Radiotherapy with sequential αnti-PD-1 mAb compared with concurrent αnti-PD-1 mAb for enhancing the abscopal effect by promoting the inflammatory tumor microenvironment.

Abstract

e15122 Background: Radiotherapy (RT) is one of the main approaches for treatment of lung cancer. A large number of preclinical and several clinical studies revealed that RT can activate the host’s anti-tumor immune system and induce abscopal effect. However, the optimal timing and its underlying mechanism of the combination of RT and immune checkpoint inhibitor (CHI) are unclear. Methods: MC38 cells were used for subcutaneous tumor formation in the left lower limbs (irradiation field) and the right lower limbs (non-irradiation field) of six-eight weeks old male C57BL/6 mice. When the tumor volume reached 150mm3, the mice were randomly assigned to receive RT (8Gy × 3F) with concurrent anti-PD-1 monoclonal antibody (mAb) or RT with sequential (1 week after the first irradiation) anti-PD-1 mAb (group B). The changes of immune microenvironment in irradiated and non-irradiated tumors in the two groups were analyzed by flow cytometry. Results: Both RT with concurrent anti-PD-1 mAb and RT with sequential anti-PD-1 mAb could inhibit the growth of tumors in non-irradiation field. The number of CD4+T cells (CD45+CD44+CD4+T) and effector CD4+T cells (CD44+CD4+IFNγ + T) in non-irradiated tumors in the sequential group was significantly higher than that in the concurrent group, but which showed no significant difference between the two groups in irradiated tumors. Interestingly, CD8+T cells (CD45+CD44+CD8+T) and effector CD8+T cells (CD44+CD8+IFNγ + T) increased both in the irradiation and non-irradiation field in the sequential group compared with that in the concurrent group, while tumor associated macrophages (TAM, F4/80+CD11b+) and Treg cells (CD4+FoxP3+) in the sequential group decreased significantly. RT with sequential anti-PD-1 mAb reduced more exhausted CD8+T cells (PD-1+Eomes+of CD8+T) and induced more reinvigoration of exhausted T cell (Ki-67+PD-1+Emoes+CD8+T). The decrease of MDSCs (Gr1+CD11b+) in the non-irradiated tumors was observed in both groups, but there was no difference in the extent of this decrease between two groups. Conclusions: Compared with the concurrent combination, RT with sequential anti-PD-1 mAb is more effective in promoting the inflammatory tumor microenvironment in out-field tumors and inducing the abscopal effect, which provides a new perspective for further exploring the mechanism of optimal timing of combination. Funding: 81972853, 81572279, 2016J004, LC2019ZD009, 2018CR033.