THE HUMAN HAIR FOLLICLE - A RESERVOIR OF CD40(-DEFICIENT LANGERHANS CELLS THAT REPOPULATE EPIDERMIS AFTER UVB EXPOSURE() B7)

The ability of skin to maintain its protective structural and function al integrity depends on both resident and circulating cells. Until now it was thought that dendritic antigen presenting cells of epidermis ( Langerhans cells) were replaced by circulating bone marrow derived pre cursors. Here we show by immunostaining studies of timed biopsies take n from human skin after ultraviolet exposure, that hair follicle is a critical reservoir of Langerhans cells that repopulate epidermis deple ted of Langerhans cells by a single four minimal erythema dose of ultr aviolet B. Immunostaining with antibodies to thymidine dimers showed t hat ultraviolet B only penetrated the superficial hair follicle openin g, whereas deeper follicle was relatively protected. Langerhans cells migrating from hair follicle into epidermis 72 h after ultraviolet exp osure have a partial deficiency of molecules important to T cell costi mulation, We used four color flow cytometry to show that Langerhans ce lls isolated from epidermis 72 h after ultraviolet B can upregulate CD 40 but not B7-1 or B7-2 expression in culture, suggesting a different phenotype of hair follicle Langerhans cells. Therefore, the hair folli cle is a specialized immune compartment of the skin that serves as an intermediate reservoir of Langerhans cells between bone marrow and epi dermis, and that may play a critical role in immune surveillance.