HIV Researchers Struggle to Hit Their Moving Target : Disease: The virus frustrates scientists with its ability to mutate and hide for long periods in a host’s genes.
There is no cure for AIDS, no good treatment to control its symptoms for long periods and no vaccine to prevent it, for one major reason: HIV, the human immunodeficiency virus that causes the disease, is a sloppy housekeeper when it comes to tending its genetic endowment.
Every time HIV replicates, it makes mistakes, at least one error per generation of the virus. Within a few generations, it can begin to take on subtly different characteristics.
By all rights, such sloppiness should be devastating to HIV, causing it to mutate itself out of existence. Instead, it gives the virus an immense advantage in its fight for survival, not unlike the morphing android of the movie “Terminator 2.”
Every time the Arnold Schwarzeneggers of science have the virus in their sights for destruction by a drug, it transforms into something else and shrugs the drugs off. When researchers erect a vaccine to keep the virus out of the body, it changes into a form that can slip through defenses like the Terminator slides between the bars of a fence.
HIV is a moving target that is always one step ahead of scientists, says virologist Jerry Schochetman of the Centers for Disease Control and Prevention in Atlanta. “As one variant is knocked down, another pops up,” he said. “The next variant is attacked, but the virus continues to keep changing and escaping. . . . Eventually the virus wins.”
It is this mutability, combined with the ability of the virus to hide in genes--undetected by the host--for long periods, that makes HIV such a difficult and deadly opponent, experts say. “This is one of the meanest viruses of them all,” said G. Kirk Raab, president and chief executive officer of Genentech Inc. of South San Francisco, which is working on a number of approaches to AIDS drugs.
And it is that inherent difficulty that has slowed the search for an AIDS therapy and vaccine to what many consider a crawl.
“The classical approach to new diseases is to identify a viral agent and develop a vaccine,” said Irvin S. Y. Chen, director of the UCLA AIDS Center. “That’s been the case for all the viruses that we know. For AIDS, we thought early on that we would be able to do the same thing. It hasn’t turned out to be that easy because these properties make it different from all the other viruses.”
Said Dr. Jay Levy of UC San Francisco, “It is a very difficult (virus) to figure out.”
Levy should know, because he has been probing HIV from the time of its discovery about 11 years ago. It is widely acknowledged in the scientific community that Levy, unheralded and largely unfunded, isolated and identified the virus that causes AIDS in late 1983, only to be beaten into print by Dr. Robert Gallo and Dr. Luc Montagnier. “I guess I held off (on publishing results) too long,” he says cheerfully, albeit a bit ruefully.
Levy’s is one of the relatively few laboratories in the country that emphasizes study of the basic science of the virus, rather than concentrating solely on finding a cure or vaccine. Indeed, since the virus was discovered there has been little progress toward finding the “magic bullet” to cure or prevent AIDS, as HIV continues to claim succeeding generations of victims.
Levy and others now believe that a better understanding of how the virus works will lead to new ways to control it.
With a team of 20 postdoctoral researchers and technicians and a budget of nearly $1 million per year, he is continuing his long quest for an answer to AIDS. On the 12th floor of the medical center, they are examining the virus’ strengths, probing it for weaknesses and deciphering its genetics in half a dozen projects that typify HIV research around the country.
The HIV that Levy is studying is, like all viruses, a wispy organism that is neither inanimate nor fully living, but somewhere in between. Strictly speaking, viruses are not alive because they do not consume food for energy and cannot reproduce without assistance.
Viruses are simply small bundles of genetic information encapsulated in a protective protein coat and a viral membrane stolen from the last cell they visited. They are extremely simple, containing as few as four genes or perhaps as many as a dozen, compared to the estimated 100,000 genes necessary to describe a human.
But their power is disproportionate to their size. They attack by sneaking into cells and hijacking the host’s protein- and gene-producing machinery, proliferating until their sheer numbers force the cells to break open in a death spasm, freeing the newly made viruses to infect other cells and other hosts. Because viruses use the cell’s own machinery to reproduce, it has been almost impossible to develop antiviral drugs that do not kill healthy cells along with the virus.
Sore throats occur, for example, when the influenza virus kills cells lining the throat. Paralysis occurs when the polio virus bursts muscle cells. Dementia occurs when the rabies virus short-circuits brain cells. Death occurs when the hantavirus destroys lung cells.
Although HIV shares broad characteristics with other viruses, it falls into a special category called retroviruses, which makes it especially unpredictable and, therefore, deadly.
Viruses can have two types of genetic material, DNA or RNA. DNA viruses share the same deoxyribonucleic acid that encodes the genes of all living organisms. Most DNA viruses cause relatively trivial infections, ranging from warts to colds, but one family, the herpes viruses, causes illnesses ranging from cold sores to cancer.
