An important focus of the National Institute of Allergy and Infectious Diseases (NIAID) is research devoted to the pathogenesis of human immunodeficiency virus (HIV) disease--the complex mechanisms that result in the destruction of the immune system of an HIV-infected persons. A detailed understanding of how HIV causes the acquired immune deficiency syndrome (AIDS) is crucial to identifying and developing effective drugs or vaccines to fight HIV and AIDS.

OVERVIEW: HIV DESTROYS THE IMMUNE SYSTEM

HIV disease is characterized by a gradual deterioration of immune function. During the course of infection, specific cells of the immune system called CD4+ T cells are disabled and killed, and their numbers progressively decline. CD4+ T cells play a crucial role in the immune response, signaling other cells in the immune system to perform their special defensive functions.

A healthy, uninfected person usually has 800 to 1,200 CD4+ T cells per microliter (one millionth of a liter, abbreviated ul) of blood. When an HIV-infected person's CD4+ T cell count falls below 200/ul, he or she becomes particularly vulnerable to the opportunistic infections and cancers that typify AIDS, the end stage of HIV disease.

The period between infection with HIV and the onset of AIDS averages 10 years in adults in the United States. People with AIDS often suffer infections of the lungs, brain, eyes, and other organs and frequently suffer debilitating weight loss, diarrhea and a type of cancer called Kaposi's sarcoma. Even with treatment, most people with AIDS die within a few years of developing infections or cancers that take advantage of their weakened immune systems.

THE SCOPE OF THE HIV EPIDEMIC

HIV was first isolated in the laboratory in 1983 and shown to be the cause of AIDS in 1984. The World Health Organization (WHO) estimates some 14 million people worldwide are infected with the virus, and more than 3 million people had developed AIDS by mid-1993. By the year 2000, between 30 and 40 million people worldwide will be HIV-infected, according to WHO estimates.

In the United States, at least 1 million people are infected with HIV, according to the Centers for Disease Control and Prevention (CDC) estimates. Through August 30, 1993, 331,845 cases of AIDS in the United States had been reported to the CDC.

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HIV, THE VIRUS THAT CAUSES AIDS

Like all viruses, HIV can replicate only inside cells, commandeering the cell's machinery to reproduce. HIV carries its genetic material in the form of RNA. Like other retroviruses, HIV uses an enzyme called reverse transcriptase to convert its RNA into DNA that is then integrated into the host cell DNA.

HIV belongs to a subgroup of retroviruses known as the lentiviruses. The course of infection with these viruses is characterized by a long interval between initial infection and the onset of serious symptoms. Other lentiviruses infect non-human species. For example, the feline immunodeficiency virus (FIV) infects cats and the simian immunodeficiency virus (SIV) infects monkeys and other non human primates. Scientists use these and other viruses along with their animal hosts as models of HIV disease.

THE VIRAL ENVELOPE. HIV is spherical in shape with a diameter of 1/10,000 of a millimeter. This is far too small to be seen even with a light microscope. HIV can be observed only with a special instrument called an electron microscope. The outer coat, or envelope, is composed of two layers of fat-like molecules called lipids, derived from the membranes of human cells. Embedded in the envelope are numerous cellular proteins, as well as mushroom-shaped HIV proteins that protrude from the surface. Each "mushroom" is thought to consist of a cap made of four HIV molecules called gp120, and a stem consisting of four gp41 molecules embedded in the envelope. The virus uses these proteins to attach to and infect CD4+ T cells.

THE VIRAL CORE. Within the envelope is a bullet-shaped core made of another HIV protein,p24, that surrounds two copies of the viral RNA. Each copy of HIV RNA contains the virus's nine genes. Three of these, gag,pol and env, are genes that contain information needed to make structural proteins. The env gene, for example, codes for gp 160, the envelope precursor protein that is later broken down to gp120 and gp41.

Three regulatory genes, tat, rev, and nef, and three so-called auxiliary genes, vif,vpr, and vpu, contain information necessary for the production of proteins that control the virus's ability to infect a cell, produce new copies of virus, or cause disease. The tat gene, for instance, is thought to enhance viral replication.

