AIDS

AIDS stands for Acquired Immune Deficiency Syndrome.

It represents the late stages of infection by a retrovirus called Human Immunodeficiency Virus (HIV).

The immune deficiency is caused by the loss of the CD4⁺ T cells that are essential for both cell-mediated immunity and antibody-mediated immunity.

HIV — Human Immunodeficiency Viruses

• HIV-1 — the cause of AIDS in the Western Hemisphere and in Europe
• HIV-2 — the major cause of AIDS in Africa and Southeast Asia.

Infection

• the transmembrane protein CD4 found on helper T cells and
• a second "coreceptor" on these cells:
  • Strains of HIV (designated "R5") bind the coreceptor CCR5. These are the strains that are most infectious.
  • Strains of HIV (designated "X4") bind the coreceptor CXCR4.
  • Both strains usually coexist in an ongoing infection with X4 tending to dominate in the final stages of AIDS.

The virion binds to both CD4 and either coreceptor with molecules on its surface called glycoprotein 120 (gp120).

This binding triggers an allosteric change in a second molecule, called glycoprotein 41 (gp41), which penetrates the host plasma membrane allowing the virion to get inside.

When HIV infects a cell

• its molecules of reverse transcriptase and integrase are carried into the cell attached to the viral RNA molecules.
• The reverse transcriptase synthesizes DNA copies of the RNA.
• These enter the nucleus where the integrase catalyzes their insertion into the DNA of the host's chromosomes.
• The HIV DNA is transcribed into fresh RNA molecules which reenter the cytosol where
  • some are translated by host ribosomes.
    • The env gene is translated into molecules of the envelope protein (gp160).
      Proteases of the host cell then cut gp160 into
      • gp120 which sits on the surface of the virions (and is the target of most of the vaccines currently being tested).
      • gp41, a transmembrane protein associated with gp120.
    • the gag and pol genes are translated into a single protein molecule which is cleaved by the viral protease into
      • 6 different capsid proteins
      • the protease
      • reverse transcriptase
      • the integrase
  • other RNA molecules become incorporated into fresh virus particles

Disease Transmission

• especially blood and semen
• especially in the early and late phases of the disease

Breaks or abrasions in mucous membranes and skin allow the virus in.

• between men when one ejaculates into the rectum (or mouth — the adenoids and tonsils are filled with dendritic cells) of the other
• among intravenous drug users who share needles
• in women who are the sexual partners of bisexual men or i.v. drug users
• in the newborn babies of these women
• in recipients of infected blood or blood products. This last category accounted for a devastating epidemic among hemophiliacs in the 1980s who unknowingly used HIV-contaminated preparations of factor VIII. In some areas, 90% or more of the hemophiliacs developed AIDS.

  That risk, and the risk from blood transfusions, is now virtually zero because

  • all donated blood is now tested to see if the donor has been infected with HIV (as well as some other viruses);
  • plasma-derived preparations of factors 8 and 9 are now treated with heat and/or solvents to destroy any viruses that might be present;
  • recombinant factor 8 and recombinant factor 9 made by genetic engineering are now available.

Disease Progression

• an early phase that
  • lasts about 2 weeks
  • is accompanied by fever, aches, and other flu-like symptoms
  • is accompanied by high levels of virus in the blood.
• a middle phase with these features:
  • lasts for months or even years
  • produces few, if any, symptoms
  • The patient's blood contains few viruses, but contains antibodies to the virus. These antibodies are the basis of the most common test for HIV infection.
  • Continuous infection, death, and replacement of CD4⁺ T cells. The T cells are probably killed by the patient's CD8⁺ cytotoxic T cells (CTL). Some may die from apoptosis.
• It is the late phase that is called AIDS. It has these features:
  • A rapid decline in the number of CD4⁺ T cells. When these drop below about 400 per µl (normal is >1000), the patients immunity is sufficiently weakened that opportunistic infections begin. These are infections caused by organisms that ordinarily do not cause disease symptoms in immunocompetent people. They include:
    • viruses, e.g., herpes simplex, herpes varicella-zoster, Epstein-Barr virus (EBV)
    • bacteria, e.g., Mycobacterium tuberculosis
    • fungi, e.g. Candida albicans (the cause of "thrush"), Pneumocystis jirovecii (causes pneumonia)
    • protozoans, e.g., Microsporidia
  • When the CD4⁺ count drops below 200 per µl, opportunistic infections become more severe and cancer (e.g., lymphoma, Kaposi's sarcoma) may develop. These usually kill the patient within a year or so.

Treatment

In affluent countries, the progression of HIV disease has been markedly slowed by the use of HAART (= Highly Active AntiRetroviral Therapy).

This refers to combined therapy with three or more drugs, usually two that target the reverse transcriptase and one that targets the viral protease.

Reverse Transcriptase Inhibitors

• Nucleoside analogs. Examples:
  • zidovudine (AZT)(Retrovir®)
  • lamivudine (Epivir®)
  • didanosine (Videx®)

  Each of these drugs "fools" the reverse transcriptase into incorporating it into the growing DNA strand which then halts further DNA synthesis.

