What is Viral Tropism?
HIV needs our cells to make copies of itself -- also known as replication. The type of cell the HIV needs is called the CD4 cell. HIV attaches to CD4 cells and transfers its genetic material into the cell. The virus attaches to the CD4 receptor itself as well as a second co-receptor. These receptor molecules provide a place to which HIV attaches. There are two types of these receptor sites:- CCR5 receptors
- CXCR4 receptors
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A person's HIV usually uses one receptor or the other, meaning certain people have an HIV type that uses CCR5 receptors to attach to the cell and others have an HIV types that uses CXCR4 receptors. However, there are even people whose HIV can uses either CCR5 or CXCR4 receptors to attach to the CD4 cell in order to replicate.
Why Does It Matter Which Receptor Your Virus Is?
So we know that people can have HIV that uses on of two receptor types; or in some cases can use both. But why is this significant? It's important because there is now a drug that interrupts attachment to the CCR5 receptor. This drug is the first in the new class of HIV medications called CCR5 Inhibitors. The drug, Maraviroc, blocks the attachment of HIV to the CCR5 receptor, halting HIV replication.CCR5 inhibitors like Maraviroc are only effective in blocking the CCR5 receptor site. Therefore, any HIV that uses the CCR5 receptor will not be able to replicate. However, the HIV that uses CXCR4 receptors will be unaffected, meaning replication can continue. Simply put, CCR5 inhibitors like Maraviroc will only be effective in people with HIV that uses the CCR5 receptor for replication.
How Can You Tell Which Receptor Your HIV Uses?
Since CCR5 inhibitors only work on virus that uses the CCR5 receptor, it makes sense that your doctor would have to know which type of virus your HIV is; one that uses CCR5 or one that uses CXCR4. There is a way to tell which type of virus you have using a blood test called the Trofile Assay.
The Trofile Assay
The trofile assay is a type of genetic testing of a person's blood. The virus in a blood sample is tested against cells that are CCR5 and CXCR4. If the HIV only replicates in the presence of the CCR5 cells, it is said to be CCR5 tropic or R5 for short. If, on the other hand, the virus only replicates in the presence of the CXCR4 cells, it is said to be CXCR4 tropic or X4 for short. On those occasions when HIV replicates in the presence of both CXCR4 and CCR5 cells, it is said to be dual tropic.Once a person's HIV is identified as either R5 of X4, an HIV medication regimen can be developed accordingly; a Maraviroc containing regimen for people who are R5 and a regimen without Maraviroc for X4. For those people dual tropic, a regimen may or may not contain Maraviroc.
Currently the Trofile Assay is the only test available to type a person R5, X4, or dual tropic. At $1,500, the price is a bit steep especially for people without insurance. Other companies are at work trying to develop cheaper and more sensitive HIV tropism tests.
Source: aidsinfonet; "HIV Tropism Tests - Fact Sheet Number 129"; New Mexico AIDS education and Training Center; 4 Oct 2007.
