Immune enhancement by conditioning of senescent mice. Comparison of old and young mice in learning ability and in ability to increase natural killer cell activity and other host defense reactions in response to a conditioned stimulus.

It has been clearly demonstrated that immune responses may be conditioned in a manner similar to that of the classical Pavlovian experiments. Evidence of impaired immune function in aging has raised the question of whether psychological conditioning of an immune response can also be effective in old age. The knowledge that aged mice have decreased spleen cell natural killer (NK) activity and that NK cytotoxicity, at least in young mice, can be psychologically conditioned led us to explore in old mice the possibility of conditioning the response of NK cell activity using the odor of camphor as the conditioned stimulus (CS) and the injection of Poly I:C as the unconditioned stimulus (US). Young and old male mice were divided into five and six groups, respectively. They received the CS and/or the US in association (conditioning) trials (sessions 1-9). Mice were exposed to the camphor odor alone at 72 hours after the final association trial to observe the conditioning phenomenon (session 10). The group conditioned with Poly I:C and camphor and receiving the CS at session 10 showed statistically significant increases in spleen cell NK activity over those of the control groups that did not receive the CS treatment at session 10 (2.6- and 4.0-fold increase in young and old, respectively). Treatment with camphor odor alone had no effect on boosting NK cell activity. These findings demonstrate the possibility of conditioning immune responses in old age, offering a valuable tool for attenuating age-related immune deterioration in various species, including the human. In addition, these results again confirm highly significant immune enhancement by classical conditioning and extend previous findings from female mice to males as well.