Pinning down T cell death in the thymus.

Though an inconspicuous organ located behind the breastbone, the thymus is the place where the immune system makes peace with the rest of the body. Here, white cells called T cells -- key soldiers in the body's cellular army -- proliferate and mature. But if all these soldiers were allowed to roam free through the blood, they would destroy the body's own tissues as well as foreign invaders. So here many immature T cells also meet their demise, two immunologists have now demonstrated.

A staining technique for marking dying cells made possible this demonstration, says Charles D. Surh of the Scripps Research Institute in La Jolla, Calif. Until now, scientists only suspected that this breeding ground for T cells was also a burial ground, he adds.

Surh and Scripps colleague Jonathan Sprent monitored the thymus for a particular type of disintegration called apoptosis (SN: 1/15/94, p.44), a programmed cell death in which the cell membrane shrivels and its genetic material crumbles.

Throughout the body, cells activate this program when damaged beyond repair or when their particular jobs are done. But in this act of cellular suicide, as with suicide in the traditional sense, outside signals can trigger the process.

For years, immunologists have tried to determine exactly where in the thymus and why about 95 percent of the T cells there call it quits before ever leaving. In this organ, each immature T cell develops a unique molecular docking site, called a receptor, on its surface. That receptor recognizes a particular MHC (major histocompatibility complex) molecule with a specific protein fragment attached. The thymus keeps the immune system in check by getting rid of T cells that recognize and subsequently react to MHC molecules carrying fragments of the body's own proteins, Surh explains. That's where apoptosis comes in handy.

To "see" appoptosis, Surh and Sprent first injected lots of a particular enzyme into slices of mouse thymus. This enzyme puts a nucleotide building block onto the end of the DNA pieces created early in apoptosis. By then adding antibodies -- first some that recognize this building block and then others that carry a different enzyme and link up with the first antibodies -- the researchers can mark dying cells in red, Surh explains.

In the part of the thymus called the cortex, positive selection occurs: Only new T cells that recognize and link with MHC molecules seem to avoid apoptosis, Surh says. Then the thymus somehow weeds out and triggers cell death in T cells that would bind to MHC molecules carrying the body's own protein pieces.

Based on the staining patterns observed in the thymuses of mice genetically altered to make no MHC molecules, "we're thinking that most of the [T cell] death occurs for lack of [the] positive selection," Surh says.

Researchers had not seen this massive destruction before, because other immune system cells called macrophages gobble up the dying cells quickly and efficiently, he and Sprent report in the Nov. 3 NATURE. They determined this by irradiating thymuses and monitoring them for 24 hours. Within an hour, macrophages had ingested the T cells, disposing of them by the next day, they report.