Lupus erythematosus-like imiquimod reaction: a diagnostic pitfall

Imiquimod (AldaraTM), a Toll-like receptor 7 agonist (TLR7), is known for its unique properties of being an immune response modifier and stimulator. Upon topical application, this TLR7 agonist triggers a cell-mediated immune response predominantly expressed by dendritic cells and monocytes. Local skin irritation at the application site involving erythema, pain, crusting and erosions is common and well documented. On the contrary, the specific histopathologic features associated with these treatment site reactions is not. Herein reported is a case where historical omission of imiquimod use for actinic keratosis complicated the histologic interpretation. We highlight a lupus erythematosus-like microscopic pattern and explore histopathologic features that could help in avoiding a diagnostic pitfall, as well as the relationship between TLR activation, cell-mediated immunity and skin histology.