Research Update
Attack of the killer lymphocytes

CMAJ 2000;162:1035

"We've found this evidence that the immune system can be fooled, and it suggests subtle changes that could underlie many autoimmune diseases," says Dr. Mark Soloski, an associate professor of medicine at Johns Hopkins.

The researchers infected mice with salmonella bacteria, then observed the behaviour of a typical bacteria-fighting immune cell, the cytotoxic lymphocyte (CTL), as it approached infected body cells. Cells invaded by bacteria normally give clear signals that they're infected. Attracted by this protein "flag," CTLs dock with the infected cells and trigger their rapid self-destruction. (This protein flag in infected mice is one that is common to certain bacteria associated with human arthritis, including Borrelia burgdorferi, which causes Lyme disease.) However, the researchers also found that the bacterial flag was almost identical to parts of a "universal housekeeping molecule" found in humans, mice and all living organisms that help proteins keep their shape. When the researchers coaxed the mouse cells to display this salmonella flag, their CTLs would readily attack. But they also went into attack mode if the cells displayed a piece of the mouse's own housekeeping molecule or the identical human version. "These cells have the potential to go over and destroy our own cells," notes Soloski. "Surprisingly, we found these cells were a dominant response in over half the animals. . . . There are hints the same thing is going on in humans."

It is still too early to assume a cause-and-effect relation between certain bacteria and specific autoimmune diseases, he adds. "We're not moving yet to state that arthritis is caused by a bacterial infection, but we're looking closely [at the idea] that some forms of arthritis may be triggered by this." — Donalee Moulton, Halifax

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