AIDS is an epidemic of global proportions. Whereas about 1 million people in the United States (about 0.5% of the total population of the US) are currently infected with the Human Immunodeficiency Virus (HIV) which causes AIDS, the infection rates are far higher and growing rapidly in other parts of the world such as sub-Saharan Africa, the Indian sub-continent, and Asia. As many as 25% of the sexually active populations of some cities in Rwanda may be infected, and at least 90% of the female prostitutes in Nairobi, Kenya are HIV infected today. In India, it is estimated that there are 700,000 to 1 million people infected with HIV, and that number is increasing unchecked. By 1993, over 2.5 million people worldwide were infected with HIV. Statistics from September, 1994, state that 401,789 cases of AIDS and 250,000 deaths from AIDS had been reported in the United States at this time (up from 240,000 and 160,000 respectively in October of 1992). By the end of this decade, a cumulative worldwide total of 40 million people will likely be infected--more than all those killed in World War II.
Figure 1.1. Graph of AIDS cases in US.
Figure 1.2. Newspapers 1 (statistics and global issues--choose some)
The spread of AIDS will ultimately have a devastating social and economic impact. In the African countries that have been hit hardest by this disease, whole generations have been lost. Orphanages are filled with children whose parents are too sick to care for them, and grandparents and children live together in poverty because the wage earners in the family have died.
Figure 1.3. Newspapers 2 (Social implications, etc of AIDS--choose some)
In Africa, the AIDS epidemic which is mostly concentrated in cities and urban areas probably spread along truck routes. In the United States, a report in 1981 by Dr. Michael Gottlieb at the University of California at Los Angeles described a rare form of pneumonia occurring in homosexual men. The symptoms were consistent with damage to the immune system, and the disease appeared to be acquired rather than inherited. Other reports from around the country noted similar developments, and it soon became apparent that a new disease had surfaced.
In addition to pneumonia, AIDS is associated with numerous other infections. These secondary infections are caused by bacteria, fungi, protozoa, and other viruses. It is these secondary infections (called opportunistic infections) which usually cause death in AIDS patients. Cancers including lymphomas and Kaposi's sarcoma are also frequently developed by people with AIDS. HIV infection can result in damage to brain cells which leads to loss of mental function and is referred to as AIDS dementia.
Often, no visible symptoms accompany the early stages of infection by HIV. The infected person may look and feel completely normal at this time, although such a person can transmit the infection. This incubation period is on average 8 to 10 years. This makes studying and controlling AIDS very difficult, since many people infected with the virus have not yet developed the disease.
Figure 1.4. Symptom development Timelines.
HIV is actually not a very robust virus. Transmission isn't all that easy, and the virus becomes inactive quickly when exposed to air, light, or water. The three main modes of transmission are birth, blood, and sex. Current statistics indicate that there is about a 15-30% chance that a child born from an infected mother will be infected. Sharing needles during intravenous (IV) drug use can transmit HIV since infected blood can be directly injected into the bloodstream. After 1985, when screening of the blood supply for HIV was begun, the risk of contracting the HIV virus from a blood transfusion dropped to 2.25 out of 100,000. Intimate sexual contact with an HIV-infected person is the most likely source of HIV infection for most people in the general public. Casual contacts such as shaking hands, hugging, kissing, sharing eating utensils, sharing towels or napkins, and using the same telephone pose no risk for HIV infection.
Table 1.1. HIV in body fluids (131 boa)
| Degree of association | Body Fluid |
|---|---|
| very high | blood, semen, vaginal/cervical secretion (include. menstrul fluid) |
| high | breast milk |
| low or no | saliva, tears, persiration/sweat, urine, feces |
Although AIDS is a terrible disease for which there is currently no cure, three equally important approaches are being used to help combat AIDS: education, research, and treatment. Education in high-risk groups is critical to reducing the spread of HIV; education of the general public will reduce some of the irrational fears and reduce discrimination toward individuals with HIV and AIDS. Research will focus on both prevention of infection by HIV via the development of a vaccine and on finding new drugs to arrest the symptoms of (or, ideally, cure) AIDS and the opportunistic infections which accompany it. Treatment of the large number of people likely to develop the disease in the future will greatly strain our current health-care resources, and society will have to develop appropriate methods to care for future AIDS patients.
