Published in the Bulletin of Experimental Treatments for AIDS April 1998 issue, by the San Francisco AIDS Foundation.

April 1998 Table of Contents

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DNA and Other Technologies for HIV Vaccines

Mark Bowers

DNA-based vaccines are the newest technology that may hold the key to controlling the spread of HIV and other infectious diseases. All of an organism's genetic information is stored in DNA. Because of technological advances, it has become possible to find and clone stretches of DNA that contain the instructions for producing specific molecules useful in immunizations. Short stretches of viral DNA are selected because they do not cause disease and because they represent the whole virus to the immune system. These DNA instructions are then inserted into plasmids, small circular constructs of bacterial DNA. These plasmids can enter human cells and immediately begin to produce the target molecules. They are introduced into the person receiving vaccination by either attaching them to gold beads and propelling them through the skin using a "gene gun" or by mixing them in saline and injecting them into muscle through a hypodermic needle. It is human cells that actually make the viral protein and mobilize an immune response.

These molecules stimulate the immune system to produce antibodies and cellular (T-cell) responses that may prevent or control infectious diseases such as HIV disease. DNA technology may provide the answer to the urgent demand for an HIV vaccine that is safe, inexpensive and easy to produce. In animals, DNA vaccines have already achieved protective immunity for diarrhea-causing viruses, bacteria that cause tuberculosis and parasites that cause malaria. Will the new technology be equal to the challenge of HIV?

5 Ways to Make a Vaccine

There are now 5 technologies for making vaccines: live attenuated (weakened) vaccines, killed whole viruses, purified component vaccines (not currently used to make HIV vaccine candidates), genetically engineered vaccines and DNA vaccines. Each technique has produced a small number of successful vaccines, but there are many human diseases, including HIV disease, for which no vaccine exists. Following are brief descriptions of current efforts to harness each method to produce an effective HIV vaccine.

Live attenuated vaccines

Live attenuated vaccine technology was the first route to successful vaccination, pioneered by Edward Jenner in the 18th century. This technology led to the eventual successful eradication of smallpox and containment of polio in the developed world. The possibility of testing a live attenuated HIV vaccine candidate garnered media attention last summer when the International Association of Physicians in AIDS Care (IAPAC) announced that they had recruited 50 potential volunteers for a vaccine study from among the physician and activist communities. The wisdom of pursuing such a course has been hotly debated, despite the obvious obstacle that no live attenuated HIV vaccine candidate yet exists.

Several companies are seriously considering developing live attenuated HIV as a vaccine candidate because infection with an attenuated strain of HIV is believed to be the reason that some HIV-infected people are long-term non-progressors. Research is currently underway at the Macfarlane Burnet Centre in Australia to develop a vaccine candidate based on a mutant strain of HIV that infected 9 Australians as long as 17 years ago. Six of the original 9 are long-term non-progressors, 2 died in their eighties, presumably of old age, and 1 elderly woman with severe systemic lupus erythematosus (an immune system disorder unrelated to HIV) died of Pneumocystis carinii pneumonia (PCP).

Debate about the underlying cause of this woman's death hinges on the fact that lupus is characterized by general immune activation, not restricted to the loss of T-cells. She had been on prolonged therapy with prednisone, an immunosuppressive drug sometimes used to combat the immune activation and autoimmunity characteristic of systemic lupus. Blood samples from the period prior to her death were not retained for later evaluation, so it is a matter of speculation and debate whether HIV played a role in her development of PCP infection and in her subsequent death, or if prednisone and lupus completely account for her death.

The strain of virus that was transmitted to these 9 individuals lacks a large segment of HIV's nef gene, a lack which apparently limits the damage the virus can inflict on the immune system. A vaccine candidate based on this deletion mutant strain could be available for human testing in 18-24 months, according to John Mills, MD, the Centre's director. Whether this crippled HIV can revert to virulence and cause progressive HIV disease is the wider question raised by the death of the woman described above.

