Lichen planopilaris (LPP) is a lymphocyte-mediated cicatricial alopecia mostly involving the bulge region of the hair follicle. The origin of LPP is unknown. Therapy for LPP often does not prevent disease progression. We describe histologic and immunohistologic features that aid in diagnosis and provide an explanation for disease progression in LPP.

We sought to demonstrate a decrease in the number of catagen-/telogen-phase follicles and to confirm the loss of cytokeratin 15 (CK15) expression in the stem cells of LPP-affected follicles.

In all, 144 LPP cases were retrieved; 55 cases were stained immunohistochemically, targeting the CK15 antigen with 40 cases ultimately analyzed for CK15 expression.

Catagen/telogen phase was significantly decreased or absent in all cases of LPP, a novel clue useful in histologic diagnostics. The loss of CK15⁺ stem cells in most affected follicles in LPP was also confirmed, with unaffected follicles retaining CK15⁺ stem cells.

Limited tissue for analysis remained in the clinical sample tissue blocks.

Damaged follicles that have lost their CK15⁺ stem cells disappear when they enter catagen phase. CK15⁺ stem cell loss explains the clinical observation that LPP progresses despite immunosuppressive therapies. Finally, the absence of catagen/telogen hair follicles is a helpful diagnostic clue for LPP.