Introduction: Photoimmunotherapy is an investigational cancer treatment platform that utilizes an antibody conjugated to a light activated dye IRDye® 700DX (IR700) combined with illumination with a non-thermal, red light for selective cell killing. CTLA-4 is a lymphocyte exhaustion marker targeted by anti-CTLA-4 checkpoint inhibitors to induce anticancer immune response and is highly expressed on intratumoral regulatory T cells (Tregs). In this study we used anti-CTLA-4-IR700 photoimmunotherapy (CTLA-4 photoimmunotherapy), which involves intratumoral Treg depletion with checkpoint inhibition, in syngeneic mouse models, including models refractory to immune checkpoint inhibition, to assess anticancer activity and immune response.

Methods: Mice inoculated with CT26, LL/2, MCA205, or 4T1 tumors were treated with CTLA-4 photoimmunotherapy. Tumor volume was determined by caliper measurements in target (illuminated) and distal (non-illuminated) tumors. Intratumoral immune responses were characterized by flow cytometry. To evaluate the activation of systemic immune responses, cytotoxic T lymphocyte assays were conducted with splenocytes derived from mice treated with CTLA-4 photoimmunotherapy. Tumor re-challenge studies were conducted to assess for tumor-specific immune memory responses.

Results: CTLA-4 photoimmunotherapy induced notable anticancer responses including complete responses (CRs), and enhanced survival of mice in multiple tumor models, including tumors resistant to anti-PD-1 or anti-CTLA-4 treatment. CTLA-4 photoimmunotherapy resulted in a reduction of intratumoral Tregs, an increase of the CD8/Treg ratio, and an increase of intratumoral NK and CD8 T cell activation in comparison to control groups. All CR mice (n=7) rejected new tumor inoculations, suggesting enhancement of systemic immune memory. Splenocytes derived from CTLA-4 photoimmunotherapy-treated mice displayed an expansion of tumor antigen-specific lymphocytes in comparison to control groups. In mice bearing bilateral CT26 tumors, tumor regression was observed in target and distal tumors after CTLA-4 photoimmunotherapy, demonstrating systemic anticancer immunity and potential abscopal effects.

Conclusion: CTLA-4 photoimmunotherapy elicited anticancer responses in a variety of tumor models, including tumors resistant to anti-PD-1 or anti-CTLA-4 treatment. Depletion of intratumoral Tregs by CTLA-4 photoimmunotherapy resulted in the local activation of CD8 T cell-mediated anticancer activity and systemic tumor specific immune response. These results indicate that treatment with CTLA-4 photoimmunotherapy can induce anticancer responses in checkpoint resistant mouse tumor models by combining rapid Treg depletion with anti-CTLA-4 checkpoint inhibition into a single method of treatment.

Citation Format: C. Daniel de Magalhaes Filho, Stephanie M. Okamura, Daniele M. Bergeron, Ahiram Rodriguez, Abram Lozano, Jerry J. Fong, Miguel Garcia-Guzman. CTLA-4 photoimmunotherapy depletes intratumoral regulatory T cells (Tregs) to augment local and systemic immunity for anticancer activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1798.