The Human Immune System: Resilience in Spaceflight

Last year, results from the landmark NASA’s “Twin Study” were published in the scientific journal Science (April 12, 2019). The study was carried out in 2015/2016 to understand the health impact of long-duration spaceflight, and centered around astronauts Scott and Mark Kelly, who are identical twins. One of the twins, Scott Kelly, was monitored before, during, and after a 1-year mission onboard the International Space Station, whereas his twin, Mark Kelly, stayed on Earth and served as a genetically matched control subject. The study results help to understand how the human body responds to the spaceflight environment, and how to maintain crew health during human expeditions to the Moon and Mars.

Photo by JJ Shev on Unsplash

The published paper (The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight) details the integrated work of 10 research teams on the effects of the spaceflight environment on human health and performance, physiology, and cellular and molecular processes. The paper points out that “The space environment is made harsh and challenging by multiple factors, including confinement, isolation, and exposure to environmental stressors such as microgravity, radiation, and noise.” Therefore, during and after the year in space, the researchers collected physiological, telomeric, transcriptomic, epigenetic, proteomic, metabolomic, immune, microbiomic, cardiovascular, vision-related, and cognitive data. The researchers observed significant changes in multiple data types in association with the spaceflight period. However, most observed changes eventually returned to the preflight state, showing the human body’s resilience.

Jennifer Fogarty, chief scientist of the Human Research Program at NASA’s Johnson Space Center in Houston, said: “A number of physiological and cellular changes take place during spaceflight. We have only scratched the surface of knowledge about the body in space. The Twins Study gave us the first integrated molecular view into genetic changes, and demonstrated how a human body adapts and remains robust and resilient even after spending nearly a year aboard the International Space Station. The data captured from integrated investigations like the NASA Twins Study will be explored for years to come.”

The researchers found changes in telomere length, gene regulation measured in both epigenetic and transcriptional data, gut microbiome composition, body weight, carotid artery dimensions, subfoveal choroidal thickness and peripapillary total retinal thickness, and serum metabolites. Cognitive performance was significantly affected by the stress of returning to Earth. For a few measures, persistent changes were observed even after 6 months on Earth, including the expression levels of some genes, increased DNA damage from chromosomal inversions, increased numbers of short telomeres, and attenuated cognitive function.

What about the immune system? The researchers found that many immune-related pathways were significantly changed inflight across all cell types. Adaptive immune system, innate immune response, and natural killer cell-mediated immunity were all affected. Notably, inflammatory cytokines and immune response gene networks were significantly influenced by the stress of returning to Earth. However, the researchers also found that the flu vaccine administered in space worked exactly as it does on Earth. A fully functioning immune system during long-duration space missions is critical to protecting astronaut health from opportunistic microbes in the spacecraft environment.

In the published paper, the researchers conclude that astronauts conducting exploration-class missions could experience risks from mitochondrial dysfunction, immunological stress, vascular changes and fluid shifts, cognitive performance decline, as well as alterations in telomere length, gene regulation, and genome integrity. However, the researchers point out that given the limitations related to the analysis of a single spaceflight individual, further studies are needed, including studies of additional astronauts on long-duration (≥12 month) missions.

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