Alopecia is an autoimmune condition in which the hair follicles are erroneously targeting, which causes hair loss. Researchers investigating this condition have now revealed a surprising link between the immune system and hair growth. Reporting in Nature Immunology, scientists found that the interaction between regulatory T cells, which are immune cells, and skin cells is mediated by a hormone, and can lead to hair follicle generation and hair growth.
"For the longest time, regulatory T cells have been studied for how they decrease excessive immune reactions in autoimmune diseases," said corresponding study author Ye Zheng, associate professor in Salk's NOMIS Center for Immunobiology and Microbial Pathogenesis. "Now we've identified the upstream hormonal signal and downstream growth factor that actually promote hair growth and regeneration completely separate from suppressing immune response."
This research began with a study of how glucocorticoids and regulatory T cells were related to autoimmune disorders. Glucocorticoids are steroid hormones that can help control inflammation; they are used as a treatment for autoimmune diseases in some cases.
The scientists determined that the relationship between glucocorticoids and regulatory T cells don't appear to be involved in several disorders, including asthma, multiple sclerosis, and Crohn's disease.
They turned their attention to places where regulatory T cells can be found carrying high levels of glucocorticoid receptors, like the skin. In both normal mice and a mouse model that lacked glucocorticoid receptors, hair loss was induced. The researchers found that normal mice could easily regrow their hair after two weeks. But noticeably, the mice without glucocorticoid receptors could barely regrow any, said first study author Zhi Liu, PhD. "It was very striking, and it showed us the right direction for moving forward."
By observing signaling between cells, the researchers found that glucocorticoids can trigger regulatory T cells to stimulate hair follicle stem cells, leading to hair growth. The connection between T cells and hair follicle stem cells depends on the production of a molecule called TGF-beta3 in regulatory T cells, caused by glucocorticoid receptors. Hair follicle stem cells will differentiate into new hair follicles, and promote hair growth because they've been activated by TGF-beta3.
Regulatory T cells don't typically generate TGF-beta3, though they do in muscle and heart tissue has been injured.
Glucocorticoids can be used to prevent immune cells from attacking hair follicles in the skin of alopecia patients. The glucocorticoids also have a ".. double benefit of triggering the regulatory T cells in the skin to produce TGF-beta3, stimulating the activation of the hair follicle stem cells," said corresponding study author Ye Zheng, an associate professor at the Salk Institute.