Photothermal Therapy-Induced Immunogenic Cell Death Synergistically Enhances the Therapeutic Effect of Immune Checkpoint Inhibitors

Background/Objectives: To improve the response rate of immune checkpoint inhibitors (ICIs), inducing immunogenic cell death (ICD) is a promising approach. Photothermal therapy (PTT) induces immunogenic cell death and activates anti-tumor immunity. While there are various ICD inducers, the difference in ICD induction by various modalities is poorly understood. In this study, we found previously unrecognized advantages of PTT compared to anti-cancer drugs and showed the usefulness of PTT as an anti-cancer drug-free approach to be combined with immunotherapy. Methods: Gold nanorods were synthesized as photothermal agents and added to culture medium or locally administered to tumor tissues. Mitoxantrone (MIT), an ICD inducer, and cisplatin (CDDP), a non-ICD inducer, were compared with PTT. To assess the induction of ICD, the subcellular localization and amounts of high mobility group box 1 (HMGB1) and calreticulin (CRT) were observed using immunofluorescent staining. FM3A tumor-bearing mice were treated with PTT or anti-cancer drugs, and cell death and DAMPs localization in tumor tissues were analyzed. Also, the supra-additive effect of PTT on ICI was observed. Tumor-infiltrating CD8+ T cells were examined to evaluate the immune status in tumor tissues. Results: In vivo assays showed that PTT induces HMGB1 release and increased expression of CRT on the cell membrane. Moreover, PTT showed a supra-additive effect in terms of therapeutic effect and anti-tumor activation when combined with an immune checkpoint inhibitor. Conclusions: In this study, we demonstrated that PTT induced ICD-related signaling and improved the response rate of ICI, which means PTT is a promising combination therapy with ICI.