Abstract
Background
While cancer treatment efficacy exhibits circadian differences, the role of circadian rhythms in photodynamic therapy (PDT) remains unclear. Understanding this relationship is crucial for treatment optimization.Methods
Using B16F10 and 4T1 tumor-bearing mice, PDT was administered at different Zeitgeber times (ZT). Immune cell infiltration was analyzed using flow cytometry and immunofluorescence. We also compared the effects of PDT treatment between NSG mice and CD11c-DTR mice. Transcriptomic profiling of sorted CD11c⁺MHCII⁺ DCs was performed via RNA-seq. In vitro assays measured immunogenic cell death (ICD) markers and dendritic cells (DC) maturation. A PDT vaccine model was also evaluated.Results
PDT efficacy showed strong circadian dependence, optimal at ZT10 and weakest at ZT22 in immunocompetent mice-a rhythm maintained in constant darkness but inverted with light-cycle reversal, confirming endogenous control. The effect was abrogated in immunodeficient NSG mice, pinpointing an immune-mediated mechanism. Further analysis revealed circadian oscillations in the infiltration of CD4⁺, and CD8⁺ T cells into tumors and spleens, peaking at ZT10. Crucially, the rhythmic abundance and function of CD11c⁺MHCII⁺ DCs were identified as the pivotal regulator. Specific DC ablation eliminated both the circadian efficacy of PDT and the rhythmic T-cell infiltration. Transcriptomics revealed ZT10 DCs were polarized toward adaptive immunity, while ZT22 DCs favored pro-tumorigenic pathways. PDT-induced ICD promoted DC maturation in vitro and enhanced antitumor responses in vivo.Conclusion
The intrinsic circadian rhythm of DCs is a master regulator of PDT efficacy, rhythmically governing adaptive antitumor immunity. These findings advocate chrono-immunotherapy through precise treatment timing to maximize clinical benefits.