Evidence that the bulge region is a site of relative immune privilege in human hair follicles

Background Recent gene profiling data suggest that, besides the anagen hair bulb, the epithelial stem cell region in the outer root sheath of hair follicles (HFs), termed the bulge, may also represent an area of relative immune privilege (IP). Objectives To investigate whether the human HF bulge is a site of relative IP within anagen VI HFs. Methods Anagen VI HFs from normal human scalp skin were analysed using immunohistological staining techniques, quantitative histomorphometry and statistical analysis. For functional evidence we performed full‐thickness human scalp skin organ cultures to investigate whether interferon (IFN)‐γ, a key inducer of IP collapse in hair bulbs, has a similar effect on the putative bulge IP. Results Major histocompatibility complex (MHC) class Ia, β₂‐microglobulin and MHC class II immunoreactivity are downregulated in the human bulge. The immunosuppressants α‐melanocyte stimulating hormone, transforming growth factor‐β2, macrophage migration inhibitory factor and indoleamine‐2,3‐dioxygenase (IDO) are upregulated in the CD200+, stem cell‐rich bulge region. These CD200+ cells also co‐express HLA‐E. Furthermore, IFN‐γ induces significant ectopic MHC class Ia expression in bulge cells of organ‐cultured human scalp skin. Conclusions These data suggest that the bulge of human anagen HFs represents a hitherto unrecognized site of relative IP in human skin. Simultaneously, we present the first evidence of IDO and HLA‐E protein expression in normal human HFs. Bulge IP presumably protects the HF epithelial stem cell reservoir from autoaggressive immune attack whereas a loss of bulge IP may play a central role in the pathogenesis of cicatricial alopecias.