Ultrastructural studies of spontaneously regressing plane warts. Macrophages attack verruca-epidermal cells.

Plane warts were examined at the earliest phase of spontaneous involution using an electron microscopy. Macrophages outnumbered lymphocytes in invading the epidermis. Lymphocytes exhibited pseudopods on the surface which occasionally reached into the epidermal cells. The cell membranes of both macrophages and epidermal cells frequently disappeared on the contacting surface and on occasion part of the cytoplasm and even the nucleus of macrophages invaded the epidermal cells. The macrophages showed an independent partial loss of their cell membranes with a subsequent outpour of their contents. This ultrastructural feature indicates that in spontaneously regressing plane warts the depressed membrane stability of macrophages as observed in BCG and other-induced tumor cell destruction also occurs in the presence of lymphocytes. Therefore, the spontaneous involution may represent a rejection phenomenon.