Sequences of E/NS1 gene junction from four dengue-2 viruses of northeastern Thailand and their evolutionary relationships with other dengue-2 viruses

Viremia and immunogenicity in nonhuman primates of a tetravalent yellow fever-dengue chimeric vaccine: genetic reconstructions, dose adjustment, and antibody responses against wild-type dengue virus isolates

Chimeric yellow fever (YF)-dengue (DEN) viruses (ChimeriVax-DEN) were reconstructed to correct amino acid substitutions within the envelope genes of original constructs described by Guirakhoo et al. (2001, J. Virol. 75, 7290-7304). Viruses were analyzed and compared to the previous constructs containing mutations in terms of their growth kinetics in Vero cells, neurovirulence in mice, and immunogenicity in monkeys as monovalent or tetravalent formulations. All chimeras grew to high titers [ approximately 7 to 8 log(10), plaque-forming units (PFU)/ml] in Vero cells and were less neurovirulent than YF 17D vaccine in mice. For monkey experiments, the dose of DEN2 chimera was lowered to 3 log(10) PFU in the tetravalent mixture in an effort to reduce its dominant immunogenicity. The magnitude of viremia in ChimeriVax-DEN immunized monkeys was similar to that of YF-VAX, but significantly lower than those induced by wild-type DEN viruses. All monkeys developed high levels of neutralizing antibodies against homologous (chimeras) or heterologous (wild-type DEN viruses isolated from different geographical regions) viruses after a single dose of monovalent or tetravalent vaccine. Administration of a second dose of tetravalent vaccine 2 months later increased titers to both homologous and heterologous viruses. A dose adjustment for dengue 2 chimera resulted in a more balanced response against dengue 1, 2, and 3 viruses, but a somewhat higher response against chimeric dengue 4 virus. This indicates that further formulations for dose adjustments need to be tested in monkeys to identify an optimal formulation for humans.

Phylogenetic relationships and differential selection pressures among genotypes of dengue-2 virus

To elucidate the processes controlling the emergence and spread of dengue-2 virus (DEN-2) we examined the evolution of viral isolates sampled from both local (Viet Nam) and global populations. Our phylogenetic analysis, incorporating envelope (E) glycoprotein sequences from 147 isolates of DEN-2, provided a more complete picture of viral diversity, with a newly defined "Cosmopolitan" genotype having a near global distribution and two other genotypes restricted to Asia. By analyzing rates of synonymous and nonsynonymous substitution we determined that genotypes have experienced different selection pressures, with some evidence of positive selection in the Cosmopolitan genotype and one of the two Asian genotypes, but that the transition from sylvatic to human transmission was not accompanied by adaptive evolution of the E gene. Although there was no association between selection pressures acting on the E gene and proposed virulence differences among genotypes, some putatively selected amino acid sites have previously been implicated in changing viral pathogenicity, most notably E-390, and may also affect transmittability. These findings have implications for the future spread of DEN-2.

Molecular and in vitro analysis of eight dengue type 2 viruses isolated from patients exhibiting different disease severities

Potential genetic determinants of dengue virulence were studied by sequencing the entire genomes of eight dengue 2 virus strains isolated from patients exhibiting different disease severities during an epidemic season in northeastern Thailand in 1993. The isolates came from one dengue shock syndrome (ThNH-7/93), three dengue hemorrhagic fever, and four dengue fever patients. Phylogenetic analysis showed that the isolates belonged to the Southeast Asian genotype. The 3' noncoding regions showed distinctive secondary structures, with one specific structure for the isolate ThNH-7/93. Analysis of the predicted polyprotein showed several amino acid (aa) changes scattered mostly in the nonstructural region. Of 30 positions with aa changes, 7 were unique to the isolate ThNH-7/93 and 3 of those led to radical alterations in aa character. Several aa changes coincided with previous studies relating genome sequence and virulence. Minimal changes in computer-predicted protein secondary structures were observed. Infective particles in the inoculum for all isolates were approximately equal as measured by focus formation on BHK-21 cells, but this did not correlate with the number of plaques formed on LLC-MK2 cells. Isolates from patients that experienced secondary infection were shown to have significantly larger plaques than the isolates from primary infection patients.

Impact of dengue virus infection and its control

Dengue virus infection has been counted among emerging and re-emerging diseases because of (1) the increasing number of patients, (2) the expansion of epidemic areas, and (3) the appearance of severe clinical manifestation of dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS), which is often fatal if not properly treated. In the meantime, there are no effective dengue control measures: a dengue vaccine is still under development and vector control does not provide a long-lasting effect. In order to obtain direct evidence for the virulent virus theory concerning the pathogenesis of DHF/DSS, type 2 dengue virus strains isolated from patients with different clinical severities in the same epidemic area in northeast Thailand, during the same season, were comparatively sequenced. The result revealed a DF strain specific amino acid substitution from I to R in the PrM, and a DSS strain specific amino acid substitution from D to G in the NS1 gene regions, which could significantly alter the nature of these proteins. Moreover, DF strain specific nucleotide substitutions in the 3' noncoding region were predicted to alter its secondary structure. These amino acid and nucleotide substitutions in other strains isolated in different epidemic areas during other seasons, together with their biological significance, remain to be confirmed. In order to innovate dengue vector control, field tests were carried out in dengue epidemic areas in Vietnam to examine the efficacy of Olyset Net screen, which is a wide-mesh net made of polyethylene thread impregnated with permethrin. The results show that Olyset Net (1) reduced the number of principal dengue vector species, Aedes aegypti, (2) interrupted the silent transmission of dengue viruses and (3) was highly appreciated by the local people as a convenient and comfortable vector control method. This encouraging evaluation of the Olyset Net screen should be confirmed further by other tests under different settings.

Detection of the disease severity-related molecular differences among new Thai dengue-2 isolates in 1993, based on their structural proteins and major non-structural protein NS1 sequences

We determined the nucleotide sequences of the whole structural protein gene of four new dengue-2 viruses by the primer extension dideoxy chain termination method, using multiple cDNA clones for six overlapping gene regions. The nucleotide sequences of the major non-structural protein NS1 gene of these viruses were also determined by direct sequencing of the reverse-transcription polymerase chain reaction products. These viruses were isolated from dengue patients with different clinical severities in Nakhon Phanom, Northeastern Thailand in 1993. The results were compared with the sequences of prototype New Guinea C strain and other reference strains. All four viruses revealed highest homology to New Guinea C strain. The homology between each of the four strains and New Guinea C strain varies from 95.09% to 95.29% in its nucleotide sequences, and from 97.24% to 97.78% in its amino acid sequences covering all structural proteins and NS1 protein. The PreM region shows the highest divergence (6.59% to 7.32%) in its nucleotide sequence, whereas C protein is most highly conserved (only 1.75% to 2.63% divergence). Our data showed that there are certain molecular differences in the genomic structure of these four new isolates, which indicate the possibility that these changes are related with the virulence of the virus strains.