Questions, Questions: The Future of HIV Research
It may sound hard to believe, but 25 years into the epidemic we still don't know exactly how HIV causes disease. We know that HIV infects immune cells and that after prolonged damage to the immune system people with HIV can get sick and die from one the illnesses that make up the acquired immune deficiency syndrome (AIDS). But we don't know exactly how this damage to the immune system occurs. Dr. Coffin's talk surveyed this question and several other key mysteries in the basic science of HIV.
Though we don't know enough about how HIV behaves in the body, we do know a great deal about how HIV replicates in cells. For example, to infect a new cell, the virus must bind to a CD4 receptor on a target cell's surface. This is why HIV infection is mostly limited to the CD4-bearing T cells of the immune system.
Most people count T cells in the tens or hundreds, but these are the number of cells found in just a small sample of blood. There are actually 5,000 times that number in the body's total blood supply and many millions more living in tissue, totaling perhaps a billion CD4 T cells -- many of them targets for HIV. Most of the CD4 T cells that HIV targets typically have a very short lifespan, and once infected, most T cells will die within a day or two. Meanwhile, free-floating HIV may survive in the blood only for a few hours, with between 100 million and 10 billion virions formed and removed per day.
In the absence of treatment, an estimated 10 million to 100 million CD4 T cells become infected nearly every day, and the infected cells make enough new HIV to infect all of the next generation of CD4 T cells. This constant daily cycle of CD4 cell infection, production of new virus, and cell death occurs continuously in most infected people who have not suppressed the virus with antiretroviral drugs.
While the production and destruction of virus and CD4 cells proceeds at a near steady state in this furious but silent cycle, over much longer periods of time, the cycle becomes unbalanced and the total CD4 cell count begins to decline.
The biggest single unanswered question about HIV is how this slow loss of CD4 cells occurs. A newly infected person experiences an extremely high spike in viral load within the first few months. This is probably due to the abundant supply of target cells and the lack of any immune control at that early stage. This early phase of high viremia may also be a time when a person is especially likely to pass the virus on to others.
Surprisingly, the body is often able to mount an initially effective immune response to HIV with immune cells that recognize and kill infected CD4 cells. After the sharp initial rise in viral load and an equally sharp decline in the CD4 cell count, the appearance of this immune response about three months into the infection signals the beginning of a quieter phase, when viral load remains relatively steady and CD4 cell counts decline gradually. This phase can last for years, but eventually, in most people, the slow but steady loss of CD4 cells leaves them without immune protection from everyday pathogens that healthy people never notice but are deadly to those with compromised immunity. At that point, during late-phase AIDS, the viral load may once again reach extremely high levels. Fortunately, effective antiretroviral therapy can lower virus levels dramatically well before this point, putting the disease on hold and allowing the body's immune capacity to recover.
Because the pace of HIV replication is so brisk in untreated people and because so many millions of virus copies are made each day, it is not surprising that mistakes are made as the viral genetic material is copied and processed in the cell. Most copying mistakes (transcription errors) probably result in a fatally flawed virus. But, rarely, a "mistake" is able to adapt and thrive better than the parent virus. For example, a virus might be better able to evade the body's immune defenses or to escape from control by drugs. Over time these mistakes, or mutations, accumulate. Eventually the dominant viral strain found in the blood can differ significantly from the HIV the person was originally infected with. It's likely that this is how HIV gradually escapes from immune control.
The viral load test measures the number of copies of HIV RNA in the blood, but it can also be seen as an indirect measurement of the number of productively infected CD4 T cells at any one time. When the viral load is very low it means that few cells are making new copies of HIV. The viral load test is also a quick way of telling if antiretroviral therapy is working effectively.
Although antiretroviral therapy can reduce the number of infected CD4 cells to almost zero, studies have found one or two copies of HIV RNA in the blood even after seven years of continually suppressed viral load. These viral embers are hidden in a very small number of infected CD4 cells thought to exist in a long-lived resting state, where they are unreachable by drugs. If antiretroviral drugs are removed, these embers can flare up to become a raging, active HIV infection. Unfortunately, at this time there is no known way to reach and eliminate these latently infected cells. In other words, there is no cure.
Dr. Coffin outlined several major unanswered questions that young scientists will have to wrestle with: Most importantly, what is responsible for CD4 cell depletion? Is the virus killing the CD4 cells directly? Or are toxic viral proteins killing them indirectly? Is it the anti-HIV immune activity of the body's own killer cells that is doing the job? Maybe it is some byproduct of a highly activated immune system that is exhausting the CD4 cell supply. Perhaps it is a combination of several of these factors.