The ends of each strand of HIV RNA contain a duplicated RNA sequence called the long terminal repeat (LTR). Regions in the LTR act as switches to control viral expression and are triggered by proteins from both HIV and the host cell.

The core of the virus also includes proteins and enzymes that carry out later steps in the virus's life cycle. Among these are the enzyme reverse transcriptase, which HIV uses to convert its RNA into DNA after the virus has entered a cell, and the enzyme integrase, used to splice HIV's genes into the DNA of the host cell.

HIV infects CD4+ cells. HIV's preferred targets are cells that have a receptor or docking molecule on their surface called cluster designation 4 (CD4). Cells with this molecule are known as CD4+ cells.

Although white blood cells known as CD4+ T lymphocytes appear to be HIV's main target, other immune cells with CD4 molecules are infected as well. Among these are macrophages, which devour invading pathogens and stimulate T cells by presenting them with small pieces of the invaders, and dendritic cells, which pick up foreign particles as they enter the body and alert the rest of the immune system. Macrophages can harbor large quantities of the virus without being immediately killed, thus acting as reservoirs of HIV.

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THE LIFE CYCLE OF HIV

HIV's ENTRY INTO CELLS. Infection typically begins when HIV encounters a CD4+ cell. The HIV surface protein gp120 binds tightly to the CD4 molecule on the cell's surface. The membranes of the virus and the cell fuse, a process also requiring gp41. The viral core, containing HIV's RNA, proteins and enzymes, is released into the cell.

REVERSE TRANSCRIPTION. The virus's unique enzyme, reverse transcriptase, converts the single-stranded viral RNA into DNA. Human cells normally do not contain reverse transcriptase. The antiviral drugs approved in the United States for the treatment of HIV infection all work by interfering with reverse transcriptase at this stage of the viral life cycle. The three approved drugs are zidovudine (AZT), didanosine (ddI), and zalcitabine (ddC).

INTEGRATION. The viral DNA migrates to the cell's nucleus, where it is spliced into the host's DNA with the help of viral integrase. Once integrated into the host's chromosome, HIV DNA is called the provirus and is duplicated together with the cell's genes every time the cell divides.

Recent reports suggest that HIV's DNA also can integrate into the DNA of non-dividing cells such as macrophages and brain and nerve cells.

TRANSCRIPTION. For the provirus to produce new viruses, RNA copies called messenger RNA must be made that can be read by the host cell's protein-making machinery. This process is called transcription and is facilitated by cellular enzymes, including RNA polymerase II. The viral genes may partly control this process: tat, for example, encodes a protein that accelerates the transcription process by binding to and stabilizing a section of the newly made viral RNA.

Other factors may initiate transcription. Cytokines, immune system proteins involved in the normal regulation of the immune response, also may help to activate HIV. Tumor necrosis factor-alpha (TNF-alpha) and other cytokines, secreted in elevated levels during HIV infection, may initiate HIV transcription. Research suggests that infections with other microorganisms such as herpes viruses and mycoplasmas also may help to activate HIV.

TRANSLATION. HIV messenger RNA is processed in a cell's nucleus (where host genetic material is stored) and transported to the cytoplasm, the cellular material outside the nucleus. In the cytoplasm, the cell's protein-making machinery translates the messenger RNA into viral proteins and enzymes.

In the absence of the viral protein rev, HIV messenger RNA produces mostly regulatory proteins. The presence of rev allows the processing of messenger RNA for HIV structural proteins and enzymes.

ASSEMBLY AND BUDDING. Viral core proteins, enzymes, and RNA gather just inside the cell's membrane, while the viral envelope proteins aggregate within the membrane. An immature viral particle if formed and then pinches off from the cell, acquiring an envelope and the cellular and HIV proteins from the cell membrane. The immature viral particle then undergoes processing by an HIV enzyme called protease to become an infectious virus.

Cell-to-cell spread of HIV, independent of virus release, also can occur through the fusion of an infected cell with an uninfected cell.

