Abstract 2260: Radiotherapy enhances newly infiltrating CD8+T cells into tumor tissue to increase intratumor CD8+T cells and exert an anti-tumor effects
Authors: Yamashita Kimihiro , Fukuoka EIJI , Sugita Yutaka , Arimoto Akira , Watanabe Akihiro , Takiguchi Gousuke , Urakawa Naoki , Hasegawa Hiroshi , Yamamoto Masashi , Kanaji Shingo , Matsuda Yoshiko , Matsuda Takeru , Matsuda Takeru , Oshikiri Taro , Nakamura Tetsu , Suzuki Satoshi , Kakeji Yoshihiro 
Published: September 2020

Abstract
Background: The enhancement of antitumor immune response with radiotherapy was reported recently, but there are few reports on the mechanism of the effect in rectal cancer. In neoadjuvant chemoradiotherapy (NACRT) for rectal cancer, we reported that CD8+T cells in tumor tissues were highly induced and the density of CD8+T cells were a favorable prognostic factor. The aim of the present study was to elucidate the dynamics of CD8+T cells in the tumor after irradiation.
Methods: For this purpose, we established a therapeutic model of irradiation for BALB/c mice inoculated with CT26 colon cancer cell line. They are irradiated with a single dose of 10Gy after 14 days. We analyzed CD8+ T cells infiltration of both irradiated (mice) and no treated (control) mice by flow cytometry. Intratumor CD8+ T cells were fractionated into three populations according to the expression of PD-1 and Tim-3. The numbers and ratios were measured and their dynamics were analyzed. Next, purified CD8+T cells from the tumor with or without irradiation and intracellular cytokines such as IFNγ, TNF-α, and IL-2 were measured. Finally, we analyzed mice treated with FTY720 to test newly infiltrating T cell infiltration after irradiation delivered locally into tumors.
Results: In our model, elevated numbers of tumor-infiltrating CD8+ T cells between day11 and day18 after irradiation were observed in tumors of mice compared to control. We observed that the number of PD-1+Tim-3+ CD8+ T cells, that comprised the major population among intratumoral CD8+ T cells, were markedly increasing in irradiated mice. Functionally, intratumoral CD8+ T cells in irradiated mice produced more INF-γ compared to control. Especially, PD-1+Tim-3+ CD8+ T cells enhanced INF-γ production. Among the three populations, PD-1+Tim-3+ CD8+ T cells contained the largest fraction of effector memory (CD44hiCD62Llow) cells, but the lowest fraction of central memory (CD44hiCD62Lhi) cells. Finally, after administration of FTY-720, the decrease rate of intratumoral CD8+ T cells in irradiated mice was much higher than in control. This results suggest that newly infiltrating CD8+ T cell number in irradiated mice was higher than in control.
Conclusions: In mouse model of irradiation, there were newly infiltrating CD8+ T cells which was markedly increased compared to the control. This population may be involved in the increase of intratumor CD8+ T cells and may contribute to antitumor effects. This change is expected to trigger further therapeutic intervention.
Citation Format: Kimihiro Yamashita, EIJI Fukuoka, Yutaka Sugita, Akira Arimoto, Akihiro Watanabe, Gousuke Takiguchi, Naoki Urakawa, Hiroshi Hasegawa, Masashi Yamamoto, Shingo Kanaji, Yoshiko Matsuda, Takeru Matsuda, Takeru Matsuda, Taro Oshikiri, Tetsu Nakamura, Satoshi Suzuki, Yoshihiro Kakeji. Radiotherapy enhances newly infiltrating CD8+T cells into tumor tissue to increase intratumor CD8+T cells and exert an anti-tumor effects [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 2260.