild-type WN virus NY99 (RT-PCR) (XL-PCR kit; Applied Biosystems, Foster City, Calif.). The 5 end of the WN virus prM gene was cloned precisely at the 3 end of the YF 17D capsid strain previously described by Lanciotti et al. (19). The result- gene by overlap-extension PCR using Pwo polymerase (Roche Applied Science, ing YF/WN chimera lacked the mouse neuroinvasive property Indianapolis, Ind.). This cloning step maintained integrity of the cleavage/pro- of WN virus and is less neurovirulent than YF 17D vaccine in cessing signal encoded at the 3 end of the YF capsid gene. The 3 end of the E gene was cloned at the 5 end of the YF NS1 protein coding sequence by overlap- extension PCR. The two-plasmid system used to clone the prME region of WN * Corresponding author. Mailing address: Acambis, Inc., 38 Sidney virus into the YF 17D backbone was described previously (4). Silent mutations St., Cambridge, MA 02139. Phone: (617) 761-4200. Fax: (617) 494- were introduced in the sequences of prM and E to create unique BspEI and EagI 1741. E-mail: tom.monath@acambis.com. restriction sites. Digestion of the two plasmids with these restriction nucleases 12497
  2. 2. 12498 ARROYO ET AL. J. VIROL. TABLE 1. Switch oligonucleotides used for site mutagenesis a E protein position and residue Primerb Marker site 107L3F 5 -CAACGGCTGCGGATTTTTTGGCAAAGGATCCATTGACACATGCGCC-3 BamHI 138E3K 5 -GAAAGAGAATATTAAGTACAAAGTGGCCATTTTTGTCC-3 SspI *176V 5 -GCCCTCGAGCGGCCGATTCAGCATCACTCCTGCTGCGCCTTCAGTCACAC-3 *176Y 5 -GCCCTCGAGCGGCCGATTCAGCATCAC-3 280K3M 5 -GCAACACTGTCATGTTAACGTCGGGTCATTTG-3 HpaI 316A3V 5 -CTTGGGACTCCCGTGGACACCGGTCACGGCAC-3 AgeI 440K3R 5 -GGGGTGTTCACTAGTGGTTGGGCGGGCTGTCCATCAAGTG-3 SpeI a Primers indicated with an asterisk are cloning primers used in fragment subcloning. One incorporates a change to valine as indicated. b Primers for site-directed mutagenesis to create YF/WN chimeric viruses. Nucleotide changes that switch to a new amino acid are indicated in bold. Silent restriction (marker) sites introduced are underlined. generated DNA fragments that were gel purified and ligated in vitro to produce Neutralizing antibody titers were determined by a constant virus-serum dilu- a full-length chimeric cDNA. The cDNA was linearized with XhoI to facilitate in tion 50% plaque reduction neutralization assay test (PRNT50) in Vero cells, as vitro transcription by SP6 polymerase (Epicentre, Madison, Wis.). previously described (24). An equal volume (0.1 ml) of virus suspension contain- Point mutations were introduced into various E gene codons to produce ing 700 PFU/ml and serial twofold dilutions of heat-inactivated serum were variants of the original chimera coding for wild-type WN virus prME genes incubated overnight at 4°C, and the serum-virus mixture was inoculated onto (Transformer site-directed mutagenesis kit; Clontech, Palo Alto, Calif.). Table 1 Vero cell monolayers grown in 12-well plates. An overlay of methylcellulose in shows the mutation target sites and the oligonucleotide sequences used to create minimal essential medium was added before incubation of the cultures at 37°C all of the YF/WN chimeras. Site mutations were confirmed by sequencing of the for 3 to 4 days prior to fixation and crystal violet staining for plaque count de- envelope proteins (prME region) of the resulting viruses. Virus cDNA templates termination. The endpoint neutralization titer was the highest dilution of serum for sequencing originated from RNA extraction of virus containing infected Vero that reduced plaques by 50% compared to a mouse hyperimmune serum control. cell supernatants (Trizol LS; Invitrogen, Carlsbad, Calif.) followed by RT-PCR Nonhuman primate studies. Neurovirulence tests in rhesus macaques were Downloaded from jvi.asm.org by on April 13, 2007 (XL-PCR kit; Applied Biosystems) and sequencing with a CEQ 2000XL nucleic performed according to World Health Organization (WHO) guidelines for test- acid sequencer (Beckman-Coulter, Fullerton, Calif.). ing YF vaccine (36) and as described previously for safety tests of ChimeriVax-JE Viruses and cell lines. The wild-type WN virus used in animal challenge vaccine (24). Animals were inoculated with specific virus candidates by inocula- studies is the NY99 strain (NY99-35262-11 flamingo isolate, a homolog of the tion of the frontal lobe of the brain (see Table 7). Blood samples were obtained virus used to build chimeras) obtained from the Centers for Disease Control and daily for the first 10 days following inoculation, and serum viremia was measured Prevention, Fort Collins, Colo. (CDC stock designation B82332W) with two by plaque assay on Vero cells. Animals were observed daily for clinical signs of additional passages in Vero E6 cells to produce a master virus bank. YF 17D is encephalitis and associated symptoms such as fever or tremors. Animals were a commercial vaccine (YF-VAX; Aventis Pasteur, Swiftwater, Pa.) used after euthanized 30 days after infection, and the brain and spinal cord tissues were reconstitution of the lyophilized product or after one passage (P1) in Vero E6 removed for histopathology. Slides were prepared from tissues of the frontal and (American Type Culture Collection [ATCC] origin; Acambis, Inc., cell bank, temporal cortex, basal ganglia/thalamus (two levels), midbrain, pons, cerebellum Cambridge, Mass.). Chimeric YF/WN (i.e., ChimeriVax-WN) viruses were pre- (two levels of the nuclei and cortex), medulla oblongata, and six levels of each of pared by RNA transfection (P1 virus) of Vero E6 cells (ATCC origin, CIDVR cervical and lumbar enlargements of the spinal cord. Sections were stained with University of Massachusetts Medical Center cell bank, Worcester, Mass.). Re- gallocyanine. Histological lesions were analyzed and scored for pathology rela- search master seeds (RMS) were prepared by additional amplifications (either tive to that of the YF 17D according to the criteria for evaluation of neuroviru- passage 2 or 3 at a 0.001 multiplicity of infection [MOI]) in Vero E6 cells. Vero lence in rhesus monkeys proposed by the current WHO requirements. Mean E6 cells were maintained in minimal essential medium (Invitrogen) containing lesion scores for individual monkeys were calculated for “target” (substantia 10% heat-inactivated fetal bovine serum (HI-FBS) (HyClone, Logan, Utah). nigra) and “discriminator” (basal ganglia/thalamus and the spinal cord) areas Preparation of pre-master seeds (PMS) for manufacture of the vaccine was individually and for the target and discriminator areas combined. initiated by RNA transfection of serum-free