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THE COURSE OF HIV INFECTION

TRANSMISSION. HIV is spread most commonly by sexual contact with an infected partner. The virus can enter the body through the mucosal linings of the vagina, vulva, penis, rectum or, rarely, the mouth during sex. The likelihood of transmission is increased by factors that may damage these linings, especially other sexually transmitted diseases that cause ulcers or inflammation. Studies of SIV infection of the genital membranes of non-human primates suggest that sentinel cells known as mucosal dendritic cells may be the first cells infected. Infected dendritic cells may migrate to lymph nodes and infect other cells.

HIV also is spread through contact with infected blood, most often by the sharing of drug needles or syringes contaminated with minute quantities of blood containing the virus.

Children can contract HIV from their infected mothers either during pregnancy or birth. Approximately 15-25% all pregnant women with HIV will pass the infection to their newborns. HIV also can be spread to babies through the breast milk of mothers infected with the virus. [AZT therapy may greatly reduce the chance of the child being born with HIV.

THE ACUTE INFECTION. Once it enters the body, HIV infects a large number of CD4+ T cells and replicates rapidly. During this acute or primary phase of infection, the blood contains many viral particles that spread throughout the body, seeding themselves in various organs, particularly the lymphoid tissues.

Lymphoid tissues include the lymph nodes, spleen, tonsils, and adenoids. Within these tissues, immune activity is concentrated in regions called germinal centers, where the thread-like tentacles of follicular dendritic cells (FDCs) form networks that trap invaders and present them to immune cells that congregate there.

During the acute phase of infection, the number of CD4+ T cells in the bloodstream decreases by 20 to 40 percent. It is not known whether these cells are killed by HIV or if they leave the blood and go to the lymphoid tissues in preparation to mount an immune response.

Two or four weeks after exposure to the virus, up to 70 percent of HIV- infected persons suffer flu-like symptoms related to the acute infection, such as moderate to severe fever, severe headaches, rash, sore throat, muscle and joint pain, nausea, vomiting, mouth ulcers, and swollen lymph nodes. The patient's immune system responds: B cells produce antibodies that neutralize some of the free virus and killer T cells destroy many HIV- infected cells. A patient's CD4+ T count may rebound to 80 to 90 percent of its original level, and he or she generally goes into a symptomless stage of infection that may last 10 years or longer.

CLINICAL LATENCY. Although infected individuals usually exhibit a period of clinical latency with little evidence of disease, the virus is never truly latent. NIAID researchers have shown that throughout the course of infection, even early in infection, HIV is active within the lymphoid organs, where large amounts of virus become trapped in the FDC networks. Surrounding the germinal centers are areas rich in CD4+ T cells. These cells increasingly become infected, and viral particles accumulate, both in infected cells and as free virus.

In and around the germinal centers, many CD4+ T cells are probably activated by the increased production of cytokines such as TNF-alpha and IL-6, possibly secreted by B cells and macrophages. Activation allows uninfected cells to be more easily infected and causes increased replication of HIV in already infected cells.

Other components of the immune system also are chronically activated, with negative consequences. For example, HIV-infected individuals exhibit a massive stimulation of B cells, resulting in an impaired ability to initiate a new antibody response. Immune activation can also result in the suicide of cells by a process known as apoptosis, and an increased production of cytokines that boost HIV replication and may have other deleterious effects. Increased levels of TNF-alpha, for example, may be at least partly responsible for severe weight loss or wasting syndrome seen in may HIV-infected individuals.

Over a period of years, even when little virus is readily detectable in the blood, significant amounts of virus accumulate in the germinal centers, both within infected cells and as free virus. Paradoxically, the FDC networks in the germinal centers, which normally have the job of trapping pathogens and initiating an immune response, may be an important reason why HIV is so effective at destroying the immune system. A steady stream of CD4+ T cells probably become infected with HIV as they move to the lymphoid tissues in response to other infections.

BREAKDOWN OF FDC NETWORKS. Ultimately, HIV overwhelms the germinal centers. The FDC networks breakdown in late-stage disease and virus-trapping is impaired, allowing entry of large quantities of virus into the bloodstream.The destruction of the lymph node structure seen late in HIV disease may preclude a successful immune response against not only HIV but other pathogens as well. This devastation heralds the onset of the opportunistic infections and cancers that characterize AIDS. Although it remains unclear why FDCs die and the FDC networks dissolve, some scientists think that this process may be as important as the loss of CD4+ T cells in HIV pathogenensis.

