2805 - Mechanistic Study of the Anti-Tumor Effects of PRaG Therapy in a Mouse Colorectal Cancer Tumor Model

Y. He^1,2, Y. Ma^1,2, J. Zhang^1,2, and L. Zhang^1,2; ¹Center for Cancer Diagnosis and Treatment, The Second Af?liated Hospital of Soochow University, Suzhou, China, ²Center of PRaG therapy, The Second Affiliated Hospital of Soochow University, Suzhou, China

Purpose/Objective(s): The efficacy of PD-1 inhibitors is closely related to the tumor microenvironment (TME). The TME consists of a variety of immune cells, including CD8+ T cells, which play a vital anti-tumor role, and myeloid-derived suppressor cells (MDSCs), a key driver of immunosuppression. Recent studies in both pre-clinical and clinical settings have demonstrated that PRaG therapy has the power to trigger immunological responses that can influence disease outcomes. However, the impact of PRaG therapy on the TME is unclear. Therefore, the study explored the changes of immune cell components in the TME of mice, in addition to the type of immune cells through which the anti-tumor effect of PRaG therapy is mainly achieved. Materials/

Methods: We established bilateral subcutaneous tumor model of a mouse colorectal cancer (MC38), 36 female C57BL/6N mice aged 6-8 weeks were divided into six groups: control group, radiation group, radiation + PD-1 inhibitor group, PD-1 inhibitor + Radiation + GM-CSF (PRaG therapy) group, PRaG + anti-CD4 (aCD4) group and PRaG + anti-CD8 (aCD8) group. aCD4 or aCD8 antibodies (100ug, i.p.) were injected before treatment. GM-CSF (200ng, s.c.) was injected around the tumor two days before radiotherapy and continued for 9 days. Right subcutaneous tumor was selected for radiotherapy of 8 Gy×3F. On day 26, the right tumor and the right inguinal lymph node of mice were collected. The proportion of immune cells was detected by flow cytometry. Meanwhile, the effects of different treatments on tumor burden in mice were compared.
Results: Compared with other groups, PRaG therapy reduced the proportion of intratumoral granulocyte-like MDSC and enhances the effector function of CD8+ T cells in tumor-draining lymph nodes. Depletion of CD8+T cells in vivo, but not CD4+T cells, impaired the antitumor effect of PRaG therapy in primary tumor and secondary tumor.
Conclusion: PRaG therapy improves the immune microenvironment in the tumor and tumor-draining lymph nodes of mice with a bilateral subcutaneous tumor model of colon cancer to achieve a better antitumor effect, which is primarily mediated by CD8+T cells.