Researchers at UC Davis and Mount Sinai School of Medicine have captured the first video footage of human immunodeficiency virus (HIV) spreading between T cells through contact interfaces called virological synapses.
The groundbreaking study, which was published in the Mar. 27 issue of Science, provides evidence backing virological synapses (VS) as viable targets for developing new drugs to combat HIV infection.
There is currently no vaccine for HIV, the virus that causes AIDS, and standard anti-viral drug therapies can help manage but not cure the disease.
The new study suggests that one reason treatments so far cannot completely eliminate HIV is that some of the virus is harbored by and transmitted directly through living T cells – vital components of the immune system that kick start the body’s defense mechanisms.
While the predominant belief held that HIV spreads inside the body as free particles after reproducing and bursting from infected T cells, lead author Benjamin Chen and colleagues at Mount Sinai discovered that stable adhesive contacts can dramatically enhance HIV transfer from infected to uninfected T cells through a distinct process involving VS.
“In lymphoid tissues such as lymph nodes … there is likely to be a great deal of T cell interaction that could facilitate VS [formation],” said Chen, an assistant professor at Mount Sinai.
When protected inside VS structures, the researchers argue, HIV can evade both the immune system and drugs that cannot penetrate the cell surface.
“Even though those drugs will kill off the free virus, they do not affect the virus that’s hiding in the cell,” said study co-author Thomas Huser, an associate professor and chief scientist at the UC Davis Center for Biophotonics Science and Technology. “So we need a combination [of drugs] that prevents the synapses or somehow targets synaptic transfer through the VS, and inhibits the free virus.“
To better understand HIV transmission through VS, Chen adapted the virus for imaging by creating a fully infectious clone that expresses green fluorescent protein (GFP) “tagged” onto a structural protein needed for making new virus particles. The GFP molecule, originally isolated from a species of jellyfish, glows bright green when exposed to blue light and appears inside VS as green “buttons“ of concentrated viral proteins protruding from T cells infected with the HIV clone.
Using specialized microscopy methods developed at CBST, Huser and his team were able to visually track the movement of fluorescent HIV when infected cells stick to healthy cells by continuously collecting high-speed snapshots from different depths of field that were later merged to create 3D videos.
From hours of footage, the researchers observed viral proteins rapidly migrating inside infected cells to the contact sites within minutes. The VS form budding structures that then penetrate and release the accumulated viruses into attached target cells.
Furthermore, the newly infected cells appear to initially contain viruses inside distinct compartments called endosomes, mechanisms involving the internal scaffolding of cells that “nobody had thought before that HIV would use,” Huser said.
“This work is only the beginning as there are loads of questions we have yet to ask that imaging and tracking may resolve,” said study co-author Gregory McNerney, a biophysics graduate student at UC Davis. “We were very fortunate to have various specialists come together with the right tools and infrastructure to lift this project off the ground.“
The researchers will next verify the process and identify specific components involved in VS mediated HIV transfer using longer-term image recording. For this purpose, they have created multiple HIV clones, each carrying modified versions of different viral proteins that have been tagged for fluorescence imaging to follow their fate after VS transmission into newly infected cells.
The work could translate over to other viruses known to spread in the same cell-to-cell route, such as HTLV-1, which can cause leukemia or an autoimmune syndrome, the researchers said.
The imaging methods will also be used to screen for drugs that can effectively target VS or inhibit HIV transfer, Huser said.
ELAINE HSIA can be reached at email@example.com.