Sometimes, brain tumors are so hard to detect that the immune system can't fight them. Researchers at the University of Michigan Medical School may have just figured out why.

In an accidental research finding from a study done with rats and mice, the team learned that brain tumors coat their tumor cells with excess of a protein that hides the cells from the body's immune system fighters.

These fighters, called natural killer (NK) cells, should be able to spot the tumor cells and kill them immediately--but that wasn't happening. Pedro Lowenstein of the University of Michigan Department of Neurosurgery led a team of researchers to find out why.

The study didn't start out with that goal in mind. Rather, the team was trying to understand the role of a protein, galectin-1. They wanted to see how the extra production of this protein by tumor cells affects cancer growth.

But when they blocked the cancer cells' ability to produce extra galectin-1, they found that the tumors disappeared. It turned out that without the extra protein coating, the immune system's NK cells were capable of detecting the tumors and eradicating them, leading to this surprising finding.

"This is an incredibly novel and exciting development, and shows that in science we must always be open-minded and go where the science takes us; no matter where we thought we wanted to go," Lowenstein said.

When they allowed the tumor cells to produce their necessary amount of galectin-1, the immune system wasn't able to get its T cells (considered the second line of defense after the NK cells) to eradicate the tumors until they were much too large.

"By the time it's detected," Lowenstein explained, "the battle is already lost."

Previously, researchers had believed that galectin-1 was more involved with evading T cells, rather than NK cells, and that the protein was important just in helping gliomas spread. Gliomas are tumors that arise in glial cells, the cells in the brain that are not neurons. Over 24,000 people in the U.S. each year are diagnosed with primary malignant brain tumors, 80 percent of which are gliomas. 

One of the best ways we know of keeping people safe from tumors is early detection, but finding gliomas in their early stages is so difficult. Now that the reason, or at least one of the reasons, for this difficulty has been revealed, scientists can go further and see what happens when galectin-1 is blocked in patients. Hopefully future studies will help the immune system catch the culprits earlier, one day saving countless lives.

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