  Link to illustrated discussion of how, zidovudine (AZT), does this.

  
  
• Other Reverse Transcriptase Inhibitors

  These drugs, e.g., Viramune® inhibit the enzyme by other mechanisms.

Protease Inhibitors

• indinavir (Crixivan®)
• saquinavir (Invirase®)
• ritonavir (Norvir®)

Fusion Inhibitors

The allosteric change that enables gp41 to penetrate the host plasma membrane involves noncovalent binding between two segments of its chain designated HR1 and HR2.

Enfuvirtide, a synthetic polypeptide containing 36 of the amino acids present in the HR2 segment, interferes with this process. It probably acts as a kind of competitive inhibitor, binding to HR1 thus preventing HR2 from binding HR1.

Enfuvirtide (Fuzeon®) has shown promise in phase III clinical trials.

Integrase Inhibitors

A drug that inhibits the HIV-1 integrase has safely slowed disease progression in experimental animals (monkeys).

Problems with drug treatment

• slowing the progression of the disease
• reversing — at least for a time — the symptoms of the late stages of the disease
• preventing the infection of babies born to infected mothers

• The drugs are so expensive ($7,000 to $10,000 per year) that they not only drain resources in affluent countries but are simply unavailable in the many poor countries where the epidemic rages.
• They have many unpleasant side-effects (e.g., nausea, diarrhea, liver damage).
• They demand a very complicated dosing regimen: over a dozen pills a day (not counting those needed to cope with the accompanying opportunistic infections).
• They often lose effectiveness as they select for the emergence of drug-resistant virions in the patient. This latter problem is particularly serious because of the speed at which mutations occur in HIV (as we shall now see).

Genetic Variability of HIV

• The population of viruses in a single patient becomes genetically more diverse as time goes by. This can lead to:
  • appearance of strains that invade other types of cells such as X4 strains that target T cells and strains that target cells of the brain, etc.
  • Development of resistance to the anti-viral drugs being used.
• New strains and subtypes of HIV-1 and HIV-2 arise in the human population.
  • These complicate the efforts to develop a vaccine against HIV
  • But — as we shall now see — have helped to unravel the origins of the disease.

Origin of HIV

• HIV-1 is most closely related to a SIV found in chimpanzees (Pan troglodytes troglodytes)
• HIV-2 is most closely related to a SIV that occurs in the sooty mangabey (Cercocebus atys).

• HIV-1 appears to have jumped from chimpanzees to human on at least 3 separate occasions (there are three clades; M, N, and O). Except in parts of West Africa, most human cases are caused by members of Group M.
• HIV-2 appears to have jumped from sooty magabeys to humans on at least 4 different occasions (there are 4 clades).
• How? These (and other) primates are often slaughtered for food and exposure to their blood and tissues is probably the route of transmission. In fact the chimpanzee SIV that gave rise to HIV-1 appears to be itself the product of recombination between two monkey SIVs that infected chimpanzees. (Chimps often eat monkeys.)

Just as with other evolutionary trees, one can also estimate from genome sequences the time of divergence of two branches. This evidence indicates that the Group M clade of HIV-1 invaded humans sometime early in the 20th century perhaps around 1930.

But the worldwide epidemic of AIDS did not get its start until the 1980s.

• The ease of transmission. The transmissibility of HIV is very low. HIV is not influenza or measles which spread like wildfire.
• The length of time the host remains contagious. Again, HIV is not like influenza or measles where the period of contagiousness is just a few days. For HIV, it can be years.
• The number of susceptible contacts; that is, the proximity of potential new hosts. For sexually-transmitted diseases (STDs), that means the number of sexual contacts.

Prevention of AIDS

Vaccines

Over two dozen experimental anti-HIV vaccines have been developed and clinical trials of some of these have been — and are presently being — undertaken.

So far, the results have been disappointing. There are probably several reasons.

• The gene (env) encoding the envelope protein mutates too rapidly.
• The virus may be able to stay within cells out of the reach of circulating antibodies. High levels of antibodies (the basis of the most common test of infection) persist even while the disease pursues its inexorable course.

Many of these are DNA vaccines — molecules of DNA that incorporate

• a plasmid or live virus such as canarypox (a harmless relative of smallpox) which serves as
• a vector for introducing HIV genes.

It is hoped that expression of these genes within the cells (e.g., muscle) of the subject will induce a protective immune response.

Behavior

• Reducing the number of sexual partners.
• If injecting drugs cannot be stopped, then using sterile needles (thus not sharing them) would prevent infection.
• Using condoms and/or other (e.g., chemical) barriers to prevent contact with infectious semen.

"Unlike microbial scourges, such as malaria and tuberculosis (among many others), for which there is very little that people can do to prevent infection, HIV infection in adults is entirely preventable by behavior modification" [emphasis added].