With the help of a special microscope that lights only the surface of the cell, researchers from the Rockefeller University have observed the first real time images of HIV particles assembling on the surface of a living cell to form a single particle of the HIV virus. This deadly virus causing the AIDS disease has claimed the lives of more than 25 million people over the last two and a half decades, thus learning how it assembles may prove useful for developing new treatments for the millions of people still living with the virus.

"The use of this technique is almost unlimited. Now that we can actually see a virus being born, it gives us the opportunity to answer previously unanswered questions, not only in virology but in biology in general", says Nolwenn Jouvenet, a postdoc from the Rockefeller University, who carried out the study with the help of HIV expert Paul Bieniasz from the Laboratory of Retrovirology and cellular biophysicist at the Laboratory of Cellular Biophysics, Sandy Simon.

Typical optical microscopes light the living cell as a whole, but the technique Simon has been experimenting with since the early 1990s only reflects light off the surface of the cell, because this is where HIV particles assemble.

"The result is that you can see, in exquisite detail, only events at the cell surface. You never even illuminate anything inside of the cell so you can focus on what you are interested in seeing the moment it is happening", says Simon.

By shinning light on the cell at a much steeper angle, part of it will pass right through while a small part will be reflected back off the cell's surface. Measurements reveal that each HIV particle, known as a viron, requires between five and six minutes to assemble on the surface of the cell. "At first, we had no idea whether it would take milliseconds or hours. We just didn't know", Jouvenet explains.

"This is the first time anyone has seen a virus particle being born. Not just HIV, any virus", says Bieniasz, who is also a scientist at the Aaron Diamond AIDS Research Center.

During the experiment, in order to make sure that they were observing the virus particle assembling on the cell surface, the team used a Gag protein which has molecules that experience fluorescence and change their color when being packed closer. As the HIV particles attach to the inner membrane of the cell and when Gag molecules flood the area, they spontaneously come to form a sphere.

When Gag molecules are recruited from the cell, they start traveling to the membrane where they form a virion infectious particle that will no longer exchange resource with the cell. Basically, the researchers were able to view the birth of a virus from beginning to the end.

"I think that you can begin to understand events on a different level if you actually watch them happen instead of inferring that they might occur using other techniques. This technique and this collaboration made that possible", says Bieniasz.