P3.02c-071 Spatially Selective Depletion of Regulatory T Cells with Near Infrared Photoimmunotherapy for Syngeneic Lewis Lung Carcinoma: Topic: IT Biomarkers

CD4+CD25+Foxp3+ regulatory T-cells (Tregs) are known to suppress immune responses. Treg-mediated immunosuppression is a key mechanism for tumor immune-evasion, which could lead cancer immunotherapies to failures. Systemic depletion of Tregs has been tried; however, intravenously delivering antibodies against Tregs might not sufficiently deplete Tregs in tumor-microenvironment or could deplete effector cells. Moreover, auto-immune responses could cause potential side effects. To overcome these problems, we exploited near infrared photoimmunotherapy (NIR-PIT) at the tumor to deplete only intratumoral-Tregs. F(ab’)2 fragments of an anti-mouse CD25 antibody, PC61.3, were generated and conjugated with a phthalocyanine dye, IRDye-700DX (αnti-CD25-F(ab’)2-IR700). In vitro NIR-PIT effect was examined against CD25-expressing-T-lymphocytes, HT2-clone-A5E-cells. In vivo CD25-target-NIR-PIT was performed after an intravenous injection of the conjugate to mice bearing subcutaneous, luciferase transfected, LL/2 (Lewis lung carcinoma, LL/2-luc). Tumor-volume, bioluminescence signals (BLI), and Immune responses following the therapy were examined. In vitro NIR-PIT-induced cytotoxicity was light-dose-dependent. Local CD25-target-NIR-PIT selectively depleted intratumoral-Tregs; yet, Tregs in any other organs were not affected. The local CD25-target-NIR-PIT induced a intratumoral rapid activation and cytotoxic action of CD8-T cells and NK-cells. This led to significant reductions of tumor-volume (p < 0.0001) and BLI (p < 0.05) and prolonged survival (p < 0.0001) compared to non-treated controls. Intriguingly, this local CD25-target-NIR-PIT induced a transient systemic cytokine storm and antitumor-effects on distant non-irradiated specific tumors. Effects of local CD25-target-NIR-PIT were significantly (p < 0.0001) inhibited by a CD8-, NK-, or INFg-depletion, suggesting the anti-tumor roles of CD8 T-cells and NK-cells. Depletion of intratumoral-Tregs with a local CD25-target-NIR-PIT rapidly induced CD8 T- and NK-cell activation, thereby restoring local anti-tumor immunity. Consequently, activated immunity led to regression of not only NIR-PIT-treated-tumors but also non-NIR-light-exposed-tumors in separate parts of the body (Fig). These observations suggest that local CD25-target-NIR-PIT may be a promising new strategy for cancer immunotherapy.