Impact of PRaG Therapy on Immune Microenvironment of Bilateral Subcutaneous Tumor Model of Colon Cancer

Abstract

Immune microenvironment is closely related to the efficacy of PD-1 inhibitors. The immune microenvironment contains a variety of immune cells, including effector T cells (cytotoxic CD8+T cells and effector CD4+T cells), dendritic cells (DC), and Myeloid-derived suppressor cells (MDSCs). The antitumor effects of PRaG therapy have been confirmed in bilateral subcutaneous transplantation tumor model of colon cancer. But the impact of PRaG therapy on immune microenvironment of such model is unclear. Therefore, the study continued to reveal the changes of immune microenvironment in mice. 80 male Balb/c mice aged 6-8 weeks were divided into five groups: control group, PD-1 inhibitor group, radiation group, radiation + PD-1 inhibitor group, and radiation + PD-1 inhibitor +GM-CSF (PRaG therapy) group. Bilateral subcutaneous tumor model of colon cancer in mice was constructed. 3×105 CT26.WT cells were inoculated subcutaneously in the right thigh root, and then the left thigh root 3 days later. Right subcutaneous tumor was selected for radiotherapy of 8 Gy×3. GM-CSF (100ng, i.p.) was given on the 1st day and PD-1 inhibitor (0.25mg/kg, i.p.) was given on the 2nd day after radiotherapy with one cycle every 3 days. On day 15, the spleen, left inguinal lymph node and left subcutaneous tumor of mice were collected. The proportion of immune cells was detected by flow cytometry. Compared with other groups, PRaG therapy decreased the proportion of cDC1 in left inguinal lymph node, increased the proportion of cDC2 in left subcutaneous tumor and left inguinal lymph node. Moreover, PRaG therapy increased the proportion of CD8+ effector memory T cells and CD226+CD8+T cells in left inguinal lymph nodes. Finally, PRaG therapy increased the proportion of CD4+, CD8+ central memory T cells and CD69+CD8+T cells and reduced the proportion of M-MDSCs in spleen. PRaG therapy can improve the immune microenvironment of spleen, unirradiated tumors and inguinal draining lymph nodes of bilateral subcutaneous tumor model of colon cancer in mice.