Joseph E. Blaney

Prospects for a dengue virus vaccine

The number of cases of severe dengue disease continues to grow in endemic areas of southeast Asia, Central and South America, and other subtropical regions. Children bear the greatest burden of disease, and the development of an effective vaccine remains a global public health priority. A tetravalent vaccine is urgently needed and must be effective against all four dengue virus serotypes, be cost-effective and provide long-term protection. In this Review we discuss the unique immunological concerns in dengue virus vaccine development and the current prospects for the development of an acceptable vaccine, a goal that is likely to be reached in the near future.

Recombinant, Live-Attenuated Tetravalent Dengue Virus Vaccine Formulations Induce a Balanced, Broad, and Protective Neutralizing Antibody Response against Each of the Four Serotypes in Rhesus Monkeys

ABSTRACT Three tetravalent vaccine (TV) formulations of previously described monovalent dengue (DEN) virus vaccine candidates were compared to a tetravalent formulation of wild-type DEN viruses (T-wt) for replication in SCID mice transplanted with human liver cells (SCID-HuH-7) or for replication and immunogenicity in rhesus monkeys. TV-1 consists of recombinant DEN1, -2, -3, and -4, each with a 30-nucleotide deletion in the 3′ untranslated region (Δ30). TV-2 consists of rDEN1Δ30, rDEN4Δ30, and two antigenic chimeric viruses, rDEN2/4Δ30 and rDEN3/4Δ30, both also bearing the Δ30 mutation. TV-3 consists of rDEN1Δ30, rDEN2Δ30, rDEN4Δ30, and a 10-fold higher dose of rDEN3/4Δ30. TV-1 and TV-2 were attenuated in SCID-HuH-7 mice with minimal interference in replication among the virus components. TV-1, -2, and -3 were attenuated in rhesus monkeys as measured by duration and peak of viremia. Each monkey immunized with TV-1 and TV-3 seroconverted to the four DEN components by day 28 with neutralization titers ranging from 1:52 to 1:273 and 1:59 to 1:144 for TV-1 and TV-3, respectively. TV-2 induced low antibody titers to DEN2 and DEN3, but a booster immunization after 4 months increased the neutralizing antibody titers to greater than 1:100 against each serotype and elicited broad neutralizing activity against 19 of 20 DEN subtypes. A single dose of TV-2 induced protection against wild-type DEN1, DEN3, and DEN4 challenge, but not DEN2. However, two doses of TV-2 or TV-3 induced protection against DEN2 challenge. Two tetravalent formulations, TV-2 and TV-3, possess properties of a successful DEN vaccine and can be considered for evaluation in clinical trials.

A Live, Attenuated Dengue Virus Type 1 Vaccine Candidate with a 30-Nucleotide Deletion in the 3′ Untranslated Region Is Highly Attenuated and Immunogenic in Monkeys

ABSTRACT The Δ30 deletion mutation, which was originally created in dengue virus type 4 (DEN4) by the removal of nucleotides 172 to 143 from the 3′ untranslated region (3′ UTR), was introduced into a homologous region of wild-type (wt) dengue virus type 1 (DEN1). The resulting virus, rDEN1Δ30, was attenuated in rhesus monkeys to a level similar to that of the rDEN4Δ30 vaccine candidate. rDEN1Δ30 was more attenuated in rhesus monkeys than the previously described vaccine candidate, rDEN1mutF, which also contains mutations in the 3′ UTR, and both vaccines were highly protective against challenge with wt DEN1. Both rDEN1Δ30 and rDEN1mutF were also attenuated in HuH-7-SCID mice. However, neither rDEN1Δ30 nor rDEN1mutF showed restricted replication following intrathoracic inoculation in the mosquito Toxorhynchites splendens. The ability of the Δ30 mutation to attenuate both DEN1 and DEN4 viruses suggests that a tetravalent DEN vaccine could be generated by introduction of the Δ30 mutation into wt DEN viruses belonging to each of the four serotypes.

rDEN4Δ30, a Live Attenuated Dengue Virus Type 4 Vaccine Candidate, Is Safe, Immunogenic, and Highly Infectious in Healthy Adult Volunteers