In order to understand AIDS, it is important to understand the basic workings of the immune system. First, consider blood. One major function of blood is protection from infection by foreign agents, and the cells of the blood responsible for this task make up the immune system. The immune system protects us from infectious agents which include viruses, bacteria, protozoa, fungi, and multicellular parasites. Also, the immune system is important in fighting cancer. Cells of the immune system are divided into two classes; those that respond to a specific foreign agent (lymphocytes) and those that are not specific for the agent they attack (phagocytes, mast cells, eosinophils, and natural killer cells). Phagocytes, mast cells, and eosinophils are intructed by antibodies to attact particular cells. Antibodies, proteins produced by certain lymphocytes, bind specifically to a foreign agent or antigen. This allows the phagocytes, mast cells, and eosinophils to recognize the target, and they attack.
Figure 1.5. Immune system at work (31 boa)
Focusing on the lymphocytes which respond to specific antigens (foreign invaders) in the body, the two classes of cells are B-lymphocytes (B-cells) and T-lymphocytes (T-cells). B-lymphocytes are the cells which secrete the antibodies that recognize and bind to specific antigens. Each B-cell makes only one kind of antibody. The body's immune response is based on generation of many B-cells with different antibody specificities and rapid production of B-cells that recognize their specific antigen when infection occurs. Antibodies generated by B-cells fight infections by direct neutralization of viruses, binding to targets and signaling phagocytes or other white blood cells to attack, or binding to target cells and signaling for other host defense mechanisms.
T-cells are similar to antibodies in that they also bind to the surfaces of specific antigens. Unlike B-cells which generate antibodies to bind to the antigens, the T-cells themselves are involved in binding. Tkiller-cells bind to the surfaces of and then eliminate foreign cells. Thelper-cells signal to B-cells or Tkiller-cells and help them to respond to antigens. All T-cells have characteristic proteins on their surfaces: the CD8 protein is present on Tkiller-cells and the CD4 protein is present on Thelper-cells. We will learn more about virus activity shortly, but at this point it is important to point out that, when HIV infection occurs, the virus recognizes and binds to the CD4 protein which is found on Thelper-cells. Eventually, the virus causes these cells to die.
Figure 1.6. T-cell and HIV (45 boa)
In healthy people, the number of Thelper-cells per cubic millimeter of blood are typically over 1000. In AIDS patients, there may be well under 100 Thelper-cells per cubic millimeter. Currently, a Thelper-cell count of under 200 is used as the defining mark for people with full-blown AIDS. People with a Thelper-count of more than 200 are said to be HIV-infected; people with a Thelper-count of less than 200 are said to have advanced to AIDS.
Viruses are among the simplest life forms. They are parasites; they cannot replicate and make more of themselves outside of cells. In humans, this means that viruses must replicate in some tissue or cell type in our bodies.
Virus particles consist of just two main components--genetic material and a system for protecting this material and introducing it into a cell (Figure 7). The viral genetic material is carried in the form of either DNA or RNA which are chemically related compounds made of nucleic acids. The genetic material of a virus specifies virus proteins which may be regulatory proteins that help the virus to take over a host cell, structural proteins that help with the growth of the virus particles, or enzymes that help carry out biochemical processes necessary to the virus. Because this genetic material is quite fragile, viruses carry genes that direct production of a protein coat to surround the genetic material. Some viruses also direct synthesis of a viral envelope that surrounds the virus's genetic information and protein coat. For many basic functions including energy metabolism, protein synthesis, and nucleic acid synthesis, viruses depend on cells.
Figure 1.7. Virus cartoon
The typical virus infection cycle is shown in Figure 8. First, the virus binds to the cell by interacting with a specific protein on the cell surface. Viruses have evolved so that they can bind to a protein that is normally present on an uninfected cell. Second, the virus must penetrate the cell. Once inside the cell, the protective protein coat is removed from the virus and the genetic material is released. Next, expression of the viral genetic material occurs.. This is accomplished by organization of the infected cell, replication of the viral genetic material, and synthesis of proteins for virus particles. Once the proteins have been synthesized, all the parts necessary for formation of a new virus particle are present in the infected cell. In the final stage of the infection cycle, assembly of the virus particles and release from the cell, hundreds or thousands of new virus particles can be released. This spreads the infection to other cells.