Vero (SF-Vero) cells obtained from A second neurovirulence test was performed with cynomolgus monkeys and a cell bank that had been manufactured and controlled to meet current Food and inoculation with the YF/WNFVR vaccine candidate (ChimeriVax-WN02) produc- Drug Administration guidelines for cell culture vaccines. (The cells were ob- tion virus seed (P4). The study was conducted according to good laboratory tained from an ATCC strain predating 1980, and the cell bank was made by practices (GLP) standards (14a). Eleven monkeys were inoculated i.c. with YF/ Baxter/Immuno, Orth, Austria.) Progeny virus from the transfection step was WNFVR production virus seed (P4), 11 positive control monkeys received YF- amplified by a single passage in the same SF-Vero cell line to produce P2, which VAX, and 5 negative control monkeys received diluent. The monkeys were eval- was designated the PMS for subsequent manufacture of clinical-grade vaccine. uated for changes in clinical signs (twice daily), body weight (weekly), and food con- The SF-Vero cell line is propagated and maintained in a serum-free, animal sumption (daily). Clinical signs were assigned scores according to a clinical scoring protein-free medium formulation, VT-Media (Baxter/Immuno). Viruses for an- system, based on the WHO requirements for YF vaccine (36). Blood samples imal experiments were diluted in M199 with HEPES buffer (Invitrogen) and 20% were collected preinoculation on day 1 and on days 3, 5, 7, 15, and 31 for clinical HI-FBS (HyClone) unless otherwise indicated. Plaque assays to verify the titer of pathology analysis (serum chemistry and hematology parameters). Additional blood virus inoculi were performed in a Vero cell substrate as previously described (24). samples were collected preinoculation on day 1 and on days 2 to 11 for viremia Mouse studies. Protocols for mouse experiments were approved by the Insti- analysis, and on days 1 (predose) and 31 for antiviral antibody titer analyses. tutional Animal Care and Use Committees at both University of Massachusetts To determine immunogenicity, rhesus monkeys were inoculated by the sub- Medical Center (Worcester, Mass.) and Acambis, Inc. (Cambridge, Mass.). Re- cutaneous (s.c.) route with a single 0.5-ml dose containing 4 log10 PFU of search was conducted in compliance with the Animal Welfare Act and other chimeric vaccine. Control animals received undiluted YF-VAX containing 4.49 federal statutes and regulations relating to animals and experiments involving log10 PFU in a 0.25-ml volume. Each vaccine dose was back titrated following animals and adhered to principles set forth in the Guide for the Care and Use of immunization. Serum viremia was measured daily by plaque assay through day 10 Laboratory Animals (27a). Female ICR mice (Taconic, Germantown, N.Y., or after vaccination. Neutralizing antibody levels were measured by PRNT50 on days Harlan Sprague-Dawley, Indianapolis, Ind.) were inoculated intraperitoneally (i.p.) 14, 30, and 63 after vaccination. Animals were challenged 64 days after vaccination with 100 to 200 l of wild-type WN virus NY99 for neuroinvasion tests or post- by i.c. inoculation of 125 l containing 2.4 105 PFU of wild-type WN NY99 vaccination challenge experiments (titers of inoculated viruses are indicated in the suspended in M199 with HEPES buffer (Invitrogen) and 10% sorbitol (Sigma). Results section and in the tables presented). ICR strain adult (3 to 4 weeks of age) Monkeys were observed for viremia, clinical illness, and antibody response;se- and suckling (2 and 8 days of age) mice were inoculated intracerebrally (i.c.) on verely ill animals were euthanized. The i.c. challenge model closely followed the the right side of the brain as previously described (24) and using a 20- l volume of model established during the development of ChimeriVax-JE vaccine (24, 26). YF 17D or chimeric YF/WN constructs for neurovirulence testing (titers of inocu- Genetic stability (in vivo and in vitro passage) and sequencing. The chimeric lated viruses are indicated in the Results section and in the tables presented). YF/WN virus containing unmodified, wild-type WN virus prME sequence (des- Mice were observed daily for 21 days following inoculation to determine ignated ChimeriVax-WN01) was passed six times in Vero E6 cells followed by survival ratio and average survival time (AST) after virus challenge. six passages in suckling mice by the i.c. route. The chimeric YF/WN virus
  3. 3. VOL. 78, 2004 ChimeriVax-WEST NILE VACCINE PRECLINICAL EVALUATION 12499 TABLE 2. Neuroinvasiveness of ChimeriVax-WN01 relative to The strategy for this mutagenesis approach was to design a safe YF 17D based on dose response in ICR micea attenuated WN vaccine; this strategy was first discussed in an Back titration dose % Mortality earlier publication (4). Briefly, the selection of specific amino Test article i.p. (log10 PFU) (no. dead/no. tested)d acid residues for mutagenesis was defined by previous studies ChimeriVax-WN01 (P2)b 0.89 0 (0/5) of the attenuating mutations in a vaccine strain of JE virus 2.23 0 (0/5) (SA14-14-2) (3). Since the wild-type JE and WN viral E gene 3.24 0 (0/5) sequences are identical at the residues implicated in attenua- 4.06 0 (0/5) 5.45 0 (0/5) tion of JE (SA14-14-2) vaccine, with one exception at residue 6.51 0 (0/5) 176, we postulated that introduction of mutations at the ma- jority of these sites into wild-type WN virus prME genes would YF17D (ATCC) 2.78 0 (0/3) 4.48 0 (0/3) result in a similar attenuation of the WN phenotype. Amino acid residues mapping to the wild-type WN envelope (E) gene Negative control NAc 0 (0/3) positions 107, 138, 176, and 280 were all mutated in a single a Harlan-Sprague, ICR strain (3 to 4-week-old female mice). construct to encode amino acid residues F, K, V, and M, respec- b P2 indicates a second-generation passage virus on Vero cells. West Nile virus tively. The new chimeric virus was identified as YF/WNFKVM. strains are typically neuroinvasive after i.p. inoculation as shown by others (5). Chimeras were constructed in which each amino acid residue in c NA, not applicable. d AST was not determined. the FKVM group was individually mutated to produce single-site mutants and to assess their individual roles in neurovirulence (Table 4). The dissection of the FKVM group into single site containing three mutations introduced by site-directed mutagenesis (designated ChimeriVax-WN02) at the P2 level (PMS) and P3 level (RMS) were passed 12 mutations identified only residue 107 as reducing virulence and 10 times, respectively, in