611 Painless Photodynamic Therapy for Actinic Keratosis: Enhancement of Innate and Adaptive Immune Responses by 5-Fluorouracil Pretreatment in a Murine Model

Painless photodynamic therapy (pPDT) to treat actinic keratoses (AK) involves application of a photosensitizer followed by immediate exposure to light; this causes little-to-no pain, yet results in effective lesion clearance by inducing long-term immune responses. In this investigation, we show that pretreatment of AK lesions with 5-fluorouracil (5FU), a popular chemotherapeutic agent with immune-modulatory effects, causes significant enhancement of PDT-related immune responses in 5FU-treated lesions. Hairless mice with AK lesions (generated by repeated UVB exposure for 20 weeks) were treated topically with 5FU or with vehicle for three days prior to application of topical ALA followed immediately by light (Blu-U) exposure. Lesions were harvested for time-course analyses of intra-tumoral immune responses after PDT with or without 5FU. Our immunofluorescence data showed increased recruitment of innate immune cells, i.e., neutrophils (Ly6G+) and macrophages (F4/80+), which peaked at 72 hours and 1-week post pPDT, respectively, and was greater in 5FU treated lesions. Also, enhanced infiltration of activated T cells (CD3+) throughout the time course, and of cytotoxic T cells (CD8+) approximately 1 - 2 weeks post pPDT, was observed in 5FU treated lesions. In addition to its effects on innate and adaptive immunity, 5FU pretreatment significantly reduced the presence of cells expressing, the immune checkpoint marker PD1, at ∼72 hours post pPDT, which should favor an anti-tumor immune response by promoting inflammatory, cytotoxic T cell activity. Our results suggest that a combination of these two cancer therapies (5FU and pPDT), each individually known to induce long-term anti-tumor immune responses in addition to their primary effects on cancer cells, may synergize to provide better management of AK in the dermatology clinic.