March 30, 2009
Sacramento, Calif. -- For the first time, scientists at UC Davis and Mount Sinai School of Medicine have captured video footage showing the transfer of the human immunodeficiency virus (HIV) from an infected T-cell to an uninfected one through an adhesive structure called a virological synapse. The researchers hope their breakthrough findings, which appear in the March 27 issue of Science, will lead to a new era in HIV treatment and revive hope of developing a vaccine to halt the virus that leads to AIDS.
"Our findings may explain why attempts to develop an HIV vaccine have so far been unsuccessful," said Thomas Huser, one of the study's authors and chief scientist at the UC Davis Center for Biophotonics Science and Technology (CBST), where the video images were produced using advanced, live-cell video imaging microscopy.
While previous efforts to create an HIV vaccine have focused on priming the immune system to recognize and attack surface proteins of free-circulating virus, the current results indicate that HIV avoids recognition by being directly transferred between cells.
"We should be developing vaccines that help the immune system recognize proteins involved in virological synapse formation and antiviral drugs that target the factors required for synapse formation," explained Huser, who is also an associate professor in the UC Davis Department of Internal Medicine.
For decades, scientists believed that HIV mainly spreads in the body through free-circulating particles that attach to a cell, take over its replication machinery and make multiple copies of themselves. Once in the bloodstream, the new particles attach to target cells and continue the process.
In 2004, scientists discovered that cell-to-cell transfer of HIV also occurred via virological synapses. This was considered to be an effective method of transferring the infection, but the reasons were unclear. The current study, however, reveals that the synapse is providing the essential structure by which viral proteins are gathered and relocated to uninfected cells.
"Direct T-cell to T-cell transfer through a virological synapse is a highly efficient avenue of HIV infection, and it could be the predominant mode of dissemination," said study senior author Benjamin Chen, assistant professor of medicine and infectious diseases at the Mount Sinai School of Medicine.
Chen made the study possible when he created a molecular clone of infectious HIV by inserting into its genetic code a gene that codes for the green fluorescent protein (GFP), a molecule originally isolated from a species of jellyfish. The protein glows when exposed to blue light, making it visible on digital video.
Chen, however, required the expertise of UC Davis researchers to record the behavior of live cells. Using the video microscopy developed at CBST, the researchers filmed interactions between cells infected with GFP-labeled HIV-infected T cells and uninfected ones, generating 19 hours of video footage.
In the videos, the researchers observed an infected cell coming into contact with a healthy one. Huser said that the two cells appear to struggle as a virological synapse is formed at the point where they connect, and, within minutes, fluorescent viral particles in the infected cell move toward the synapse and into the healthy cell.
According to Huser, the technology developed at CBST and the unique infrastructure available at the Oak Park Research Building on the Sacramento campus of UC Davis made the current study possible.
"We were able to determine the physical parameters of HIV transfer because of our ability to observe these interactions in real time and in three dimensions," Huser said.
In creating a key technology used in this study, Huser and his colleagues modified a commercially available spinning disk confocal microscope so that it could be used to image live infected cells. The technology allowed the capture of 30 images per second from various angles and at various depths of field. Computer programs then merged the images, creating a 3-D movie.
"CBST's interdisciplinary team was key to the successful collaboration with Mount Sinai," Huser said. "Few facilities have both virologists with experience handling live HIV samples and experts in high-speed, high-resolution imaging."
The Mount Sinai-UC Davis team is now planning to find out as much as they can about how HIV transfer through virological synapses works. For example, the team will be working to identify the exact process involved in the movement of viral particles, which they believe involves the scaffolding of the cell -- or its cytoskeleton. New super-resolution microscopy at CBST will make that possible.
They will also be working out the details of what happens to viral particles once they are transferred into a newly infected cell.
"We saw that particles are released into compartments," Huser said. "We will be looking carefully at how the virus particles are freed from these compartments, since it could be another mechanism to target with new treatments."
The overall goal for Huser, Chen and their colleagues is to fully understand the process of HIV transfer so scientists can use this knowledge in the fight against AIDS.
"The more we know about this mode of transfer, the better chance we have of figuring out how to block it and the spread of HIV and AIDS," Huser said.
In addition to Chen and Huser, study authors were Wolfgang Hübner, Ping Chen, Benjamin Dale and Ronald Gordon of the Mount Sinai School of Medicine; and Gregory McNerney, Frank Y. S. Chuang, Xiao-Dong Li and David Asmuth of UC Davis. The study was funded by the National Institutes of Health, Burroughs Wellcome, Hirschl Weill-Caulier, the National Science Foundation through the Center for Biophotonics Science and Technology, Mount Sinai School of Medicine and UC Davis.