David Baltimore, MD, who heads the AIDS Vaccine Research Committee of the National Institutes of Health, expressed concern about the attenuated virus approach. He cited unpublished results of 2 vaccine studies in monkeys in which some vaccinated animals went on to develop disease, given enough time. For Baltimore, live attenuated viruses have not yet been proven safe enough to consider proceeding to human trials.

Therion Biologics of Massachusetts is currently outlining a plan to develop a live attenuated HIV vaccine candidate that the company will present to the Food and Drug Administration (FDA) later this year. The template for this vaccine candidate was supplied by Ronald Desrosiers, MD, a primate researcher at Harvard Medical School. Desrosiers' live attenuated simian immunodeficiency virus (SIV) vaccine has protected monkeys from SIV infection for more than 7 years. Biostratum of Research Triangle, North Carolina is developing a live attenuated vaccine candidate in which HIV genes other than nef have been deleted. Key questions of safety and liability remain to be worked out before human testing can be contemplated in the U.S., despite the existence of an apparently willing cohort of potential volunteers for a trial of a live attenuated vaccine.

Whole killed virus vaccines

Vaccines made from whole "killed" viruses have been useful in preventing disease since the 19th century. The Salk polio vaccine was "killed" (technically speaking, inactivated) chemically with formaldehyde. Burt Dorman at Acrogen, a small vaccine development company in Oakland, California, is currently updating the technology used to inactivate viruses before seeking permission to test a whole killed HIV vaccine candidate in humans. The strongest argument for a whole killed virus approach is that other strategies do not include the preserved outer coat of HIV (the envelope). The immune system recognizes HIV's outer coat most frequently when it encounters intact, infectious virus, the population of virus that rapidly spreads from cell to cell. Sub-unit vaccines made from parts of the envelope may be insufficient to provoke a broad-based immune response. The immune system may need to see the whole virus in order to respond effectively.

Genetically engineered vaccines

Genetically engineered vaccines have only recently been developed, and none are yet approved for human use. Several HIV vaccine candidates have been developed using recombinant DNA technology. Examples include the gp120 and gp160 sub-unit vaccines that were made of parts of the outer coat of HIV. In 1994, FDA denied the California biotechnology companies that made these candidate vaccines, Chiron and Genentech, permission to progress to Phase III clinical testing. Early clinical study results did not seem promising enough to counterbalance the risks of wide-scale public exposure to the vaccines. Based on strains of HIV that grow well under laboratory conditions, these candidate vaccines failed the crucial test of neutralizing HIV taken from the blood of infected individuals. HIV taken from an infected individual is called a primary isolate. Laboratory strains of HIV have adapted to laboratory cultures and differ from primary isolates in significant ways.

Work on the Genentech candidate vaccine was then moved to a separate but affiliated company, VaxGen, and Donald Francis, MD, continued to seek funding for further testing and for development of a hybrid vaccine candidate called a bivalent vaccine. By early 1998, $20 million was secured from private sources and a 3-year Phase III clinical trial involving 7,500 volunteers is scheduled to begin this year in Thailand and the U.S. Both studies await formal approval from FDA and Thai authorities. In the U.S., confirmatory Phase I and II studies of the bivalent candidate vaccine (AIDSVAX) have already received FDA approval.

A recent series of wire stories and newspaper articles erroneously suggested that FDA had reversed its 1994 decision and was ready to fund wide-scale testing of the VaxGen vaccine candidate. Individual vaccine candidates that VaxGen expects to test soon in the U.S. and Thailand differ from the original candidate and from one another in that a component of each has been created from primary isolates common among infected people in each country. This combined strategy addresses earlier problems with the vaccine, and suggests that human immune responses to these vaccines will be broader and better focused on the different strains of HIV that currently infect people in the 2 target populations.

