Effects on immune responses by chronic stress are modulated by aging.

Male Fischer-344 rats at 5, 11, and 18 months of age were exposed to chronic stress for 6 months and subsequently allowed a 1-month interval with no stress before examination of splenic lymphocyte proliferative responses, IL-2 secretion by T cells, and NK cell activity in stressed and age- and sex-matched control animals. All four responses declined as a function of age in control rats. Stress exposure significantly decreased concanavalin A and lipopolysaccharide proliferative responses in 12- and 18-month-old compared to control rats without altering IL-2 secretory capacity. NK cell activity was slightly depressed by stress only in 18-month-old rats. By contrast, in 25-month-old animals that already demonstrated immune response levels lower than those of younger animals, stress did not significantly affect the responses examined. Thus, younger rats were more susceptible to a decline in host-defense responses induced by long-term chronic stress than older rats. Overall, the data suggest that aging significantly and differentially modulates the ability of environmental stress to influence the immunocompetent status of the organism.