serum-free, protein-free SF-Vero cell substrate. All significantly (0% mortality in three mouse neurovirulence tests in vitro virus passages were performed with an initial MOI of 0.01 PFU/cell presented). Residue 280 followed with 0% mortality after a 105 Downloaded from jvi.asm.org by on April 13, 2007 followed by harvest of the virus on the third day after infection. Passages in vivo viral dose; however, inconsistency of this attenuated phenotype were performed by initial i.c. inoculation of 105 PFU; brain tissue from ICR mice (Taconic) was harvested 3 days after inoculation and homogenized, and the (i.e., mortality ratios of 40 to 89%) was observed in the lower- clarified homogenate was used for passage to a new group of mice. Virus titers viral-dose groups tested. A mutation at residue 138 resulted in at each passage were determined by plaque assay. Neurovirulence of the pas- minimal reduction of virulence ( 60% mortality), while a mu- saged viruses was determined by i.c. inoculation of adult or suckling mice (see tation at residue 176 showed no impact. The neurovirulence of Tables 13 and 14). Sequencing of viral RNA was performed with Superscript II the multisite YF/WNFKVM construct resulted in 0 to 20% mor- reverse transcriptase and XL-PCR; products were purified by QIAGEN gel extraction (QIAGEN, Valencia, Calif.). Sequencing reactions were prepared and tality. In later studies, amino acid residues 316 and 440 were analyzed using the standard Beckman CEQ 2000XL protocol and equipment mutated to V and R, respectively, based on previous data in- (Beckman-Coulter). For virus passages, at least two independent sequencing dicating mutations in the E protein which mapped to these reactions were executed per RT-PCR product strand sequenced; sense and regions thought to function in the biology of the E protein antisense strands were sequenced each time. Mutation acceptance criteria needed a positive identity in at least three of four sequencing reactions analyzed; third domain (1, 32). Changes in neurovirulence of these in addition, two independent operators read sequence chromatographs. mutants with respect to parental ChimeriVax-WN01 were evaluated in the mouse model as for the previous groups above (Table 5). A single mutation at residue 316 resulted in a RESULTS greater attenuation ( 30% mortality) than residue 440 but not Virulence phenotype of chimeric YF/WN containing wild- type WN prME genes relative to YF 17D (YF-VAX). The initial WN virus chimera encoded the envelope and premem- TABLE 3. Neurovirulence of ChimeriVax-WN01 relative to brane protein genes of the WN NY99 wild-type strain (desig- YF 17D based on dose response in ICR micea nated ChimeriVax-WN01). This chimeric virus did not cause encephalitis after i.p. inoculation at doses of 106 PFU in 3- to Back % Mortality AST Test article i.c. titration dose (no. dead/no. 4-week-old adult ICR mice (Table 2). Encephalitis was as- (days) (log10 PFU)b tested) sessed by daily observations for illness, paralysis, and death. ChimeriVax-WN01 (P2) 2 0 (0/5) ChimeriVax-WN01 resembles YF 17D vaccine (33) in being 0.30 0 (0/5) nonneuroinvasive in adult mice. In contrast, the WN NY99 0.89 20 (1/5) 11 wild-type virus was lethal for mice when inoculated by the i.p. 2.23 0 (0/5) route with as few as 1 to 4 PFU (5; unpublished results). 3.24 20 (1/5) 10 4.06 60 (3/5) 9 ChimeriVax-WN01 retained the ability to cause lethal en- 5.45 20 (1/5) 9 cephalitis after i.c. inoculation, a property consistent with that of YF 17D virus (10). We estimated the i.c. 50% lethal YF17D (ATCC) 0 20 (1/5) 9 0 60 (3/5) 10.3 dose (LD50) of ChimeriVax-WN01 to be between 103 and 105 0.9 100 (5/5) 9.2 PFU. The neurovirulence phenotype of ChimeriVax-WN01 is 0.98 100 (5/5) 8.2 lower than that of YF 17D virus, for which the i.c. LD50 is 2.78 100 (5/5) 8 between 101 and 102 PFU (Table 3). Negative control NAc 0 (0/3) Evaluation of the multisite mutagenesis for attenuation. a Amino acids in the envelope protein previously established Harlan-Sprague, ICR strain (3 to 4-week-old female mice). b Actual dose delivered i.c. assumed to be 20 l for the back titration calcu- as genetic determinates of virulence for ChimeriVax-JE lations shown. were changed to reduce the virulence of YF/WN chimeras. c NA, not applicable.
  4. 4. 12500 ARROYO ET AL. J. VIROL. TABLE 4. Neurovirulence of ChimeriVax-WN01 (YF/WN) TABLE 5. Neurovirulence of ChimeriVax-WN01 site-directed site-directed mutagenesis variants at E protein residues 1073F, mutagenesis variants at E protein residues 1073F, 1383K, 1763V, 2803M, tested by i.c. inoculation in adult micea 3163V, and 4403R tested in adult micea Back % Mortality Back % Mortality Test article Target dose AST Test article i.c. AST titration dose (no. dead/no. titration dose (no. dead/no. (Vero passage)b (log10 PFU) (days) (Vero passage) (days) (log10 PFU) tested) (log10 PFU) tested) ChimeriVax-WN01 (P3) 4 4.87 100 (5/5) 8.60 ChimeriVax-WN01 (P3) 4.11 83 (10/12) 9.20 5 6.09 60 (3/5) 9 4.74 60 (3/5) 10.33 4.83 100 (8/8) 10.63 YF/WN107F (P2) 4 4.22 0 (0/5) 4 4.42 0 (0/8) YF/WN316V (P3) 4.09 25 (3/12) 12.33 5 4.99 0 (0/5) 4.67 38 (3/8) 10.67 4.57 38 (9/24) 11.22 YF/WN138K (P3) 4 4.26 60 (3/5) 10.33 4 4.41 63 (5/8) 11.40 YF/WN440R (P3) 4.17 83 (10/12) 9.22 5 5.48 60 (3/5) 9.33 4.60 38 (3/8) 10.33 4.35 56 (14/25) 11.21 YF/WN176V (P3) 4 4.42 80 (4/5) 12.50 5 5.54 80 (4/5) 11 YF/WN316V440R (P3) 3.90 17 (2/12) 16.5 4.12 40 (2/5) 13 YF/WN280M (P3) 4 4.14 40 (2/5) 9 3.71 36 (9/25) 12 4 4.55 89 (7/8) 11.86 5 5.14 0 (0/5) YF/WN107F316V440R (P4) 3.72 0 (0/12) YF/107F138K280M (P2) 4 3.70 0 (0/5) 5.54 0 (0/12) 5 4.81 0 (0/5) a Taconic, ICR strain, 3 to 4-week-old female mice. Results of independent experiments are shown. Downloaded from jvi.asm.org by on April 13, 2007 YF/107F138K176V280M (P3) 4 4.13 0 (0/5) 5 5.10 20 (1/5) 7 YF-VAX 3 2.77 100 (5/5) 9 lence of the good manufacturing practice (GMP) manufac- WN NY99 4 3.90 100 (5/5) 5 tured ChimeriVax-WN02 production virus seed (P4) and a vaccine lot (P5) prepared for clinical trials. Four litters (32 a Taconic, ICR strain (3 to 4-week-old female mice). mice) of 8-day-old suckling mice were inoculated by the i.c. b P2 and P3 indicate second and third generation virus passage on Vero cells, respectively. route with 20 l containing 103, 104, or 105 PFU of either production virus seed (P4) or vaccine (P5) virus. Control ani- mals of the same age received either 103or 105 PFU of YF- as significant as residue 107. The single mutation at residue 440 VAX. Negative controls were inoculated with diluent. The