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HIV Vaccine: RPK-8129
Background
The global HIV/AIDS epidemic is one of the greatest challenges facing the human race. An estimated 38.6 million people worldwide were living with HIV/AIDS in 2005, 64% of these in Sub-Saharan Africa. In some southern African countries HIV/AIDS is responsible for the erosion of life expectancy by between 25% and 45% over the last 20 years. HIV/AIDS related health effects are compounded by damaging social and economic impacts. Although prevention strategies have slowed the progression of HIV infection, approximately 4.1 million new infections were reported in 2005.
Current Treatments
Despite ongoing efforts there is no prophylactic vaccine for HIV.
Currently, FDA approved treatments are restricted to four classes of anti-retroviral therapies:
- nucleoside reverse transcriptase inhibitors (NRTI)
- non-nucleoside reverse transcriptase inhibitors (NNRTI)
- protease inhibitors (PI)
- fusion inhibitor
Resistance or multi-resistance to members of approved classes of HIV therapeutics is well documented and as a result they are routinely administered in combination, a regime referred to as highly active antiretroviral therapy (HAART). Although, HAART can reduce viral load to undetectable levels, anti-retroviral treatment cannot eradicate cellular reservoirs of latent HIV infection. The widespread use of HAART in developed countries has been highly successful in improving quality of life, increasing life expectancy and is attributed with the significant decline in AIDS incidence and deaths in the United States (CDC). However, HAART is associated with severe side effects, involving metabolic complications and mitochondrial dysfunction and increased risk of cardiovascular diseases. Moreover, poor patient compliance can promote accumulation of virus mutations and the development of drug resistance.
RPK-8129: HIV Vaccine
Replikun has developed a Kunjin replicon HIV vaccine candidate (RPK-8129), which encodes the HIV-1 gag gene. HIV gag proteins are relatively conserved amongst HIV isolates may provide cross clade immunity and are incorporated into most HIV vaccines.
Expression Studies Back to Top
The results of electron microscopy and radioimmunoprecipitation analysis demonstrated that the HIV-1 gag protein produced from the Kunjin replicon in transfected BHK-21 cells was correctly processed and assembled into secreted gag particles. Total gag expression was approximately 600 ng per 106 initially transfected cells (60 h post transfection), with approximately one third of this protein secreted into the culture fluid (see figure below).
Production and secretion of Gag protein produced from the Kunjin Replicon.
(A) Radioimmunoprecipitation Assay. Radiolabeled culture fluid and cell lysate, collected 0.5 and 4 h after labeling with [35S]methionine/cysteine from RPK-8129 RNA electroporated cells, was immunoprecipitated with anti-pr55gag antibody.
(B) Culture fluid and cell lysate were serially collected from RPK-8129 electroporated cells and the amount of gag protein was determined. The total amount of cell-associated (cell lysate [black circles]) and secreted (culture fluid [black squares]) pr55gag produced per 106 initially transfected BHK cells at different times post-transfection was calculated.
Preclinical Studies Back to Top
In pre-clinical murine studies, immunization of animals once or twice (2 weeks apart) with RPK-8129 delivered as a VLP, RNA or DNA, induced significant gag-specific CD8+ T cells (as measured by IFN-gamma ELISPOT and chromium release assays) and anti-gag antibodies. Importantly, Kunjin replicon VLPs induced gag-specific CD8+ T lymphocytes responses in excess of those generated following vaccination with replication-competent recombinant vaccinia virus expressing gag (rVVgag) (see Figure below).
T-cell responses specific for a CD8+ T-cell epitope induced by different delivery modalities of KUN replicons carrying the HIV-1 gag gene or with recombinant vaccinia virus carrying the HIV-1 gag gene (rVVgag).
(A) ELISPOT analysis of BALB/c mice responses after immunization with the indicated vaccine modalities. Mice received a single immunization (white bars)
or two immunizations (2 weeks between the two immunizations) (black bars).
(B) Standard 6-h 51Cr release assay using restimulated splenocytes from BALB/c mice immunized twice with KUNgag RNA, KUNgag VLPs, or rVVgag. 51Cr-labeled target cells were sensitized with (black squares) or without (white squares) response peptide.
Kunjin replicon VLP vaccines elicit long-term CD8+ T-cell responses. In mice immunized once with RPK-8129 VLPs, the gag-specific CD8+ T cell responses assessed by ELISPOT 6 months later, were not significantly different from those seen after 2 weeks, illustrating the induction of long-lasting CD8+ T-cell responses by RPK-8129 VLP immunization (see Figure below). Furthermore, when mice that had been immunized twice with RPK-8129 VLPs, were challenged 10 months later with a rVV expressing gag, substantial protection was still apparent, as assessed by measurement of ovary vaccinia titres (see Figure below).
Long-term immune responses elicited by RPK-8129 VLP vaccines.
Top: ELISPOT response to an epitope of gag in BALB/c mice 2 weeks and 6 months after a single immunization with 106 IU of RPK-8129 VLP.
Bottom: BALB/c mice vaccinated twice with 106 IU of RPK-8129 VLPs were challenged 10 months later with 106 PFU of recombinant vaccinia virus encoding gag. Vaccinia virus titers in ovaries were measured at day 4 after infection. All control mice were mock immunized with PBS.
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