Figure 8 Infection cycle of a virus.
Once a virus infection has become established, it is very difficult to eliminate. Only a few antivirals, compounds that specifically inhibit a viral process, have been identified. Azidothymidine (AZT) which is used to treat AIDS is one example of an antiviral. Management of symptoms caused by viral infections includes treatment to reduce fevers, classical antibiotics, and bed rest.
Human Immunodeficiency Virus (HIV) belongs to a class of viruses called retroviruses. The retrovirus (Figure 9) is much like a normal virus; the main difference is in the direction of flow of genetic information. Normally, genetic information flows in this direction:
DNA --> RNA --> Protein
In a retrovirus, genetic information is stored in the form of RNA. During the life cycle of the retrovirus, the RNA is converted to DNA by a unique virus-specified enzyme called reverse trascriptase. In a retrovirus, the flow of genetic information is not in the customary order.
Figure 1.9 Retrovirus cartoon, AIDS virus pictures (61, 65 boa; 23 who)
The life cycle of a typical retrovirus is shown in Figure 10. First, the retrovirus binds to the surface of an uninfected cell by recognizing a cell receptor (a protein on the cell surface). After binding, the virus particle is brought into the cytoplasm of the cell and the viral envelope is removed. The reverse transcriptase enzyme is activated to read viral RNA, and in this way viral DNA is made. The viral DNA makes its way into the nucleus of the cell and is incorporated into the chromosome. Viral DNA now resembles any other cell gene. As a result, the normal cell machinery reads the integrated viral DNA to make more copies of viral RNA. This viral RNA is then used for two purposes. It is used in the cytoplasm as a viral messenger RNA to program the formation of viral proteins and it is used to become genetic material for new virus particles by combining with viral proteins in the cytoplasm. These virus particles are formed at the surface of the cell and leave the cell by a process called budding.
Figure 1.10 Life cycle of retrovirus (63 boa)
HIV is a retrovirus that begins its infection cycle by recognizing and binding to CD4 proteins on the surfaces of uninfected Thelper-cells. CD4 proteins are also found on the surfaces of macrophages, and these cells are also infected by HIV. Most other cells in the body do not have CD4 surface proteins, so HIV infection is specific for these two types of cells from the immune system.
In addition to the three genes that all retroviruses carry (for coat proteins, reverse transcriptase, and envelope proteins), HIV contains genes that specify six regulatory proteins which give HIV finer levels of contol and a more versatile life cycle. These extra genes may be important in allowing the virus to establish a latent or inactive state in some infected cells followed by reactivation at later times.
Figure 1.11 Special features of HIV retrovirus (cartoon) (66 boa)
Although the most effective prevention of HIV infection would be a vaccine that blocks virus infection, such a vaccine may be difficult to develop. HIV has a high mutation rate, can establish a latent state in some cells, and can spread by cell-to-cell contact. HIV evades the immune system very efficiently, so a vaccine whose goal is to raise a protective immune response to the infectious agent will be hard to make. Further, a second strain of HIV has been detected in some areas of Africa. The common virus is HIV-1; the new virus, HIV-2, resembles HIV-1 structurally and also causes AIDS, but one vaccine for both strains of HIV may not be obtainable. In spite of these hurdles, several different vaccines are nearly ready for trials in humans.
Figure 1.12 Newspapers 3 (vaccines)
Figure 1.13 Newpapers 3 (treatment)
Understanding the life cycle of HIV (Figure10) allows scientists to use different approaches to stop viral replication in people who are already infected with HIV. The first point in the life cycle that could be targeted is the entry of the virus into the cell. Some work has been done in this area focusing on the surface protein, CD4, which is the receptor site for the virus. Test-tube experiments have shown that adding large amounts of CD4 to a mixture of virus and T-cells prevents infection of the T-cells by HIV. The many CD4 proteins bind to HIV and effectively cap the ends that would normally bind to CD4 on the surface of a T-cell. Also, a drug used as a blood anticoagulant called dextran sulfate has been shown to interfere with viral entry, although the mechanism of action is not understood.
