Voluntary Exercise Training, Stress Biomarkers, and Immune Dysregulation During a 45-Day Human Exploration Research Analog (HERA) Mission

Abstract

NASA considers spaceflight an isolated, confined, and extreme environment (ICE), which consequently embodies multiple hazards that may impair both the physical and psychological well-being of the astronauts, implying that robust countermeasures to minimize health risks to the crew are vital. Immunity may be dysregulated due to various ICE factors, particularly chronic stress. This is a critical concern as it amplifies health risks such as viral reactivation and impaired anti-tumor immune surveillance, especially in combination with other ICE aspects. An abundance of literature has shown that exercise has a mitigating effect on chronic stress and can also preserve and improve the normal functioning of the immune system, but it is not known if exercise performed in an ICE analog can impact potential changes in biomarkers of stress and immune function. The aim of this study was to determine the effects of a 45-day terrestrial Human Exploration Research Analog (HERA) mission on immune cell phenotypes and function (NK-cell cytotoxicity), blood biomarkers of stress (cortisol, epinephrine, norepinephrine, DHEA-S) and cognitive function (BDNF). We also sought to determine voluntary exercise patterns among the crew and their associations with stress and immune biomarkers. Surprisingly, we found that biomarkers of stress and cognitive function did not change significantly in response to the HERA mission. There was, however, a decrease in NK-cell cytotoxicity just before the late stage of the mission, which is similar to a previous documentation of astronauts onboard the International Space Station. The crew were found to perform aerobic exercise at a consistent volume (intensity x duration) throughout the mission, which resulted in discernible improvements in cardiorespiratory fitness (V̇O2max). No correlations were found between the change in aerobic fitness or the cumulative exercise volume and the observed decline in NK-cell cytotoxicity. We conclude that a 45-day HERA mission may not accurately capture the stressful environment experienced by astronauts during spaceflight but can cause transient immune dysregulation. Additional data acquired over future HERA missions are required to fully understand the impact that prescribed exercise training may have on immune function during ICE.