Enhanced systemic antitumor efficacy of PD‐1/PD‐L1 blockade with immunological response induced by photodynamic therapy

Abstract Background Photodynamic therapy (PDT) is an antitumor therapy and has traditionally been regarded as a localized therapy in itself. However, recent reports have shown that it not only exerts a direct cytotoxic effect on cancer cells but also enhances body's tumor immunity. We hypothesized that the immunological response induced by PDT could potentially enhance the efficacy of programmed death‐1 (PD‐1) / programmed death‐ligand 1 (PD‐L1) blockade. Methods The cytotoxic effects of PDT on colon 26 cells were investigated in vitro using the WST assay. We investigated whether the antitumor effect of anti‐PD‐1 antibodies could be amplified by the addition of PDT. We performed combination therapy by randomly allocating tumor‐bearing mice to four treatment groups: control, anti‐PD‐1 antibodies, PDT, and a combination of anti‐PD‐1 antibodies and PDT. To analyze the tumor microenvironment after treatment, the tumors were resected and pathologically evaluated. Results The viability rate of colon 26 cells decreased proportionally with the laser dose. In vivo experiments for combined PDT and anti‐PD‐1 antibody treatment, combination therapy showed an enhanced antitumor effect compared with the control. Immunohistochemical findings of the tumor microenvironment 10 days after PDT indicated that the number of CD8+ cells, the area of Iba‐1+ cells and the area expressing PD‐L1 were significantly higher in tumors treated with combination therapy than in tumors treated with anti‐PD‐1 antibody alone, PDT alone, or the control. Conclusions PDT increased immune cell infiltration into the tumor microenvironment. The immunological response induced by PDT may enhance the efficacy of PD‐1/PD‐L1 blockade.