Most perplexing, how can the virus depend on, yet evade destruction by, the immune cells that should be attacking it? One remarkable adaptation is the camouflage of the virus's external proteins by clouds of sugar molecules. These envelope proteins are the best available target for immune system recognition, but they are hidden by the sugars. Finding a chink in this sugarcoated armor would be great news. But HIV is so changeable that even when these viral proteins become temporarily exposed when HIV binds to a cell, immune cells may not recognize their target.
Scientists are also studying similar viruses that cause AIDS-like disease in monkeys, but not humans, to identify crucial differences and similarities. One key finding may be that the disease-causing viruses all tend to produce high levels of immune activation, much as HIV does. Some have compared the explosion of target CD4 cells produced by runaway immune activation to pouring gasoline on a fire. Finding a way to tamp down immune activation may be one avenue of treatment.
Where does the virus in latently infected cells come from? Are they archived from early permutations of the virus that evolved before suppressive therapy was begun? Do these resting cells divide periodically and pass on the viral genome to their progeny? Would it be possible to wake these cells up and make them go through a cycle of viral production and cell death? If so, and if all new infections could be blocked by drugs, then HIV might actually be curable, according to some theorists. But we still don't know if infected cells in protected parts of the body, such as the brain, harbor enough HIV to stage a comeback when the drugs are stopped.
It should be evident from this long list of questions that young scientists coming into the field of HIV research can expect to find a lot of exciting and important work to do.
Policy makers, politicians, and public health officials love vaccines because they promise a cheap and easy way to eliminate major public health care problems with a single shot. Finding a vaccine for HIV has been one of most difficult scientific challenges in fighting this disease. Beginning in the mid 1980s, various leaders have periodically said to expect an AIDS vaccine within five to 10 years. Current estimates bandied about at the Denver conference say it will be at least ten years before a vaccine can be expected. But work continues, and vaccine research is increasingly finding the funding it deserves.
Koup described vaccines as "the most powerful biomedical intervention" ever developed. Koup suggested that we may be on the brink of a vaccine revolution as a host of new technologies based on manipulating genetic material are perfected.
A vaccine works by exposing the body's immune system to a bit of material that "looks like" a pathogen but does not cause disease itself. Pathogens can be any disease-causing bug, such as a bacterium, virus, or fungus. Once the immune system has been primed to recognize this characteristic bit of matter, it will respond much faster and more effectively if and when the pathogen bearing that characteristic shows up in person. Traditional vaccines use killed or weakened versions of pathogens to educate the immune system to recognize the real danger.
This simple approach hasn't worked with HIV because the virus is particularly adept at hiding from the immune system. Meanwhile HIV targets and destroys the very immune cells that should be fighting it. Natural immunity does seem to provide some control over HIV, at least in the beginning, but because HIV is so changeable, over time this control declines as the immune system no longer recognizes its foe.
There are two main wings to the immune system: defense by neutralizing antibodies, which attack pathogens directly; and defense by killer T cells, which mainly destroy cells that have become infected. It's probable that a truly effective vaccine will need to engage both divisions of the immune system in fighting HIV. While there has been some modest success in stimulating the cell mediated wing to recognize HIV, the challenge for antibodies has been much greater.
Remember that the proteins carried on the outside of HIV are highly variable and are shrouded in sugar. We know very well what certain key parts of these proteins look like, and we can make antibodies to attack them, but they are as effective as an assassin seeking his victim at a costume party where everyone is wearing a mask -- and the masks are constantly changing.
The challenge is to make an antibody that will recognize a vulnerable feature of the exposed HIV protein that is stable both physically and genetically. A few artificially constructed antibodies have been able to achieve some success, but getting the body to generate such broadly neutralizing antibodies on its own in sufficient quantities remains a challenge with no solution in sight.
Much more is known about inducing cell-mediated immune responses to HIV -- and if HIV would stand still, these responses might offer very effective viral suppression. Unfortunately the experimental evidence so far suggests that cellular immunity is not very effective at blocking HIV from establishing an infection in the first place. While a therapeutic vaccine could be very important for people living with HIV (and this is likely the first HIV vaccine product we will see) what the policy makers and public health officials really want is a preventative vaccine.
One recent startling finding about the impact of HIV in the gut has stimulated much interest in the role of mucosal immunity. It's been shown that shortly after infection, HIV rapidly and dramatically destroys vast numbers of CD4 T cells that reside in lymphoid tissue lining the intestines. Because this wholesale destruction happens within the first two or three weeks of infection, an immune system that had been primed to recognize HIV might be able to minimize the damage and possibly alter the natural course of HIV disease. Other avenues of research are looking at ways to stimulate immune responses in these vulnerable mucosal tissues.
