Human and mouse skin contains many tissue-resident regulatory T cells (Treg cells) that localize mostly around the hair follicles. In Cell, Ali et al. show that Treg cells enhance the activation and differentiation of hair-follicle stem cells (HFSCs) and hair growth. In mice, the abundance and activation of Treg cells increase during the telogen (quiescent) stage of the hair follicle cycle compared with the anagen (proliferation) stage. Depletion of Treg cells results in slower hair regeneration and attenuates the telogen-to-anagen transition and the proliferation of bulge HFSCs after depilation or during normal growing cycles, without affecting their steady-state numbers or inducing an inflammatory environment. Treg cells in the skin have higher expression of the Notch1 ligand Jagged1 than that of Treg cells in the draining lymph nodes, and depletion of Treg cells changes the Notch-dependent transcriptional signature in HFSCs. Treg cell enhancement is known to promote hair regeneration in patients with alopecia areata.

Cell 169, 1119–1129 (2017)