Virus-based IFNγ gene delivery and photodynamic therapy cooperate to remodel the tumor microenvironment and suppress breast cancer

This study demonstrates a potent synergistic anti-tumor effect of photodynamic therapy (PDT) based on chlorin e6 (Ce6) combined with Semliki Forest virus-mediated IFNγ (SFV/IFNγ) delivery in 4T1 mouse breast cancer model. We demonstrate that Ce6-PDT promotes macrophage-mediated phagocytosis of cancer cells but fails to reprogram macrophages into an M1-like phenotype. <i>In vitro</i>, in a 3D spheroid model, the Ce6-PDT combination with SFV/IFNγ reduces cancer spheroid viability more effectively than monotherapies. <i>In vivo</i>, sequential administration of PDT followed by SFV/IFNγ significantly reduces tumor burden, reducing 4T1 tumors by 87.5%, surpassing monotherapies (∼40%). Spectral flow cytometry and high-dimensional analysis of tumor immune cell infiltrate revealed that PDT boosts innate immune activation (DCs, NKs, iNOS⁺, MHC II^high cells), while SFV/IFNγ diminishes immune suppression (Th, Tregs, and CD206⁺ macrophages, MDSCs), creating a more immunostimulatory tumor microenvironment. This research contributes to our understanding of the synergistic potential between SFV/IFNγ therapy and PDT in breast cancer treatment.