Annular lichen planus: study of the cellular mechanisms of annularity.

Annular lichen planus is a rare, unique subtype of lichen planus. We report a 34-year-old Japanese male who had multiple, dark red to purple, annular macules with slightly raised borders. Histopathological examination of a skin biopsy specimen from the peripheral region of the macule showed the typical features of lichen planus. Immunohistochemical stainings revealed that a number of CD1a-positive, S-100-protein-positive Langerhans cells were present at the border zone of the annular lesion and most of the infiltrating cells were CD4-positive, CD8-negative lymphocytes. Conversely, inside the annular lesion, the number of Langerhans cells was decreased, and an equal number of CD4-positive cells and CD8-positive cells was present in the sparse infiltration. These results suggest that activation of Langerhans cells initiated a lichenoid tissue reaction but a subsequent depletion of Langerhans cells suppressed the lymphocyte infiltration. These sequential events might be involved in the formation of the unique annular structure in this condition.