5-fluorouracil enhances the immune response triggered by photodynamic therapy in murine cutaneous squamous cell carcinoma: Transcriptomic, flow cytometric and immunohistochemical analyses

Photodynamic therapy (PDT) for squamous cell carcinoma (SCC) involves the application of a photosensitizer (aminolevulinic acid, ALA) followed by exposure to light, inducing cell death and long-term anti-tumor immune responses. Here, we show that application of 5-fluorouracil (5FU) as a pretreatment significantly enhances PDT-induced immune responses. Mice bearing UV-induced SCC were treated with 5FU for three days prior to ALA-PDT. Samples were harvested for time-course analyses of systemic and intra-tumoral immune responses after PDT. RNA-Seq analysis identified significant differences in pathways and differentially expressed genes (DEGs) involved in immune responses within the tumor. Flow cytometry and immunofluorescence showed that 5FU increased recruitment of innate immune cells, activated T cells, cytotoxic T cells. Also, 5FU reduced the presence of cells expressing the immune checkpoint marker PD-1. A combination of 5FU and PDT may provide better management of SCCs in the dermatology clinic.