DIFFERENTIAL-EFFECTS OF EXERCISE AND HOUSING CONDITION ON MURINE NATURAL-KILLER-CELL ACTIVITY AND TUMOR-GROWTH

Acute exercise and exercise conditioning have been shown to affect the activity of natural killer (NK) cells as well as the growth of experimentally induced tumors in animals. Since psychosocial factors are also known to alter NK activity and tumor growth, isolation, a known psychosocial stressor of mice, was also investigated to see if housing condition could alter exercise-induced changes in NK cell activity and tumor growth. NK cell activity and concentration of asialo GM1 (ASGM1) positive splenocytes were measured in male C3H mice inoculated i.v. with CIRAS 3 tumor cells. Mice were housed individually or in groups of four and trained to run for eight weeks on a rodent treadmill; controls remained sedentary throughout the experimental period. At four weeks into the training protocol, mice were injected with the tumor cells and continued to run for four weeks after tumor exposure. There was a significant effect of physical activity (p < 0.019) but not of housing on splenic NK cytotoxicity against tumor targets in vitro. When the data were analyzed by presence or absence of lung metastases, only those animals without visible lung tumors had significantly higher NK activity as a function of exercise relative to sedentary controls. There were no significant differences in the frequency of ASGM1+ splenocytes between trained and untrained animals, irrespective of presence or absence of lung tumor colonies. There was a significant effect of housing (p < 0.02), but not of physical activity, in mice with successful tumor takes with greater numbers of group housed animals (29/59) with tumor relative to individually housed animals (13/60). The data reveal that environmental factors such as housing conditions can modify the effects of exercise on natural tumor immunity. Also, the effect of exercise conditioning on NK cells in tumor and nontumor bearing animals differs significantly and, further, raises the underlying question of the physiological significance of exercise-induced changes in immune function.