Because HIV is constantly evolving in the body, the HIV transmitted from one person to another may have a different genetic signature from the virus the first person was infected with. As the virus passes from person to person, over time the genetic "distance" from the original virus increases. If an infected person moves to a different part of the world and introduces HIV there, eventually different identifiable subtypes of HIV will be associated with different global regions. The evolving genetics may not greatly affect how the virus works or how it causes disease as much as they represent changes in the fine details of viral proteins that could be targets for immune recognition. For example, people have unique faces that identify them as individuals, and the variation in facial appearance around the world is great, but everyone still has eyes and ears that perform consistent functions. The challenge for designing a vaccine that will work for everyone all around the world is to find the key unchanging features across all viral strains that can serve as immune targets.
So to sum up the situation for HIV vaccines, there are a few vaccine candidates moving forward to clinical trials that may stimulate some cellular immune response, but while these might offer modest viral suppression after an infection has taken hold, they will probably provide only minimal protection from transmission. To do that, vaccines that stimulate neutralizing antibodies will probably be needed, and currently those remain out of reach.
Our bodies actually have certain systems of innate immunity that can block viruses, including HIV, from infecting cells. These systems don't recognize specific pathogens but rather tend to reject foreign materials such as viral DNA or double stranded RNA that don't belong in the cell. Nevertheless, HIV has evolved adaptations that have defeated these systems too.
The viral protein called "Vif" was identified during the 1980s as a "virion infectivity factor." In most cells, if Vif is deleted, HIV is unable to replicate. Cells that need Vif to produce viable HIV are called nonpermissive, because they don't permit replication unless Vif is present. This suggests that blocking Vif might be a possible treatment strategy. Experiments that joined a permissive and a nonpermissive cell together caused the cell to become nonpermissive, which suggested that there is some natural factor in T cells that inhibits HIV replication. In 2002 this factor was identified as a protein called APOBEC3G, one of a family of proteins that performs DNA processing in cells. In the absence of Vif, the APOBEC3G protein is packaged into newly formed HIV virions and is carried along when the virus subsequently infects a new host cell. After HIV enters a target cell, one of the first replication steps is to copy the HIV RNA into DNA. But if APOBEC3G is present, then the DNA copy of the viral genes is inappropriately processed and degraded, which effectively stops HIV replication from continuing.
So in the absence of Vif, APOBEC3G acts as a kind of natural antiviral mechanism. But if Vif is present, it binds to APOBEC3G and the protein is degraded before it can be packaged into the virus. Thus Vif defeats one of the cell's natural antiviral defenses. Investigators are now looking at other members of the APOBEC protein family for antiviral activity and at the role of cellular APOBEC3G in preventing new HIV infections in resting T cells. It's long been observed that only activated T cells can be infected by HIV and that resting T cells resist infection. One research group has recently shown that if APOBEC3G can be depleted from a resting T cell, then it becomes susceptible to infection.
In 2004 another cellular protein with potentially natural anti-HIV activity, called TRIM5, was discovered. Although the monkey version of TRIM5 can block HIV replication, human TRIM5 allows HIV replication to proceed. Some scientists suspect that the TRIM protein causes an invading virus to fall apart just after entry, before it can successfully unpack its contents. Others think that TRIM5 triggers degradation of certain HIV proteins or that it possibly causes the viral machinery to be delivered to the wrong place in the cell. These unsolved questions are typical of biological science at the leading edge, and represent opportunities for discovery that will hopefully attract young scientists into HIV research.
The best-attended sessions at the Retrovirus Conference are devoted to antiretroviral drug therapy -- mainly because the biggest and splashiest research comes from drug trials. A significant market for antiretroviral drugs has developed in the past decade, and the pharmaceutical industry is willing to invest large amounts of money in research to refine and develop new antiretroviral drugs.
This year is the tenth anniversary of learning that triple combination drug therapy can suppress HIV replication and prolong life. There are now over 20 drugs approved in the three main classes, and new drugs are being developed to block the virus at other points in its life cycle. Several entry inhibitors are well along in development, and one, Fuzeon, has already been approved as the first of this fourth class. Another new class of drugs is meant to block the viral protein integrase, which is responsible for stitching the viral genes into the cell's DNA. Other targets for drug development include the packaging and maturation of newly formed virions. For the most part, antiretroviral drug research is alive and well.
