ESA VIVALDI Dry Immersion Microgravity Simulations Induce Increases in Immune Biomarkers Associated With Physical and Psychological Stress, and Sex‐Specific Factors

With future manned space projects involving missions of unprecedented duration, multisystem deconditioning induced by spaceflight could seriously affect the well-being and health of astronauts. Safe and easily determined in-flight biomarkers are therefore needed to monitor health status. In this study, we simulated space deconditioning with a 5-day dry immersion (DI) of 18 healthy women and 19 healthy men and evaluated the effects of this protocol on three biomarkers: the neutrophil-to-lymphocyte ratio (NLR), the granulocyte-to-lymphocyte ratio (GLR) and the platelet-to-lymphocyte ratio (PLR). Increases in all three ratios were observed in both men and women, as also observed at the end of a space mission or after exposure to simulated microgravity. These increases were associated with physical and psychological stress in both sexes. Furthermore, our work suggested a positive link between NLR increase and cardiovascular system alteration in women, whereas in men, there would be a positive relationship between NLR, GLR, PLR, and inflammation. Thus, in addition to physical and psychological stress, sex-specific factors could contribute to increases in NLR, GLR, and PLR ratios during DI. As for the increase in PLR, it did not predict the development of long-lasting immune diseases during DI, in contrast to 2 months of head-down tilt bed rest (HDBR), another spaceflight analog. These data show that the NLR, GLR, and PLR ratios are promising biomarkers that deserve further study to determine the relationships between their increase and microgravity-induced deconditioning. These dry immersion investigations are registered at clinicaltrials.gov as NCT05043974 for women and NCT05493176 for men.