Understanding how the human immunodeficiency virus (HIV) works inside the human cell gives scientists important clues about how to attack it at its most vulnerable points. Knowing the secrets of how the virus functions and reproduces itself -- a process called its life cycle -- can help scientists design new drugs that are more effective at suppressing HIV and have fewer side effects. For people with HIV, knowing how HIV works can make it easier to understand the way the drugs work in the body.
Viruses cannot reproduce without the aid of a living cell. Although HIV can infect a number of cells in the body, the main target is an immune cell called a lymphocyte, more specifically a CD4 helper cell, a type of T-cell. T-cells are an important part of the immune system because they help facilitate the body's response to many common but potentially fatal infections. Without enough T-cells, the body's immune system is unable to defend itself against many infections. By ways that are not yet completely understood, HIV's life cycle directly or indirectly causes a reduction in the number of T-cells in the body, eventually resulting in an increased risk of infections.
After HIV enters the body -- through unsafe sex, contaminated needles, blood transfusions or from mother to child (vertical or perinatal transmission) -- it comes in contact with its favorite host cell - the T-cell. When this happens, HIV will hijack the host cell's cellular machinery to reproduce thousands of copies of itself. HIV has to complete many steps in order for this to happen. At each step of HIV's life cycle, it is theoretically possible to design a drug that will stop the virus. Designing drugs to interfere with specific steps in the viral life cycle is called rational drug design.
The following sections outline some of the better understood steps in the viral life cycle, along with the classes of drugs that inhibit these steps. Scientists are just now uncovering the ways HIV manipulates the immune system to spread its infection throughout the body. This review will focus on events that take place when virus and cell are in close proximity.
Once HIV comes into contact with a T-cell, it must attach itself to the cell so that it can fuse with the cell and inject its genetic material (a blueprint for making more HIV) into it. Attachment is a specific binding between proteins on the surface of the virus and proteins that serve as receptors on the surface of the T-cell. Normally, these receptors help the cell communicate with other cells. Two receptors in particular, CD4 and a beta-chemokine receptor (either CCR5 or CXCR4), are used by HIV to latch onto the cell. On the surface of the viral envelope, two sets of proteins (also known as antireceptors) called gp120 and gp41 attach to CD4 and CCR5/CXCR4.
Drugs called attachment or entry inhibitors are currently being studied in clinical trials. These drugs block the interaction between the cellular receptors and the antireceptor on the virus by binding to or altering the receptor sites. Scientists have found that people who naturally lack these cellular receptors because of a genetic mutation, or those who have them blocked by natural chemokines (chemical messengers), may not get infected as readily with HIV or may progress more slowly to AIDS. Scientists are also examining vaccines that may help the body block these receptors.
After attachment is completed, viral penetration occurs. Penetration allows the nucleocapsid -- the genetic core -- of the virus to be injected directly into the cell's cytoplasm. gp120 actually contains three sugar-coated proteins (glycoproteins) and, once gp120 attaches itself to CD4, these three proteins spread apart. This allows the gp41 protein, which is normally hidden by the gp120 proteins, to become exposed and bind to the chemokine receptor. Once this has occurred, the viral envelope and the cell membrane are brought into direct contact and essentially melt into each other.
Drugs called fusion inhibitors prevent the binding of gp41 and the chemokine receptor. T-20 (enfuvirtide, Fuzeon), an experimental fusion inhibitor that is nearing FDA approval, binds to a portion of gp41, preventing it from binding to the chemokine receptor.
Once HIV has penetrated the cell membrane, it is ready to release its genetic information (RNA) into the cell. The viral RNA is protected in the nucleocapsid. The nucleocapsid needs to be partially dissolved so that the virus's RNA can be converted into DNA, a necessary step if HIV's genetic material is to be incorporated into the T-cell's genetic core.
The process by which HIV's RNA is converted to DNA is called reverse transcription. This transcription process happens in almost every human cell, but in the opposite direction -- from DNA to RNA. DNA from the cell nucleus is transcribed into messenger RNA, which then directs the cell's various metabolic functions needed to do its job in the body. HIV uses an enzyme called reverse transcriptase to accomplish this transcription. The single-stranded viral RNA is transcribed into a double strand of DNA, which contains the instructions HIV needs to hijack a T-cell's genetic machinery in order to reproduce itself. Reverse transcriptase uses nucleotides -- building blocks of DNA -- from the cell cytoplasm to make this process possible.
