Sci. Aging Knowl. Environ., 8 September 2004
Some old mice fight parasitic infection better than young mice do
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/2004/36/nf82
Most of the people who die from flu each winter are older than 65. But a new study suggests that sometimes age bolsters, rather than cripples, the immune system. In at least one strain of mice, geriatric rodents resist a parasitic skin infection better than youngsters do. The finding highlights the convoluted changes that time imparts on the immune system, although researchers don't yet know how the finding relates to human health.
As people age, the immune system's careful choreography breaks down (see "Immunity Challenge"). T cells--which destroy host cells infected by invaders and organize immune responses--diminish in number and don't work properly, and antibodies that mark trespassers aren't as finely tuned. As a result, the elderly succumb to diseases like the flu more readily than younger persons do, and vaccines don't protect older people from disease as well as they protect younger folks (see "All Pain, No Gain"). In addition, autoimmune diseases become more common. Researchers aren't sure why immunity unravels, but they've found that the balance among immune-triggering molecules called cytokines changes with age. For instance, older creatures tend to produce fewer type 1 cytokines, which prod immune cells to engulf pathogens or destroy infected host cells, compared with type 2 cytokines, which spark antibody production. However, some studies show the opposite pattern.
To further investigate aging-related changes in immunity, Ehrchen and colleagues turned to a quirky strain of mice that exhibits an unusually strong type 2 immune response. These animals are susceptible to Leishmania major, a parasite that causes a skin disease; the mice typically die from the infestation, whereas other mouse lines fight it off. In young animals infected with Leishmania, foot pads swelled and developed open sores. In 18-month-old animals--equivalent to 60-year-old humans--paws didn't swell as quickly, and the sores began to heal; in the older mice the bug also didn't spread to internal organs. To figure out why the geezer mice fought the disease more successfully, the team measured quantities of particular immune molecules. Compared to youthful rodents, older mice produced more of the type 1 cytokine interferon and less of the type 2 cytokine interleukin. The older animals might resist infection better because aging boosts their type 1 immune response, posit the authors.
"It's a little surprising," says immunologist Raymond Yung of the University of Michigan, Ann Arbor. "It's the first paper that shows aging associated with improved outcome from an infection, but it's a preliminary study." He'd like to know how even older animals respond to infection. The study doesn't nail down a mechanism that explains why the older mice warded off Leishmania, says cellular immunologist Yuping Deng of Eastern Virginia Medical School in Norfolk. But by showing that in some cases aging enhances, rather than dampens, type 1 immunity, the work "will help people to think in a different way," she says. Studies that further define the alterations that aging causes in the immune system might make winter a gentler season for seniors.
September 8, 2004
Suggested by Jennifer Fuller.
Science of Aging Knowledge Environment. ISSN 1539-6150