resulted in a greater level of attenuation over those at residues results are shown in Table 6. There were no differences across 138 and 176, but only in two of the three independent tests dose groups in the mortality ratios, and therefore data from performed (i.e., 40% mortality observed with a mutation at dose groups for each test article were combined for statistical residue 440). In summary, neurovirulence of the YF/WN chi- analysis. There was no difference in the mortality ratio of meras in which modified amino acids were inserted in the E animals infected with P4 or P5. Both the production virus seed protein at residues 107, 316, and 440 were the most important (P4) and the vaccine (P5) were highly attenuated compared to contributors to neurovirulence. Based on this information, a YF-VAX. The neurovirulence profile of the WN vaccine is multisite YF/WN107F316V440R construct was selected as our therefore similar to that of the ChimeriVax-JE vaccine, which vaccine candidate (ChimeriVax-WN02). is currently in phase II clinical trials (25). Neurovirulence studies in mice and in rhesus and cynomolgus In a pilot monkey neurovirulence study, the ChimeriVax- macaques. Neurovirulence of viruses with single or multisite mu- WN01 construct was compared to that of the YF 17D vaccine. tations in the YF/WN virus E gene was measured in 21-day-old Rhesus macaques were screened and found negative for flavi- mice inoculated by the i.c. route with doses between 104 and 105 virus antibodies by hemagglutination-inhibition (HI) test PFU. This assessment identified only residues 107 and 280 (Ta- (kindly performed by Robert Shope, University of Texas Med- ble 4) and the combination of residues 316 and 440 (Table 5) ical Branch, Galveston, Tex.). Groups of three young adult as the dominant attenuating mutations as measured by mor- tality and AST. The chimera selected as our vaccine candidate had mutations F, V, and R at residues 107, 316, and 440, respec- tively, and was avirulent for the adult mouse (Table 5). However, TABLE 6. Comparative neurovirulence of the ChimeriVax-WN02 (YF/WN107F316V440R) vaccine candidate (P5), production virus seed this virus was neurovirulent for a 2-day-old suckling mouse (data (P4), and YF-VAX in 8-day-old suckling ICR mice (GLP study) not shown). Because mice become resistant to flavivirus infection in an age-dependent manner, the suckling mouse is the most % Mortality Test article (no. dead/no. tested)a sensitive host for determining subtle differences in neuroviru- lence. Preliminary studies with ChimeriVax-WN02 virus in suck- Negative control 0 (0/32) ChimeriVax-WN02 P4 production virus seed 1 (1/96) ling mice of various ages showed that mice 8 days of age were able ChimeriVax-WN02 P5 vaccine lot 02K01 4 (4/96) to discriminate differences in neurovirulence, whereas younger YF-VAX 98 (63/64) mice were too susceptible to differentiate the attenuation phe- a Statistical significance was determined by Fisher’s exact test (two sided). notype of the ChimeriVax-WN02 vaccine candidate. P 0.0001 for ChimeriVax-WN02 P5 versus YF-VAX, and P 0.3684 for A GLP study was undertaken to characterize the neuroviru- ChimeriVax-WN02 P4 versus P5.
  5. 5. VOL. 78, 2004 ChimeriVax-WEST NILE VACCINE PRECLINICAL EVALUATION 12501 TABLE 7. Pilot study with rhesus monkeys of neurovirulence of ChimeriVax-WN01 relative to YF-VAX based on neuropathological evaluation at 30 days post-i.c. inoculations Back titration dose Individual histopathological score Test article Monkey Sexa (log10 PFU) Target area Discriminator area Sum of areas YF-VAX G211 M 4.40 0.5 0.64 0.59 P417 F 4.40 0 0.43 0.28 N555 F 4.40 1.5 0.66 0.94 Mean SD 0.67 0.76 0.58 0.13 0.60 0.33 ChimeriVax-WN01 N525 M 5.07 1.0 0.58 0.72 D402 M 4.99 0.5 0.48 0.48 C358 F 5.06 0 0.42 0.28 Mean SD 0.50 0.50 0.49 0.08 0.49 0.22 a M, male; F, female. rhesus monkeys were inoculated by the i.c. route with 5 log10 PFU/ml. The number of viremic days did not differ between PFU of ChimeriVax-WN01 or 4.4 log10 PFU of commercial YF treatment groups (P 0.4067; analysis of variance [AVOVA]). 17D vaccine (YF-VAX) (Table 7). Monkeys inoculated with A higher proportion of monkeys (91%) was viremic on the first the chimera had a mean peak viremia titer of 1.85 0.9 log10 day after inoculation than that seen in the YF-VAX group PFU/ml with a mean duration of 4.5 days. Monkeys inoculated (27%). On days 2 to 3 after inoculation, the proportion of viremic Downloaded from jvi.asm.org by on April 13, 2007 with YF-VAX had a similar viremia profile (mean peak vire- monkeys (82%) was the same as for YF-VAX. The mean peak mia titer of 2.65 0.1 log10 PFU/ml and a mean duration of viremia was 2,097 1,845 PFU/ml. Although the mean peak vire- 4.5 days). Histological scores induced by ChimeriVax-WN01 mia titers for ChimeriVax-WN02 production virus seed (P4) were lower than those of a higher dose of YF-VAX (Table 7). were higher than that of the reference YF-VAX vaccine (P Histological lesions in all six monkeys were mildly inflamma- 0.0073; ANOVA), individual monkey and group viremia titers tory, predominantly small perivascular infiltrates. The vast ma- for ChimeriVax-WN vaccine remained within acceptable group jority of them were scored as grade 1 on a scale of 1 to 4. No and individual monkey specifications, based upon WHO require- involvement of neurons was seen. The lesions were located ments for YF 17D vaccine (36). The WHO specifications stip- mostly in YF vaccine discriminator centers (the basal ganglia/ ulate that no individual monkey will have a viremia exceeding thalamus areas and both enlargements of the spinal cord). 