DEATH OF PRECURSOR CELLS. Recent evidence suggests that HIV also destroys precursor cells that mature to have special immune system functions, aswell as the microenvironment in the bone marrow and the thymus needed for the development of such cells. These organs probably lose the ability to regenerate, further compounding the suppression of the immune system.

CENTRAL NERVOUS SYSTEM DAMAGE. Although monocytes and marophages can be infected by HIV, they appear to be relatively resistant to killing.However, these cells travel throughout the body, and carry HIV to various organs, especially the lungs and brain. People infected with HIV often experience abnormalities in the central nervous system, such as HIV dementia. Neurological manifestations of HIV disease are the subject of an accumulation of HIV in brain and nerve cells or the inappropriate release of cytokines or toxic byproducts by these cells may be to blame.

LOW LEVELS OF CYTOKINES. While greater quantities of certain cytokines are secreted during HIV infection, others with key roles in the regulation of normal immune function may be secreted in decreased amounts. For example, CD4+ T cells may lose their capacity to produce interleukin 2 (IL-2), a cytokine that enhances the growth of other T cells and helps to stimulate other cells' response to invaders. Infected cells also have low levels of receptors for IL-2, which may reduce their ability to respond to signals from other cells.

MANY STRAINS OF HIV. HIV mutates rapidly. During the course of HIV disease, viral strains may emerge in an infected individual that differs widely in their ability to infect and kill different cell types, as well as in their rate of replication. Strains of HIV from patients with advanced disease appear to be more virulent and infect more cell types than strains obtained earlier from the same individual.

Tropism, or preference of particular viral strains for certain cell types, is due to variations within or adjacent to a region of gp120 called the V3 loop. Although individuals with late-stage HIV disease may carry many strains of the virus, the strain transmitted to an uninfected individual usually is a variant with macrophage tropism.

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THEORIES OF IMMUNE CELL LOSS IN HIV INFECTION

Researchers around the world are studying how HIV destroys or disables CD4+ T cells, and many think that a number of mechanisms may occur simultaneously in an HIV-infected individual. These theories are not backed up by scientific evidence. These are all areas of current research.

DIRECT CELL KILLING. Infected CD4+ T cells may be killed directly when large amounts of virus are produced and bud off from the cell surface, disrupting the cell membrane, or when viral proteins and nucleic acids collect inside the cell, interfering with normal cellular function.

SYNCTIA FORMATION. Infected cells also may fuse with nearby uninfected cells, forming balloon like giant cells called syncytia. In test tube experiments these giant cells have been associated with the death of uninfected cells. The presence of so-called synctia-including variants of HIV has been correlated with rapid disease progression in HIV-infected individuals.

APOPTOSIS. Infected CD4+ T cells may be killed when cellular regulation is distorted by HIV proteins, probably leading to their suicide by a process known as programmed cell death or apoptosis.

Uninfected cells also may undergo apoptosis. Normally, when CD4+ T cells mature in the thymus gland, a small proportion of these cells are unable to distinguish self from non-self. Because these cells would otherwise attack the body's own tissues, they receive a biochemical signal from other cells that results in apoptosis. Investigators have shown in cell cultures that gp120 alone or bound to gp120 antibodies sends a similar but inappropriate signal to CD4+ T cells causing them to die even if not infected by HIV. In addition, T cells from HIV-infected individuals appear more susceptible to apoptosis than those from uninfected individuals.

INNOCENT BYSTANDERS. Uninfected cells may also die in an "innocent bystander" scenario : HIV particles may bind to the cell surface, giving them the appearance of an infected cell and marking them for destruction by killing T cells.

"SHEEP IN WOLVES' CLOTHING". Killer T cells also may mistakenly destroy uninfected CD4+ T cells that have consumed HIV particles and that display HIV fragments on their surfaces. Alternatively, because HIV envelope proteins bear some resemblance to certain molecules that may appear on CD4+ T cells, the body's immune responses may mistakenly damage such cells as well.

ANERGY. Researchers have shown in cell cultures that CD4+ T cells can be turned off by a signal from HIV that leaves them unable to respond to further immune stimulation. This inactivated state is known as anergy.

