Evidence is mounting that astronauts are more at risk of infections while in space. For instance, astronauts on board the International Space Station (ISS) commonly suffer from skin rashes, in addition to respiratory and non-respiratory diseases. Astronauts are also known to shed more live virus particles, for instance Epstein-Barr virus, varicella-zoster accountable for shingles, herpes-simplex-1 accountable for sores, and cytomegalovirus. These observations suggest that our immune system is perhaps weakened by space travel. But what could cause such an immune deficit?

“Here we show that the expression of many genes related to immune functions rapidly decreases when astronauts reach space, while the other happens once they return to Earth after six months aboard the ISS,” said Dr Odette Laneuville, an associate professor on the Department of Biology of the University of Ottawa, leading creator of a recent study in Frontiers in Immunology. The research was funded by the Canadian Space Agency.

The research was funded by the Canadian Space Agency.

The researchers studied gene expression in leukocytes (white blood cells) in a cohort of 14 astronauts, including three women and 11 men, who resided on board the ISS for between 4.5 and 6.5 months between 2015 and 2019. Leukocytes were isolated from 4 milliliters blood drawn from each astronaut at 10 time points: once pre-flight, 4 times in flight, and five times back on Earth.

There and back again

15,410 genes were found to be differentially expressed in leukocytes. Amongst these genes, the researchers identified two clusters, with 247 and 29 genes respectively, which modified their expression in tandem along the studied timeline.

Genes in the primary cluster were dialed down when reaching space and back up when returning to Earth, while genes within the second followed the other pattern. Each 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 primary cluster, and to cellular structures and functions for the second.

These results suggest that when someone travels to space, these changes in gene expression cause a rapid decrease within the strength of their immune system.

“A weaker immunity increases the chance 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 on the Department of Cellular and Molecular Medicine of the University of Ottawa.

Return to usual levels back on Earth

But there may be a silver lining to this cloud: the information showed that the majority genes in either cluster returned to their pre-flight level of expression inside one 12 months after return on Earth, and typically much sooner – on average, after a number of weeks. These results suggest that returning astronauts run an elevated risk of infection for not less than one month after landing back on Earth.

In contrast, the authors don’t yet understand how long it takes before immune resistance is fully back to its pre-flight strength: the length of this era is more likely to rely on age, sex, genetic differences, and childhood exposure to pathogens.

The authors hypothesized 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 a part of the body, including the lymphatic system. This causes a discount in plasma volume by between 10% and 15% throughout the first few days in space. Fluid shift is thought to be accompanied by large-scale physiological adaptations, apparently including altered gene expression.

Prophylaxis against immune deficits in space

“The following query is the way to apply our findings to guide the design of countermeasures that may prevent immune suppression while in space particularly for long duration flight,” said Laneuville.

The health of astronauts while in space, especially during long missions, would profit from detecting each immune dysfunction and sub-clinical inflammation. Early detection provides opportunities for intervention, with the aim to forestall a progression towards severe symptoms.”

Dr Odette Laneuville, Associate Professor, Department of Biology, University of Ottawa

Source:

Journal reference:

Stratis, D., et al. (2023) The transcriptome response of astronaut leukocytes to long missions aboard the International Space Station reveals immune modulation. Frontiers in Immunology. doi.org/10.3389/fimmu.2023.1171103.