When potent antiretroviral drugs are taken consistently, there is usually an initial rapid decline in viral load that occurs within the first week or two. Then, during the next eight weeks or so, virus levels continue to drop to where they are no longer detectable by the standard HIV RNA assay. If a person can develop good habits for taking the drugs on time and can tolerate any side effects, then there is a good chance that he or she can continue with undetectable virus indefinitely -- with no, or only minimal, evolution occurring among the few viral holdouts. But not everyone can achieve this optimal outcome, and more research is need on strategies to extend the benefits of viral suppression to everyone.
Drug development is largely financed by industry, but government-funded research is also proceeding on strategies for using the drugs and treating people who have not responded to their first regimens or who may need to take a break from therapy. When therapy can safely be interrupted and for how long remain viable questions despite the recent, early closing of the large SMART study that compared continuous treatment to intermittent therapy guided by CD4 count. Because significantly more AIDS illnesses occurred in the intermittent group than in the continuous treatment group, the trial was halted. SMART was the largest and best-organized trial of this treatment strategy, yet other, smaller studies that used higher and safer CD4 counts for triggering interruption have showed promising results. While interruption can't be recommended as a strategy, the fact that many people will require interruption due to fatigue, toxicity, or other life issues means that interruption strategies still deserve to be studied. Knowing the best way to manage these patients remains a significant unanswered question.
Another strategy under investigation is simplification of treatment by reducing the number of drugs in the regimen once viral suppression has been achieved. Studies of this strategy using single-drug therapy with boosted Kaletra or Reyataz are under way. There is also a critical, unmet need for drug regimens that can reliably suppress virus in people with extensive treatment histories who have accumulated multiple drug-resistant mutations. Some of the most exciting research is focusing on the possibility of forcing drug-resistant virus into a weakened state by using nonsuppressive drugs to maintain mutations that hobble the virus.
Much basic science is being done to understand the impact and interplay of drug resistance mutations. In the future it will be important to track the impact of resistance around the globe as antiretroviral drugs come into broader usage, particularly in the developing world. Patterns of drug resistance and response may be different in different parts of the world, depending on which HIV subtype is prevalent. Other questions from the developing world that need attention are how best to prevent mother-to-child transmission of HIV with simple regimens and how best to treat infants and children who do become infected. One novel drug study is investigating if taking Viread daily as a preventative measure can prevent HIV infection in healthy people who are at risk for becoming infected. While this does not seem like an optimal approach to prevention, given the state of vaccine research, it may well be a viable stopgap measure to help hold down new infection rates in vulnerable populations until something better arrives.
In discussing the complications of HIV disease, Dr. Currier opened up a world of unanswered and intriguing questions on everything from the impact of sex and the environment on the course of an infection to the litany of side effects that can accompany therapy.
Complications make treating HIV more difficult and there is often a question of whether to first treat the complication or the HIV. Many of these questions are most acutely encountered in resource-limited settings where diagnostics and monitoring of complex medical conditions are not easily done. Yet studies coming from the developing world are showing excellent results in initiating and maintaining therapy, even when complications such as giving tuberculosis (TB) treatment are thrown into the mix.
Because TB is endemic in so many parts of the world and because TB is a particularly serious infection in HIV-infected people, it is now considered the most important complication of HIV. Research is needed to develop new drugs, new diagnostics, and new treatment strategies for identifying and treating TB in cooperation with emerging HIV treatment programs.
Immune reconstitution inflammatory syndrome is a phenomenon that can emerge a month or two after starting antiretroviral therapy. It is particularly common in people who start therapy with very low CD4 cell counts. The acute symptoms of immune reconstitution syndrome can appear virtually the same as underlying diseases such as TB or hepatitis C infection. The cause of this syndrome is not well understood, but some studies point to the reactivation of pathogen specific immune responses that make it appear the body is fighting off an infection that has already been cured.
The metabolic and fat wasting side effects of antiretroviral therapy have also opened up new areas of basic research into fat metabolism, drugs to mitigate metabolic disorders, and strategy studies on switching drugs to remove suspected lipotoxic agents from regimens. Again, the impact of these problems in the developing world is just starting to be investigated.
From the earliest days of AIDS, people attempting to understand the disease have found a web of associations reaching into almost every aspect of human knowledge. In the scientific and medical sphere alone the breadth of HIV research continues to illuminate not only this disease but the fields of immunology, infectious disease, cancer, and many others as well. Unfortunately we have not arrived at a point when AIDS researchers can relax. Because of its global spread, the vast number of infected people, and the many difficulties to overcome before the disease can be effectively prevented and treated, young investigators entering HIV research today will find themselves busy for decades to come.
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This article was provided by Gay Men's Health Crisis. It is a part of the publication GMHC Treatment Issues. Visit GMHC's website to find out more about their activities, publications and services.