Wednesday, November 22, 2006

Cannibals in the Body

Your tummy may be grumbling for a Thanksgiving feast, but scientists have found cells in our bodies that are even hungrier. As this ScienCentral News video explains, the discovery could lead to more effective flu shots and other vaccines.

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."

"This information is actually starting to develop into a new area of investigation where people are studying this phenomenon of T-cells cannibalizing other cells," he says. "And we still don't know really why they're doing this."

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. Vaccines, like the flu shot given each year, contain dead or weakened germs that give your immune system a pattern to recognize stronger versions of that virus in the future. Vaccines spur the development of T-cells floating around in the body.

Vaccine Microscope
By color coding the killer T-cells, researchers can track the effectiveness of a vaccine.
"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.

While he says that there are other ways of measuring T-cells response to infection, Slifka says that this is a more precise way of measuring that response. Different types of T-cells have a range of functions for the immune system, from producing antibodies, which are proteins that identify threats, to killing infected cells. "Every T-cell will be a little bit different in terms of how it reacts to virus-specific cells. And what we're able to do is measure all the T-cells regardless of what their individual roles are," 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.

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