Wed, 27 Aug 2014
Over the past two years, the phrase “HIV cure” has flashed repeatedly across newspaper headlines. In March 2013, doctors from Mississippi reported that the disease had vanished in a toddler who was infected at birth. Four months later, researchers in Boston reported a similar finding in two previously HIV-positive men. All three were no longer required to take any drug treatments. The media heralded the breakthrough, and there was anxious optimism among HIV researchers. Millions of dollars of grant funds were earmarked to bring this work to more patients.
But in December 2013, the optimism evaporated. HIV had returned in both of the Boston men. Then, just this summer, researchers announced the same grim results for the child from Mississippi. The inevitable questions mounted from the baffled public. Will there ever be a cure for this disease? As a scientist researching HIV/AIDS, I can tell you there’s no straightforward answer. HIV is a notoriously tricky virus, one that’s eluded promising treatments before. But perhaps just as problematic is the word “cure” itself.
Science has its fair share of trigger words. Biologists prickle at the words “vegetable” and “fruit”—culinary terms which are used without a botanical basis—chemists wrinkle their noses at “chemical free,” and physicists dislike calling “centrifugal” a force—it’s not; it only feels like one. If you ask an HIV researcher about a cure for the disease, you’ll almost certainly be chastised. What makes “cure” such a heated word?
HIV hijacks the body’s immune system by attacking T cells.
It all started with a promise. In the early 1980s, doctors and public health officials noticed large clusters of previously healthy people whose immune systems were completely failing. The new condition became known as AIDS, for “acquired immunodeficiency syndrome.” A few years later, in 1984, researchers discovered the cause—the human immunodeficiency virus, now known commonly as HIV. On the day this breakthrough was announced, health officials assured the public that a vaccine to protect against the dreaded infection was only two years away. Yet here we are, 30 years later, and there’s still no vaccine. This turned out to be the first of many overzealous predictions about controlling the HIV epidemic or curing infected patients.
The progression from HIV infection to AIDS and eventual death occurs in over 99% of untreated cases—making it more deadly than Ebola or the plague. Despite being identified only a few decades ago, AIDS has already killed 25 million people and currently infects another 35 million, and the World Health Organization lists it as the sixth leading cause of death worldwide.
HIV disrupts the body’s natural disease-fighting mechanisms, which makes it particularly deadly and complicates efforts to develop a vaccine against it. Like all viruses, HIV gets inside individual cells in the body and highjacks their machinery to make thousands of copies of itself. HIV replication is especially hard for the body to control because the white blood cells it infects, and eventually kills, are a critical part of the immune system. Additionally, when HIV copies its genes, it does so sloppily. This causes it to quickly mutate into many different strains. As a result, the virus easily outwits the body’s immune defenses, eventually throwing the immune system into disarray. That gives other obscure or otherwise innocuous infections a chance to flourish in the body—a defining feature of AIDS.
In 1987, the FDA approved AZT as the first drug to treat HIV. With only two years between when the drug was identified in the lab and when it was available for doctors to prescribe, it was—and remains—the fastest approval process in the history of the FDA. AZT was widely heralded as a breakthrough. But as the movie The Dallas Buyer’s Club poignantly retells, AZT was not the miracle drug many hoped. Early prescriptions often elicited toxic side-effects and only offered a temporary benefit, as the virus quickly mutated to become resistant to the treatment. (Today, the toxicity problems have been significantly reduced, thanks to lower doses.) AZT remains a shining example of scientific bravura and is still an important tool to slow the infection, but it is far from the cure the world had hoped for.
In three decades, over 25 highly-potent drugs have been developed and FDA-approved to treat HIV.
Then, in the mid-1990s, some mathematicians began probing the data. Together with HIV scientists, they suggested that by taking three drugs together, we could avoid the problem of drug resistance. The chance that the virus would have enough mutations to allow it to avoid all drugs at once, they calculated, would simply be too low to worry about. When the first clinical trials of these “drug cocktails” began, both mathematical and laboratory researchers watched the levels of virus drop steadily in patients until they were undetectable. They extrapolated this decline downwards and calculated that, after two to three years of treatment, all traces of the virus should be gone from a patient’s body. When that happened, scientists believed, drugs could be withdrawn, and finally, a cure achieved. But when the time came for the first patients to stop their drugs, the virus again seemed to outwit modern medicine. Within a few weeks of the last pill, virus levels in patients’ blood sprang up to pre-treatment levels—and stayed there.
In the three decades since, over 25 more highly-potent drugs have been developed and FDA-approved to treat HIV. When two to five of them are combined into a drug cocktail, the mixture can shut down the virus’s replication, prevent the onset of AIDS, and return life expectancy to a normal level. However, patients must continue taking these treatments for their entire lives. Though better than the alternative, drug regimens are still inconvenient and expensive, especially for patients living in the developing world.
