Immune privilege and alopecia areata

Abstract

Hair follicles (HFs) are chiefly characterized by an absent or very low level of expression of MHC class Ia antigens, which is complemented by a number of factors, such as the local production of potent immunosuppressive agents, dysfunction of professional antigen-presenting cells and inhibition of natural killer cell activities. In the hair bulb, immune privilege (IP) is seen only in the anagen stage of HF cycling. The (‘noncycling’) bulge area of human scalp HFs also enjoy some relative, stably maintained IP. Hair bulge IP is constituted not only by factors of HF-IP, but also indoleamine 2,3-dioxygenase and CD200. A collapse of HF (hair bulb and hair bulge) probably plays an important role in the pathogenesis of two of the most common organ-specific autoimmune diseases, alopecia areata and cicatricial alopecia, respectively. Therefore, the therapeutic restoration of IP collapse promises to be a particularly effective future strategy for the treatment of these hair-loss diseases.