When scientists at Oregon Health and Science University started experimenting with immune system cells, they witnessed a surprising new way that our body defends itself against viruses. Under the microscope, they watched immune cells, called "killer T-cells," munching on bits of their enemy.
Killer T-cells target body cells that have become virus factories, churning out copies of viruses that have infected them. Immunologist Mark Slifka and his colleagues marked these infected cells with a special green dye. Researchers knew that when killer T-cells attack these virus factories, they spew destructive chemicals, such as cytokines. "If they come up to a cell and they can recognize that it's infected with a virus that they know, they will attack that cell and actually deliver a lethal payload to that cell causing it to self-destruct," Slifka says.
But surprisingly, these cells were doing much more. "When we threw in troops of these killer T-cells ... when they recognized these virus-infected cells that were green, they themselves began to turn green," he says.
This means that the killer T-cells were literally taking a bite out of the membrane or skin, of the infected cell. "This is truly a case of microscopic cannibalism," Slifka says. "And this is the first time we've seen virus-specific killer T-cells ingest parts of infected cells."
Slifka thinks the immune system cells are actually using the infected cells as a food source, which may be what makes them so effective. "So not only do you have this warrior cell coming in and attacking these virus factories, but it's able to take away nourishment from this in order to help it to continue the fight against the infection," he says.
As he wrote in the journal Nature Medicine, drug researchers can use this discovery to measure how well a new vaccine works. "Now that we know that you can detect virus-specific T-cells by the fact that they will tear off and eat colored infected cells we can now measure T-cell responses not only after a natural infection but also after a vaccine," he says.
Slifka and his colleagues also found that the killer T-cell can be a pretty picky eater, choosing certain kinds of cells to munch on, but not others. While researchers are still unclear why, killer T-cells will eat infected white blood cells, but refuse to eat infected fibroblasts, a type of cell that provides structure to connective tissue. "It's like a child who has a choice between sugar cookies and Brussels sprouts — they'll take one over the other every time," he says.