IL-15 Prolongs Hair Growth and Acts as a Guardian of Human Hair Follicle Immune Privilege

The study "056 IL-15 prolongs hair growth and operates as a guardian of human hair follicle immune privilege" investigates the role of interleukin-15 (IL-15) in alopecia areata (AA), a condition characterized by hair follicle immune privilege (HF IP) collapse and premature catagen. The research found that AA patients' lesional skin had an increased number of perifollicular IL-15+ T and NK cells. However, the expression of the IL-15 receptor and the common g chain was significantly decreased in these patients' HFs. When healthy human anagen scalp HFs were treated with rhIL-15, it stimulated hair matrix keratinocyte proliferation and inhibited catagen development. Moreover, IL-15 provided relative protection from HF IP collapse and could partially restore HF IP. On the other hand, silencing IL-15Ra accelerated catagen development and inhibited hair matrix proliferation. Therefore, while IL-15 may promote AA-associated T and NK cell infiltration, IL-15Ra-mediated signaling acts as a critical HF IP guardian and hair growth-promoting cytokine. These novel IL-15 functions are blocked by JAK1/3 inhibitors, which may explain why AA relapses rapidly after therapy discontinuation.