More Evidence that Space Travel Is Bad for Immune Systems

For the foreseeable future, humans in space still means human bodies in space. And human bodies are crappy, ungainly skin bags prone to self-destruct mechanisms like cancer, heart disease, and depression. Sci-fi often suggests ways around this, usually involving some flawless robot doctor or health fixit pod ready to diagnose and repair just like that. In the real world, however, we aren’t much better up there than we are down here, perhaps worse. During 1968’s Apollo VII mission, for example, all three astronauts came down with fierce head colds, forcing a dangerous helmetless reentry and effectively ending the astronaut careers of all three.

A 2012 study found that of 106 flights and 742 crewmembers, there were 29 cases of infectious diseases being transmitted aboard spacecraft. The illnesses ranged from head colds to fungal infections to gastroenteritis. Note that there is no bedrest with chicken noodle soup and Netflix available on a multi-million dollar space mission.

As far as space dangers go, illness doesn’t get much attention, which is kinda strange given that one of the most distinct effects of microgravity on the human body are tanking immune systems. A 2012 piece in Time reports, “the immune system can go on the fritz in space: wounds heal more slowly; infection-fighting T-cells send signals less efficiently; bone marrow replenishes itself less effectively; killer cells— another key immune system player—fight less energetically.” Meanwhile, many pathogens have an awesome time in space, growing stronger and increasing their resistance to antimicrobials. In particular, both herpes and staph have been shown to thrive in the gravity-free, hyper-sterile environment of a space vessel.

A study out this week examining space-born Drosophila flies—often studied because of the similarity between the flies’ immune systems and that of humans—found that in the case of fungal infections, microgravity effectively nullified the immune response. Interestingly, the study also looked at centrifuge-induced hypergravity, finding that  the flies’ immune responses to fungi increased beyond normal as gravity increased beyond normal. Meanwhile, on a somewhat positive note, the space flies’ immune responses to bacteria were found to be “robust,” though that’s probably a slim comfort to an astronaut trapped in a tight space capsule on a month’s-long voyage to Mars with a rapidly multiplying staph population.

Immune responses to fungal infections and bacterial infections in humans and Drosophila flies are mediated by two different cellular pathways: for fungus, the pathway is governed by Toll receptors (imagine security alarms for pathogens) and the other via a gene known as imd. The Toll pathway appears to function somewhat in proportion to gravitational field strength, while the imd pathway doesn’t seem to care either way.

One theory is that space travel involves the production of “heat-shock” proteins, which bind to Toll pathway receptors and limit the immune systems’ real estate for detecting pathogens. The result is a disproportionately small response to a pathogen threat.

The answer to the immune system problem still seems to be induced gravity via centrifuge—a concept so far just considered useful for maintaining bone and muscle mass. So boosting astronaut immune systems would really just be a bonus of that well-established space travel concept. The team behind this week’s study, led by UC Davis’s Deborah Kimbrell, hopes to continue their fly-based research aboard the International Space Station.

Just one more thing: I'm never going into space. And I also happen to be as susceptible to infectious disease as the next Earthling (maybe just a bit less). Will this knowledge that gravity boosts the immune system have some future application in Earthbound medicine? I asked Kimbrell just that question on Friday. "It is always speculative to go from model organism research to human treatments, but …," she replied cautiously. "Our hypergravity data show that Drosophila fared better against the fungal infection at high g. If humans are similar, then perhaps treatment at high g, i.e. human centrifuges, would bolster the immune response."

It's an interesting idea. Imagine some future hospital as one giant rotating torus, boosting gravity just enough to add a worthwhile immune system edge. Or, if you're feeling a bit under the weather, maybe just sip some tea in the nearest Gravitron.