Tregs, a type of immune cell, may be the key in promoting healthy hair growth in humans.

Researchers from the University of California San Francisco have discovered that when Tregs, which are associated with controlling inflammation and directly trigger stem cells in the skin are absent, the stem cells are unable to regenerate hair follicles, leading to baldness.

“Our hair follicles are constantly recycling: when a hair falls out, the whole hair follicle has to grow back,” Dr. Michael Rosenblum, Ph.D., an assistant professor of dermatology at UCSF and senior author on the new paper, said in a statement. “This has been thought to be an entirely stem cell-dependent process but it turns out Tregs are essential.

“If you knock out this one immune cell type, hair just doesn't grow.”

According to the researchers, defects in Tregs might be responsible for alopecia areata—an autoimmune disorder that causes hair loss and could play a role in other forms of baldness including male pattern baldness.

The researchers developed a technique for temporarily removing Tregs from the skin and tested it on mice. However, when they shaved patches of hair from the mice to make observations of the affected skin they found that the hair never grew back.

After conducting imaging experiments, the researchers discovered that Tregs has a close relationship with the stem cells that reside within hair follicles and allow them to regenerate.

By removing the T cells from the skin, hair growth was blocked only if it was done within the first three days after shaving a patch of skin, when follicle regeneration would normally be activated.

However, if Tregs is removed once the regeneration had already started there was no impact on hair regrowth.

Tregs inform the rest of the immune system of the difference between harmful entities in the body and helpful ones. When the cells do not properly function, a person may develop allergies to harmless substances including peanut protein or cat dander or possibly suffer from autoimmune disorders where the immune system turns on the body’s own tissues.

Tregs is native to the body’s lymph nodes but can live permanently in other tissues where they seem to have evolved to assist with local metabolic functions as well as playing their normal roles as an anti-inflammatory.

The researchers discovered that Tregs trigger stem cell activation directly through a cell-to-cell communication system and found that Tregs in the skin express unusually high levels of communication with a signaling protein called Jagged 1, compared to Tregs elsewhere in the body.

They also found that by removing Tregs with microscopic beads covered in the protein they were able to restore communicating signaling in the stem cells and successfully active follicle regeneration.

“It's as if the skin stem cells and Tregs have co-evolved, so that the Tregs not only guard the stem cells against inflammation but also take part in their regenerative work,” Rosenblum said. "Now the stem cells rely on the Tregs completely to know when it's time to start regenerating."

The researchers also theorized that Tregs could play a role in wound repair because the same stem cells are responsible for helping heal the skin after an injury.