Spaceflight leads to the deconditioning of multiple body systems, including the immune system. But what could cause such an immune deficit?
A study from the University of Ottawa and the Ottawa Hospital characterized the molecular response involved by capturing changes in leukocyte transcriptomes from astronauts transitioning to and from long-duration spaceflight.
Scientists have demonstrated that when astronauts reach the International Space Station, the expression of white blood cells (mainly protein-coding) genes changes quickly. This explains why, when in space, astronauts seem to be more prone to infectious diseases.
Most genes resumed their pre-flight expression level within a few weeks to a year after landing, indicating that astronauts’ immune systems require Earth-level gravity for optimal performance.
Three women and 11 men who were part of a cohort of 14 astronauts who spent 4.5 and 6.5 months on board the International Space Station (ISS) between 2015 and 2019 were evaluated for their gene expression in leukocytes (white blood cells). Four milliliters of blood were taken from each astronaut ten times: once before takeoff, four times during the flight, and five times after landing to isolate leukocytes.
Leukocytes showed an unusual expression of 15,410 genes. The researchers found two clusters of genes among them, 247 and 29 genes, respectively, whose expression changed simultaneously across the timeline under investigation.
Genes in the first cluster were turned down when they left Earth and back up when they returned, whereas genes in the second cluster behaved oppositely. Both gene clusters were primarily composed of protein-coding genes. However, the genes in the first cluster had a predominant function linked to immunity. In contrast, the genes in the second cluster had a predominate function connected to cellular structures and functions.
According to these findings, the immune system’s potency rapidly declines in space travelers due to these alterations in gene expression.
Dr. Guy Trudelnorth_eastexternal link, a rehabilitation physician and researcher at The Ottawa Hospital and professor at the Department of Cellular and Molecular Medicine of the University of Ottawa, said, “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.”
However, the data showed that- on average, after a few weeks- most of the genes in each cluster resumed their pre-flight expression level within a year of their return to Earth. According to these findings, returning astronauts face a higher risk of illness for at least a month after touching down on Earth.
The length of this period is expected to depend on age, sex, genetic differences, and early exposure to infections. In contrast, scientists are unsure how long it takes to restore immune resistance to its pre-flight strength.
The scientists proposed that “fluid shift,” in which blood plasma is redistributed from the lower to the upper section of the body, including the lymphatic system, causes the change in leukocyte gene expression during microgravity. Within the first several days in space, this results in a 10%–15% decrease in plasma volume. Large-scale physiological changes, including changed gene expression, are known to coexist with fluid shift.