Persistent memories of a T cell

VACCINATIONS ARE A TRAINING exercise for the immune system: they let it learn to recognize a foreign invader, or antigen, before the real assault comes. But just how does the immune system remember, sometimes for years, the identity of, say, a poliovirus? Must bits of the virus be continuously present in the bloodstream? Recent research suggests not, at least in some cases, and that some cells in the immune system have long memories.

Rafi Ahmed, an immunologist at UCLA, and his colleagues Lisa Lau and Beth Jamieson, have been studying mice to acquire a better understanding of how immune systems remember past threats. The researchers first exposed one group of mice to a common mouse virus. After the mice were infected, their immune systems started churning out specialized virus-fighting cells called CD8 T cells. The researchers collected these cells, carefully checking to make sure they weren't contaminated with stray bits of virus. Then they injected the cells into mice that had never been exposed to the virus.

The question was whether the CD8 T cells would grant their new hosts the same degree of immunity the original group of mice enjoyed. Would the cells retain their antiviral potency even in the absence of an immediate threat, or would the new group of mice have to develop their own resistance from scratch?

After the CD8 cells had been in their new hosts for varying periods of time, the researchers exposed the previously uninfected mice to the virus. They found that even if the virus was first injected after 28 months--near the end of a mouse's natural life span--the rodents fended it off as rapidly as if they had been vaccinated and were familiar with the virus. The CD8 cells apparently don't require a clear and present danger to remain effective.

Ahmed cautions that the rules may well be different for other types of immune cells. But he believes his work should at least lead vaccine researchers to reconsider their assumptions about how vaccines work--in particular the assumption that a vaccine dose must be large enough to ensure the continued presence of antigen in the body. "People have thought that vaccinations work because you have bits of antigen floating about," says Ahmed, "and that when you lose immunity, it is because of lack of antigen. Our results show that at least for the CD8 cells, that's not true."

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