Therapeutic vaccines

Immune Response Corporation is one of 2 companies that have developed and tested a treatment vaccine specifically for individuals who are HIV infected. Their vaccine candidate is a sub-unit vaccine made from proteins that are found in the inner core of HIV. Currently in Phase III testing, Remune is an immune-based therapy based on whole killed HIV stripped of envelope proteins. The current study is expected to determine whether Remune can delay HIV disease progression by beefing up immune responses in HIV positive individuals with CD4 cell counts between 300 and 549 cells/mm3. Researchers hope to demonstrate that Remune brings forth strong HIV-specific immune responses and beta chemokine production.

AIDS ReSEARCH Alliance in Los Angeles reported earlier this year that a recent clinical trial of Cel-Sci Corporation's therapeutic vaccine candidate, HGP-30, a sub-unit vaccine based on a different core protein of HIV, has shown mostly disappointing results.

Hybrid approaches

Much interest has been generated by a new, non-traditional approach to vectors -- bacteria, other viruses or plasmids that are used to introduce DNA or recombinant vaccines into individuals (a recombinant is a protein made by genetic engineering). Canarypox virus is the vector for several candidate vaccines now in human testing. The advantage of combining a live virus with a genetically modified protein lies in the ability to provide immunity against viruses that cannot be reliably attenuated or inactivated without distorting them so grossly that they no longer resemble live virus.

Early Successes of DNA Vaccines

DNA vaccines have generated excitement since 1993, when the first reports of immune responses to naked DNA vaccines in mice were reported. The ability of one DNA vaccine to prevent malaria in animals boosted research efforts and led to safety trials in human volunteers. Also in 1993, a DNA vaccine developed at Merck Research Laboratories protected mice against lethal doses of influenza A.

Theoretically, these vaccines are successful where others have failed because they awaken a broad immune response, including not only antibodies but also cytotoxic (killer) T-cells that can then seek out and destroy cells that are already infected. In this respect, DNA vaccines operate like live attenuated vaccines, but without the risk. However, the exact mechanism of DNA vaccines is not known with certainty, and theories -- although based on the best knowledge now available -- remain unproven.

DNA vaccines are easier to make and cheaper than other vaccine technologies. A candidate vaccine for malaria, containing a single gene from the organism that causes the disease, took only 3 months to prepare. The important decision is to select the appropriate gene to make a candidate vaccine. Such decisions are frequently made empirically.

Research has shown that the cells that pick up naked DNA genes are usually dendritic cells, which are cells of the immune system that systematically patrol the body on the lookout for foreign antigens. Dendritic cells are antigen-presenting cells that trap foreign substances on their mop-like surfaces and carry them to lymph nodes for further identification and processing.

Research on a DNA vaccine candidate for HIV is being conducted at the University of Pennsylvania. Widely publicized research completed last year involved attenuated HIV genes inserted into a plasmid, mixed with the local anesthetic bupivicaine and injected into 3 chimpanzees. Two of these chimps were then "challenged" (exposed to massive doses of HIV) but both remained free of infection. The third chimp was kept as a negative control and received no challenge. A fourth control chimp who did not receive any vaccination was challenged and became infected. Sensitive laboratory assays were able to detect HIV in the blood of each of the protected chimps, but only briefly. The implication was that the chimps' immune systems were ultimately able to clear the infection. Neither chimp mounted an antibody response. The importance of killer T-cell responses and the relative unimportance of antibodies were strongly suggested by this study. The study left some questions entirely unanswered. Eight booster shots were given to each vaccinated chimp. Are this many boosters needed? If not, how many is enough?

Research at the University of Pennsylvania now includes a dose-ranging study of a plasmid containing DNA for the HIV genes env and rev that contain codes for the envelope and reverse transcriptase enzyme. Fifteen HIV-positive volunteers in this study were given increasingly larger intramuscular injections of the vaccine candidate (30, 100 and 300 micrograms). After 4-8 months, 12 received a 100 microgram booster. No pattern of changes in CD4 cell counts or viral load was noted. The vaccine and booster injections were well tolerated. Comparisons of the effects of route of administration, whether by jet-injection or needle injection, on antibody and cytotoxic lymphocyte responses are now being evaluated.

