Increased number of mast cells in the dermis in actinic keratosis lesions effectively treated with imiquimod

Actinic keratosis (AK) is a cutaneous cancer in situ which develops as a result of excessive exposure to ultraviolet (UV). Toll‐like receptor (TLR)7 agonist imiquimod is a topical immune response modifier and is effective for the treatment of non‐melanoma skin cancers. Recently, the diagnostic role of the dermatoscope has been reported in the course of treatment of AK. In addition, mast cells are now considered to contribute to both the innate and adaptive immune systems in topical imiquimod therapy. We assessed the effect of imiquimod treatment by dermatoscopic and immunohistochemical findings in 14 patients with a total of 21 AK lesions. With the dermatoscope, though the mean erythema score was not significantly different between the cured lesions and the unresponsive lesions, the erythema/red pseudo‐network (“strawberry”) pattern was decreased significantly in the cured lesions. By immunohistochemistry, the number of Ki‐67‐positive proliferative cells in the epidermis was decreased and that of CD117‐positive mast cells in the dermis was increased in the responding lesions. To the best of our knowledge, this is the first study demonstrating that the number of mast cells in the dermis was increased in AK lesions effectively treated with imiquimod. Our present result suggests that mast cells may contribute an antitumor effect in human skin treated with topical imiquimod.