Conditioned immunological memory: implication for cognition of immune cells

Associative learning refers to the process by which animals or human beings build the new relationship between two stimuli or events, which has been found throughout the animal kingdom. However, no studies have reported the associative learning in cells in the body to date. In the study, we focused on immune cells, as immunological memory is considered as one of the cardinal features of the immune system. As for a classical conditioning procedure, we designed the analogues of ginsenoside Rh1 (e.g., protopanaxadiol and panaxadiol) as the conditioned stimulus (CS) and the mcKLH-Rh1 conjugate as the unconditioned stimulus (US) and determined the CS-US association by detecting the dynamics of anti-Rh1 antibody production in serum. To test the hypothesis that immune cells also have the same associative learning ability as animals or human beings, mice were divided into control and CS group. Based on above CS and US, two groups of mice were subjected to conditioned learning, conditional memory recall, and deconditioning. When we used protopanaxadiol as the CS, the antibody response to Rh1 in the mice of CS group was slower and weaker than the control mice during conditional memory recall. Interestingly, the difference between two groups in antibody response to the immunogen disappeared during deconditioning. When we used panaxadiol as the CS, the antibody response to Rh1 in the mice of two groups was similar with that in studies regarding protopanaxadiol as the CS throughout three experimental phases. These results suggested that immune cells could acquire new association between a hapten analogue and a hapten-protein conjugate after undergoing a classical conditioning. Moreover, the conditioned immunological memory enables immune cells to make a conditioned response of humoral immunosuppression to prevent US induced immune damage. Our findings open a new frontier to explore cognitive functions of cells in the body and contribute to a novel understanding of cellular bioactivities.