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The elusive AIDS vaccine

In the 25 years since HIV was discovered, researchers have made many strides in the fight against AIDS. One notable advance was the development of combination therapy for HIV. This has led to prolonged survival for HIV positive people who have access to and who can adhere to these medications.

Although the AIDS crisis is generally considered a problem affecting low- and middle-income countries, particularly in sub-Saharan Africa, HIV continues to spread in the high-income regions of North America and Western Europe. As with many infectious diseases, a vaccine that could prevent the spread of HIV would be ideal. Recognizing this, even in the early days of the HIV epidemic, scientists have pursued vaccine research.

Yet, despite many clinical trials with at least 30 different potential HIV vaccines, all have failed. Years of effort and work are put into the design, creation and testing of a potential vaccine, so to have such a large number of failures is disheartening.

Challenged by complexity

One reason that it is so difficult to find an effective HIV vaccine is that this virus is like no other; it attacks and disables the body’s defenses against germs—the immune system. Moreover, what we call the immune system is really a complex network of organs, tissues and cells scattered throughout the body. How this network functions is not fully understood. Throw HIV into this mix, and you have the makings of a giant puzzle.

The string of failures that litter the AIDS vaccine landscape should serve as a starting point where immunologists might review, reflect upon and reconsider past and current vaccine strategies. Perhaps the time has come to review research that may have been forgotten, overlooked or avoided in the race to an AIDS vaccine. In such an exercise, clues about successful immunity against HIV might emerge and this could incite new research on HIV vaccines.

Below is a quick review of some of the challenges faced by HIV vaccine designers, followed by areas of research that may be useful for a potential vaccine.

With T cells, timing is everything

In cases of SIV (simian immunodeficiency virus) or HIV infection, the immune system’s premier infection-fighting cells—CD8+ T cells—are able to attack virus-producing cells and reduce the amount of virus in the blood. However, these cells are unable to stop the spread of HIV (or SIV) from the mucosa to the lymph nodes and tissues lining the gut, where most T cells reside.

Still, vaccine designers have focused on stimulating CD8+ cells to respond to viral invasion. However, several research teams have found that the immune system takes too long, perhaps two to three weeks, to respond with large numbers of CD8+ cells that can contain HIV infection. Moreover, the few cells that are readily available to fight HIV have a limited capacity to do so. So, vaccines that depend on CD8+ cells to prevent HIV infection may not work, as suggested by experiments in monkeys and in the lab. Other teams of researchers have focused on trying to get HIV vaccines to stimulate antibodies that can attack or neutralize HIV.

Antibodies—too little, too late?

Experiments on monkeys deliberately infected with SIV suggest that any vaccine based on antibody stimulation would need to trigger a massive output of antibodies from the immune system. Such production would have to occur quickly, within six hours of exposure to SIV, to be effective. Also, the vaccine must be able to maintain high levels of neutralizing antibodies for SIV or HIV infection to be controlled.

The production of antibodies or CD8+ cells that can attack a specific germ, such as HIV, relies on one part of the immune system called adaptive immunity. This type of immunity takes time to reach sufficient levels that can control an infection. So, antibody-based HIV vaccines are unlikely to be able to quickly staunch HIV.

What is to be done?

The ideal type of immune response to HIV would be one that could be invoked within seconds or minutes after exposure to this virus. The immune system is capable of responding quickly, as happens in some people who have rapid allergic reactions to certain foods or insect venom. Harnessing such a response for an HIV vaccine might be useful. However, to date, vaccine designers have not concentrated on such approaches.

An ancient defense system

Part of the immune system that evolved many millions of years ago is called innate immunity. These defenses rely on recognizing patterns in the cells of germs that make them appear different from the body’s cells. The immune system can then quickly respond to the invading germs. It is beyond the scope of this report to delve into the complexities of innate immunity. However, this type of immunity in part relies on cells such as natural killer cells, macrophages and dendritic cells. So far, most HIV research has focused on B cells and T cells. Clearly, a different research focus is needed if innate immunity is to be exploited.

