Growing Hair Switches On Ancient Viral DNA in the Follicle

When a mouse hair follicle enters its growth phase, the stem cells inside it start transcribing retroviruses that have been parked in the mammalian genome for millions of years. Nothing bad happens. That is the strange part.

In a mouse study at the National Institute of Biological Sciences, Beijing, researchers stripped out regulatory T cells, the immune system's peacekeepers, at different points in the hair cycle. During the resting phase, the animals shrugged it off. During the growth phase, killer CD8 T cells swarmed the follicles, destroyed the stem cells and the hair came out.

The difference traced back to nerves. Sympathetic nerve activity runs higher while hair is growing, and cutting that input protected the mice. The reverse worked too: stimulating those nerves, or subjecting the animals to psychological stress, left resting follicles vulnerable. The signal lands on a receptor called ADRB2 on the stem cells themselves. Delete it and the attack mostly stops.

That same nerve signal is what wakes the ancient retroviruses. A DNA sensor inside the cell named AIM2 registers the result, dendritic cells get pulled in, and the team identified a viral peptide the attacking T cells recognize.

They describe it as a two-hit model, an accelerator and a brake. Regeneration presses the accelerator by switching on the viral sequences. Regulatory T cells are the brake. Release the brake during the resting phase and nothing happens. Release it mid-growth and the stem cells are gone.

The model fits certain scarring forms of hair loss most closely, and the authors suggest the logic may extend to other tissues. Whether it does is unsettled.

The brake usually holds. The accelerator gets pressed every time a hair decides to grow.

Read the full story at National Institute of Biological Sciences, October 8, 2026

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