Influence of stress on lymphocyte subset distribution--a flow cytometric study in young student pilots

Stressors can induce sizable modifications on immunocompetent cells. Major circulating lymphocyte subsets were quantitated in Italian Air Force student pilots undergoing intensive training and continuous evaluation, a stressful situation both physically and psychologically. Instructor pilots matched for age and assayed in parallel were used as controls. A typical flight training session was not able per se to induce immediate significant modifications of the lymphocyte subset distribution either in the students or instructors, although it did affect plasma levels of stress-related hormones such as growth hormone, prolactin and cortisol in the former. Irrespective of the time of flying, however, the percentage of CD4+ lymphocytes and the CD4/CD8 ratio were lower in students than in instructors, and the absolute number of CD8+ lymphocytes was higher in students than in instructors. In a second series of experiments, 30 student pilots were tested at the beginning and at the end of a flight course (duration 30 days). Although the percentage of CD29+ lymphocytes comprised in the CD8+ subset was reduced at the end of the course in all individuals, such a reduction was more evident in those students who failed to pass the final examination, an additional cause of psychological stress. In light of the functional significance of the lymphocyte subsets investigated, it is suggested that the present stress-induced alterations may have practical implications.