Slippery when wet

HIV’s first contact with the immune system is usually in the wet tissues of the mucosa—the anus, penis and vagina—and the mucosal immune system has developed to protect these sites. The immune system in the mucosa behaves somewhat differently from the immune system in the rest of the body, so research to understand how the mucosal immune system interacts with HIV is vital. Unfortunately, no one likes to have samples of mucosal tissue removed for study, and progress uncovering the mysteries of this part of the body has been slow.

Exposed yet seronegative

Scientists in laboratories in Winnipeg, Nairobi, Milan, Toronto and Stockholm have found that a very small number of people who have been sexually exposed to HIV have managed to remain HIV negative. Yet their immune systems, particularly in the mucosa, show traces of having encountered HIV. Research into why they have managed to fight off HIV might come in handy when designing a vaccine.

Not Invented Here

The American National Institutes of Health (NIH) in Bethesda, Maryland, is the largest bio-medical funding and research body in the world. The NIH spends about $600 million dollars every year on HIV vaccine research. Nearly one-third of this goes toward developing and testing potential vaccines.

At the recent American Conference on Retroviruses and Opportunistic Infections (CROI), held February 3 to 6, 2008, in Boston, HIV vaccine research came under scrutiny. Dr. Ron Desrosiers, head of the New England Primate Research Center, highlighted the problems with the current approach toward an HIV vaccine fostered by the NIH. He stated that “no [HIV vaccine] now under development has any reasonable hope of being effective.”

Dr. Desrosiers was not the only prominent researcher to challenge the NIH. Virus expert Dr. Neal Nathason, a former head of the NIH’s office of AIDS research, voiced similar concerns at CROI. Both scientists suggest that rather than continue to test potential vaccines that are unlikely to work more studies need to be done to understand the immune system and how it interacts with HIV. Indeed, readers should note that no research team has any idea as to what is the ideal immune response needed to protect the body from HIV infection.

Meeting in March

In January 2008, Dr. Desrosiers and 13 other researchers wrote a letter to Dr. Anthony Fauci, the head of the National Institutes of Allergy and Infectious Diseases (NIAID, an arm of the NIH). Their letter articulated their concerns regarding the NIH’s research priorities when it comes to HIV vaccine research. As a result, the NIH plans to hold a daylong meeting on March 25, 2008, in Bethesda to consider the scope and balance of its research activities regarding HIV vaccines. Now all eyes in the vaccine world will be looking toward NIAID and this meeting to see if the American research effort on HIV vaccines can be rescued.

Not giving up

Much work remains to be done in understanding the immune system and how to protect it from HIV. Given the complexity of the task, this effort will take many years. Unless there is a breakthrough, an effective AIDS vaccine is not likely to appear for at least a decade, and perhaps much longer. Dr. Anthony Fauci put it this way in a recent report in the New England Journal of Medicine:

“To be brutally honest with ourselves we have to leave open the possibility that we might not ever get a vaccine for HIV. People are afraid to say that because it would then indicate that maybe we are giving up. We are not giving up. We are going to push this agenda as aggressively and energetically as we always have. But there is a possibility—a clear and finite possibility—that that’s the case.”

In Canada

The Canadian government, together with the Bill and Melinda Gates Foundation, has formed a partnership to strengthen Canada’s contribution to the search for an AIDS vaccine. The partnership is called Canadian HIV Vaccine Initiative (CHVI). It brings together important ministries and agencies—such as the Public Health Agency of Canada, Health Canada, Canadian International Development Agency and Industry Canada—with Canada’s outstanding research agency, the Canadian Institutes of Health Research (CIHR). To find out more about CHVI, visit its website at:

http://www.chvi-icvv.gc.ca/index-eng.html

In the meantime…

As a vaccine is unlikely in the foreseeable future, the HIV pandemic will continue, and so HIV prevention research needs to be strengthened. In high-income countries, a portfolio of HIV prevention activities needs to be explored, enhanced and field-tested, including the following issues:

• safer-sex messages
• addiction prevention and treatment
• increased public recognition, acceptance and treatment of mental health conditions
• harm reduction
• more widespread HIV testing
• post-exposure prophylaxis
• pre-exposure prophylaxis
• microbicide development and testing
• education on the prevention of sexually transmitted infections
• community engagement and capacity building

—Sean R. Hosein

Created on: 03/04/2008

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