500 i.c. adult mouse LD50/ml and that no more than 10% of the Comparison of the two groups of monkeys for the severity and animals will have a viremia exceeding 100 i.c. mouse LD50/ml. distribution of lesions did not reveal any noticeable difference. We have determined that these limits correspond to 20,000 Vero On a second neurovirulence study, cynomolgus mon- PFU/0.03 ml and 4,000 PFU/0.03 ml, respectively, in the case keys were inoculated with YF/WNFVR vaccine candidate of YF-VAX (an LD50 for ChimeriVax-WN02 cannot be deter- (ChimeriVax-WN02) production virus seed (P4). These ma- mined). The monkey viremias observed following ChimeriVax- caques were screened and found negative for flavivirus anti- WN02 do not exceed the limits set for YF vaccine. bodies by HI test (kindly performed by Robert Shope). Eleven There were no abnormalities in hematology or clinical chem- monkeys were inoculated i.c. with 4.74 log10 PFU of YF/WN- istry values associated with treatment. A complete necropsy FVR production virus seed (P4), 11 reference control monkeys was performed on day 31, and tissues were prepared for his- received 5.34 log10 PFU of YF-VAX, and 5 negative control topathology. There were no ChimeriVax-WN02 production monkeys received diluent. The monkeys were evaluated for seed (P4)-related histopathologic changes in kidney, heart, changes in clinical signs (twice daily), body weight (weekly), liver, adrenal glands, or spleen. and food consumption (daily). Clinical signs were assigned scores according to a clinical scoring system based on the WHO requirements for YF vaccine (36). TABLE 8. Summary of CNS histopathologic lesion scores in YF 17D vaccine virus was detected in the sera of 10 of 11 cynomolgus monkeys inoculated by the i.c. route with ChimeriVax- monkeys inoculated with YF-VAX. The mean peak viremia WN02 production virus seed (P4), YF-VAX, or negative control standard deviation (SD) was 357 579 PFU/ml, and the mean Mean SD lesion scores number of viremic days was 2.45 1.13. Monkey viremia titers Treatment group n were below the 500 and 100 YF-VAX mouse i.c. LD50 values, Target Discriminator Combined areas areas score which are the maximum acceptable titers for individual mon- key and group viremia titers (i.e., present in no more than 10% Negative control 5 0 0 0 of the monkeys), respectively, as established under the WHO ChimeriVax-WN02 11 0.12 0.11 0.13 0.13 0.13 0.09 production virus requirements for YF 17D vaccine. seed (P4) ChimeriVax-WN vaccine virus was detected in the sera of 10 YF-VAX 11 0.5 0.22 0.54 0.23 0.52 0.2 of 11 monkeys inoculated with ChimeriVax-WN02 vaccine pro- P-valuea 0.000476 0.000357 0.000122 duction seed bank (P4). The duration of viremia was 1 to 5 days a The Kruskall-Wallis test was used for comparison of the ChimeriVax and (mean, 2.9 1.38) with peak titers ranging from 180 to 6,400 YF-VAX groups.
  6. 6. 12502 ARROYO ET AL. J. VIROL. TABLE 9. Neutralizing antibody titers (PRNT50) and protective high titers of neutralizing antibodies to the respective virus activity of ChimeriVax-WN candidate vaccines in with which they were inoculated (data not shown). adult ICR mice challenged by the i.p. routea Immunogenicity and efficacy studies in mice and rhe- PRNT50 Wild-type sus monkeys. The immunogenicity of ChimeriVax-WN01 and WN NY99 % Survival ChimeriVax-WN02 was evaluated in adult ICR mice inoculated s.c. dose GMT SD Vaccine challenge (no. live/ (log10 PFU) (4 wk post- by the s.c. route. Serum neutralizing antibodies were measured i.p. dose total) s.c. vaccine) (log10 PFU) by PRNT50 4 weeks after vaccination with a single dose, and titers ChimeriVax-WN01 3.48 197 93 3 100 (8/8) were expressed as the geometric mean titer (GMT) (Table 9). In mice, ChimeriVax-WN02 vaccine elicited antibody titers ChimeriVax-WN02 2.64 20 0 3 40 (4/10) that were approximately 10-fold lower than those elicited by 5.01 37 45 3 100 (9/9) ChimeriVax-WN01 virus, reflecting the greater attenuation of Negative control NAb 0 3 0 (0/5) this virus. However, when mice were challenged i.p. with 1,000 a LD50 of wild-type WN NY99, mice that had been immunized Mice were challenged 4 weeks after s.c. vaccination (challenge titer was not back titrated). with either ChimeriVax-WN01 or -02 were protected in a dose- b NA, not applicable. dependent manner. A vaccine dose of 105 PFU of ChimeriVax- WN02 protected all animals, whereas a dose of 103 PFU pro- tected only 40% of the animals. Histopathology of the brain and spinal cord was performed Young adult rhesus monkeys seronegative for WN neutral- according to the methods described by Levenbook et al. (20) izing antibodies were vaccinated by the s.c. route with three dif- and incorporated into the WHO requirements for YF vaccine ferent chimeric vaccines: (i) a chimera containing the E107 (L3 (36). Central nervous system (CNS) lesions were noted in 11 of F) single-site mutation (YF/WNF); (ii) a chimera containing two 11 and 10 of 11 of YF-VAX-treated and ChimeriVax-WN02 mutations at E316 (A3V) and E440 (K3R) (YF/WNVR); and Downloaded from jvi.asm.org by on April 13, 2007 vaccine-treated monkeys, respectively, and there were no CNS (iii) ChimeriVax-WN02 containing all three mutations. lesions in the vehicle control monkeys. Inflammatory lesions Viremia in the monkeys immunized with the different induced by both viruses in the meninges and the brain and ChimeriVax-WN viruses following s.c. inoculation was longer spinal cord matter were minimal to mild (grades 1 or 2) and relative to YF-VAX in some animals, although the levels de- composed of scanty, mostly perivascular infiltrates of mononu- tected at later time points were very low (Table 10). Viremias clear cells. There was no involvement of neurons in any of the in monkeys receiving the ChimeriVax-WN vaccines ranged ChimeriVax-WN02- or YF-VAX-treated monkeys. Summary from 1.0 to 2.3 log10 PFU/ml, with a mean duration of 3.5 to 5 data are presented in Table 8. ChimeriVax-WN production days. The mean peak titers of the viremia in monkeys given virus seed (P4) was significantly less neurovirulent (P 0.05) YF-VAX were approximately the same as those receiving the than the reference article, YF-VAX, in the target, discrimina- WN vaccines. Among the ChimeriVax-WN vaccines, the vire- tor, and combined mean lesion scores. All monkeys developed mia titers measured suggest an inverse relationship between TABLE 10. Viremia in rhesus monkeys inoculated by the s.c. route with YF-VAX, ChimeriVax-WN virus constructs with single or double mutations, and the ChimeriVax-WN02 vaccine candidate Vaccine and Vaccine dose Viremia (log10 PFU/ml) at day postinoculationa: Mean peak Mean duration monkey (log10 PFU) 1 2 3 4 5 6 7 8 9 10 titer SD (days) YF-VAX M017 4.49 0 1.0 2.1 2.9 2.4 0 0 0 0 0 2.4 0.5 3.5 B101 4.49 0 1.6 2.0 1.9 0 0 0 0 0 0 R286 4.49 0 1.8 2.8 2.6 0 0 0 0 0 0 T081 4.49 1.3 1.0 1.5 2.0 0 0 0 0 0 0 YF/WN107F N313 4.19 1.6 2.0 1.0 1.3 1.0 0 0 0 0 0 2.2 0.2 5 P367 4.19 0 1.7 1.6 1.8 2.3 1.6 0 0 1.3 0 T087 4.19 2.3 2.3 1.3 1.3 0 0 0 0 0 0 AE81 4.19 2.3 2.1 1.6 1.3 0 0 0 1.0 0 0 YF/WN316V440R R918 4.0 0 2.0 1.5 1.7 0 0 0 0 0 0 1.8 0.2 3.5 N577 4.0 1.0 1.9 1.5 1.0 0 1.0 0 0 0 0 M233 4.0 0 0 1.0 1.0 0 0 0 1.0 1.6 1.8 T757 4.0 0 0 0 1.6 0 0 0 0 0 0 YF/WNFVR J729 3.92 1.0 0 0 1.0 1.3 1.0 0 1.0 0 1.0 1.4 0.2 4.5 T445 3.92 1.0 1.6 1.5 0 0 1.0 0 0 0 1.0 T086 3.92 1.0 0 1.3 1.3 0 0 0 0 0 0 T491 3.92 0 1.5 0 1.0 0 0 1.0 0 1.0 0 a No virus was detected in the assay at day 0 (preinoculation); 1.0 log10 PFU/ml is the assay lower limit.
