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HIV and AIDS
A Global Health Pandemic
By Scientific American
Scientific AmericanCopyright © 2012 Scientific American, a division of Nature America, Inc.
All rights reserved.
The AIDS Virus
by Robert C. Gallo
It is a modern plague: the first great pandemic of the second half of the 20th century. The flat, clinical sounding name given to the disease by epidemiologists-acquired immune deficiency syndrome-has been shortened to the chilling acronym AIDS. First described in 1981, AIDS is probably the result of a new infection of human beings that began in central Africa, perhaps as recently as the 1950's. From there it probably spread to the Caribbean and then to the U.S. and Europe. By now as many as two million people in the U.S. may be infected. In the endemic areas of Africa and the Caribbean the situation is much worse. Indeed, in some areas it may be too late to prevent a disturbingly high number of people from dying.
In sharp contrast to the bleak epidemiological picture of AIDS, the accumulation of knowledge about its cause has been remarkably quick. Only three years after the disease was described its cause was conclusively shown to be the third human retrovirus: human T-lymphotropic virus III (HTLV-III), which is also called human immunodeficiency virus (HIV). Like other retroviruses, HTLV-III has RNA as its genetic material. When the virus enters its host cell, a viral enzyme called reverse transcriptase exploits the viral RNA as a template to assemble a corresponding molecule of DNA. The DNA travels to the cell nucleus and inserts itself among the host's chromosomes, where it provides the basis for viral replication.
In the case of HTLV-III the host cell is often a T4 lymphocyte, a white blood cell that has a central role in regulating the immune system. Once it is inside a T4 cell, the virus may remain latent until the lymphocyte is immunologically stimulated by a secondary infection. Then the virus bursts into action, reproducing itself so furiously that the new virus particles escaping from the cell riddle the cellular membrane with holes and the lymphocyte dies. The resulting depletion of T4 cells-the hallmark of AIDs-leaves the patient vulnerable to "opportunistic" infections by agents that would not harm a healthy person.
How HTLV-III manages to replicate in a single burst after lying low, sometimes for years, is one of the most fundamental questions confronting AIDS researchers. Another important question is the full spectrum of diseases with which the virus is associated. Although most of the attention given to the virus has gone to AIDS, HTLV-III is also associated with brain disease and several types of cancer. In spite of such lingering questions, more is known about the AIDS virus than is known about any other retrovirus. The rapidity of that scientific advance was made possible partly by the discovery in 1978 of the first human retrovirus, HTLV-I, which causes leukemia. In its turn the new knowledge is making possible the measures that are desperately needed to treat AIDS and prevent its spread.
HTLV-III VIRION, or virus particle, is a sphere that is roughly 1,000 angstrom units (one ten-thousandth of a millimeter) across. The particle is covered by a membrane, made up of two layers of lipid (fatty) material, that is derived from the outer membrane of the host cell. Studding the membrane are glycoproteins (proteins with sugar chains attached). Each glycoprotein has two components: gp41 spans the membrane and gp120 extends beyond it. The membrane-and-protein envelope covers a core made up of proteins designated p24 and p18. The viral RNA is carried in the core, along with several copies of the enzyme reverse transcriptase, which catalyzes the assembly of the viral DNA.
The first sign that a new disease was afoot was the appearance of a rare cancer called Kaposi's sarcoma among the "wrong" patients. Kaposi's sarcoma is a tumor of blood-vessel tissue in the skin or internal organs that had been known mainly among older Italian and Jewish men and in Africa. In the late 1970's, however, a more aggressive form of the same cancer began to appear among young white middle-class males, a group in which it had been extremely rare. Many of the new Kaposi's sarcoma patients turned out to have a history of homosexuality, and these young men provided the basis for the first reports of a new syndrome, which came in 1981 from Michael S. Gottlieb of the University of California at Los Angeles School of Medicine, Frederick P. Siegal of the Mount Sinai Medical Center and Henry Masur of New York Hospital.
Seen mainly among young homosexual men, the new syndrome included opportunistic infections and a depletion of T4 cells as well as, in some cases, Kaposi's sarcoma. Soon epidemiologists at the U.S. Centers for Disease Control (CDC) noted a dramatic increase in pneumonia caused by Pneumocystis carinii, a widespread but generally harmless protozoan. It seemed clear that an infectious form of immune deficiency was on the rise, and the name AIDS was coined to describe it. AIDS was quickly found to be spreading among users of intravenous drugs, recipients of frequent blood transfusions and Haitians. A mysterious and fatal illness, apparently associated with life-style, had appeared.
