Abstract
M.Sc.
Therapeutic and preventative treatment are continually being sought to cease or curtail
the worldwide HIV-1 epidemic. At present, therapeutic drug strategies such as highly
active anti-retroviral therapy (HAART) have been particularly successful in slowing
disease progression and reducing the incidence of AIDS and AIDS related mortality
(Detels et al., 1998; Mocroft et al., 1998; Palella et al., 1998). However, the high costs,
intricate dosing regimens and limited availability of the HAART drugs (Butera, 2000) has
restricted its efficacy in developing and third world countries. As such, available and
future drugs will remain inaccessible to the regions that are profoundly affected by the
epidemic. An effective vaccine presents a viable solution to the HIV-1 epidemic in these
countries.
Approximately 70 vaccines are presently in various stages of clinical trials, the majority
of which are subtype B specific (Johnston and Flores, 2001). This prevents their use in
the predominantly subtype C infected sub-Saharan region of Africa, which accounts for
50% of the global HIV / AIDS population and includes South Africa, statistically the
country with the highest number of people living with HIV / AIDS of any country in the
world (UNAIDS, 2002).
Presently there is no HIV-1 vaccine, regardless of subtype, in clinical use. This owes to
several difficulties that hinder the progression of vaccine development, including the lack
of predictive animal models, the establishment of viral latency and the difficulty involved
in overcoming HIV-1 genetic diversity (Klein, 1999).
The expansive HIV-1 genetic variation exhibited by HIV-1 is attributed to a high number
of errors made by the reverse transcriptase (RT) enzyme (Coffin, 1992) and the absence
of RT proofreading mechanisms during HIV-1 replication (Roberts et al., 1988; Bebenek
et al., 1989). The HIV-1 nucleotide sequence drift is most frequently observed in the
envelope (env) gene and expressed in env gene products (Shafer et al., 1999). Expression
of the variable genome results in the production of progeny strains that are not identical
to the parent strain (i.e. HIV-1 exists as a quasispecies within each seropositive individual
and between individuals) and contributes to the diverse collection of viral strains in
global circulation that vary across and within subtypes. Thus, for an HIV-1 vaccine to be
efficient and truly functional it would be required to target this observed hypervariability
and be effective against a multitude of currently circulating strains, exhibit cross-clade
specificity and remain viable despite the emergence of variant strains.
In this study we describe the design, synthesis and immunological ability of a multiple
epitope immunogen (MEI) that mimics the hypervariability observed within the third
variable (V3) loop of the envelope gp120 region of HIV-1 subtype C. Conjugation to a
multiple antigenic peptide (MAP) produces a four -branched (b4) tetrameric peptide
construct, designated MEIV3b4. This construct was characterized by theoretical and
analytical techniques, tested in a variety of immunological assays and assessed for its
potential as a candidate vaccine component. The construct was comparatively analysed
through evaluation of three comparison peptides, two of which are hypervariable and
based on the V3 region, the other representing a conserved region of HIV-1 envelope.
The V3 peptides, named b-MEI-s and poly-L-MEI, differ from the MEIV3b4 construct in
that they are less variable and less branched or conjugated to a traditional carrier rather
than to a MAP system, respectively. The conserved peptide, designated CCD4 allowed
for comparative evaluation between conserved and variable peptides as potential vaccine
components.
Dr. Debra Meyer