In what may be a significant advance in HIV research, scientists at the University of Colorado Health Sciences Center have discovered a "flag" present in HIV-infected cells that will allow doctors and scientists to pinpoint exactly which cells are infected. Before the discovery, HIV only was able to be detected through the presence of HIV antibodies and no test was available to determine which cells were infected.
"As an analogy," said Alex Franzusoff, Ph.D., associate professor of cellular and structural biology at the University of Colorado Health Sciences Center, and principal investigator of the study, "imagine you have all these mailboxes. From a distance, you can’t determine which of them have mail in them. But if you can see which ones have a red flag up, you now know which ones do."
The discovery relied on a new method called "cell-sorting," which, according to Franzusoff, uses a machine able to sort cells by the types of proteins on their surfaces. The study, which was funded by the National Institutes of Health and published in the June 11 issue of Proceedings of the National Academy of Sciences, used the machine to isolate and examine HIV-infected CD4 T-cells. CD4 T-cells are the cells that become infected by the HIV virus.
Although HIV replicates in these T-cells, prior studies examined other cells because, according to Franzusoff, "it’s hard to do these experiments on T-cells. But when you don’t, you get wrong answers. Since we took the time to do the experiments with T-cells, we found things that were overlooked."
Franzusoff started the study because he realized "something was wrong with the way they said (HIV) replicated."
It was thought that HIV sent an envelope protein to the surface of the T-cell in order to continue its development. The problem with this is that it damages the T-cell and "T-cells are sensitive cells that would rather die than live as a damaged cell," said Franzusoff.
"They’re very prone to suicide. HIV had to evolve to use the cells. It had to develop special tricks so that it could replicate."
By studying T-cells, though, Franzusoff discovered that "HIV would store bits of itself in cellular compartments until it was completed and then make these compartments go to the surface." By keeping all of the activities for virus production inside the cell until the last moments, HIV is able to remain hidden from the body’s immune system.
But the research also revealed a way to determine which cells have HIV replicating inside them. "We have discovered the ‘flag’ that identifies which cells in the blood are actively making new virus, " said Franzusoff. It was concluded that a protein, CTLA4, shows up on the surface of infected T-cells at the time they are making new virus. Clinical studies from other groups have proven that people with higher levels of virus in their blood also have higher numbers of T-cells that show CTLA4 on their surface.
"This method could change how clinicians evaluate HIV infection and monitor the effects of anti-viral drugs," Franzusoff said.
"It can also be used as a reliable marker for monitoring progression to AIDS.
Since individual HIV-infected cells can now be isolated, we can search for new ways to kill these cells before they have a chance to spread the virus." The new method of detecting does not require specialized equipment. "It’s just a matter of adding another antibody to the tube," Franzusoff said.
"This technique of flagging the cells will allow us to specifically kill or remove them before they replicate, and that could never be done before," said Franzusoff.
Franzusoff plans to continue studying T-cells and will now research techniques to either kill or remove the HIV-infected cells.
Co-authors on the study include Luis R. Miranda, Brian Schaefer, Abraham Kupfer, and Zixin Hu, representing the CU Health Sciences Center, the CU Cancer Center, National Jewish Medical and Research Center and GlobeImmune Inc. The study is available at www.pnas.org.