Visualizing the Viral Kiss of Death

Using time-lapse confocal laser-scanning microscopy,NIH researchers have captured on video human T-cells zeroing in for the kill on viruses, revealing for the first time that killer T-cells take far longer to dispatch their viral enemies than was generally believed. 

The very idea threw the Victorian poet Alfred Lord Tennyson into a funk. Nature, red in tooth and claw, Tennyson called the nightmarish idea that life was an unending battle to eat or be eaten. If only Tennyson could have seen the latest self-defense videos made by Daniela Malide and others at the National Institutes of Health in Bethesda, Md. Using time-lapse confocal laser scanning microscopy, Malide captured human T-cells picking up distress signals from cells infected with a virus and zeroing in for the kill.

Scientists have worked out experimentally many of the mechanisms and tactics of the immuno-surveillance system but Malides real-time videos show the action as it unfolds. The videos also revealed for the first time that killer T-cells take far longer to dispatch their viral enemies than was generally believed. Instead of a brisk 10-minute rubout, these killer T-cells can take up to two hours to mount a fatal assault.

The struggle begins when MHC (Major Histocompatibility Complex) Class I molecules inside cells infected with vaccinia virus, the virus used for small pox vaccination, grab bits of scrap viral protein and present them on the cell surface to flag down passing Tcd8+ killer cells. Nearly all cell types in the body have MHC class I molecules and the small pieces of viral proteins or peptides that they collect in the cytosol are a byproduct of imperfect protein synthesis by the virus. To passing T-cells, these viral peptides are highly suspect and call for a closer look. The T-cells arrive ready to kill, packing death-inducing proteins (perforin and granzymes) in their lytic granules.

On the video, the T-cells make contact, fluorescent labels marking their cell surface and interior poison granules. Inside the infected cells, another fluorescent label shows the vaccinia virus glowing brightly as it goes about its own deadly business of replicating. Then comes the surprise. Says Malide, We found that contrary to the general notion that target cell lysis often occurs within 10 minutes of establishing firm contact with T-cells, lysis occurs with much greater delay, generally 45 to 120 minutes. During this time, many T-cells remain in contact with target cells, and we frequently see the transfer of viral proteins and MHC Class I molecules to T-cells.

Only after this prolonged interaction does the killer cell kill. The perforin and granzyme explode inside the infected cell, the glowing vaccinia virus goes abruptly dark, and the target cell disintegrates. Even viewed through a microscope, it is a violent ending, revealing nature to be, if not red in tooth and claw, then fluorescent green in killer T-cells.

Reference:

Real Time Visualization of Cytotoxic T Lymphocyte Killing of Vaccinia Virus Infected Target Cells, D. Malide,1 S. Basta,2 J. R. Bennink,2 J. W. Yewdell2 ; 1 Light Microscopy Facility, NIH-NHLBI, Bethesda, MD, 2 Laboratory of Viral Diseases, NIH-NIAID, Bethesda, Md.

Source: American Society for Cell Biology (ASCB)