A non-replicating adenoviral vector as a potential HIV vaccine.

Abstract

HIV-specific T cell immune responses will play an important role in any HIV vaccine paradigm. Studies in rhesus monkeys have shown that significant and persistent virus-specific T cell responses can be elicited with vaccines incorporating viral genetic sequences and that these responses are primarily mediated by CD8 T cells. Benefits such as stable CD4 levels and viral control have resulted. Two vaccine candidates developed by Merck and Co., Inc., including a non-replicating adenoviral vector, have been studied in animals and are now being studied in Phase I clinical trials in humans. Important considerations include crossclade reactivity (effectiveness in diverse HIV-infected populations), tolerability, and durability of response. Ongoing studies are looking at responses in both uninfected and infected individuals. Optimal vaccine combinations as well as the development and testing of vaccines with multiple genetic targets are part of future plans investigating this vaccine strategy.

Presentation Summary

The first speaker in the afternoon session was John Shiver, PhD, a researcher with Merck and Co., Inc. His presentation, "A non-replicating adenoviral vector as a potential HIV vaccine," began with an outline of the clinical and immunologic goals of the Merck vaccine research program. Clinical goals include decreasing the likelihood of persistent virus infection and establishing a clinically significant lower viral load subsequent to infection. Immunologic goals include eliciting HIV-l-specific CD8 (cytotoxic or CTL) and CD4 (helper) T cell immune responses and directing a broad response against multiple viral determinants in the infected host. Shiver's group has focused on the cellular immune response for several reasons. First, research has generally shown that neutralizing antibody responses are weak and virus type specific. Second, the work of several groups demonstrates that control of initial viremia following infection in humans correlates with the detection of anti-HIV CTL responses, but not with antibodies. Finally, experimental data from studies in rhesus monkeys infected with Simian Immunodeficiency Virus (SIV) indicate that viral control is a function of the antiviral CD8 "F cell response.

Merck is currently investigating 2 vaccine candidates that encode codon-optimized HIV-1 gag, pol, and nef genes based on consensus sequences. The first candidate is a plasmid DNA vaccine encoding flail-length p55 gag (no poll that is delivered in saline intramuscularly with or without adjuvant-alum or a "CRL1005" polymer. A 5-mg dose that uses the CRL 1005 adjuvant consistently showed the best results, with CD8 T cell responses in particular, in rhesus monkeys immunized at 0, 4, and 8 weeks. The second vaccine candidate uses a non-replicating or "replication-defective" adenovirus type-5 vector (Ad5) containing an optimized gag sequence. In several studies, a dose of 10 (11) virus particles (vp) or 24 [mu]g of adenovirus protein, which appears to be the upper limit of tolerability in the animals studied, elicited strong anti-gag responses in blood samples from animals vaccinated at 0 and 24 weeks. In addition, CD8 T cell responses (interferon-[gamma] production) were much stronger than CD4 responses. The Merck group also has looked at a "prime/ boost" strategy using these vaccines. The best T cell responses were found when the DNA/adjuvant vaccine was used first as a primer, followed by the adenovirus vaccine later on as a boost. One additional consideration is pre-immunity to Ad5, which can lower T cell responses when the Ad5 vaccine is used; Ad5 immunity is fairly common in humans.

In another group of studies, the researchers have used several vaccine groups in rhesus monkeys infected with a chimeric HIV-SIV virus, known as SHIV. All vaccines encoded the same codon-optimized SIV p55 gag gene. Envelope (env) was specifically excluded to separate the contribution of neutralizing antibodies, even for priming, versus the challenge virus. In each animal, a challenge with SHIV occurred 3 months after the last immunization. In the animals given DNA vaccines, a transient loss of CD4 cells occurred (lymphopenia) that was not seen in animals given adenoviral vector vaccines. Viral control was best in the animals given the Ad5 and DNA+CRL1005 vaccines. In addition, animals given the Ad5 vaccine have shown stable CD4 T cell levels and viral control out to 2 years after SHIV challenge (see Figure).

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In humans, one important consideration for making an effective vaccine is genetic variability of HIV. Epidemiologic comparison studies show that the viral genes gag, nef, and pol are the most conserved across the various viral clades found in different parts of the world. Therefore these genes represent major targets of T cell immune responses in HIV-infected humans. In addition, there seem to be substantial cross-clade T cell responses for gag and nef in humans.

The second part of Shiver's presentation was an update on Merck's vaccine clinical program looking at vaccine candidates in Phase I study in HIV-infected and uninfected human subjects. At this meeting, he was able to present data on the HIV-1 gag-expressing DNA vaccine with just saline (no adjuvant) and on the HIV-1 gag-expressing Ad5 vaccine in uninfected humans. For the DNA vaccine group (n=109), injections of placebo (n=24), 1 mg of vaccine (n=42), or 5 mg of vaccine (n=43) were administered at 0, 4, 8, and 26 weeks. The vaccine was generally well tolerated with some injection site tenderness and a few complaints of headache and muscle aches. The best responses were seen in the group given the 5-mg dose of vaccine, but even at week 30, fewer than half (42.1%) of the subjects in that group had gag-specific T cell responses. In the Ad5 vaccine studies, uninfected volunteers were given doses at 0, 4, and 26 weeks with placebo (n=22) or different concentrations of viral particles: [10.sup.8] (n=17), [10.sup.9] (n=16), [10.sup.10] (n=24),and [10.sup.11] (n=26). The vaccine was well tolerated overall, but adverse events (mild or moderate injection site pain, fever less than 102[degrees]F often with malaise or chills lasting about 24 hours) occurred at higher doses, with fewer symptoms after subsequent injections (rechallenge). In general, about two-thirds of the vaccinated subjects had significant anti-gag T cell responses. Overall, this preliminary Phase I clinical data from uninfected human subjects show that the Ad5 vaccine is more immunogenic (67% responders across all doses) than the DNA vaccine and that cross-clade anti-gag responses can be induced using these vaccines.

Shiver concluded by pointing to the next steps in Merck's vaccine research program. First, the researchers are working to complete the current clinical studies to allow selection of the best vaccine combination for continued trials. Second, work is underway to introduce additional vaccine components, such as pol and nef Third, the clinical trials will be expanded internationally and vaccine evaluations will be continued in HIV-infected subjects.

Further Reading

Barouch DH, Fu TM, Montefiori DC, Lewis MG, Shiver JW, Letvin NL. Vaccine-elicited immune responses prevent clinical AIDS in SHIV(89.6P)-infected rhesus monkeys. Immunol Lett. 2001;79(1-2):57-61.

Barouch DH, Craiu A, Santra S, et al. Elicitation of high-frequency cytotoxic T-lymphocyte responses against both dominant and subdominant simian-human immunodeficiency virus epitopes by DNA vaccination of rhesus monkeys. J Virol. 2001;75(5):2462-2467.

Caulfield MJ, Wang S, Smith JG, et al. Sustained peptide-specific gamma interferon T-cell response in rhesus macaques immunized with human immunodeficiency virus gag DNA vaccines. J Virol. 2002;76(19):10038-10043.

Shiver JW, Fu TM, Chen L, et al. Replication-incompetent adenoviral vaccine vector elicits effective anti-immunodeficiency-virus immunity. Nature. 2002;415(6869):331-5.

Zhang ZQ, Fu TM, Casimiro DR, et al. Mamu-A*01 allele-mediated attenuation of disease progression in simian-human immunodeficiency virus infection. J Virol, 2002;76(24): 12845-12854.

Shiver, John PhD

Merck & Co., Inc.