How HIV works

Posted by Dave on February 17, 2010 | 4 Comments

In order to understand how to fight HIV, you need to know how it works. As a reminder, here’s a schematic of an HIV virus from my post two weeks ago:

Remember I told you the HIV virus hijacks the host cells to reproduce? Here’s how that happens.

First, the GP 120 protein on the surface of the HIV virus bonds with a CD4 molecule and another molecule called a coreceptor found on the surface of some T-cells (white blood cells critical to the human immune system). It fuses with the cell and injects its genetic material, RNA, into the T-cell. Below is a diagram of the full HIV life cycle (Note: it’s not to scale. An HIV virus is about 70 times smaller in diameter than a T-cell):

Humans reproduce their genes in the form of DNA, not RNA. All those pretty pictures of cell division in your high school biology textbook are representations of DNA reproducing. We convert DNA into RNA to make proteins, but when we want to make a copy of our genes, we need DNA. Since humans don’t have a way to copy RNA, the HIV virus needs to change its RNA to DNA in order to hijack our replication system. It carries its own personal copy machine, the enzyme reverse transcriptase, to do just that.

Reverse transcriptase is like a little tool that crawls along the RNA molecule and makes a matching strand of DNA, one bit at a time. The bits are individual DNA nucleotides that float around in within human cells. These four nucleotides correspond to four similar nucleotides on the RNA molecule, and when assembled into a chain, they contain all the information necessary to create a new HIV virus, reverse transcriptase and all. Then the human cell finishes the job, creating a matching pair for the DNA strand to turn it into the well-known double helix.

Next another viral enzyme, integrase, is used to fuse the viral DNA into the blood cell’s DNA. Once it becomes a part of the human cell’s own genetic material, the natural protein synthesis system of the cell is used to create many copies of the virus. The same mechanism we use to build all the things we need to survive—bones, muscles, nerves, and so on—is now reappropriated to spread the disease. Finally the new virus buds away from the human cell, building a shell based on the human cell membrane.

Once a T-cell has been infected by a virus, its normal operations are impaired. Ultimately the number of T-cells in the body diminishes, and since these cells are critical for immune system response, it’s easy for people with AIDS to catch diseases that would normally be easily defeated by the natural immune response. And because the HIV virus mutates readily, it’s difficult to formulate an effective vaccine.

Fortunately, because there are several steps involved in HIV spreading through the body, there are also many ways to attack it. The many antiretroviral drugs now on the market attack different parts of that process, and the most successful therapy combines several different drugs. Later this week I’ll discuss some of the promising new avenues of research.

Comments

2 Tweets