Healthy Wheel-Spinners: Mice that jog regularly kill off fewer immune cells (Immunology/Exercise)
Authors: Miller Katharine 
Published: August 2003

Mice might not race in the Boston Marathon, but they do run, given the chance. And the workout boosts their health: By keeping fit, the critters build better defenses against both disease and oxidative stress, according to a new study. Although the result is not surprising, it appears to be the first definitive demonstration that habitual moderate exercise decreases the suicide rates of immune cells. The result supports one reason for less frequent illness among people who exert themselves regularly: better immune cell survival.

Those who study the consequences of exercise have often focused on its extreme forms. Rats forced to run to the point of exhaustion suffer increased oxidative damage from free radicals, molecules likely responsible for many aspects of aging (see
"The Two Faces of Oxygen"
). The oxidative stress in turn accelerates the destruction of immune cells by a process known as apoptosis. Although apoptosis benefits animals--it kills off cells that weren't made properly or have already performed their jobs--defects in the process correlate with increased autoimmune disorders and susceptibility to infection, as has been seen in athletes who push themselves too hard and in old people. Regular moderate exercise, on the other hand, reduces the risk of infection and improves disease recovery in humans. It also increases the amounts of muscle cell antioxidants, molecules that eliminate free radicals. But little is known about the effects of moderate long-term exercise on immune cells. Avula and colleagues wondered whether the additional antioxidants available to mice on a jogging regimen might provide long-term protection from excessive immune cell suicide.

To study the effect of voluntary workouts on the immune system, the researchers housed female mice with and without exercise wheels. After 10 months, during which the sedentary mice grew slowly fatter and the 33-kilometer-per-week joggers stayed slim, the researchers drew the animals' blood, killed them, and removed their spleens for study. Immune cells from the blood and spleens of fit animals didn't show any more oxidative damage than did cells from the idle mice, a result possibly caused by the unusually large amounts of antioxidants in the blood of active mice. In addition, immune cells from active mice were less susceptible to apoptosis when exposed to hydrogen peroxide and other T cell activators, as well as when left untreated.

The work suggests that consistent moderate exercise hardens immune cells against cell death, unlike exhaustive exercise, which can increase the extent of apoptosis. Further studies are needed to assess whether the altered immune status translates into improved ability to respond to infection and whether the findings apply to older mice--and to humans. But if stints on the treadmill do keep immune cells going, perhaps we'll all have an added incentive to hit the gym.

--Katharine Miller; suggested by James M. Harper

C. P. Avula, A. R. Muthukumar, K. Zaman, R. McCarter, G. Fernandes, Inhibitory effects of voluntary wheel exercise on apoptosis in splenic lymphocyte subsets of C57BL/6 mice.
J. Appl. Physiol.
91
, 2546-2552 (2001).
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