Important note: Information in this article was accurate in October 2001. The state of the art may have changed since the publication date. IAVI Report - October - November 2001
Patricia Kahn and Ian Grubb
One of the field's major new meetings, "AIDS Vaccine 2001" was held in Philadelphia on 5-8 September 2001 and attracted a crowd of over 1,000 attendees. Co-sponsored by several US NIH entities together with UNAIDS, the US CDC and the French ANRS, the conference featured a packed program on topics ranging from vaccine design and basic virology to plans for clinical trials. Here we present selected scientific highlights; for coverage of Phase III vaccine trials, see the VaxGen, Thailand and HVTN reports in this issue.
Last year, Dan Barouch, Norman Letvin and colleagues at Harvard helped dispel some of the pessimism surrounding AIDS vaccines when they published encouraging results from a study of vaccinated macaques (Science 2000 Oct 20;290(5491):486-92). In Philadelphia, Barouch presented longer-term follow-up data from these experiments (600 days, compared with the original 140), which are evaluating protection by an HIV-DNA vaccine given with IL-2 (a cytokine that enhances T-cell responses) in animals challenged with the pathogenic HIV/SIV chimera SHIV89.6P.
All four macaques given the HIV-DNA/ IL-2 plasmid continued to preserve their CD4 counts and control viral replication. One animal given DNA plus IL2 protein had a late breakthrough of viremia after 300 days, with a decline in CD4 count and onset of simian AIDS. [Note added in proof: This proved to be due to Nature 2002 Jan 17;415(6869):335-9 and article at www.aidsinfonyc.org/tag]. Two animals that received the DNA vaccines alone developed signs of AIDS during this time period, and one of these animals died at day 500 after challenge.
In contrast, four of eight control monkeys died within the initial 140 days, and two additional control animals died by day 500 after challenge. Searching for correlates of continued viral load control, preservation of CD4 counts and clinical health, the researchers found that proliferative responses to SIV-Gag were the strongest predictor (with the highly significant p value of 0.0001), suggesting that T-cell help is crucial for long-term control of viremia in this model.
Some researchers have criticized the use of SHIV89.6P as a challenge virus based on the suspicion that, once initially controlled, the virus is unlikely to rebound and cause disease (though Barouch mentioned two cases of late progression in his talk, which represented longer follow-up than previously reported). These criticisms were aired publicly in a much talked-about Newsday article by Laurie Garrett, published on the first day of the Philadelphia conference.(http://www.aegis.com/news/newsday/2001/ND010901.html). Countering the argument that SHIV89.6P is "too easy" to protect against, Barouch also presented 600-day follow-up data from a previously published study ( J Virol 2000 Aug;74(16):7485-95) showing similar control of viral loads in gag-DNA vaccinated monkeys challenged with SIVE660, a strain which induces a more AIDS-like disease and which many researchers consider a more stringent challenge. To shed further light on this issue, researchers at Merck are collaborating with Letvin to test the ability of their DNA/adenovirus-based vaccine to protect against several different challenge viruses.
Based on these findings, Barouch, Letvin and colleagues are now moving the DNA/IL-2 plasmid vaccine into human studies. NIH-sponsored Phase I trials are planned through the HIV Vaccine Trials Network (HVTN 044) and the Vaccine Research Center (VRC 003). Dose-escalation protocols will examine the safety, tolerability and immunogenicity of the DNA/IL-2 construct. (The cytokine is approved for use in treating renal carcinoma, but in protein rather than DNA plasmid form.)
Harriet Robinson (Emory University, Atlanta) gave an update on another DNA-based vaccine approach moving towards clinical trials. In a widely publicized study published earlier this year (Science 2001 Apr 6;292(5514):69-74), Robinson, Bernard Moss (NIAID) and colleagues analyzed a prime-boost combination designed to induce the broadest possible T-cell responses: a DNA construct containing eight viral genes (SIV gag, pol, vif, vpx and vpr and HIV-1 env, tat and rev) and an MVA vector (modified vaccinia virus Ankara strain) with HIV env and SIV-gag and pol. Monkeys were divided into four groups of six animals each, plus four controls, and immunized with DNA at different doses and routes, then boosted with MVA at 24 weeks and challenged mucosally with SHIV89.6P seven months later.
Presenting a more detailed ELISPOT analysis of pre-challenge T-cell responses than appeared in the published paper, Robinson reported that, on average, the macaques responded to one CD4 T-cell epitope per 105 amino acids of vaccine-encoded protein, and one CD8 epitope per 238 amino acids. She also observed that "if we had truncated any portion of our Gag-Pol construct we would have compromised the ability of some animals to mount a T-cell response."
