Topical tretinoin replenishes CD1a-positive epidermal Langerhans cells in chronically photodamaged human skin

Excessive exposure to sunlight results in drastic degradative changes in the constitutive cells of the epidermis. Among these is a profound abrogation of Langerhans cell number and function, leading to compromised immunologic competency. Retinoids have recently been shown to restore immunologic function in the setting of iatrogenic immunosuppression. We asked whether topical tretinoin would reverse Langerhans cell depletion in chronically photodamaged skin. We examined the skin of 8 volunteers immunohistochemically before and after 6 months of daily applications of tretinoin. At baseline, there was a profound depletion of CD1a-positive Langerhans cells in the interfollicular epidermis, but not in the adjoining follicular epithelium. After tretinoin, all patients demonstrated replenishment of interfollicular epidermis by CD1a-positive Langerhans cells. This was associated with induction of HLA-DR expression on infundibular keratinocytes, as well as the appearance of CD1a dendritic cells in the papillary dermis. Thus, the enhancement of epidermal immunity in photodamaged skin may reflect restoration of antigen-presenting Langerhans cells. The source of this renewed dendritic cell population is likely to be follicular infundibulum and the papillary dermis.