BACKGROUND The live attenuated dengue virus type 4 (DEN-4) vaccine candidate virus rDEN4 Delta 30 was previously found to be safe and immunogenic at a dose of 10(5) plaque-forming units (pfu). METHODS In a follow-up placebo-controlled phase 2 clinical trial, rDEN4 Delta 30 was administered as a single inoculation to 3 separate dose cohorts (10(3) pfu, 10(2) pfu, or 10(1) pfu), for further evaluation. Each dose cohort consisted of 20 vaccinees and 4 placebo recipients. Volunteers were monitored closely for adverse events, and serum was collected on study days 28 and 42 for determination of neutralizing antibody titer. RESULTS The vaccine was well tolerated at all doses studied. The most common adverse events observed were a transient asymptomatic rash in >50% of vaccinees and a mild neutropenia in approximately 20% of vaccinees. No vaccinee developed a dengue-like illness. The vaccine was highly infectious and immunogenic, with 95%-100% of vaccinees in each dose cohort developing a >/=4-fold increase in titers of serum neutralizing antibodies against DEN-4. CONCLUSIONS The rDEN4 Delta 30 vaccine is safe and induced an antibody response that was broadly neutralizing against genotypically diverse DEN-4 viruses. It is a promising vaccine candidate for inclusion in a tetravalent dengue vaccine formulation.

Paired Charge-to-Alanine Mutagenesis of Dengue Virus Type 4 NS5 Generates Mutants with Temperature-Sensitive, Host Range, and Mouse Attenuation Phenotypes

ABSTRACT Charge-to-alanine mutagenesis of dengue virus type 4 (DEN4) NS5 gene generated a collection of attenuating mutations for potential use in a recombinant live attenuated DEN vaccine. Codons for 80 contiguous pairs of charged amino acids in NS5 were individually mutagenized to create uncharged pairs of alanine residues, and 32 recombinant mutant viruses were recovered from the 80 full-length mutant DEN4 cDNA constructs. These mutant viruses were tested for temperature-sensitive (ts) replication in both Vero cells and HuH-7 human hepatoma cells. Of the 32 mutants, 13 were temperature sensitive (ts) in both cell lines, 11 were not ts in either cell line, and 8 exhibited a host range (tshr) phenotype. One tshr mutant was ts only in Vero cells, and seven were ts only in HuH-7 cells. Nineteen of the 32 mutants were 10-fold or more restricted in replication in the brains of suckling mice compared to that of wild-type DEN4, and three mutants were approximately 10,000-fold restricted in replication. The level of temperature sensitivity of replication in vitro did not correlate with attenuation in vivo. A virus bearing two pairs of charge-to-alanine mutations was constructed and demonstrated increased temperature sensitivity and attenuation relative to either parent virus. This large set of charge-to-alanine mutations specifying a wide range of attenuation for mouse brain should prove useful in fine-tuning recombinant live attenuated DEN vaccines.

rDEN2/4Δ30(ME), a Live Attenuated Chimeric Dengue Serotype 2 Vaccine, is Safe and Highly Immunogenic in Healthy Dengue-Naïve Adults

rDEN2/4Δ30(ME) is an attenuated chimeric dengue virus in which the prM and E structural proteins of the DEN4 candidate vaccine rDEN4∆30 have been replaced by those of the prototypic DEN2 NGC virus. rDEN2/4Δ30(ME) was evaluated at a dose of 1,000 PFU in 20 healthy dengue-naïve adult volunteers. Eight volunteers received placebo. Volunteers were monitored closely for adverse events and serum was collected for determination of the level and duration of viremia and neutralizing antibody response. The vaccine was well tolerated by all volunteers. The most common adverse events observed were a transient asymptomatic rash and mild neutropenia. All vaccinees seroconverted to DEN2 and maintained significant antibody titers throughout the six-month trial duration. Eleven vaccinees had vaccine virus recovered from the blood during the study. RNA derived from virus isolates obtained from viremic volunteers was sequenced for confirmation of retention of the ∆30 mutation in the 3´ UTR. The Δ30 mutation remained unchanged in each isolate, confirming the stability of the Δ30 mutation. Further evaluation of this vaccine in a tetravalent formulation is warranted.

A trade-off in replication in mosquito versus mammalian systems conferred by a point mutation in the NS4B protein of dengue virus type 4

