What is HIV tropism?
Tropism refers to the way HIV enters cells. The virus enters the human cell by attaching to a protein on the cell. Some types of HIV attach to a protein called CCR5 and others attach to one called CXCR4 – ‘tropism’ refers to which of the two proteins a virus “likes.” Humans have both kinds of proteins on their cells.
Most HIV infections are caused by CCR5-using virus. However, as HIV reproduces it may change from a CCR5-using virus to a CXCR4-using virus. As this transition is taking place the patient can have both types of virus at the same time (dual tropism). Without testing, there is no way to tell whether a patient has CCR5-using, CXCR4-using, or dual tropic virus.
Why test for HIV tropism?
One kind of anti-HIV drug works by blocking the CCR5 protein on the patient’s cells. If HIV is not able to enter the cell it cannot grow. This type of drug is called a “CCR5 antagonist”. Currently, the only drug of this type is maraviroc (Celsentri).
Celsentri prevents CCR5-using virus from entering the cell, but cannot stop the CXCR4-using virus. This is why knowing which tropism the virus has is important. Patients with dual tropism are not good candidates for Celsentri because CXCR4-using virus would continue to grow. Celsentri is best used in patients infected with only the CCR5-using virus.
When should the tropism test be done?
As noted above, HIV tropism can change as the virus reproduces and mutates over time. If the last tropism test is “too old” the test result may not give a true picture of the patient’s current virus. Tropism testing should be done just prior to starting Celsentri, and should be repeated if this drug treatment begins to fail.
How does tropism testing work?
The BC Centre for Excellence in HIV/AIDS laboratory extracts genetic material from the HIV in the patient’s blood and uses sequencing technology (similar to DNA fingerprinting of the virus) to identify which type of virus the patient has.
There are two types of tropism testing. Deciding on which test to order should be based on the patient’s current viral load. If the viral load is > 500 copies/mL, a regular (plasma) HIV Tropism Test should be ordered. If the patient has had HIV viral load testing at St. Paul’s Hospital, this type of tropism testing can be done on stored blood and the patient does not need to have extra blood drawn.
If the viral load is undetectable or < 500 copies/mL, a Proviral HIV DNA Tropism Test should be ordered. This type of tropism testing requires drawing an additional tube of blood.
