Regulatory T cells (Tregs) play pivotal roles in inhibiting anti-tumor immunity at different phases and locations (priming phase in lymph node and effector phase in tumor tissue) through their ability to suppress the function of T, NK, and antigen-presenting cells. However, our knowledge about the roles of Tregs in tumor immunity has largely derived from analyses after systemic depletion of Tregs or systemic manipulation of Treg functions in murine tumor models. The specific roles that intra- and extra-tumor Tregs play remain unclear due to technical difficulties to deplete or manipulate Tregs in a spatially specific manner. In order to achieve spatially specific depletion of Tregs in a clinically applicable manner, we have previously developed an anti-CD25 F(ab')2 near-infrared photoimmunotherapy (NIR-PIT), in which an intravenous injection of IR700-conjugated anti-CD25 F(ab')2 followed by irradiation of near-infrared light on tumors achieves selective depletion of intratumor Tregs and suppresses tumor growth. The suppression of tumor growth by the anti-CD25 F(ab')2 NIR-PIT depends on CD8 T and NK cells and their production of IFN-γ; however, the exact target of IFN-γ and the mechanisms of tumor regression remain to be elucidated. In this study, we aimed to dissect the mechanisms of the anti-tumor effect of intratumor Treg depletion by anti-CD25 F(ab')2 NIR-PIT using a series of Ifngr1 knockout mice models including bone-marrow chimera and conditional knockout models. Unexpectedly, we found that the target of IFN-γ in anti-CD25 F(ab')2 NIR-PIT was endothelial cells of tumor vessels. In both MC38 and EO771 tumor models (murine colon and breast cancer, respectively), CD8 T cells, NK cells, and Tregs were mainly located along the tumor vessels, and depletion of intratumor Tregs by anti-CD25 F(ab')2 NIR-PIT induced IFN-γ expression from these perivascular CD8 T and NK cells for up to 6 hours. This transient but synchronized induction of IFN-γ/STAT1 signaling along tumor vessels caused rapid vessel regression, intratumor ischemia, and apoptotic death of tumor cells. Combination with IL-15 treatment further eradicated residual tumor cells to achieve complete remission of MC38 and EO771 tumors.

Citation Format: Yutaka Kurebayashi, Peter L. Choyke, Hisataka Kobayashi, Noriko Sato. Intratumor regulatory T cells prevent IFN-gamma-dependent tumor vessel regression and can be selectively targeted by anti-CD25 near-infrared photoimmunotherapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3444.