Highlights of AIDS Vaccine Meeting
The Conference on Advances in AIDS Vaccine Development: 1996 drew 750 people to Natcher Conference Center on the NIH campus February 11 to 15. NIAID Director Anthony S. Fauci, M.D., presented a special lecture the first full day, and plenary speakers, workshops and brief talks on selected posters rounded out the program. Meeting highlights follow, except those describing international efforts to develop HIV prevention strategies.
Government and Industry Cooperation Stressed
Opening speaker Patsy Fleming, director of the White House Office of National AIDS Policy, praised Congressional leaders for recognizing the importance of biomedical research and increasing the NIH budget overall. But she noted that research progress depends on more than just adequate resources, and that the government cannot by itself develop vaccines or drugs for AIDS.
Therefore, she said, the Administration had planned an unprecedented meeting between leaders of the pharmaceutical industry and government scientists "to discuss ways to accelerate the search for new treatments, a cure and a vaccine, and for prevention methods -- including microbicides." The meeting, led by Vice President Al Gore, took place on February 20.
Another opening night speaker, Dino Dina, M.D., president of Chiron Biocine of Emeryville, Calif., echoed Ms. Fleming's call for better relations between government and industry.
He said the private and public sectors must develop mutually agreed upon development plans with timelines and criteria for moving experimental vaccines from phase I to phase II and, ultimately, to phase III clinical trials. Dr. Fauci expanded on the theme of government-industry cooperation the next morning when he outlined NIAIDs new strategy for negotiating vaccine development plans with industrial partners.
HIV RecombinationCA Potential Complication for Vaccine Development?
The rapid mutation rate of HIV-1 has resulted in a genetically diverse mix of strains worldwide. To help make sense of this diversity, scientists have devised a genetic tree that groups the known HIV-1 isolates into the major (M) viruses, consisting of at least nine subtypes, A-I, and the outlier (O) viruses, consisting of at least two distinct subtypes. Isolates are grouped together based on the degree of similarity among the amino acids in their envelope genes.
This scheme has been complicated by new evidence suggesting that recombination between subtypes existing in the same geographic area occurs frequently, contributing more to HIV-1's genetic diversity than previously thought.
Beatrice Hahn, M.D., of the University of Alabama at Birmingham and her colleagues analyzed the 1995 database of HIV-1 genomes of viruses taken from patients worldwide. Following earlier results from Thailand, Dr. Hahn found that E viruses from both Thailand and the Central African Republic actually are A/E recombinants. The heterosexual epidemic in Thailand, commonly attributed to transmission of E virus, is therefore caused by a mosaic virus that Dr. Hahn said likely evolved from a single recombinant ancestor virus from Africa. Their analyses also revealed that all currently known G viruses are actually G/A or G/H recombinants.
Dr. Hahn concluded that recombination is common and may influence the transmissibility and pathogenicity of these viruses.
Is It the Assay or the Vaccine?
Debate continues about whether the inability of HIV vaccines to neutralize human virus isolates has been a failure of the vaccines themselves and/or the tests used to measure virus neutralization.
Susan Zolla-Pazner, Ph.D., of the Veterans Affairs Medical Center in New York City, described a new "resting cell assay" that has yielded promising neutralization results. Unlike conventional assays, which use cell lines or mitogen-activated T cells, the resting cell assay uses unstimulated peripheral blood mononuclear cells as target cells.
With this new test, neutralizing antibody against a subtype B primary isolate could be found in the sera of 10 of 16 people immunized with the Genentech MN recombinant gp120 vaccine. The assay also detected significant cross-reactive neutralizing antibody to a subtype B primary isolate at multiple time points in three chimpanzees primed with an adenovirus vector vaccine and boosted with Chiron Biocine's SF-2 recombinant gp120 vaccine. All tests were performed using panels of coded sera provided by NIH and Genentech. The studies must now be extended to determine if the immune response generated by these vaccines is capable of neutralizing many or only a few primary isolates.
In a related talk, James E. K. Hildreth, Ph.D., M.D., of The Johns Hopkins Medical School showed that neutralization of both laboratory and clinical isolates of HIV can be increased by orders of magnitude by adding monoclonal antibody against the adhesion molecule LFA-1. These findings, said Dr. Hildreth, implicate adhesion molecules in the resistance of clinical isolates to antibody neutralization.