An acceptable live-attenuated dengue virus vaccine candidate should have low potential for transmission by mosquitoes. We have identified and characterized a mutation in dengue virus type 4 (DEN4) that decreases the ability of the virus to infect mosquitoes. A panel of 1248 mutagenized virus clones generated previously by chemical mutagenesis was screened for decreased replication in mosquito C6/36 cells but efficient replication in simian Vero cells. One virus met these criteria and contained a single coding mutation: a C-to-U mutation at nucleotide 7129 resulting in a Pro-to-Leu change in amino acid 101 of the nonstructural 4B gene (NS4B P101L). This mutation results in decreased replication in C6/36 cells relative to wild-type DEN4, decreased infectivity for mosquitoes, enhanced replication in Vero and human HuH-7 cells, and enhanced replication in SCID mice implanted with HuH-7 cells (SCID-HuH-7 mice). A recombinant DEN4 virus (rDEN4) bearing this mutation exhibited the same set of phenotypes. Addition of the NS4B P101L mutation to rDEN4 bearing a 30 nucleotide deletion (Delta30) decreased the ability of the double-mutant virus to infect mosquitoes but increased its ability to replicate in SCID-HuH-7 mice. Although the NS4B P101L mutation decreases infectivity of DEN4 for mosquitoes, its ability to enhance replication in SCID-HuH-7 mice suggests that it might not be advantageous to include this specific mutation in an rDEN4 vaccine. The opposing effects of the NS4B P101L mutation in mosquito and vertebrate systems suggest that the NS4B protein is involved in maintaining the balance between efficient replication in the mosquito vector and the human host.

The Live Attenuated Dengue Serotype 1 Vaccine rDEN1Δ30 is Safe and Highly Immunogenic in Healthy Adult Volunteers

Background: The live attenuated DEN1 vaccine candidate virus rDEN1Δ30 has been evaluated in preclinical animal models and found to be attenuated and immunogenic. These promising preclinical studies have identified rDEN1Δ30 as a candidate DEN1 vaccine virus for further testing in a human Phase I clinical trial. Methods: rDEN1Δ30 at a dose of 103 pfu was administered as a single inoculation to twenty healthy adult volunteers. Eight additional volunteers received placebo. Volunteers were monitored closely for adverse events and serum was collected on study days 0, 28, 42, and 180 for determination of neutralizing antibody titer. Results: The vaccine was well tolerated by the vaccinees. The most common adverse events observed were a transient asymptomatic rash in 40% of vaccinees and a mild neutropenia in 45% of vaccinees. No vaccinee developed a dengue-like illness. The vaccine was highly infectious and immunogenic with 95% of vaccinees developing a ? 4-fold rise in serum neutralizing antibody titer against DEN1 that persisted throughout the six month duration of the trial. Conclusions: The rDEN1Δ30 vaccine is safe and induced a potent and durable antibody response against DEN1. It is a promising vaccine candidate for inclusion in a tetravalent dengue vaccine formulation.

Antibody quality and protection from lethal Ebola virus challenge in nonhuman primates immunized with rabies virus based bivalent vaccine.

We have previously described the generation of a novel Ebola virus (EBOV) vaccine platform based on (a) replication-competent rabies virus (RABV), (b) replication-deficient RABV, or (c) chemically inactivated RABV expressing EBOV glycoprotein (GP). Mouse studies demonstrated safety, immunogenicity, and protective efficacy of these live or inactivated RABV/EBOV vaccines. Here, we evaluated these vaccines in nonhuman primates. Our results indicate that all three vaccines do induce potent immune responses against both RABV and EBOV, while the protection of immunized animals against EBOV was largely dependent on the quality of humoral immune response against EBOV GP. We also determined if the induced antibodies against EBOV GP differ in their target, affinity, or the isotype. Our results show that IgG1-biased humoral responses as well as high levels of GP-specific antibodies were beneficial for the control of EBOV infection after immunization. These results further support the concept that a successful EBOV vaccine needs to induce strong antibodies against EBOV. We also showed that a dual vaccine against RABV and filoviruses is achievable; therefore addressing concerns for the marketability of this urgently needed vaccine.

Mutations which enhance the replication of dengue virus type 4 and an antigenic chimeric dengue virus type 2/4 vaccine candidate in Vero cells

Mutations which increase the replication of dengue viruses in cell culture would greatly facilitate the manufacture of both a live attenuated or inactivated dengue virus vaccine. We have identified eight missense mutations in dengue virus type 4 (DEN4) that increase the plaque size and kinetics of replication of recombinant DEN4 virus in Vero cells. DEN4 viruses bearing these Vero cell adaptation mutations were also evaluated for the level of replication in the brains of mice. Two of these eight recombinant viruses expressing distinct mutations in NS3 were both restricted in replication in the brains of suckling mice. In contrast, six recombinant viruses, each encoding individual mutations in NS4B (five) or in NS5 (one), were not attenuated in mouse brain. Recombinant viruses encoding various combinations of these Vero cell adaptation mutations did not demonstrate enhanced replication in Vero cells over that exhibited by the single mutations. Finally, addition of a subset of the above non-attenuating, adaptation mutations to a DEN2/4 chimeric vaccine candidate was found to increase the virus yield in Vero cells by up to 500-fold. The importance of these Vero cell adaptation mutations in flavivirus vaccine design and development is discussed.