IL9 and melanoma: The topical immunotherapy agent, DPCP, was first reported as a treatment for melanoma metastases in 1989.8 As a hapten, DPCP elicits a CD8+ T-cell-mediated, delayed hypersensitivity response, which in turn stimulates the release of cytokines, specifically IL-24 and IL-9, which are known to act in melanoma as tumor-suppressor cytokines promoting lymphocyte-mediated tumor destruction.9