A surprising number of people—about one-third of the world’s population—may already carry a tiny parasite in their brains.
This parasite, called Toxoplasma gondii, can be dangerous, but most people don’t even know they have it. New research from UVA Health has now uncovered how our immune system manages to keep it under control.
Toxoplasma gondii is spread mainly through cats or by eating undercooked meat or unwashed fruits and vegetables.
Once it gets into your body, it can move around and eventually settle in the brain. There, it can live quietly for many years. In most people, it causes no problems, but for people with weak immune systems, it can lead to a serious illness called toxoplasmosis.
The research team, led by Dr. Tajie Harris from the University of Virginia School of Medicine, found that the parasite can even infect the very immune cells meant to fight it. These cells, called CD8+ T cells, usually help the body fight infections by killing infected cells. But when they themselves are infected by T. gondii, a special backup defense kicks in.
Dr. Harris and her team discovered that CD8+ T cells have a secret weapon: an enzyme called caspase-8. This enzyme helps the infected immune cell self-destruct before the parasite can fully take over.
Since the parasite needs a live host cell to survive, the death of the T cell also kills the parasite inside. It’s a kind of “if I go down, you go down with me” move by the immune system.
To test the role of this enzyme, the researchers used mice that lacked caspase-8 in their CD8+ T cells. These mice had a much harder time fighting off the parasite.
Even though their immune systems responded to the infection, they got sicker, and many died. When scientists looked at their brains, they found many more parasites and many more infected T cells. This showed how important caspase-8 is in controlling the infection.
Before this study, scientists had found very few examples of parasites or germs that could live inside T cells. Now we may know why: most T cells self-destruct with the help of caspase-8 before the invader can spread. Only the most advanced pathogens have found ways to stop or avoid this process.
Dr. Harris and her team believe that this discovery could help improve treatments for people with weak immune systems. By learning more about how caspase-8 works, doctors may be able to protect the brain better from infections like toxoplasmosis.
This research also shows how smart and powerful our immune systems are, even when facing parasites that have evolved to hide inside our cells. And it highlights caspase-8 as a key part of that defense system—not just for this parasite, but possibly for fighting other infections as well.
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The study is published in Science Advances.
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