Most serious diseases, including measles, mumps, encephalitis, polio, hepatitis and rabies, are caused by RNA viruses. Ribodeoxynucleic acids, which are chemically similar to DNA, are used for carrying messages inside cells of all living organisms, but in viruses they store genetic information.
HIV and other retroviruses are RNA viruses, but they are unusual ones because they contain an enzyme called reverse transcriptase, or RT, which makes a DNA copy of the virus’ RNA genes. This DNA “transcript” can then be used as a pattern for making more viruses. Reverse transcriptase is the sloppy housekeeper in HIV, introducing genetic errors and then failing to correct them.
All other organisms, including other viruses, also make mistakes when replicating their genetic information. But they typically have a mechanism, perhaps a second enzyme, that identifies and corrects mistakes like the spell-check function on a word processor.
Retroviruses don’t have such a mechanism, UCLA’s Chen said. As a result, HIV makes mistakes a million times more often than other cells, or about one per generation.
Those miscues give the virus a wealth of subtle variants to use in adapting to changing conditions--and in eluding therapeutic drugs. AZT, the most successful anti-HIV drug, inhibits the replication of the virus by blocking the activity of reverse transcriptase. But lurking somewhere in each HIV-infected person is at least one mutant form of the virus with a slightly different RT that is not blocked by the drug. That variant eventually predominates, killing the victim.
“This virus has impressed us again and again with its ability to change,” said Dr. David Ho, director of the Aaron Diamond Center in New York City. “It always has a new (variant) to counter our efforts.”
But mutation is not the only unique--and ultimately, lethal--characteristic of retroviruses. At the beginning of an HIV infection, another enzyme produced by one of the virus’ genes inserts the DNA copy into the cell’s own DNA. This copy of the virus’ genetic information becomes permanently hidden from the host’s immune system, until it re-emerges months or years later to cause disease.
The ability of HIV to lie dormant for long periods presents the body’s immune system with a seemingly impossible task.
“How does the defense mechanism of an organism recognize this foreign DNA from its own?” asked Schochetman. “It can’t. If the virus is not functioning, there’s nothing for the immune system to recognize. The virus can actually lie there hidden for long periods of time, and neither we nor the body are used to dealing with such long-term infections.”
“It becomes very tricky to attack the virus,” said epidemiologist Jonathan Kaplan of the CDC. “How do you get it out of the genetic material? The answer is, we don’t know any way to do that. You have to kill the cell.”
The final and crushing blow is the nature of the virus’ target. It is not a relatively minor throat cell, or even something as vital as a liver cell. Instead, it is one of the most crucial cells involved in leading the fight against infections: a white blood cell called CD4.
CD4 is “the conductor of the orchestra of cells in the immune system,” Schochetman said. “While the immune system is trying to galvanize itself to deal with the virus, the virus undermines the system by destroying the leader.” Without direction, the immune system is lost.
“There aren’t very many reasons why progress against AIDS is so difficult, but they are very big reasons,” Levy said.
The key to understanding HIV is understanding retroviruses. So far, not much progress has been made.
The first retrovirus was discovered in 1911 by Peyton Rous, a biologist at Columbia University. He identified a simple virus, containing only four genes, that causes tumors in chickens. Rous believed that a similar virus caused cancer in humans, but was never able to find one. Other scientists scoffed and few joined the search. Researchers looking back today acknowledge that that was clearly a big mistake.
“We were probably very naive to think that retroviruses wouldn’t be in humans,” Levy said. “They’re everywhere else in nature. Why not in humans as well?”
But it was not until 1980 that Gallo of the National Cancer Institute identified the first human retrovirus, now called human T-cell lymphotrophic virus-I or HTLV-I. That virus, which is common in Japan, the Caribbean and parts of Africa, causes a rare form of adult leukemia in about 5% of the people it infects.
Some researchers suspect that HTLV-I and a more recently identified companion, HTLV-II--or viruses very similar to them--may be responsible for a wide variety of human illness, including chronic fatigue syndrome and multiple sclerosis, as well as some forms of cancer.
HIV is very similar to the HTLVs--so similar, in fact, that Gallo initially named it HTLV-III. The main differences are that HIV has more genes than HTLVs, nine versus four, and causes disease in a much higher percentage of the people it infects.
Because the viruses are so much alike, researchers throughout the country are now studying the HTLVs carefully in the hope of gaining new insights into HIV.
“There are lessons to be learned from HTLV-I and II,” said William Blattner of the National Cancer Institute. “They may be the Rosetta stone” for understanding HIV.
Levy thinks researchers will find other new retroviruses as well that will also aid the understanding of AIDS. “Every new virus opens up incredible vistas of knowledge,” he said. He is looking for such viruses, but most of his effort is spent on HIV.
Among other things, Levy and his colleagues are working out the mechanism of Kaposi’s sarcoma, a tumor that frequently strikes AIDS patients; studying ways to block transmission from infected mothers to infants; exploring the function of one of the HIV genes whose role in reproduction is still a mystery; and probing relationships between strains of viruses in different parts of the world.
