Space travel has long been known to have profound effects on the human body, and a new study has shed light on how it can alter gene expression in white blood cells, potentially weakening the immune system. This could explain why astronauts appear more susceptible to infectious diseases while in space.
The study, led by Dr. Odette Laneuville, an associate professor at the Department of Biology of the University of Ottawa, and funded by the Canadian Space Agency, examined gene expression in leukocytes (white blood cells) in a cohort of 14 astronauts who resided on board the International Space Station (ISS) for between 4.5 and 6.5 months between 2015 and 2019.
The researchers found that 15,410 genes were differentially expressed in leukocytes. Among these genes, two clusters, with 247 and 29 genes respectively, changed their expression in tandem along the studied timeline. Genes in the first cluster were dialed down when reaching space and back up when returning to Earth, while genes in the second followed the opposite pattern. Both clusters mostly consisted of genes that code for proteins, but with a difference: their predominant function was related to immunity for the genes in the first cluster, and to cellular structures and functions for the second.
These findings suggest that when someone travels to space, these changes in gene expression cause a rapid decrease in the strength of their immune system. “A weaker immunity increases the risk of infectious diseases, limiting astronauts’ ability to perform their demanding missions in space. If an infection or an immune-related condition was to evolve to a severe state requiring medical care, astronauts while in space would have limited access to care, medication, or evacuation,” said Dr. Guy Trudel, a rehabilitation physician and researcher at The Ottawa Hospital and professor at the Department of Cellular and Molecular Medicine of the University of Ottawa.
The study also revealed that most genes in either cluster returned to their pre-flight level of expression within one year after return on Earth, and typically much sooner – on average, after a few weeks. These results suggest that returning astronauts run an elevated risk of infection for at least one month after landing back on Earth.
The researchers hypothesize that the change in gene expression of leukocytes under microgravity is triggered by ‘fluid shift’, where blood plasma is redistributed from the lower to the upper part of the body, including the lymphatic system. This causes a reduction in plasma volume by between 10% and 15% within the first few days in space. Fluid shift is known to be accompanied by large-scale physiological adaptations, apparently including altered gene expression.
The findings of this study underscore the importance of further research into the effects of space travel on the human immune system, with the aim of developing countermeasures to prevent immune suppression in space, particularly for long-duration flights. Early detection of immune dysfunction and sub-clinical inflammation could provide opportunities for intervention, preventing a progression towards severe symptoms.