  7. 7. VOL. 78, 2004 ChimeriVax-WEST NILE VACCINE PRECLINICAL EVALUATION 12503 TABLE 11. Reciprocal neutralizing antibody titers (PRNT50) lenge with WN NY99 (Table 12). It is noteworthy that 50% of against ChimeriVax-WN virus, rhesus monkeys inoculated by the the animals vaccinated with ChimeriVax-WN developed fever s.c. route with YF-VAX or ChimeriVax-WN vaccine candidates after challenge, with an average duration of 5 days postchal- PRNT50 on day b: lenge, suggesting that they sustained subclinical infections. An Vaccine and Dose Postimmunization Postchallenge i.c. challenge with WN virus is extremely aggressive and is the monkeya (log10 PFU) only route of challenge tested to induce WN virus disease in 14 30 63 15 31–34 naïve rhesus monkeys. It is likely that virus replication occurs YF-VAX in brain tissue after i.c. inoculation and before a specific im- M017 4.49 320 640 5,120 NAd NA mune response in the brain can be recruited for clearance of B101 320 640 2,560 1,280 2,560 R286 320 640 640 NA NA the virus. In the case of a human peripherally challenged by a T081 640 640 2,560 10,240 5,120 mosquito bite, preexisting immunity would rapidly neutralize the virus and fever is unlikely to occur. However, none of the GMT 380 640 2,153 3,620 3,620 ChimeriVax-WN-immunized animals developed detectable viremia after challenge, none developed signs of illness (aside YF/WN107F from fever), and none died. Vaccinated animals showed an N313 4.19 160 640 640 2,560 5,120 P367 40 640 640 5,120 2,560 increase in antibody levels postchallenge (Table 11), suggesting T087 40 640 640 2,560 1,280 that viral replication and antigenic stimulation occurred with- AE81 40 640 160 10,240 20,480 out associated illness. Postchallenge viremias ( 102 to 103 PFU/ml) were detected GMT 57 640 453 4,305 4,305 in the control monkeys that had previously been immunized with YF-VAX (Table 12). Two out of four monkeys vaccinated YF/WN316V440 with YF-VAX (M017 and R286) developed a high fever and Downloaded from jvi.asm.org by on April 13, 2007 R918 4.0 40 320 640 1,280 2,560 N577 40 160–320c 320 1,280 2,560 signs of encephalitis: muscle tremors, anorexia, and spasticity. M233 40 160–320c 320 640 1,280 These two animals were euthanized between days 9 and 11 T757 40 40 640 1,280 5,120 after challenge. The other two YF-VAX-vaccinated animals GMT 40 135 453 1,076 2,560 developed fever and survived i.c. challenge with WN NY99 strain without any clinical symptoms; this finding is attributed to cross-protection across the two flaviviruses. YF/WNFVR J729 3.92 40 320 80 5,120 5,120 Two monkeys without any prior vaccination were also chal- T445 80 640 160 640 5,120 T086 160 320–640c 640 1,280 5,120 T491 80 320 160 2,560 5,120 TABLE 12. Viremia and clinical outcome in rhesus monkeys GMT 80 381 190 1,280 5,120 immunized with ChimeriVax-WN or YF-VAX and challenged 63 days later by the i.c. route with a For GMT, if an endpoint was not reached, the assay limit titer was used in the 5.38 log10 PFU of wild-type WN NY99 virus calculation (e.g., 640 taken as 640 and 40 was taken as 40). b PRNT50 was calculated after subtraction of the PRNT from day 0 serum Viremia by day post-i.c. No. of monkeys with samples. Vaccine and challenge (log10 PFU/ml) outcome/total (%) c PRNT50 calculation fell between the titers shown. The lower titer was used monkey for the GMT calculation. 1 2 3 4 5 Illness Death d NA, not applicable. Neg control K396 2.0 3.1 2.6 2.4 0 2/2 (100) 2/2 (100) P500 2.0 3.1 2.6 2.2 1.0 the number of attenuating mutations in the chimera and the YF-VAX peak titer of viremia (Table 10), but small sample size pre- M017 2.5 2.6 1.7 1.3 0 2/4 (50) 2/4 (50) cludes definitive characterization of these differences. B101 2.6 2.5 1.7 0 0 The immunogenicities of vaccine candidates with one, two, R286 3.3 3.3 1.7 0 0 T081 2.4 3.0 2.4 0.5 0 or three attenuating mutations were similar (Table 11). Neu- tralizing antibody titers ranged from 40 to 640 depending YF/WN107F upon the vaccine. There were no significant differences in N313 0 0 0 0 0 0/4 (0) 0/4 (0) P367 0 0 0 0 0 neutralizing antibody response between treatment groups (Ta- T087 0 0 0 0 0 ble 11). High titers of neutralizing antibodies ( 100 PRNT50) AE81 0 0 0 0 0 were present 30 and 63 days after vaccination. The observation that monkeys developed neutralizing antibodies by day 14 in- YF/WN316V440R R918 0 0 0 0 0 0/4 (0) 0/4 (0) dicates that ChimeriVax-WN02 elicits rapid onset of protective N577 0 0 0 0 0 immunity. M233 0 0 0 0 0 Rhesus monkeys vaccinated with YF-VAX developed neu- T757 0 0 0 0 0 tralizing antibodies against YF 17D with GMTs of 380 on day ChimeriVax-WN02 14 postvaccination and 2,153 by day 63, which was 1 day before J729 0 0 0 0 0 0/4 (0) 0/4 (0) challenge with the virulent WN NY99 virus. T445 0 0 0 0 0 Monkeys immunized with ChimeriVax-WN single, double or T086 0 0 0 0 0 T491 0 0 0 0 0 triple mutants were uniformly protected against lethal i.c. chal-
