Radiotherapy Combined with PD-L1 Antibody Exerts a Synergistic Anti-tumor Effect in a Mouse Model of Esophageal Squamous Cell Carcinoma

Abstract

The combination of radiotherapy and immune checkpoint inhibitors (ICIs) has been shown to exert synergistic anti-tumor effects in various tumors. In esophageal squamous cell carcinoma (ESCC), several clinical trials of radiotherapy combined with ICIs are undergoing or have been finished. However, the efficacy and action mechanisms of radiotherapy combined with ICIs is still not clear in ESCC. Our study aimed to investigate whether radiotherapy improved the anti-cancer effect of PD-L1 antibody and the effect of this combination therapy on tumor-immune microenvironment. The mouse esophageal cancer cell line mEC25 was implanted into the armpits of female C57 mice. When the tumor volume grew to 150-250 mm3, the mice were randomly divided into different groups including control group (administration of IgG antibody), radiotherapy group (12 Gy at 4 Gy per fraction), PD-L1 antibody group (200 mg/kg i.p. for three times) and combination group (fractionated radiation combined with PD-L1 antibody was delivered on day 1, day 4 and day 7, the day when fractionated radiation began was recorded as day 1).The mice were sacrificed on the 1st, 3rd and 7th day after the treatment was ended. The tumor, spleen and draining lymph nodes of the mice were collected and analyzed using flow cytometry (FCM), and the tumor volume and survival time of mice were calculated. Compared with radiotherapy or PD-L1 antibody alone, the combination therapy significantly prolonged the survival time of mice and decreased the growth rate of xenograft tumors FCM analysis showed that the combination therapy significantly increased the infiltration and cytotoxicity of effector T cells and reduced the proportion of M2 macrophages in tumor, spleen and lymph nodes. In addition, in tumor and spleen, the proportion of regulatory T cells (Tregs) decreased in the group of combination therapy. In tumor, spleen, and lymph nodes, the proportion of central memory T cell (TCM) was increased in the group of combination therapy. The anti-tumor effect of radiotherapy combined with PD-L1 antibody is superior to single treatment in the mouse model of esophageal cancer. Radiotherapy can shape tumor-immune microenvironment, allowing the infiltration of effector T cells and exclusion of immune-suppressive cells such as M2 macrophages and Tregs.