Given modern medicine’s success in curing other diseases, what makes HIV different? By definition, an infection is cured if treatment can be stopped without the risk of it resurfacing. When you take a week-long course of antibiotics for strep throat, for example, you can rest assured that the infection is on track to be cleared out of your body. But not with HIV.
A Bad Memory
The secret to why HIV is so hard to cure lies in a quirk of the type of cell it infects. Our immune system is designed to store information about infections we have had in the past; this property is called “immunologic memory.” That’s why you’re unlikely to be infected with chickenpox a second time or catch a disease you were vaccinated against. When an infection grows in the body, the white blood cells that are best able to fight it multiply repeatedly, perfecting their infection-fighting properties with each new generation. After the infection is cleared, most of these cells will die off, since they are no longer needed. However, to speed the counter-attack if the same infection returns, some white blood cells will transition to a hibernation state. They don’t do much in this state but can live for an extremely long time, thereby storing the “memory” of past infections. If provoked by a recurrence, these dormant cells will reactivate quickly.
This near-immortal, sleep-like state allows HIV to persist in white blood cells in a patient’s body for decades. White blood cells infected with HIV will occasionally transition to the dormant state before the virus kills them. In the process, the virus also goes temporarily inactive. By the time drugs are started, a typical infected person contains millions of these cells with this “latent” HIV in them. Drug cocktails can prevent the virus from replicating, but they do nothing to the latent virus. Every day, some of the dormant white blood cells wake up. If drug treatment is halted, the latent virus particles can restart the infection.
Latent HIV’s near-immortal, sleep-like state allows it to persist in white blood cells in a patient’s body for decades.
HIV researchers call this huge pool of latent virus the “barrier to a cure.” Everyone’s looking for ways to get rid of it. It’s a daunting task, because although a million HIV-infected cells may seem like a lot, there are around a million times that many dormant white blood cells in the whole body. Finding the ones that contain HIV is a true needle-in-a-haystack problem. All that remains of a latent virus is its DNA, which is extremely tiny compared to the entire human genome inside every cell (about 0.001% of the size).
Defining a Cure
Around a decade ago, scientists began to talk amongst themselves about what a hypothetical cure could look like. They settled on two approaches. The first would involve purging the body of latent virus so that if drugs were stopped, there would be nothing left to restart the infection. This was often called a “sterilizing cure.” It would have to be done in a more targeted and less toxic way than previous attempts of the late 1990s, which, because they attempted to “wake up” all of the body’s dormant white blood cells, pushed the immune system into a self-destructive overdrive. The second approach would instead equip the body with the ability to control the virus on its own. In this case, even if treatment was stopped and latent virus reemerged, it would be unable to produce a self-sustaining, high-level infection. This approach was referred to as a “functional cure.”
The functional cure approach acknowledged that latency alone was not the barrier to a cure for HIV. There are other common viruses that have a long-lived latent state, such as the Epstein-Barr virus that causes infectious mononucleosis (“mono”), but they rarely cause full-blown disease when reactivated. HIV is, of course, different because the immune system in most people is unable to control the infection.
The first hint that a cure for HIV might be more than a pipe-dream came in 2008 in a fortuitous human experiment later known as the “Berlin patient.” The Berlin patient was an HIV-positive man who had also developed leukemia, a blood cancer to which HIV patients are susceptible. His cancer was advanced, so in a last-ditch effort, doctors completely cleared his bone marrow of all cells, cancerous and healthy. They then transplanted new bone marrow cells from a donor.
Fortunately for the Berlin patient, doctors were able to find a compatible bone marrow donor who carried a unique HIV-resistance mutation in a gene known as CCR5. They completed the transplant with these cells and waited.
For the last five years, the Berlin patient has remained off treatment without any sign of infection. Doctors still cannot detect any HIV in his body. While the Berlin patient may be cured, this approach cannot be used for most HIV-infected patients. Bone marrow transplants are extremely risky and expensive, and they would never be conducted in someone who wasn’t terminally ill—especially since current anti-HIV drugs are so good at keeping the infection in check.
Still, the Berlin patient was an important proof-of-principle case. Most of the latent virus was likely cleared out during the transplant, and even if the virus remained, most strains couldn’t replicate efficiently given the new cells with the CCR5 mutation. The Berlin patient case provides evidence that at least one of the two cure methods (sterilizing or functional), or perhaps a combination of them, is effective.
Researchers have continued to try to find more practical ways to rid patients of the latent virus in safe and targeted ways. In the past five years, they have identified multiple anti-latency drug candidates in the lab. Many have already begun clinical trials. Each time, people grow optimistic that a cure will be found. But so far, the results have been disappointing. None of the drugs have been able to significantly lower levels of latent virus.
