No Gravity? Exhausted Immune System

By: Helen Beilinson

In 1970, astronaut Fred Haise fell ill thousands of miles from home aboard Apollo 13. The culprit? A bacterium that, on Earth, causes no symptoms in healthy individuals. The extended health screens taken before his flight indicated that Haise’s health was in impeccable condition and his doctors had no reason to believe that Haise’s immune system couldn’t battle the bacteria. But, as we have learned over the last fifty years, weak gravity, or microgravity, impairs the immune system. Although Haise survived and returned to Earth safely, his infection was one of the first indications that our immune systems function under different rules in space.

Strengthening weakened immune systems is not an impossible task on Earth, it has been done with everything from battling infections to battling cancer. However, to help individuals’ bodies fight infections in microgravity is still a difficult thing to do because it is still unclear how microgravity changes the way the cells of our immune systems function. Last year, a study published in Life Sciences in Space Research provides a basis for how future astronauts can aid their bodies function at full capacity by finding a unique feature of immune cells in microgravity.

Dr. Jillian Bradley and her colleagues studied an immune cell type called T cells, which are fundamental in fighting infections. When an infection occurs in the body, other immune cells sense the intrusion and signal to the T cells that they need to activate to fight off the infection. T cells are also capable of killing our own cells that aren’t ideal—for example, if they are infected with a virus or if they are cancer cells.

Bradley compared T cells grown in normal gravity conditions and in microgravity, by placing the T cells in a spinning chamber that drops the level of gravity the cells experience. T cells need to be turned on, or activated, to handle invading bacteria or other infectious agents. When Bradley starting the signaling to the T cells in microgravity, they became more active more quickly compared to the T cells receiving the same signals in normal gravity.

However, something surprising happened if she looked at the T cells after a few days of being activated. After three days, these activated T cells lose their muster. The longer duration in microgravity made these T cells activate more sluggishly, as if they couldn’t receive the signals to turn on.

Slow T cell responses are potentially dangerous and could be the reason Haise got sick in space. T cells are activated a few days after an infection starts, giving the bacteria time to multiply before these heavy hitting cells start attacking them. If the activated T cells are slow in their response time to the infection, the bacteria have more opportunity to expand and cause symptoms of being sick. Slow immune responses in space could put astronauts at risk for infections by bacteria that on Earth our immune responses respond rapidly to, like the bacteria Haise was infected with.

When investigating why microgravity is debilitating for T cells, Bradley found that T cells without gravity very quickly make a protein that inhibits their function. This protein is seen in the T cells that enter a state of exhaustion, where they have been told to activate to much that they lose the ability to quickly respond. Typically, this protein is seen when T cells are exposed to very extended periods of activation, however, in microgravity, the amount of time it takes to exhaust a T cell is significantly shorter.

It may not be all bad news, however, because this mark of immune response dampening is infamous in another location where turning T cells back into full swing is already being done—tumors.

In addition to their ability to eliminate infections, T cells are important in battling cancers. However, when tumors are large enough, they exhaust T cells, causing to express the same inhibitory protein as microgravity does. The newest wave of cancer treatments, termed cancer immunotherapies, are focused on eliminating or suppressing the function of the inhibitory proteins, causing the T cells to be more active and able to eliminate the cancer cells more readily.

Could the same technology be applied in space? Scientists don’t know yet. But it could be a potential way that astronauts can utilize existing technology to prevent them from getting sick in space.

With eyes on missions to Mars and beyond, understanding how microgravity affects our bodies is critical to ensure the health of those leaving our stratosphere and defining the changes occurring in our immune cells provides physicians with the information they need to design novel treatments for astronauts dealing with infections in space. Fascinatingly, the parallels between T cells in microgravity and in cancer could provide insights to keeping bugs at bay in space.