B. Berkhout et al., Genetic instability of live, attenuated human immunodeficiency virus type 1 vaccine strains, J VIROLOGY, 73(2), 1999, pp. 1138-1145
Live, attenuated viruses have been the most successful vaccines in monkey m
odels of human immunodeficiency virus type 1 (HIV-1) infection. However, th
ere are several safety concerns about using such an anti-HIV vaccine in hum
ans, including reversion of the vaccine strain to virulence and recombinati
on with endogenous retroviral sequences to produce new infectious and poten
tially pathogenic viruses. Because testing in humans would inevitably carry
a substantial risk, we set out to test the genetic stability of multiply d
eleted HIV constructs in perpetuated tissue culture infections. The Delta 3
candidate vaccine strain of HIV-1 contains deletions in the viral long ter
minal repeat (LTR) promoter and the vpr and nef genes. This virus replicate
s with delayed kinetics, but a profound enhancement of virus replication wa
s observed after approximately 2 months of culturing. Analysis of the rever
tant viral genome indicated that the three introduced deletions were mainta
ined but a 39-nucleotide sequence was inserted in the LTR promoter region.
This insert was formed by duplication of the region encoding three binding
sites for the Sp1 transcription factor. The duplicated Sp1 region was demon
strated to increase the LTR promoter activity, and a concomitant increase i
n the virus replication rate was measured. In fact, duplication of the Sp1
sites increased the fitness of the Delta 3 virus (Vpr/Nef/U3) to levels hig
her than that of the singly deleted Delta Vpr virus. These results indicate
that deleted HIV-1 vaccine strains can evolve into fast-replicating varian
ts by multiplication of remaining sequence motifs, and their safety is ther
efore not guaranteed. This insight may guide future efforts to develop more
stable anti-HIV vaccines.