Wild dengue virus types 1, 2 and 3 viremia in rhesus monkeys

Among the flaviviruses, dengue, with its four serotypes, has spread throughout the tropics. The most advanced vaccines developed so far include live attenuated viruses, which have been tested in humans but none has been licensed. Preclinical testing of dengue vaccine candidates is performed initially in mice and in nonhuman primates. In the latter the main criteria used to assay protection are neutralizing antibodies elicited by the vaccine candidate and the magnitude and duration of peripheral viremia upon challenge of previously immunized animals. Towards the identification of wild-type viruses that could be used in challenge experiments a total of 31 rhesus monkeys were inoculated subcutaneously of wild dengue types 1, 2, and 3 viruses. The viremia caused by the different viruses was variable but it was possible to identify dengue viruses useful as challenge strains.

Attenuation and immunogenicity of recombinant yellow fever 17D-dengue type 2 virus for rhesus monkeys

A chimeric yellow fever (YF)-dengue serotype 2 (dengue 2) virus was constructed by replacing the premembrane and envelope genes of the YF 17D virus with those from dengue 2 virus strains of Southeast Asian genotype. The virus grew to high titers in Vero cells and, after passage 2, was used for immunogenicity and attenuation studies in rhesus monkeys. Subcutaneous immunization of naive rhesus monkeys with the 17D-D2 chimeric virus induced a neutralizing antibody response associated with the protection of 6 of 7 monkeys against viremia by wild-type dengue 2 virus. Neutralizing antibody titers to dengue 2 were significantly lower in YF-immune animals than in YF-naive monkeys and protection against challenge with wild-type dengue 2 virus was observed in only 2 of 11 YF-immune monkeys. An anamnestic response to dengue 2, indicated by a sharp increase of neutralizing antibody titers, was observed in the majority of the monkeys after challenge with wild-type virus. Virus attenuation was demonstrated using the standard monkey neurovirulence test. The 17D-D2 chimera caused significantly fewer histological lesions than the YF 17DD virus. The attenuated phenotype could also be inferred from the limited viremias compared to the YF 17DD vaccine. Overall, these results provide further support for the use of chimeric viruses for the development of a new live tetravalent dengue vaccine.

Identification, sequencing and structural analysis of a nifA-like gene of Acetobacter diazotrophicus

A recombinant plasmid, pAD101, containing a DNA fragment of Acetobacter diazotrophicus strain PAL5 was isolated by its ability to restore Nif+ phenotype to a nifA- ntrC- double mutant of Azotobacter vinelandii. Hybridization with the nifA genes of Azospirillum brasilense located the nifA gene more precisely to specific fragments of pAD101. DNA sequencing of appropriate subclones of pAD101 revealed that the nifA gene was adjacent to the nifB gene in A. diazotrophicus, and the 5' end of the nifB gene was located downstream of the nitrogenase MoFe subunit gene, nifK. The deduced aminoacid sequence of A. diazotrophicus nifA and nifB gene were most similar to the NifA and NifB proteins of Azorhizobium caulinodans and Rhodobacter capsulatus, respectively. In addition, nucleotide sequences upstream of the A. diazotrophicus nifA-encoding region indicate features similar to those in the A. caulinodans nifA promoter region involved in O2 and fixed N regulation of nifA expression.

The yellow fever 17D vaccine virus: molecular basis of viral attennuation and it use as an expression vector

The yellow fever (YF) virus is the prototype flavivirus. The use of molecular techniques has unraveled the basic mechanisms of viral genome structure and expression. Recent trends in flavivirus research include the use of infectious clone technology with which it is possible to recover virus from cloned cDNA. Using this technique, mutations can be introduced at any point of the viral genome and their resulting effect on virus phenotype can be assessed. This approach has opened new possibilities to study several biological viral features with special emphasis on the issue of virulence/attenuation of the YF virus. The feasibility of using YF virus 17D vaccine strain, for which infectious cDNA is available, as a vector for the expression of heterologous antigens is reviewed.