Dynamic Tracking of Tumor Microenvironment Modulation Using Kaede Photoconvertible Transgenic Mice Unveils New Biological Properties of Viral Immunotherapy.

abstractCAN-2409 is a replication-defective adenovirus that delivers the herpes simplex virus–thymidine kinase gene to infected cells. Intratumoral administration of CAN-2409, followed by prodrug, results in the formation of a toxic metabolite able to induce immunogenic cell death, exposure of tumor-associated antigens, and activation of local and systemic immune responses. We used a dynamic labeling model with MC38 tumor cells implanted in photoconvertible Kaede mice. Violet light was used to label the tumor microenvironment (TME), distinguishing retained versus newly entering cells and allowing real-time monitoring of immune compartment changes within tumors. Administration of CAN-2409 + prodrug led to control of tumor growth and significantly increased effector CD8+ T-cell responses. Photolabeling of the TME revealed that rather than enhancing recruitment of T cells to the tumor, CAN-2409 altered the TME whereby newly entering and retained CD8+ T cells were significantly more proliferative. CAN-2409 supported reinvigoration of tumor-associated antigen–specific CD8+ T cells and expansion of regulatory T cells of an altered phenotype. Moreover, the combination of CAN-2409 + prodrug and anti–CTLA-4 antibody treatment further improved control of tumor growth, in part by the enhanced CD8+ T-cell–mediated effector function and diminished regulatory T cell–mediated immunosuppression. Collectively, these data defined at least two temporally distinct pathways underpinning the mechanism of action of CAN-2409 that overcome cell exhaustion and decrease immune suppression. The results also support the rationale for future clinical trials of CAN-2409 treatment combined with anti–CTLA-4 antibody therapy. SIGNIFICANCE: This study utilized a novel photoconvertible mouse tumor model to track immune cell trafficking upon treatment with an investigational viral immunotherapy (CAN-2409), revealing enhanced T-cell responses after viral immunotherapy associated with local proliferation of T cells within tumors that could further enhance antitumor efficacy in combination with immune checkpoint inhibitors. These findings define temporally and spatially distinct interactions of immune cells that could be harnessed by novel therapeutics.