  8. 8. 12504 ARROYO ET AL. J. VIROL. TABLE 13. Neurovirulence of YF/WNFVR RMS (P4 and P11)a relative to YF/WNFVR PMS (Table 13). During all serial pas- and PMS (P2 and P10)a in 2-day-old ICR strain sages of the virus in Vero cells or brain tissue, no reversions mice relative to YF 17Db were detected at target E protein amino acid residues 107F, Back % Mortality AST 316V, or 440R, the attenuation markers for the vaccine Virus i.c. Mutation titration dose candidate. Additionally, during scale-up manufacturing of the (no. dead/total) (days) (log10 PFU) ChimeriVax-WN02 vaccine, no reversions at these critical res- ChimeriVax-WN02 idues were detected. RMS P4 None 1.73 80 (8/10) 14.5 The GMP manufactured ChimeriVax-WN02 production vi- P11 E336C3S 2.08 60 (6/10) 13.67 rus seed (P4) was used for inoculation of large-scale Vero-SF cultures grown on microcarrier beads in 100-liter bioreactors. ChimeriVax-WN02 An additional mutation (L3P) occurred in the vaccine at PMS P2 None 2.10 60 (6/10) 13 position 66 in the M protein. This mutation was associated with P10 E313G3R 1.88 70 (7/10) 13.67 production of slightly smaller plaque size. The vaccine lot (P5) contained equal ratios of small and large plaques. Virus pop- YF-VAX NAc 1.90 100 (10/10) 10.6 ulations with and without the M66 mutation were isolated by Negative control NA 0 (0/10) plaque purification and compared to the PMS (no detectable a Viruses were passed in a serum-free (SF-Vero) stationary-cell substrate. mutations) and the vaccine lot in the suckling mouse model. b Taconic, ICR strain, mice. One litter (10 mice) of 8-day-old mice was inoculated by the i.c. c NA, not applicable. route with 20 l containing 2, 3, or 4 log10 PFU of either large- plaque or small-plaque virus and observed for 21 days for signs lenged with WN NY99 virus. The two challenge control ani- of illness and death. For comparative purposes, litters of mice were inoculated with similar doses of the PMS (P2) and vac- Downloaded from jvi.asm.org by on April 13, 2007 mals developed fever between days 5 and 9 postchallenge, with slight tremors progressing to ataxia and spasticity between days cine lot (P5) viruses. Mice of the same age were also inoculated 10 and 11, and were euthanized between days 10 and 12. with 2 log10 PFU of YF-VAX. Negative controls were inocu- Genetic stability. In vitro and in vivo substrate-passage stud- lated with diluent (Table 14). There were no differences in ies with ChimeriVax-WN01 or the YF/WNFVR chimeric vac- mortality ratios across dose groups, and data were combined cine candidate (ChimeriVax-WN02) were conducted to deter- for analysis. Since the mortality ratio across all treatment mine genetic stability of the constructs when grown in groups differed (P 0.0001), pairwise comparisons were per- stationary cell cultures and in brain tissue. After six in vitro formed. The M66 mutation had no effect on mouse neuroviru- Vero E6 cell passages of the virus followed by six in vivo ICR lence. adult mouse brain passages of ChimeriVax-WN01, no muta- tions were selected relative to the wild-type sequence of the DISCUSSION prM and E genes in the ChimeriVax-WN01 construct nor was The original YF/WN chimeric virus constructed by insertion there an increase in mouse neurovirulence (data not shown). A of the prME genes from a wild-type WN virus strain was heterozygous mutation in the E protein at position E336 re- attenuated with respect to the parental YF 17D virus vector, sulting in a cysteine-to-serine change was identified following but retained a degree of neurovirulence for adult mice. To 10 in vitro passages of the YF/WNFVR virus in Vero E6 cells. develop a vaccine candidate with a wider margin of safety, we In a separate study, in vitro passage of YF/WNFVR in SF-Vero selectively introduced mutations in the donor WN virus. Mu- cells (manufacturing substrate) resulted in selection of a mu- tations introduced into the E protein of the WN donor virus tation at position E313 that changed the amino acid at that utilized a strategy based on the previous construction of position from glycine to arginine. Neurovirulence of these pas- ChimeriVax-JE vaccine, which contained donor prME genes saged viruses for the 2-day-old suckling mice (n 10) inocu- from an attenuated vaccine strain of JE (SA14-14-2 virus) (3, lated with a nominal 2-log10 PFU dose of viruses including 4, 28). The SA14-14-2 virus contains mutations at six amino E313 and E336 mutations showed no increase in virulence acid residues (E107, E138, E176, E279, E315, and E439) that TABLE 14. Neurovirulence of small- and large-plaque viruses isolated from ChimeriVax-WN02 P5 vaccine in 8-day-old ICR mice inoculated i.c.a P value for b: % Mortality Test article Mutation Test article vs Large plaque vs (no. dead/no. tested) Negative control YF-VAX small plaque Sham (negative control) 0 (0/10) 0.0001 PMS (P2) None 13 (4/30) 0.5558 0.0001 Vaccine lot (P5; large and small plaque) E313G3R, M66L3P 23 (7/30) 0.1612 0.0001 Large plaque E313G3R 3 (1/30) 1.000 0.0001 0.3533 Small plaque E313G3R, M66L3P 13 (4/30) 0.5558 0.0001 YF-VAX 100 (10/10) 0.0001 a All ChimeriVax-WN02 viruses used in the evaluation contained the site-directed attenuating mutations 107F316V440R. Additional mutations appearing during Vero cell passage are shown in the table. b Fisher’s exact test (two sided).