Drugs called reverse transcriptase inhibitors block HIV's reverse transcriptase from using these nucleotides. Nucleoside and nucleotide analog reverse transcriptase inhibitors (NRTIs) -- such as Zerit, Epivir, and Viread -- contain faulty imitations of the nucleotides found in a T-cell's cytoplasm. Instead of incorporating a nucleotide into the growing chain of DNA, the imitation building blocks in NRTIs are inserted, which prevents the double strand of DNA from becoming fully formed. Non-nucleoside reverse transcriptase inhibitors (NNRTIs) -- such as Viramune and Sustiva -- block reverse transcription by attaching to the enzyme in a way that prevents it from functioning.
If HIV succeeds in translating its instructions from RNA to DNA, HIV must then insert its DNA (also called the preintegration complex) into the cell's DNA. This process is called integration. In most human cells, there is a structure called the cell nucleus, where the cell's DNA is stored. In order for integration to occur, the newly translated DNA must be transported across the nuclear membrane into the nucleus.
Although the exact mechanism that HIV uses to transport its genetic cargo into the cell nucleus is still unclear, viral protein R (VPR), which is carried by HIV, may facilitate the movement of the preintegration complex to the nucleus. Once the viral RNA has successfully bridged the nuclear membrane and been escorted to the nucleus, HIV uses an enzyme called integrase to insert HIV's double-stranded DNA into the cell's existing DNA.
Drugs that inhibit the HIV preintegration complex from traveling to the nucleus -- integrase inhibitors -- are currently in early clinical trials.
Viral Latency and Protein Synthesis
After successful integration of the viral DNA, the host cell is now latently infected with HIV. This viral DNA is referred to as provirus. The HIV provirus now awaits activation. When the immune cell becomes activated, this latent provirus awakens and instructs the cellular machinery to produce the necessary components of HIV, like plastic pieces of a model airplane. From the viral DNA, two strands of RNA are constructed and transported out of the nucleus. One strand is translated into subunits of HIV such as protease, reverse transcriptase, integrase, and structural proteins. The other strand becomes the genetic material for the new viruses. Compounds that inhibit or alter viral RNA have been identified as potential antiviral agents.
Cleavage and Viral Assembly
Once the various viral subunits have been produced and processed, they must be separated for the final assembly into new virus. This separation, or cleavage, is accomplished by the viral protease enzyme.
Drugs called protease inhibitors -- such as Kaletra, Crixivan, and Viracept -- bind to the protease enzyme and prevent it from separating, or cleaving, the subunits.
If cleavage is successfully completed, the HIV subunits combine to make up the content of the new virons. In the next step of the viral life cycle, the structural subunits of HIV mesh with the cell's membrane and begin to deform a section of the membrane. This allows the nucleocapsid to take shape and viral RNA is wound tightly to fit inside the nucleocapsid. Researchers are looking at drugs called zinc finger inhibitors, which interfere with the packaging of the viral RNA into the nucleocapsid.
The final step of the viral life cycle is called budding. In this process, the genetic material enclosed in the nucleocapsid merges with the deformed cell membrane to form the new viral envelope. With its genetic material tucked away in its nucleocapsid and a new outer coat made from the host cell's membrane, the newly formed HIV pinches off and enters into circulation, ready to start the whole process again.
During HIV's life cycle, the T-cell, known as the host cell, is altered and perhaps damaged, causing the death of the cell. Scientists are not sure exactly how the cell dies but have come up with a number of scenarios. First, after the cell becomes infected with a virus or other pathogen, internal signals may tell it to commit suicide. This is known as apoptosis or programmed cell death -- a self-destruct program intended to kill the cell with the hopes of killing the virus as well. A second possible mechanism for the death of the cell is that, as thousands of HIV particles bud or escape from the cell, they severely damage the cell's membrane, resulting in the loss of the cell. Another possible cause for the cell's death is that other cells of the immune system, known as killer cells, recognize that the cell is infected and inject it with chemicals that destroy it.
Whatever the mechanism of the cell's death, there is one less T-cell in the body, and with this happening on a monumental scale, T-cells begin to decline. Over time, there are not enough T-cells to defend the body. At this stage, a person is said to have acquired immunodeficiency syndrome, or AIDS, and becomes susceptible to infections that a healthy immune system could deal with. If this process of immune destruction is halted, a weakened immune system may be able to repair some of the damage over time.
There is still much that is not known about HIV's life cycle. More research will enable scientists to coax HIV into giving up more secrets of how it survives and spreads in the body. In turn, this will allow for the development of new drugs and vaccines designed to stop it.
David Pieribone is Associate Director of Education at AIDS Project Los Angeles.