Vitamin D and alopecia areata: possible roles in pathogenesis and potential implications for therapy

Alopecia areata is characterized by the loss of hair on the scalp and elsewhere on the body. It affects approximately 2% of the general population. It is believed to be an autoimmune disease. However, its pathogenesis remains incompletely understood. Recent studies have revealed a substantial link between vitamin D and alopecia areata. But the underlying mechanism still yet to be deciphered. This article reviews the current literature and discusses the possible roles of vitamin D in the pathogenesis of alopecia areata in the context of (1) loss of immune privilege in hair follicle, (2) autoreactive effector T cells and mast cells, (3) nature killer group 2 member d-positive cytotoxic T cells, (4) Janus kinase/signal transducers and activators of transcriptional signaling pathway, (5) regulatory T cells, (6) immune checkpoints, and (7) oxidative stress, which are believed to play important roles in autoimmunity in AA. This paper provides new insights into research directions to elucidate the exact mechanisms of vitamin D in the pathogenesis. Calcipotriol, a vitamin D analog, has been reported to be topically used in treating alopecia areata with promising results. Combination therapy of vitamin D analogs with corticosteroids might also be used in treating alopecia areata.