After the virus has entered the cell and the protective coat has been removed, one could envision interfering with functions necessary to the virus such as reverse transcription. Indeed, the first and still most widely used treatment of HIV targets reverse transcriptase, the enzyme that synthesizes viral DNA. Zidovudine (AZT), Zalcitabine (ddC), and Didanosine (ddI) are three drugs that have been approved by the FDA which, although the exact mechanism of action is unknown, are believed to interfere with reverse transcriptase. These drugs can be incorporated into a chain of DNA generated by reverse transcriptase in place of the usual nucleotides that make up DNA, but they lack the 3'-hydroxyl required for DNA chain extension. At this point, the virus life cycle is effectively shut down because the genes are incompletely formed.
Figure 1.14 AZT, ddC, ddI
The problem with drugs like AZT is that they are not completely selective for incorporation by viral reverse transcriptase. The host's cellular enzymes may also incorporate AZT, ddI, or ddC into DNA. This halts production of cellular DNA, and the host cell dies. AZT is about 100 times more readily incorporated by reverse transcriptase than by normal cellular machinery, but some damage will inevitably occur to the host cells, especially at high dosages. For this reason, many other compounds similar to the three in Figure 14 have also been synthesized, but no miracle drug has yet been found to target reverse transcriptase.
Scientists have also targeted other viral proteins. Viral proteins are attractive targets because they are vital to the virus but not necessary for the survival of the host. One viral protein that is being intensively investigated as a target for antiviral therapy is HIV protease. Several protease inhibitors are being tested in clinical trials on HIV-infected people, and protease inhibitors will be discussed in much more detail shortly.
The final noteworthy place at which to interrupt the virus life cycle is the deactivation of viral RNA. Compounds called antisense molecules are being studied for this role. Antisense molecules are small pieces of single-stranded DNA or RNA that can form double-stranded complexes with HIV viral RNA. Formation of double-stranded complexes can lead to destruction of viral RNA which means that viral protein, viral RNA, and virus particles cannot be produced. Other compounds called ribozymes which are very specialized antisense RNA molecules are also being studied. Ribozymes attack HIV RNA and cause cutting at particular sites. This inactivates the virus.
One big problem with arresting the life cycle of HIV is that the virus life cycle is intertwined with the cell life cycle. It is difficult to stop one life cycle and keep the other functioning. For this reason, the viral proteins are very attractive targets. Specific inhibition of viral enzymes is an area of much current focus. And, although there is no immediate obvious solution to the problem of halting the HIV life cycle, many strides have been made. Compare the timeline of progress against HIV with the timelines of other infectious diseases as shown in Table 2.
Table 1.2 Timeline (p157 boa)
| Disease | 1st Documented Epidemic | Isolation of Agent | First Therapy |
|---|---|---|---|
| Plague (yersinia pestis) | 560 AD | 1894 | 1940's (antibiotics) |
| Polio (polio virus) | 1885 AD | 1909 identified 1949 isolated | 1953 (Salk vaccine) |
| AIDS (HIV) | 1981 AD | 1984 | 1986 (AZT, partially effective) |
Altman, L. K. New York Times January 31, 1995, B8. “AIDS is Now the Leading Killer of Americans From 25 to 44.
Bode, E.; Wright, M. “HIV-1 Proteases: An Enzyme at Work” (UW Chem. Dept. video and handout)
Cohen, J. Science 1994, 266, 1154. “At Conference, Hope for Success is Further Attenuated”
Fan H. The Biology of AIDS 3rd ed. Jones and Bartlett Publishers, Boston, 1994.
Huryn, D.; Okabe, M. Chem. Rev. 1992, 92, 1745-1768. “AIDS-Driven Nucleoside Chemistry”
New York Times March 17, 1994, B10. “Benefits of Often-Used AIDS Drug Are Questioned”
Science 1993, 260, 1253-1286. “AIDS: The Unanswered Questions”
