Invading Immune Cells May Damage Older Brains

It’s an accepted part of life that as we get older, our bodies and brains don’t function as well as they did when we were young. Yet few of us ask why that is. We don’t question why our health and mind don’t bounce back or perform the same as they did when we were in our twenties. However, researchers who won’t take, “I’m just getting old,” as an excuse are looking at the brain and have made an exciting find. The immune system may speed aging in the brain.

Scientists at Stanford Univ. saw that T cells, known as killer cells, can gather in the brain and hamper the production of new nerve cells. These T cells, which usually attack infections, should not be where they are in the brain at all. The brain has “nerve cell nurseries” when T cells infiltrate these nurseries, they secrete a chemical that stops new cell production. The research was performed in living mice. One group of mice were three months old, the others were two years old.

The young mice had brains filled with new neurons, while the older mice had very few. The older mice had a lot of T cells, while the young mice had hardly any at all. The researchers’ finds were backed up with tissue samples from autopsied human brains that showed the same process. The human samples showed tissue from people ages 79-93 had many, many more T cells than the tissue from people ages 20-44. This research may help scientists find a way to slow or even reverse the damage.

Moreover, this study disproves the belief that healthy brains cannot be invaded by the types of immune cells that shouldn’t be there. The so-called blood-brain barrier is not as impenetrable as thought. “The textbooks say that immune cells can’t easily get into the healthy brain, and that’s largely true,” said Prof. Anne Brunet, senior author of the study. “But we’ve shown that not only do they get into otherwise healthy aging brains — including human brains — but they reach the very part of the brain where new neurons arise.

The scientists believe that there may be brain-based antigens that the T cells are attacking and that those antigens, “May bear some responsibility for the disruption of new neuron production in the aging brain’s neurogenic niches,” according to Prof. Brunet. T cells are supposed to attack cells antigens they don’t recognize. So, it’s not that the T cells are malfunctioning; they’re just lost. According to Prof. Burnet, their next move to find out which antigens in the brain are luring the T cells away from their beneficial function. If that can be identified, researchers may be able to fight against the age-related problems we currently accept as being inevitable.

Banner image: T cells (white) are absent, and neural stem cells (purple) much more prone to multiply (green), in the brains of young mice (left) than in older mice (right). Image: Paloma Navarro, Stanford

July 12, 2019
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