IL-15 Prolongs Anagen, Stimulates Proliferation, and Suppresses Apoptosis in the Hair Matrix of Human Scalp Hair Follicles

The hair cycle clock that controls human hair follicle (HF) cycling remains elusive (Al-Nuaimi et al., 2014; Nicu et al., 2020; Paus and Foitzik, 2004). However, it is well appreciated that the massive HF transformations from growth (anagen) to apoptosis-driven regression (catagen) and relative quiescence (telogen) back to anagen (Geyfman et al., 2015; Paus and Cotsarelis, 1999; Paus et al., 1999) are mainly driven by cyclic changes in the intrafollicular production and activity of potent hair growth–regulatory growth factors, such as insulin-like growth factor 1 (IGF-1) (Chéret et al., 2018; Philpott et al., 1994; Rudman et al., 1997), TGFβ2 (Hibino and Nishiyama, 2004; Soma et al., 2002; Tsuji et al., 2003), HGF (Nicu et al., 2021), and Wnt signals (Hawkshaw et al., 2018).