  9. 9. VOL. 78, 2004 ChimeriVax-WEST NILE VACCINE PRECLINICAL EVALUATION 12505 play a role in neurovirulence (3). The WN and JE wild-type by three features: (i) loss of neuroinvasion relative to wild-type gene sequences are conserved at most of these residues (except WN virus; (ii) introduction of three site-directed mutations in 176), suggesting that mutations introduced at these sites in WN two E protein domains, each independently associated with virus could have the same attenuating effect as they did in the attenuation; and (iii) conservation of the FVR mutations after case of JE SA14-14-2. As predicted, we found that mutagenesis in vitro passage in manufacturing-related substrates. of the WN E residues E107, E280 (corresponding to E279 in The safety of ChimeriVax-WN02 was evaluated in a sensitive JE virus), and E316 (corresponding to E315 in JE virus) caused 8-day-old suckling mouse model and in rhesus monkeys and in attenuation of the YF/WN virus chimera. Surprisingly, while cynomolgous macaques inoculated by the i.c. route. In all host- an E138 mutation, E3K, was associated with a marked atten- virus pairings, the chimeric virus proved to be significantly less uation of JE virus (3, 34), a corresponding mutation in the WN neurovirulent than the licensed YF-VAX vaccine. The monkey gene did not reduce the neurovirulence of the YF/WN virus to safety test was performed as prescribed by current regulations the expected 0% mortality by the mouse neurovirulence test. applicable to YF vaccines (36) and showed that the vaccine was Mutation of the E protein at E440 (corresponding to E439 in significantly less virulent than YF-VAX. The nonhuman pri- JE virus) from K3R, a conservative residue change, also re- mate model has been previously used to assess the safety of duced neurovirulence for mice. A construct with the three other chimeric vaccines against JE and dengue virus (6, 18, 24). mutations of F, V, and R at positions E107, E316, and E440, After s.c. inoculation of rhesus monkeys, viremias were more respectively, was designated ChimeriVax-WN02 and was se- erratic and of longer duration in animals immunized with the lected as the candidate for manufacture of the vaccine for ChimeriVax-WN vaccines than in animals given YF-VAX (Ta- clinical studies. ChimeriVax-WN02 was not neuroinvasive com- ble 10). The mean peak titer viremia for YF-VAX-vaccinated pared to WN NY99 virus and had reduced neurovirulence monkeys was 1 log higher than that for the ChimeriVax- compared to YF 17D vaccine virus. Attenuation of this virus WN02 (triple mutant) vaccine candidate. The longer viremia was conferred by the mutation at E107, which maps to the observed after immunization with the chimeric viruses suggests Downloaded from jvi.asm.org by on April 13, 2007 fusion peptide in the second domain as predicted in the crystal that the viruses replicate in different tissues had different re- structure of the E protein (1, 12, 32). This amino acid is ticuloendothelial clearance rates from the parental YF 17D thought to reduce virulence by altering the function of the virus or had different kinetics of immune response. We are fusion peptide in the natural cycle of the virus replication. The currently studying the sites of replication of ChimeriVax-WN02 additional ChimeriVax-WN02 mutations at positions E316 and and YF-VAX in tissues of cynomolgus macaques and will E440 map in domain III on the crystal structure of the E report results in a future publication. In addition, future clin- protein. Residue E316 is thought to be involved in binding of ical trials will assess the magnitude and duration of viremia tick-borne encephalitis virus to the virus receptor on the cell following ChimeriVax-WN02 and YF-VAX and establish cor- plasma membrane (1, 32) and thus may play a role in WN virus relations between viremia and adverse events. The low titer of cell entry. Residue E440 is in the transmembrane region of the the viremia observed in rhesus monkeys after s.c. vaccination E protein and is believed to be involved in anchoring the E with the chimeric vaccine candidates suggests that ChimeriVax- protein during its translation in the endoplasmic reticulum; WN02 vaccine has an acceptable phenotype for trials in hu- hence, a mutation at E440 may be altering the natural associ- mans. ation of the E protein with prM (2). The K-to-M mutation at The triply mutated virus (ChimeriVax-WN02) vaccine ap- position E280 that attenuated neurovirulence for mice was not peared to be less immunogenic than the wild-type chimera in included in the final vaccine because it appeared unstable, mice, but performed satisfactorily in nonhuman primates. similar to the corresponding residue in JE virus E protein ChimeriVax-WN02 vaccine rapidly elicited a neutralizing anti- sequence (i.e., E279) shown to be unstable during in vitro body response in all rhesus monkeys and provided solid pro- passage. A reversion to K at position 279 in the JE virus E tection against an aggressive i.c. challenge with 5 log10 PFU of protein occurred after less than five passages of the virus in WN NY99 virus. MRC-5 cells (22). Mutation of residue E176 from Y in the WN A partially protective immune response was observed in two virus sequence to either V or I, as seen in JE strains, did not of the four rhesus monkeys immunized with YF 17D and suggest a significant change in neurovirulence; therefore, po- subsequently challenged with wild-type WN virus. Previous sition E176 was not changed in the final vaccine candidate observations by others have shown the cross-protective effect sequence (unpublished results). This observation contrasts to of prior exposure to phylogenetically related flaviviruses and the previously published results linking a mutation from I to V concluded that potential for protective cross-reactivity is un- at position E176 in the JE virus envelope protein to neuro- likely to prevent infection and only likely to prevent disease virulence (3, 28). Other approaches to flavivirus chimeras em- (16). Similarly, we observed that prior YF immunization of ployed an attenuated dengue virus genome backbone to pro- monkeys did not prevent infection (viremia) after WN virus duce chimeric dengue virus vaccine candidates against the four challenge, but may have provided an element of protection major serotypes (15); similarly, a dengue virus has been used to against death. It should be pointed out that the interval be- deliver the prM and E genes of WN virus, producing an atten- tween YF immunization and challenge was relatively brief and uated vaccine candidate shown protective in a nonhuman pri- that cross-protection between heterologous flaviviruses often mate model (30). This dengue/WN virus chimeric construct diminishes over time, probably due to affinity maturation of the was attenuated by virtue of the chimeric nature and as a result antibody response and waning of T-cell immunity. It is highly of a 30-nucleotide deletion in the 3 end noncoding region unlikely that YF immunity would provide reliable cross-pro- (untranslated region) of the virus genome. tection of humans and therefore a specific, homologous (WN) Safety of ChimeriVax WN02 (YF/WNFVR) is characterized vaccination strategy must be pursued. This observation is sim-
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D. J. Gubler. 1999. Origin of the West Nile virus responsible for an outbreak We would like to acknowledge contributions from the following: of encephalitis in the northeastern United States. Science 286:2333–2337. 20. Levenbook, I. S., L. J. Pelleu, and B. L. Elisberg. 1987. The monkey safety independent contributor Inessa Levenbook; from University of Mas- test for neurovirulence of yellow fever vaccines: the utility of quantitative sachusetts Medical School, Worcester, Sharone Green, Francis Ennis, clinical evaluation and histological examination. J. Biol. Stand. 15:305–313. and John Cruz; from the Centers for Disease Control and Prevention, 21. Marfin, A. A., and D. J. Gubler. 2001. West Nile encephalitis: an emerging John Roehrig; from Sierra Division, Charles River Laboratories, Ken disease in the United States. Clin. Infect. Dis. 33:1713–1719. Draper; from University of Texas, Galveston, Amelia P. Travassos da 22. Monath, T. P., J. Arroyo, I. Levenbook, Z.-X. Zhang, J. Catalan, K. Draper,

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