Life Cycle of the Human Immunodeficiency Virus
HIV binds to the outer surface of CD4+ cells, enters the cells, and then remains hidden and protected from the other immune system cells. Safely inside the cell, it duplicates its RNA. RNA is very similar to DNA. It serves the same genetic function, but has a slightly different molecular makeup. The new viral DNA is integrated into the host cell's DNA where it governs the production of new HIV virions (single viruses). The new virions leave the host cell to infect other cells, and the host CD4+ cell dies.
The body produces about 10 billion new virions daily, and the immune system destroys and removes all but about 100 million of them, which are infectious. An equal number of CD4+ cells are produced and destroyed by the virions, creating a balance of power struggle between the virus and the CD4+ cells.
The HIV life cycle requires specific enzymes, which serve as targets for antiretroviral drug therapy:
- Reverse transcriptase helps create DNA copies of HIV's RNA. Nucleoside and non-nucleoside antiretroviral drugs block reverse transcriptase, preventing HIV from copying its RNA into DNA.
- Integrase helps integrate the viral DNA into the host cell's DNA. Integrase is a potential target for drug therapy, and scientists are hoping to find a way to block it to prevent viral DNA from being integrated into the host cell's DNA.
- Protease helps assemble the new virions. Protease inhibitors prevent protease from performing this function.
In some cases, HIV does not start replicating immediately upon entering a new host cell. Once the DNA enters the host cell's genome, HIV can persist for years inside the body without causing the symptoms that define AIDS. But even at this stage (called latency), the virus can still be transmitted to others. Latency is one of the greatest challenges to finding a cure or vaccine for AIDS and is why people with AIDS must take antiretroviral drugs for life. Currently, there is no way to get rid of HIV once it has entered the body.
Other Immune Cells
Despite the critical role that CD4+ cells play in the HIV life cycle and the progression of AIDS, it is not entirely true that destruction of CD4+ cells causes AIDS. Although its greatest effect is on the CD4+ cells, HIV infects and damages other immune system cells as well.