Blackwell Publishing Journal Backfiles 1879-2005
The hair follicle offers an exquisite model for the experimental exploration of key issues of cutaneous neuroimmunology, for example, how local, intracutaneous and systemic stress–response systems are integrated with the skin immune system and with epithelial–mesenchymal interactions (as they occur during hair follicle growth and cycling). Previously, we had shown that skin mast cells, which operate as central switchboards of inflammation and tissue remodelling, also are important regulators of hair growth in mice and that endogenous, immunomodulatory mast cell secretagogues are potent hair growth modulators. This is true both for secretagogues that are generated by the hair follicle epithelium itself (e.g. ACTH) and for mast cell-activating neuropeptides synthesized by the sensory hair follicle innervation (e.g. SP). Also, we had shown that the prototypic stress-associated neuropeptide, SP, plays a crucial role in mediating the hair growth-inhibitory, mast cell-activating, inflammation- and catagen-promoting properties of chronic psychoemotional stress on murine hair follicles. Now, we show that the immunomodulatory and mast cell-activating neurotrophin, NGF, is also crucially involved in mediating the inhibitory effects of stress on murine hair growth. Furthermore, the central, stress-related neurohormone CRH, a recognized mast cell secretagogue which is expressed by the hair follicle epithelium, also is a hair growth inhibitor and activates a fully functional peripheral equivalent of the hypothalamic-pituitary-adrenal axis within organ-cultured human scalp hair follicles, including the synthesis and secretion of cortisol as well as the induction of classical feedback loops. We also demonstrate that one of the melanocortins whose intrafollicular synthesis is stimulated by CRH (α-MSH) is a potent suppressor of MHC class I expression in situ and is thus capable of restoring the collapsed immune privilege of human anagen hair bulbs, while SP upregulates the ectopic expression of MHC class I, thus endangering the hair follicle immune privilege. Finally, we show that vanilloids long exploited as experimental tools for neuroimmunological research in the skin (capsaicin) can, in fact, directly modulate human hair growth via the stimulation of vanilloid receptors (VR1) expressed by the follicle epithelium, in addition to stimulating vanilloid expressed by skin mast cells. Therefore, the hair follicle offers an ideal, highly instructive and clinically most relevant research model for dissecting how nervous system, central and peripheral (neuro-) endocrine signalling loops and the immune system interact in order to adapt skin functions to changing environmental conditions (e.g. in response to external stressors, by alterating, e.g. keratinocyte proliferation/apoptosis, skin immune status, as well as defined cutaneous metabolic and endocrine activities).
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