The Future of Vaccines for HIV

The future of vaccines for HIV is dependent on a host of political, economic and scientific variables. National political will to see the development of an effective HIV vaccine by the year 2007 was expressed by President Bill Clinton; unfortunately, the statement was accompanied by no dramatic increase in direct funding for vaccine research and development. Other political pressures have been exerted by the International Association of Physicians in AIDS Care's ongoing advocacy for testing a live attenuated vaccine candidate soon. Political opposition to the IAPAC position is provided by Nobel laureate Baltimore, who says that the development of an AIDS vaccine is at least a decade away. Expressing a view somewhere between these positions is Anthony Fauci, MD, director of HIV research at National Institute of Allergy and Infectious Diseases. He said last December that empirical decisions will have to be made about testing an HIV vaccine without all the scientific answers being available.

The economics of an HIV vaccine are out of balance as well. Biotechnology firms backed away from HIV vaccine research 3 years ago because the return on initial investments seemed disappointingly small. Meanwhile, small non-profits have channeled needed funds to a multinational study of the VaxGen vaccine candidate. Larger organizations such as the International AIDS Vaccine Initiative (IAVI) will award $30,000 to Ronald Desrosiers, MD, at the New England Regional Primate Center to stu;dy the safety of live-attenuated vaccines in monkeys, more than $400,000 to Macfarlane Burnet Center in Australia to study a DNA-based, live-attenuated vaccine in animals, and almost $500,000 to Dan Farber Cancer Research Institute to develop hybrid viruses. A $4 million grant from Starr Foundation to IAVI increased funds available for additional grants. The AIDS Vaccine Research Committee has targeted specific areas of vaccine research, culminating in 49 widely publicized NIH Innovation Grants totaling $11.8 million, awarded last autumn.

Other international efforts include a collaborative Indian-U.S. vaccine initiative, the Vaccine Initiative Program, that has adopted as its mandate the creation of an HIV vaccine that targets the strain of HIV most prevalent in India. Infrastructure is being developed to study and test candidate vaccines at all phases, including selection of testing sites, special diagnostic kits specific to India and basic research. The rapid spread of HIV in India, where 2-5 million people are already infected in a country of more than 900 million, makes this effort a national priority.

Increased collaborations and private interest and investment will eventually speed the discovery of a successful vaccine for HIV. Questions that remain are when to move to wide-scale testing, as directly addressed by Donald Francis in his urgent efforts to see the VaxGen vaccine candidates widely tested. Increased attention to the problem of HIV variation and construction of an HIV vaccine that provides immunity to a wide spectrum of strains is another recent trend in vaccine research. A related question is whether private industry will significantly contribute to research and development of an HIV vaccine. Current interest at Chiron seems high, and the technology of choice, DNA vaccines, may effectively address concerns about wide supply at low cost. Hybrid vaccine approaches are also receiving increased attention, and new vectors such as live viruses genetically engineered to deliver vaccines are generating great interest.

Scientific interest was generated by David Baltimore's remark at the 5th Conference on Retroviruses and Opportunistic Infections in Chicago in February, that an effective vaccine may call forth more than the 2 recognized types of immune response, humoral and cellular. The third type of immune response he characterized as "superinfection," explaining that a infection with benign strain of virus (such as a weakened or attenuated virus administered as a vaccine) can block later infection with a virulent strain by physically excluding the virulent virus, perhaps through the work of an as-yet-unidentified cell. Since Baltimore heads the AIDS Vaccine Research Committee at the NIH, his remarks are bound to reflect the shape of future basic research in the quest for an AIDS vaccine.

On-line Resources

• International AIDS Vaccine Initiative (IAVI): www.iavi.org
• Office of AIDS Research: www.nih.gov/od/oar
• The DNA Vaccine Web: www.genweb.com/Dnavax/dnavax.html

References

Page last updated 5 May 1998