A Novel Approach to Attenuation
In 1992, scientists first reported complete protection in monkeys against simian immunodeficiency virus (SIV) using a live-attenuated vaccine made by deleting the nef gene. However, further research showed that the vaccine caused disease in newborn macaque monkeys, all of which subsequently died. Since then, some studies have found that modifying the vector or lowering the dose can improve the vaccine's safety.
Tackling the problem in a different way, Tilahun D. Yilma, D.V.M., Ph.D., of the University of California Davis, described experiments with a nef-deleted SIV vector vaccine into which he and his colleagues inserted the gene for gamma interferon.
Juvenile macaques vaccinated with the modified vaccine had significantly lower or undetectable viral loads and reduced viral persistence compared with those animals vaccinated with the original nef-deleted vaccine. Both groups were challenged intravenously with a highly infectious dose of pathogenic SIV six months post-vaccination. Compared with the macaques vaccinated with the original nef-deleted vaccine, those vaccinated with the modified vaccine had reduced virus loads and greater resistance to challenge. In addition, the construct was not pathogenic to newborn macaques inoculated orally at 12 and 24 hours after birth. According to Dr. Yilma, these macaques had transient, low-titer viremia that cleared in less than six weeks, and they have remained healthy for 28 weeks.
Progress In DNA Vaccine Research
Stephen Johnston, Ph.D., Ph.D., of the University of Texas-Southwestern Medical Center in Dallas and several other speakers addressed progress in investigations of genetic, or DNA, vaccines, a new approach that attempts to preserve the immunogenicity of an attenuated HIV vaccine without having to use live virus. Dr. Johnston and his colleagues have developed a method to systematically break down a pathogen into its protective epitopes.
Rather than incorporating whole HIV genes into DNA vaccines, they first divided both HIV and SIV genomes into 32 to 34 random DNA fragments. Each fragment was then cloned into various vectors that can direct the fragment to specific locations in the cell. The investigators subsequently injected these vectors into mice and characterized the immune responses elicited. Various combinations of these sub-gene vaccines can now be tested for protective efficacy. The Dallas group will make these sub-gene libraries available to other researchers on request.
In separate talks, Susan Barnett, Ph.D., of Chiron Biocine and Deborah Fuller of Agracetus, Inc., in Middleton, Wis., described experiments in mice, guinea pigs and monkeys that demonstrate the feasibility of using DNA vaccines with an rgp120 vaccine made by Chiron Biocine in a prime-boost vaccine strategy. Priming with DNA yielded strong, durable cytotoxic T lymphocyte (CTL) responses in mice and moderate antibody responses in all animals. A single rgp120 boost given to the same animals substantially elevated antibody titers. The studies in rhesus macaques described by Ms. Fuller using gene-gun based DNA vaccines were conducted in cooperation with Michael Murphey-Corb, Ph.D., at the Tulane Regional Primate Center in Covington, La.
David Weiner, Ph.D., of the University of Pennsylvania in Philadelphia discussed his group's work in primates using a multi-plasmid cassette system for DNA immunization. Immunization of infected chimpanzees with the cassette induced boosting of immune responses and a decrease in viral load. Immunization of three naive chimpanzees induced high neutralizing antibodies in one and low neutralizing responses in the others; the lowest antibody responder developed the highest CTL responses.
He also reported on the safety of an ongoing therapeutic DNA vaccine trial at the University of Pennsylvania (see AIDS Agenda, September 1995). There have been no serious adverse side effects in the 15 volunteers immunized to date. Finally, he announced that the Food and Drug Administration will allow the first HIV DNA vaccine trial in healthy uninfected volunteers to go forward at the NIH Clinical Center using an HIV DNA vaccine developed by his group in collaboration with Apollon, Inc. of Malvern, Pa.
Clinical Trials Results
Robert B. Belshe, M.D., of the St. Louis University Health Sciences Center AIDS Vaccine Evaluation Group (AVEG) site, presented an overview of candidate HIV vaccines available for expanded clinical trials. Since 1988, he said, AVEG has tested more than 15 vaccines in some 25 trials involving about 2000 volunteers.
