How Space Travel May Weaken Astronauts’ Immune Systems

A study of astronauts on the International Space Station (ISS) shows that space travel can rapidly change the expression of genes in our white blood cells. This may explain why astronauts become more susceptible to infections while in space and suggests that Earth’s gravity may be essential to keep our immune systems functioning properly.

The research is published in Frontiers in Immunology.

The effects of spaceflight

We may think that life on board the ISS is a sterile, Star Trek-like environment, but the evidence suggesting that spaceflight can temporarily weaken our immune systems and increase our susceptibility to infections is building.

Astronauts on board the ISS frequently develop skin rashes and respiratory diseases. They also shed more live virus particles than they do on Earth, such as the Epstein-Barr virus (which causes mononucleosis), the varicella-zoster virus (responsible for chickenpox and shingles) and the herpes simplex 1 virus (responsible for skin infections and cold sores).

Science hasn’t yet provided us with an answer for what causes these immune deficits, but it’s not rocket science – or is it?

Altered gene expression

To investigate further, the researchers selected a cohort of 14 astronauts – 3 women and 11 men – who resided on board the ISS for 4.5–6.5 months between 2015 and 2019.

The researchers studied gene expression in a type of white blood cell called leukocytes, which help to fight infection. These cells were isolated from blood samples drawn from each astronaut at 10 different time points: one pre-flight, four during flight and five after they returned to Earth.

They found that the expression levels of 15,410 genes in leukocytes changed over these 10 time points. Of these, two important clusters of genes were identified – one of 247 genes, and the other of 29 genes.

The expression of the first cluster decreased in space and then increased back on Earth, while the second cluster did the opposite. The researchers discovered that most of these genes form the blueprints for proteins, but that each cluster was involved in a different role – the first was related to the immune system, whereas the second was related to cellular structures.

This suggests that spaceflight leads to altered gene expression and a rapid decrease in the strength of astronauts’ immune systems.

“A weaker immunity increases the risk of infectious diseases, limiting astronauts’ ability to perform their demanding missions in space,” said Dr. Guy Trudel, rehabilitation physician and researcher at The Ottawa Hospital, and professor at the Department of Cellular and Molecular Medicine of the University of Ottawa. “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.”

Fortunately for the astronauts, the data also revealed that most genes returned to their pre-flight expression levels within a year of returning to Earth, though typically much sooner – just a few weeks, on average. However, returning astronauts’ risk of infection is still heightened for at least one month after landing.

The authors hypothesized that altered gene expression in space is triggered by a phenomenon called “fluid shift” that occurs in microgravity, where blood plasma – the liquid portion of blood – shifts from the lower body to the upper body. This includes the lymphatic system, which is itself an important part of the immune system. The volume of blood plasma decreases by 10–15% within the first few days in space and is accompanied by large-scale physiological adaptations, and possibly the altered gene expression observed by the researchers.

Informing the design of long-duration missions

Dr. Odette Laneuville, senior author of the study and associate professor at the Department of Biology of the University of Ottawa, summarized the study’s findings: “Here we show that the expression of many genes related to immune functions rapidly decreases when astronauts reach space, while the opposite happens when they return to Earth after six months aboard the ISS.”

Nevertheless, the jury is still out on how long it takes before immunity fully returns to its pre-flight strength. It is likely to depend on age, sex, genetics and exposure to pathogens in childhood.

“The next question is how to apply our findings to guide the design of countermeasures that will prevent immune suppression while in space in particular for long-duration flight,” added Laneuville. “The health of astronauts while in space, especially during long missions, would benefit from detecting both immune dysfunction and sub-clinical inflammation. Early detection provides opportunities for intervention, with the aim to prevent a progression towards severe symptoms.”

Reference: Stratis D, Trudel G, Rocheleau L, Pelchat M, Laneuville O. The transcriptome response of astronaut leukocytes to long missions aboard the International Space Station reveals immune modulation. Front. Immunol. 2023;14. doi: 10.3389/fimmu.2023.1171103

This article is a rework of a press release issued by Frontiers. Material has been edited for length and content.