Hypotheses about the cause of AIDS proliferated rapidly. It was suggested that the disease resulted from exposure to sperm or to amyl nitrate, a stimulant used by some homosexuals. It was even proposed that AIDS had no specific etiologic agent: the patients' immune systems had simply broken down under chronic overexposure to foreign proteins carried by other people's white blood cells or by infectious agents. Yet it seemed more plausible to think of a single cause, and several workers suggested known viruses such as Epstein-Barr virus or cytomegalovirus, which are members of the herpes virus family. Both were long-established viruses, however, whereas AIDS seemed to be a new disease. Moreover, neither virus has an affinity for T cells.
James W. Curran of the CDC and his colleagues, who had been following the nascent epidemic, clearly favored the notion of a new infectious agent. In late 1981, as I listened to Curran outline what was known about the epidemiology of AIDS, I was already in agreement with him. A clue as to what the new agent might be came from the fact that some hemophiliacs had developed AIDS after receiving infusions of a preparation called Factor VIII, prepared from the plasma of many blood donors. In preparing Factor VIII the plasma is passed through filters fine enough to remove fungi and bacteria — but not viruses.
That observation supported those who had argued in favor of a virus. Yet if one could not look to established viruses as the cause, how could the culprit be identified? Any virus that was a candidate would have to fit what was known about the agent, which included the following. It was present in whole blood, plasma and semen as well as in Factor VIII. The epidemiological pattern showed that it could be transmitted by sexual contact, blood and congenital infection. Infection led, directly or indirectly, to the loss of T4 cells.
As it happened, that pattern was familiar to me and my co-workers, because HTLV-I had been isolated in my laboratory in 1978. HTLV-I can be transmitted by blood, intimate contact and congenital infection; it has a strong affinity for T cells. Furthermore, although the chief effect of HTLV-I is leukemia, the virus can also cause a mild immune deficiency in some patients. Accordingly, in the spring of 1982 I proposed that the cause of AIDS was likely to be a new human retrovirus.
To refine and test the retrovirus hypothesis I assembled a small working group of scientists, each chosen for a specific expertise. Along with clinicians, epidemiologists, immunologists and molecular biologists were investigators experienced in animal retrovirology. One of the retrovirologists, Myron Essex of the Harvard Medical School, had published results lending support to the idea that a human retrovirus might cause AIDS. Essex had shown that a retrovirus called feline leukemia virus (FeL V) can cause either leukemia or immune deficiency in cats. A minor variation in the virus's outer envelope, it was later shown, determines whether infection leads to immune suppression or to cancer.
These suggestive results made it seem even more plausible that a variant of HTLV-I (or its near relative HTLV-II, isolated in 1982) might be the AIDS agent. Essex's group and my own quickly began searching for such a virus. Soon we were joined by a third group, led by Luc Montagnier of the Pasteur Institute, who had been stimulated by the retrovirus hypothesis. All three groups employed the methods that my colleagues and I had developed for isolating HTLV-I: the virus was cultured in T cells stimulated by the growth factor called IL-2 and its presence was detected by sensitive assays for the viral reverse transcriptase.
Those methods quickly produced results. Beginning in late 1982 and continuing throughout 1983 my coworkers and I found preliminary evidence of retroviruses different from HTLV-I or II in tissues from people with AIDS or pre-AIDS conditions. Then in May of 1983 Montagnier and his colleagues Françoise Barré-Sinoussi and Jean-Claude Chermann published the first report of a new retrovirus from a patient with the lymphadenopathy ("swollen glands") typical of some pre-AIDS cases. The French investigators later gave their find the name lymphadenopathy-associated virus (LAV).
The initial report of LAV was intriguing, but it was hardly a conclusive identification of the cause of AIDS. The reason is that the methods then available (reverse-transcriptase assays accompanied by electron microscopy) can show that a retrovirus is present in a tissue sample but cannot specify the precise type of virus. Unique identification is possible only if reagents (such as antibodies) are available that react with the proteins of that virus and no other. Making such reagents requires large quantities of purified viral proteins; to obtain them the virus must be grown in the laboratory.
The new virus (or viruses), however, resisted the early attempts at laboratory culture: when they were put in T cells, the cells died. Hence no specific reagents to the new isolates could be made. We had previously learned how to culture HTLV-I and II, and reagents to those viruses were available. As a result it was possible to show that the viruses present in AIDS patients were not HTLV-I or HTLV-II, but throughout most of 1983 it was not possible to make a positive identification because specific reagents were lacking. Moreover, in the absence of reagents one could not say that any two of the new isolates were the same, which was clearly a requirement for showing that AIDS has a single cause.
The answer to such difficulties was to find a way to grow the virus. In the fall of 1983 my colleague Mika Popovic identified several cell lines that could be infected with the virus but resisted being killed. To obtain them the blood cells of a person with leukemia were separated and allowed to proliferate into clones of genetically identical cells. Many clones were screened, and several were found to have the right combination of qualities; the most productive of them was the clone designated H9. All the resistant lines are made up of leukemic T4 cells that are immortal in culture and therefore an endless source of virus.
