Effect and mechanism of fluence rate on immunity of photodynamic therapy mediated by hematoporphyrin derivatives in murine colorectal cancer

Background: Colorectal cancer (CRC) is the second most common cause of cancer-related death globally, characterized by high rates of recurrence and metastasis. Photodynamic therapy (PDT) is a promising CRC treatment that controls the primary tumor effectively and triggers host immunity to prevent tumor recurrence and metastasis. Fluence rate is a crucial regulator of tissue oxygenation and inflammatory response and may play a critical role in antitumor immunity associated with PDT. This study examined the effect of fluence rate on antitumor immunity of PDT mediated by hematoporphyrin derivatives (HpD) in the CT26 tumor model. Methods: Mice with subcutaneous CT26 tumors were subjected to an 80 J/cm² light dose administered at fluence rates of 10, 50, 100, and 150 mW/cm². All PDT-treated tumor-free mice were rechallenged with the same tumor cell line in the contralateral flank 90 days after PDT. Treatment outcome and immune infiltration in tumors were examined to investigate the antitumor immunity of HpD-PDT at varying fluence rates. Transcriptomic sequencing analysis and exposure of danger-associated molecular patterns (DAMPs) on tumors were also investigated to study the mechanism of antitumor immunity. Results: HpD-PDT at 10–150 mW/cm² induced systemic immune responses that mediated protection against tumor rechallenge in survivor mice (p < 0.05). Analysis of infiltrating lymphocytes revealed that HpD-PDT increased T-cell infiltration and enhanced the cytotoxicity of CD8⁺ T cells in tumors and spleens (p < 0.05), which was shown to be fluence rate-dependent. As the fluence rate increased from 10 mW/cm² to 150 mW/cm², the antitumor immunity of HpD-PDT showed a trend of first decreasing and then increasing. Furthermore, the expression of DAMPs on tumor cells was upregulated following HpD-PDT (p < 0.05). Conclusions: These results indicate that fluence rate affects HpD-PDT-induced immune response by provoking systemic antitumor immunity and reshaping the tumor immune microenvironment. Our findings establish a correlation between fluence rate and antitumor immunity of PDT in the CT26 tumor model and provide an experimental basis for optimizing the PDT regimen.