Abstract: Objective To compare 3 commercial immunogenic cell death (ICD) inducers, namely Chlorin e6 (Ce6), Neutral Red (NR), and Rose Bengal Sodium salt (RB), for their photosensitive properties, efficacy for photodynamic therapy (PDT) and ICD induction efficiency in antitumor immunotherapy. Methods Reactive oxygen species (ROS) probes were used to evaluate the photosensitivity of the 3 ICD inducers, and their capacity for inducing intracellular ROS production was evaluated using a DCFH-DA probe. The cytotoxicity and biocompatibility of the 3 photosensitizers were compared using a CCK-8 kit, and their ICD-inducing efficiency was assessed by detecting the levels of surface-exposed calreticulin (ecto-CRT), high mobility group protein 1 (HMGB1) and adenosine triphosphate (ATP). In the animal experiment, BALB/c mouse models bearing 4T1 cell-derived subcutaneous tumor were given intratumoral injection of Ce6 or NR solution (30 μL, 5 mg/mL), followed 2 h later by white light irradiation for 10 min (400 mW/cm2). Body weight and tumor size changes of the mice were monitored, and the percentage of CD8+ T cells in the tumor and IFN-γ+ CD8+ T cells in the spleen were analyzed by flow cytometry 14 days after the treatment. HE and TUNEL staining was used to analyze tumor cell apoptosis in the mice. Results Among the 3 photosensitizers, Ce6 exhibited the strongest ROS-inducing capability and killing effect on the tumor cells. The results of ecto-CRT, HMGB1 and ATP level detection all demonstrated a stronger ICD induction ability of Ce6. In the tumor-bearing mice, the tumor growth in Ce6 and NR groups was significantly inhibited after the treatment. The percentages of CD8+ T cells and IFN-γ+ CD8+ T cells were 12.7% and 7.1% in Ce6 group, respectively, significantly higher than those in NR group (6.1% and 2.8%, respectively; P<0.05). HE and TUNEL staining revealed obvious tumor cell apoptosis in the tumor tissues in both Ce6 and NR groups, but the therapeutic effect was more prominent in Ce6 group. Conclusion Among the 3 photosensitizers, Ce6 has the highest efficiency for inducing ROS production with the strongest PDT efficacy and ICD induction capability. Ce6 can also increase the number and function of CD8+ T cells in anti-tumor immunotherapy to initiate robust adaptive immune response.

Key words: chlorin e6; Neutral Red; Rose Bengal sodium salt; photodynamic therapy; immunogenic cell death; anti-tumor immunotherapy