Primary tissue culture of spontaneously regressing flat warts. In vitro attack by mononuclear cells against wart‐derived epidermal cells

Although tumors may be resolved due to host immune response, it is difficult to obtain direct evidence of this in man. Numerous flat warts, human papilloma virus type 3‐induced papillomas, disappear systemically and simultaneously after showing inflammatory changes. Histologically, there is a dense cellular infiltration composed of lymphocytes and mononuclear phagocytes as identified by α‐naphthyl acetate esterase staining in situ, the former being predominant in most cases. The primary tissue culture of such inflamed flat warts from ten cases revealed a proliferation of wart‐derived keratinocytes as is the case with ordinary flat warts. However, in nine of the ten cases, massive mononuclear cells, most of which were T‐lymphocytes, migrated out of the explants and began to attack these keratinocytes, inducing degenerative changes. These findings indicate that cell‐mediated tumor cell destruction rather than antiviral reaction induces systemic spontaneous regression of multiple papillomas in man.