SUPERANTIGENS. Other investigators have proposed that a molecule known as a superantigen, either made by HIV or an unrelated agent, may stimulate massive quantities of CD4+ T cells at once, rendering them highly susceptible to HIV infection and subsequent cell death.

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FURTHER READING

NIAID's Giuseppe Pantaleo, M.D., Cecilia Graziosi, Ph.D., and NIAID Director Anthony S. Fauci, M.D. review the HIV disease process in "The Immunopathogenesis of Human Immunodeficiency Virus Infection," New England Journal of Medicine 328: 327-335 (Feb. 4, 1993).

Greene, Warner C., "AIDS and the Immune System," Scientific American 269, No. 4: 99-105 (Sept. 1993).

Special issue "AIDS, the Unanswered questions," Science 260: 1209-1396 (May 28, 1993).

The NIAID HIV/AIDS Research Agenda and backgrounders on clinical trials for AIDS therapies and on AIDS-related opportunistic infections are available from the NIAID Office of Communications. To receive free copies, call (301) 496-5717, Monday through Friday, 8:30 a.m. to 5:00 p.m., Eastern Time.

GLOSSARY

Apoptosis: Cellular suicide, also known as programmed cell death. HIV may induce apoptosis in both infected and uninfected immune system cells.

B cell: A white blood cell of the immune system that produces infection- fighting proteins called antibodies.

CD4+ T cell: White blood cells killed or disabled during HIV infection. These cells normally orchestrate the immune response, signaling other cells in the immune system to perform their special functions. Also known as "T helper cells."

Cytokines: Proteins used for communication by cells of the immune system. Central to the normal regulation of the immune response.

Cytoplasm: The living matter within a cell surrounding the nucleus.

Enzyme: A protein that accelerates a specific chemical reaction without altering itself.

Follicular dendritic cells (FDCs): Cells found in the germinal centers of lymphoid organs. FDCs have thread-like tentacles that form a web-like network to trap invaders and present them to other cells of the immune system.

Germinal centers: Structures within lymphoid tissues that contain FDCs and in which immune response are initiated.

gp41: Glycoprotein 41, a protein embedded in the outer envelope of HIV. Plays a key role in HIV's infection of CD4+ T cells by facilitating the fusion of the viral and cell membranes.

gp120: Glycoprotein 120, a protein that protrudes from the surface of HIV and binds to CD4+ T cells.

Intergrase: An HIV enzyme used by the virus to integrate its genetic material into the host cell's DNA.

Kaposi's sarcoma: A type of cancer characterized by abnormal growths of blood vessels which develop into purplish or brown lesions, usually in the skin or mouth, but arising in internal organs also.

Killer T cell: Kills cells infected with HIV or other viruses or transformed by cancer. Also known as cytotoxic T cells.

Lentivirus: "Slow" virus characterized by a long interval between infection and the onset of symptoms. HIV is a lentivirus as is the simian immunodeficiency virus (SIV), which infects non-human primates.

LTR. Long terminal repeat, the RNA sequences repeated at both ends of HIV's genetic material. These act as regulatory switches which are thought to control viral transcription.

Lymphoid organs: Include tonsils, adenoids, lymph nodes, spleen, and other tissues. Act as the body's filtering system, trapping invaders and presenting them to immune cells that congregate there.

Macrophage: A large immune cell that devours invading pathogens and other intruders. Stimulates other immune cells by presenting them with small pieces of the invaders.

Monocyte: A large white blood cell that ingests microbes or other cells and foreign particles. When a monocyte enters tissues, it develops into a macrophage.

Mycoplasma: Smallest free-living organisms known to infect humans. Mycoplasma cause a variety of illnesses, especially of the lungs and sexual organs.

Opportunistic infection: An illness caused by an organism that usually does not cause disease in a person with normal immune system. People with advanced HIV infection suffer opportunistic infections of the lungs, brain, eyes, and the other organs.

Retrovirus: HIV and other viruses that carry their genetic material in the form of RNA and that have the enzyme reverse transcriptase.

Reverse transcriptase: The enzyme produced by HIV and other retroviruses that allows them to synthesize DNA from their RNA.

Syncytia: Giant cells formed by the fusion of other cells.

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