Follow-up is now out to one year post-challenge, and all animals in the high-dose DNA and low dose i.d. DNA groups remain healthy, maintain high CD4 counts and are controlling viral load to around 1,000 copies or less. One animal in the low-dose i.m. DNA group progressed to AIDS and died since the Science paper appeared, while the other five remain healthy. Control animals all showed precipitous declines in CD4 counts, and three of the four died by 23 weeks.
Robinson and colleagues also investigated the importance of including env in the DNA/MVA vaccines. Using the same immunization and challenge schedule, an additional group of 12 animals was given vaccine lacking env and two more animals were added to the controls. Four of the 12 failed to effectively control viral load, and all 12 showed CD4 declines after challenge; CD4 counts later showed a partial recovery but achieved pre-challenge levels in only one of the 12 animals. Viral loads have remained high in the four animals that failed to control virus. One of the two additional controls has succumbed to AIDS. Robinson noted that the absence of env did not affect T-cell responses to Gag.
Lastly, working with Moss's group and Janet McNicholl's laboratory (CDC, Atlanta), the researchers tested whether a gp120 boost given twice (with the second DNA and the rMVA inoculations) enhanced protection. Perhaps surprisingly, they found that, whereas all eight of the non-gp120-boosted animals controlled the challenge virus, three of eight gp120-boosted animals did not. While the gp120 boosts raised pre-challenge antibody responses as measured by ELISA, they did not increase the levels of post-challenge ELISA or neutralizing antibodies. Preparations are now underway for moving into Phase I clinical trials. Robinson's group is preparing a DNA vaccine that includes HIV-1 (subtype B) gag, pol, env, vpu, tat and rev, while Moss's laboratory is producing a matched rMVA booster with gag, pol and env. Both are expected to enter Phase I trials through the HVTN in 2002. In addition, a collaborative project with Sal Butera, Dennis Ellenberger and John Nkengasong of the CDC is developing multiprotein DNA and MVA vaccines based on subtype AG isolates from the Ivory Coast, where the CDC Project Retro-CI has HIV laboratory facilities that could support future clinical trials. The Ivoirian government has expressed interest in helping to test promising candidate vaccines.
Bette Korber from Los Alamos National Laboratory took the audience on a whirlwind tour through a forest of HIV-1 genetic trees, stopping along the way to raise points relevant to vaccine design. Looking first at the variability of HIV proteins across clades, Korber pointed out that some of them are highly conserved, including integrase, p24, reverse transcriptase and protease. This relative conservation appears linked to the number and location of T-cell epitopes within the protein: these proteins have epitopes distributed throughout, while the more variable HIV proteins (e.g., Env, Nef and p17) tend to have epitopes clustered in the least variable regions. Another factor influencing the location of T-cell epitopes is the presence of certain amino acids that fit poorly into the C-terminal F pocket of class I HLA molecules. Patterns such as these allow immunogenic regions to be predicted, with Korber's methodology identifying 9 of 11 known epitopes in Rev, Tat and Vif.
Hypothesizing that these epitope-rich regions may be critical to include in vaccine constructs, Korber also discussed how to select vaccine strains that achieve the broadest possible compatibility across clades. Focusing on epitope-rich regions among clade C HIV-1 isolates, her team asked whether it is better to base vaccines on consensus sequences derived from multiple clade C isolates or on a single "reference strain." They found that the difference between any two reference strains is typically twice as large as that between a consensus sequence and any one reference strain (usually around 5%). Studying multiple subtype C isolates, Korber also reported that, for the epitope-rich regions, picking consensus sequences originating in a specific country (e.g., Botswana, Brazil, South Africa or India) had only a "marginal" effect on divergence from the overall clade C consensus sequence. The important implication is that designing vaccines from a single reference strain may be far less effective than using a consensus sequence either for that country or for the entire clade.
Looking at the issue of cross-clade compatibility, Korber compared key sites within Gag and env (where cleavage into class I epitopes occurs). The encouraging results: a good correlation between clade B and C sequences, an even better one among clade C reference strains; and closer again between clade C consensus sequences and reference strains. In contrast, Korber predicted that clade B and C envelope proteins may have different folding (and therefore antigenic) properties, which argues for using clade-specific immunogens when aiming to induce antibody responses.