From the results of these trials, he drew the following general conclusions: 1) after three doses, as a group, gp120 vaccines induce more neutralizing antibody than gp160 vaccines but gp160 vaccines induce more vigorous proliferation of lymphocytes; 2) subunit vaccines made in mammalian cells perform better than those made in yeast or baculovirus systems; and 3) more antigen stimulates better immune responses than a smaller quantity of antigen.
Dr. Belshe said that using a live vector virus vaccine primer and a recombinant gp120 booster has shown the most promising results of any vaccine strategy tested. This strategy induces both CD8+ CTLs and neutralizing antibodies, whereas the live vector vaccines alone primarily induce CTLs and the subunit vaccines alone primarily induce neutralizing antibodies.
Pasteur-Merieux-Connaught's canarypox (tradename ALVAC) vaccines are currently being tested in three prime-boost phase I AVEG trials. Early results from two trials were presented by Kent Weinhold, Ph.D., of Duke University and AVEG's Central Immunology Laboratory, reporting on a study known as AVEG 012; and Lawrence Corey, M.D., of the University of Washington AVEG site, study chair of AVEG 022.
Both studies use Chiron Biocine's SF-2 gp120 vaccine as the booster immunogen. For the primer doses, AVEG 012 is evaluating two different doses of ALVAC-HIV (vCP125), the gp160 gene spliced into the canarypox vector. AVEG 022, begun last May, is evaluating ALVAC-HIV (vCP205), a more complex antigen consisting of the canarypox vector into which the genes for envelope, gag and protease have been inserted. A third trial, AVEG 026, is testing ALVAC-HIV (vCP300). It contains the same genes as vCP205 but in addition, CTL determinants in the pol and nef genes in order to elicit an even broader response.
The vCP125 and vCP205 vaccines both appear to be safe and so far have induced, to varying degrees, CTL and antibody responses. Higher doses of the ALVAC vaccines are more effective than lower doses at stimulating CTLs. Having previously received a smallpox vaccination does not influence responses to canarypox vectors.
Preparation for Prevention Trials in the United States
Eight domestic sites -- two in New York and one each in Seattle, Denver, San Francisco, Chicago, Boston and Philadelphia -- have enrolled 4,884 people at high risk of HIV infection in preparatory studies for HIV prevention trials. The NIAID-sponsored Vaccine Preparedness Study, part of HIVNET, was described by Cladd Stevens, M.D., of the New York Blood Center and David Metzger, Ph.D., of the University of Pennsylvania in Philadelphia, both principal investigators.
Dr. Metzger said their goals are to design and implement prevention studies in preparation for phase III vaccine efficacy trials. Specifically, the objectives of HIVNET are to enroll and retain cohorts of volunteers at high risk of HIV infection; to carefully monitor risk behavior and early HIV infection; to evaluate strategies for insuring informed consent; to engage target communities as active participants in the planning of prevention trials; and to evaluate behavioral interventions that could augment future vaccine trials. Several studies are in progress or ready to start, including phase I trials of topical microbicides.
About 77 percent of the participants have expressed a willingness to enroll in a vaccine trial. The researchers expect that the volunteers required for an intermediate or large-scale efficacy trial could be recruited from these populations. Dr. Stevens said this group may also join with the AVEG in conducting phase II clinical vaccine trials.
Intermediate Trial Design
In a separate talk, HIVNET statistician Tom Fleming, Ph.D., from the University of Washington explained the concept of intermediate trials of prevention strategies. Such trials would occur between a phase II and phase III trial and would provide "a less expensive option to move ahead in those situations where there is more uncertainty." He said they also would be useful for screening microbicides to determine which should be advanced into phase III efficacy trials.
Assuming a population with a 2 percent annual rate of new HIV infections, an intermediate trial would require 3,000 volunteers, half the number of a definitive large-scale trial, and would require two years versus three-and-a-half years, said Dr. Fleming.
Scientists acknowledge that it may be unrealistic to expect a single prevention strategy to completely protect against HIV. However, this year's meeting presented encouraging evidence that combining the best available prevention strategies -- intervening perinatally using immunologic interventions and drugs like HIVIG and AZT, thwarting HIV and other STDs with microbicides or other drugs, and stimulating protective immune responses with vaccines -- could provide an effective solution to controlling AIDS.
--Laurie K. Doepel