Consequences of simulated spaceflight stressors on the cell mediated immunity: How does the immune system cope with microgravity and psychological stress?

Evidence obtained during spaceflight and from experiments under simulated space conditions, strongly indicates that microgravity exerts a broad range of effects on mammalian tissues and cells, including impairment of immune cell function. The reduced gravitational vector present in space disturbs homeostasis of the human body and impairs the immune reaction towards several external stimuli. These changes can result in a severely imbalanced inflammatory response. In addition, astronauts residing in space are subjected to increased psychological stress due to the isolation, confinement and heavy workload associated with missions in space. Elevated stress levels cause a disturbance in the hypothalamic–pituitary–adrenal axis, leading to a heightened production of stress hormones, which in turn influence the immune responsiveness. The purpose of this project was to better understand the cellular changes of the human cell-mediated immunity (CMI) under various simulated space conditions, including microgravity and stress hormones. Therefore, ground-based in vitro experiments were performed to examine the effect of simulated microgravity (s-µg) and stress hormones on the CMI using the in vitro delayed-type hypersensitivity (DTH) assay, a standardized tool to monitor the overall status of the immune system. Blood samples were taken from healthy volunteers and exposed to s-µg using the 3-Dimensional Random Positioning Machine (3D-RPM). In addition, cell viability was also evaluated to check whether the closed tubes, microgravity or the added stimulants might increase cell death and thereby influence the cytokine response. Via Luminex and ELISA technologies, the concentration of four cytokines (IL-2, IL-10, TNF-α and INF-ƴ) was measured after 24 h stimulation with recall antigens or mitogens. To investigate the impact of endocrine modifications on the CMI response, additional different concentrations of hydrocortisone and noradrenaline were added to the samples during incubation. Changes in cytokine concentrations indicated alterations of the immune response to external stressors. More knowledge on the specific and combined effects of s-µg and stress hormones will lead to a better insight into the altered immune response during spaceflight.