Cytokeratin 15 expression in trichoepitheliomas and a subset of basal cellcarcinomas suggests they originate from hair follicle stem cells

Trichoepitheliomas and many basal cell carcinomas appear to arise from the hair follicle, and in particular from the hair follicle bulge. This histoge nesis is suggested from both morphological and immunohistochemical studies on tumor cells and stroma. Epithelial stem cells are thought to be importan t in tumorigenesis, and we previously localized a population of stem cells to the bulge area of the outer root sheath. We recently identified an anti- CD8 monoclonal antibody (DAKO clone C8/144B) that cross-reacts with cytoker atin 15 (K15), and serves as a specific marker for the bulge. In this study , we screened a series of trichoepitheliomas (n = 13), basal cell carcinoma s (n = 37) and a variety of other skin tumors with this antibody. All trich oepitheliomas (100%) showed keratin 15 expression, while only a subset of b asal cell carcinomas (27%) was K15-positive. Epidermal tumors, including sq uamous cell carcinomas, were K15-negative. Tumors of follicular derivation such as proliferating trichilemmal cysts were also K15-positive, while othe rs such as pilomatricoma were K15-negative. Expression of K15 in trichoepit heliomas, some basal cell carcinomas and other follicular tumors suggests t hat these tumors are related to hair follicle stem cells in the bulge.