Apoptosis is a process by which cells in multi-cellular organisms undergo a normal and programmed death. In brief, the cell's nucleus becomes condensed as the cell itself shrivels up, and this remnant is then engulfed and processed by other nearby cells. These cell deaths occur in a completely predictable pattern and do not result in any harmful byproducts being released among neighboring cells. However, many viruses can regulate apoptosis in their host cells to serve their own purposes.
The Process of Apoptosis
Apoptosis is essentially a combination of biochemical events that force a cell to undergo a type of "suicide". These events lead to several key morphological changes in the cell, such as its nucleus and chromatin condensing and fragmenting, an overall shrinking in size, and the cell membrane losing its ability to adhere to surfaces. Cells undergoing apoptosis also change their surface chemistry so that other "disposal" type cells, macrophages, can recognize them. Once recognized, the dying cell will be taken up by a macrophage and then digested.
Apoptosis vs Necrosis
Programmed cell death by apoptosis is a regular facet of normal animal development, and is the ultimate fate of many of the cells generated in most animals. However, cells which die accidentally, such as by physical damage or poisoning, generally undergo a process called "necrosis". In necrosis, a cell will usually swell up and burst, spilling its components and possibly harmful substances into the surrounding extra-cellular space. This triggers an inflammatory response in the area and can lead to further problems in the specific tissue or organ.
Apoptosis and Development
The process of apoptosis and its regulation have been highly conserved in evolution across the entire animal kingdom, as it is an extremely important process in animal development. For example, in order for the fingers and toes of a human embryo to develop normally, apoptosis must occur in the cells between the specific finger and toe areas; this allows the digits to separate properly. Apoptosis also gets rid of cells which have finished their job, such as when a tadpole loses its tail as it changes into a frog.
Apoptosis and Viral Infection
While viruses can trigger apoptosis in host cells by a number of ways, many times a virus will inhibit the cell's programmed cell death so that it can grow and multiply without killing its host. For example, numerous viruses can generate a molecule similar to Bcl-2, a protein which normally inhibits the start of apoptosis in a cell. This Bcl-2-homologue can then prevent pro-apoptosis proteins in the host cell from starting the mechanism of programmed cell death.
Apoptosis and HIV
In order for an infection resulting from the HIV virus to progress to AIDS, an individual's CD4+ T-helper lymphocyte population must be substantially decreased. This results in a compromised immune system, and the HIV virus itself can actually induce apoptosis in these immune cells through several biochemical means. HIV enzymes can activate pro-apoptosis proteins in the cell, such as procaspase-8. Alternatively, HIV enzymes can also inactivate the host cell's Bcl-2 proteins, molecules which prevent apoptosis from occurring.