Summary Statement: ADARC Program Project on HIV/SIV Vaccine Development

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1 P01 A1430412-01 13 SEP
Ho, David D.
REVIEWER A: In this well thought ou t,p al, modifications will be
introduced into the envelope glycoprotein gene in an attempt to increase
immunogenicity, and these concepts will be tested in an SHIV/macaque model.
The choice of a strain for the development of a candidate HIV vaccine will be
critical to its ultimate success. Several criteria for the selection of the
ideal strain are suggested in this project. The decision to concentrate on
subtype C seems sound because of the frequency of occurrence and the potential
impact if an effective vaccine can be developed. Similarly, the choice of a
non-syncytium-inducing macrophage-tropic strain using the CCR5 co-receptor
also seems logical since this is the phenotype that is most often transmitted.
Arguments area presented regarding the role of the variable loops and
glycosylation sites in blocking the development of an effective neutralizing
antibody response against the envelope glycoprotein. This argument lead to
the hypothesis that the amino acids in the V1/V2 and V3 loops shield critical
and conserved functional domains from antibody binding, and that deletion of
those loops may expose conserved underlying structures and result in a better
antibody response to block virus interactions with receptors. Elimination of
some glycosylation sites may also expose epitopes and increase neutralization
sensitivity. It is also argued that the SHIV-macaque model is the most
relevant current model for vaccine development. Preliminary results are
presented regarding the phylogenetic analysis, growth characteristics,
phenotype, and co-receptor usage of 21 subtype C isolates. This work may set
the stage for the choice of subtype C candidates for further development.
Also, studies are presented showing that removal of a glycosylation site in
the C2 region of a primary strain increases its sensitivity to neutralization.
Results are also presented on viability and growth characteristics of loop
deletion mutants of an HIV isolate. These results suggest that the strategies
to be investigated may well be fruitful. Finally, results are shown regarding
the pathogenicity of SHIV constructs in rhesus macaques. These preliminary
results constitute a firm basis for the experiments proposed in this project.
Specific Aim 1 is aimed at the genotypic, phenotypic, and antigenic
characterization of subtype C viruses and selection of vaccine strains.
Strains already in hand and additional newly acquired strains will be further
characterized, and two vaccine strains will be chosen for further development.
The strains to be selected will have the following characteristics: (1) they
will be close to a node in the phylogenetic analysis; (2) the strains should
contain known epitopes for broadly neutralizing monoclonal antibodies; (3)
Sthe strains will be macrophage-tropic; and (4) they will utilize the CCR5 coreceptor. The rationale for these criteria seems logical and reasonableg and
the proposed experiments to investigate them seem straightforward.
The Specific Aim 2 is designed to explore the expression and modification of
the envelope genes of the two strains. The genes will be cloned in a plasmid
vector so that they can be expressed by the CMV promoter when transfected into
293T cells. Loop deletion and glycosylation site mutations will be introduced
into the cloned genes. The ability of the modified genes to produce a

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