One of his most promising leads involves the discovery by molecular biologist Chris Walker, a former postdoctoral associate of Levy’s, that a white blood cell called CD8 holds HIV in check by preventing viral replication while HIV is hidden in the host genome, the complete genetic blueprint of an organism. People who have been infected by HIV for many years but who still have not developed full-blown AIDS may possess a stronger or more resilient CD8, scientists suggest.
It appears that the loss of this ability to restrain the virus contributes to the development of full-blown AIDS.
Simply transfusing AIDS patients with CD8 cells from long-term survivors to slow HIV infection would not work because the cells themselves would trigger an adverse immune reaction. So researchers are instead exploring a protein produced by CD8--called CD8 antiviral factor, or CAF--that they believe is the key component in checking the progression of the disease.
Molecular biologists Ed Barker and Carl Mackewicz are now trying to collect and purify enough of this protein so they can identify its composition and produce large quantities by genetic engineering techniques. That’s a daunting task, Levy said, requiring 100 liters of fluid from CD8 cells, which costs them $2,000 per liter just for materials to produce the fluid.
Nonetheless, Levy said he is “very encouraged by this approach,” because the virus doesn’t seem to be able to evade this protein by mutating into a different strain. “We haven’t met a virus yet that can escape it,” he said.
The approach may even have the potential to completely rid the body of an HIV infection, Mackewicz said. All white blood cells die off over a period of a few years to be replaced with new ones. If viral replication could be suppressed by CAF, perhaps in conjunction with drugs, until all the infected cells die naturally, theoretically the infection would be over.
Mackewicz concedes that such a scenario may be overly optimistic. But even if CAF controlled only 90% of replication, he said, “you could get a situation like herpes, where you are infected for life but don’t die from it.”
But “the most exciting thing we have going on right now,” Levy said, is the development of a new animal model for AIDS. Although primates have their own version of HIV--called the simian immunodeficiency virus, or SIV--it is distinctly different from HIV and does not provide the best possible gauge for testing the effectiveness of new drugs and vaccines.
Animal models are crucial in studying AIDS, according to virologist Stephen Morse of Rockefeller University in New York City, because it is the interaction of the virus with the entire organism that produces its adverse effects, and this interaction cannot be mimicked by growing the virus in cells in the laboratory. “The fact that we haven’t had a good animal model is a tremendous disadvantage,” Morse said.
Chimpanzees can be infected with HIV, but the infection is not pathogenic--the animals do not get sick and die. This makes it impossible to tell if potential vaccines or drugs tested in the animals are effective. Chimps are also rare and expensive. A far better model would be the baboon, which is more common and less expensive.
Scientists have long believed that baboons cannot be infected with HIV, much less develop AIDS. But David Blackbourn in Levy’s laboratory has proved them wrong. He has infected several baboons on a farm in Texas with HIV-2, a less virulent strain of HIV that is most common in West Africa. Moreover, Levy will report this week at the 10th International Conference on AIDS in Japan that two of the baboons have now developed symptoms of AIDS, including lymph node destruction and fibromatosis, a tumor similar to Kaposi’s sarcoma.
That could open up a whole new track of research. “When I said I was going to work in baboons, they laughed at me,” Levy says now with a glint in his eye. “It’s extra pleasant to get these results because we stuck with it.”
A sense of urgency pervades the laboratory, more so than in a cancer research center, according to Barker, who has worked in both. “Perhaps it’s because AIDS affects younger people who are . . . educated and more vocal,” he said.
It may also be because many people with AIDS come through the laboratory regularly, donating blood for research and becoming, in Levy’s words, “a part of the laboratory.” Sooner or later those donors will die.
But perhaps the ultimate difference is that the number of cancer victims has stabilized while the number of AIDS victims is still growing. “I don’t know where AIDS is going,” Levy said. “But if we don’t stop it soon, it may spread beyond all our imagining.”
Next: The search for an AIDS vaccine
A Stealthy Virus
The AIDS virus evades efforts to control it because of its unique cycle of replication, which occurs in a white blood cell called a T-cell.
1) Infection begins when the virus binds to the outside of a healthy T-cell and injects its genetic material into the cell.
2) Inside the cell, viral RNA is converted to “proviral” DNA by an enzyme called reverse transcriptase. This enzyme makes at least one error every time it reproduces the RNA. These errors allow the virus to mutate rapidly, giving it the abiity to develop resistance to drugs.
3) Another enzyme unique to retroviruses integrates the proviral DNA into the host cell’s own DNA, where it can remain hidden for years. Eventually, it serves as a template for the production of more viral RNA. This material forms a “bud” that is released from the cell.
4) The enzyme proteinase completes the formation of the new infectious HIV particles, which is then free to continue the cycle by infecting new T-cells.