Chris Locher (University of California at San Francisco) reported on a DNA vaccine produced by Vical, Inc. and tested in a novel baboon (Papio cynocephalus hamadryas) model. The challenge virus was an HIV-2 isolate that causes AIDS in this monkey species over 3-7 years, derived from a West African individual with symptomatic HIV-2 infection and passaged just once in baboons. Locher's DNA vaccine construct encodes HIV-2 tat, nef, p55-gag, and gp140 env, with or without additional genetic adjuvants (1 mg of DNA encoding the cytokine GM-CSF and the co-stimulatory molecule B7-2). GM-CSF is thought to mobilize antigen-presenting dendritic cells, while B7-2 interacts with the CD28 molecule on T-cells to enhance activation.
The investigators immunized four animals with HIV-DNA alone and four with HIV-DNA plus genetic adjuvants, while two controls received an empty DNA vector and two received adjuvants only. Immunizations were given at months 0, 1, 2 and 6 via multiple routes (i.m., i.d. and intranasally using an Accuspray device), and baboons were challenged vaginally one month later with 100 baboon infectious doses (BID) of HIV-2.
Based on still-limited post-challenge data, Locher found that addition of genetic adjuvants to the vaccine enhanced virus-specific cytotoxic T-lymphocyte (CTL) activity compared to DNA alone, particularly in the CD4+ T-cell population. (Virus-specific CD4+ CTLs have been reported for several human viruses, including herpes simplex, Epstein-Barr and HIV; see J. Virol. 2001;75:9771. All four animals in the vaccine plus adjuvant group, and one animal given DNA vaccine alone, cleared the infection within three weeks, as measured by PCR, while HIV-2 DNA remained detectable in all control animals. Follow-up is continuing to monitor CD4 counts and signs of disease progression.
George Makedonas (McGill University, Montreal) reported on a small cohort of HIV-exposed but uninfected individuals whose risk factor is injection drug use (IDU). Twenty-eight individuals with documented exposures to HIV (through needle sharing with a positive partner or partners) were enrolled. Eighteen individuals were persistently seronegative (group 1) while ten seroconverted within three months of study entry (group 2). Analysis of blood samples taken prior to seroconversion revealed that none of the individuals in group 2 showed evidence of HIV-specific CD8 T-cells (as measured by ELISPOT), similar to a low-risk control group. In contrast, 12/18 individuals from group 1 had T-cell responses to one or more class I-restricted HIV peptide(s). Follow-up of 11 volunteers is continuing (ranging from 84-722 days thus far); despite an average of eight new exposures to HIV (range 1-50), no seroconversions have occurred. Monoclonal Antibody Protection Dennis Burton (Scripps Institute, La Jolla) reviewed data that might illuminate ways to enhance antibody-mediated defenses against HIV. The study built on previous findings that treatment of monkeys with rare monoclonal antibodies capable of neutralizing HIV can offer dose-dependent protection against vaginal challenge (J Virol 2001 Sep;75(17):8340-7). Analyzing the data, Burton calculated that antibody titers on the order of 1:400 were required to achieve complete neutralization of the challenge virus and full protection. But Burton pointed out the unlikelihood that even effective, licensed vaccines achieve such complete neutralization, since a "good" antibody titer is typically considered to be 1:40. So he speculated that these vaccines work at less-than-optimal neutralizing antibody titers because they also stimulate cellular immune responses—a "major unknown," he said, and a subject of ongoing studies in his group.
Burton has also studied the molecular structure of the b12 antibody to elucidate how it interacts with HIV. The work has revealed a long "finger-like" structure that extends into the CD4 binding site of gp120, overcoming the notorious inaccessibility of this region to antibodies. The next challenge is to design an immunogen that incorporates this structural information and might induce antibodies which mimic b12's neutralizing capabilities.
011010
IAVI2001-1009
©2001. The IAVI Report.
AEGiS is made possible through unrestricted grants from Boehringer Ingelheim, iMetrikus, Inc., John M. Lloyd Foundation, the National Library of Medicine, and donations from users like you. Always watch for outdated information. This article first appeared in 2001. This material is designed to support, not replace, the relationship that exists between you and your doctor.
AEGiS presents published material, reprinted with permission and neither endorses nor opposes any material. All information contained on this website, including information relating to health conditions, products, and treatments, is for informational purposes only. It is often presented in summary or aggregate form. It is not meant to be a substitute for the advice provided by your own physician or other medical professionals. Always discuss treatment options with a doctor who specializes in treating HIV.
Copyright ©1990, 2001. AEGiS. All materials appearing on AEGiS are protected by copyright as a collective work or compilation under U.S. copyright and other laws and are the property of AEGiS, or the party credited as the provider of the content.