Why certain T4 cell lines should resist the cytopathic effects of the virus is a significant question that has not been answered. In the winter of 1983-84, however, my colleagues and I had little time for that puzzle because we were concentrating on growing the virus. By December substantial quantities were being grown, and soon afterward reagent production was under way. With reagents in hand, we could go back and identify the many stored viral isolates. Initial testing showed that 48 isolates from AIDS patients or members of risk groups were of the same type. In contrast, the virus so identified was not found in any members of a control group of 124 healthy heterosexuals.
Continuous production of the virus also yielded enough viral proteins to provide the basis of a blood test. (Although there are several methods of testing blood for the AIDS agent, all of them rely on the reaction between viral proteins and antibodies in the infected person's blood.) The first blood testing was done by my colleague M. G. Sarngadharan, working on serum identified only by a code. By means of such "blind" testing Sarngadharan found virus in the serum of from 88 to 100 percent of AIDS patients (depending on the study), in a high but varying proportion of people in risk groups and in almost no healthy individuals outside the risk groups. The cause of AIDS had been established.
My colleagues and I reported these results in a series of publications in May, 1984. The retrovirus we had identified showed an affinity for T4 cells and also killed those cells. In accord with the prevailing conventions of virus nomenclature, the isolates were given the generic name HTLV-III and individual strains were distinguished by the initials of the patient from which they had come. Later it was shown that LAV is a different strain of the same virus. Later still, the name HIV was coined by a committee set up to resolve the problems caused by the existence of multiple names for the same biologic object.
Demonstrating the cause of AIDS was a fundamental step. Perhaps equally important from the viewpoint of public health was the fact that growing the virus had provided the basis for a practical blood test. The infected H9 line was given to several biotechnology companies, which used it as a source of viral proteins for a commercial blood test. The commercial test, marketed in 1985, virtually eliminated the risk of contracting AIDS through blood tranfusion.
Although only three years have elapsed since the cause of AIDS was identified, much has been learned about how the virus gives rise to disease. When a person is first infected, his (or her) immune system does respond by making antibodies. That response is clearly not adequate, however, and the virus takes hold. In many cases lymphocytes then begin to proliferate abnormally in the lymph nodes. Thereafter the node's intricate structure collapses, and a decline in the number of lymphocytes in the node follows. Soon the number of lymphocytes in the blood also decreases, leaving the patient open to opportunistic infections.
What events at the cellular level underlie this clinical catastrophe? It seems infection may be initiated by free virus or by virus carried in infected cells. Once the virus is inside the body its target consists of cells bearing the T4 molecule in their outer membrane. That molecule defines the category of T4 lymphocytes, but it is also found on cells called monocytes and macro- phages, and it appears that T4-carrying monocytes and macrophages are among the first targets of infection by the AIDS virus.
Monocytes and macrophages arise from the same bone-marrow precursors as lymphocytes, but they have different roles in the immune response. Among the roles of the macrophage are interactions with T4 lymphocytes that stimulate the T4 cells to undertake their tasks. Some of the interactions occur in the lymph node, and observations by Peter Biberfeld of the Karolinska Institute in Stockholm and Claudio Baroni of the University of Rome suggest that many T4 cells are infected in the lymph node during contact with a macrophage. After a variable latency the infected lymphocyte may be killed by viral replication.
Clearly the T4 population is reduced by the death of infected cells. The effect is compounded by the fact that the killing halts the normal proliferation of the lymphocytes that accompanies their immune functions. In the interaction with a macrophage the T4 cell not only is primed to respond to a particular protein but also is activated. Growth factors secreted by the macrophage cause it to begin a process of cell division that ultimately yields a clone of perhaps 1,000 descendants, all programmed to respond to the same antigen (protein). The descendants circulate in the blood and, on encountering the antigen they are programmed for, they induce the maturation of cells called B lymphocytes and T8 cytotoxic cells that attack pathogens directly. In this way the "memory clone" provides part of the basis of lasting immunity.
When a T4 cell infected with the AIDS virus is activated, however, the result is quite different, as Daniel Zagury of the University of Paris has shown in collaboration with me. Instead of yielding 1,000 progeny, the infected T cell proliferates into a stunted clone with perhaps as few as 10 members. When those 10 reach the bloodstream and are stimulated by antigen, they begin producing virus and die. Other suggestions have been made, but I think the direct killing of infected lymphocytes and the abortive expansion of the memory clones are largely responsible for the profound depletion of T4 cells observed in AIDS.
Excerpted from HIV and AIDS by Scientific American. Copyright © 2012 Scientific American, a division of Nature America, Inc.. Excerpted by permission of Scientific American.
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