Expression of the hair stem cell-specific keratin 15 in pilar tumors of the skin.

Keratin 15 (K15) was recently shown to be a specific marker of stem cells of the hair-follicle bulge. We studied the reactivity of an antibody to the CD8 antigen (C8/144B), recognizing K15, on 66 cutaneous tumors with known or alleged pilar differentiation, in order to assess its usefulness in the diagnosis of this group of tumors. 2/2 basal cell nevi, 5/8 trichoepitheliomas and 1/3 trichofolliculomas showed substantial reactivity. Much weaker reactivity was observed in cases of trichilemmal tumors (trichilemmomas and trichilemmal cysts); by contrast, all cases of pilomatricomas, basal cell carcinomas and epidermoid cysts were completely unreactive. These results are in keeping with the admitted differentiation of the tumors studied, and suggest further that basal cell carcinomas do not differentiate towards hair bulge cells. From a practical point of view, immunostaining for K15 seems to be an additional useful adjunct for the differential diagnosis between basal cell carcinoma and trichoepithelioma.