Efficacy of combining radiotherapy and immunotherapy for metastatic colorectal cancer and the modulation of tumor microenvironment

Abstract

Immune checkpoint blockade antibodies specific to programed death-1/ligand-1 (PD-1/PD-L1) shows durable clinical responses in various cancers but benefit only to a small portion of patients. The failure of immunotherapy is likely due to the complex network of immunosuppressive mechanisms present in advanced tumors. Radiation (RT) modifies tumor microenvironment and causes tumor antigen release, and thus augment efficacy of immunotherapy. Interleukin 12 (IL-12) and granulocyte macrophage colony-stimulator factor (GM-CSF) have shown potent antitumor activities through activating innate and adaptive immunities, as well as reprogramming myeloid cells to a phenotype that favors immune activation. The aim of this project is to compare the therapeutic efficacy of two types of combination therapy: RT combined with IL-12/GM-CSF (RT/12GM) versus RT combined with anti-PD-L1 antibody (RT/PD-L1), and to dissect the molecular mechanisms of the antitumor responses. Our data showed that RT/12GM led to tumor regression in a majority of mice with large established tumors and suppressed liver metastasis, while RT/PD-L1 could only suppress primary tumors but not metastatic tumor. Mechanistic analysis showed that RT/12GM was superior to RT/PD-L1 in inducing accumulation of CD8⁺ T cells in tumor and in augmenting both NK and CD8⁺ T cells cytotoxicity in tumor and liver. Interestingly, RT/12GM also induced infiltration of tumor-associated neutrophils (TAN) and impaired their immunosuppressive functions. Depletion experiments showed that both CD8⁺ T cell and TAN were required for tumor regression in RT/12GM. Together, our results demonstrate that RT/12GM represent a powerful alternative therapy against both primary and metastatic tumors.