Effects of mission duration on neuroimmune responses in astronauts.

BACKGROUND Spaceflight poses a unique stress to humans that can impair cellular immune responses and reactivate latent herpes viruses. Notably, prior studies have suggested that mission length may be an important factor in the variability of immune alterations observed after spaceflight. In this study, adrenocortical responses and circulating leukocytes were compared between astronauts who participated in either 9- or 16-d missions. HYPOTHESIS Mission duration will differentially affect neuroimmune responses after spaceflight. METHODS Blood and urine samples, collected from 28 crew-members who flew on 5 Space Shuttle missions, were analyzed for levels of plasma and urinary cortisol, urinary catecholamines, leukocyte and lymphocyte subsets, and total IgE. RESULTS After spaceflight, plasma cortisol was significantly decreased after the 9-d missions but increased after the 16-d missions. In contrast urinary epinephrine and norepinephrine levels were greater after the 9-d missions than the 16-d missions. Total IgE was significantly increased after the 16-d missions and correlated with urinary cortisol. The number of white blood cells, polymorphonuclear leukocytes, and CD4+ T cells were significantly increased postflight. After the 9-d missions, monocytes were increased while natural killer cells were decreased. However, monocytes were decreased after the 16-d missions; no change occurred in natural killer cells. CONCLUSIONS These results suggest that sympathetic nervous system responses predominate after shorter spaceflights, while longer flights are characterized by glucocorticoid-mediated changes at landing that may result from the accumulative effects of microgravity (i.e., physiological deconditioning) over time.