In the meantime, doctors in Boston have attempted to tease out which of the two cure methods was at work in the Berlin patient. They conducted bone marrow transplants on two HIV-infected men with cancer—but this time, since HIV-resistant donor cells were not available, they just used typical cells. Both patients continued their drug cocktails during and after the transplant in the hopes that the new cells would remain HIV-free. After the transplants, no HIV was detectable, but the real test came when these patients volunteered to stop their drug regimens. When they remained HIV-free a few months later, the results were presented at the International AIDS Society meeting in July 2013. News outlets around the world declared that two more individuals had been cured of HIV.
Latent virus had likely escaped the detection methods available.
It quickly became clear that everyone had spoken too soon. Six months later, researchers reported that the virus had suddenly and rapidly returned in both individuals. Latent virus had likely escaped the detection methods available—which are not sensitive enough—and persisted at low, but significant levels. Disappointment was widespread. The findings showed that even very small amounts of latent virus could restart an infection. It also meant meant that the anti-latency drugs in development would need to be extremely potent to give any hope of a cure.
But there was one more hope—the “Mississippi baby.” A baby was born to an HIV-infected mother who had not received any routine prenatal testing or treatment. Tests revealed high levels of HIV in the baby’s blood, so doctors immediately started the infant on a drug cocktail, to be continued for life.
The mother and child soon lost touch with their health care providers. When they were relocated a few years later, doctors learned that the mother had stopped giving drugs to the child several months prior. The doctors administered all possible tests to look for signs of the virus, both latent and active, but they didn’t find any evidence. They chose not to re-administer drugs, and a year later, when the virus was still nowhere to be found, they presented the findings to the public. It was once again heralded as a cure.
Again, it was not to be. Just last month, the child’s doctors announced that the virus had sprung back unexpectedly. It seemed that even starting drugs as soon as infection was detected in the newborn could not prevent the infection from returning over two years later.
Despite our grim track record with the disease, HIV is probably not incurable. Although we don’t have a cure yet, we’ve learned many lessons along the way. Most importantly, we should be extremely careful about using the word “cure,” because for now, we’ll never know if a person is cured until they’re not cured.
Clearing out latent virus may still be a feasible approach to a cure, but the purge will have to be extremely thorough. We need drugs that can carefully reactivate or remove latent HIV, leaving minimal surviving virus while avoiding the problems that befell earlier tests that reactivated the entire immune system. Scientists have proposed multiple, cutting-edge techniques to engineer “smart” drugs for this purpose, but we don’t yet know how to deliver this type of treatment safely or effectively.
As a result, most investigations focus on traditional types of drugs. Researchers have developed ways to rapidly scan huge repositories of existing medicines for their ability to target latent HIV. These methods have already identified compounds that were previously used to treat alcoholism, cancer, and epilepsy, and researchers are repurposing them to be tested in HIV-infected patients.
The less latent virus that remains, the less chance there is that the virus will win the game of chance.
Mathematicians are also helping HIV researchers evaluate new treatments. My colleagues and I use math to take data collected from just a few individuals and fill in the gaps. One question we’re focusing on is exactly how much latent virus must be removed to cure a patient, or at least to let them stop their drug cocktails for a few years. Each cell harboring latent virus is a potential spark that could restart the infection. But we don’t know when the virus will reactivate. Even once a single latent virus awakens, there are still many barriers it must overcome to restart a full-blown infection. The less latent virus that remains, the less chance there is that the virus will win this game of chance. Math allows us to work out these odds very precisely.
Our calculations show that “apparent cures”—where patients with latent virus levels low enough to escape detection for months or years without treatment—are not a medical anomaly. In fact, math tells us that they are an expected result of these chance dynamics. It can also help researchers determine how good an anti-latency drug should be before it’s worth testing in a clinical trial.
Many researchers are working to augment the body’s ability to control the infection, providing a functional cure rather than a sterilizing one. Studies are underway to render anyone’s immune cells resistant to HIV, mimicking the CCR5 mutation that gives some people natural resistance. Vaccines that could be given after infection, to boost the immune response or protect the body from the virus’s ill effects, are also in development.
In the meantime, treating all HIV-infected individuals—which has the added benefit of preventing new transmissions—remains the best way to control the epidemic and reduce mortality. But the promise of “universal treatment” has also not materialized. Currently, even in the U.S., only 25% of HIV-positive people have their viral levels adequately suppressed by treatment. Worldwide, for every two individuals starting treatment, three are newly infected. While there’s no doubt that we’ve made tremendous progress in fighting the virus, we have a long way to go before the word “cure” is not taboo when it comes to HIV/AIDS.
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